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Detrital thermochronology—Provenance analysis, exhumation, and landscape evolution of mountain belts
Edited by Matthias Bernet Laboratoire de Géodynamique des Chaînes Alpines Université Joseph Fourier Maison des Géosciences 1381 rue de la piscine BP 53, 38041 Grenoble Cedex 9 France and Cornelia Spiegel School of Earth Sciences University of Melbourne Victoria 3010 Australia
Special Paper 378
3300 Penrose Place, P.O. Box 9140
Boulder, Colorado 80301-9140 USA
2004
Copyright © 2004, The Geological Society of America, Inc. (GSA). All rights reserved. GSA grants permission to individual scientists to make unlimited photocopies of one or more items from this volume for noncommercial purposes advancing science or education, including classroom use. For permission to make photocopies of any item in this volume for other noncommercial, nonprofit purposes, contact the Geological Society of America. Written permission is required from GSA for all other forms of capture or reproduction of any item in the volume including, but not limited to, all types of electronic or digital scanning or other digital or manual transformation of articles or any portion thereof, such as abstracts, into computer-readable and/or transmittable form for personal or corporate use, either noncommercial or commercial, for-profit or otherwise. Send permission requests to GSA Copyrights Permissions, 3300 Penrose Place, P.O. Box 9140, Boulder, Colorado 80301-9140, USA. Copyright is not claimed on any material prepared wholly by government employees within the scope of their employment. Published by The Geological Society of America, Inc. 3300 Penrose Place, P.O. Box 9140, Boulder, Colorado 80301-9140, USA www.geosociety.org Printed in U.S.A. GSA Books Science Editor: Abhijit Basu Library of Congress Cataloging-in-Publication Data Detrital thermochronology : provenance analysis, exhumation, and landscape evolution of mountain belts / edited by Matthias Bernet, Cornelia Spiegel. p.cm. -- (Special paper ; 378) Includes bibliographical references. ISBN 0-8137-2378-7 (pbk.) 1. Fission track dating. 2. Argon-Argon dating. #. Zircon--Alps--Analysis. 4. Geology, Structural--Europe, Central. 5. Earth temperature. 6. Marine sediments--Alps, Western. 7. Orogeny--Alps, Western. 8. Orogeny--Antarctica. I. Bernet, Matthias, 1967- II. Spiegel, Cornelia, 1971- III. Special papers (Geological Society of America) ; 378. QE508.D47 2004 551.7’01—dc22 2004040371 Cover, front: Liebig Range of the Southern Alps on the South Island of New Zealand in the Mount Cook National Park, looking to the north, with the Pukaki River in the foreground. Back: Southern Alps on the South Island of New Zealand near Lewis Pass looking to the east, with the Hope River in the foreground. Photos by M. Bernet, 2003.
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Contents
Introduction: Detrital thermochronology Matthias Bernet and Cornelia Spiegel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.
Characterizing the significance of provenance on the inference of thermal history models from apatite fission-track data—A synthetic data study. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Andrew Carter and Kerry Gallagher
2.
Fundamentals of detrital zircon fission-track analysis for provenance and exhumation studies with examples from the European Alps. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 Matthias Bernet, Mark T. Brandon, John I. Garver, and Brandi R. Molitor
3.
Toward a comprehensive provenance analysis: A multi-method approach and its implications for the evolution of the Central Alps . . . . . . . . . . . . . . . . . . . . . . 37 Cornelia Spiegel, Wolfgang Siebel, Joachim Kuhlemann, and Wolfgang Frisch
4.
Miocene siliciclastic deposits of Naxos Island: Geodynamic and environmental implications for the evolution of the southern Aegean Sea (Greece). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 J. Kuhlemann, W. Frisch, I. Dunkl, M. Kázmér, and G. Schmiedl
5.
Detecting provenance variations and cooling patterns within the western Alpine orogen through 40Ar/39Ar geochronology on detrital sediments: The Tertiary Piedmont Basin, northwest Italy . . . . . . . . . . . . . . . . . . . . . . . . 67 B. Carrapa, J. Wijbrans, and G. Bertotti
6.
Siliclastic record of rapid denudation in response to convergent-margin orogenesis, Ross orogen, Antarctica . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 John W. Goodge, Paul Myrow, David Phillips, C. Mark Fanning, and Ian S. Williams
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Geological Society of America Special Paper 378 2004
Introduction: Detrital thermochronology Matthias Bernet* University of Canterbury, Department of Geological Sciences, Christchurch, New Zealand Cornelia Spiegel* University of Melbourne, School of Earth Sciences, Victoria 3010, Australia
INTRODUCTION
in combination with other techniques. The paper by Spiegel et al. (Chapter 3) deals with the exhumation history of the Central European Alps based on the combination of zircon fission-track data, Nd isotope ratios of detrital epidote and sediment accumulation rates in the foreland basin. Another multidisciplinary study, using detrital zircon fission-track analysis combined with grain-size analysis and petrology, is presented by Kuhlemann et al. (Chapter 4) to reveal the history of Miocene geodynamic and environmental changes on Naxos, in the southern Aegean Sea. The study by Carrapa et al. (Chapter 5) focuses on the provenance aspect of detrital white mica 40Ar-39Ar analysis, using the Tertiary Piedmont basin and the eroding Western and Ligurian Alps as an example. The contribution by Goodge et al. (Chapter 6) combines U-Pb ages on detrital zircon with 40Ar-39Ar analysis on detrital white mica to examine the exhumational history of the Ross Orogen in Antarctica.
In recent years, an increasing number of studies have used single-grain age dating of detrital sediments to obtain information on sediment provenance, the thermal history and exhumation of sediment source areas, and landscape evolution. In order to highlight the value of this current development in geological research, we based this volume on papers presented in a session on detrital thermochonology at the 2002 Geological Society of America Annual Meeting held in Denver, Colorado. This Special Paper provides a short overview of the different, nowadays commonly used, dating techniques in detrital thermochronology, followed by discussion of some of the methodological aspects of detrital thermochronology (in the first two manuscripts) and of a variety of applications of detrital thermochronology, demonstrated in several regional case studies. The contributions in this special paper are as follows. The paper by Carter and Gallagher (Chapter 1) examines the role of provenance and inherited information in the inference and resolution of thermal histories from detrital apatite fission-track data. This is done on a set of synthetic samples with variable predepositional and postdepositional components in the total thermal history. The second methodological contribution, by Bernet et al. (Chapter 2), addresses some of the fundamental aspects of detrital zircon fission-track analysis for provenance and exhumation studies using zircon from sediment of rivers that drain the European Alps. These authors put particular emphasis on the detection of grain-age components and the possible influence of etching of zircon fission tracks on the observed grain-age distribution. The following manuscripts deal with the application of detrital thermochronology in various geologic settings, often
DATING TECHNIQUES Detrital thermochronology relies on dating techniques which allow dating of single grains. Most commonly used are U-Pb dating of zircon, 40Ar-39Ar dating of white mica, fissiontrack analysis of apatite and zircon, and more recently (U-Th)/He dating of apatite and zircon. While U-Pb dating on zircons provides crystallization ages, 40Ar-39Ar, fission-track, and (U-Th)/He dating yield cooling ages. Interpretation of such cooling ages is based on the closure temperature concept of Dodson (1973, 1979). This concept states that decay products such as daughter isotopes from radioactive decay or fission tracks from spontaneous fission events within a crystal are lost as long as the crystal is above a critical temperature, the closure or blocking temperature
*Present address, Bernet: LGCA, Université Joseph Fourier, 38041 Grenoble Cedex 9, France,
[email protected]. Present address, Spiegel: Geologisches Institut, Universität Tübingen, Sigwart Str. 10, D-72076 Tübingen, Germany,
[email protected]. Bernet, M., and Spiegel, C., 2004, Introduction: Detrital thermochronology, in Bernet, M., and Spiegel, C., eds., Detrital thermochronology—Provenance analysis, exhumation, and landscape evolution of mountain belts: Boulder, Colorado, Geological Society of America Special Paper 378, p. 1–6. For permission to copy, contact
[email protected]. © 2004 Geological Society of America.
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of the specific isotopic system. That means the rate at which the daughter isotopes diffuse out of the system or fission tracks are annealed is faster than the rate at which they are produced. The decay products are retained in the crystal lattice as soon as the crystal cools below the closure temperature. Cooling can be postmagmatic or caused by tectonic and erosional exhumation. Here, we give a short overview of the different dating techniques and their principal applications in detrital thermochronology to provide the reader with some background information. We refer the interested reader to the references mentioned in the text for further information on individual techniques. U-Pb Geochronology The U-Pb technique was first applied for dating zircons in the 1950s (Vinogradov et al., 1952; Tilton and Aldrich, 1955) and is therefore one of the most established dating methods. Nevertheless, development and refinement are still in progress. This technique is based on the radioactive decay of U and Th, which can be described by the following simplified equations: U = 84He + 6β + 206Pb
(1)
U = 74He + 4β + 207Pb
(2)
Th = 64He + 4β + 208Pb
(3)
238
235
232
The fact that U-Pb dating involves two different U isotopes decaying at different rates to two different Pb isotopes provides the possibility to detect potential isotopic disturbances. In an undisturbed system, both the 238U-206Pb- and 235U-207Pb-decay should have consistent ratios yielding the same ages and plot on a so-called Concordia diagram (Wetherill, 1956). Isotopic disturbances, in contrast, result in discordant ages, which are mainly caused by Pb losses because the Pb atom is too large and of incorrect charge to be easily incorporated in the zircon crystal lattice. Development of single grain and in situ measurement techniques in the 1980s and 1990s enlarged the application of U-Pb dating to sedimentary rocks. Most commonly used are ion-microprobe based techniques, such as SHRIMP (sensitive high-resolution ion microprobe) dating (Compston et al., 1982). Because SHRIMP dating involves only small areas of the zircon grain (~30 µm diameter; Williams, 1992) the problem of discordant ages is less pronounced than for conventional isotope dilution methods. Another rapidly developing in situ dating method is the laser ablation–inductively coupled plasma mass spectrometry (LA-ICPMS; Feng et al., 1993; Fryer et al., 1993; Li et al., 2001; Košler et al., 2002; Tiepolo, 2003). Compared to SHRIMP dating, LA-ICPMS is less precise but faster and cheaper. Therefore, LA-ICPMS is especially interesting for provenance studies, which usually require the measurement of 50–100 grains per sample in order to include all major sedimentary source components (Dodson et al., 1988; Bernet et al., this volume). The main problems of LA-ICPMS are an instrumental mass bias and
a laser-induced elemental fractionation of U and Pb due to volatility differences. These can be corrected externally by repeated measurements of a standard (e.g., Tiepolo, 2003) or internally by aspirating a Tl-U tracer with known ratios coeval with laser ablation (Chenery and Cook, 1993; Parrish et al., 1999; Horn et al., 2000; Košler et al., 2002). Ar-39Ar Analysis
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The 40Ar-39Ar dating method was first described by Merrihue and Turner (1966). Information on this dating technique can be found in McDougall and Harrison (1999). Like the K-Ar dating technique, the 40Ar-39Ar method is principally based on the decay of 40K to 40Ar but 40K is measured in an indirect way: K-bearing minerals, such as K-feldspars, amphiboles, or white mica, are irradiated with fast neutrons in a nuclear reactor. Irradiation causes the formation of 39Ar by a neutron capture, proton emission reaction . 39Ar can be used as a measure 39 39/40 for K, and, since the K ratio is constant in the crust, as a measure for 40K. The 40Ar-39Ar method has the advantage over K-Ar dating in that K and Ar are determined on the same sample and that only measurements of Ar isotope ratios instead of absolute concentrations are required. Therefore, this method can be used to date relatively small samples. The most commonly used mineral in 40 Ar-39Ar studies of detrital grains is white mica (e.g., von Eynatten et al. 1996; Najman et al., 2001; Carrapa, 2002; von Eynatten and Wijbrans, 2003; Carrapa et al., this volume). The closure temperature of the 40Ar-39Ar system for white mica is ~350–420 °C, depending on crystal size, cooling rate, and the kind of mica (phengite, muscovite, etc.) used (McDougall and Harrison, 1999; von Eynatten and Wijbrans, 2003). In the 1980s, laser microbeam technologies came into general use, allowing outgassing of Ar from small multigrain samples, single crystals, or even parts thereof. Ar is released either by total fusion or by step heating. The latter allows detection of potential Ar loss due to temporary opening of the system (e.g., during heating events). The degassing spectra yielded by step heating may also help to indicate excess Ar, unless the excess Ar is not homogeneously distributed with respect to K. Fission-Track Analysis Fission tracks are damaged zones of the crystal lattice caused by the spontaneous nuclear fission of 238U. The first descriptions of dating minerals using fission-track techniques date back to the 1960s (see summary in Fleischer et al., 1975). A summary of the technique is given in Wagner and van den Haute (1992). In contrast to other thermochronological methods, products of radiogenic decay are not measured by mass spectrometry but by optical determination (i.e., counting of etched fission tracks under a microscope with high [>1000×] magnification). Fissiontrack dating can be performed on every U-bearing mineral, but in practice it is mainly applied to apatite and zircon. If exposed to
Introduction elevated temperatures, fission tracks start to anneal by shortening until above a certain temperature—the total annealing temperature—they disappear completely, which resets the fission-track age to zero. The total annealing temperature is ~110 °C for apatite (Gleadow and Duddy, 1981) and ~240 °C for zircon (Hurford, 1986; Brandon et al., 1998; Bernet et al., 2002). Dating of single detrital grains was enabled by the development of the external detector method (Gleadow, 1981; Hurford and Green, 1983). For this method, a low-uranium mica detector is attached to the mounted apatite or zircon sample and irradiated by thermal neutrons in a nuclear reactor. Irradiation induces the fission of 235U. The induced fission tracks are monitored on the mica detector. The external detector method therefore allows determining the density of spontaneous fission tracks as well as the U-content of the same area of a mineral grain, which in turn allows calculating fission-track ages of individual grains. Because of the relatively low total annealing temperature of apatite, postdepositional processes such as heating during basin subsidence may overprint the provenance ages. Therefore, zircon fission-track dating is more commonly used for provenance studies, but as demonstrated by Carter and Gallagher (this volume) detrital apatite fission-track dating is in principle also suitable for provenance analysis. (U-Th)/He Thermochronology The (U-Th)/He method is again based on the decays of 238U, U and 232Th (see Equations 1, 2, and 3). (U-Th)/He ages are calculated from the accumulation of radiogenic He. The history of the (U-Th)/He dating method was somewhat more turbulent than for the other thermochronometers. The possibility of dating minerals by the accumulation of radiogenic He has been described early (e.g., Strutt, 1905), but the revealed ages were apparently too young and were therefore interpreted as meaningless. Only recently, (U-Th)/He ages were recognized as low-temperature cooling ages (Zeitler et al., 1987), which led to an impressive comeback of the (U-Th)/He method in the past few years (e.g., Wolf et al., 1996; Farley et al., 1996; Reiners and Farley, 1999; Farley, 2000; Reiners et al., 2000, 2002; Reiners, 2002). Today, (U-Th)/He dating is applied on several different minerals but with a clear focus on apatite (closure temperature: ~45–85 °C, Wolf et al., 1996; Farley, 2000, and references therein) and zircon (closure temperature: 180–200 °C, Reiners et al., 2002). An outline of the method is given by Reiners (2002) and Ehlers and Farley (2003). For age measurements, He is first extracted by a furnace or a laser and analyzed by quadropole mass spectrometry. The U and Th contents of the degassed grains are then measured by ICPMS. Finally, the raw ages are corrected for He loss by alphaejection at the rims of the grains (Farley et al., 1996). While zircon (U-Th)/He ages can be used for provenance studies (Rahl et al., 2003), the apatite (U-Th)/He system with its low closure temperature is potentially responsive to changes in topography 235
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and can therefore be used to reconstruct paleorelief (e.g., House et al., 1998). APPLICATIONS OF DETRITAL THERMOCHRONOLOGY The three main applications of detrital thermochronology—provenance analysis, landscape evolution, and exhumation studies—are closely related to each other, and one hardly occurs without the others. However, the most fundamental application is the provenance aspect, from which landscape evolution and orogenic exhumation studies are commonly the derivatives. Provenance Analysis Sediment provenance analysis is one of the most important applications of detrital thermochronology (e.g., Hurford and Carter, 1991; Ireland, 1992; Gehrels et al. 1999; Carter, 1999; Gehrels, 2000). The reason for this is that sediment source areas, especially in active mountain belts, show distinct patterns of UPb crystallization, or Ar-Ar, fission-track, and (U-Th)/He cooling ages. It is very common that cooling ages derived from bedrock samples tend to cluster in certain age groups instead of representing a continuum of ages across the orogen. To identify the different age clusters, 50–100 grains per detrital sample should be dated (Dodson et al., 1988; Bernet et al., this volume). From the attained single-age distributions, major-age components are derived (e.g., Brandon, 1996; Sircombe, 2000). These age components are compared with the age patterns of the hinterland and correlated with specific source areas. While detrital grain ages provide valuable provenance information, new efforts are being made to combine different dating techniques like fission-track dating and U-Pb dating of the same zircon suites (Carter and Moss, 1999; Carter and Bristow, 2000), or U-Pb and (U-Th)/He dating of the same individual zircons (Rahl et al., 2003) to optimize provenance information. An additional detrital dating technique is single pebble or pebble population dating with the fission-track method using conglomerates from foreland basin deposits (Dunkl et al., 1998; Spiegel et al., 2001; Brügel et al., 2004). This technique has the advantage that, besides the cooling ages, the additional petrographic information helps to identify lithotectonic units in the source area. Another aspect of detrital thermochronology in provenance analysis is its application in landscape evolution studies. Provenance information can be used to trace pathways of sediment transport (Cawood and Nemchin, 2001) and detect positions of drainage divides relative to distinct tectonic units (von Eynatten et al., 1999; Spiegel et al., 2001; von Eynatten and Wijbrans, 2003). Such information can be invaluable in reconstructing the dynamic evolution of orogens through time. Furthermore, this information can help the reconstruction of paleocurrent directions and identify marine connections, which in turn have
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cations on paleoecology and climatic evolution (Kuhlemann et al., 1999 and this volume). Information on landscape evolution can also be revealed in a more direct approach. Systems such as apatite fission-track and apatite (U-Th)/He, which close at shallow crustal levels, hold a high potential to detect changes in landscape and topography evolution (Stuewe et al., 1994; House et al., 1998; Braun, 2002), while 40Ar-39Ar, zircon fission-track and zircon (U-Th)/He have relatively high closure temperatures, as mentioned above, and are somewhat inert to detect changes in topography. However, this aspect of detrital thermochronology has not been intensely studied so far and could easily be combined with cosmogenic dating of detrital sediment (Schaller et al., 2001, 2002). Thermal and Exhumational Evolution of Orogens For studying the thermal history of orogens, the focus has been mainly on thermochronology of currently exposed bedrock. While in situ thermochronological analysis of bedrock was very successful for determining the tectonic and exhumational history of local areas in orogens like the Himalayas or European Alps (e.g., Treloar et al., 1989; Hunziker et al., 1992), detrital thermochronology allows studying the long-term record of orogenic exhumation and landscape evolution on a local and regional scale. Synorogenic sedimentary rocks are the remnants of rock exposed at Earth’s surface in the past that have since been eroded. The thermal information of previously exposed bedrock is retained in sedimentary basins adjacent and connected to orogenic highlands. Dating detrital grains with the above-mentioned techniques provides information on exhumation of orogenic sediment sources (Zeitler et al., 1986; Cerveny et al., 1988; von Eynatten et al., 1996; Lonergan and Johnson, 1998; von Eynatten and Gaupp, 1999; von Eynatten et al., 1999; Garver et al., 1999; Spiegel et al., 2000; Bernet et al., 2001; Carrapa, 2002). If the time of deposition of synorogenic sediment is known, it can be subtracted from the cooling ages derived from 40Ar-39Ar, fission-track, and (U-Th)/He dating. The time difference between the cooling age and the depositional age is the so-called lag time (e.g., Garver et al., 1999). Lag time integrates between the time of cooling to the closure temperature of the used dating technique, over the time needed for exhumation of deep seated crustal rocks to the surface, to the time of erosion, transport, and deposition. The time for erosion and transport is usually regarded as geologically instantaneous compared to the time needed for exhumation. The calculated lag time can be converted into long-term average exhumation rates (Garver et al., 1999), which can be different from short-term exhumation or erosion rates, as determined by cosmogenic dating (Schaller et al., 2001, 2002). Overall, every cooling age can be converted into an exhumation rate knowing the closure temperature of the system used and assuming a temporally constant geothermal gradient. Dating techniques with lower closure temperatures such as apatite fission-track and apatite (U-Th)/He dating, which close at lower
crustal levels, are much more sensitive to short-term variations in surface erosion rates than techniques with higher closure temperatures, such as 40Ar-39Ar dating. U-Pb zircon ages are crystallization ages and therefore are mainly used for provenance analysis or to determine exhumation rates of synorogenic crystalline rocks. CONCLUSIONS Detrital thermochronology provides a variety of opportunities to study long-term evolution of mountain belts and their cooling history, as well as changes in mountainous topography and shifts in major drainage divides. While each individual dating technique offers unique information with respect to provenance and exhumation, the combination of different dating techniques on the same samples, or if possible on the same grains, holds great potential for future research in deciphering the record of orogenic exhumation and evolution, which is preserved in sediments and sedimentary rocks. Insights gained from detrital thermochronology can be complemented with additional data from other geological observations, including, but not limited to, other provenance techniques, sediment budget calculations, convergence rates, dynamic modeling, or erosion rates determined from cosmogenic radionuclide dating. The options seem to be unlimited. Improvement of existing techniques and the development of new and combined approaches will help to improve our understanding of how orogens and their associated sedimentary basins evolve through time. ACKNOWLEDGMENTS We are thankful to Geological Society of America Books Science Editor Abhijit Basu for inviting us to compile this Geological Society of America Special Paper, and for his enthusiastic support, continuous valuable advice, and patience, especially during the final stages of this project. We are also grateful to Peter van der Beek, Manfred Brix, Bernhard Fügenschuh, Matt Heizler, Richard Ketcham, John Miller, Nancy Naeser, Yani Najman, Meinert Rahn, Stuart Thomson, and Barbara Ventura who provided us with quick and critical yet constructive reviews of all the manuscripts. We are also thankful to George Gehrels, Hilmar von Eynatten, Peter van der Beek, Roland Maas, and Wolfgang Siebel, for discussions, critical comments, and improvements on this introduction. REFERENCES CITED Bernet, M., Zattin, M., Garver, J.I., Brandon, M.T., and Vance, J.A., 2001, Steady-state exhumation of the European Alps: Geology, v. 29, p. 35–38. Bernet, M., Brandon, M.T., Garver, J.I., Reiners, P.W., and Fitzgerald, P.G., 2002, Determining the zircon fission-track closure temperature: Geological Society of America, Cordilleran Section, 98th Annual Meeting: Geological Society of America Abstracts with Programs, v. 34, no. 5, p. 18. Bernet, M., Brandon, M.T., Garver, J.I., and Molitor, B., 2004, Fundamentals of detrital zircon fission-track analysis for provenance and exhumation studies with examples from the European Alps, in Bernet, M., and Spiegel, C.,
Introduction eds., Detrital thermochronology—Provenance analysis, exhumation, and landscape evolution of mountain belts: Geological Society of America Special Paper 378, p. 25–36 (this volume). Brandon, M.T., 1996, Probability density plot for fission track grain-age samples. Radiation Measurements, v. 26, p. 663–676. Brandon, M.T., Roden-Tice, M.K., and Garver, J.I., 1998, Late Cenozoic exhumation of the Cascadia accretionary wedge in the Olympic Mountains, northwest Washington State: Geological Society of America Bulletin, v. 110, p. 985–1009. Braun, J., 2002, Estimating exhumation rate and relief evolution by spectral analysis of age-elevation datasets: Terra Nova, v. 14, p. 210–214. Brügel, A., Dunkl, I., Frisch, W., Kuhlemann, J., and Balogh, K., 2004, Geochemistry and geochronology of gneiss pebbles from foreland molasse conglomerates: Geodynamic and paleogeographic implications for the Oligo-Miocene evolution of the Eastern Alps: Journal of Geology, v. 111, p. 543–563. Carrapa, B., 2002, Tectonic evolution of an active orogen as reflected by its sedimentary record [Ph.D. thesis]: Amsterdam, Vrije Universiteit, 177 p. Carrapa, B., Wijbrans, J., and Bertotti, G., 2004, Detecting provenance variations and cooling patterns within the Western Alpine orogen through 40 Ar/39Ar geochronology on detrital sediments: The Tertiary Piedmont Basin, NW Italy, in Bernet, M., and Spiegel, C., eds., Detrital thermochronology—Provenance analysis, exhumation, and landscape evolution of mountain belts: Geological Society of America Special Paper 378, p. 67–103 (this volume). Carter, A., 1999, Present status and future avenues of source region discrimination and characterization using fission-track analysis: Sedimentary Geology, v. 124, p. 31–45. Carter, A., and Moss, S.J., 1999, Combined detrital-zircon fission-track and U-Pb dating: A new approach to understanding hinterland evolution: Geology, v. 27, p. 235–238. Carter, A., and Bristow, C.S., 2000, Detrital zircon geochronology: Enhancing the quality of sedimentary source information through improved methodology and combined U-Pb and fission-track techniques: Basin Research, v. 12, p. 47–57. Carter, A., and Gallagher, K., 2004, Characterizing the significance of provenance on the inference of thermal history models from apatite fissiontrack data—A synthetic data study, in Bernet, M., and Spiegel, C., eds., Detrital thermochronology—Provenance analysis, exhumation, and landscape evolution of mountain belts: Geological Society of America Special Paper 378, p. 7–23 (this volume). Cawood, P.A., and Nemchin, A.A., 2001, Paleogeographic development of the east Laurentian margin: Constraints from U-Pb dating of detrital zircon in the Newfoundland Appalachians: Geological Society of America Bulletin, v. 113, p. 1234–1246. Cerveny, P.F., Naeser, N.D., Zeitler, P.K., Naeser, C.W., and Johnson, N.M., 1988, History of uplift and relief of the Himalaya during the past 18 million years: Evidence from fission-track ages of detrital zircons from sandstones of the Siwalik Group, in Kleinspehn, K., and Paola, C., eds., New perspectives in basin analysis: New York, Springer-Verlag, p. 43–61. Chenery, S., and Cook, J.M., 1993, Determination of rare earth elements in single mineral grains by laser ablation microprobe-inductively coupled plasma-mass spectrometry—Preliminary study: Journal of Analytical Atomic Spectrometry, v. 8, p. 299–303. Compston, W., Williams, I.S., and Clement, S.W., 1982, U-Pb ages within single zircons using a sensitive high mass-resolution ion microprobe: Abstracts, 30th American Society of Mass Spectrometry Conference, 593-5. Dodson, M.H., 1973, Closure temperature in cooling geochronological and petrological systems: Contributions to Mineralogy and Petrology, v. 40, p. 259–274. Dodson, M.H., 1979, Theory of cooling ages, in Jaeger, E., and Hunziker, J.C., eds., Lectures in isotope geology: Berlin, Springer-Verlag, p. 207–214. Dodson, M.H., Compston, W., Williams, I.S., and Wilson, J.F., 1988, A search for ancient detrital zircons from Zimbabwean sediments: Journal of the Geological Society of London, v. 145, p. 977–983. Dunkl, I., Frisch, W., Kuhlemann, J., and Brügel, A., 1998, Pebble-population-dating: A new method for provenance analysis: Terra Nostra, v. 98/1, p. 45. Ehlers, T.A., Farley, K.A., 2003, Apatite (U-Th)/He thermochronometry: Methods and applications to problems in tectonics and surface processes: Earth and Planetary Science Letters, Frontiers, v. 206, p. 1–14.
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MANUSCRIPT ACCEPTED BY THE SOCIETY OCTOBER 21, 2003
Printed in the USA
Geological Society of America Special Paper 378 2004
Characterizing the significance of provenance on the inference of thermal history models from apatite fission-track data—A synthetic data study Andrew Carter* School of Earth Sciences, University & Birkbeck College, Gower St., London WC1E 6BT, England Kerry Gallagher* Department of Earth Sciences and Engineering, Imperial College of Science, Technology and Medicine, South Kensington, London SW7 2AS, England ABSTRACT The role of provenance and inherited information in the inference and resolution of thermal histories from fission-track data from detrital apatite is examined in a set of synthetic samples with variable predepositional (provenance) and postdepositional (burial) components in the total thermal history. The models and data are used to show how partially reset samples with protracted provenance history lead to underprediction of the maximum burial temperature. Neglect of provenance effects can therefore lead to misinterpretation of postdepositional thermal histories. To avoid this problem, the sample depositional (stratigraphic) age should be routinely used where available to constrain the modeling procedure. We also show how provenance thermal histories can be recovered over a greater temperature range than previously considered. In practice, this will depend on the annealing model appropriate for a given apatite composition. For common fluorapatite samples with a protracted but simple provenance thermal history, this can be as high as 100 °C, rather than just up to 60 °C, as is often inferred Keywords: apatite, thermal histories, modeling procedures, provenance. INTRODUCTION
the last decade, zircon fission-track studies have been directed at monitoring source exhumation, particularly in young orogenic belts where sediment routing systems are known and the time lag between exhumation and deposition is short (Lonergan and Johnson, 1998; Garver et al., 1999). This information can be used to define when an exhumational steady-state was first reached and is diagnostic of maturity within the orogenic system. Apatite fission-track data have seen little use in provenance studies as a consequence of the considerably lower temperature sensitivity range of the fission-track annealing (e.g., <120 °C for common compositions of apatite). This sensitivity to low temperatures can be a potential problem for recovery of provenance thermal
From its inception, fission-track analysis has been recognized as a powerful tool for detrital studies because the method produces age data gathered from single grains. Common applications related to provenance include constraining the age of biostratigraphically barren sediments and characterization of source terrain in terms of postmetamorphic cooling (exhumation) history (Hurford and Carter, 1991; Carter 1999; Gallagher et al., 1998; Garver et al., 1999). Most detrital fission-track studies use zircon as a consequence of its relative high temperature partial annealing range (200–310 °C; Tagami et al., 1998). Over *E-mails:
[email protected];
[email protected].
Carter, A., and Gallagher, K., 2004, Characterizing the significance of provenance on the inference of thermal history models from apatite fission-track data—A synthetic data study, in Bernet, M., and Spiegel, C., eds., Detrital thermochronology—Provenance analysis, exhumation, and landscape evolution of mountain belts: Boulder, Colorado, Geological Society of America Special Paper 378, p. 7–23. For permission to copy, contact
[email protected]. © 2004 Geological Society of America.
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history information as burial of the host sediment causes partial or total resetting of the population of tracks formed prior to burial (provenance or inherited tracks). Based on predictions from empirical annealing models, it is generally assumed that exposure of detrital apatite to temperatures above ~60 °C (for 106–107 yr) is enough to cause sufficient track shortening (annealing) as to degrade the provenance signal beyond meaningful recovery. Thus, basin sediments from depths below ~1–2 km tend to be rejected for study as they are considered beyond the scope of apatite fission-track data. However, this assumption has never been systematically tested even with synthetic data. Another aspect regarding provenance tracks that has also not seen any systematic study is the influence that inherited tracks have on the inference of thermal histories from apatite fissiontrack data, one of the methods major applications (Gallagher et al., 1998). Evidence from sedimentary basin maturation studies suggest that detrital apatite, under certain conditions, can retain a significant component of fission tracks formed prior to deposition. A number of factors related to provenance have the potential to degrade thermal history reconstruction from fission-track data obtained from sedimentary rocks, even for monocompositional apatites. These include multiple source ages with differing styles of predepositional thermal history, and the relative duration of the predepositional and postdepositional thermal histories. This aspect is particularly relevant to passive margin studies (e.g., Gallagher and Brown, 1997, 1999), where there may be a significant time lag between pre-breakup thermal resetting and later erosion and burial in an offshore basin, potentially leading to a nontrivial provenance component in the detrital apatites. Table 1 provides a summary of apatite fission-track ages from the onshore region of various passive margins relative to the time of breakup to illustrate this. The difference between the fission-track age of the onshore basement rocks and the time of breakup can be used as an indication of the minimum duration of track accumulation prior to post-breakup erosion and subsequent deposition offshore.
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The influence of inherited tracks on modeled postdepositional thermal histories, especially in the absence of independent thermal maturation indicators such as vitrinite reflectance data, is unclear. Currently, studies that use detrital apatite to monitor sediment postdepositional thermal histories generally apply the rule that a component of inheritance or provenance-related tracks may be present if the measured fission-track age is older than the stratigraphic age of the host sediment. However, the converse is not true in that a measured fission-track age younger than the stratigraphic age does not necessarily imply the provenance signature has been eliminated. Use of vitrinite reflectance data (see Arne and Zentilli, 1994) to constrain maximum depositional temperatures independently, although useful, cannot provide direct information on how many inherited tracks survived after deposition. Although this contribution is primarily aimed at exploring the potential for extended recovery of apatite fission-track source information from basin sediments, we also consider the issue of how provenance related (inherited) tracks influence thermal history modeling. It may be a case of severely degrading the resolution of paleotemperatures or the timing of maximum paleotemperatures or perhaps introducing systematic bias in the inferred thermal history. In the context of provenance studies, current modeling procedures are often ad hoc and not based on any quantitative understanding of the role of the thermal history. Thus, any guidelines for appropriate strategies to adopt when modeling sedimentary samples are desirable. The following questions clearly require further investigation. • Is there a critical postdepositional temperature above which the inherited (source) signal becomes modified beyond recognition? • How does the shape of the inherited track length distribution influence the inferred postdepositional thermal history? • How significant are the relative proportions of predepositional to postdepositional tracks for the inference of the thermal history?
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A synthetic data study • What is the relationship between the maximum predepositional, postdepositional temperatures and the observed fission-track age and track length distribution? • How important are independent geological constraints in a predominantly data driven approach to thermal history modeling when provenance effects are significant? MODELING APATITE FISSION-TRACK DATA As a consequence of postdepositional annealing, the potential for the recovery of source information from detrital apatite fission-track data requires understanding their thermal history. The data constituting a typical fission-track analysis are considered in terms of two basic components. The first is the fission-track age, which is typically some average estimate of single grain ages from 20 to 30 individual apatite crystals (for provenance studies the number of grains analyzed will generally be higher in order to detect multiple sources). The simplest interpretation of the fission-track age is as a measure of the duration of the thermal history recorded by the sample. But this is only a reliable measure if the sample thermal history involved rapid cooling from high (>100 °C) to low temperature (<60 °C), and the measured single grain ages are statistically consistent with a single parent population (a Poissonian distribution is appropriate for fission-track age counting statistics). In most cases, the fission-track age is a minimum measure of the thermal history duration as a consequence of annealing: Old ages indicate longterm residence at moderately low temperatures. In sediments, the apatite grains derived from multiple source regions can lead to a spread of single grain ages beyond that expected for purely Poissonian variation. Consequently, it has become common in the past 10 years to consider the distribution of single grain ages in terms of the number of possible age components contributing to the overall spread in ages (Galbraith and Green, 1990; Brandon, 1992; Sambridge and Compston, 1994) or in terms of a minimum age component (Galbraith and Laslett, 1993), with the remaining spread being attributed to factors such as multiple provenance or variation in apatite chemical composition (e.g., Cl, F, OH, REE substitutions [Carlson et al., 1999; Barbarand et al., 2003]). The second component of a fission-track analysis is the track length distribution. This typically is characterized by 50–200 confined horizontal fission-track length measurements, and is often reported as the mean track length (MTL) and standard deviation of the distribution and/or a track length histogram. The measured fission-track age depends on the underlying track length distribution, and so track length measurements are essential to the interpretation of fission-track age data. The complete track length data are also essential for modeling as the shape of the distribution provides diagnostic information on the thermal history. For mixed provenance data, the track length data need to be linked to each individual age component and modeled individually. The two basic requirements for deriving quantitative thermal history information from apatite fission-track data are (1) a
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forward model relating track length to temperature and time (i.e., an annealing model), and (2) a methodology to find the optimal data-fitting thermal history (and its uncertainties). Annealing Models A variety of different laboratory calibrated annealing models have been published (Laslett et al., 1987; Crowley et al., 1991; Carlson, 1990; Ketcham et al., 1999). These differ in detail, but given the uncertainties on the data and the models themselves (Barbarand et al., 2003), they produce broadly similar solutions when extrapolated to geological time scales. It is not our goal to address the differences between these various models, as the merit of any particular model is secondary for our simulations. For this study, we adopt the commonly used mono-compositional (Durango apatite) model of Laslett et al. (1987). Variable annealing between grains in response to apatite crystal structure and chemistry (particularly halogen and hydroxyl substitution) can lead to differences in annealing rates (Carlson et al., 1999; Ketcham et al., 1999; Barbarand et al., 2003). The advantage of using a single composition model in this study is that we avoid complexity and uncertainty inherent in current multicomposition and/or multikinetic models (e.g., Ketcham et al., 1999; Barbarand et al., 2003). However, while the general inferences made with a single compositional model will be generally applicable for any apatite population with a restricted compositional range, we acknowledge that variations in annealing properties arising from composition or crystallographic influences are likely to require additional consideration in practice. Modeling Thermal Histories A range of strategies incorporating one or more of the empirical annealing models has been described to derive thermal history information from observed data. These strategies can be classed as forward modeling (Duddy et al., 1988; Green et al., 1989a, 1989b; Willet, 1992; Crowley, 1993) or as optimization procedures using the observed data to drive the sampling of the possible thermal history scenarios (Corrigan, 1991; Gallagher, 1995; Lutz and Omar, 1991; Willet 1998). Forward modeling is useful to assess the viability of particular thermal history scenarios. However, when modeling real data, it is preferable to use some form of optimization strategy. In this case, the data determine the form of the thermal history explicitly, subject to varying levels of independent geological constraints, as available. As a consequence of the nonlinear nature of the annealing models, the optimization methods tend to rely on some form of stochastic sampling. Most of the optimization methods also allow the resolution of the thermal history to be determined in a relatively straightforward fashion (Gallagher, 1995). In the context of the present study, the problem is to assess how provenance-related thermal history information can be extracted from apatite fission-track data. Alternately, we need to consider how the provenance information maps into the thermal history model
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and the consequences of ignoring the possibility of a provenance signal. PROVENANCE SIGNATURES AND THERMAL HISTORY MODELING In this study, we concentrate on how the shape of a track length distribution and the relative durations of predepositional to postdepositional annealing maps into the inferred thermal history, as these are the principal factors used in modeling procedures. We consider only synthetic data, where we prescribe both the predepositional and postdepositional thermal histories. In this situation, we can readily address the relationship between the data, the inferred, and the true thermal histories. This is required in order to define some objective quantitative criteria for dealing with real data in future. Throughout this paper, we refer to modeling in terms of a sedimentary analog whereby the inherited component of the fission-track data reflects the tracks formed (and annealed) during the predepositional phase, and the subsequent heating episode refers to tracks which accumulate during the postdepositional phase, the minimum temperature generally occurring at the time indicated by the stratigraphic age. However, the general concept is also relevant to basement rocks which have been brought close to, or exposed at, Earth’s surface (after some protracted cooling history) and then experienced a later reheating episode due to reburial by later sediments or lava flows, which has occurred in southeast Brazil (Gallagher et al., 1994). Track length reduction at elevated temperatures does not occur at an equivalent rate for tracks of different length and crystallographic orientation. Short tracks perpendicular to the “c” crystallographic axis shorten and disappear more rapidly than longer tracks at high angles to c. Thus, it is intuitive that the form of a track length distribution, particularly the proportion of shorter tracks, will govern the extent to which postdepositional annealing will degrade a provenance signature. But which type of inherited length distribution (type of source cooling history) is less affected by postdepositional heating is unknown. To quantify this, we consider four different track length distributions for the predepositional (source or inherited) component, representative of the range found in nature (e.g., Gleadow et al., 1983). Model 1. A unimodal distribution typical of rapid cooling (“volcanic type” distribution). Model 2. A negatively skewed form consistent with a slow cooling history (“basement type” distribution). Model 3. A bimodal distribution typical of heating/residence at elevated temperature followed by slow cooling. Model 4. A positively skewed distribution typical of heating followed by rapid cooling from temperatures close to a value appropriate for total annealing. Each of these length distributions was used in turn as the starting condition for model thermal histories simulating burial and heating to progressively increasing temperatures. We initially want to examine the variation in the predicted final (or presentday) fission-track age and length distribution as a function of
maximum postdepositional temperature for each model. Also, we consider the relative proportion of predepositional tracks preserved at the present day, relative to the true proportions specified initially. The control models and their predepositional track length distributions calculated for a duration of 100 m.y. are shown in Figure 1. Also shown in this figure are two forms of postdepositional thermal history to demonstrate the effect of different styles of postdepositional annealing on the inherited distribution. The first postdepositional thermal history has heating to 90 °C over 50 m.y. followed by cooling to 0 °C over 50 m.y. The second only has heating over 100 m.y., reaching 90 °C at the present day. Thus, the total duration of the thermal history is 200 m.y. (100 my. of predepositional heating and 100 m.y. of burial) in all cases shown in Figure 1. The two different postdepositional thermal history scenarios suggest that the proportion of inherited tracks preserved in the final length distribution varies little for the four input distributions and is controlled by the maximum postdeposition temperature, at least when it reaches 90–100 °C. Influence of Inherited Tracks on Fission-Track Data We generalized the models shown in Figure 1 by considering a wider range of maximum burial temperature. In Figure 2, we show the fission-track age and the mean track length and standard deviation of the final length distributions for the four models as a function of maximum postdepositional temperature for the two postdepositional thermal histories. In all cases, the predepositional signature is totally lost once the maximum depositional temperature reaches 110–115 °C. The trends in the different parameters (age, mean track length) are broadly similar, but with a small offset between each model, essentially reflecting the differences in the initial track length distribution at the time of deposition. Up to temperatures of 90–100 °C, the offsets are maintained, but decrease in magnitude as the depositional temperature increases. The fission-track age is relatively insensitive to the two different styles of postdepositional history, but the mean track length and standard deviation (and therefore length distributions) do show considerable variation between the two. For a given heating scenario, the fission-track age and mean track length respond to increased burial temperature in a broadly similar way, which is not surprising given the observable track density (i.e., measurable age is linked to track length [Laslett et al., 1984]). As temperature increases, both fission-track age and mean track length decrease accompanied by a convergence between paths for each model. Above ~90–100 °C, the fissiontrack age and length parameters converge for all models, reflecting severe track shortening (annealing) of remnant predepositional tracks, including some track loss. This suggests that above ~90–100 °C, inherited tracks have become annealed to a level where the form of the initial distribution is not important. In Figure 3, we show the proportion of preserved predepositional tracks for each model with respect to burial temperature
A synthetic data study
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Figure 1. The four 100 m.y. control models on the left show the different predepositional thermal histories. The next panels show the control track length distributions for these models with no postdepositional annealing. The third panel shows the effect of postdepositional heating to 90 °C within 50 m.y., followed by cooling to 0 °C in the final 50 m.y. The last panel on the right shows the track length distribution for continuous linear postdepositional heating to 90 °C over 100 m.y., with maximum temperature at the present day. The (unshaded) track length distributions on the left represent the provenance signature at the start of the heating episode. The final track length distribution is comprised of a mixture of preserved (but partially annealed) predepositional (white) and postdepositional (shaded) tracks. The components are stacked on one another.
for the two postdepositional scenarios. The trends are similar for both cases, as is the relative variation for the four initial length distribution models. Not surprisingly, an initial distribution with relatively more long tracks contributes a greater proportion of tracks to the final distribution. Similarly, models converge at high temperatures due to increased annealing, especially above
~90 °C as progressively more tracks are shortened and erased, causing a rapid reversal in the proportion trend. Above 100 °C, the proportion of preserved tracks falls rapidly as tracks become totally annealed. These results again show that the response of the inherited length distribution to later burial and reheating is reflected in the final length distribution, and the preservation of
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Figure 2. Predicted fission-track (FT) age, mean track length, and standard deviation of the length distribution with increasing levels of postdepositional heating (burial temperature) for the four control models and two postdepositional thermal histories shown in Figure 1. The predictions for just the post-burial thermal history are also shown for comparison. Note all models converge as the maximum temperature increases.
predepositional tracks is significant up to temperatures of 90 °C or so (i.e., relatively high degrees of annealing, as the actual temperature will depend on the annealing algorithm adopted for modeling). Overall, models that start with a larger component of long tracks remain older than samples initially containing a higher proportion of shorter tracks. The proportions of short and long tracks in a sample prior to burial will therefore influence the final length distribution (and fission-track age), depending on the level of postdepositional heating. Effect of Relative Duration of Predeposition and Postdeposition Track Accumulation Up to this point, we have only considered the response of the fission-track parameters (age and length distribution) to variation
in burial temperature under conditions where the predepositional and postdepositional periods are of equal duration (i.e., 100 m.y. and a total duration of 200 m.y.). Clearly, as this will commonly vary significantly in nature (e.g., Table 1), it is important to consider the effect of varying the proportion of predepositional to postdepositional tracks (i.e., the potential for swamping the postdepositional track distribution with inherited tracks). Figure 4 summarizes the range of proportions of preserved tracks for each of the four initial length distribution models as a function of the duration of the predepositional history up to 1100 m.y. (providing an effective dilution factor of 10:1 or ten times more predepositional tracks). The results are not significantly different for the two thermal history scenarios (Figs. 4A and 4B). Models 1–3 display similar trends in the proportion of inherited tracks starting with a rapid and then more gradual increase through time, as more predepositional tracks are accumulated
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Figure 4 (on this and following page). A: The proportion of inherited tracks preserved at the present day as a function of increasing pre-burial duration (horizontal axis) and postdepositional burial temperature (vertical axis). The postdepositional duration Fig 3. is kept at a constant 100 m.y. with linear heating to maximum temperatures at 50 m.y., followed by linear cooling to 0 °C at the present day. Track production rate is also constant for all models. B: As in A but with continuous linear heating only and maximum temperatures achieved at the present day.
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Fig.4b and retained in the postdepositional phase. Once the maximum heating temperature reaches 100–110 °C, the proportion of preserved tracks reduces rapidly as all tracks are heavily annealed. The small differences among models 1–3 are caused by variation in timing of cooling from peak predepositional temperatures, which governs the proportion of long tracks, again demonstrating how the shape of the inherited length distribution can influence the response to postdepositional annealing. In contrast, model 4 stays close to a “steady-state” ratio of 0.5 (equal proportions), at least up to a maximum temperature of 100 °C and for short durations (<500 m.y.). This reflects the rapid predepositional cooling causing the relative duration of predepositional and postdepositional heating to be effectively the same for all durations of pre-depositional history (i.e., the maximum predepositional temperature of 100 °C for model 4 always occurs between 100 and 200 m.y.). Observed small rises and falls in the proportion of predepositional tracks over longer durations (>500 m.y.) relate to the rapid annealing and disappearance of very old short tracks. Overall, the results show that changes in the proportion of predepositional tracks associated with increased burial temperature are minor compared to the effect of increased duration of
pre-burial history, provided the burial temperature remains less than ~100 °C. The model track length distributions derived from a monocompositional, isotropic annealing algorithm imply that the most recent tracks are the longest since these will be the least annealed. In contrast, shorter tracks may reflect either predepositional or postdepositional periods, depending on the postdepositional thermal history. The dominance of either component will depend on the relative duration of the predepositional and postdepositional periods (Fig. 4). Clearly, when the duration of the predeposition period is substantially longer than postdepositional duration, the older tracks tend to dominate the distribution. This can be seen in Figures 5A and 5B, which show how the form of the length distributions for models 1 and 4 change with different duration of predepositional history for the same maximum burial temperature (100 °C) (with the two heating scenarios considered previously). Models 2 and 3 are similar to model 1 (as implied from the earlier figures) and are not shown here. In both figures, model 4 shows little sensitivity to duration because the pre-burial heating duration remains effectively constant and similar to the subsequent postdeposition duration.
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Figure 5 (on this and following page). A: Predicted length distributions for models 1 and 4, with a linear heating then cooling postdepositional history. Total duration of thermal histories are 200, 500, and 1000 m.y. Predicted present-day track length distribution is divided into the predeposition component (white), and the postdeposition component (shaded). B: As in A but for the continuous linear heating postdepositional history.
The Effect of Inherited Tracks on Modeled Thermal Histories A noticeable feature of the length distributions in Figures 5A and 5B is the difference in form associated with predepositional tracks. In Figure 5A, for both models, postdepositional tracks have a distinctly different distribution that overlaps with the modified (by the postdepositional heating) predepositional distribution. In contrast, for Figure 5B the two length populations have similar distributions. An implication from these observations is that modeling may not easily be able to distinguish between the predepositional and postdepositional contributions to the final length distribution. This will depend to some extent on how well the stratigraphic age and thermal history of the sediment is known.
One outstanding question is how the form of the initial length distribution can influence the resolution of modeled thermal histories. In particular, we are interested in how independent geological information, such as the stratigraphic age of the host sediment, contributes to improving the resolution of the thermal history. The fact that the stratigraphic age generally represents the time at which the sediment is at its lowest temperature may intuitively suggest that any later burial and heating will lead to annealing and obliteration of inherited tracks. However, this will depend on the extent of annealing achieved prior to deposition. In general, some stratigraphic age information is available and
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this is unlikely to be a major issue. However, there are situations where this may be poorly defined, such as barren red beds (Carter et al., 1995). For rocks that may have undergone several cycles of burial and later exhumation, with no record of the age of younger sediments, there is virtually no control on the time of minimum temperature. To investigate this aspect further, a selection of fissiontrack age and track length distributions characteristic of each inherited distribution model was treated as “observed” data. These synthetic data were modeled without any extra information on postdepositional thermal history (either the effective stratigraphic age, or time of maximum temperature). Thus, we can regard this as a worst-case scenario where we know nothing of the stratigraphic constraints. We are interested in how well
we can recover the maximum postdepositional temperature, a parameter of considerable importance to thermal history modeling in sedimentary basins and studies of hydrocarbon generation (Gleadow et al., 1983; Green et al., 1989a; Naeser, 1993). The modeling approach used in this study (i.e., in Figures 6, 7, and 8) follows that described by Gallagher (1995), where a stochastic search method (genetic algorithm) is used to explore the parameter space for good data fitting solutions (i.e., thermal histories parameterized as a series of time-temperature points, with linear interpolation between them). Having found an acceptable solution, we used the likelihood ratio test to determine the 95% confidence intervals about this solution. This involves perturbing each parameter (i.e., temperature or time) in turn until the data fit changes by an amount equivalent to the 95% confidence interval
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Fig 6. for a chi-squared distribution with one degree of freedom (see Gallagher, 1995, for more details). In Figure 6, the inferred maximum postdepositional temperatures between 60 and 100 °C are plotted relative to the true value for each model for the three different predepositional durations considered in Figure 5. The results shown are one realization for each model run, but they do demonstrate a tendency to deviate
significantly (beyond model uncertainties) from the true maximum temperatures. For most models and burial history scenarios, this deviation consists of an underestimation of maximum temperature. This relationship appears more strongly developed for the heating and cooling scenario than for continuous heating and the degree of deviation appears largest for the longer predepositional duration. This reflects the greater separation between the
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predepositional and postdepositional track length components in the heating-cooling scenario. In addition to underestimation of maximum burial temperature, it is also important to consider potential for degrading the resolution of the timing of maximum paleotemperature. We only consider the heating and cooling scenario because the linear heating scenario has the maximum temperature at the present day. Figure 7 shows the inferred timing and value of the maximum postdepositional temperature for the situation where no strati-
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Fig. 8a track annealing. At higher temperatures, the uncertainty on timing would be expected to decrease. This suggests that the input of some additional geological information, such as depositional age, to help modeling separate the two populations of tracks, may be important. Willet (1998) noted the significance of building prior constraints on time and temperature on modeling to reduce the range of potential solutions, eliminate improbable histories, and to improve model resolution. The results from this study suggest that geological information could have a more fundamental role. To explore this aspect further, we re-modeled the data used in Figure 7A using depositional age as a single time-temperature constraint (i.e., the thermal history has to satisfy a temperature between 0 °C and 20 °C at this time). The results in Figure 7B show a significant improvement on the results of Figure 7A in terms of the reduced uncertainty on the estimate of timing and improvement in the accuracy of the inferred temperature. Thus, routine incorporation of sample depositional age into the modeling procedure will limit the unwanted influence of predepositional tracks.
Recovery of Predepositional (Provenance) Thermal History In situations where we want to link denudation chronologies to a basin’s depositional history (Gallagher et al., 1998; Rohrman et al., 1996) it is necessary to identify potential sediment source regions. In this case, it may be desirable to use the predepositional thermal history to constrain the cooling history of sediment’s potential source regions. The current view indicates using only data obtained from apatites that have not experienced significant burial and heating (i.e., from samples that have remained below 50–60 °C), because above this the predepositional length distribution may become significantly altered by postdepositional annealing (Mitchell, 1997). However, our study has demonstrated that a significant proportion of inherited tracks can be preserved, even when exposed to temperatures at the upper end of the partial annealing zone (e.g., 90–100 °C for Durango composition apatite, and potentially higher for other, less readily annealed, apatites). However, whether these inherited
20
A. Carter and K. Gallagher
Heating and cooling Model 3.
Model 4
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Figure 8 (continued).
500 m.y.
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tracks contain any useful information concerning the predepositional thermal history is unclear. Figures 8A–8D provide example thermal histories to illustrate how well the different inherited track distribution models reproduce their predepositional thermal histories for maximum burial temperatures of 60 °C and 100 °C for a duration of 200, 500, and 1000 m.y. Overall, these results show that the resolution of the predepositional thermal history depends on relative magnitude of the maximum predepositional and postdepositional temperatures. Thus, for model 1, the inferred predepositional thermal histories are always at a lower temperature than the subsequent maximum and generally less than 50–60 °C, the temperature below which little annealing occurs with the Laslett et al. (1987) annealing model. The inferred predepositional thermal histories, therefore, are in agreement with the original model, particularly
Fig. 8b
where the duration prior to deposition is relatively long. This is a consequence of the reduced dilution of the predepositional tracks (as shown in Figure 5). Results for model 2 imply predepositional cooling histories consistent with the original model, generally requiring maximum temperatures at least as high as the postdepositional value, and fairly reliable rates of cooling. The results for models 3 and 4 are variable, in that some capture the form of heating-cooling predepositional history, while others do not. For model 3, most of the models reproduce the cooling part of the thermal history, with approximately the correct cooling rate. Results for model 4 also show cooling, but the resolution of the cooling rate (or timing of predepositional temperature) is not particularly good. The relative insensitivity of the final track length distribution for model 4 to the duration of the predepositional history is reflected in poor resolution of this duration.
21
A synthetic data study
Linear Heating Model 1.
Model 2.
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Figure 8 (continued).
500 m.y.
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CONCLUSIONS At the outset of this study, we asked five questions concerning the preservation potential for provenance-related fission tracks in detrital apatite subjected to postdepositional heating. Our modeling study allows us to provide answers to these as follows. 1. Is there a critical postdepositional temperature above which the inherited (source) signal becomes modified beyond recognition? We found that high burial temperatures (i.e., at the higher end of the apatite partial annealing zone) do not necessarily remove the thermal history signature from predepositional tracks and, in these cases, exert a secondary control on their preservation. However, peak burial temperature is not the only factor that governs recovery of source information. 2. How does the form of the inherited track length distribution influence the inferred postdepositional thermal history? A fundamental control that governs how many preserved predepositional tracks remain for any given burial-related ther-
mal history is the initial predepositional track length distribution—those dominated by longer lengths (e.g., model 1) have greater preservation potential than those with many short lengths (e.g., model 4). This conclusion can be generalized to different compositions, as the longer lengths will reflect a tendency to resist annealing and so be preserved. 3. How significant are the relative proportions of predepositional to postdepositional tracks for the recovery of source thermal histories? The relative duration (proportions) of the predepositional Fig. 8c phases influences the preservation potenand postdepositional tial. With increasing numbers of predepositional tracks relative to postdeposition, there is a tendency to increase the proportion of preserved inherited tracks. 4. What is the relationship between the maximum predepositional, postdepositional temperatures and observed fission-track age and track length distribution? The style of burial-related thermal history does not appear to have a significant influence on the inferred predepositional
22
A. Carter and K. Gallagher
Model 3.
Linear Heating Model 4.
200 m.y.
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200 m.y.
20 40 60 80
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Figure 8 (continued).
500 m.y. 100°C
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Fig. 8d thermal history unless total annealing occurs, in which case the predepositional information is totally lost. 5. How important are independent geological constraints in a predominantly data driven modeling approach? The impact of the inherited tracks on resolution of inferred thermal histories is important when the fission-track data are considered in isolation. Thus, not incorporating simple geological information such as stratigraphic age can lead to significant under prediction of later peak burial temperature. However, this problem can be overcome if appropriate geological information is incorporated into the modeling procedure explicitly. A key measure of inherited tracks and their significant potential to degrade modeled thermal histories is a fission-track age more than twice the depositional age with a moderately long (>12µm) mean track length. Samples with these characteristics also provide a measure of the potential for recovery of meaningful predepositional thermal histories.
REFERENCES CITED Arne, A., and Zentilli, M., 1994, Apatite fission track thermochronology integrated with vitrinite reflectance, in Mukhopadhyay, P.K., and Dow, W.G., eds., Vitrinite reflectance as a maturity parameter: American Chemical Society Symposium Series No. 570, p. 250–268. Barbarand, J., Carter, A., Wood, I., and Hurford, T., 2003, Compositional and structural control of fission-track annealing in apatite: Chemical Geology, v. 198, p. 107–137. Brandon, M.T., 1992, Decomposition of fission track grain-ages distributions: American Journal of Science, v. 292, p. 535–564. Carlson, W.D., 1990. Mechanisms and kinetics of apatite fission-track annealing: American Mineralogist, v. 75, p. 1120–1139. Carlson, W.D., Donelick, R., and Ketcham, R.A., 1999, Variability of apatite fission-track annealing experiments: I. Experimental results: American Mineralogist, v. 84, p. 1213–1223. Carter, A., 1999, Present status and future avenues of source region discrimination and characterization using fission track analysis: Sedimentary Geology, v. 124, p. 31–45. Carter, A., Bristow, C.S., and Hurford, A.J., 1995, The application of fissiontrack analysis to the dating of barren sequences: examples from red beds in Scotland and Thailand, in Dunay, R.E., and Hailwood, E.A., eds., Non-
A synthetic data study biostratigraphical methods of dating and correlation: Geological Society [London] Special Publication 89, p. 57–68. Corrigan, J., 1991, Inversion of fission track data for thermal history information: Journal of Geophysical Research, v. 96, p. 10,347–10,360. Crowley, K.D., 1993, Lenmodel: A forward model for calculating length distributions and fission track ages in apatite: Computational Geoscience, v. 19, p. 619–626. Crowley, K.D., Cameron, M., and Schaefer, R.L., 1991, Experimental studies of annealing of etched tracks in flourapatite: Geochimica et Cosmochimica Acta, v. 55, p. 1449–1465. Duddy, I.R., Green, P.F., and Laslett, G.M., 1988, Thermal annealing of fissiontracks in apatite: 3. Variable temperature behaviour: Chemical Geology, v. 73, p. 25–38. Galbraith, R.F., and Green, P.F., 1990, Estimating the component ages in a finite mixture: Nuclear Tracks, v. 17, p. 197–206. Galbraith, R.F., and Laslett, G.M., 1993, Statistical models for mixed fission track ages: Nuclear Tracks, v. 21, p. 459–470. Gallagher, K., 1995, Evolving temperature histories from apatite FT data: Earth and Planetary Science Letters, v. 136, p. 421–435. Gallagher, K., and Brown, R., 1997, The onshore record of passive margin evolution: Journal of the Geological Society of London, v. 154, p. 451–457. Gallagher, K., and Brown, R., 1999, Denudation and uplift at passive margins: the record on the Atlantic Margin of southern Africa: Philosophical Transactions of the Royal Society of London A, v. 357, p. 835–859. Gallagher, K.L., Hawksworth, C.J., and Mantovani, M., 1994, The denudation history of the onshore continental margin of S.E. Brazil inferred from fission track data: Journal of Geophysical Research, v. 99, p. 18,117– 18,145. Gallagher, K., Brown, R., and Johnson, C., 1998, Fission track analysis and its application to geological problems: Annual Reviews in Earth and Planetary Science, v. 26, p. 519–572. Garver, J.I., Brandon M.T., Rice, M.T., and Kamp, P.J., 1999, Exhumation history of orogenic highlands determined by detrital fission-track thermochronology, in Ring, U., Brandon, M.T., Lister, G.S., and Willet, S.D., eds., Exhumation processes: Normal faulting, ductile flow and erosion: Geological Society [London] Special Publication 154, p. 283–304. Gleadow, A.J.W., Duddy I.R., and Lovering, J.F., 1983, Fission track analysis: a new tool for the evaluation of thermal histories and hydrocarbon potential: Australian Petroleum Exploration Association, v. 23, p. 93–102. Green, P.F., Duddy, I.R, Gleadow, A.J.W., and Lovering, J.F., 1989a, Apatite fission-track analysis as palaeotemperature indicator for hydrocarbon exploration, in Naeser, N.D., McCulloh, T.H., eds., Thermal history of sedimentary basins: New York, Springer-Verlag, p. 181–195. Green, P.F., Duddy, I.R., Laslett, G.M., Hegarty, K.A., Gleadow, A.J.W., and Lovering, J.F., 1989b, Thermal annealing of fission tracks in apatite: 4. Quantitative modeling techniques and extension to geological timescales: Chemical Geology (Isotope Geoscience Section), v. 79, p. 155–182. Hurford, A.J., and Carter, A., 1991, The role of FT dating in discrimination of provenance, in Morton, A.C., Todd, S.P., and Haughton, P.D.W., eds., Developments in sedimentary provenance studies: Geological Society [London] Special Publication 57, p. 67–78.
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Ketcham, R.A., Donelick, R.A., and Carlson, W.D., 1999. Variability of apatite fission-track annealing kinetics: III. Extrapolation to geological timescales: American Mineralogist, v. 84, p. 1235–1255. Laslett, G.M., Gleadow, A.J.W., and Duddy, I.R., 1984, The relationship between fission track length and density in apatite: Nuclear Tracks, v. 9, p. 29–38. Laslett, G.M., Green, P.F., Duddy I.R., and Gleadow, A.J.W., 1987, Thermal annealing of fission tracks in apatite: 2. A quantitative analysis: Chemical Geology (Isotope Geoscience Section), v. 65, p. 1–13. Lonergan, L., and Johnson, C., 1998, A novel approach for reconstructing the denudation histories of mountain belts: with an example from the Betic Cordillera (S. Spain): Basin Research, v. 10, p. 353–364. Lutz, T.M., and Omar, G., 1991, An inverse method of modeling thermal histories from apatite fission-track data: Earth and Planetary Science Letters, v. 104, p. 181–195. Mitchell, M., 1997, Identification of multiple detrital sources for Otway Supergroup sedimentary rocks: implications for basin models and chronostratigraphic correlations: Australian Journal of Earth Sciences, v. 44, p. 743–750. Moore, M., Gleadow, A.J.W., and Lovering, J.F., 1986, Thermal evolution of rifted continental margins: new evidence from fission tracks in basement apatites in south-eastern Australia: Earth and Planetary Science Letters, v. 78, p. 255–270. Naeser, N.D., 1993, Apatite fission-track analysis in sedimentary basins—a critical appraisal, in Doré, A.G., Auguston, J.H., Hermanrud, C., Stewart, D.J., and Sylta, Ø., eds., Basin modeling: Advances and applications: Norwegian Petroleum Society Special Publication 3, p. 147–160. Rohrman, M., Andreissen, P., and van der Beek, P., 1996, The relationship between basin and margin thermal evolution assessed by FT thermochronology: an application to offshore southern Norway: Basin Research, v. 8, p. 45–63. Sambridge, M.S., and Compston, W., 1994, Mixture modeling of multi-component data sets with application to ion-probe zircon ages: Earth and Planetary Science Letters, v. 128, p. 373–390. Steckler, M.S., and Omar, G.I., 1994, Controls on erosional retreat of the uplifted rift flanks at the Gulf of Suez and northern Red Sea: Journal of Geophysical Research, v. 99, p. 12,159–12,173. Tagami, T., Galbraith, R.F., Yamada, G.M., and Laslett, G.M., 1998, Revised annealing kinetics of fission-tracks in zircon and geological implications, in Van den Haute, P., and De Corte, F., eds., Advances in fission-track geochronology: Amsterdam, Kluwer Academic Press, p. 99–112. Willet, S.D., 1992, Modeling thermal annealing of fission tracks in apatite, in Zentill, M., ed., Short course on low temperature thermochronology: Techniques and applications: Mineralogical Association of Canada, p. 43–72. Willet, S.J., 1998, Inverse modeling of annealing of fission-tracks in apatite: 1. A controlled random search method: American Journal of Science, v. 297, p. 939–969. MANUSCRIPT ACCEPTED BY THE SOCIETY OCTOBER 21, 2003
Printed in the USA
Geological Society of America Special Paper 378 2004
Fundamentals of detrital zircon fission-track analysis for provenance and exhumation studies with examples from the European Alps Matthias Bernet* Mark T. Brandon Department of Geology & Geophysics, Yale University, New Haven, Connecticut, 06520–8109, USA John I. Garver Brandi R. Molitor* Geology Department, Olin Building, Union College, Schenectady, New York, 12308–3107, USA ABSTRACT Fundamental aspects of detrital zircon fission-track analysis in provenance and exhumation studies include etching of fission tracks in zircon, decomposition of grain-age distributions, detection of major bedrock age components, and reproducibility of results. In this study, we present new detrital zircon fission-track data of sediment samples from eight Italian rivers draining the European Alps and previously published data from the Rhône delta in southeastern France. These samples are used to demonstrate that variable etching rates in detrital zircon, which have been shown elsewhere to necessitate a multi-etch procedure during sample preparation, are not a significant problem for zircons from the Alps. Etching response in zircon is a function of radiation damage, principally caused by α-decay. Spontaneous fissiontrack density can be used as a proxy for total radiation damage. We use spontaneous track density, fission-track cooling age, and uranium content to define a “window of countability” for detrital zircon. We also show that detrital zircon fission-track results are reproducible by comparing results from modern sediments from the same river drainage. The results also compare well with the known distribution of bedrock cooling ages in each drainage area. On a regional scale, our data illustrate that a few samples can provide an overview of the fission-track age pattern of a whole orogen, which is useful for exhumation and provenance studies. Keywords: zircon, fission-track, provenance, exhumation, European Alps. INTRODUCTION
Kamp, 2002). In this paper, we examine the ability of fissiontrack grain-age distributions of detrital zircon from modern river sediments to resolve bedrock cooling ages in an orogenic source region, and we consider the procedures used in this kind of study. A detrital sample commonly contains a variety of zircons with different cooling ages and uranium contents. In active orogenic settings, zircons cool in the crust by tectonic and erosional exhumation or by conductive cooling following volcanism or shallow plutonism. Therefore, in geologic settings with little or no
Fission-track analysis of detrital zircon has become an important tool for the study of sediment provenance and longterm exhumation of orogenic mountain belts (Hurford et al., 1984; Zeitler et al., 1986; Cerveny et al., 1988; Hurford and Carter, 1991, Brandon and Vance, 1992; Garver and Brandon, 1994a, 1994b; Lonergan and Johnson, 1998; Carter, 1999; Garver et al., 1999; Spiegel et al., 2000; Bernet et al., 2001; Garver and
*Present addresses: Bernet— LGCA, Université Joseph Fourier, 38041 Grenoble Cedex 9, France,
[email protected]; Molitor—Western Washington University, Bellingham, Washington 98225, USA. Bernet, M., Brandon, M.T., Garver, J.I., and Molitor, B.R., 2004, Fundamentals of detrital zircon fission-track analysis for provenance and exhumation studies with examples from the European Alps, in Bernet, M., and Spiegel, C., eds., Detrital thermochronology—Provenance analysis, exhumation, and landscape evolution of mountain belts: Boulder, Colorado, Geological Society of America Special Paper 378, p. 25–36. For permission to copy, contact
[email protected]. © 2004 Geological Society of America.
25
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M. Bernet et al. igneous activity, zircon fission-track ages can be used as a proxy for long-term exhumation (Cerveny et al., 1988; Garver et al., 1999). Many orogens have deeply exhumed metamorphic internal zones where the fission-track ages for zircon provide information about postmetamorphic cooling and exhumation. The main agents of erosional exhumation are rivers that flank the orogenic system. These rivers tend to sample the landscape of their drainage areas by yield, which means that faster eroding areas potentially deliver more material to nearby basins. In addition to sediment yield, which is a function of erosion rate, the effective zircon yield also varies with lithology (Poldervaart, 1955, 1956; Deer et al., 1992). The purpose of this study is to examine whether it is possible to detect all major cooling age components in a drainage area with one detrital sample, despite the variable etching response of detrital zircon (Naeser et al., 1987; Kasuya and Naeser, 1988). Furthermore, we are also interested to see if detrital zircon fission-track results are reproducible. The European Alps are ideal for this study because the thermochronology of the bedrock has been thoroughly investigated over the past 40 years. Using the large data set of bedrock cooling ages in the European Alps available from the literature (Hunziker et al., 1992; Bernet et al., 2001, and references therein), we are able to address the questions above. For the Alps, the temperature for effective closure of the zircon fission-track system is ~240 °C (Hurford, 1986).
1000 N = 654 FT grain ages detrital zircons from Mesozoic and Cenozoic strata of Washington State
Uranium (ppm)
800
contours are for spontaneous track density 6 -2 (units of 10 tracks cm )
600
400
200
30
20
0
5
1
0
10
100 200 Fission-track age (Ma)
300
Figure 1. Plot showing the range of datable zircons by the fissiontrack method as a function of age, U, and spontaneous track density. The points are based on 654 fission-track (FT) grain ages for detrital zircons from Mesozoic and Cenozoic strata of Washington State (Brandon and Vance, 1992; Garver and Brandon, 1994a, 1994b; and Stewart and Brandon, 2004). The data reflect the fact that it is difficult to count grains with spontaneous track densities greater than ~3 × 107 tracks/cm2. This effect is largely due tracks. Figto2 overlapping Bernet et al.
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ETCHING OF ZIRCON FISSION TRACKS An important analytical challenge in sample preparation of zircon is the variable etching rates of tracks caused by intergrain
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Figure 2. Overview map of the Rhône River drainage with its major tributaries and the Rhône delta in southeastern France. The Rhône delta samples, labeled A, B, and C, were collected east of, west of, and close to the main channel of the modern Rhône River. Also shown are locations of samples collected from eight Italian rivers draining the southern flank of the European Alps.
27
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������������������������������������������������������������������������������������������������������������������������������������� ������������������������������������������������������������������������������������������������������������������������������������������ �� � �������������������������������������������������������������������������������������������������������� ����� ������������������������ ������������������������������������������������������������������������������������������������������������������������������������� �������������������������������������������������������������������������������
and intragrain variation in radiation damage. This is of particular concern in detrital suites, which may be made up of grains with a variety of cooling ages and uranium concentrations (Fig 1). Revelation of fission tracks in zircon is accomplished by etching grains in a chemical etchant (most laboratories use a NaOH/ KOH eutectic melt; Gleadow et al., 1976). Etching for short times reveals tracks in grains with high radiation damage (generally older grains), and etching for a long time reveals tracks in grains with low radiation damage (generally younger grains). To ensure that none of the major age groups is excluded from detection, the “multi-mount technique” is used (Naeser et al., 1987). This method utilizes at least two mounts per sample, one for a long etch and one for a short etch. Therefore, two mounts were processed for each Italian river sample in this study (Fig. 2; Table 1). Short etch times were selected between 7 and 15 hours, while long etch times ranged between 15 and 24 hours for these samples, which were all etched at 228 ºC in a laboratory oven. Each mount generally contained between 500 and 1000 zircons, depending on the amount of available sample material. The range of uranium content against cooling age for all zircons counted in this study from Italian river samples is shown in Figure 3. Sample preparation and etching of Rhône delta samples is described in Bernet et al. (2004). The ideal situation would be to have a short etch with no over-etched grains and a long etch with no under-etched grains, so that the differently etched mounts would overlap in their fission-track grain-age distributions. In most cases, the selected etch times were able to produce short-etch and long-etch mounts that conform to this objective.
A bias introduced by etching could cause problems if etchability was correlated with specific sources. Comparison of fission-track grain-age distributions from paired mounts can be used to check for this problem. For our study here, we found that each mount contains the full spectrum of grain ages. There is a bias, however, in the relative sizes of peaks in these distributions. For example, the size of a young peak might be smaller in the fission-track grain-age distribution for a short-etch mount than that for the distribution from the corresponding long-etch mount. Alpha Damage and Etching Response Etch time influences which grains will be countable because it affects how fission tracks are revealed. The etching response of zircon is related to radiation damage in the grain (Gleadow, 1978; Kasuya and Naeser, 1988; Garver et al., 2000a). The result of the radiation damage is that grains with high radiation damage etch easily, and tracks are quickly revealed in a few hours. Grains with low radiation damage are less chemically reactive and need longer etch times, up to 40–100 h. The main source of radiation damage is the production of α-particles associated with the decay series for 238U, 235U, and 232 Th (e.g., Palenik et al., 2003). The number of α−decay events per gram, Dα, is given by
(1)
28
M. Bernet et al. �������������������������������������������������
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Density (% /ppm )
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0.3 0.2
U content by U/Pb method Median = 248 ppm Range = 10 to 7880 ppm N = 679
0.1 0.0
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Figure 3. Uranium content–fission-track age relation of all zircons dated from the Italian river samples in this study.
where τ is the amount of time for α−production, [U] and [Th] are the fractional concentrations�������������������� by mass of U and Th in zircon, mU and mTh are atomic masses for U and Th (238.0289 and 232.038 g/mol, respectively), N is Avogadro’s number (6.022 × 1023 mol1), c238, c235, and c232 are the fractional abundances of 238U, 235U, and 232Th (0.992743, 0.7200, and 1.000, respectively), and the λα variables are the rate constants for alpha decay for 238U, 235U, and 232 Th, with λα238 = 1.55125 × 10–10 yr–1, λα235 = 9.8485 × 10–10 yr–1, and λα232 = 0.49475 × 10–10 yr–1. The integer values in equation (1) represent the total number of α-particles ejected during the decay series for each of the U and Th isotopes. This equation shows that the contribution of Th to α-production is small, mainly due to the much slower decay of 232Th. The α-production rate for Th is only 5% relative to that for an equivalent mass of U. A compilation of zircon analyses (discussed below) indicates that the ratio Th/U in natural zircons ranges from 0 to 2.9, with a mean of 0.5 (Fig. 4; see Garver and Kamp, 2002, for details). Thus, U content is the primary factor for assessing radiation damage in zircon. Fission decay will also produce radiation damage, but 238U is the only isotope of the U and Th series that has a significant fission decay rate, λf238 = 8.45 × 10–17 yr–1. The number of fission decay events per gram of zircon is given by .
(2)
Fission decay produces highly energetic particles and visible damage zones, but α-decay occurs much more frequently, as indicated by .
(3)
Density (% /ppm )
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U content by FT method Median = 202 ppm Range = 17 to 1048 ppm N = 654
0.1 0.0
Cumulative Probability (%)
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C
80
U/Pb method
60
FT method
40 20 0 0
500 1000 1500 2000 2500 3000 3500 4000 U content (ppm)
Figure 4. A comparison of U content in detrital zircons as measured by sensitive high-resolution ion microprobe (SHRIMP) dating of detrital from Garver and Kamp, 2002), and the Fig 4 zircons Bernet(compilation et al. fission-track (FT) method (compilation of detrital zircons dated in the Washington State from Mesozoic and Cenozoic strata [see Figure 1 for references]). The SHRIMP determinations show the true range for U contents, whereas the more limited range indicated by the FT method is due to difficulties in counting zircons with high track densities.
This equation ignores the contribution of Th decay and uses an approximation λτ ≈ exp[λτ] − 1, which is precise to within 5% for τ < 100 m.y. The spontaneous track density ρs is used in fission-track dating as a measure of 238U fission events in zircon. Spontaneous track density is related to Df according to
29
Fundamentals of detrital zircon fission-track analysis
with ρs given in tracks per cm2. The other variables are L, the etchable length of unannealed spontaneous fission tracks (10.6 μm for spontaneous tracks in Fish Canyon Tuff zircon, Brix et al., 2002), and ρz, the density of zircon (4.65 g cm–3). Combining equations (3) and (4) gives the following relationship of α-decay to spontaneous track density .
(5)
1e+17
alpha events/mg
1e+16
N = 336 zircons 1e+15
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Full relationship assuming 10 Ma (lower line) and 500 Ma (upper line) 1e+6
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Approximate relationship
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Zircons assuming
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This relationship suggests that ρs might be used as a proxy for Dα. A critical assumption is that fission tracks and α-damage should have a similar sensitivity to thermal annealing. Tagami et al. (1996) argue that α-damage is annealed at temperatures just below those needed to start annealing of fission tracks. However, their evidence is indirect, in that it was based on changes in the etching behavior of zircon, and not on a direct measure of αdamage. Garver and Kamp (2002) show evidence that color in zircon, which is a manifestation of radiation damage, requires temperatures up to 400 °C to be fully annealed on geologic time scales, whereas fission tracks are fully annealed at temperatures of 250–300 °C. These observations suggest that α-damage and fission tracks are annealed at similar temperatures, but the partial retention zone for α-damage spans a broader temperature range than that for fission tracks. Thus, we propose that ρs is in fact a good proxy for Dα. Comparison of the different estimates of the relationship between ρs and Dα are shown in Figure 5. The dashed line shows
the approximate equation (5). The points are based on U and Th measurements for SHRIMP-dated zircon grains compiled from the literature (336 zircons from Ireland, 1992; Zhao et al., 1992; Schäfer et al., 1995; Gray and Zeitler, 1997). The estimated ρs and Dα values were determined using the full equations (1, 2, 4), and a representative cooling age of 100 Ma. The solid lines show the trend for these data for different cooling ages, 10 and 500 Ma. The conclusion is that cooling age and Th/U ratio have only a minor influence on the relationship between ρs and Dα. With increasing radiation damage, the crystalline structure of zircon is gradually transformed into an increasingly disordered structure (Palenik et al., 2003). The metamict state starts at ~3.5 × 1015 α-decay events/mg, which corresponds to ρs ≈ 2 × 108 cm–2. Compilation of our fission-track zircon ages indicates that zircons have to have ρs < 3 × 107 cm–2 to be dated using standard techniques (Fig. 1). Thus, the etching bias is associated with radiation damage well below the metamict state. Nonetheless, the influence of radiation damage on etching rates is dramatic. A zircon with an old cooling age or high-U content can be etched in a few hours, whereas zircons with young cooling ages and low-U
������������������������������� � �
(4)
,
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2
Spontaneous track density (t/cm )
Figure 5. Relationship of alpha decay to spontaneous fission-track density in zircon. The dashed line shows the approximate relationship given by equation (5). TheFigfilled circlesetwere . 5 Bernet al. calculated for 336 zircons using U and Th measurements determined by sensitive highresolution ion microprobe (SHRIMP) analyses from several unrelated studies (Ireland, 1992; Zhao et al., 1992; Schäfer et al., 1995; Gray and Zeitler, 1997). The calculation is based on the full equations (1, 2, 4), and a representative cooling age of 100 Ma. The solid lines show the trend for these data for cooling ages of 10 and 500 Ma.
Figure 6. The “window of countability” of fission tracks in zircon is shown as a function of spontaneous track density, uranium content, and cooling age. The zircon fission-track grain-age data for the Rhône delta and Italian��������������������� River samples were used to define the size of the window of countability. The grains represent etching conditions ranging from 6 to 60 h, but most were etched between 15 and 24 h. The horizontal contours show the cumulative probability distribution (1%–99%) for spontaneous track density in zircons from our study etched for 15 h. The vertical contours shows the cumulative probability distribution for U content in natural zircons, based on sensitive high-resolution ion microprobe (SHRIMP) analyses from several unrelated studies (Ireland, 1992; Zhao et al., 1992; Schäfer et al., 1995; Gray and Zeitler, 1997).
30
M. Bernet et al.
content are less chemically reactive and need longer etch times, up to 40–100 h. The zircon fission-track grain-age from our Rhône delta and Italian river samples are used to illustrate the “window of countability” (Fig. 6). The graph includes determinations from all etches, both long and short. However, the contours showing the distribution of spontaneous track densities are based on average etching conditions of a 15 h etch time at 228 ºC. The zircons in this diagram appear to have a sufficient range of properties to highlight the area covered by the etchability window. Other studies may not find the same range, but this would reflect a more limited range of properties for the zircons being studied. Grain-Age Distributions and Spontaneous Track Density As noted above, the etching bias will be a significant problem in cases where etchability correlates strongly with zircon fission-track age. For example, Naeser et al. (1987) and Cerveny et al. (1988) showed a strong bias for very young zircon fission-track ages from sediments shed from the Himalayas. In areas with cooling ages older than ca. 5 Ma, this aspect of the etching bias seems to be less of a problem (Garver et al., 2000a). To illustrate, we considered an example from one of our Rhône delta samples (Fig. 2, sample B, Stes. Maries-de-la-Mer, Table 2). For this sample, five mounts were prepared and etched for 6, 10, 15, 30, and 60 hours each. About 20 grains were dated per mount. The results are illustrated in Figure 7 by comparing the two mounts that have the largest difference in etch time (6 and 60 h) and the two mounts with the smallest difference in etch time (10 to 15 h). Figure 7A and 7B show ρs for the grains in terms of cumulative probability, ranging from 0 to 100%. The Kolmogorov-Smirnov (KS) test (Press et al., 1992, p. 614–617) is used to assess the statistical significance of the difference between the distributions. A low probability on the test, such as P(KS) < 5%, would indicate that the differences between the two distributions are significant. If P(KS) >> 5%, then the differences could be due to random chance alone. The test indicates a significant dif-
ference between the 60 h and 6 h etches, with P(KS) < 1%, but no significant difference between the 10 and 15 h etches, where P(KS) = 96%. The next step is to examine probability density plots for the fission-track grain-age distributions for each of the four mounts (Fig. 7C–D). The density plots were constructed from grain age data using the method of Brandon (1996). The density plots indicate that all four etches sampled the same range of grain ages, even though there are only 20 grains dated per mount. To apply the KS test, we recast the grain-age data as cumulative probability plots (Fig. 7E–F). The KS probabilities of 99% and 86% for these comparisons indicate that there is no significant difference between the fission-track grain-age distributions, despite the differences in etching times. Furthermore, comparing the range of detrital zircon fission-track ages with known bedrock zircon fission-track ages in the source area of the Rhône River system (Bernet et al., 2004), we find that all major bedrock-age components are covered. The conclusion is that for the Alps, we can easily reveal countable fission tracks in all major grain-age components of the total grain-age distribution. There are three reasons for this outcome. (1) The Alps source regions are not dominated by very young grain ages (<5 Ma), so that the etching process is able to reveal tracks in almost all zircons. (2) Each zircon source appears to supply zircons with a wide range of uranium concentrations, which can be properly etched with different etch times. (3) Zircons from the Alps show no strong spatial correlation in the bedrock between the U and Th content and zircons cooling ages. This result was expected given that many of the metamorphic rocks in the source region are derived from sedimentary protoliths, so that zircon properties are already randomized in the source region. The conclusion from this exercise is that for Alpine zircons etching does not have a large influence on the sampled grain-age distribution. A single mount, etched anywhere between 6 and 60 h, would have covered the expected range of grain ages. Nevertheless, it is strongly recommended that at least two mounts with different etch times are used for detrital samples. This is
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Figure 7. Spontaneous track density and fission-track grain-age distribution comparison for different etch times. Examination of the influence that the etching bias might have on a typical detrital zircon fission-track grain-age distribution. The plots are based on fission-track grain-age distributions from sample B collected from modern sediments of the Rhône delta. The four distributions were treated with different etch times, 6, 10, 15, and 60 h. Cumulative probability plots of spontaneous track density distributions show (A) significant differences between the grains dated from the shortest ������������������� and longest etches (6 and 60 h), but (B) little difference between the two closest etches (10 and 15 h). The fission-track grain-age distributions for these four mounts are presented as probability density plots in (C) and (D), and cumulative probability plots in (E) and (F). The main conclusion is that etching bias observed in (A) appears to have little if any bias on the fission-track grain-age distributions of these samples.
32
M. Bernet et al. �
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Figure 8. Bedrock zircon fission-track contour maps of (A) Sesia River drainage, Western Alps; (B) Dora Baltea River drainage; Western Alps; and (C) Ticino River drainage, Central and Southern Alps. Contours were constructed from published zircon fission-track data (Hunziker et al., 1992; Bernet et al., 2001, and references therein).
especially true for detrital zircon suites that are dominated by very young (<5 Ma) or very old (>300 Ma) cooling ages, to fully understand the grain-age/U content range and to select appropriate etch times (Garver et al., 2000a, 2000b). COUNTING CRITERIA In most bedrock zircon fission-track studies, 10–20 clear, well-etched grains are typically dated to determine a cooling age of a single-source sample (i.e., a granitic intrusion). More zircon grains need to be dated for detrital samples because they usually contain a mixture of zircons with a variety of different cooling ages. Only a random selection of countable zircons
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should be analyzed from a randomly mixed suite of zircons in the Teflon mount. Grains should only be selected by their countability and not by their shape, color, or other attributes. Grains with well-exposed and polished surfaces parallel to the zircon C-axis, containing well-etched fission tracks, with some tracks parallel to the C-axis, were considered countable in the present study. Under-etched or over-etched grains were not counted. In addition, metamict grains and grains with very high spontaneous track densities (>~3 × 107 tracks/cm2) or strong zonation, as well as grains with uneven surfaces, cracks or very small counting areas of less than 270 µm2, were not counted. If possible, enough grains per sample should be dated to ensure that the major components—those that make up more than 20% of the
33
Fundamentals of detrital zircon fission-track analysis
GRAIN-AGE PEAKS All observed fission-track grain-age distributions were decomposed into their main grain-age components or peaks. We followed the approach of Galbraith and Green (1990) in using their binomial peak-fit method (Brandon, 1996, 2002). Zircon fission-track peak ages are a proxy for long-term exhumation rates, where cooling occurs by erosion or normal faulting, and not following magmatic events (Cerveny et al., 1988; Garver et al., 1999). The Alps are basically free of recent volcanism. The last major magmatic activity occurred ca. 30 Ma (von Blanckenburg, 1992; Dunkl et al., 2000). Peak ages tend to remain fairly constant within their error range, whereas peak sizes are much more variable from sample to sample and through time. Peak age reflects the amount of time needed to exhume the zircons from the depth of the zircon fission-track closure temperature. Thus, peak age provides an estimate of long-term exhumation rates. Peak size, however, is influenced not only by the long-term erosion rate, but also by short-term variations in erosion rates (e.g., storms, rock slides, etc.) and also by spatial variations in zircon concentrations in the source region. Peak size can also be influenced by the etching bias discussed above. In general, we find that peak ages tend to be a more robust feature of a fission-track grain-age distribution, whereas peak sizes can vary considerably within replicated distributions. DETECTION OF BEDROCK COOLING AGES One detrital sample can provide a remarkably representative picture of the bedrock fission-track cooling-age distribution in a drainage area, as first shown by Zeitler et al. (1986) and Cerveny et al. (1988). This initial picture can be further refined by comparison of the detrital zircon fission-track grain-age distributions with the distribution of bedrock zircon fission-track ages in river drainage areas in the European Alps. Because there have been over four decades of bedrock fission-track analysis in the European Alps, a large data set of low-temperature cooling ages is now available (e.g., Hunziker et al., 1992). We selected the Sesia River, Dora Baltea River, and Ticino River in the Western, Central, and Southern Alps (Fig. 2, Table 1) to make the proposed comparison because these drainages have different sizes, they drain areas with diverse exhumation rates, and the bedrock cooling-age data set has the highest density in these areas. We constructed contour maps for zircon fission-track ages (Fig. 8) from published fission-track data (Hurford and Hunziker, 1985; Flisch, 1986; Hurford, 1986; Giger and Hurford, 1989; Michalski and Soom, 1990; Hurford et al., 1991; Hunziker et al., 1992;
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distribution—are represented (see discussion on detection limits in Stewart and Brandon, 2004). In practice, this means that, if a sufficient number of grains were available, 60–100 countable grains per sample were dated in this study; otherwise, all possible grains were counted (Tables 1 and 2).
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Figure 9. Probability density plots of fission-track grain-age distributions of eight Italian modern river samples in west to east order (Fig. 2), reflecting the regional trend of faster exhumation in the Western and Central Alps and slower exhumation in the Eastern and Southern Alps.
Seward and Mancktelow, 1994; Bertotti et al., 1999; Fügenschuh ������������������� et al., 1999; Bernet et al., 2001). Here, we review the results for all three river samples. A. The Sesia River, the smallest of these three drainages, drains part of the Monte Rosa massif, the northern part of the Sesia-Lanzo Zone, and the Ivrea Zone (Fig. 8A). Bedrock zircon fission-track ages in this drainage range from 25 to 130 Ma with the majority of ages ca. 35 Ma for the Sesia-Lanzo Zone, but the area also has a number of zircon fission-track ages between 45 and 60 Ma (Hurford and Hunziker, 1985; Hurford and Hunziker, 1989; Hurford et al., 1991; Hunziker et al., 1992). This bedrock age pattern is reflected in the detrital sample, which has two major peaks at 34.2 ± 2.7 Ma and 62.9 ± 15.1 Ma. B. The Dora Baltea River, which reaches the southern flank of the Mont Blanc massif, drains parts of the Dent Blanche nappe, the Aosta Valley, and Sesia-Lanzo Zone. Published bedrock zircon fission-track ages range between 12 and 190 Ma. The fission-track contour map (Fig. 8B) shows that there are three major cooling-age components in this drainage, <20, 20–40, and >100 Ma (Hurford et al., 1991; Hunziker et al., 1992). These age components are detected in the detrital sample with peak ages of 18.0 ± 2.0, 34.0 ± 5.6, and 101.1 ± 22.5 Ma. C. The Ticino River drains the Central Alps (Lepontine dome) and parts of the Southern Alps, including the northern end of the Ivrea Zone. This drainage allows the best comparison between bedrock ages and detrital age components, because it has the highest density of bedrock zircon fission-track ages of the examples presented in this paper (Hurford, 1986; Giger and Hurford, 1989; Michalski and Soom, 1990; Hunziker et al., 1992). The bedrock zircon fission-track ages show the largest range, between 8 and >200 Ma, and the ages can be divided into groups of <10, 10–20, 20–50, and >100 Ma on the fission-track contour map (Fig. 8C). In the detrital sample, components were detected as peaks at 8.6 ± 1.4, 15.6 ± 1.8, 25.6 ± 4.8, and 140.1 ± 19.0 Ma, which is representative of the bedrock pattern despite its apparent complexity.
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M. Bernet et al. �
�����������
to the east (0.2 km/m.y. and less). In these areas, the proportion of young to old grains decreases.
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CONSISTENCY OF DETRITAL ZIRCON FISSIONTRACK RESULTS
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We noted above that a single detrital sample could be used to resolve the major fission-track age components in a drainage �� area, but an important assumption in many studies of detrital minerals is that a single sample from a small part of a depositional system is representative of all the sediment in that system. �� Few studies have addressed this issue. Here, we present three samples from the Rhône River delta in southeastern France (Fig. � 2, Table 2; Bernet et al., 2004), which were selected to test for ��� �� � variability between samples collected from the same general ���������������������������� part of a depositional system. The samples were collected to the � east (Fig. 2, sample A, Fos-sur-Mer), the west (Fig. 2, sample ���������������������� B, Stes. Maries-de-la-Mer), and near the modern main channel ��������������������������������������������� of the Rhône delta (Fig. 2, sample C, Plage de Piémanson). All � � samples were collected from heavy mineral placer deposits along the shore face. �� The fission-track grain-age distributions for these three samples have a similar range in ages, but the cumulative probability �� plots (Fig. 10A) show that sample A is significantly different from samples B and C, as indicated by the KS test. The character of each fission-track grain-age distribution is illustrated in more ��� detail by the probability density plots (Fig. 10B) and by the bestfit peaks (Table 1). Each distribution contains a similar set of four peaks. The main thing that distinguishes sample A from the other ������������������������� ���������������������� ��������������� two samples is that its young peaks are relatively small and its old peaks relative to large. This difference may merely reflect short������������������� term fluctuations in yield, as discussed above. Note, however, Figure 10. A: Cumulative probability plot of fission-track grain-age that the two young peaks in sample A are significantly older, by distributions of the Rhône delta, southeast France, samples Fos- at least 1 m.y., than the ages of the two young peaks in the other sur-Mer (A), Stes. Maries-de-la-Mer (B), Plage de Piémanson (C). B: Probability density plots of best-fit peaks of zircon fission-track two samples. This difference may reflect the fact that the sedi���2, ������������������ grain-age distributions in the three Rhône delta samples (Fig. Table ments for sample A came from a different part of the drainage 2). Comparison of peaks indicates similarity of detrital fission-track relative to those represented by samples B and C. Despite these results for samples from the same depositional setting. differences, the fission-track grain-age distributions for samples A, B, and C are remarkably similar. As result, we conclude that The above-mentioned results demonstrate the reliability of zircons moving through the Rhône drainage are well mixed and the detrital fission-track method to represent the zircon fission- thus are able to deliver a fairly complete representation of the track age distribution in a drainage area. To expand this approach distribution of zircon fission-track cooling ages in the drainage. from single, local drainages to a regional scale, we examined results from five more river drainages: the Orco, Adda, Adige, CONCLUSIONS Brenta, and Piave rivers, all of which drain the southern flank of the European Alps. Probability density plots showing the The data from the Rhône delta samples indicate that detrital observed fission-track grain-age distributions of all river samples zircon fission-track analysis gives similar results for different are presented in Figure 9. Fission-track peak ages are given in samples within a single depositional system. By dating 60–100 Table 2. All of the fission-track grain-age distributions are con- single grains of a detrital sample, using the multi-mount techsistent with the cooling history and exhumational evolution of nique with different etch times, it is possible to detect all of the the Alps. Essentially, the detrital fission-track data reflect: (a) principle cooling-age components in a drainage area. The relative fast exhumation of the Central Alps (~0.4–0.7 km/m.y.), where sizes of the peaks in the distribution are based on yield, which the metamorphic internal zone of the Alps is exposed; and (b) includes the erosion rate, lithology, and the size of area of the zirslower exhumation of the southern flank of the Alps and farther con sources at that time of erosion. In a number of drainage-spe�
Fundamentals of detrital zircon fission-track analysis cific case studies, the detrital fission-track results make sense on a local as well as on a regional scale, so we are encouraged that this technique can be applied to other sedimentary sequences. In a broader context, the observations in our study on zircons of modern river sediment from an orogenic belt support the results of other workers from ancient stratigraphic sequences. One of the real promises of the fission-track grain-age technique is its ability to address the long-term exhumation of orogenic belts, because sediments in a stratigraphic sequence capture a representative picture of source exhumation through time (Cerveny et al., 1988; Brandon and Vance, 1992; Garver and Brandon, 1994b; Lonergan and Johnson, 1998; Carter and Moss, 1999; Garver et al., 1999; Carter and Bristow, 2000; Spiegel et al., 2000; Bernet et al., 2001). At this point, we can be reasonably assured that the basin strata captures representative samples of the orogenic belt through time, and there is a reasonably quantitative transfer of material. An outstanding issue for those studies that utilize stratigraphic sequences is the temporal variation of drainage basins and the effects of long-term sediment storage within or adjacent to the orogenic belt. Nonetheless, it seems clear that future studies will continue to advance our understanding of orogenic exhumation from cooling ages in the sedimentary record, and therefore it is important that the potential influences are understood and quantified. ACKNOWLEDGMENTS This research was supported by a Geological Society of America student grant (Bernet), an Enders Summer-Research Fellowship from Yale University (Bernet), a National Science Foundation grant (EAR-9614730, Garver); grants from the Union College Faculty Research and Internal Education funds (Garver and Molitor). The Reactor Use Sharing Program (U.S. Department of Energy) granted to the Oregon State University Nuclear Reactor subsidized some of the neutron irradiations. We also thank Nick Meyer for his help with sample preparation. Nancy Naeser and Manfred Blix are gratefully acknowledged for constructive reviews that helped to improve this manuscript. REFERENCES CITED Bernet, M., Zattin, M., Garver, J.I., Brandon, M.T., and Vance, J.A., 2001, Steady-state exhumation of the European Alps: Geology, v. 29, p. 35–38. Bernet, M., Brandon, M.T., Garver, J.I., and Molitor, B., 2004, Downstream changes of Alpine zircon fission-track ages in the Rhône and Rhine rivers: Journal of Sedimentary Research, v. 74, p. 82–94. Bertotti, G., Seward, D., Wijbrans, J., ter Voorde, M., and Hurford, A.J., 1999, Crustal thermal regime prior to, during, and after rifting: A geochronological and modeling study of the Mesozoic South Alpine rifted margin: Tectonics, v. 18, p. 185–200. von Blanckenburg, F., 1992, Combined high-precision chronometry and geochemical tracing using accessory minerals; applied to the Central-Alpine Bergell Intrusion (Central Europe): Chemical Geology, v. 100, p. 19–40. Brandon, M.T., 1992, Decomposition of fission-track grain-age distributions: American Journal of Science, v. 292, p. 535–564. Brandon, M.T., 1996, Probability density plot for fission track grain-age samples: Radiation Measurements, v. 26, p. 663–676. Brandon, M.T., 2002, Decomposition of mixed grain age distributions using BINOMFIT: On Track, v. 24, p. 13–18.
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Brandon, M.T., and Vance, J.A., 1992, New statistical methods for analysis of fission track grain-age distributions with applications to detrital zircon ages from the Olympic subduction complex, western Washington State: American Journal of Science, v. 292, p. 565–636. Brix, M.R., Stöckhert, B., Seidel, E., Theye, T., Thompson, S.T., and Küster, M., 2002, Thermobarometric data from a fossil zircon partial annealing zone in high pressure–low temperature rocks of eastern and central Crete, Greece: Tectonophysics, v. 349, p. 309–326. Carter, A., 1999, Present status and future avenues of source region discrimination and characterization using fission-track analysis: Sedimentary Geology, v. 124, p. 31–45. Carter, A., and Bristow, C.S., 2000, Detrital zircon geochronology: Enhancing the quality of sedimentary source information through improved methodology and combined U-Pb and fission-track techniques: Basin Research, v. 12, p. 47–57. Carter, A., and Moss, S.J., 1999, Combined detrital-zircon fission-track and U-Pb dating: A new approach to understanding hinterland evolution: Geology, v. 27, p. 235–238. Cerveny, P.F., Naeser, N.D., Zeitler, P.K., Naeser, C.W., and Johnson, N.M., 1988, History of uplift and relief of the Himalaya during the past 18 million years: Evidence from fission-track ages of detrital zircons from sandstones of the Siwalik Group, in Kleinspehn, K., and Paola, C., eds., New perspectives in basin analysis: New York, Springer-Verlag, p. 43–61. Deer, W.A., Howie, R.A., and Zussman, J., 1992, An introduction to the rockforming minerals (2nd ed.): Essex, England, Longman Scientific and Technical, 696 p. Dunkl, I., Spiegel, C., Kuhlemann, J., and Frisch, W., 2000, Impact of the volcanism on age-provenance studies—The Periadriatic event in the Alpine Molasse, in Noble, W.P., O’Sullivan, P.B., and Brown, R.W., eds., The 9th International Conference of Fission-track Dating and Thermochronology, Lorne Australia, February 6–11, 2000: Geological Society of Australia Abstracts, v. 58, p. 75–76. Flisch, M., 1986, Die Hebungsgeschichte der oberostalpinen Silvretta-Decke seit der mittleren Kreide: Bulletin Vereinigung Schweizer PetroleumGeologen und Ingenieure, v. 53, p. 23–49. Fügenschuh, B., Loprieno, A., Ceriani, S., and Schmid, S.M., 1999, Structural analysis of the Subbriabçonnais and Valais units in the area of Moûtiers (Savoy, Western Alps): Paleogeographic and tectonic consequences: International Journal of Earth Sciences, v. 88, p. 201–218. Galbraith, R.F., and Green, P.F., 1990, Estimating the component ages in a finite mixture: Nuclear Tracks and Radiation Measurements, v. 17, p. 197–206. Garver, J.I., and Brandon, M.T., 1994a, Fission-track ages of detrital zircon from Cretaceous strata, southern British Columbia: Implications for the Baja BC hypothesis: Tectonics, v. 13, p. 401–420. Garver, J.I., and Brandon, M.T., 1994b, Erosional denudation of the British Columbia Coast Ranges as determined from fission-track ages of detrital zircon from the Tofino Basin, Olympic Peninsula, Washington: Geological Society of America Bulletin, v. 106, p. 1398–1412. Garver, J.I., and Kamp, P.J.J., 2002, Integration of zircon color and zircon fission track zonation patterns in Orogenic belts: Application of the Southern Alps, New Zealand: Tectonophysics, v. 349, p. 203–219. Garver, J.I., Brandon, M.T., Roden-Tice, M.K., and Kamp, P.J.J., 1999, Exhumation history of orogenic highlands determined by detrital fission track thermochronology, in Ring, U., Brandon, M.T., Willett, S.D., and Lister, G.S., eds., Exhumation processes: Normal faulting, ductile flow, and erosion: Geological Society [London] Special Publication 154, p. 283–304. Garver, J.I., Brandon, M.T., Bernet, M., Brewer, I., Soloviev, A.V., Kamp, P.J.J., and Meyer, N., 2000a, Practical considerations for using detrital zircon fission track thermochronology for provenance, exhumation studies, and dating sediments, in Noble, W.P., O’Sullivan, P.B., and Brown, R.W., eds., The 9th International Conference of Fission-track Dating and Thermochronology, Lorne Australia, February 6–11, 2000: Geological Society of Australia Abstracts, v. 58, p. 109–111. Garver, J.I., Soloviev, A.V., Bullen, M.E., and Brandon, M.T., 2000b, Towards a more complete record of magmatism and exhumation in continental arcs using detrital fission-track thermochronometry: Physics and Chemistry of the Earth, Part A, v. 25, no. 6–7, p. 565–570. Gleadow, A.J.W., 1978, Comparison of fission-track dating methods: effects of anisotropic etching and accumulated alpha-damage, in Zartman, R.E., ed., Short papers of the 4th International Conference on Geochronology, Cosmochronology and Isotope Geology, Snowmass, Colorado, August 1978: U.S. Geological Survey, Open File Report 78-701.
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Naeser, N.D., Zeitler, P.K., Naeser, C.W., and Cerveny, P.F., 1987, Provenance studies by fission track dating of zircon—Etching and counting procedures: Nuclear Tracks and Radiation Measurements, v. 13, p. 121–126. Palenik, C.S., Nasdala, L., and Ewing, R.C., 2003, Radiation damage in zircon: American Mineralogist, v. 88, p. 770–781. Poldervaart, A., 1955, Zircon in rocks 1, Sedimentary rocks: American Journal of Science, v. 235, p. 433–461. Poldervaart, A., 1956, Zircon in rocks 2, Igneous rocks: American Journal of Science, v. 234, p. 521–554. Press, W.H., Teukolsky, S.A., Vetterling, W.T., and Flannery, B.P., 1992, Numerical recipes in FORTRAN, 2nd ed.: New York, Cambridge University Press, 963 p. Schäfer, H.-J., Gebauer, D., Naegler, T., and Eguiluz, 1995, Conventional and ion-microprobe U-Pb dating of detrital zircon of the Tentudia Group (Serie Negra, SW Spain): Implications for zircon systematics, stratigraphy, tectonics, and the Precambrian/Cambrian boundary: Contributions to Mineralogy and Petrology, v. 113, p. 289–299. Seward, D., and Mancktelow, N.S., 1994, Neogene kinematics of the central and western Alps: Evidence from fission-track dating: Geology, v. 22, p. 803–806. Spiegel, C., Kuhlemann, J., Dunkl, I., Frisch, W., von Eynatten, H., and Balogh, K., 2000, The erosion history of the Central Alps: Evidence from zircon fission-track data of the foreland basin sediments: Terra Nova, v. 12, p. 163–170. Stewart, R.J., and Brandon, M.T., 2004, Detrital-zircon fission-track ages for the “Hoh Formation”: Implications for late Cenozoic evolution of the Cascadia subduction wedge: Geological Society of America Bulletin, v. 116, p. 60–75. Tagami, T., Carter, A., and Hurford, A.J., 1996, Natural long term annealing of the zircon fission-track system in Vienna Basin deep borehole samples: Constraints upon the partial annealing zone and closure temperature: Chemical Geology, v. 130, p. 147–157. Zeitler, P.K., Johnson, M.N., Briggs, N.D., and Naeser, C.W., 1986, Uplift history of the NW Himalaya as recorded by fission-track ages of detrital Siwalik zircons, in Jiqing, H., ed., Proceedings of the Symposium on Mesozoic and Cenozoic Geology: Beijing, Geological Publishing House, p. 481–494. Zhao, J.X., McCulloch, M.T., and Bennett, V.C., 1992, Sm-Nd and U-Pb zircon isotopic constraints on the provenance of sediments from the Amadeus basin, Central Australia: evidence for REE fractionation: Geochimica et Cosmochimica Acta, v. 56, p. 921–940.
MANUSCRIPT ACCEPTED BY THE SOCIETY OCTOBER 21, 2003
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Geological Society of America Special Paper 378 2004
Toward a comprehensive provenance analysis: A multi-method approach and its implications for the evolution of the Central Alps Cornelia Spiegel* School of Earth Sciences, University of Melbourne, 3010 Victoria, Australia Wolfgang Siebel Mineralogisches Institut, Universität Tübingen, Wilhelmstrasse 56, D-72074 Tübingen, Germany Joachim Kuhlemann Wolfgang Frisch Geologisches Institut, Universität Tübingen, Sigwartstrasse 10, D-72076 Tübingen, Germany ABSTRACT In this study, we discuss potential problems connected with using geochronological data from foreland basins to unravel exhumation histories of the hinterland. In particular, we compare the results of a provenance analysis solely based on zircon fission-track ages from the foreland basin with a multi-method approach based on (i) the aforementioned zircon fission-track data, (ii) Nd isotope ratios of detrital epidote, and (iii) sediment accumulation rates in the foreland basins. For the example of the Central European Alps, we demonstrate that the multi-method approach can lead to highly different interpretations in terms of hinterland exhumation and geodynamic evolution. This is due to the fact that fission-track dating on detrital zircons alone only monitors the exhumation and erosion of zircon-containing lithologies and therefore only of restricted areas of the hinterland while the combination with Nd isotope ratios on detrital epidote also includes the erosion of zircon-free or -poor units such as basic magmatic rocks. A comparison of zircon fission-track and epidote Nd data with the sediment accumulation curve shows whether hinterland exhumation was predominantly caused by tectonic or by erosional denudation. Furthermore, we discuss some problems that may arise from using geochronological data from foreland basins to assess the maturity of a mountain belt in the hinterland. Applied to the Central Alps, our combined approach shows that the metamorphic core became exposed simultaneously over large areas by one sudden pulse of exhumation between 21 and 20 Ma. The main trigger for that exhumation event was tectonic denudation which is consistent with a geodynamic setting of large-scale extension. The Central Alps did not achieve exhumational steady-state conditions before 14 Ma. Keywords: zircon fission-track dating, Nd isotopic signature, sediment budget, Swiss Molasse Basin, steady-state exhumation.
Present address: Geologisches Institut, Universität Tübingen, Sigwart Str. 10, D-72076 Tübingen, Germany,
[email protected]. Spiegel, C., Siebel, W., Kuhlemann, J., and Frisch, W., 2004, Toward a comprehensive provenance analysis: A multi-method approach and its implications for the evolution of the Central Alps, in Bernet, M., and Spiegel, C., eds., Detrital thermochronology—Provenance analysis, exhumation, and landscape evolution of mountain belts: Boulder, Colorado, Geological Society of America Special Paper 378, p. 37–50. For permission to copy, contact
[email protected]. © 2004 Geological Society of America.
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INTRODUCTION Dating detrital minerals from synorogenic sediments has turned out to be a powerful tool for the reconstruction of longterm thermal and denudation histories of mountain belts (e.g., von Eynatten et al., 1999; Köppen and Carter, 2000; Krapez et al., 2000). For this approach, the most frequently dated minerals are white mica (40Ar/39Ar dating) and zircon (U/Pb, fission-track, and (U-Th)/He dating), because both minerals are relatively stable against chemical alteration by weathering and mechanical destruction during sediment transport. Zircon especially can survive several cycles of redeposition. With closure temperatures of 420–350 °C for 40Ar/39Ar white mica dating (McDougall and Harrison, 1988; Kirschner et al., 1996) and ~240 °C for zircon fission-track dating (Hurford, 1986) the minerals give information on the thermal evolution of the upper crust during mountain building processes and are likely to retain their provenance signal during burial in most sedimentary settings. However, reconstructions of orogenic evolutions based exclusively on dating detrital zircon and/or white mica bear the following problems. 1. Both minerals are absent or occur only in very small amounts in basic magmatic lithologies. Therefore, the erosion of ophiolitic units, for example, will not be monitored. Metabasic rocks in general and ophiolitic rocks in particular are important components of orogens and are often associated with suture zones. Neglecting them may lead to incomplete or even wrong interpretations in terms of geodynamics. 2. Dating detrital minerals from foreland basin sediments basically yields average cooling rates integrated over a certain drainage area and cooling period in the hinterland. If the paleogeothermal gradient is known, these cooling rates can be transformed into exhumation rates. However, exhumation rates are not
Figure 1. Digital elevation model of the Alps and the adjacent regions (Székely, 2001). Numbers refer to the different studied sections of the foreland basin. See also captions of Figure 2. GLF—Gonfolite Lombarda Formation; TPB—Tertiary Piedmont Basin.
necessarily equivalent to erosion rates. Therefore, the detrital age record does not allow estimating how much exhumation is due to erosional denudation and how much has to be attributed to tectonic denudation. This study aims to meet these problems by combining fission-track data of detrital zircons with Nd isotope analyses of detrital epidotes and sediment accumulation data from the foreland basins. The Nd data allow specifying the provenance of epidote and thus give evidence for the erosion of epidote-bearing ophiolitic rocks in the hinterland, while the sediment accumulation data provide an estimate for the amount of material removed from the source area by erosion alone. For the example of the Central Alps and their northern foreland basin, we will discuss advantages and disadvantages of each method and highlight the different interpretations resulting from a single-method approach compared to a multi-method approach. GEOLOGICAL SETTING Central Alps The Central Alps (Figs. 1 and 2) basically consist of three different tectonic mega-units: the Austroalpine units at the top, thrust over Penninic units, which in turn overlie the Helvetic units (Figs. 2 and 3). Austroalpine units represent the former margin of the African continent. Penninic units comprise continental crust as well as oceanic remnants. Ophiolitic rocks are frequent (Fig. 3), especially at the top of the Penninic sequence. These ophiolites are remnants of the South-Penninic ocean, which was situated between the European and the African continent. The Helvetic units belong to the former southern margin of the European continent. Each of these units experienced an individual metamorphic and tectonic history. In this study, we present only a short, simplified outline and refer to Steck and Hunziker (1994) and Schmid et al. (1996), for example, for more detailed information. Only the Austroalpine units, which are widely exposed in the Eastern Alps but only sparsely preserved in the Central Alps, experienced an early orogenic phase in Cretaceous times, which culminated in metamorphism up to eclogite and amphibolite facies at ca. 100 Ma in parts of the eastern Alpine crystalline basement (e.g., Thöni, 1981; Frank et al., 1987). During the Tertiary orogeny of the Alps, the Austroalpine unit acted as a rigid orogenic lid (Laubscher, 1983) and remained largely undeformed. Therefore, zircon fission-track cooling ages of Austroalpine units mainly cluster between ~90–60 Ma (Frank et al., 1987; Hunziker et al., 1992). In contrast to the Eastern Alps, Cretaceous metamorphism of Austroalpine units in the Central Alps only reached temperatures around or slightly below the zircon fissiontrack closure temperature (Spiegel et al., 2001). Therefore, many older fission-track cooling ages (Variscan, Triassic, Jurassic, and Early Cretaceous) are also preserved. The Tertiary orogeny involved ~500 km of north-south convergence between Africa and Europe and is characterized by
Implications for the evolution of the Central Alps
39
Figure 2. Geological map of the Central Alps and the Swiss Molasse Basin. Numbers refer to the different studied sections. 1—Pfänder system; 2—Kronberg-Gäbris system; 3—Speer system; 4—Hörnli system; 5—Rigi-Höhrone system; 6—Honegg-Napf system; 7—axial drainage system. NCA—Northern Calcareous Alps; DB—Dent Blanche unit; SF—Simplon normal fault; FF—Forcola normal fault (Meyre et al., 1998); PAL—Periadriatic lineament; EL—Engadine line. Inset: Bs—Basel; Be—Berne; Zh—Zurich; Ge—Geneva; Lc—Locarno.
top-to-the-north and north-northwest movements (Schmid et al., 1996). Between ~65 and 35 Ma, convergence, subduction, and finally, continent-continent collision took place with the Austroalpine units acting as overriding upper plate and the Penninic units as downgoing lower plate (see Figure 4). At ca. 45 Ma, the subducting slab is assumed to break off, resulting in an upwelling of the asthenospheric mantle (Davies and von Blanckenburg, 1995). The subsequently upward migrating heat front caused melting in the lithospheric mantle. Magmatic activity first took place in Eocene times (Villa, 1983; Dunkl, 1990) and culminated between 32 and 30 Ma (von Blanckenburg, 1992). During this
time, the Bergell plutonic body intruded, and volcanic activity was widespread in the area of the Periadriatic lineament (Fig. 2; Ruffini et al., 1997; Brügel et al., 2000). Magmatic activity was accompanied by enhanced heat flow (Davies and von Blanckenburg, 1995), affecting large areas of the present-day Central and Western Alps and resetting parts of the Austroalpine basement of the Western Alps (Hurford et al., 1991; Dunkl et al., 2001). Between 40 and 30 Ma, the Penninic units of the Central Alps underwent greenschist to amphibolite-facies metamorphism (Steck and Hunziker, 1994; Gebauer, 1999), resulting in Oligocene or younger cooling ages (Hunziker et al., 1992). The lower Penninic units form the Lepontine structural dome (Fig. 2), which yields zircon fission-track cooling ages mainly younger than 15 Ma, and experienced fast cooling during mid-Tertiary times with rates up to 80 °C/m.y. (Hurford, 1986). In contrast, the cooling rates of the upper to middle Penninic hanging wall of the dome are only in the range of 10 °C/m.y. for the same time (Markley et al., 1998; Fig. 3). Our study focuses on timing and processes that led to the successive removal of the Austroalpine upper plate and the exposure of the Penninic lower plate Swiss Molasse Basin
Figure 3. Highly schematic sketch of the Austroalpine-Penninic nappe system and some of its important characteristics concerning zircon fission-track (Zr FT) ages, mid-Tertiary cooling rate, and epidote content. For legend see Figure 2.
The Swiss Molasse Basin is a flexural basin formed due to tectonic loading of the evolving Alps. Coarse molasse sedimentation started in late Rupelian times (ca. 31 Ma) and lasted until at
40
C. Spiegel et al. tions, see, e.g., Tanner, 1944; Matter, 1964; Schiemenz, 1960; Schlunegger et al., 1998; Kempf, 1998). Major components of these conglomerates are flysch and limestone pebbles, but crystalline clasts (mainly granites, granitic gneisses, and quartzites) are also present in most of the fan systems. The most striking feature of the molasse sandstones is their heavy mineral compositions. While the older molasse sandstones mainly contain garnet, apatite, zircon, tourmaline and rutile, the younger sandstones show a pronounced change toward epidote dominance with up to 90% epidote of the total amount of heavy minerals (Füchtbauer, 1964, Schlunegger et al., 1997). This change happened diachronously in the molasse basin (i.e., several million years earlier in the western part of the basin than in the eastern part [Fig. 5; Schlunegger et al., 1997; Kempf et al., 1999; Strunck, 2001]). The provenance of the epidote is a long-standing problem in the literature. Most researchers attribute its occurrence to the onset of erosion of the upper Penninic ophiolites in the hinterland (e.g., Renz, 1937; Dietrich, 1969). Another possible source is greenschist-facies metagranites, which are common in the lower Austroalpine units (Füchtbauer, 1964). In this study, we mainly focus on samples from the alluvial fan systems (sections 1–6 in Figure 2), because sediment input from sources other than the direct Alpine hinterland can be excluded, due to the proximity of the fans to the Alpine front. In the following, we refer to sections 1 to 3 as eastern fans, sections 4 and 5 as central fans, and section 6 as western fan. For local names, see caption of Figure 2. Only section 7 is situated in a more distal position within the axial drainage system of the basin. THE MULTI-METHOD APPROACH Zircon Fission-Track Dating
Figure 4. Simplified evolution of the Central Alps and the Swiss Molasse Basin after Schmid et al. (1996, Cretaceous and Tertiary orogeny), Davies and von Blanckenburg (1995, slab breakoff and magmatism), Steck and Hunziker (1994, Lepontine metamorphism), Gebauer (1999, Lepontine metamorphism), Frisch et al. (2000, Tertiary extension), Matter and Weidmann (1992, molasse evolution).
least Langhian to Serravallian times (Pfiffner, 1986; Matter and Weidmann, 1992). The sedimentary succession is characterized by two coarsening and shallowing upward sequences reflecting changes from shallow marine to fluvial environments. During the first sequence (Rupelian and Chattian times), the axial sediment transport was directed from southwest to northeast (e.g., Berger, 1996). During that early shallow marine stage, the molasse basin was connected to the Rhine graben in the north (Kuhlemann et al., 1999). The second sedimentary cycle (Burdigalian to Langhian/Serravallian) was associated with northeast-southwest directed currents along the axis of the basin (e.g., Berger 1996). The proximal part of the molasse basin consists of large alluvial fan systems (Figs. 1 and 2) which are composed of conglomerates, sandstones, and mudstones (for more detailed descrip-
Method The fission-track method is based on the spontaneous decay of uranium which causes defects in the zircon crystal lattice. At temperatures above ~240 °C (zircon fission-track closure temperature; Hurford, 1986) these defects or spontaneous fission tracks anneal after their formation while at temperatures below ~240 °C they are retained. Tracks are made visible by etching and are counted under an optical microscope at high magnification (>1000×). To determine their U-content, zircons are irradiated by thermal neutrons, which induces the fissioning of 235U. The induced tracks are monitored and counted on a low-uranium mica detector, which is attached to the zircon mount during irradiation (external detector method; see Gleadow [1981] for more detailed description). From the ratio between spontaneous and induced track density, the time which has passed since the sample cooled below ~240 °C is calculated. The external detector method allows the dating of single detrital zircon grains. About 60 grains per sample are dated. From the attained age distributions, single age populations are derived by fitting to a set of Gaussian distribution functions (Brandon, 1992).
Implications for the evolution of the Central Alps
41
Figure 5. Temporal relationship of the different stratigraphic profiles from the Swiss Molasse Basin and the investigated samples therein. The columns symbolize the drainage systems of the Swiss Molasse Basin according to their east-west position and their time of activity. Stratigraphic positions of samples used for fission-track (FT) and Nd analyses are indicated, as well as the change of the heavy mineral composition and the first occurrence of zircons from the Penninic lower plate and the time difference between the two latter events (Δt). The youngest sample from section 7 is not dated by the fission-track method, but cooling ages of detrital white mica (von Eynatten and Wijbrans, 2003) suggest the presence of zircons derived from metamorphosed Penninic units.
To use zircon fission-track dating in provenance studies, the following requirements should be met. (1) The different tectonic units exposed in the hinterland should have contrasting age patterns. (2) For a precise timing of exhumation processes in the hinterland, a good stratigraphic control of the foreland basin sediments is important. (3) The postdepositional thermal history of the foreland basin sediments should be known to make sure that heating during burial did not reach the closure temperature of the dated mineral. (4) For samples from distal basin positions, information on paleocurrents and directions of sediment transport is needed for a correct interpretation of the sediment provenance. For the Central Alps–Swiss Molasse Basin system all these prerequisites are fulfilled: With zircon fission-track ages <30 Ma and >60 Ma for Penninic and most of the Austroalpine units, respectively, cooling patterns in the hinterland are highly contrasting (Frank et al., 1987, Hunziker et al., 1992). Stratigraphic control is excellent due to biostratigraphic and magnetostratigraphic calibrations, except for section 1, which is only dated by biostratigraphy (Berger, 1992; Bolliger, 1992; Schlunegger et al., 1997; Kempf et al., 1997; Strunck, 2001). Errors for most of the stratigraphic ages are only in the range of a few hundred thousand years. Vitrinite reflection data (Schegg, 1992; Schegg et al., 1997; Erdelbrock, 1994) show that postdepositional tem-
peratures were well below the zircon fission-track closure temperature. The foreland basin evolution in terms of sedimentology and paleogeography has been extensively studied (e.g., Pfiffner, 1986; Homewood et al., 1986; Berger, 1996; Kuhlemann and Kempf, 2002). Results Figure 6 shows the modeled fission-track age populations from the Swiss Molasse Basin (after Spiegel et al., 2000, 2001, 2002). The majority of the populations are of Triassic, Jurassic, and Cretaceous age. The relation of the pre-Cenozoic cooling ages to thermal events in the Alpine hinterland are discussed by von Eynatten et al. (1999), Spiegel et al. (2000), and Dunkl et al. (2001). They reflect the erosion of Austroalpine basement units and to a large part the recycling of sedimentary units in the hinterland. In this study, we focus on the Cenozoic cooling ages. They can be subdivided into (i) Eocene and (ii) Oligo-Miocene cooling ages. (i) Eocene age groups in the molasse sediments cluster between 55 and 40 Ma (Fig. 6 and Table 1). Most of them contain only two to three grains, resulting in ill-constrained mean ages with large errors. Only in section 3 and 5 do Eocene age groups contain a significant number of grains (Fig. 6). Their
42
C. Spiegel et al. detrital record with cooling ages becoming continuously younger upsection. Section 5 shows the opposite trend with the youngest age population becoming older upsection, while in section 3, the Eocene age group completely disappears upsection. (2) Dating of flysch pebble populations from the molasse basin showed that the flysch contains some zircon grains with ages between 50 and 40 Ma (Spiegel et al., 2000), similar to what is contained in the Oligocene molasse sandstone. Furthermore, flysch nappes from the present-day exposures of the Central Alps also contain an Eocene (volcanic) age component (Winkler et al., 1990). Therefore, we assume that the Eocene ages are recycled ages and do not give evidence on hinterland exhumation processes during the time of molasse sedimentation. (ii) The first Oligocene fission-track cooling ages (32 Ma) are found in the easternmost and western fan (sections 1 and 6) with deposition ages of 21–20 Ma (Fig. 6). From the difference between fission-track age and deposition age, an average hinterland cooling rate of ~20 °C/m.y. is calculated. These young zircons are interpreted to be derived from the metamorphosed upper to middle Penninic units of the Central Alps and therefore reflect the exposure of the Penninic lower plate in the Central Alps. At the same time, the other sections (2 and 4, eastern and central fans) did not receive any Oligocene zircon grains. In a Middle Miocene sandstone of section 6 (14 Ma deposition age), zircons with fission-track ages of 20 Ma suggest an average cooling rate in the range of 40 °C/m.y. in the hinterland (Fig. 6). This cooling rate is too high to be derived from the erosion of the upper to middle Penninic hanging wall of the Lepontine Dome. Therefore, we assume that the lower Penninic Lepontine Dome became exposed in Middle Miocene times in the hinterland of the western molasse fan. At the same time the central fan system (4) did not receive zircons from the Lepontine Dome but from its hanging wall, as suggested by a fission-track age group of 32 Ma in the sandstones of this fan.
Figure 6. Modeled fission-track age groups of detrital zircons (given in Ma ±1σ) from the Swiss Molasse Basin listed according to section number and deposition age. The numbers in italics refer to the number of dated grains per modeled age group. The grey-shaded area shows the range of ages expected for zircons derived from the metamorphosed Penninic lower plate (= Oligo-Miocene ages). Modeling was performed by BinomFit (Brandon, 1992), based on the binomial model of Galbraith and Green (1990). For location of the dated sections, see Figures 1 and 2. Data is compiled after Spiegel et al. (2000, 2001, 2002).
interpretation is difficult because Eocene ages are scarce in the hinterland today. They could be related to an Eocene exhumation period (Dunkl et al., 2002) or to volcanic activity (Winkler et al., 1990; Dunkl, 1990). However, we suggest that the Eocene ages of the molasse sediments do not directly result from incision into a crystalline basement in the hinterland but are recycled ages due to the erosion of flysch nappes. This assumption is based on the following. (1) Incision into basement rocks should result in a
Discussion As outlined in the introduction of this paper, exhumation histories solely based on age-provenance studies bear the risk of neglecting the erosion of lithologies, which are devoid of the dated mineral phase. In the following, we summarize an exhumation history of the Central Alps as suggested by the fission-track data; later, we compare this to an interpretation based on a combination of different methods. According to our fissiontrack data, the Penninic lower plate was exposed at 21–20 Ma in the hinterland of the easternmost and western fan and only several millions years later in the hinterland of the central fan. Exhumation leading to its exposure was apparently a process which took place diachronously over a long period of time. This would fit to a setting where moderate erosion is the driving force for exhumation. If we compare the fission-track data with the heavy mineral compositions of the molasse sandstones, we find a considerable time gap of 4–10 m.y. between the first occurrence of large amounts of epidote and the occurrence of young zircons with
378-03
43
Implications for the evolution of the Central Alps
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Penninic provenance (Fig. 5). If the epidote is in fact derived from the erosion of upper Penninic ophiolites, then the exposure of the lower plate would accordingly have taken place several million years earlier than recorded by the fission-track data. For the Central Alps, this would mean that conventional heavy mineral analysis give a much better clue about timing of exhumation processes in the hinterland than the geochronological approach. If, in contrast, the epidote is derived from greenschist facies lower Austroalpine metagranites, as suggested by Füchtbauer (1964), no contradiction between heavy mineral and fission-track data would result. Nd Isotope Ratios on Detrital Epidote Method Nd isotopic ratios give evidence on a crustal or mantle origin of rocks and permit calculation of crustal residence ages. Nd iso-
tope studies on sediments have been successfully used for provenance analysis (e.g., Richard et al., 1976; Miller and O’Nions, 1984; Basu et al., 1990; Henry et al., 1997; and, more recently, Najman et al., 2000; Clift et al., 2001; and Robinson et al., 2001). In most of the provenance studies, whole-rock data are used, but Nd ratios of pebbles or single mineral phases also turned out to be useful in attaining more specific information on distinct lithologies of the hinterland (see, e.g., Henry et al., 1997). In the Central Alps, we measured Nd ratios of detrital epidote because of the dominance of epidote in the heavy mineral spectra of the molasse sandstones (see Spiegel et al. [2002] for base data). However, in other orogen-sedimentary basin systems, it may make sense to combine geochronological data with isotopic signatures of other heavy mineral phases and/or whole-rock data. In the hinterland of the Swiss Molasse Basins, two lithologies are potentially able to supply the huge amount of epidote: Austroalpine metagranites and Penninic ophiolites (Fig. 3). The
44
C. Spiegel et al.
different chemical compositions and origins of these potential source rocks should be reflected by their Nd isotopic signature. Because Nd isotope ratios had not been used before to specify the provenance of detrital epidote, we first tested whether this approach is suitable for our purpose (Spiegel et al., 2002). We sampled Penninic metabasitic and Austroalpine metagranitic rocks from the present-day exposures as well as metabasitic and metagranitic pebbles from the molasse basin, separated the epidote and measured the Nd ratios. In addition, we measured Sr isotopic ratios and a variety of trace elements. We found that the Nd data allow a very good distinction between the two different lithologies (Fig. 7), while the Sr ratios largely overlap. The contents of Ba, U, Rb, Nb, and Th correlate well with the εNd data (Spiegel et al., 2002). The good discrimination by Nd ratios was used to define a “crustal source” and a “mantle source” range. Results The results of the Nd measurements of the detrital epidote are plotted in Figure 7. Epidotes from Lower Miocene sandstones of sections 2 and 4 (eastern and central) clearly show a provenance from a metabasic source rock. Although these sandstones do not contain young zircons of Penninic provenance, the isotopic signature of the epidote points to an exposure of Penninic ophiolites in the hinterland. The western and the easternmost fan (sections 6 and 1—those which received “Penninic zircons” during Lower Miocene times) yield a more complex picture. The oldest epidotes (25 Ma deposition age) clearly plot within the crust-derived field and are therefore interpreted to be derived from lower Austroalpine metagranitic lithologies. This means
Figure 7. Variations of Nd isotopic ratios of epidote from the different molasse fans through time. Ranges of Nd ratios for the “mantle source” and “crustal source” fields are derived from measuring epidote from the present-day Penninic and Austroalpine hinterland as well as from ophiolitic and metagranitic pebbles from the Swiss Molasse Basin. Filled symbols indicate sandstones with reset zircon grains from the Penninic lower plate. Note that sandstones from section 3, 5, and 8 do not contain significant amounts of epidote. For base data, see Spiegel et al. (2002).
that even though the first large amounts of epidote appeared in the molasse in Oligocene times (Fig. 5), the Penninic lower plate did not become exposed before the end of Oligocene times in the Central Alps. Epidote from the western fan (6), deposited at around 20 Ma, plots in an intermediate position. We interpret this to reflect a mechanical mixture of epidote derived from both Austroalpine metagranites and Penninic ophiolites. Although the detrital epidote looks basically homogeneous, we tried to verify this assumption by separating two different epidote populations from the sample of the easternmost fan (1). These two populations were distinguished due to slight differences in color, shape, and zonation (Spiegel et al., 2002). One population plots in the crustal source field while the other plots close to the mantle source field. This seems to corroborate the assumption of a mechanical mixture from two different sources. Epidote from the oldest sandstones from section 7 also plots in an intermediate position, which means that this part of the Swiss Molasse Basin received a mixture of granitic and ophiolitic detritus as early as 27 Ma. For the interpretation of this data, we have to consider that section 7 is situated in the axial drainage system of the basin with southwest to northeast directed currents. In the Western Alps, Penninic ophiolitic rocks were already exposed during Chattian to Aquitanian times (Mange-Rajetzky and Oberhänsli, 1982). Due to the currents prevailing in the molasse basin, we suggest that the epidote from section 7 was derived from the Western Alps, and was transported toward the east and redeposited in the foreland basin of the Central Alps. Discussion The Nd data show that the molasse epidote is derived from two different sources—Austroalpine metagranitoids and Penninic ophiolites—belonging to the upper and lower plates, respectively. Because conventional heavy mineral analysis alone cannot distinguish between different types of epidote, it is not suited for timing exhumation processes in the hinterland. For the hinterland of the eastern and central sections (2 and 4) Nd isotope ratios suggest that the Penninic lower plate became exposed 7 m.y. earlier than indicated by the fission-track data. For epidote plotting in an intermediate position (section 1 and 6), an admixture from ophiolitic source rocks seems likely but cannot ultimately be proven by the Nd data alone, at least if we do not measure Nd ratios on single epidote grains. The combination of Nd and fission-track data shows that, at 21–20 Ma, each molasse fan received detritus from Penninic units, either zircon or epidote. This leads to an entirely different picture of the Oligo-Miocene Central Alps. Instead of a relatively slow unroofing over several million years, the lower plate was exhumed to the surface by a single pulse of exhumation which caused the simultaneous exposure of Penninic units over large areas of the Central Alps. This simultaneous exposure can be bracketed between 21 and 20 Ma. However, we still cannot answer the question of the geodynamic framework. In other words, was the sudden pulse of exhumation caused by tectonic denudation or by a phase of enhanced erosional denudation?
Implications for the evolution of the Central Alps
45
Sediment Budget of the Foreland Basins Method The calculation of sediment accumulation rates provides a direct estimate of the surface erosion in the source area and an indirect measure of the topographic evolution of the hinterland (Kuhlemann, 2000). It is based on the calculation of sediment volumes of the circum-Alpine basins by digitizing the available thickness maps of strata and base contour lines of sedimentary basins, as well as planimetry of geological profiles. The export of dissolved material is estimated on the basis of recent catchment settings and includes areal estimates of recent and ancient rocks exposed at the surface to account for highly variable ratios of solid versus dissolved rock (e.g., quartzite vs. limestone). The calculated sediment volumes were recompacted to a porosity equivalent to the solid rock of the source area and plotted for 1 m.y. steps (Kuhlemann et al., 2001). The 1 m.y. steps provide a good time resolution but are also large enough to exclude disturbances by local shortterm events like storms or landslides. Therefore, calculation of the sediment budget monitors the long-term erosional flux (see also Willett and Brandon, 2002). The calculation is corrected for recycling of orogenic sediments due to cannibalism in thrust sheets of the subalpine molasse but did not take into account the different erodibilities of the bedrock lithologies (Kuhlemann, 2000). The disadvantages of this approach are the relatively high uncertainties and the poor spatial resolution, which means that erosion rates cannot be specified for smaller catchment areas. Results Figure 8 shows the sediment accumulation data in the foreland basins of the Central and Western Alps, separated for south/east and north/west directed catchments (after Kuhlemann, 2000). Between 30 and 21 Ma, sediment accumulation rates were continuously rising. This is interpreted to reflect the buildup of relief in the hinterland and is in line with the onset of coarse clastic sedimentation in the foreland basins at ca. 30 Ma. At 21 Ma, sediment discharge rates decreased dramatically, which suggests the collapse of the relief. This drop is not restricted to the Central Alps, but is independently calculated for the entire Alps (Hay et al., 1992; Kuhlemann, 2000). After a short period of enhanced sediment accumulation rates between 18 and 15 Ma, discharge rates dropped again. Between 5 Ma and the Quaternary, sediment accumulation rates were again strongly increasing. This is in line with the up to ten times enhanced erosion rates estimated for the Alps during late Pleistocene times (Hinderer, 2001). Discussion The most striking and, for our study, most important feature of the sediment accumulation curve is the sharp drop at 21 Ma (i.e., exactly contemporaneous with the exposure of the Penninic lower plate over large areas of the Central Alps). Combining fission-track, Nd, and sedimentary data reveals the following.
Figure 8. Sediment accumulation rates in Alpine foreland basins between Oligocene times and the Quaternary (Kuhlemann, 2000) compared to estimated north-south convergence (Schmid et al., 1996). *—convergence since Late Eocene times (40 Ma); **—0.5 cm/yr if deformation stopped at 7 Ma.
1. Keeping in mind that the erodibilities of the bedrock lithologies were not taken into account for the sediment budget calculation, the decrease of the sediment supply could be at least partly caused by a change of the bedrock lithology to rocks with low erodibility. However, the Nd data indicate the exposure of upper Penninic ophiolitic rocks over significant hinterland areas. These upper Penninic ophiolites are generally associated with fine-grained schist (Bündnerschiefer), which is characterized as “highly erodable” due to its lithology and pervasive cleavage (Kühni and Pfiffner, 2001). In this context, the drop of the sediment supply seems to be even more dramatic. 2. The combination of enhanced hinterland exhumation and reduced erosion rates suggests that exhumation and unroofing of the Alpine metamorphosed lower plate was triggered by tectonic denudation rather than by erosion. This would fit well to a geodynamic setting of large-scale lateral extension processes affecting the Eastern and also the Central Alps in Oligocene-Miocene times, as suggested by Frisch et al. (2000). IMPLICATIONS FOR THE ALPINE EVOLUTION Combining all our data as well as data from the literature, we propose the following evolution for the postcollisional Central Alps. At ca. 30 Ma, the Central Alps started to develop significant relief, which caused enhanced erosion and therefore the onset of coarse clastic molasse sedimentation in the foreland basin. During upper Oligocene times, mainly sedimentary cover units and increasingly Austroalpine basement units were eroded and subsequently deposited in the molasse basin. At 25 Ma, erosion of epidote-rich greenschist-facies metagranitoids belonging to the lower Austroalpine units caused the influx of large amounts of
46
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Figure 9. Relationship between population ages, sedimentation ages, and derived lag times. A: The two youngest modeled age groups of zircon fission-track age distributions from sandstones of the Swiss Molasse Basin, plotted against their deposition ages. Oblique lines mark contours of constant lag times. For comparison, the youngest 40Ar/39Ar ages of white mica from the same sandstones were also plotted (*—after von Eynatten and Wijbrans, 2003). B: Youngest modeled zircon fission-track age groups from sandstones of the Oligo-Miocene Gonfolite Lombarda Formation south of the Central Alps. Modeled age groups are previously unpublished; for the base data, see Spiegel et al. (2001). C: Youngest modeled 40 Ar/39Ar age groups of detrital white mica from the Tertiary Piedmont Basin, which drained the internal retro wedge of the Western Alps. The same 38 Ma age group also persists in sediment younger than 12 Ma and even in recent river sediments (**—after Carrapa, 2002).
Implications for the evolution of the Central Alps epidote into the molasse basin. Around the Oligocene-Miocene boundary, large-scale orogen-parallel extension started in the Eastern and Central Alps as a result of ongoing north-south convergence, tectonic escape toward the unconstrained margin in the east (Pannonian basin), and a topographic west-east gradient (Ratschbacher et al., 1991; Frisch et al., 1998, 2000). This extension caused (i) an east-west stretching of more than 200 km in the Eastern Alps and ~100 km in the Central Alps (Frisch et al., 2000), (ii) the collapse of the relief, as indicated by the strong decrease of the sediment discharge (Kuhlemann, 2000), and (iii) a pulse of enhanced exhumation in the Central Alps, which led to the simultaneous exposure of Penninic units over large areas of the hinterland at 21–20 Ma, as indicated by the occurrence of zircon and epidote from the Penninic lower plate in molasse sandstones. Extension processes and normal faulting continued until ca. 13 Ma, causing the contemporaneous exposure of the Tauern window in the Eastern Alps (Brügel, 1998; Frisch et al., 1998, 2000) and the Lepontine Dome in the Central Alps between 14 and 13 Ma. The exhumation of the Lepontine Dome is reflected by zircon fission-track ages in the foreland basin, showing a sharp increase in the average cooling rates of the hinterland from ~20 °C/m.y. in Lower Miocene times to ~40 °C/m.y. in Middle Miocene times (Spiegel et al., 2000). The same is observed for detrital white micas from the molasse basin, which indicate a continuous rise of the hinterland cooling rate from ~20 °C/m.y. to ~28 °C/m.y. between 21 and 15 Ma, and a sharp increase of the cooling rate to ~40 °C/m.y. between 15 and 14 Ma (von Eynatten and Wijbrans, 2003). At 5 Ma, erosion rates in the Alps strongly increased. The reason for this increase is unknown, but it may have been caused by the increasing importance of orographic precipitation in the course of Pliocene southward migration and intensification of the westerlies (Kuhlemann, 2000). Since ca. 2.7 Ma, late Neogene climatic changes caused glaciations in the northern hemisphere, which resulted in an increase of valley erosion, relief formation, and isostatically forced mountain top uplift. These processes eventually shaped the Alps into the mountain chain we know today. Toward Steady State? The reconstruction of the postcollisional Alpine history leads to the question whether the Central Alps reached steadystate conditions at any point of their postcollisional evolution, as proposed by Bernet et al. (2001). Steady state of orogenic systems has been the subject of many studies in recent times because it is considered to provide a measure for the maturity of a mountain belt (Willett and Brandon, 2002). In theory, convergent orogens experience three different stages during their lifetimes: a constructional phase, a steady state, and a decay phase (Howard, 1965; Jamieson and Beaumont, 1989). Willett and Brandon (2002) defined four different forms of steady state: topographic, thermal, exhumational, and flux. For this study, only the latter two are relevant. Flux steady state is characterized by a dynamic equilibrium between accretional flux into the orogenic
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system and erosional flux out of the system (i.e., erosion balances accretion). Exhumational steady state refers to constant hinterland exhumation rates, which are thought to be reflected by constant lag times of detrital minerals from synorogenic sediments (Garver et al., 1999). “Lag time” means the time difference between cooling age and depositional age, i.e., the time a certain mineral takes to be exhumed from the depth of its closure temperature to the surface plus the time of erosion, transport and deposition in the foreland basin. The latter term is assumed to be negligible compared to the time required for the exhumation to the surface (Brandon and Vance, 1992). Orogens in a constructional phase should yield sediments with decreasing lag times upsection, while steady state is expected to result in constant lag times. The decay phase is associated with increasing lag times upsection (Brandon and Vance, 1992; Garver et al., 1999). Figure 9A shows a plot of the two youngest modeled age populations from the molasse basin against their deposition ages. For comparison, we also show detrital white mica ages from the same molasse sandstones (von Eynatten and Wijbrans, 2003), zircon fission-track data from the southern foreland of the Central Alps (Fig. 9B and Table 1; Gonfolite Lombarda Formation), and detrital white mica ages from the foreland of the Western Alps (Fig. 9C; Tertiary Piedmont Basin; Carrapa, 2002). For steady-state conditions, the respective modeled ages should plot on the same contour line of constant lag time (e.g., Bernet et al., 2001). Before we discuss the data in terms of potential steadystate conditions, we will first discuss the problems which may arise from the choice of the age groups in this kind of plot. For population 1 (P1), we chose all Cenozoic cooling ages, while population 2 (P2) was assigned to Late Cretaceous cooling ages (Fig. 9A). These Late Cretaceous ages are interpreted as to be initially derived from the erosion of Austroalpine nappes. However, we cannot assess how many of these Cretaceous zircons were directly derived from incision into the Austroalpine basement rocks and how many were stored in unmetamorphosed flysch nappes in an intermediate stage. The latter zircons do not provide any information in terms of hinterland exhumation during the time of molasse sedimentation. Furthermore, Cretaceous metamorphism of the eroded Austroalpine units of the Central Alps only reached temperatures in the range of the zircon fission-track closure temperature (Spiegel et al., 2001). Therefore, we cannot be sure whether the fission-track ages are fully reset cooling ages or if we are dealing with partly reset mixed ages. In that respect, measuring the track length distribution would be helpful. The P1 age groups contain Eocene and Oligo-Miocene cooling ages. As discussed before, for the Eocene ages, again the problem of recycling arises so they cannot be used to assess hinterland exhumation. In the southern foreland of the Central Alps, we found Oligocene (P1) and Eocene (P2) component ages (Fig. 9B). This is in line with the data of Dunkl et al. (2001) from the Chattian to Aquitanian Macigno formation, which is also situated south of the Central Alps. For the southern flank of the Central Alps, the problem of recycled ages is less pronounced because there
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were fewer sedimentary cover units exposed than on the northern flank (Longo, 1968; Giger, 1991). The problem of cooling age distributions from the southern foreland basin is that the modeled age groups cannot be clearly attributed to a special source area. In the southern Central Alps, a variety of potential sources with roughly the same age but entirely different thermal histories were exposed: (i) the Bergell plutonic body with an intrusion age of ca. 32 Ma (von Blanckenburg, 1992); (ii) reworked volcanic detritus from Periadriatic Oligocene volcanism and its Eocene precursors (Giger and Hurford, 1989; Dunkl, 1990; Ruffini et al., 1997, Brügel et al., 2000); (iii) the Austroalpine orogenic lid, which was largely thermally overprinted by enhanced Mid-Oligocene heat flow (Hurford et al., 1991); (iv) Penninic units from the Lepontine area, which experienced Tertiary metamorphism and exhumation (e.g., Schmid et al., 1996); and (v) the South-Alpine Ivrea zone, which yields in the present-day exposures Eocene zircon fission-track cooling ages (Hurford, 1986). Because we are not able to define the provenance of the age groups from the Gonfolite Lombarda Formation more precisely, an interpretation of this data in terms of a potential steady state is impossible. After a closer examination, most of the fission-track data seem to be insufficient for applying the lag-time concept because of mixed ages, recycled ages, or uncertainties in terms of the provenance of the age groups. What remains are the Oligocene cooling ages from the Swiss Molasse Basin, in combination with detrital white mica ages from the same samples (Fig. 9A). Their provenance is well defined: Between 21 and 15 Ma, P1 was derived from the upper to middle Penninic hanging wall; after 15 Ma, P1 was derived from the lower Penninic footwall of the Lepontine Dome. The source area should have remained roughly the same during lower to middle Miocene times, although we assume some changes of the main drainage divide at 17 Ma (Kuhlemann et al., 2001). Therefore, testing a potential steady state on the base of these zircon and white mica ages seems reasonable. Between 21 and 19 Ma, lag times deduced from the zircon fission-track data seem to be more or less constant at ca. 10 Ma, while between 19 and 14 Ma, the lag time decreases to ca. 6 Ma. For the same period of time (21 to 14 Ma), the lag time deduced from 40Ar/39Ar ages continuously decreases from 21 to 10 Ma. Hence, the geochronological data is in line with a constructional orogenic state. Therefore, we can clearly exclude exhumational steady-state conditions in the Central Alps before 14 Ma. For the time after 14 Ma until the present, Bernet et al. (2001) proposed exhumational steady state based on zircon fission-track data from the southern foreland of the Central Alps that show a constant exhumation of the footwall of the Lepontine Dome. However, the Central Alps did not achieve a flux steady state in postcollisional times, because this would require constant erosion rates (Willett and Brandon, 2002), which is clearly not the case for the Central Alps (see Figure 8). Instead, the orogenic wedge was still growing between ca. 9 and 4 Ma (thrusting of the Jura mountains; Becker, 2000), corresponding to a constructional state. In the Tertiary Piedmont Basin, the foreland of the adjacent Western Alps, detrital white mica yield a youngest modeled age
group of 38 Ma (Fig. 9C; Carrapa, 2002). This youngest age group is persistent from Late Oligocene times until the present. It is interpreted to reflect very fast post-Eocene exhumation and subsequently slow and continuous erosion for more than 25 m.y. (Carrapa, 2002). According to the lag-time concept, the detrital white mica ages are consistent with a decay phase for the internal zone of the Western Alps. A comparison of the Central and Western Alps shows that the Alps as a whole did not reach steady-state conditions at a certain time but that the different parts of the Alps may have attained regional equilibria at different points in time. CONCLUSIONS From this study, we can draw two main conclusions for the use of age data from detrital minerals. 1. The sole use of age provenance studies for reconstructions of hinterland denudation histories can result in misleading interpretations. Instead, geochronological data should be combined with other methods that are able to add information on the erosion of basic magmatic lithologies and on the geodynamic framework, such as geochemical or isotopic fingerprints of heavy mineral phases and the sediment budget of the foreland basins. The importance of a combined approach is demonstrated for the example of the Central Alps, where we tried to reconstruct timing and processes leading to the exposure of the Alpine metamorphosed lower plate by the sedimentary record. According to the fission-track data alone, the lower plate became exposed by relatively slow erosion processes over several million years. In contrast, the combined approach showed that it became exposed by one fast exhumation pulse mainly triggered by tectonic denudation. Similar combined approaches may be transferred to foreland studies on other orogenic systems. 2. Using the lag-time concept to recognize steady-state conditions of an orogen requires detailed knowledge on the provenance of the single modeled age groups. For the example of the Central Alps, we had to deal with the following problems. (i) Recycling of zircon grains from sedimentary cover units. “Recycled” age groups do not give direct evidence on hinterland exhumation. (ii) Partial resetting of cooling ages. These ages are geologically meaningless mixed ages that, again, cannot be used to decipher hinterland exhumation rates. (iii) Different provenances of similar age groups. In the southern Central Alps, similar age groups were derived from different tectonic units and thermal settings (magmatic ages, thermally reset ages due to enhanced heat flow and metamorphic cooling ages). They cannot be combined with each other for the lag-time concept. From our fission-track data of sandstones from the Swiss Molasse Basin, we can conclude that the Central Alps did not reach exhumational steady state before 14 Ma. ACKNOWLEDGMENTS This study was financed by the German Science Foundation in the framework of the collaborative research centre SFB 275.
Implications for the evolution of the Central Alps Thanks to Oliver Kempf, Fritz Schlunegger, and Peter Strunck for guidance and discussions in the field and to Istvan Dunkl and Hilmar von Eynatten for ongoing scientific exchange. The manuscript benefited from the thorough reviews of Peter van der Beek and Meinert Rahn. REFERENCES CITED Basu, A., Sharma, M., and DeCelles, P., 1990, Nd, Sr-isotopic provenance and trace element geochemistry of Amazonian foreland basin fluvial sands, Bolivia and Peru: implications for ensialic Andean orogeny: Earth and Planetary Science Letters, v. 100, p. 1–17. Becker, A., 2000, The Jura Mountains—an active foreland fold-and-thrust belt?: Tectonophysics, v. 321, p. 381–406. Berger, J.-P., 1992, Correlative chart of the European Oligocene and Miocene: application to the Swiss Molasse Basin: Eclogae Geologicae Helvetiae, v. 85, p. 573–609. Berger, J.-P., 1996, Cartes paléogéographiques-palinspastiques du bassin molassique suisse (Oligocène inférieur-Miocène moyen): Neues Jahrbuch für Geologie und Paläontologie, Abhandlungen, v. 202, p. 1–44. Bernet, M., Zattin, M., Garver, J., Brandon, M., and Vance, J., 2001, Steadystate exhumation of the European Alps: Geology, v. 29, p. 35–38. Bolliger, T., 1992, Kleinsäugerstratigraphie in der lithologischen Abfolge der miozänen Hörnlischüttung (Ostschweiz) von MN3 bis MN7: Eclogae Geologicae Helvetiae, v. 85, p. 961–1000. Brandon, M., 1992, Decomposition of fission-track grain-age distributions: American Journal of Science, v. 292, p. 535–564. Brandon, M., and Vance, J., 1992, Fission-track ages of detrital zircon grains: implications for the tectonic evolution of the Cenozoic Olympic subduction complex: American Journal of Science, v. 292, p. 565–636. Brügel, A., 1998, Provenances of alluvial conglomerates from the Eastalpine foreland: Oligo-Miocene denudation history and drainage evolution of the Eastern Alps [Ph.D. thesis]: Tübinger Geowissenschaftlich Arbeiten, Reihe A40, p. 1–168. Brügel, A., Dunkl, I., Frisch, W., Kuhlemann, J., and Balogh, K., 2000, The record of Periadriatic volcanism in the Eastern Alpine Molasse zone and its paleogeographic implications: Terra Nova, v. 12, p. 42–47. Carrapa, B., 2002, Tectonic evolution of an active orogen as reflected by its sedimentary record. An integrated study of the Tertiary Piedmont Basin (Internal Western Alps, NW Italy) [Ph.D. thesis]: Vrije Universiteit Amsterdam, The Netherlands. Clift, P., Shimizu, N., Layne, G., and Blusztaju, J., 2001, Tracing pattern of erosion and drainage in the Paleogene Himalaya through ion probe Pb isotope analysis of detrital K-feldspars in the Indus molasse, India: Earth and Planetary Science Letters, v. 188, p. 475–491. Davies, J., and von Blanckenburg, F., 1995, Slab breakoff: A model of lithosphere detachment and its test in magmatism and deformation of collisional orogens: Earth and Planetary Science letters, v. 129, p. 85–102. Dietrich, V., 1969, Die Oberhalbsteiner Talbildung im Tertiär—ein Vergleich zwischen den Ophioliten und deren Detritus in der ostschweizerischen Molasse: Eclogae Geologicae Helvetiae, v. 62, p. 637–641. Dunkl, I., 1990, Fission track dating of tuffaceous Eocene formations of the North Bakony Mountains (Transdanubia, Hungary): Acta Geologica Hungarica, v. 33, p. 13–30. Dunkl, I., Di Giulio, A., and Kuhlemann, J., 2001, Combination of single-grain fission track chronology and morphological analysis of detrital zircon crystals in provenance studies—sources of the Macigno formation (Apennines, Italy): Journal of Sedimentary Research, v. 71, p. 516–525. Dunkl., I., Frisch, W., and Kuhlemann, J., 2002, A “mini-orogen” mirrored in detrital apatite FT ages of Alpine Miocene Molasse—the obscure PostGosau cooling event of the Eastern Alps, in Casado, J.M.G., Segura, M., and Pinol, F.C., eds., International Workshop on Fission Track Analysis: Theory and Applications: Geotemas v. 4, p. 61–62. Erdelbrock, K., 1994, Diagenese und schwache Metamorphose im Helvetikum der Ostschweiz (Inkohlung und Illit-Kristallinität) [Ph.D. thesis]: Technische Hochschule Aachen, Germany. Frank, W., Kralik, M. Scharbert, S., and Thöni, M., 1987, Geochronological data from the Eastern Alps, in Flügel, H., and Faupl, P., eds., Geodynamics of the Eastern Alps: Deuticke, Wien, p. 272–281.
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Schegg, R., 1992, Thermal maturity of the Swiss Molasse Basin; indications for paleogeothermal anomalies?: Eclogae Geologicae Helvetiae, v. 85, p. 745–764. Schegg, R., Leu, W., Cornford, C., and Allen, P., 1997, New coalification profiles in the molasse basin of western Switzerland: Implications for the thermal and geodynamic evolution of the Alpine foreland: Eclogae Geologicae Helvetiae, v. 90, p. 79–96. Schiemenz, S., 1960, Fazies und Paläogeographie der subalpinen Molasse zwischen Bodensee und Isar: Beihefte Geologisches Jahrbuch, v. 38, p. 1–119. Schlunegger, F., Matter, A., Burbank, D., and Klaper, E., 1997, Magnetostratigraphic constraints on relationships between evolution of the central Swiss Molasse basin and Alpine orogenic events: Geological Society of America Bulletin, v. 109, p. 225–241. Schlunegger, F., Slingerland, R., and Matter, A., 1998, Crustal thickening and crustal extension as controls on the evolution of the drainage network of the central Swiss Alps between 30 Ma and the present: constraints from the stratigraphy of the North Alpine Foreland Basin and the structural evolution of the Alps: Basin Research, v. 10, p. 197–212. Schmid, S.M., Pfiffner, O.A., Froitzheim, N., Schönborn, G., and Kissling, E., 1996, Geophysical-geological transect and tectonic evolution of the Swiss-Italian Alps: Tectonics, v. 15, p. 1036–1064. Spiegel, C., Kuhlemann, J., Dunkl, I., Frisch, W., von Eynatten, H., and Kadosa, B., 2000, Erosion history of the Central Alps: evidence from zircon fission track data of the foreland basin sediments: Terra Nova, v. 12, p. 163–170. Spiegel, C., Kuhlemann, J., Dunkl, I., and Frisch, W., 2001, Paleogeography and catchment evolution in a mobile orogenic belt: The Central Alps in Oligo-Miocene times: Tectonophysics, v. 341, no. 1–4, p. 33–47. Spiegel, C., Siebel, W., Frisch, W., and Berner, Z., 2002, Sr and Nd isotope ratios and trace element geochemistry of detrital epidote as provenance indicators: implications for the reconstruction of the exhumation history of the Central Alps: Chemical Geology, v. 189, p. 231–250. Steck, A., and Hunziker, J., 1994, The Tertiary structural and thermal evolution of the Central Alps—compressional and extensional structures in an orogenic belt: Tectonophysics, v. 238, p. 229–254. Strunck, P., 2001, The Molasse of Western Switzerland [Ph.D. thesis]: Universität Bern, Switzerland. Székely, B., 2001, On the surface of the Eastern Alps—a DEM study [Ph.D. thesis]: Tübinger Geowissenschaftliche Arbeiten, v. 60A, p. 1–157. Tanner, H., 1944, Beitrag zur Geologie der Molasse zwischen Ricken und Hörnli: Thurgauer Naturforschende Gesellschaft, v. 33, p. 1–108. Thöni, M., 1981, Degree and evolution of the Alpine metamorphism in the Austroalpine unit W of the Hohe Tauern in the light of K/Ar and Rb/Sr age determinations of micas: Jahrbuch der Geologischen Bundesanstalt Wien, v. 124, p. 111–174. Villa, I., 1983, 40Ar/39Ar chronology of the Adamello gabbros, southern Alps: Società Geologica Italiana, Memorie, v. 26, p. 309–318. von Blanckenburg, F., 1992, Combined high-precision chronometry and geochemical tracing using accessory minerals: applied to the Central-Alpine Bergell intrusion (central Europe): Chemical Geology, v. 100, p. 19–40. von Eynatten, H., and Wijbrans, J., 2003, Precise tracing of exhumation and provenance using 40Ar/39Ar geochronology of detrital white mica: the example of the Central Alps, in McCann, T., and Saintot, A., eds., Tracing tectonic deformation using the sedimentary record: Geological Society of [London] Special Publication 208, p. 289–305. von Eynatten, H., Schlunegger, F., Gaupp, R., and Wijbrans, J., 1999, Exhumation of the Central Alps: Evidence from 40Ar/39Ar laserprobe dating of detrital white micas from the Swiss Molasse basin: Terra Nova, v. 11, p. 284–289. Willett, S., and Brandon, M., 2002, On steady states in mountain belts: Geology, v. 30, p. 175–178. Winkler, W., Hurford, A., von Salis Perch-Nielsen, K., and Odin, G., 1990, Fission track and nannofossil ages from a Paleocene bentonite in the Schlieren Flysch (Central Alps, Switzerland): Schweizerische Mineralogische und Petrographische Mitteilungen, v. 70, p. 389–396. MANUSCRIPT ACCEPTED BY THE SOCIETY OCTOBER 21, 2003
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Geological Society of America Special Paper 378 2004
Miocene siliciclastic deposits of Naxos Island: Geodynamic and environmental implications for the evolution of the southern Aegean Sea (Greece) J. Kuhlemann W. Frisch I. Dunkl Institute of Geology, University of Tübingen, Sigwartstrasse 10, D-72076 Tübingen, Germany M. Kázmér Department of Paleontology, Eötvös University, P.O. Box, H-1518 Budapest, Hungary G. Schmiedl Institute of Geophysics und Geology, University of Leipzig, Talstrasse 35, D-04103 Leipzig, Germany ABSTRACT An interdisciplinary study has been carried out on Naxos Island, located in the southern Aegean Sea (Greece), which shows Miocene geodynamic and environmental changes in a classic example of a collapsing orogen. Early to mid-Miocene siliciclastic deposits on Naxos have been shed from an uplifting mountainous realm in the south, which included a patchwork of at least four source terrains of different thermal histories. Petrography of pebbles suggests that the source units formed part of a passive continental margin succession (external Pelagonian unit), and an ophiolite succession mainly of deep-water cherts and limestones deposited on basalt substratum (Pindos unit). The continental margin source contributed rounded zircon crystals of Late Jurassic to Early Cretaceous age and broadly scattering Paleozoic zircon fission-track cooling ages. A distal pebble assemblage of Paleogene shallow-water carbonates passing into flysch-like, mixed calcarenitic and siliciclastic components with volcanic arc components is subordinately present. High-grade metamorphic components from the nearby metamorphic core complex are not present. The depositional evolution reflects increasing relief and, in some parts, a fluvial succession with rhythmic channel deposition, possibly due to runoff variability forced by orbital cyclicity. Upsection, the depositional trend indicates increasing seasonality and decreasing humidity in the source region. The Miocene sedimentary succession has been deposited on an ophiolite nappe. Juxtaposition of this ophiolite nappe occurred as an extensional allochthon during large-scale extension in the Aegean region at the margins of an exhuming metamorphic core complex. Keywords: Aegean, fission track, age provenance, extension, cyclicity. *E-mail:
[email protected],
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[email protected],
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[email protected] Kuhlemann, J., Frisch, W., Dunkl, I., Kázmér, M., and Schmiedl, G., 2004, Miocene siliciclastic deposits of Naxos Island: Geodynamic and environmental implications for the evolution of the southern Aegean Sea (Greece), in Bernet, M., and Spiegel, C., eds., Detrital thermochronology—Provenance analysis, exhumation, and landscape evolution of mountain belts: Boulder, Colorado, Geological Society of America Special Paper 378, p. 51–65. For permission to copy, contact
[email protected]. © 2004 Geological Society of America.
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INTRODUCTION Ancient orogenic debris records erosional unroofing of orogens through time. It preserves information on the petrography, relief, and exhumation rates of a hinterland which typically changed its geodynamic and paleogeographic context. Such information is of special importance in a classic example of a collapsing orogen such as the Hellenides in the Aegean region, which is largely submerged at present. Here, Tertiary large-scale extension has exposed numerous core complexes, which have been examined in detail by means of metamorphic petrology, geochronology and structural geology (e.g., Gautier and Brun, 1994). In contrast, postcollisional Neogene deposits in the Aegean region are poorly investigated (Böger, 1983; Jacobshagen et al., 1986; Böger and Dermitzakis, 1987). They occur mainly in the northern Aegean region, most of them subsurface (Sidiropoulos, 1980; Zygojannis and Sidiropoulos, 1981), and include local marine deposits with uncertain connections to the Mediterranean region (Rögl and Steininger, 1984; Steininger and Rögl, 1984; Steininger et al., 1985). During early to middle Miocene extension, local basins in the southern Aegean region were mainly characterized by continental deposition (Jacobshagen et al., 1986), despite a global transgressive trend (Haq et al., 1988). These rare siliciclastic deposits provide information on the provenance, paleogeography and changing climatic and environmental conditions in a mobile belt. To unravel the thermal history of the source terrains, fission-track geochronology of detrital zircon grains from the sandstone members of the Miocene deposits on the east and west coasts of the island has been carried out. The aim of this study is to highlight Neogene near-surface processes in a classical setting of spectacular deeper crustal processes.
GEODYNAMIC EVOLUTION OF THE SOUTHERN AEGEAN SEA The southern Aegean Sea is characterized by numerous islands, forming the Cyclades archipelago. These represent the largely submerged part of tectonic units striking NW-SE along the southern Balkan peninsula and bending into the W-E direction toward Turkey (Fig. 1). The Attico-Cycladic massif is part of a continuous belt of the Pelagionan-Cycladic zone (Mountrakis et al., 1987), which may be correlated with hanging-wall units of the Menderes Massif in Turkey (Ring et al., 1999). Similar lithologies and similar metamorphic overprint of the PelagionanCycladic basement and the Menderes massif reflect a common Alpine metamorphic history. The common Alpine metamorphic history includes meso-Hellenic high pressure metamorphism at ca. 45 Ma, and a neo-Hellenic Barrovian-type metamorphic phase between 23 and 16 Ma (Altherr et al., 1982; Andriessen et al., 1987; Wijbrans and McDougall, 1988; Okrusch and Bröcker, 1990; Avigad, 1998). On the Greek mainland, this Alpine-metamorphic basement domain is juxtaposed against an external arcuate belt of unmetamorphic continental units (Sub-Pelagonian and Ionian) with the Pindos ophiolite unit sandwiched in between (Jacobshagen et al., 1986; Figure 1). Miocene large-scale N-S extension in the Aegean Sea enabled updoming of numerous metamorphic core complexes in the Cyclades archipelago (Lee and Lister, 1992). During Miocene extension, the medium- to high-grade metamorphic rocks suffered ductile deformation in a W-E to NW-SE compressive and NNE-SSW extensional regime (Lister et al., 1984, 1986; Buick, 1991; Walcott, 1998). Rapid decompression caused by top-tothe-north tectonic unroofing generated migmatization within the
Figure 1. Structural map of the Aegean Sea (A), displaying the central position of Naxos (B).
Miocene siliciclastic deposits of Naxos Island core complexes and batholiths at the base of the continental crust (Jansen and Schuiling, 1976; Altherr et al., 1982; Pe-Piper et al., 1997; Pe-Piper, 2000). Tectonic unroofing continued in places until latest Miocene times, as recorded by apatite fission-track cooling ages (Hejl et al., 2002). According to Jolivet and Patriat (1999), late Oligocene to early Miocene submergence of the arcuate belt in the Aegean Sea followed the collapse of the crust, which was thickened in the Eocene. On the other hand, backarc extension related to subduction retreat, similar to the recent setting, may have started by the same time (Lee and Lister, 1992). The formation age of the isolated siliciclastic deposits of Naxos and surrounding Cycladic islands, however, has been a matter of debate, owing to the rarity of datable organic content and facies heterogeneity. Age determinations and estimates vary between Pliocene (Renz, 1928), early to middle Miocene (Rösler, 1972, 1978), early Miocene (Angelier et al., 1978), and Oligocene (Ökonomidis, 1935). Latest investigations support an early to middle Miocene time of formation of limnic-fluvial successions in the Cyclades region, based on middle Miocene limnic gastropods from Moutsouna on the east coast of Naxos (Böger, 1983). REGIONAL GEOLOGIC SETTING OF NAXOS ISLAND The dominant geologic feature of Naxos is a metamorphic core complex, in which continental basement of probable Variscan age (Altherr et al., 1982; Strumpf, 1997; Reischmann, 1998) is capped by a meso- to high-grade metamorphic cover
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succession of Mesozoic carbonates, and fine-grained siliciclastic and tuffaceous rocks (Jacobshagen, 1986). After a high-pressure event in Eocene time, the metamorphic complex experienced Barrovian-type metamorphism, which climaxed probably ca. 16 Ma (John and Howard, 1995). Only small and thin remnants of the hanging wall of the core complex are exposed on land at the western and eastern margins of the metamorphic core complex (Fig. 2A; see Jansen, 1977). Structural evidence indicates that the hanging wall of the core complex experienced large-scale top-to-the-north (010º) transport along a low-angle detachment fault (Gautier et al., 1990; Gautier and Brun, 1994; Walcott, 1998). The tectonic transport started under ductile conditions (Buick, 1991; Urai et al., 1990; Gautier et al., 1993) and continued under brittle conditions (Angelier et al., 1978; John and Howard, 1995). The only preserved and exposed part of the hanging-wall unit is an unmetamorphosed ophiolite nappe (Jansen, 1977). The studied Miocene sedimentary succession is part of this nappe, which is spread over several islands of the Cyclades (e.g., Avigad, 1998; Jolivet and Patriat, 1999). The ophiolitic succession is strongly disrupted, and only limited exposures are found on Naxos (Fig. 2). The ophiolitic members are serpentinite and basalt, with radiolarite in two spot-like outcrops. We consider the Miocene clastic succession as the neo-autochthonous cover of the imbricated ophiolite deposited after nappe formation, since both are unmetamorphosed and spatially associated (Fig. 2). The Miocene deposits contain no components from the metamorphic complex of Naxos, in contrast to the coarse Pliocene deposits (Rösler, 1978). Therefore, the Miocene succession is considered
Figure 2. Geologic sketch map of the location of the working area within Naxos (A) and the location of the Melanés Miocene siliciclastic succession (B).
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to have been deposited when the metamorphic complex was still buried. This is consistent with cooling ages of the presently exposed footwall rocks (see Pe-Piper et al., 1997). On the western side of the island of Naxos, an I-type granodiorite batholith, now in tectonic contact with the Miocene clastic succession, was emplaced ca. 11.4 Ma according to U/Pb dating on zircon (Henjes-Kunst et al., 1988). Fast cooling of the batholith is indicated by Ar-Ar ages ca. 12.5 Ma on various minerals (Wijbrans and McDougall, 1988) and K-Ar cooling ages of biotite ca. 10 Ma (Pe-Piper et al., 1997). Cooling to apatite fissiontrack stability below 120 °C occurred at 8.2 Ma (Altherr et al., 1982). Cooling occurred during ongoing top-to-the-north lowangle normal faulting. Ultramylonite, pseudotachylite, and cataclasite formation close to the detachment fault record tectonic activity lasting until nearly superficial conditions were reached (John and Howard, 1995). Pseudotachylite formation, which is locally intense in the granodiorite and along the tectonic contact below the ophiolite nappe, is estimated to have occurred at ca. 10 Ma (Andriessen et al., 1979). The reported ages yield time constraints for the final juxtaposition of the ophiolite nappe and the Miocene neoautochthonous cover sequence, which apparently show no thermal overprint (see below). RESULTS Structural Setting of the Neogene Deposits The ophiolite nappe with its Miocene sedimentary cover tectonically overlies the steep to subvertical fault contacts between the metamorphic complex and the late Middle Miocene granodiorite body on the western side of Naxos (Fig. 2). At its base to the west, the studied Miocene siliciclastic profile of Melanés is in tectonic contact with the granodiorite and a 3-m-thick sliver of dolomite derived from the metamorphic complex. The fault contact is characterized by intense cataclastic deformation and chloritization of the intrusive body. The quartz fabric is generally mylonitic in the vicinity of the tectonic contact to the ophiolite nappe and the faulted contact to the metamorphic complex, but also elsewhere in the granodiorite. It shows consistently top-to-the-north movement (010° to 025°). The mylonitic fabric is strongly cataclastically overprinted and brecciated under brittle conditions for quartz. Dynamic recrystallization of quartz to an extremely fine grain size and subsequent strong cataclastic overprint characterizes the immediate vicinity of the fault contact to the metamorphic core complex. Strong cataclasis is also observed in a local dolomite sliver between the granodiorite and the Miocene sedimentary. The sedimentary succession above the nappe boundary shows only limited deformation. The basal parts (i.e., the reddish playa-facies sandstones; see below) show lustrous shear fractures and some faulting and folding but no penetrative deformation or cataclasis. A >20 m wide subvertical ultracataclastic zone characterizes the contact between the metamorphic complex and basalts of the ophiolite nappe on the eastern side of the nappe exposure (see Figure 2B).
The deformation mechanisms along the tectonic contact of the Miocene succession show that the final juxtaposition of the ophiolite nappe occurred under brittle conditions with temperatures below 300 °C. This is in accordance with K-Ar dating on <2µm clastic mica in the basal sediments, which gave ages in the range of 80–105 Ma (Table 1) and thus do not indicate rejuvenation. The vitrinite reflectance values of the sedimentary succession (up to 1.4%; Vanderhaeghe et al., 2003) also give a limit for the postdepositional thermal overprint. Thus, the fission-track ages of the detrital zircon grains can be interpreted as unmodified cooling ages of the source areas. These data agree with the microscopic study of the basal sedimentary succession, indicating that the basal beds did not experience temperatures exceeding lower anchizonal conditions. Brittle shearing, however, was a continuous process starting with the ductile deformation of the granodiorite. The structural observations and the temperature difference between the high-grade metamorphic rocks of the core complex and the ophiolite complex and its early-to-middle Miocene sedimentary cover, respectively, suggest that large-scale extension and north-directed transport juxtaposed this hanging-wall unit in its present position (see, e.g., Walcott, 1998). Late steepening of the detachment fault has been caused by ongoing W-E to NW-SE compression, which also caused late folding in the core complex (Walcott, 1998). The studied Miocene profile extends over ~0.5 km, half of which is exposed along an unpaved road (Fig. 2). The section seems to include some repetitions due to long-wave folding in the lower section and thrusting and faulting in the central section (Fig. 3) caused by E-W compression. Strike-slip movements along meter-thick cataclastic zones are frequent within the profile, although the sense of shear is difficult to determine. The general dip of the bedding is toward the SSE (see Figure 2B). A small occurrence of early Miocene debris is located on the peninsula of Moutsouna on the east coast (Fig. 2A). This succession consists of partly red, partly gray calcareous pelites with intercalated fanglomerate beds. The red color is not strictly stratabound, and appears to be irregularly altered to gray color. Several unconformities were observed in this section. The succession is intensely faulted and imbricated with slices of serpentinite and radiolarite. The relics of the ophiolite unit and its Miocene cover are unconformably overlain by coarse Pliocene
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Miocene siliciclastic deposits of Naxos Island
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conglomerates and thus mirror the setting on the west coast in and near the town of Naxos. Facies and Depositional Setting of the Miocene Clastic Sediments (Melanés) Owing to limited exposure of outcrops and tectonic fragmentation, no depositional geometry or lateral facies change of the Miocene siliciclastic deposits could be reconstructed. The base of the Melanés sedimentary succession is exposed at the northwestern end of the profile. The succession is composed of three subunits (Fig. 3). Subunit 1 at the base of the succession consists of grayishviolet playa-type pelites with abundant white mica flakes and occasional centimeter-thick sandstone and microconglomerate layers. This facies is dominated by siltstones that frequently contain burrows (Fig. 3). Sandstone and microconglomerate layers occasionally display planar bedding. Imbrication of micropebbles is rarely observed and indicates transport from roughly southeasterly directions in the recent geographic frame. The overall depositional trend is coarsening-upward. Fine-grained yellow sandstones, interpreted as overbank deposits, dominate subunit 2. Channel levee deposits have not been clearly identified. In medium- to coarse-grained sandstones, cross-bedding is more frequent than planar bedding. Channels with coarse sandstone, pebbly sandstone, and microconglomerate largely display normal and subordinately inverse graded bedding. Channel transport from the southeast has been deduced from imbricated micropebbles and a sole mark leeward of an isolated 4-cm-sized pebble, which was probably released as a dropstone from floating plants. All coarse layers display channel deposits of only a few meters’ width. Amalgamated channels are frequent in more coarse-grained parts of the profile. In the middle part of the profile, thin caliche crusts (0.5–3 cm) with pisoids (up to 12 mm) are frequent and are typically found within or above pelite intercalations. The caliche horizons reflect periods of weak clastic supply, which favored the formation of soil. The source of calcite both for the formation of caliche and for later cementation of pore space is limestone pebbles (see below). At the top of the profile, the fluvial succession passes into subunit 3, which consists almost exclusively of poorly sorted, coarse red fanglomerate facies with very few intercalated relics of sandy channel infills. The matrix of the grain-supported fanglomerate is typically pelitic. Sieve deposits, typical of minor but fairly constant water supply, have not been found. Depositional cycles, possibly indicated by variations in grain size, have not been observed. In order to reconstruct environmental changes in the depositional system, the variation in maximum grain size has been registered every 4 cm in the field to record practically all coarse sandy or microconglomerate layers (Fig. 3). Maximum grain sizes of <1 mm have been estimated. The grain size variation reflects several hierarchic orders of depositional cycles. The smaller cycles show variable spacing throughout the profile and are often truncated
Figure 3. Detailed depositional sequence of the Miocene sediments; the base is located at the bottom left row.
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Figure 4. Maximum grain size within the profile, displaying rhythmic variations.
at the top. The record displays generally increasing maximum grain sizes toward ~50 m above the base and a rapid decrease of maximum grain sizes 80 m above the base (Fig. 4). Between ~112 and 125 m increased maximum grain sizes occur again. The distribution of grain-size peaks suggests that there might be a rhythmic variation and not just random scatter. Cyclic deposition cannot necessarily be expected in a fluvial environment, because redeposition and frequent periods of non-deposition, as proved by caliche horizons, are typical. Spectral analysis was carried out on the grain-size record in order to discriminate cycles (Fig. 5). The spectra were calculated with the Blackman-Tukey method using the AnalySeries program of Paillard et al. (1996). Surprisingly, significant cyclic events can be observed between 45 and 85 m, and 110 and 130 m, respectively, whereas in the remaining profile such sections are lacking. Prominent cycles occur at distances of 2.4 to 2.5 m, and 1.2 to 1.4 m. Smaller-spaced cycles of ~0.85 m and 0.65 m are less prominent. Petrographic Composition and Provenance of the Miocene Deposits Heavy Mineral Composition The heavy mineral composition can provide information on rocks from distal source terrains that are not present in the pebble spectrum. Abundant spinel, magnetite, zoisite, epidote, and green, chlorite-rich lithic fragments (Table 2) indicate a strong contribution from an ophiolitic source (see below). Garnet, kyanite, and chloritoid, present only in the eastern occurrence (Moutsouna), reflect a distal upper greenschist to lower amphibolite facies source terrain of Barrovian-type metamorphism. The stable minerals zircon, tourmaline, and rutile, which are typically rounded, probably derive from a mature continental source. However, euhedral zircons lacking abrasion, which are present also in the heavy mineral spectrum, probably derive directly from magmatic sources. Petrography of Microconglomerate Layers in the Melanés Profile In the basal section, a playa-type sandstone with a 3-cmthick microconglomerate layer (M1-1) has been analyzed. The
Figure 5. Power spectrum of rhythmic grain size variations in cycles per m. A potential equivalence to orbital cycles is indicated in kilo years (ka), addressed with question marks according to its speculative status (no age control).
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pebbly components are sedimentary rocks, mainly carbonate, chert, and quartz grains, whereas quartzite, chlorite-bearing basaltic volcanics and colorless mica flakes are subordinate (Fig. 6). Ophitic structures in the basalt micropebbles are typical. The carbonates are mainly micritic limestones, some of which contain radiolaria. There are all mixtures of pelagic siliceous limestone and chert with calcite-rich nodules. Remnants of algae as indicators of shallow-water carbonates are rare. The cherts and radiolaria-bearing micritic limestones represent pelagic deposits. These unmetamorphic sedimentary rock components are typical of late Mesozoic ophiolite domains exposed in the arcuate belt of the Hellenides. Quartz occurs as angular, well-rounded, or corroded grains and as polycrystalline micropebbles. Usually the quartz micro-
Figure 6. Gross petrographic composition of microconglomerates from the Miocene profile; sampling location indicated in Figure 3.
57
structures and microtextures indicate temperatures near the brittle-ductile boundary and slightly above (~270–400 ºC). Quartz in the rock fragments and in single grains shows typical features of very low-grade to low-grade metamorphic terrains like undulatory extinction and limited annealing. White mica occurs as rare coarse single grains or in rock fragments, where its size is indicative of low-grade metamorphism. A fluvial yellowish microconglomerate (M2-1) with calcite cement from the lower part of subunit 2 contains less siliceous pelagic limestone but much more sandstone, polycrystalline quartz, chert, and few basalt, quartzite, and marble pebbles, which are in general cherty, and mica flakes (Fig. 6). Single quartz grains of different size are frequent, as are a few plagioclase grains and white mica aggregates. A few clear and euhedral single quartz crystals show a hexagonal shape and are of volcanic origin. Together with intermediate to acid plagioclase crystals (mostly andesine and oligoclase), which reveal complex twinning, they indicate a mainly dacitic to rhyolitic source. The petrographic composition is more polymict than that of the older layer (M1-1). The non-metamorphic pelagic carbonates are often red and black, but a dark gray type is also observed. Pebbles of lagoonal facies with lumps of algae are common Additional metamorphic components comprise graphitic quartzite, phyllite and mainly dolomitic marble. According to their microstructures and microtextures, these components mostly formed during dynamic metamorphism at up to higher greenschist facies conditions. The sandstones and metasandstones, however, indicate very low-grade metamorphic and unmetamorphic source terrains. The largest micropebbles are the sandstones. The secondlargest in size is the group of chert micropebbles, which partly show angular edges, possibly caused by breaking during torrential transport. Shallow-water carbonate pebbles are somewhat larger than the deep-water carbonates. The smallest micropebbles are the basalts and higher-grade metamorphic particles. With the exception of cherts, all pebbles are fairly well rounded. The contrasts in size indicate that the sandstones derive from a relatively proximal source, whereas basalt pebbles, marbles, and quartzites derived from greater distance A microbreccia from the upper part of subfacies 2 (M2-5) is mainly composed of reddish and gray cherts, goethite-limonite aggregates, slightly recrystallized light gray carbonates, and basalt (Fig. 6). Many of the limestone and marble pebbles are cherty. Most carbonate components are unmetamorphosed, although some metamorphic fabrics, including foliation and crystal plasticity are observed. Quartz, sandstones, and quartzites are much less frequent than in the samples from the lower part of the section but display similar features. The metamorphosed quartzand calcite-rich components uniformly indicate that metamorphism did not exceed lower or middle greenschist facies. The overall petrographic trend within the subunits 1 and 2 shows that in the lower and middle parts of the profile, about half of the debris derived from an ophiolite unit containing variable amounts of chert, deep-water limestone, and basalt. Toward
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J. Kuhlemann et al.
the top, the contribution from the ophiolite unit increases at the expense of a mature continental source. The continental source is represented by sandstone and quartzite, and polycrystalline quartz components derived from schists rich in quartz nodules mobilized under weak metamorphic conditions. The volcanic source is inferred to represent arc-type magmatism. Petrography of Single Pebbles from the Fanglomerate Member (Subunit 3) Pebbles provide more specific information about sediment structures and organic content of the source-rock units. Some may represent marker pebbles of lithologies of small distinctive source regions. Sampling of single pebbles from subunit 3, both from Melanés and Moutsouna (80 thin sections), focused on light-colored and reddish cherty carbonates, Mesozoic shallowwater carbonates, fossiliferous carbonates, and siliciclastic sandstones and graywackes. Abundant reddish chert, metasandstones, quartzites, and basalt pebbles were not investigated. Deep-water carbonates and cherty limestones. These lithologies display all transitions from light gray limestone to reddish cherty limestone. The organic content typically displays relics of radiolaria and filaments. Some red-colored pebbles resembling sandstones turned out to be winnowed radiolaria grainstones cemented by calcite sparite. A red limestone dissolution breccia with a few pellets and filaments and late-diagenetic quartz cement is typical of Ammonitico Rosso swell facies. Although datable microfauna have not been found, these lithotypes widely formed during Late Jurassic to Early Cretaceous times in the Tethys realm. Reddish cherty limestones are much more frequent at Melanés than at Moutsouna. More obvious than in the microconglomerate (M2-5) is a mild metamorphic overprint, as proven by white mica seams, which affected many of the pebbles at Melanés but fewer pebbles at Moutsouna on the east coast. Mesozoic shallow-water carbonates. The shallow-water carbonates show a variety of lagoonal facies, typical of the Upper Triassic and Jurassic platform carbonates that are widespread in the Tethys realm. Lumps of algae and peloids are frequent. Subtidal wackestones are rich in organic components: corals, crinoids, thin-shelled gastropods, embryonal bivalves, foraminifera, and filaments. Pure micrites are rare. Arenites of high-energy facies are abundant and include intraformational breccia of micrite and fossiliferous packstone as well as peloidooid-biosparite grainstone, with coral, echinoderm and algal fragments, and foraminifera as main organic constituents. Intratidal facies are much less abundant and include microkarst and bird’s-eye structures. All these facies types were dolomitized to different, generally minor, degrees. A very mild metamorphic overprint, as indicated by homogenization and increase of grain size and polygonal grain boundaries in micrite, affected about one third of the shallow-water carbonates. The corresponding lithology within micropebbles of subunit 1 and 2 is unmetamorphic, which may indicate progressive exhumation during the sedimentary record.
Light-colored fossiliferous Late Cretaceous–Paleogene limestone. These limestone pebbles are easily recognizable components which make up ~1%–2% of the pebble spectrum of the Melanés fanglomerate. At Mountsouna peninsula, light colored fossiliferous limestones are less easily recognizable due to the abundance of older light-colored carbonates and calcarenites. The fossiliferous limestones display a large variety of shallow-water facies and organic content, with abundant nummulites, corals, bryozoa, benthic foraminifera, gastropods, and echinoderm bioclasts. Two lithologies can be differentiated. 1. A Late Cretaceous–Paleocene coral reef facies is composed of colonial corals encrusted by corallinacean and peyssonneliacean algae and acervulinid foraminifers. A variety of reef-dwelling organisms (sphinctozoan sponges, Tubiphytes, “pseudoostracods”) is present. This facies is similar to the coral reefs of the same age in the Northern Calcareous Alps (Tragelehn, 1996) and in the Western Carpathians (Samuel et al., 1972). 2. A middle-to-late Eocene shallow-water limestone is composed of crusts and clasts of corallinacean algae, orthophragminid and nummulitid larger foraminifers, encrusting foraminifers, corals, and bryozoans in a micrite matrix. The sediments were deposited in a quiet, neritic environment. This facies is similar to the Priabonian limestone in the Transdanubian Central Range near Budapest (Kázmér, 1985). The fauna of the middle to upper Eocene shallow-water limestone pebbles differs from the Alveolina-rich facies exposed nearby in isolated fragments associated with the ophiolite unit. Several Paleogene biodetritic grainstones and packstones contain extraclasts of basalt, detrital quartz, mica, low-grade metamorphic cherty carbonate, and volcanic quartz phenocrysts. The extraclast composition is largely similar to that found in the early Miocene microconglomerate samples (see above), indicating that the same source terrain continuously shed clastic material from Paleogene to mid-Miocene times. Since one of the nummulitic limestones experienced a very low-grade metamorphic overprint, the Paleocene deposits were not only recycled near surface but were partially influenced by a mild metamorphic overprint, followed by fairly fast exhumation. Siliciclastic sandstones and graywackes. Immature sandstones are represented by nine thin sections, four of which are mixed carbonate-siliciclastic sediments and contain determinable microfauna. Two of nine pebbles experienced a very low-grade metamorphic overprint. All pebbles are tightly cemented. Datable bioclasts are usually lacking. In one pebble, a few miliolid foraminifera indicate Eocene depositional age. The graywacke pebbles appear to be the siliciclastic equivalents of the more-calcareous components within a Paleogene succession. The suggested age of deposition is also supported by the lack of Pithonella, typical of Cretaceous deposits in the western Tethys.
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Miocene siliciclastic deposits of Naxos Island TABLE 3: �������������������������������������� Lat.
Long.
Locality
Petrography
Cryst.
Spontaneous �s (Ns)
Induced �i (Ni)
Dosimeter �d (Nd)
2
P(c ) (%)
FT age* (Ma ± 1s)
36°06' 25°25.5' Melanés sandstone 61 144 (6153) 56 (2386) 10.5 (7041) <1 172 ± 15 36°05' 25°35.5' Montsouna sandstone 60 126 (4596) 76 (2746) 9.9 (7041) <1 107 ± 9 5 2 Note: Cryst. is number of dated zircon crystals. Track densities (�) are as measured (x 10 tr/cm ); number of tracks counted (N) shown in 2 brackets. FTfission track. P(c ): probability obtaining Chi-square value for n degree of freedom (where n = no. crystals-1). *Central ages calculated using dosimeter glass: CN 2 with zCN2 = 127.8 ± 1.6.
In summary, the marker pebbles have been shed from a Tertiary active margin in the south. Upper Triassic and Jurassic platform carbonates of the Hellenides, Paleogene shallow-water limestones, and graywackes have been buried and re-exhumed within the Paleogene and early Miocene. Geochronology K-Ar dating of detrital white mica. Field evidence of a contact-metamorphic overprint of the siliciclastic sedimentary deposits in excess of 200 ºC is lacking, although the presence of very fine sericite might have formed after deposition. Rare small white mica flakes in the pelitic basal section of the Miocene succession appear to be of detrital origin. The clay mineral composition of the playa facies (subunit 1) is half illite and half chlorite. Four samples selected for K-Ar dating contain trace amounts of feldspar. K-Ar ages have been determined on the sericite fraction <2 µm size. The ages scatter between 82 and 103 Ma, displaying provenance ages (Table 2).
Fission-track geochronology. Fission-track geochronology on detrital zircon grains of the sandstone members of the Miocene successions near the east and west coasts of the island reflects the cooling history and thus exhumation of source terrains. Fission-track dating was performed by the external detector method (Fleischer et al., 1964; Gleadow, 1981) and the zeta calibration approach (Hurford and Green, 1983; Galbraith and Green, 1990). The component analysis of the fission-track age distributions has been performed using the BINOMFIT and POPSHARE procedures (Brandon, 1992; Dunkl and Székely, 2002). The track counts are listed in Table 3, and the details of the analytical technique can be found in Dunkl et al. (2001). The single-grain ages range from 30 to 500 Ma and are thus older than the sedimentation age. The age distributions are very broad and complex; the multi-component character is obvious (Fig. 7; Table 4). Microscopic parameters like color and shape were also registered. The majority of the zircons are colorless, whereas
Figure 7. Zircon fission-track results from sand sampled in the Melanés profile and from Moutsouna on the eastern coast, displayed (A) as singlegrain age bar plots and age spectra (generated according to Hurford et al., 1984), and (B) segregated into ages of euhedral and rounded grain populations.
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J. Kuhlemann et al. ����������������������������������������� ����������������������������������� �����������
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������� ����������������������� ������������ ������� ��� ������������ ��������� ��� ������������ ���������� ��� ���������������������� ������������ �������� ��� ������������ ��������� ��� ��������� ����������������������� ������������ �������� ��� ������������ ��������� ��� �������������������������������������������������������������������� �������������������������������������������������������������������� ������������������������������������������������������������������� ��������������������������������������������������������������� �������������������������������������������
around one sixth of the dated population is red. These grains are commonly rounded and occur only in the age clusters older than 90 Ma. The shape and the surface of the crystals carry important age-independent information. Fifty percent of the datable grains are euhedral, but the majority of this population is slightly corroded and chipped along the edges, and only a small number have well-preserved crystal faces and edges (Fig. 8). All these completely intact grains are colorless. About a quarter of the grains are well rounded. The samples from Melanés and Moutsouna show similarities: in both samples clusters of fission-track ages ca. 30–40 Ma and ca. 120–150 Ma are present. An age group ca. 300 Ma is much more pronounced in the Melanés sample. No zircon crystals with Miocene fission-track ages are found, and the proportion of latest Cretaceous–Paleocene fission-track ages is relatively low. The euhedral and rounded crystals form distinct clusters in both samples (Fig. 7). The youngest age group is exclusively composed of euhedral zircon crystals. In the Melanés sample, the Mesozoic ages can be split into a younger, rounded cluster (ca. 90–120 Ma) and an older, euhedral cluster (ca. 120–180 Ma). The crystals with pre-Triassic ages are dominantly rounded. These include a broadly scattering Variscan age group with some surprisingly old outliers. A similar trend to the Melanés sample was detected in the age distribution of the Moutsouna sample. DISCUSSION Environment and Climate The depositional environment of the early to middle Miocene sedimentary deposits is characterized by a superimposed coarsening upward trend, which indicates increasing relief. Dur-
Figure 8. Microphotos of typical euhedral and rounded zircon grains, separated for fission-track age determination (see Figure 7B). Figure A is 0.25 mm wide; Figure B is 0.5 mm wide.
ing the early phase of playa deposition (subunit 1), relief could hardly have exceeded hill-size elevation. Maximum pebble sizes of more than 10 cm occur in the middle of the fluvial succession (subunit 2), which is slightly less than maximum pebble size of the fanglomerate succession above (subunit 3). Pebble size, composition, and the degree of rounding indicate that a local relief of intermediate mountainous character was established during deposition of the fluvial facies, which only slightly increased with time. Thus, we can conclude that the nearby hinterland showed a general trend of increasing relief and increasing proximity ending up in a scenery with intermediate relief but steep gorges in the immediate vicinity of the depositional area. The most important difference between the subunits 2 and 3 is the rareness of conglomerate beds in the fluvial facies. The alternation of siltstones and channel deposits in subunit 2 appears to be partly climate controlled. We conclude that the seasonality of precipitation increased suddenly at the onset of fanglomerate
Miocene siliciclastic deposits of Naxos Island deposition in subunit 3. Subtropical conditions certainly governed the entire period (Gregor and Velitzelos, 1987), but during deposition of the fluvial facies, the season of excess evaporation to form caliche horizons may have been fairly short and the runoff fairly stable, whereas during fanglomerate deposition, seasons of precipitation and runoff were probably short and intense. This climatic change may be related to mid-Miocene global cooling at ca. 14 Ma recorded simultaneously in the Mediterranean realm (Vergnaud-Grazzini, 1985), which increased seasonality in terrestrial circum-Alpine basins (Bruch, 1998). The occurrence of cyclic deposition, mainly in distances of 2.4–2.5 m and 1.2–1.4 m may be interpreted as a climatic signal, possibly due to precipitation changes, triggered by orbital (Milankovitch-type) cycles. We speculate that the 2.4–2.5 m cycle may correspond to the 41 ka obliquity cycle, whereas the 1.2–1.4 m cycle would correspond to the 19/23 ka precession cycle. The signal of the 100 ka eccentricity period is poorly fitting. If this speculation is correct, the whole succession would cover ~2.5 m.y. at depositional rates of ~60 mm/ka. The interpretation of cyclic coarse-grained deposition in terms of orbitally controlled precipitation changes is, of course, highly disputable. Instead, irregular or autocyclic deposition, both in temporal and spatial terms, is typical within a fluvial environment (e.g., Miall and Bridge, 1995). Nevertheless, formation of overbank deposits may have happened fairly regularly during constant and slow subsidence, even if such depositional events were separated by long periods of non-deposition. The migrating channel paths appear to have been largely preserved within the overbank background sediments. The pebble size in the channels reflects the intensity of runoff which may be controlled by Milankovitch-type climatic cycles. The dominance of the 41 ka obliquity and the precession cycles as found in this region in the Pliocene (Cramp and O’Sullivan, 1999) may also be applicable to the Miocene (e.g., Juhasz et al., 1997; Hilgen et al., 2000; van Vugt et al., 2001). A cyclic variation of precipitation and runoff may be related to an early precursor of the African monsoon, which is dominantly forced by insolation and responds both to the 41 ka obliquity cycle and the 19/23 ka precession cycle (Rossignol-Strick, 1983). An impact of the African monsoon on southeastern Mediterranean precipitation and sapropel formation in Pliocene time is evident (Rohling, 1994). Provenance The transport direction from the southeast in the recent geographic frame has to be corrected for the paleo-transport direction, due to counterclockwise (CCW) rotation of the eastern and central Aegean blocks (e.g., Morris and Anderson, 1996; Kondopoulou, 2000). This CCW rotation can be separated into a latest Miocene to recent CCW component of ~15° of the central Aegean block and a Middle to Late Miocene CCW rotation of ~19° (Walcott and White, 1998). Thus, the paleo-transport direction was roughly from the south to the north, which is in line with an orogen-perpendicular (radial) northward dewatering system.
61
This does not exclude, however, lateral orogen-parallel transport particularly along zones of weakness and high erodibility. The lack of Miocene zircon fission-track ages indicates that large-scale tectonic unroofing had not yet exposed freshly cooled metamorphic footwall rocks of the Cycladic core complexes within the catchment area of the Melanés and Moutsouna sediments. This matches with the petrography of the siliciclastic material, in which footwall components are completely lacking (see Rösler, 1978). Nevertheless, such a coincidence is not trivial, since recent studies in the Miocene succession of the Northern Alpine Foreland Basin have shown that detrital zircon fission-track ages, heavy mineral composition, and pebble assemblage monitor source-rock units of contrasting exhumation history (Spiegel et al., this volume). The source terrains supplying material from the south were two main lithotectonic units, represented by a proximal continental source and a more distal ophiolitic source of upsectionincreasing importance. On the basis of pebbles derived from former source terrains now eroded or buried in the Aegean Sea under sediments, we cannot differentiate between the Vardar or Pindos domains, although in the recent lithotectonic frame, the southern part of the former ocean (Pindos domain) appears to be the more likely source region (see Aubouin et al., 1970; Jones and Robertson, 1991; Schermer, 1993; Doutsos et al., 1993). The quartzites, the single quartz grains, and rare sandstone micropebbles appear to represent a relatively mature continental low-grade metamorphic unit and possibly also redeposited material. If this source supplied all zircons that yielded Cretaceous and older zircon fission-track cooling ages, a very complex cooling history would have to be assumed for this unit. This passive margin source is probably represented by an upper Pelagonian unit, which may have covered the crystalline Cycladic basement, and the Subpelagonian unit, which supplied Mesozoic carbonate rocks. The rounded zircon crystals forming an age cluster ca. 110 Ma appear to be derived from source areas dominated by parametamorphic lithologies, such as quartzites. Their cooling history is probably related to the extensional period following the Eohellenic thrusting. The 90 Ma sericite K-Ar ages from Melanés probably also formed during this tectonic phase in metapelitic material. The Variscan age group of the Melanés sample, composed mainly of rounded grains, indicates sources which escaped thermal reset during both the Mesozoic (Vardar) rifting and the Eohellenic metamorphism. The rounded shape may indicate repeated sediment redeposition and thus could imply Paleozoic or late Precambrian metasedimentary passive margin sources (e.g., De Bono, 1998), possibly even of African origin (Keay and Lister, 2002). We suppose that crystalline areas have also contributed to the sand fraction of the Miocene sedimentary rocks. Granitic and other feldspar-containing lithologies disintegrate relatively quickly under subtropical conditions, but numerous euhedral
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grains indicate acid or intermediate igneous rocks in the source areas. For these zircon populations, the Late Jurassic to Early Cretaceous age clusters are characteristic. The potential source units of this material are Pelagonian crystalline slices displaying Mesozoic thermal resetting during extension in the course of the opening of the Pindos-Vadar ocean (Jacobshagen et al., 1986; Robertson and Karamata, 1994; Most et al., 2001). A third, quantitatively unimportant but geodynamically highly relevant source terrain is represented by a distal active margin with Paleogene volcanic activity, as recorded by volcanic fragments of intermediate to acidic character in Eocene fossiliferous pebbles, euhedral hexagonal quartz grains, and clear, colorless, euhedral zircon grains with Paleogene fission-track ages. This material may have derived by orogen-parallel transport from western Anatolia, where calc-alkaline volcanic activity increased after Oligocene collision between the Sakarya continent and the Tauride-Anatolide platform (Genc, 1998), or from a later submerged volcanic arc in the southern Aegean Sea, which was related to this collision zone. We cannot exclude the derivation from the Balkan peninsula, because zircon-rich acid and intermediate volcanic centers are known in the Rodope Mountains (e.g., Yanev, 1998). Despite the similarities in provenance of the two sampling locations, some important differences should be noted for Moutsouna: (1) presence of an upper greenschist to lower amphibolite Barrovian-type metamorphic source; (2) a higher relative contribution from the Paleogene volcanic arc; and (3) a higher relative contribution from granitic slices with Early Cretaceous zircon fission-track cooling ages. Although Moutsouna today is located only ~14 km east of Melanés, the paleogeographic position was distant enough to cause significant paleogeologic differences in the catchment areas. However, this assumes that both localities represent the same time of deposition, which is not clear. To compare, the pebble spectrum of Miocene deposits of eastern Paros ~15 km west of Melanés displays high contents of crystalline rocks, including gneisses, which were not derived from the nearby core complex but from an unknown source terrain (Sanchez-Gomez et al., 1999). Thus, the paleogeology in the catchment area appears to be a complex patchwork of the mentioned source-rock units. The increasing contribution from the ophiolitic source at Melanés may be interpreted by normal erosional unroofing, although the change of the pebble spectrum may also be the result of catchment enlargement. Thus, more provenance studies will be required to reconstruct the Neogene paleogeographic and paleogeologic evolution of the southern Aegean realm in more detail. CONCLUSIONS On the basis of geochronology and the reported structural data, a scenario of the juxtaposition of the ophiolite nappe as an extensional allochthon is proposed. Compressional nappe thrusting and internal imbrication must already have occurred before
the deposition of the Miocene succession. The local Miocene basins were supplied from source areas in the south including continental sources which did not experience heating to temperatures higher than the zircon fission-track partial annealing zone for more than 250 Ma, but also weakly metamorphic terranes (partly belonging to Pelagonian hanging-wall units according to the age patterns), and the unmetamorphic ophiolite nappe. During Miocene sedimentation, the ophiolite nappe was positioned some distance to the south of the actual metamorphic complex of Naxos and formed the basement to the Miocene succession. Miocene sedimentation occurred syntectonically to crustal stretching, which enabled fast unroofing of footwall rock from higher amphibolite-grade metamorphic conditions associated with decompressional melting to the near surface. Figure 9 shows the possible juxtaposition scenario of the extensional allochthon in middle to upper Miocene times. We propose the beginning of extension and basin formation on Naxos at 17 Ma according to the conclusions drawn by John and Howard (1995) at a time of widespread extension observed in many parts of the Tethyan belt (Kuhlemann, 2003). The final juxtaposition of the allochthon into its present position relative to the granodiorite and the metamorphic complex occurred at ca. 10 Ma. We conclude the following. 1. Late early-to-middle Miocene deposition of fluvial sediments in the central Cyclades records the establishment of intermediate mountainous relief of increasing proximity south of the basin and a change from regular, rhythmic, perhaps partly orbitally triggered fluvial deposition to seasonal fanglomeratic deposition. 2. A complex patchwork of source-rock units in the catchment area includes a Late Jurassic ophiolite unit with deep-water cover rocks, probably belonging to the Pindos realm, and a Pelagonian Mesozoic passive margin sequence. Zircon fission-track ages of rounded grains from this passive margin reflect a stable hanging wall–source terrain with Paleozoic cooling and a source terrain with Late Jurassic to Early Cretaceous cooling, which was affected by increased heatflow during opening of the PindosVadar ocean. A distant active margin with Paleogene volcanic activity shed material to the north. 3. The Miocene sedimentary succession was deposited on the ophiolite nappe, which was juxtaposed after the middle Miocene as an extensional allochthon during large-scale extension in the Aegean region on top of an exhuming metamorphic core complex. The Miocene land-based sedimentary record in the southern Aegean Sea on Naxos monitors a very conservative near-surface response to the spectacular deeper crustal processes. Low-angle normal faulting of hanging-wall units kept the footwall units covered until the end of the Miocene sedimentary record. The contribution of surface erosion to the exhumation of the footwall units was probably unimportant, owing to only moderate relief. The Pliocene clastic debris monitors a depositional environment and provenance quite similar to the present setting. Therefore, the
Miocene siliciclastic deposits of Naxos Island
63
Figure 9. Sketch profile of the crustal deformation since the Miocene and a reconstruction of the potential locus of deposition of Miocene sediments on an extensional allochthon.
fundamental paleogeographic change from a northward-drained, low mountainous belt to fairly steep islands in the sea probably happened in the late Miocene, although the contemporaneous record on land is lacking. ACKNOWLEDGMENTS We are especially grateful to C. Hemleben for determining foraminifera in thin sections and to M. Satir for providing K-Ar data of sericite from playa sediments, to T. Most for supplying literature and overview maps and to I. Gill-Kopp for producing numerous thin sections. Comments by Y. Dilek, J. Urai, N.W. Rogers, and particularly A. Robertson on an early draft of the manuscript were very helpful. Constructive reviews of S. Thomson and B. Fügenschuh are gratefully acknowledged. Special thanks are dedicated to S. Thomson for detailed improvement of the language and for supporting the interdisciplinary concept of the paper.
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MANUSCRIPT ACCEPTED BY THE SOCIETY OCTOBER 21, 2003
Printed in the USA
Geological Society of America Special Paper 378 2004
Detecting provenance variations and cooling patterns within the western Alpine orogen through 40Ar/ 39Ar geochronology on detrital sediments: The Tertiary Piedmont Basin, northwest Italy B. Carrapa* J. Wijbrans* G. Bertotti* Vrije Universiteit Amsterdam, Faculteit der Aardwetenschappen, De Boelelaan 1085, 1081HV Amsterdam, The Netherlands ABSTRACT The Tertiary Piedmont Basin is a synorogenic basin located on the internal side of the Western Alps. Because of its key position, the Tertiary Piedmont Basin represents an important record of processes occurring in the Alpine retrowedge for over the last 30 m.y. 40Ar/39Ar geochronology has been applied to detrital white micas as a provenance tool and to derive information on cooling and exhumation patterns within the surrounding orogen. The age distribution in the detritus shows that in the Oligocene the clastic sediments were fed mainly from a southern source area (Ligurian Alps) that widely records high pressure (HP) Alpine metamorphism (40–50 Ma) and, in part, Variscan metamorphism (ca. 320 Ma). From the Miocene, the main source area gradually moved from the south to a western Alpine provenance characterized by strong Late Cretaceous (70 Ma) and Early Cretaceous (120 Ma) signals. This enlargement in the source is likely linked to an evolution of the main paleodrainage system into the basin. From the Serravallian, Variscan ages reappear; this is attributed to the exposure of the Argentera Massif as a new source for the Tertiary Piedmont Basin. The lack of thermal overprinting of the main detrital signals through time suggests that the western Alpine orogen has been regulated by episodic fast cooling and exhumation events followed by periods of slower erosion. Also, detrital 40Ar/39Ar Cretaceous signals in Miocene and Present sediments suggest the presence of real Eoalpine events in the Alps. Keywords: Western Alps, provenance, 40Ar/39Ar geochronology, cooling, exhumation. INTRODUCTION
of sediment deposition. Therefore, they provide a record of the original setting of mountain belts through time. The Tertiary Piedmont Basin, located within the internal western Alpine arc (retrowedge; Beaumont et al., 1996) in northwest Italy (Fig. 1), contains up to 4 km of clastic sediments (Fig. 2). The Tertiary Piedmont Basin and the western Alpine arc formed as a result of the Tertiary European-African plate collision (e.g., Platt et al., 1989). The Tertiary Piedmont
Synorogenic clastic sediments contained in sedimentary basins preserve a record of the exhumation kinematics of an orogen. Because of erosional and tectonic processes, the original rocks outcropping in the orogen no longer exist. As a consequence, clastic sediments are the only remaining direct evidence of the original source rocks outcropping at the time
*Present address, Carrapa (corresponding author)—Universität Potsdam, Institut für Geowissenschaften, Postfach 601553, 14415 Potsdam, Germany,
[email protected]. E-mails: Wijbrans—
[email protected]; Bertotti—
[email protected]. Carrapa, B., Wijbrans, J., and Bertotti, G., 2004, Detecting provenance variations and cooling patterns within the western Alpine orogen through 40Ar/39Ar geochronology on detrital sediments: The Tertiary Piedmont Basin, northwest Italy, in Bernet, M., and Spiegel, C., eds., Detrital thermochronology—Provenance analysis, exhumation, and landscape evolution of mountain belts: Boulder, Colorado, Geological Society of America Special Paper 378, p. 67–103. For permission to copy, contact
[email protected]. © 2004 Geological Society of America.
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Figure 1. Geological map of the study area; TPB—Tertiary Piedmont Basin (study area), VG—Voltri Group, LA—Ligurian Alps, AGM—Argentera Massif, DM— Dora Maira, GP—Gran Paradiso, SL—Sesia Lanzo.
Basin is well suited for investigation of a substantial part of the complex Alpine orogen because it is one of the main sedimentary basins collecting clastic sediments produced by cooling and exhumation and erosion of the internal Western Alps (including the Ligurian Alps). The Tertiary Piedmont Basin stratigraphy is well preserved, exposed, and documented (e.g., Gnaccolini et al., 1998, and references therein; Fig. 2). These data, together with paleocurrent directions data (Fig. 3A) on the study area, allowed a good sample strategy (Fig. 3B). For the present study, we have applied 40Ar/39Ar geochronology to detrital white micas derived from samples selected from the entire Tertiary Piedmont Basin stratigraphy (Lower Oligocene–Upper Miocene). To extend the covered time interval from the oldest Tertiary Piedmont Basin sediments to the present, we sampled sands from three of the main rivers currently draining the internal side of the southwestern Alps, which are the main sources of Tertiary Piedmont Basin sediments (Fig. 3B). Detrital mineral thermochronology on present-day river sands has been largely applied to different tectonic settings to characterize the present river drainage pattern (e.g., Heller et al., 1992). Dating minerals from present-day river sediments provides new information on the geochronological signal currently recorded at the surface in the southwestern margin of the Dora Massif, the Argentera Massif, and on the Ligurian Alps. The major objective of this work is to obtain new information on: (1) provenance of the clastic sediments; (2) cooling patterns due to unroofing of the original source rocks related to past denudation and/or tectonic and erosional processes; and (3) potential information on the timing of HP and ultra-high pressure (UHP) metamorphism in the Western Alps. White micas are well suited for this geochronological approach because of their high K content, their widespread occurrence in different lithologies, and their resistance against mechanical breakdown. Furthermore, the closure temperature
of white micas (350–420 °C; e.g., Hames and Bowring, 1994; Kirschner et al., 1996; Kohn et al., 1995; von Blanckenburg et al., 1989) is high enough to avoid substantial overprinting due to short-lived thermal disturbances and sedimentary burial. Also, 40 Ar/39Ar dating of white mica is a well-tested technique that records the time in which the investigated minerals pass through a closure temperature of 350–420 °C (Najman et al., 1997; von Eynatten and Wijbrans, 2003; White et al., 2002). Research in the Himalayas (e.g., Harrison et al., 1993; Najman et al., 2001; White et al., 2002), in the Central Alps (e.g., Bernet et al., 2001; Spiegel et al., 2001; von Eynatten and Wijbrans, 2003), and in western North America (e.g Heller et al., 1992) has demonstrated the potential of the geochronological approach for both provenance and tectonothermal evolution studies. In particular, studies on the Central Alpine sedimentary record and on the exhumation of crystalline rocks from the Alpine orogen have suggested rapid episodic exhumation (e.g., Hurford et al., 1991) followed by a relatively steady state of exhumation (e.g., Bernet et al., 2001; Schlunegger and Willett, 1999). So far, however, little data exist (Carrapa et al., 2003) on the depositional counterpart of the western Alpine erosion. Our data set of over 500 individual white mica analyses is well constrained due to the proximity of the source to the basin and sheds new light on the reconstruction of the thermotectonic evolution of the western Alpine arc for a time period of over 30 m.y. GEOLOGICAL SETTING The Tertiary Piedmont Basin is located on the boundary between the internal Alpine domain, consisting of deep crustal rocks, and the Apennine domain, constituted mainly by upper Cretaceous–Eocene flysch (Fig. 1). The Tertiary Piedmont Basin is flanked to the south by the Ligurian Alps and to the west and north by Plio-Quaternary sediments, which in turn are bounded
Detecting provenance variations and cooling patterns to the west by metamorphic units belonging to the internal western Alpine domain (Fig. 1). The Orogen Surrounding the Tertiary Piedmont Basin The western Alpine arc surrounding the Tertiary Piedmont Basin includes the Ligurian Alps to the south and the Western
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Alps (sensu stricto) to the west. The western Alpine arc contains HP and UHP rocks that experienced deep burial during the Alpine orogeny and subsequent rapid exhumation, such as the Dora Maira and Gran Paradiso massifs to the west (Hurford et al., 1991; Hurford and Hunziker, 1989; Rubatto and Hermann, 2001) and the Voltri Group to the south (Brouwer, 2000). All these rocks constitute potential sources of sediments for the Tertiary Piedmont Basin.
Figure 2 (on this and following page). A: Geological framework of the study area modified after Gnaccolini et al. (1998). 1—Pliocene to Recent deposits; 2—Messinian deposits; 3—Langhian to Tortonian siliciclastic and carbonate shelf to slope deposits; 4—Late Burdigalian to Tortonian mainly turbidite succession (only Burdigalian in the eastern sector of the figure); 5—Late Oligocene to Burdigalian turbidite systems and hemipelagic mudstones; 6—Late Eocene to Early Oligocene deposits: (a) alluvial to coastal conglomerates, shallow marine sandstones and hemipelagic mudstones, (b) slope and base-of-slope, resedimented conglomerates, and (c) mainly turbidites; 7—Late Eocene to Tortonian siliciclastic deposits of the northwestern Apennines–Monferrato–Torino Hill wedge; 8—Alpine and Apennine allochthonous units; 9—depocenter axis of the Plio-Quaternary basins; 10—buried thrust-front of the Torino Hill–Monferrato–northwestern Apennine wedge; 11—buried, southvergent backthrusts of the Monferrato, active from Messinian onward; 12—buried, Pre-Burdigalian backthrusts of the Western Alps (as inferred from Roure et al., 1990); 13—faults: SV—Sestri-Voltaggio; VVL—Villalvernia-Varzi line; I: Bagnasco–Ceva–Bastia Mondovì transect; II: Millesimo–Monesiglio–Somano transect; III: Dego–Torre Bormida–Alberetto della Torre transect; IV: Spigno Monferrato–Cessole transect; V: Montechiaro d’Acqui; VI: Cavatore; VII: Visone. B: Oligo-Miocene depositional sequences (A, B1–B6, C1–C6) in the study area after Gelati et al. (1993). 1—mudstones, locally with thin-bedded turbidites (a); 2—turbidite systems (sand/mud ratio from very high to = 1; locally, conglomerates); 3—resedimented ophiolite breccia; 4—olistolith-bearing pebbly mudstones; 5—shallow-water carbonates; 6—alluvial conglomerates, coastal sandstones, and conglomerates, freshwater mudstones (a); 7—Pre-Cenozoic rocks; 8—sequence boundary; 9—fault. The square inset is mainly based on Cazzola et al. (1981), Cazzola and Sgavetti (1984), and Cazzola and Fornaciari (1990). The ages of the unconformities and sequence boundaries are based on planktonic foraminifers and calcareous nannofossils and correlate with the third order global cycles boundaries of Haq et al. (1988). C: Stratigraphic scheme modified after Gelati (1968).
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Figure 2 (continued).
The Ligurian Alps The Ligurian Alps bound the Tertiary Piedmont Basin to the south. They consist of a nappe stack (Vanossi et al., 1984), including the following: 1. The meta-ophiolite Voltri Group, which is a remnant of the Piedmont-Ligurian ocean and consists of a cover sequence constituted by metasedimentary rocks (calc-schists, mica schists, quartz schists, and metabasalts) and a basement sequence of Mggabbros, Fe-Ti–gabbros, and serpentinites (Messiga, 1984). The Voltri group experienced a complex retrograde metamorphism ranging from peak eclogitic and blueschist facies toward greenschist facies (Cimmino et al., 1981; D’Antonio et al., 1984; Messiga et al., 1989; Messiga and Scambelluri, 1991). Scarce K-Ar geochronology on white micas from presently outcropping gneisses of the Voltri group gives cooling ages of 31–41 Ma (Hunziker et al., 1992; Schamel and Hunziker, 1977; Fig. 4). 40Ar/39Ar dating on white mica from the Beigua serpentinite unit (Voltri Group) gives ages of 45 ± 2 Ma (data cited in Brouwer, 2000).
2. The Montenotte Nappe (Piemontese unit), which is derived from the transitional domain between the ocean and the paleo-European continental margin (Vanossi et al., 1984). This unit experienced HP/low temperature (LT) Alpine metamorphism with a re-equilibration in blueschist facies (Messiga, 1981). No chronological data is available on the age of the metamorphism. 3. The Briançonnais complex, which is derived from thinned paleo-European continental crust and includes the Variscan Crystalline Massifs (e.g., Calizzano–Savona Massifs). These units have been affected by Alpine greenschist facies metamorphism (e.g., Messiga et al., 1992, and references therein). The Variscan Crystalline Massifs are characterized by 40Ar/39Ar ages ca. 380– 240 Ma (Hunziker et al., 1992; Fig. 4). Zircon fission-track ages on the Briançonnais domain yielded ages between 180–125 and 31 Ma (Vance, 1999). The Western Alps The Western Alps consist of different domains including numerous units. In the following, we will consider only those
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Alpine units affected by cooling and exhumation during the formation of the Tertiary Piedmont Basin and that therefore constitute a possible source for the Tertiary Piedmont Basin sediments. The Internal Massifs (e.g., Dora Maira, Gran Paradiso; Fig.1) belong to the Penninic units, which represent the European continental basement and form the core of the western Alpine chain. They were affected by Eoalpine HP or UHP metamorphism during Alpine subduction (e.g., Chopin, 1984; Chopin and Monié, 1984; Compagnoni and Lombardo, 1974; Dal Piaz, 1999; Monié and Chopin, 1991; Paquette et al., 1989; Polino et al., 1990; Scaillet et al., 1992) and subsequent exhumation. The timing and mechanism of the HP metamorphism and kinematics of exhumation of the Internal Massifs are still debated (e.g., Brouwer, 2000; Compagnoni et al., 1995; Hurford et al., 1991; Michard et al., 1993; Rubatto et al., 1999; Rubatto and Hermann, 2001; von Blanckenburg et al., 1989). The Dora Maira Massif is mainly characterized by 40Ar/39Ar age ranges of 45–30 Ma, 85–60 Ma, and ≥120 Ma (Monié and Chopin, 1991; Scaillet et al., 1992; Scaillet et al., 1990; Fig. 4). Determination of cooling and relative exhumation kinematics for the UHP rocks of the Dora Maira has not been totally resolved (Gebauer et al., 1997; Hurford et al., 1991; Michard et al., 1993). One important issue in the Western Alps exhumation evolution is the interpretation of ages ~70–120 Ma. Different geochronometers (U-Th-Pb on zircons and monazites, U/Pb on whole rock, and 40Ar/39Ar on white micas) from HP rocks of the Dora Maira record ages ranging between 70–120 Ma (Monié and
Figure 3. A: Paleocurrent directions modified after Gnaccolini and Rossi (1994); location given in B. B: Enlargement of the study area with location of the analyzed samples. The inset area corresponds to the study on paleocurrent directions of Gnaccolini and Rossi (1994) (Fig. 3A).
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Probability
Figure 4. Probability distribution diagrams of 40Ar/39Ar ages on white micas (single/total fusion ages) in the Tertiary Piedmont Basin surrounding source areas based on literature data from Stöckhert et al. (1986), Hurford et al. (1991), Hunziker et al. (1992), Scaillet et al. (1992), Ruffet et al. (1997), and Cortiana et al. (1998). For legend, see Figure 1.
Detrital Ages (Ma) Chopin, 1991; Paquette et al., 1989; Scaillet et al., 1992; Scaillet et al., 1990), partially reset by a later Eocene-Oligocene greenschist phase (Chopin and Maluski, 1980; Monié, 1985). A contrasting view is that cooling of the UHP in the Dora Maira Massif occurred at 35–38 Ma (Gebauer et al., 1997; Rubatto and Hermann, 2001) while 70–120 Ma 40Ar/39Ar ages are ascribed to incorporation of excess 40Ar (Arnaud and Kelley, 1995; Kelley et al., 1994). The Gran Paradiso Massif was affected by HP metamorphism during the Alpine orogeny. Rb-Sr dating on white micas yielded ages of ca. 34 Ma (Freeman et al., 1997), while zircon fissiontrack (ZFT) ages are ca. 30 Ma (Hurford and Hunziker, 1989). Apatite fission-track (AFT) ages are ca. 20 Ma, indicating that the massif was at a few kilometers depth in the early Miocene (Hurford and Hunziker, 1989), possibly constituting a source for Tertiary Piedmont Basin sediments in late Miocene time. A further potential source for the Tertiary Piedmont Basin is the Sesia Lanzo zone and the Argentera Massif. The first belongs to the Austroalpine basement and was part of the Adriatic plate, while the second belongs to the Provençal Delphinois domain. The Sesia Lanzo zone was affected by Paleogene-Neogene exhumation (Hurford et al., 1991). Timing of HP metamorphism in the Sesia Lanzo Zone has been argued to be between 130 Ma (Inger et al., 1996; Oberhänsli et al., 1985; Ruffet et al., 1997;
Stöckhert et al., 1986) and 70–40 Ma (Cliff et al., 1998; Cortiana et al., 1998; Rubatto et al., 1999; Fig. 4). A phase of rapid exhumation of the Sesia Lanzo zone occurred during the Oligocene (25–30 Ma) Insubric phase (Hurford et al., 1991; Schmid et al., 1989). The Argentera Massif is one of the largest external crystalline massifs of the Western Alps, and it is the most proximal to the Tertiary Piedmont Basin. The Argentera Massif is made of Variscan basement overlain by Upper Carboniferous, Permian, and Triassic series (Menot et al., 1994). The massif was overthrust by the Penninic nappes during the Oligocene and exhumed and eroded in Miocene time (Bigot-Cormier et al., 2000). 40Ar/ 39 Ar dating on the Argentera Massif gives Variscan ages (e.g., Monié and Maluski, 1983; Fig. 4) while apatite fission-track ages show that the massif was at few kilometers from the surface during the Late Miocene–Pliocene (Bigot-Cormier et al., 2000). The Tertiary Piedmont Basin Sedimentation in the Tertiary Piedmont Basin started with a transgression dated as Late Eocene in the east and as Late Oligocene in the west (Fravega et al., 1994; Lorenz, 1984; Vannucci et al., 1997) and continued until Late Miocene (e.g., Gelati and
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Formation, Lequio Formation), tabular bodies are formed that can be traced through the whole basin, reaching a thickness of up to 2000 m (Fig. 2B, 2C). This reorganization is also recorded by uniform sandstone composition (Gnaccolini and Rossi, 1994). In particular, rock fragments of Miocene (from the Cortemilia Formation up) sediments mainly comprise quartzite, micaschist, orthogneiss, acid metavolcanic, phyllite, carbonate rock and subordinate metabasic rock, serpentinite, and phyllite, indicating mainly a western Alpine source. Also, a detailed sandstone petrographic study on the Cassinasco Formation by Caprara et al. (1985) shows that sandstones from this formation are very rich in all types of metamorphic lithics and indicates a composite collision orogen source (e.g., Dickinson and Suczek, 1979), which could correspond to the Alpine Penninic nappe. METHODOLOGY Concepts for the Interpretation of Detrital Minerals Ages Figure 5. Sketch showing: A: continuous cooling and exhumation, erosion of the source, and the correspondent geochronological signature produced in the clastic infill; B: rapid cooling and exhumation followed by slower erosion and the correspondent geochronological signature produced in the clastic infill. For both scenarios, nappe stacking took place at a temperature entirely in excess of ~350 °C.
Gnaccolini, 1996; Mutti et al., 1995). Sedimentation continued until the Plio-Pleistocene in the surrounding Po Plain. Initially, transitional (fluvial, fan deltas) environments characterized sedimentation with the deposition of a mainly conglomeratic sequence up to 600 m thick, known as the Molare Formation (Fig. 2B, 2C) (e.g., Gnaccolini et al., 1990; Turco et al., 1994). Sediments from the Molare Formation were locally sourced from the Ligurian Alps (Gnaccolini et al., 1990; Barbieri et al., 2003). These sediments pass stratigraphically into the Rocchetta and Monesiglio formations (lower Oligocene pro parte-Aquitanian; Gnaccolini et al., 1998, and references therein). These sediments consist of transitional facies (Rocchetta Formation) and turbiditic sandstones (Monesiglio Formation) in the lower part and by pelagic mudstones in the upper part, for a total of up to 1500 m (Fig. 2B, 2C). They are indicative of the progressive deepening of the Tertiary Piedmont Basin. The Rocchetta Formation in the eastern sector includes a redeposited sandy body known as Cassinelle Sandstones of Rupelian age. Facies characteristics, petrographic data, and paleocurrent directions obtained from the Rocchetta and Monesiglio formations suggest a provenance from both southern sectors, mainly the Briançonnais domain of the Ligurian Alps and the Voltri Group, and western sectors (Gelati et al., 1993; Gnaccolini and Rossi, 1994). From the Miocene, sedimentation became more homogeneous (Gelati et al., 1993) with paleocurrent directions indicating dominant flow to the east (Gnaccolini and Rossi, 1994; Fig. 3A). From the late Burdigalian (Cortemilia Formation) up to the Serravallian-Tortonian (Cassinasco Formation, Murazzano
Source rocks, each with characteristic geochronological signals, will result in the contribution of different ages to the basin infill. Potentially, mica 40Ar/39Ar ages from different stratigraphic levels of the Tertiary Piedmont Basin can be interpreted as recording variations in the original source rock cooling ages. Therefore, 40Ar/39Ar age populations reflect the contribution in ages present in the original source area surface at the time of sediment deposition. The major assumption underlying the geochronological approach is that of a short, and therefore negligible, time span between erosion in the belt and deposition of clastic sediments in the correspondent sedimentary basin (Brandon and Vance, 1992; Heller et al., 1992). This assumption is important for the calculation of cooling rates. We argue that this assumption is justified in the case of the Tertiary Piedmont Basin because of the close proximity of the source and basin. In general, two different end member scenarios can be envisaged for the potential Tertiary Piedmont Basin source area cooling and exhumation pattern and related 40Ar/39Ar ages in the Tertiary Piedmont Basin sediments. The first one involves tectonic nappe stacking taking place entirely at temperatures in excess of ~350 ºC. In this case, exhumation of the nappe stack would create a younging of cooling ages in the sediments (detrital ages) due to continuous upward movements of crustal rocks (e.g., Neubauer et al., 1996; Bernet et al., 2001) (Fig. 5A). In the sedimentary record, this would result in a decreasing of detrital ages (age populations or peaks of ages) up-sequence (e.g., “moving peaks” of Brandon and Vance, 1992). However, if cooling and exhumation ceased following an initial pulse of rapid cooling, and erosion was insufficient to unroof deeper crustal levels (recording younger cooling ages), constant cooling ages in the detritus would be detected over a substantial period of time (e.g., “static peaks” of Brandon and Vance, 1992) (Fig. 5B). The second scenario involves tectonic nappe stacking taking place entirely at temperatures less than ~350 ºC. In this case, exhumation and erosion would produce ages recording old
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cooling processes while the time of tectonic nappe stacking would not be recorded by the Ar system, because it would have occurred at a temperature lower than the closure temperature (T) of the system (T < ~350 ºC). As we are dealing with the integrated contribution of minerals over a large source area, any combination of the proposed scenarios may occur. Thus, by looking at changes in the main detrital age populations, we can obtain information on different source rock contributions and on different cooling patterns (e.g., Harrison et al., 1993; White et al., 2002; von Eynatten and Wijbrans, 2003). For provenance discrimination, we use variations in main detrital age populations. Analytical Technique Ar/39Ar geochronology has been applied to detrital phengites from over 60 samples from selected stratigraphic units of the Tertiary Piedmont Basin, ranging in age from Oligocene to Tortonian and to present-day river sands, with an average of seven samples for each formation and two samples for each river (Tables A1 and A2). Single fusion was applied on ~10 single crystals (ranging between 250–500 μm in size) for each selected sample, for a total of over 500 individual experiments. Step heating was applied to 10 single grains ranging between 500 and 1000 μm in size, derived from different formations and rivers, to check the internal distribution of radiogenic 40Ar in each sample. Only experiments concordant within 95% confidence intervals (i.e., MSWD < 2.5) have been used to derive plateau ages. The ages obtained on Tertiary Piedmont Basin clastic phengites are interpreted to represent the time of isotopic closure during cooling of the crystalline source through 350–420°C, as temperatures reached during the main metamorphic events of the basement rocks were generally higher than ~500 °C in the whole source area (e.g., Messiga and Scambelluri., 1991; Gebauer, 1999). The 40Ar/39Ar experiments were carried out with the VULKAAN laserprobe at the Isotope Geology Laboratory of the Vrije Universiteit in Amsterdam, following laser extraction and mass spectrometry methods for the laserprobe described by Wijbrans et al. (1995). The irradiation was carried out in the cadmium-lined CLICIT facility of the TRIGA reactor of the Oregon State University Radiation Center. Irradiation times were 7 h for three different irradiations, VU32, VU36, VU41. Correction factors for interferences of Ca and K isotopes were 0.000673 for 39Ar/37Ar, 0.000264 for 36Ar/37Ar, and 0.00086 for 40 Ar/39Ar, respectively. These values were determined using zero age K-feldspar and anorthite glass. After irradiation, a J curve was derived for individual samples by interpolation between five single fusion experiments on every flux monitor. As flux monitor standards for this project, we used Taylor Creek sanidine (for VU32) and DRA (Drachenfels trachite [Wijbrans et al., 1995; Steenbrink et al., 1999]) sanidine (for VU36-41; Steenbrink et al., 1999), with an age of 28.34 ± 0.16 Ma and 25.26 ± 0.14 Ma, respectively. These values are compatible with the set from Renne et al. (1998), based on biotite GA1550 (at a K/Ar age of 98.79 ± 0.69 Ma). In the present study, system blanks were determined 40
after every five unknowns. The unknowns were corrected for the interpolated blanks at the time of analysis of the unknown, and the 2σ error on the blank was further used for the error calculation of the unknown. 40Ar intensities for the analyzed samples were in the order of >100 times the blanks (see Wijbrans et al. [1995] for further details on mass spectrometer sensitivity). The discrimination factor was on average equal to 1.006 (see Kuiper [2003] for further details on discrimination factor calculation). Note that the 2σ errors reported in Table A2 do not include the uncertainties in J and uncertainties related to the age of the standards (the average of J-related errors is in the order of 0.3%). The exclusion of the Jrelated errors in the analytical errors reported in Table A2 enables a better comparison between samples (Foland, 1983). For further details on the calculation of the ages and related errors reported in Table A1, refer to Koppers (2002). Probability distribution diagrams (Sircombe, 1999; Sircombe, 2000) have been used to identify the main populations of detrital ages present in different formations of the Tertiary Piedmont Basin clastic infill and present-day river sands. The probability distribution curves are compiled by summing the Gaussian distribution of each individual measurement, which is defined by the age and its error (e.g., Sircombe, 2000). Some formations (e.g., Rocchetta-Cassinelle, Cortemilia-Paroldo, Murazzano-Cassinasco) have been combined because they are synchronous and contain similar sedimentological patterns (similar depositional environment and/or similar petrographic compositions and paleocurrent directions). As we are using the age probability distribution mainly as an indication of provenance, the major conclusions of this research are independent of any possible 40Ar excess problems in the source rocks. When addressing the question of differential exhumation and cooling of the source rocks, we assess systematic age differences in the mica populations at different stratigraphic levels. Biostratigraphic ages of the formations indicated in the following paragraphs are given using the stratigraphic scheme of Figure 2C and the geological timescale of Berggren et al. (1995). RESULTS Results from Tertiary Piedmont Basin Clastic Minerals The geochronological data set is presented in Figure 6 and Tables A1, A2, and A3. The results will be discussed in order of stratigraphic succession. The Molare Formation (Oligocene, ca. 33.7–23.8 Ma) is characterized mainly by 40Ar/39Ar ages clustering ~38–52 Ma with a strong 320 Ma signal and few ages ca. 99 Ma and 60–75 Ma (Fig. 6). We refer to Barbieri et al. (2003) for further details on this formation. The Rocchetta Formation (Lower Oligocene pro parte-Aquitanian; ca. 30–20.5 Ma) is characterized by a dominant age signal of 40–65 Ma (Fig. 6), which is similar to the main age population also found in the Molare Formation. Some ages ranging between 90 Ma and 150 Ma are present, while the Variscan signal is less pronounced than in the Molare Formation. Step heating
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analyses on single grains were carried out on four samples from the Rocchetta Formation (Fig. 7). A step heating experiment on D61a produced a discordant age spectrum with a total fusion age of 109.8 ± 1.1 Ma. This spectrum is represented by slightly higher ages at lower temperatures, which might be the result of alteration or excess 40Ar. However, in case of excess 40Ar, a much more disturbed spectrum should be expected (e.g., McDougall and Harrison, 1999). The “plateau-like” region of the spectrum has a weighted mean age of 113.5 ± 7.4 (Fig. 7; Table A3). The analysis on sample D61b gives a plateau age of 108.7 ± 1.0 Ma, suggesting that this signal is undisturbed. The analysis of sample D57 yields a slightly discordant total fusion age of 73.2 ± 3.1 Ma, whereas a plateau age of 51.4 ± 1 Ma was obtained from sample D72, showing that this signal is undisturbed. The Monesiglio Formation (Upper Oligocene–Aquitanian; ca. 28.5–20.5 Ma), laterally interfingering with the Rocchetta Formation toward the east, is characterized mainly by ages ca. 50 Ma. The other ages range between 38 Ma and 150 Ma. Step heating analysis on sample D69 produced a slightly discordant age spectrum (e.g., alteration) with a total fusion age of 51.7 ± 0.5 Ma. The “plateau-like” region of the spectrum has a weighted mean age of 51.6 ± 1.3 Ma (Fig. 7; Table A3). The Cortemilia and Paroldo formations (latest Aquitanian– Langhian; ca. 22–14.8 Ma) display a cluster of ages mainly ranging between 38 and 70 Ma, with a few ages between 100 and 180 Ma. A minor component shows ages in the range 250–300 Ma. The Cassinasco and Murazzano formations (Langhian-Serravallian; ca. 16.4–11.2 Ma) are characterized by an important contribution of ages ca. 70 Ma (Fig. 6), which is distinctive because it is older than the dominant age population (ca. 50 Ma) detected in general in the previous formations. A minor signal indicating Variscan provenance is present. These formations are further characterized by an important group of ages between 90 and 150 Ma. Step heating analyses on single grains have been carried out on three samples from the Cassinasco Formation. Samples D40 and D50 give a plateau age of 106 ± 1 Ma and 78 ± 1 Ma, respectively (Fig. 8). The third analysis, on sample D65, produced a slightly discordant age spectrum (e.g., alteration) with a total fusion age of 94.4 ± 0.7 Ma. The “plateau-like” region of the spectrum has a weighted mean age of 94.7 ± 0.9 Ma (Fig. 8; Table A3). The Lequio Formation (Serravallian-Tortonian; ca. 14.8–7.1 Ma) displays two major peaks in its detrital mica ages, one at 50 Ma and the other at 70 Ma, and a strong reappearance of Variscan ages. Results from Present-Day River Sands
Figure 6. Probability distribution diagrams of 40Ar/39Ar (detrital) ages for the samples (grouped in formations) investigated in this study. n—number of experiments; gray bars—indicative depositional ages. For further details, see Table A2.
Three samples (average of 20 grains for each sample) coming from the main rivers draining the present western Alpine arc (Tanaro, Maira, and Stura Rivers; Fig. 3) have been analyzed using single fusion analyses. Step heating analyses have been performed on a single grain from each river sample. The headwaters of the Tanaro River are in the Ligurian Alps. It drains Triassic and Permian formations belonging to the
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Figure 8. Step heating analyses of selected samples from the Cassinasco Formation. For further details, see Tables A1 and A3.
Figure 7. Step heating analyses of selected samples from the Rocchetta and Monesiglio formations. For further details, see Table A3.
Piemontese and Briançonnais domain and, further to the southwest, the Variscan Crystalline Massifs belonging to the Ligurian Alps. The sample was collected at the entrance of the Tanaro River into the Tertiary Piedmont Basin (Fig. 3A). White mica ages range mainly between 270 and 306 Ma, with only one age of 37 Ma (Fig. 9, Table A2). A step heating analysis performed on sample B16 (Fig. 10) gives a plateau age of 314 ± 3 Ma, suggesting an homogeneous signal. The Stura River drains mainly the Argentera Massif and partly the Briançonnais domain. The main age population detected falls between 204–302 Ma (Fig. 9). A step heating analysis performed on sample B18 gives a plateau age of 306 ± 2 Ma (Fig. 10, Table A2), suggesting a Variscan signal undisturbed by subsequent Alpine overprinting. The Maira River drains the poly-metamorphic HP-UHP units of the southwestern part of the Dora Maira Massif. Detrital mica ages range between 39 Ma and 159 Ma (Fig. 9), with a
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Figure 9. Probability distribution diagrams of samples coming from present-day river sands. For further details, see Tables A1 and A2.
cluster of ages between 65 and 95 Ma. A step heating analysis performed on sample B14 produced a slightly discordant spectrum with a total fusion age of 75.9 ± 0.6 Ma signal (Fig. 10). The “plateau like” region of the spectrum has a weighted mean age of 76.3 ± 0.7 Ma (Fig. 10). PROVENANCE DISCRIMINATION AND INFERENCES FOR SOURCE AREA EVOLUTION The wide range of ages in the Tertiary Piedmont Basin samples reflects the diverse provenance of the micas from different tectonic units of the surrounding belt. Discrimination between different source areas has been attempted using the main detrital age populations for each formation or group of formations as distinctive of different source area contribution. Oligocene–Early (Middle) Miocene Oligocene-Aquitanian The Molare Formation is the first Tertiary Piedmont Basin clastic infill that lies on the crystalline rocks of the Ligurian Alps, mirroring directly the source area outcropping at time of deposition (Barbieri et al., 2003; Gnaccolini, 1974). Ages range mainly ca. 40–45 Ma, which can be related to the Alpine metamorphic rocks of the Voltri Group and Montenotte Nappe, and ca. 320
Ma, which can be linked to the contribution of detritus from the Variscan Crystalline Massifs present in the Ligurian Alps (Barbieri et al., 2003). Within the Rocchetta and Monesiglio formations, depositional facies change from transitional to marine with the hemipelagic sediments and high to low density turbidity bodies (e.g., Noceto, Mazzurrini, Piantivello; Gelati and Gnaccolini, 1998, and references therein), marking a deepening of the basin. Therefore, more distal sources compared to the Molare Formation can be expected. The presence of a common (ca. 45–50 Ma) age population for the Rocchetta and Monesiglio formations could have different interpretations: (1) a primary contribution from the southern domain (e.g., Voltri Group), which reflects the signal of the crystalline basement that was widely affected by the Eocene Alpine metamorphism; (2) a contribution from western sectors, which also record the same Eocene signal (ca. 45 Ma); (3) a mix of these different sources; or (4) partial reworking of sediments from the underlying formations. The first hypothesis is less likely because in case of a primary southern source, a stronger Variscan signal might have been expected. Ages between 200 and 120 Ma are widely preserved in the western Alpine domain (e.g., Hunziker et al., 1992; Cortiana et al., 1998) but less so in the Ligurian Alps (Vance, 1999). Ages ca. 200 Ma may represent the cooling following the Middle
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B. Carrapa, J. Wijbrans, and G. Bertotti in comparison to that feeding the Molare Formation. Therefore, the Rocchetta and Monesiglio ages are interpreted to record the first signal of the erosion of western Alpine sectors. Latest Aquitanian–Langhian Facies characteristics, petrographic data, and paleocurrent directions in latest Aquitanian–Langhian sediments (Cortemila and Paroldo formations) suggest a change in sedimentary patterns with provenance mainly from western sectors (Gelati et al., 1993; Gnaccolini and Rossi, 1994). The detrital signal in the Cortemilia and Paroldo formations spans a broad range of ages, which is interpreted to indicate a wide source area (possibly wider than the one feeding the Rocchetta and Monesiglio formations), including mainly western Alpine sectors. The lack of Variscan ages discounts the southern sectors as a possible source area. The main age population (40–50 Ma) could be interpreted partially as a signal coming from the Western Alps, which could potentially also record the Eocene signal or be the result of partial reworking. Other ages, ranging between 50 and 150 Ma, suggest a contribution from the western Alpine domains. Middle–Late Miocene
Figure 10. Step heating analyses of selected samples from present river sands. For further details, see Tables A1 and A2.
Triassic thermal anomaly, which influenced the Southern Alps and the Lombardian Basin and which was possibly related to the intrusion of a magmatic body in the lower crust (Bertotti et al., 1997, and references therein). Ages ca. 85–100 and 65 Ma can potentially be linked to early exhumed oceanic units (e.g., the Sestri-Voltaggio Unit; no geochronological data are available in this particular area). The presence of different ages that can potentially be related to different sources within the same formation (e.g., D57–59, Noceto system: depositonal lobes as in Mutti and Normark, 1987), with paleocurrent directions coming from both southern and western sectors (Fig. 3), suggests mixing of sources within the same formation and possibly within the same sample. The wider range of ages present in the Rocchetta and Monesiglio formations compared to the Molare Formation suggests a wider source area (including both southern and western sectors)
This time span is characterized by a change in paleocurrent directions, sandstone composition, and sedimentary facies (Gelati et al., 1993; Gnaccolini and Rossi, 1994). With the Cassinasco and Murazzano formations (Langhian-Serravallian), an important shift in main detrital age population (from 40 to 50 Ma toward 70 Ma) occurs, with a significant cluster of ages at ca. 90–150 Ma. The stronger proportion of 70 Ma ages, which appears to be a dominant age cluster, older than the main age population (ca. 50 Ma), revealed in older formations is not compatible with a simple scenario in which continuous exhumation and erosion of a fixed source takes place (Fig. 5A). An evolution of the paleodrainage system, including more western-northwestern sectors (e.g., Sesia Lanzo), may explain the stronger contribution of the 70 Ma predominant signal. Also, in the currently exposed crystalline units of the Ligurian Alps, there is little evidence for units that have experienced cooling following a ca. 70 Ma event, while evidence for an Eocene cooling after HP metamorphism is widespread in the western Alpine domain (e.g., Cortiana et al., 1998; Hunziker et al., 1992; Ruffet et al., 1995; Agard et al., 2002, and references therein). Therefore, the shift in the main detrital age is interpreted as an evolution in sedimentary pattern and provenance related to an enlargement of the Tertiary Piedmont Basin source area toward more western-northwestern Alpine domains. This enlargement in the main Tertiary Piedmont Basin source area is probably associated with a reorganization of the paleodrainage system. Reorganization of paleodrainage systems has also been recorded for this particular time in the Central and Eastern Alps (Carrapa and Di Giulio, 2001; Spiegel et al., 2001; Frisch et al., 1998), suggesting a regional rearrangement in the erosional pattern of the Alpine chain, probably caused by a period of increased tectonic activity.
Detecting provenance variations and cooling patterns The Lequio Formation (Serravallian-Tortonian; 14.8–7.1 Ma) shows a main detrital population ca. 50–70 Ma, which can still be attributed to a western Alpine domain with a strong reappearance of Variscan ages. The Variscan ages are interpreted in this case as a signal coming from the Argentera Massif since petrographic and paleocurrent data (Gnaccolini and Rossi, 1994) do not give any support to the hypothesis of a southern source (Variscan Crystalline Massifs in the Ligurian Alps). This conclusion is well supported by a Late Miocene cooling and exhumation event recorded in the Argentera Massif (Bigot-Cormier et al., 2000). Present The main detrital 40Ar/39Ar age population in the Tanaro River clearly mirrors cooling ages of the Variscan Crystalline Massifs. Ages ranging between 204 and 302 Ma in the Stura River are representative of the Variscan metamorphic event recorded at present in the Argentera Massif. Our data agree with the conclusion reached by Monié and Maluski (1983) that the Alpine peak temperature in the Argentera Massif was <220–250 °C and consequently did not overprint the Variscan signal in white micas. The signal coming from the Tanaro and the Stura Rivers (mainly Variscan ages) could be evidence of either a geochronologically homogeneous source or of a low degree of mixing of the present drainage system. The wide range of ages in the Maira River record the heterogeneity of sources mainly characterized by middle-late Cretaceous ages and possibly also a good degree of mixing of different geochronological domains. Rocks outcropping in the present Maira drainage system provide a signal very similar to the one produced by present-day river sands (Figs. 4 and 9). By combining the age data coming from the three rivers (Fig. 9), we obtain a picture that is remarkably similar to the one observed for the Upper Miocene sediments (Lequio Formation; Fig. 6). This suggests that no major paleodrainage reorganizations occurred at least since the Late Miocene in the western Alpine catchment area. WESTERN ALPINE COOLING EXHUMATION PATTERNS In theory, a continuous pattern of exhumation of a fixed source area, represented by crustal rocks, is recorded by moving peaks up sequence (younging of the main signal) in the sedimentary infill (Brandon and Vance, 1992). Continuous exhumation through time would create a constant resetting of ages due to a steady upward movement of crustal material through the isotherms. Looking at the main detrital age populations of the Tertiary Piedmont Basin sediments (Fig. 6), a lack of moving peaks up sequence is apparent; this is mainly the result of the evolution of the paleodrainage system through time. The same ca. 45–50 Ma signal recorded as the main detrital population in Oligocene to Aquitanian sediments is very similar to outcropping ages in the
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southern domain at present (Fig. 4). The same is true for 70–120 Ma ages recorded as a strong signal in Langhian-Serravallain sediments and in the present-day sands, which is very similar to 40 Ar/39Ar data on present outcropping rocks of the western Alpine domain (e.g., Cortiana et al., 1998; Ruffet et al., 1997; Scaillet et al., 1992; Stöckhert et al., 1986; Agard et al., 2002; Fig. 4). These lines of evidence are not supported by a continuous exhumation and erosion pattern of a fixed source. This shows that a consistent amount of crustal rocks with indistinguishable 40Ar/39Ar ages has been eroded for a substantial amount of time, suggesting that the Western Alps have been regulated by episodes of fast cooling and exhumation (see also Carrapa et al., 2003). During these episodes, enough crustal material with indistinguishable 40Ar/39Ar isotopic ages was formed and then was eroded over long periods of time and at present still sheds the same signals. Also, Cretaceous ages (i.e., 120 Ma, 70 Ma) in Miocene and Present sediments derived from different western Alpine rocks could be the result of real geological events or the effect of excess 40Ar (Arnaud and Kelley, 1995; Ruffet et al., 1995; Ruffet et al., 1997). Ages ca. 120 Ma have been originally interpreted as the result of cooling following the peak of high pressure metamorphism (Monié and Chopin, 1991; Oberhänsli et al., 1985). Some authors have dismissed a Cretaceous thermal event for the internal Western Alps and consequently interpret this cluster of ages as due to excess 40Ar (e.g., Arnaud and Kelley, 1995; Kelley et al., 1994). Monié and Chopin (1991) have shown that 40Ar/39Ar ages ca. 100–110 Ma in the Dora Maira Massif are representative of a real signal. One of their major arguments was that it was highly unlikely that high pressure phengites from different internal units of the Western Alps (Monte Rosa, Sesia Lanzo, and Dora Maira) could have incorporated equivalent amounts of excess 40Ar leading to the same result. If the signal is geologically meaningful, then these ages could potentially be linked with a mid-Cretaceous metamorphic event (e.g., Cortiana et al., 1998; Monié and Chopin, 1991). Scaillet et al. (1992) show that high Mg-phengites from UHP rocks of the Dora Maira often yield Cretaceous ages, while Fe-phengites yield ages ca. 35–40 Ma. It is thus sometimes unnecessary to involve the presence of excess 40Ar to explain Cretaceous ages. Also, it is very unlikely that different rocks all experienced the same amount of excess 40 Ar leading to the same ages. Furthermore, because of the widespread occurrence of the same cluster of ages recorded by different thermochronometers in the Western Alps (Cortiana et al., 1998; Inger et al., 1996; Vance, 1999) and Central Alps (e.g., Hunziker et al., 1992), we consider the 120–70 Ma signal to be representative of important geological Eoalpine events (see also Carrapa and Wijbrans, 2003). CONCLUSIONS The general trend in the Tertiary Piedmont Basin sediments is that the main isotopic age population found in the oldest sediments (ca. 45–50 Ma) gets older toward younger sediments (ca. 70 Ma). This trend is interpreted as the result of a gradual
Figure 11. Schematic paleoreconstruction of the study area from Oligocene until post Tortonian times. + indicates uplifting area; – indicates subsiding area. Data from this work have been combined with literature data on paleogeography and paleocurrents (Gnaccolini, 1970; Lorenz, 1979; Gelati and Gnaccolini 1982; Fannucci, 1986; Dondi and D’Andrea, 1986; Gnaccolini and Rossi, 1994; Clari et al., 1995; Foeken et al., 2003).
Detecting provenance variations and cooling patterns enlargement of the catchment area to include tectonic units with progressively older isotopic ages (Fig. 11). Detrital micas from Oligocene until approximately Aquitanian time sediments in the Tertiary Piedmont Basin show a source area localized in the southern sectors (Ligurian Alps and Voltri Group), which was affected by a major Alpine metamorphic overprint at ca. 40–50 Ma. Since about Aquitanian-Burdigalian time, ages ca. 120–200 Ma and 70 Ma are widely recorded from detrital micas. This indicates a wider and mixed source area with contributions from both the Ligurian Alps and the western Alpine domain. Since the Langhian, the main detrital population shifts from 50 Ma toward 70 Ma, indicating an enlargement of the source area from southern sectors toward more western-northwestern sectors, which are characterized mainly by Eoalpine signals. This reorganization of the Tertiary Piedmont Basin source is likely linked to an evolution of the paleodrainage system most likely related to vertical tectonic movements in the hinterland. This evidence allows us to conclude that from the Early Miocene on, the Tertiary Piedmont Basin starts to record the erosion of the exhumed western Alpine arc. From Serravallian time, there is evidence of detritus coming from the Argentera Massif, characterized by a Variscan signal. The very similar signal shown by present-day river sands and Upper Miocene sediments suggests that the paleodrainage system did not continue to evolve after Miocene time in this sector of the Alps. Also, our data suggest that regional rapid and episodic Cretaceous and Middle Eocene cooling events in the Western Alps were followed by periods of relatively slow erosion, and later Mesoalpine-Neoalpine metamorphic events did not overprint the main original signals. ACKNOWLEDGMENTS This study was supported by the Netherlands Foundation of Scientific Research (NWO). Special thanks to Yani Najman, an anonymous reviewer, and Glen Murrell for their constructive advice in the preparation of the manuscript. This is NSG (Netherlands School for Sedimentary Geology) publication number 2003.05.15. REFERENCES CITED Agard, P., Monié, P., Jolivet, L., and Goffé, B., 2002, Exhumation of the Schistes Lustrés complex: in situ laser probe 40Ar/39Ar constraints and implications for the Western Alps: Journal of Metamorphic Geology, v. 20, p. 599–618. Arnaud, N.O., and Kelley, S.P., 1995, Evidence for excess argon during high pressure metamorphism in the Dora Maira Massif (Western Alps, Italy), using an ultra-violet laser ablation microprobe 40Ar/39Ar technique: Contributions to Mineralogy and Petrology, v. 121, p. 1–11. Barbieri, C., Carrapa, B., Di Giulio, A., Wijbrans, J., and Murrell, G., 2003, Provenance of Oligocene synorogenic sediments of the Ligurian Alps (NW Italy): Inferences on the belt age and its cooling history: International Journal of Earth Sciences, v. 92, p. 758–778. Beaumont, C., Ellis, S., Hamilton, J., and Fullsack, P., 1996, Mechanical model for subduction-collisional tectonics of Alpine-type compressional orogens: Geology, v. 24, p. 675–678.
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Printed in the USA
Geological Society of America Special Paper 378 2004
Siliciclastic record of rapid denudation in response to convergent-margin orogenesis, Ross Orogen, Antarctica John W. Goodge* Department of Geological Sciences, University of Minnesota, Duluth, Minnesota 55812, USA Paul Myrow Department of Geology, Colorado College, Colorado Springs, Colorado 80903, USA David Phillips* Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia C. Mark Fanning Ian S. Williams Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia ABSTRACT Siliciclastic rocks of the upper Byrd Group in the Transantarctic Mountains record rapid denudation and molasse deposition during Ross orogenesis along the early Paleozoic convergent margin of Gondwana. These rocks, which stratigraphically overlie Lower Cambrian Byrd carbonate deposits, are dominated by fresh detritus from proximal igneous and metamorphic sources within the Ross Orogen. Biostratigraphic evidence indicates that deposition of the siliciclastic succession is late Botomian or younger (<515 Ma). The largest modes of U-Pb and 40Ar/39Ar ages from detrital zircons and muscovites respectively in the siliciclastic molasse are Early to Middle Cambrian, but based on ages from crosscutting igneous bodies and neoblastic metamorphic phases, deposition of individual molasse units continued until ~490–485 Ma (earliest Ordovician). The entire episode of interrelated tectonic, denudational, sedimentary, deformational, and magmatic events is restricted to a time interval of 7–25 m.y. in the late Early Cambrian to earliest Ordovician, within the resolution of these stratigraphic and geochronologic data. Stratigraphic relationships suggest that the detrital zircon and muscovite in the sediments came from the same source terrain, consistent with large volumes of molasse having been shed into forearc and/or marginal basins at this time, primarily due to erosion of igneous rocks and metamorphic basement of the early Ross magmatic arc. Rapid erosion and unroofing in the axial Ross Orogen is consistent with a sharp carbonate-to-clastic stratigraphic transition observed in the upper Byrd Group, reflecting an outpouring of alluvial fan and fluvial-marine clastic detritus. The short time lag between tectonism and sedimentary response is similar to that determined for the corresponding section of the RossDelamerian orogen in South Australia and other continental-margin arc systems, such as in the Mesozoic Peninsular Ranges of California. Mineral cooling ages from metamorphic basement adjacent to the orogen yield a syn- to late-orogenic cooling *E-mail, Goodge:
[email protected]. Current address, Phillips: School of Earth Sciences, University of Melbourne, Melbourne, VIC 3010 Australia. Goodge, J.W., Myrow, P., Phillips, D., Fanning, C.M., and Williams, I.S., 2004, Siliciclastic record of rapid denudation in response to convergent-margin orogenesis, Ross Orogen, Antarctica, in Bernet, M., and Spiegel, C., eds., Detrital thermochronology—Provenance analysis, exhumation, and landscape evolution of mountain belts: Boulder, Colorado, Geological Society of America Special Paper 378, p. 105–126. For permission to copy, contact
[email protected]. © 2004 Geological Society of America.
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J.W. Goodge et al. rate of ~10 °C/m.y., which, combined with a known metamorphic geotherm, indicates a denudation rate of ~0.5 mm/yr. Such denudation rates are comparable to those in recent convergent or collision orogens and suggest that crustal thickening associated with both magmatic intrusion and structural shortening was balanced by near-synchronous erosional exhumation. Keywords: detrital minerals, zircon, muscovite, thermochronology, Ross Orogen.
INTRODUCTION Thermochronology is widely used to recover information about cooling histories, metamorphic P-T-t paths, and rates of tectonic displacement. Differences between the ages of minerals with different closure temperatures, as well as mineral systems that record partial thermal resetting, can be used to determine crustal cooling rates. If the regional geothermal gradient is known or can be inferred, it is also possible to estimate the rate of surficial denudation. These methods are commonly applied to igneous and metamorphic rocks in tectonically active settings, sampled from bedrock across tilted crustal sections or in deeply eroded areas of high present-day relief. This approach has some limitations, however, due to the degree of relief and the fact that part of the rock record (thereby, history) has been removed by erosion. An alternative strategy is to determine the ages of detrital minerals eroded from a mountain belt and preserved by accumulation in adjacent sedimentary basins. Detrital mineral chronometers place limits on the age and duration of sedimentation, particularly when biostratigraphic markers are lacking. In some cases they can be used to estimate rates of cooling and/or denudation, if it can be demonstrated that minerals with different closure temperatures were derived from the same source region and that those minerals have not been isotopically disturbed in the surficial and burial environments since initial closure (e.g., Copeland and Harrison, 1990; Garver et al., 1999; Bernet et al., 2001). The latter situation most likely holds in proximal “unroofing” successions where mineral provenance is well established. We have used detrital muscovite and zircon ages from Cambrian-Ordovician siliciclastic rocks of the central Ross Orogen (Fig. 1A) to evaluate tectonically induced denudation in a convergent-margin setting of Gondwana. Sandstone and conglomerate of the upper Byrd Group (Lower Cambrian to Ordovician) were deposited as forearc molasse sediments in response to structural shortening and continental-margin magmatism during the main phase of Ross tectonism (Rowell et al., 1988; Rees and Rowell, 1991). The onset of supracrustal deformation is well constrained by biostratigraphic, chemostratigraphic, and geochronological data from the region (Myrow et al., 2002b), and detrital mineral suites in the molasse deposits provide good control on their age and provenance (Goodge et al., 2002). Here we summarize previously reported U-Pb detrital zircon ages and report new 40Ar/39Ar detrital muscovite ages from the Byrd Group molasse succession that characterize the age and nature of inputs from the orogenic source region. Sandstone samples were collected for detrital zircon and muscovite analysis from the Starshot and Douglas
formations between the Byrd and Beardmore glaciers (Fig. 2), near Cape Selbourne (DIF), Mount Ubique (USF), the Holyoake Range (HSF and DCS), Cambrian Bluff (CBG), Softbed Ridges (SRG) and Dolphin Spur (DSG). We also report new age measurements of crosscutting intrusions and metamorphic mineral assemblages that bracket the duration of molasse sedimentation. Taken together, these data provide a detailed record of interrelated events that followed the first pulse of supracrustal deformation in the region and that reflect rapid denudation in the orogen. GEOLOGICAL SETTING East Antarctica is the keystone in most reconstructions of Rodinia and Gondwana. It has a long association with East Gondwana cratonic neighbors in present-day Australia, India, and Africa, which were finally amalgamated along Grenvilleage sutures during assembly of Rodinia (Fig. 1A). Breakup of Rodinia resulted in formation of a rifted margin along the paleo-Pacific sector of Australia and East Antarctica (Fig. 1B), characterized by passive-margin subsidence, sedimentation in shoreline and shallow shelf settings, and minor volcanism (Laird, 1981, 1991; Stump, 1995; Preiss, 2000). Rifting along the East Antarctic sector may have occurred as early as ca. 750 Ma, but a gabbro from the central Transantarctic Mountains dated by U-Pb zircon as 668 Ma provides a better minimum estimate of rifting age (Goodge et al., 2002). Subsequent passive-margin extension and sedimentation continued well into the late Neoproterozoic between ca. 670 and 580 Ma. By the latest Neoproterozoic to Early Cambrian, the rift margin underwent a major tectonic transformation to an active, subducting plate boundary, probably as a result of changes in global plate motions and plate-boundary stresses following initial consolidation of the central Gondwana supercontinent (Flöttmann et al., 1994; Goodge, 1997). In Antarctica, the convergent margin consisted of a continental-margin magmatic arc (Borg et al., 1987, 1990; Armienti et al., 1990; Allibone et al., 1993; Rocchi et al., 1997) constructed upon Archean-Proterozoic basement (Fig. 1C), with sinistraloblique underflow interpreted from Ross-age basement structures (Goodge et al., 1993a). Calc-alkaline magmatism indicates that subduction was initiated by at least 530 Ma (e.g., Cox et al., 2000; Allibone and Wysoczanski, 2002). Detrital zircon geochronology from lower Paleozoic rocks containing arc-derived detritus suggests that volumetrically significant magmatism occurred as early as ca. 580 Ma (Ireland et al., 1998; Goodge et al., 2002). Tectonism attributed to the Ross Orogeny is expressed by structural shortening of upper Neoproterozoic marginal-basin
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Siliciclastic record of rapid denudation A
B passive margin: ~670-580 Ma Australia
TA
shoreline & shelf deposits
WILKES LAND
East Antarctica
shield basement
NG
PCM
study area
India
• Rodinia rift margin • subsiding passive margin • minor volcanism
PM Q. MAUD LAND
SR
EAO/M Kalahari after Fitzsimons (2000)
0.55-0.48 Ga Ross-Delamerian Orogen 0.65-0.50 Ga mobile belts (Pan-African)
C syn-orogenic: ~515-490 Ma denuding arc
foreland basin ?
pluton stabilization
flooded shelf
forearc basin molasse
intra-arc deformation
sinistral transpression
1.35-0.9 Ga mobile belts (Grenville) 3.0-1.6 Ga cratons
• Gondwana active margin • sinistral-oblique subduction • forearc molasse deposition
Figure 1. Tectonic setting of the Ross Orogenic belt. A: East Gondwana assembly (modified from Fitzsimons, 2000), showing distribution of major cratonic shield areas and Mesoproterozoic to Neoproterozoic mobile belts. General position of convergent plate margin during RossDelamerian stage shown by barbed line. Study area near Nimrod Glacier indicated by circle. EAO/M—East African Orogen/Mozambique Belt; NG—Nimrod Glacier area; PCM—Prince Charles Mountains; PM—Pensacola Mountains; SR—Shackleton Range; TA—Terre Adélie. B: Passive-margin setting of rifted East Antarctic shield in late Neoproterozoic time. This stage is represented stratigraphically by mature Beardmore Group siliciclastic rocks, deposited in shoreline and shelf settings (Myrow and Goodge, 1999), and minor mafic volcanic rocks dated at 668 Ma (Goodge et al., 2002). Passive-margin deposition continued to the middle Early Cambrian (up to 515 Ma) with deposition of platform carbonate deposits of the lower Byrd Group. C: Main orogenic phase of Ross convergent margin, characterized by sinistral-oblique subduction of paleo-Pacific oceanic lithosphere beneath the East Antarctic margin (Borg et al., 1990; Goodge et al., 1993a). Basement rocks of the Nimrod Group record deep-seated metamorphism, syntectonic magmatism, and deformation in a transpressional setting. Intra-arc deformation included shortening of older marginal-basin deposits and strike-slip motion within the developing magmatic arc. The main contractional orogenic phase within the supracrustal assemblages resulted in erosion of arc and arc-basement rocks, leading to alluvial fan, fluvial, and shoreline deposition of upper Byrd Group molasse materials in a forearc basin (Myrow et al., 2002b); the present-day polar ice cap flanks the denuded orogenic belt, but a foreland basin is conjecturally illustrated here.
strata and platform carbonates of Early Cambrian age. Middle Early Cambrian to Ordovician molasse deposits of the upper Byrd Group, the subject of this paper, appear to represent alluvial-fluvial to shallow-marine siliciclastic deposits derived from the eroding Ross Orogen (Rees and Rowell, 1991; Myrow et al., 2002a). As shown in Figure 1C, these deposits are interpreted as marginal-marine and forearc-basin sediments on the basis of paleocurrent and sedimentary facies data. There may have been a corresponding foreland basin in a retroarc setting, but if it existed, the deposits would now be covered by the modern ice cap. In the central Transantarctic Mountains (Fig. 2), siliciclastic rocks of the upper Byrd Group lie east (or outboard in present coordinates) of high-grade metamorphic and igneous rocks of the Nimrod Group, representing East Antarctic shield basement and low-grade sedimentary rocks of the passive margin. The latter include late Neoproterozoic siliciclastic rocks of the Beardmore Group (following Goodge et al., 2002) and Early Cambrian carbonate strata of the lower Byrd Group (Fig. 3). The Beardmore Group deposits include quartz wacke, quartzite, carbonate grainstone, shale, diamictite, and minor mafic volcanic rocks.
The Shackleton Limestone of the lower Byrd Group includes a thick basal quartz arenite unit. Siliciclastic rocks of middle Early Cambrian or younger age (Holyoake, Starshot, and Douglas formations of the upper Byrd Group) abruptly overlie the older Goodge et al. [Fig. 1] passive-margin units (Myrow et al., 2002b). These rocks depositionally overlie the terminal Lower Cambrian carbonate platform deposits of the Shackleton Limestone and reflect tectonic drowning of the formerly quiescent platform during the early stages of Ross convergence. Faunal ages in the Holyoake and lower Starshot formations (lower part of the upper Byrd Group) date the inception of siliciclastic deposition as ca. 515 Ma (Myrow et al., 2002b). The bulk of the upper Byrd Group consists of proximal conglomerate and more distal sandstone, the latter being mainly immature feldspathic arenite and wacke associated with shale, argillite, and pebble conglomerate. Although difficult to distinguish from the older clastic rocks in the field, they are distinctive in composition, being notably rich in feldspar and detrital mica. Paleocurrent data, paleoslope data, sedimentary facies relationships, clast compositions, and detrital zircon ages from the upper Byrd units all suggest derivation and transport from igneous and
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J.W. Goodge et al.
Beacon Supergroup (Devonian-Triassic); Gondwana overlap
Polar
MO UNT AINS
81°S 155°
Byrd Group siliciclastic rocks (middle Early Cambrian to Early Ordovician); Starshot & Holyoake fms. (including former "outboard" Goldie & Dick fms.)
E
DIF
Ice Shelf
Cape Laird
Holyoake
Geologists Range
igneous & metamorphic mineral sample locations
Mt Ubique
Range
DCS
Ross
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ier lac
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82°S
Nimrod Group metamorphic complex (Archean & Early Proterozoic)
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Beardmore Group siliciclastic rocks (Neoproterozoic to Cambrian?); includes Cobham & Goldie fms.
HSF 1 A' 65°E
CBG
A
82°
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Cambrian Bluff
detrital zircon & muscovite sample locations
NIMROD GLA CIER
Panorama Point
83°S
paleocurrent direction
B'
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RA Gl.
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Softbed Ridges
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Neoproterozoic craton margin (inferred)
Figure 2. General geology of the central Transantarctic Mountains between Byrd and Beardmore glaciers, showing locations of dated samples. CBG—Starshot Fm., Cambrian Bluff; DCS—Douglas Conglomerate, Holyoake Range; DIF— Dick Fm., Cape Selbourne; DSG—Starshot Fm., Dolphin Spur; HSF—Starshot Fm., Holyoake Range; SRG—Starshot Fm., Softbed Ridges; USF—Starshot Fm., Mount Ubique. Sample information is given in Table 1. Stratigraphic assignment follows Myrow et al. (2002b) and Goodge et al. (2002), as shown in Figure 3. Schematic cross sections A–Aʹ and B–Bʹ shown in Figure 5.
nd
fault; inferred
E
Miller Range
Holla
B
trace & body fossil locality
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. g Gl
E 83 °S
Mt Hope
90°W E
90°E
Cape Selbourne
ILL
Byrd Group carbonate & carbonate-clast rocks, undivided (Lower Cambrian to Ordovician); includes Shackleton Limestone & Douglas Conglomerate
Ross Sea
IE R
Plateau
Granite Harbour Intrusives (Cambrian-Early Ordovician)
Transantarctic Mtns.
GLA C
BYRD
Weddell Sea
0° QU
85°
EE
N
A DR AN X E AL
RA
160
0
100 km
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Dolphin Spur
S °E
N
DM
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Co
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AC GL
mm
metamorphic sources to the west in present-day coordinates (Myrow et al., 2002a, 2002b; Goodge et al., 2002). Syn- to post-tectonic igneous rocks of the Granite Harbour intrusive series, emplaced between ca. 540 and 480 Ma (Borg et al., 1990; Goodge et al., 1993b), intrude all units in the region. The middle Early Cambrian to Ordovician upper Byrd Group, interpreted as forearc molasse deposits based on their sedimentary facies, detrital zircon characteristics, and position with respect to the magmatic arc (Goodge et al., 2002), therefore represents detritus derived from structurally thickened parts of the orogenic belt. These molasse units were derived in part from older metasedimentary rock units that were later thrust over them (Fig. 4). The molasse is a significant proportion of exposed rock
on
w
IE
R
lth ea
DSG
ng
e
East Antarctica
Ra
in the present-day orogen, and it is itself deformed by open, upright structures and intruded by late-stage granitoids (Fig. 5). Before the age and duration of denudation related to molasse deposition can be addressed using detrital mineral thermochronometry, it must be established that the minerals were eroded from the same source rock or from the same crustal level (Garver et al., 1999; Bernet et al., 2001). Evidence that zircon and muscovite in the upper Byrd Group molasse succession were derived from the same proximal sources includes the following: (a) the Starshot and Douglas formations Goodge etare al. proximal [Fig. 2] deposits, based on their alluvial fan and marginal-marine sedimentary facies, immature compositions, degree of grain angularity, and preservation of pristine zircon crystals and coarse detrital muscovite
Siliciclastic record of rapid denudation
109
?
?
BYRD GROUP
Late Middle Cambrian Cambrian
Starshot Fm.
late
Early Cambrian
510
?
mid
DIF USF
IV - Late orogenic DCS
CBG SRG DSG HSF
Douglas Congl.
III - Syn-orogenic
Holyoake Fm.
Shackleton Ls.
SLB
II - Platform early
540 ?
560
660 670 680
BEARDMORE GROUP
550
Neoproterozoic
cover deformation
500
530
rift-margin sedimentation
rifting
?
490
?
? ?
480
520
def'n
basement def'n
magmatism
?
Ordovician
Age (Ma) 470
?
?
Goldie Fm.
CPG1 PAC CPG2 668 Ma
CBF PAS
I - Passive
Figure 3. Stratigraphic relationships and detrital mineral samples in Neoproterozoic and lower Paleozoic sedimentary units of the central Transantarctic Mountains. Major time periods shown with Siberian biostratigraphic stages for reference. Revised stratigraphic relationships (Goodge et al., 2002; Myrow et al., 2002b) indicate passive-margin and platform deposition (stages I and II) through the middle Early Cambrian, when ramp carbonate of the Shackleton Limestone was abruptly overlapped by upward-coarsening siliciclastic deposits of the upper Byrd Group (stages III and IV; Holyoake, Starshot, and Douglas formations). Integration of biostratigraphic, chemostratigraphic, and geochronological data indicate that Shackleton carbonate deposition terminated ca. 515 Ma (Myrow et al., 2002b). Ages of geologic events in the central Ross Orogen summarized by black bars. Circles indicate units sampled for detrital mineral age determinations (see Table 1). DIF—Dick Formation, Cape Selbourne; USF— Starshot Formation, Mount Ubique; DCS—Douglas Conglomerate, Holyoake Range; CBG—Starshot Formation, Cambrian Bluff; SRG—Starshot Formation, Softbed Ridges; DSG—Starshot Formation, Dolphin Spur; HSF—Starshot Formation, Holyoake Range; SLB—Shackleton Limestone, Cotton Plateau; CPG1—Goldie Formation, Cotton Plateau; CPG2—Goldie Formation, Cotton Plateau; PAC—Goldie Formation (?) conglomerate, Princess Anne Glacier; CBF—Cobham Formation, Gargoyle Range; PAS—Goldie Formation (?) sandstone, Princess Anne Glacier.
Cobham Fm.
700
?
grains, which all suggest short transport distance; (b) the presence of limestone clasts and crystalline calcite grains in some Starshot beds, in addition to igneous and quartzite clasts, suggesting that the Shackleton Limestone and local basement rocks were eroded; (c) the Starshot appears to have a chiefly intra-arc source, based on paleocurrents, sediment composition, zircon growth and radiogenic isotope characteristics, and trace minerals such as tourmaline (a distinctive accessory mineral in Ross-age igneous rocks of the region); and (d) the deposits immediately overlie autochthonous carbonate, suggesting that they are not far-traveled. If the detrital zircon and muscovite in the Starshot strata indeed share a common source, they can provide first-order constraints on the duration of denudation and sedimentation. ANALYTICAL METHODS Ar/39Ar Methods
40
Analytical procedures followed those described by McDougall and Feibel (1999). Muscovite and biotite mineral separates
were prepared using standard crushing, desliming, heavy liquid, and paramagnetic techniques. Chips (0.5–1.0 mm) from two slate samples (CBGs1 and CBGs2) and ~30 detrital muscovite grains, from each of sandstone samples SRGm, CBGm, DSGm and USFm, were handpicked for 40Ar/39Ar analyses. The samples were ultrasonically cleaned in deionized water and acetone prior to submission for irradiation. All samples were wrapped in aluminum packets and placed in an aluminum irradiation canister together with interspersed aliquots of the flux monitor GA1550 biotite (age = 98.8 ± 0.5 Ma; Renne et al., 1998). Packets containing degassed potassium glass were placed at either end of the canister to monitor the 40Ar production from potassium. The Goodge et al. [Fig. 3] irradiation canister was irradiated for 504 hours in position X33 of the HIFAR reactor, Lucas Heights, Sydney. After irradiation, the samples were removed from their packaging and individual muscovite grains were loaded into 2-mm-diameter holes in a copper sample holder. After bake-out, single muscovite grains were fused using an argon-ion laser beam. Approximately 0.5 mg aliquots of biotite and ~1.0 mg of slate chips were wrapped in tinfoil, baked overnight, and step-heated in tantalum
W
Cambrian Bluff area, Antarctica
f
f
Figure 4. Outcrop photo mosaic of the eastern Cambrian Bluff area, southern Holyoake Range, showing older-overyounger structural relationship between Shackleton Limestone carbonate and Starshot Formation siliciclastic rocks. Massive outcrops to the west contain fault panels of different lithofacies in the Shackleton Limestone. Dark outcrops to the east are underlain by shale, argillite, sandstone, and diamictite of the Starshot Formation (formerly mapped as Goldie Formation; Laird et al., 1971). Line drawing shows interpretation of structures based on field, petrofabric, and age relationships. View to the north. Outcrop is ~800 m high. Samples CBG and 98-242 collected farther east of this outcrop area.
Middle Cambrian siliciclastics
Lower Cambrian carbonates
f
f
marble tectonite
E
f
f
f
f felsitic intrusions
talus
massive archeocyathan limestone, bedded bioturbated limestone, dolomitic limestone
shale-matrix diamictite with limestone clasts
Late Neoproterozoic marginal basin deposits A
Nimrod Glacier
Early Cambrian platform
black shale, argillite & graywacke
Cambrian-Ordovician molasse deposits Holyoake Range
Cobham Range
Algie Glacier
Nash Range
Ross Ice Shelf A'
Fig. 4
ArcheanProterozoic basement
Late Neoproterozoic marginal basin deposits Miller Range
B
Early Cambrian platform Marsh Glacier
Cotton Plateau
Cambrian-Ordovician molasse deposits Queen Elizabeth Range
Goodge et al. [Fig. 4] Endurance thrust
0
25 km
B'
Granite Harbour granitoid intrusion
Figure 5. Schematic geologic cross sections of the Ross Orogen in the Nimrod Glacier area of the Transantarctic Mountains (see Fig. 2). Sections are approximately to scale with no vertical exaggeration. Molasse deposits of the Starshot Formation in the Holyoake, Nash, and Queen Elizabeth ranges are overridden by older (Early Cambrian) carbonate of the Shackleton Limestone, yet contain clasts of the carbonate as well as quartzitic material that was probably derived from crystalline basement and marginal-basin deposits presently exposed to the west.
111
Siliciclastic record of rapid denudation resistance furnaces. 40Ar/39Ar analyses of muscovite and biotite samples were carried out at the Australian National University, whereas the two slate samples were analyzed at the University of Melbourne; all analyses were carried out on VG3600 mass spectrometers. Mass discrimination was monitored by analyses of standard air volumes. Correction factors for interfering reactions are as follows and apply to both labs: (36Ar/37Ar)Ca = 3.20 (± 0.02) × 10−4; (39Ar/37Ar)Ca = 7.54 (± 0.5) × 10−4 (Tetley et al., 1980); (40Ar/39Ar)K = 0.035 (± 0.005). K/Ca ratios were determined from the ANU laboratory hornblende standard 77-600 and were calculated as follows: K/Ca = 1.90 × 39Ar/37Ar. The reported data have been corrected for system backgrounds, mass discrimination, and radioactive decay. The 40Ar*/39Ar ratios and ages have also been corrected for fluence gradients and atmospheric contamination. Errors associated with the age determinations are 1σ uncertainties and exclude errors in the J-value estimates. The error on the J-value is ± 0.3%, excluding the uncertainty in the age of GA1550. Decay constants are those of Steiger and Jäger (1977). McDougall and Harrison (1999) describe the 40Ar/39Ar dating technique in detail. U-Pb Methods Detrital zircon procedures followed those described by Goodge et al. (2002). Heavy mineral concentrates of igneous zircon from the granitoids discussed here were prepared using standard crushing, desliming, heavy liquid, and paramagnetic techniques. Zircons were handpicked from the mineral concentrates, mounted in epoxy together with chips of the FC1 and SL13 reference zircons, sectioned approximately in half, and polished. Reflected and transmitted light photomicrographs and cathodoluminescence (CL) scanning electron microscope (SEM) images were prepared for all zircons. The CL images were used to decipher the internal structures of the sectioned grains and to target specific areas within the zircons for analysis. U-Pb analyses of zircons in the two igneous samples were made using sensi-
tive high-resolution ion microprobe (SHRIMP) II at the National Institute for Polar Research, Tokyo. The analyses consisted of 6 scans through the mass range, and data were reduced in a manner similar to that described by Williams (1998, and references therein) using the SQUID Excel macro of Ludwig (2000). The Pb/U ratios are normalized relative to a value of 0.1859 for the 206 Pb/238U ratio of the FC1 reference zircons, equivalent to an age of 1099 Ma (see Paces and Miller, 1993). Uncertainties given for individual analyses (ratios and ages) are at the 1σ level; however, the uncertainties in calculated weighted mean 206Pb/238U ages are reported as 95% confidence limits, including the uncertainty in the U-Pb calibration of the reference zircon. Tera-Wasserburg concordia plots, relative probability plots with stacked histograms, and weighted mean 206Pb/238U ages were prepared using ISOPLOT/EX (Ludwig, 1999). DETRITAL MINERAL AGES Results of detrital zircon and muscovite analyses from Starshot and Douglas samples are summarized in Table 1. Here we discuss existing U-Pb age data for detrital zircons and present new 40Ar/39Ar ages for detrital muscovites. U-Pb Ages of Detrital Zircon Detrital zircons were analyzed from 12 samples of sandstone from the central Ross Orogen that represent units deposited during the passive- to convergent-margin transition in Neoproterozoic to Ordovician time (Goodge et al., 2002; Goodge et al., 2004). This tectonic transition is well represented by the zircon age distributions, which show distinctive shifts in sedimentary provenance and depositional age. Late Neoproterozoic to Early Cambrian passive-margin and platform units of the Beardmore and lower Byrd groups, stages I and II in Figure 6, contain only Precambrian zircon. Although depositional ages are not precisely known for the passive-margin siliciclastic units, samples inferred
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112
J.W. Goodge et al.
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PAC2 CBF1 PAS2 0
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Figure 6. Summary of detrital zircon results from Beardmore and Byrd group sandstone (Goodge et al., 2002; Goodge et al., 2004), presented as relative probability histograms. Tectonic stages I–IV as in Figure 3, and detrital age results shown in order of relative depositional age based on biostratigraphic control and the youngest zircon grain populations. Provenance changes as follows: (I) passive-margin stage, characterized by Archean and Mesoproterozoic cratonic provenance; (II) platform stage, with mixed carbonate and siliciclastic deposition, the latter dominated by Mesoproterozoic and Grenville-age sources; (III) synorogenic stage, marked by near complete absence of cratonic and older orogenic (Grenville) signatures, and dominated by proximal, young material from the youthful Ross orogen; and (IV) late orogenic stage, showing the youngest detrital components with a Ross provenance, but also minor influx of older cratonic material. Samples as labeled in Figure 3. Original sources of age data as follows: 1—Goodge et al. (2002); 2—Goodge et al. (2004).
to come from stratigraphically lower units contain only Archean and Paleoproterozoic components (PAS and CBF), whereas they are succeeded by units dominated by Mesoproterozoic components, with increasing input of Grenville-orogen age (PAC, CPG, SLB). These age distributions are interpreted as the signature of a cratonic provenance, probably from the East Antarctic shield or adjacent cratonic areas (Goodge et al., 2002). Siliciclastic deposits of the upper Byrd Group are regarded as syn- to late-tectonic in origin and show markedly different detrital zircon signatures compared to the older units (stages III and IV, Fig. 6). The age distributions of these younger sandstone deposits are simpler and dominated by a composite Ross age component ranging from ca. 600 to 500 Ma (HSF, SRG, DSG, DCS). The two youngest samples (USF, DIF) also contain significant amounts of first-cycle Precambrian material, including contributions from Grenville-age (ca. 995, 1035, 1080, 1130, and 1225 Ma) and older (ca. 3.1–3.0, 2.8, and 2.5 Ga) sources. Despite the dominant Ross signature, the Grenville-type and older cratonic ages indicate either that sediment deposition occurred close to exposures of cratonic basement or that it represents deep basement erosion. The morphology, growth zoning, and isotopic compositions of the young Ross-age zircons indicate mostly an igneous origin, which we interpret as the product of erosion from a continental-margin magmatic arc that developed during the main stage of the Ross Orogeny (Fig. 1C; Goodge et al., 2002). The general age distribution in the youngest samples is quite similar to that in Ordovician sandstone units of eastern Australia (Williams, 1998; Ireland et al., 1998), New Zealand (Ireland and Gibson, 1998), and South Africa (Armstrong et al., 1998) and may indicate the widespread distribution of sediments from a common source terrain along the Pacific margin of Gondwana during late Ross and Delamerian time (Williams et al., 2002). In addition to source information, the detrital zircon ages from the upper Byrd sandstone units also confirm biostratigraphically constrained depositional ages. The youngest discrete detrital subpopulations in the Byrd Group units (ca. 550–500 Ma; Table 1) indicate maximum depositional ages between Early and Late Cambrian, consistent with the Botomian age for onset of siliciclastic deposition indicated by trilobites in the lower Starshot Formation. Radiometric dates on granitoid intrusions in the region also constrain deposition to be no younger than Early Ordovician. Detrital muscovite data, discussed below, help to further constrain the depositional ages for some of the samples. Ar/39Ar Ages of Detrital Muscovite
40
Ar/39Ar ages were measured on individual detrital muscovite grains from four sandstone samples of the upper Byrd Group. All were from the Starshot Formation (Table 1), although three (SRGm, CBGm, and DSGm) were collected from exposures originally mapped as Goldie Formation (Laird et al., 1971; Oliver, 1972).Good Detrital age data (Goodge et al., 2002; Goodge ge zircon g. 6] et al. [Fi et al., 2004) are available for samples SRGm, DSGm, and USFm. Detrital muscovite was distinguished from neoblastic muscovite 40
Siliciclastic record of rapid denudation
A
B
S1 Figure 7. Thin section photomicrographs showing textural types of muscovite in Starshot Formation sandstones and shales. A: Immature feldspathic sandstone from Mount Ubique area (sample USFm) showing angular texture of framework grains, poor sorting, and elongate, bent detrital muscovite grains. Plane-polarized light (PPL); field of view ~1 mm. B: Slate from Cambrian Bluff area (sample CBGs) showing foliation (S1) formed by fine-grained, neoblastic phyllosilicates, including colorless muscovite. Larger black clots are opaque minerals. PPL; field of view ~1 mm.
primarily by its size and shape. The grains are coarse (up to 1 mm) flakes similar in size to framework grains of quartz and feldspar, they have high aspect ratios (mostly 10:1 or greater), and they are typically bent or kinked where in contact with framework grains (Fig. 7A). Neoblastic metamorphic minerals, including muscovite, chlorite, and biotite, are finer grained and texturally distinctive (Fig. 7B). Single-grain 40Ar/39Ar fusion ages were obtained for 12–14 detrital muscovites from each sample using an Ar-ion laser. Analytical data are listed in Table 2 and the ages illustrated in Figure 8. Sample SRGm was collected from the Lowery Glacier-Softbed Ridges area (Fig. 2). It is a fine to medium-grained (0.1–0.5 mm), feldspathic graywacke containing angular grains of quartz,
113
plagioclase, muscovite, and lithic grains (slate and polycrystalline quartz), indicating a source composed of crystalline basement and low-grade metasedimentary rocks. Fourteen detrital muscovite grains yielded ages ranging from ca. 584 to 490 Ma (Fig. 8A), with two grains (1 and 11) much older than the rest (561 ± 3 Ma and 584 ± 12 Ma, respectively). The weighted mean age of the 12 younger grains is 507 ± 7 Ma, but there is considerable scatter beyond analytical error (χ2 = 23.7). This population is bimodal, with six grains yielding a weighted mean age of 497 ± 4 Ma (χ2 = 3.1) and the other six a weighted mean age of 518 ± 4 Ma (χ2 = 2.5). The mean age of the youngest grains in sample SRGm is indistinguishable from that of a nearby intrusion (see below), but the presence of some distinctly older grains suggests a maximum depositional age of 518 ± 4 Ma. If correct, this indicates that the sample has a middle Early Cambrian or younger depositional age, consistent with the less precise age limit of 531 ± 8 Ma provided by the youngest detrital zircons (Table 1). Sample CBGm is a feldspathic arenite from the northeast end of Cambrian Bluff in the southern Holyoake Range (Fig. 2). It was collected immediately north of Nimrod Glacier and ~1 km west of the Errant Glacier confluence from prominent outcrops characterized by thick tabular beds of feldspathic wacke and shale. The sample is a medium-grained (0.5–1.0 mm) arenite containing angular to sub-angular quartz, plagioclase, K-feldspar, and muscovite, with minor biotite, tourmaline, sphene, and crystalline calcite. Lithic grains include biotite-rich quartzite, fine-grained quartzite, coarse polycrystalline quartz, myrmekitic quartz, quartz–muscovite schist, matrix-supported wacke, slate, and crenulated slate. The composition and texture of the grains indicate a proximal source that includes granite, metasedimentary rocks, limestone, and older siliciclastic rocks. Eleven muscovite grains yielded ages ranging from ~515 to 480 Ma (Fig. 8B), with a weighted mean age of 502 ± 8 Ma, but significant scatter (χ2 = 7.8). One other grain (2) with a small 39Ar release, low radiogenic 40 Ar content, and a high Ca/K ratio yielded an imprecise age of 369 ± 143 Ma, likely due to alteration and/or contamination effects. This result is excluded from the population and is not shown on Figure 8. As in sample SRGm, the ages are bimodal, with seven grains yielding a weighted mean age of 486 ± 4 Ma (excluding grain 2) and four a weighted mean age of 510 ± 3 Ma. The former muscovite ages are younger than a crosscutting intrusion and probably reflect partial resetting of older detrital grains. If argon loss is minimal in the four oldest grains, then they indicate a Middle Cambrian or younger depositional age. This sample was not included in the detrital zircon study. Sample DSGm was collected from an exposure of thin-bedded sandstone and shale near the crest of Dolphin Spur, ~12 km east of Beardmore Glacier and Mount Patrick (Fig. 2). These rocks show trough cross-beds, thin graded channel deposits, desiccation cracks, and small (~1 cm) burrows, indicating a shallow marine depositional setting. The sample is a fine-grained (≤0.5 mm) quartz graywacke containing quartz, plagioclase, muscovite, and lithics (polycrystalline quartz, slate, and musGoodge et al. [Fig. 7] covite schist), with minor Fe-Ti–oxide and tourmaline. Eleven
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115
Siliciclastic record of rapid denudation A
B
CBGm, Starshot Fm. Cambrian Bluff
SRGm, Starshot Fm. Softbed Ridges error bars are 2σ
error bars are 2σ 510 ± 3
518 ± 4
Relative probability
497 ± 4
486 ± 4
Detrital muscovite Mean = 507 ± 7 Ma n = 12; MSWD = 24
M 450
DM
IZ
Detrital muscovite Mean = 502 ± 8 Ma n = 11; MSWD = 7.8
M
M
DZ
M
500
550
600
C
450
DM
IZ 500
600
550
D
USFm, Starshot Fm. Mt. Ubique
DSGm, Starshot Fm. Dolphin Spur error bars are 2σ
error bars are 2σ
Relative probability
517 ± 3 503 ± 3
530 ± 5
Detrital muscovite Mean = 506 ± 7 Ma n = 12; MSWD = 9.4
Detrital muscovite Mean = 521 ± 6 Ma n = 11; MSWD = 4.6 DM DM 450
500
DM
Age (Ma)
DZ
DZ 550
600
450
500
Age (Ma)
550
600
Figure 8. Single-grain 40Ar/39Ar laser probe fusion ages of detrital muscovites in samples of Starshot Formation sandstone. Upper panel for each sample shows individual grain analyses, with the 2σ error of each analysis (see Table 2). Lower panel shows relative probability distribution of all analyses in each sample, with ages of peaks indicated. Vertical gray bar indicates mean age of entire muscovite population, and dashed lines indicate ages of other events documented elsewhere in this paper. DM—detrital muscovite age mean; DZ—detrital zircon age mean; IZ—crosscutting igneous zircon age; M—metamorphic age from slate or neoblastic biotite. Some analyses in samples CBGm and DSGm had large errors due to alteration or contamination and were excluded (see text).
muscovite grains yielded ages ranging from ~545 to 501 Ma (Fig. 8C), with a weighted mean age of 521 ± 6 Ma, but significant scatter (χ2 = 4.6). One other grain (1) produced little 39Ar, low radiogenic 40Ar, and yielded an imprecise age of 597 ± 541 Ma, attributed to alteration or contamination. It is excluded from the population and is not shown on Figure 8. As in the other samples, the mica ages are bimodal, but both peaks are substantially older than 500 Ma: five grains yielded a weighted mean age of 532 ± 8 and six grains an age of 516 ± 5 Ma. These results are compatible with other age constraints and there is no clear evidence of thermal overprinting. Assuming minimal argon loss, the younger group places the tightest constraint on the time of deposition, indicating a middle Early Cambrian or younger age.
This is consistent with the older limit provided by detrital zircon (Table 1); excluding two grains that have probably lost Pb, the youngest detrital zircon subpopulation has an age of 547 ± 12 Ma (Goodge et al., 2002). Sample USFm was collected from the Starshot Formation near Mount Ubique (Fig. 2). The formation here contains shale and tabular interbeds of sandstone with sedimentary structures Goodge et al.currents [Fig. 8] that indicate shallow-water, wave-modified turbidity (Myrow et al., 2002a). Sample USFm, from an ~1 m thick massive sandstone bed, contains angular to subrounded quartz and feldspar (mostly plagioclase) with minor muscovite, biotite, tourmaline, crystalline calcite, and lithic grains (fine-grained quartz-muscovite schist). Thin, abraded muscovite flakes and
116
J.W. Goodge et al.
calcite grains suggest a proximal source. These, along with coarse monocrystalline quartz, plagioclase, and tourmaline, suggest a provenance that includes granite, limestone, and schistose metamorphic rocks. Twelve muscovite grains yielded ages ranging from 524 to 485 Ma (Fig. 8D), with a weighted mean age of 506 ± 7 Ma, but significant scatter (χ2 = 9.4). Excluding the youngest and two oldest grains, the remaining nine grains have a weighted mean age of 503 ± 3 Ma (χ2 = 1.2). Assuming minimal argon loss, these grains suggest a late Middle Cambrian or younger depositional age, consistent with the sample’s higher stratigraphic position and the fact that it contains the youngest detrital zircon (501 ± 5 Ma; Table 1). The similarity in detrital muscovite ages from these four Starshot samples suggests that they are from broadly coeval depositional units, despite minor sedimentary facies differences within the formation. Although there is evidence of argon loss for some detrital muscovites, the consistent presence of ca. 520 to 505 Ma muscovite in the different samples collectively indicates middle Early Cambrian or younger deposition ages (Table 1), in line with faunal ages from the basal part of the upper Byrd Group (ca. 515 Ma).
POST-DEPOSITIONAL AGE CONSTRAINTS The ages of crosscutting igneous rocks and metamorphic minerals in the Starshot Formation place upper limits on deposition age, thereby constraining the duration of sedimentation. Crosscutting Intrusions Two crosscutting igneous intrusions were dated by SHRIMP zircon U-Pb to provide younger limits for the depositional age of the Starshot Formation. In the Softbed Ridges area (SRG in Fig. 2), the Starshot is intruded by medium- to coarse-grained hornblende-pyroxene quartz gabbro. The intrusive is small (<400 m across) with a sharp western contact where the gabbro has a diabasic texture and a gradational eastern contact with hornfelsic sandstone and calc-silicate layers containing veins and apophyses of diabase. The coarser interior phase contains xenoliths of metaarenite and argillite, and it shows no evidence of post-crystallization deformation. Zircon was analyzed from gabbro sample 98-207A, collected near the margin of the intrusion (Table 3). The sample contains coarse, concentrically zoned clinopyroxene
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Siliciclastic record of rapid denudation 0.0605
A
98-207A quartz gabbro Softbed Ridges
207Pb
/ 206Pb
0.0595
0.0585
530 520
0.0575
510
500
0.0565
0.0555 11.6
0.064
/ 206Pb
0.060
207Pb
0.062
0.058
490
480
Weighted mean 206Pb/238U age = 504 ± 4 Ma MSWD = 1.09 1.8
12.0
12.2
2.4
B
12.6
2.8
13.0
to 9.1
98-242 granite aplite dike Cambrian Bluff
540
520
500
480
Age (Ma)
540
520
500 480
0.056 206Pb/238U
0.054 11.2
Weighted mean age = 494 ± 5 Ma MSWD = 0.39 1.6
12.0
12.4
238U
206Pb
/
2.8
3.2
Figure 9. Tera-Wasserburg plots of U-Pb zircon ages obtained from igneous units crosscutting Starshot Formation sandstones. General location of samples correspond to SRG and CBG as shown in Figure 1, although intrusive rock units are too small to show at that scale. A: Quartz gabbro intruding sandstone at Softbed Ridges (sample 98207A), with interpreted crystallization age of 504 ± 4 Ma from 16 zircon analyses (Table 3). B: Aplitic granite dike intruding sandstone and shale at east end of Cambrian Bluff (sample 98-242), showing two age subpopulations (see inset histogram). Separation of the ages into two populations indicates an early crystallization stage at 514 ± 5 Ma (weighted mean of 12 analyses indicated by white error ellipses) and a final crystallization age of 494 ± 5 Ma from the weighted mean of 8 zircon analyses comprising the younger population (gray error ellipses). Data presented in Table 4.
117
At the east end of Cambrian Bluff in the southern Holyoake Range (Fig. 2), folded carbonate of the Shackleton Limestone and black shale, argillite, and sandstone of the structurally underlying, but stratigraphically younger, Starshot Formation are intruded by leucocratic granite dikes up to 100 m wide (Fig. 4). The dikes have sharp contacts, fine grain size, light color, and massive texture. Sample 98-242 is from a muscovite-bearing aplitic granite dike that crosscuts black shale and sandstone sampled as CBG. Although internally massive, it becomes platy within ~2 m of its contacts, probably a result of flow while chilling. There is no textural evidence of significant subsolidus deformation, so the dike is interpreted to be post-tectonic. Zircons in the aplite are elongate euhedral crystals. CL images show that the grains have a mostly simple magmatic internal zonation; however, some of the central areas have discordant zoning that suggests an earlier phase of magmatic crystallization. Twenty zircon grains were analyzed for U-Th-Pb (Table 4, Fig. 9B). Their U and Th contents are normal for zircon from an unfractionated granitic rock. One grain (9.1) shows minor inheritance, but the other analyses are mostly concordant within error. They are not uniform in radiogenic 206Pb/238U, but the scatter is low (χ2 = 3.1). The dispersion is not simply due to Pb loss from one or two grains, but to the fact that the compositions are bimodal, as shown by the histogram in Figure 9B. There is no correlation between apparent age and U, Th, or Th/U. The weighted mean 206Pb/238U ages of the two populations are 514 ± 5 (n = 12) and 494 ± 5 (n = 8) Ma, respectively, including uncertainty in the Pb/U calibration. Assuming no pervasive Pb loss, the younger population gives the best estimate of the age of emplacement. The older population possibly records an earlier stage of magma genesis. The age of the aplite therefore limits deposition of the Starshot Formation at Cambrian Bluff to no younger than Late Cambrian. Combined with paleontologic evidence that siliciclastic deposition began at ca. 515 Ma (Myrow et al., 2002b), these igneous ages indicate that the Starshot Formation in the area of the Holyoake and Queen Elizabeth ranges is middle Early Cambrian to Late Cambrian in age. If the aplite sampled is cogenetic with other leucocratic dikes in the Cambrian Bluff outcrop that crosscut both the Shackleton and Starshot rocks, the two formations must also have been juxtaposed prior to ca. 495 Ma. Metamorphic Slate and Biotite
mantled by hornblende in a subophitic texture with interstitial quartz and plagioclase. Plagioclase and pyroxene are pervasively altered to calcite, biotite, chlorite, and epidote. The zircons are slender, prismatic grains with a simple, zoned magmatic internal structure as seen in CL imaging. They have moderate to high U and Th contents, yet all 16 analyses are concordant and have the same radiogenic 206Pb/238U within error (Fig. 9A). The weighted mean zircon age, including the error in the Pb/U calibration, is 503.5 ± 4.2 Ma. This is interpreted as the emplacement age of the gabbro. The gabbro crosscuts the sandstone sampled as SRG, constraining the deposition age to no younger than Middle Cambrian.
We determined 40Ar/39Ar ages for whole-rock slate and metamorphic biotite from samples of the Starshot Formation inGoodge order to determine the ages et al. [Fig. 9] of post-depositional metamorphism and cooling. The 40Ar/39Ar method can be applied to lowtemperature metamorphic rocks such as slates, although sometimes irradiation produces recoil loss and/or redistribution of 39 Ar in very fine-grained mineral components, resulting in discordant 40Ar/39Ar age spectra. We collected two slate samples from the eastern end of Cambrian Bluff (CBG in Fig. 2), where overturned beds of Starshot argillite, sandstone, and black shale show incipient formation of slaty cleavage, indicating lower
118
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greenschist-facies metamorphism (chlorite- to biotite-zone). Sandstone beds here show convolute bedding, load casts, and ripple-drift cross-beds, as well as graded bedding. Some of the coarse sandstone beds contain quartz granules, clasts of finegrained quartzite, and black shale clasts. Although we refer to some strata and clasts in sandstone as shale, they have macroscopic parting surfaces indicating incipient slaty or phyllitic mineral growth. Sample CBGs1 consists of small, very fine-grained slate clasts (<3 cm) collected from a sandstone bed at the base of the exposure next to Nimrod Glacier. The clasts are interpreted as forming an intraformational conglomerate because they are concentrated within certain beds and oriented generally parallel to the principal composition plane. The chips contain neoblastic muscovite, biotite, and less common chlorite; the phyllosilicates are oriented parallel to compositional layering and elongation of the chips, but small ellipsoidal opaque minerals delineate a grain-shape alignment at a small angle to that foliation, suggesting a component of shear during recrystallization. The results of conventional step-heating analysis of the chips are listed in Table 5; two sets of data were collected, one at the Australian National
University and the other at the University of Melbourne. Results of the second set of analyses are shown in Figure 10A. The stepheating data define a discordant, sigmoidal to saddle-shaped age spectrum common in fine-grained recrystallized materials that contain multiple mica components and have undergone minor recoil redistribution and/or loss of 39Ar (e.g., Fergusson and Phillips, 2001). The younger ages obtained at low temperature are probably due to argon loss, and older ages from the high-temperature steps probably come from an older detrital component. The release spectrum includes a five-step intermediate age “plateau” (representing 43% of the 39Ar released) of 492 ± 4 Ma. If the sample has undergone only recoil redistribution of 39Ar, then the age spectrum indicates an age of ca. 490 Ma for new metamorphic white mica crystallization in the slate. If the sample has suffered some recoil loss of 39Ar, then the age of metamorphism might be slightly younger (480 Ma). These ages post-date the older group of detrital muscovites from the Starshot sandstone sample at the same locality (CBGm, 510 ± 3 Ma, Fig. 8B), but they are indistinguishable from the ages of the younger muscovite grains (486 ± 4 Ma) and the crosscutting aplite dike (494 ±
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J.W. Goodge et al. 600
600
A
SRGb Starshot Fm., Softbed Ridges neoblastic biotite
500
Age (Ma)
450
Plateau age = 492 ± 4 Ma (2σ) MSWD = 1.7 39 Includes 42.8% of Ar
400 0.0 600
0.2
B
0.4
0.6
0.8
Plateau age = 488 ± 4 Ma (2σ)
400
MSWD = 1.04 Includes 55.8% of 39Ar
300
1.0
CBGs2 (98-238) Starshot Fm., Cambrian Bluff whole-rock slate
550
Age (Ma)
A
500
200 0.0
0.2
0.4
0.6
0.8
1.0
600
500
B
450
Plateau age = 485 ± 4 Ma (2σ)
400
0.0
0.2
0.4
0.6
0.8
CBGb Starshot Fm., Cambrian Bluff neoblastic biotite
500
MSWD = 1.6 39 Includes 51.6% of Ar 1.0
Age (Ma)
Age (Ma)
550
CBGs1 (98-233C) Starshot Fm., Cambrian Bluff whole-rock slate chip in graywacke
Plateau age = 481 ± 4 Ma (2σ)
400
MSWD = 1.19 Includes 63.2% of 39Ar
39
Cumulative Ar fraction
Figure 10. Ar release spectra from step-heating analyses of whole-rock slate samples obtained in the Starshot Formation at the east end of Cambrian Bluff (Fig. 2). Both samples show sigmoidal, saddle-shaped spectra with some inherited older age components, which reflect the competing influences of argon loss, argon recoil loss/redistribution, and incomplete resetting of detrital components. Plateau ages were assigned for the steps indicated in black, although the age of metamorphism may be slightly older (see text). A: Sample CBGs1 (98-233C), consisting of slate chips separated from a poorly-sorted graywacke, yielded a plateau age of 492 ± 4 Ma (2σ error). B: Sample CGBs2 (98-238), a coherent laminated slate, yielded a plateau age of 485 ± 4 Ma (2σ error).
5 Ma). We interpret the age obtained from the slate chips as an in situ metamorphic age because the chips most likely have an origin as shale intraclasts. The age of ca. 490 Ma therefore dates the maximum age of metamorphic recrystallization for CBGs1 during incipient growth of slaty minerals. Coherent, laminated black shale (sample CBGs2) was collected from an interval of interbedded Starshot shale, sandstone, and shale-matrix diamictite on top of the ridge overlooking Nimrod Glacier at the east end of Cambrian Bluff. The release spectrum from this sample is also sigmoidal (Fig. 10B), with the low-temperature steps reflecting Ar loss and the high-temperature steps affected by outgassing of coarser detrital micas. The age spectrum includes a five-step age plateau (representing 52% of the 39Ar released) with a weighted mean age of 485 ± 4 Ma. Assuming minimal recoil loss of 39Ar, we interpret the age of ca. 485 Ma as approximating the maximum age of metamorphic crystallization. The 40Ar/39Ar ages for slate formation in the
300 0.0
0.2
0.4
0.6
0.8
1.0
Cumulative 39Ar fraction
Figure 11. Ar release spectra from step-heating analyses of neoblastic biotites separated from Starshot Formation sandstone at Softbed Ridges and Cambrian Bluff (Fig. 2). Biotite is texturally fine-grained and evenly distributed in the sandstones and is interpreted as formed during greenschist-facies regional metamorphism associated with the Ross Orogeny. Plateau ages assigned for the steps indicated in black. A: Sample SRGb yielded a plateau age of 488 ± 4 Ma (2σ error). B: Sample CBGb yielded a plateau age of 481 ± 4 Ma (2σ error).
Starshot Formation therefore suggest deformation and low-grade metamorphism of these rocks by earliest Ordovician time. 40 Ar/39Ar step-heating ages were also determined for neoblastic biotite from two of the Starshot sandstone samples for which detrital minerals were dated, at Softbed Ridges (SRGb) and Cambrian Bluff (CBGb). The results are listed in Table 6 and Goodge et 11. al.Sample [Fig. 10] shown in Figure SRGb yielded a slightly discordant age spectrum (Fig. 11A), but with a plateau age of 488 ± 4 Ma (representing 56% of the 39Ar released). Sample CBGb gave a well-defined plateau from 11 increments (Fig. 11B), representing 63% of the 39Ar released and yielding an age of 481 ± 4 Ma. Together, these ages indicate that Starshot strata in the region experienced biotite-zone metamorphism prior to ca. 490 and 480 Ma, constraining deposition to earliest Ordovician or older. Biotite-zone metamorphism in these rocks was associated with a combination of late orogenic deformation and regional magmatic heating (e.g., Goodge, 1997).
G
Siliciclastic record of rapid denudation DISCUSSION Zircon and Muscovite Provenance
Detrital muscovites n = 34 X = 541 ± 5
Detrital zircons n = 122
X = 517 ± 5
Relative probability
Compared to ages of detrital zircon, the detrital muscovites in the Starshot Formation sandstones are uniformly young (580–480 Ma), indicating a proximal provenance with a dominant young source cooling age. Overlap between the youngest apparent ages of detrital muscovite and crystallization ages of nearby igneous intrusions could be interpreted as indicating thermal overprinting of the younger muscovite grains. However, it has been demonstrated that detrital muscovites remain closed systems for argon diffusion even under mid-greenschist facies conditions (e.g., Dunlap et al., 1991). As there is evidence that some grains have undergone variable alteration (CBGm grain 2; DSGm grain 1), it is more likely that the younger muscovite ages result from minor argon loss (<5%) caused by alteration and/or deformation processes. Omitting the analyses of muscovites that were altered or thermally overprinted yields an age distribution with a major peak at ca. 515 Ma (Fig. 12), composed of discrete subpeaks at 509 ± 3 and 518 ± 2 Ma that were contributed by younger and older grains, respectively. Within a given sample, however, the detrital muscovite cooling ages have a much smaller range. In contrast to detrital zircon, none of the detrital muscovites in the Starshot samples predates igneous and metamorphic
400
500
600
700
Age (Ma) Figure 12. Relative probability distribution of detrital zircon and muscovite ages obtained from the Starshot and Douglas formations, restricted to grains ≤600 Ma and excluding those muscovites interpreted to be affected by thermal resetting between 490–480 Ma. Weighted means of the two populations indicated by dashed dark gray bars. Note the similar form of the age distributions obtained for zircon and muscovite, with a steep curve toward the probability maximum followed by a trailing curve toward younger ages. The zircon distribution appears smoother because of a larger sample set. The two populations are offset by ~24–40 m.y., as indicated by the weighted means and maxima in the distributions.
121
rocks associated with the Ross Orogen (≤540 Ma). Further, most of the Ross-age detrital zircon is older than most of the muscovite (Fig. 12). Both minerals have age distributions skewed toward young ages and tailing toward old, but the maxima are offset by ~40 m.y. The difference between the muscovite and zircon age distributions is attributable to several factors: (a) muscovite is less robust mechanically than zircon and has a shorter average lifetime during long-distance transport or recycling (Kowalewski and Rimstidt, 2003); (b) muscovite is less robust chemically than zircon and can be altered by prolonged weathering (Goldich, 1938; Robertson and Eggleton, 1991; Elliott et al., 1997; Kowalewski and Rimstidt, 2003); and (c) muscovite, having a lower closure temperature than zircon, is more susceptible to isotopic resetting. Therefore, erosion of a thermally modified, polyphase metamorphic terrain, such as the nearby Nimrod Group, could yield zircons with a wide range of ages but muscovites with Ar cooling ages that are younger than the last metamorphic event that reset older muscovites or that caused growth of new muscovites (e.g., Goodge et al., 1993b; Goodge and Dallmeyer, 1992, 1996; Goodge and Fanning, 1999; Goodge et al., 2001). A notable observation is that the detrital muscovite ages, within individual samples, are bimodal (Fig. 8). This age bimodality supports the interpretation that there is a mix of variably altered grains within each sample, with the older results representing source ages and the younger ages attributable to alteration-induced argon loss. If all grains had suffered partial Ar loss, a continuum of ages would be expected. Because there are no detrital muscovite grains with ages older than known Ross events, in contrast to detrital zircons, we interpret them as discrete populations of detrital muscovite with a source in the Ross Orogen itself. A tendency to bimodality is also seen in the Rossage detrital muscovite ages from southern Australia (Turner et al., 1996). Given that muscovite is uncommon as a volcanic phase and that most of the Granite Harbour intrusives contain biotite and/or hornblende, the primary source of detrital muscovite is most likely to be older high-grade metamorphic rocks of the orogen (Nimrod Group or equivalent basement). The presence of minor tourmaline grains in the Starshot and Douglas formations is consistent with the erosion of nearby Granite Harbour plutons, the peraluminous phases of which contain tourmaline (Gunner, 1976; Borg et al. 1990) and pelitic units of the amphibolite-facies Nimrod Group. Tectonic Implications Our present understanding of the timing of depositional, deformational, igneous, and metamorphic events pertaining to the upper Byrd Group in the central Transantarctic Mountains is illustrated in Figure 13. Stratigraphic evidence from the Holyoake Range indicates that siliciclastic deposition in response to the onset of Ross deformation commenced ca. 515 Ma (Myrow et al., 2002b). Although supracrustal deformation in the region is diachronous (Rowell et al., 1992; Goodge et al., 1993b; Goodge, 1997; Encarnación et al., 1999), this phase of tectonic
122
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movement, which is biostratigraphically well-constrained, is the earliest documented along the length of the orogen in Antarctica. Detrital mineral ages from throughout the group record molasse deposition over a period of at least 25 m.y. from Botomian to Early Ordovician time. Weighted mean ages of discrete detrital muscovite populations provide the best constraint for maximum depositional age of the Starshot samples that we analyzed, which are estimated to be between ca. 520 and 505 Ma (or middle Early Cambrian to late Middle Cambrian). In other areas of the Transantarctic Mountains, deposition persisted to Early Ordovician time (Fig. 13; Goodge et al., 2002). Combining the maximum depositional age of 515 Ma with constraints from crosscutting igneous units (≤508 Ma) and metamorphic cooling ages (≤492 Ma) discussed here yields a maximum period of sedimentation of between 7 and 25 m.y. (Fig. 13). Within the framework allowed by these biostratigraphic and crosscutting age constraints, the unimodal, young populations of detrital zircon and muscovite
thus show that there was only a short time interval between formation of the youngest source rocks, regional deformation, and late-orogenic igneous intrusion, suggesting rapid erosion and short transport distances. Age data from geographically separated areas show that deposition was regionally diachronous and that events were more closely spaced in certain areas. For example, ages of crosscutting igneous units at Softbed Ridges and Cambrian Bluff indicate that deposition of sediment in those areas ended no later than ca. 500 and 489 Ma, respectively. Likewise, 40Ar/39Ar ages from metamorphic slate and biotite in these areas restrict deposition to older than ca. 484 and 488 Ma, respectively. The post-depositional age constraints indicate that the Starshot Formation in these two areas is earliest Ordovician or older, yet comparing the mean detrital muscovite ages with the crosscutting age constraints also provides an estimate for the duration of sedimentation. In the Softbed Ridges area, detrital muscovite cooling ages and the
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Siliciclastic record of rapid denudation
E. Ordovician
510
530 535 540
Starshot Fm.
Holyoake Fm.
6
22
?
post-deformation cooling & intrusion metamorphism
25 m.y. (max)
8
7 m.y. (min) onset of deformation
shale Shackleton Ls.
Early Cambrian
525
24
Other areas
intrusion
Bot.
515 520
molasse deposits
Toy.
Age (Ma)
505
Late Cambrian
500
?
?
M. Cambrian
495
Dolphin Spur
?
Adt.
490
Cambrian Bluff
platform carbonate
Tomm.
485
Softbed Ridges
Nem.-Daldyn.
480
Mt. Ubique
Douglas Conglomerate Cape Selbourne Holyoake Range
Stratigraphic constraints
detrital zircon
igneous zircon
metamorphic biotite
detrital muscovite
metamorphic muscovite
whole-rock slate
Figure 13. Diagram summarizing available age constraints on the timing of upper Byrd Group siliciclastic deposition in the areas discussed in text. General stratigraphic constraints as shown in Figure 3. Onset of siliciclastic deposition marked by shale of Holyoake Formation, at ca. 515 Ma (Myrow et al., 2002b), followed by molasse deposits of Starshot Formation; initial deposition related to a contractional pulse of supracrustal deformation in the region. Synorogenic deposition is also recorded by carbonate-clast conglomerate of the Douglas Conglomerate, but not shown here. Youngest populations (including weighted mean ages and uncertainties) of detrital zircon and muscovite from Starshot sandstone samples are indicated by black and gray bars, respectively. Texturally coarse muscovite with younger ages interpreted as a result of thermal resetting (horizontal ruling). Also shown are ages of crosscutting igneous rocks (checkerboard), metamorphic slate ages (white), and ages of neoblastic biotite (vertical ruling), which indicate the timing of post-deformation cooling and intrusion. Horizontal dashed lines indicate likely minimum and maximum durations, 7 and 25 m.y. respectively, of molasse deposition in the upper Byrd Group forearc basin, as constrained by the detrital mineral and post-depositional ages shown. Age data from individual areas differ (as shown by brackets for Softbed Ridges and Cambrian Bluff areas), constrained by crosscutting relationships and post-depositional metamorphic growth; range of possible deposition ages in each area indicated by stipple pattern. Detrital zircon data from other Byrd Group units (Douglas Conglomerate and Dick Formation) suggest deposition continued regionally into the Ordovician. Goodge
age of crosscutting gabbro bodies restrict sedimentation to 6–22 m.y., depending on treatment of uncertainties (Fig. 13). For the Starshot Formation in the area of Cambrian Bluff, the detrital muscovite and aplite dike ages similarly restrict deposition to an 8–24 m.y. period. More data will refine these estimates, but it can already be concluded that individual units within the molasse succession were deposited relatively rapidly. It appears, therefore, that siliciclastic deposition was triggered soon after the onset of regional deformation and that it occurred in brief pulses over a period of up to 7–25 m.y. Recent studies in the Ross Orogen show that most of the siliciclastic material previously assumed to be Neoproterozoic in age is in fact syn- to late-orogenic (Millar and Storey, 1995; Ireland et
al., 1998; Rowell et al., 2001; Goodge et al., 2002). By applying different mineral chronometers, the upper Byrd Group sandstone units document a short time lag of only a few million years between onset of deformation and active erosion. A similar conclusion was reached by Turner et al. (1996), who found that Ar-Ar ages of detrital muscovites in the early Paleozoic flysch of southeastern Australia are indistinguishable from their deposition ages (505–485 Ma), implying that the flysch was deposited at essentially the same time that the sediment source, the RossDelamerian orogen, was exhumed, and that exhumation was extremely rapid (5–15 mm/yr). Rapid sedimentary response to tectonism is well documented in other, younger orogenic belts. In the modern Hima-
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laya, detrital zircons obtained from the contemporary Indus River yield fission-track ages of only a few million years (Cerveny et al., 1988). Furthermore, Siwalik Group sandstones in the Himalayas contain zircons that were only a few million years old at the time of deposition, suggesting that high denudation rates were maintained since 18 Ma. Likewise, Early Cretaceous forearc deposits in Baja California reflect rapid unroofing of the Peninsular Ranges batholith and its associated arc basement rocks in a continental-margin setting (Busby et al., 1998; Kimbrough et al., 2001). Here, stratigraphic and geochronological data document denudation rates of ~1 mm/yr that are related to intra-arc deformation and magmatism, which in turn yielded forearc sedimentation rates of 1000 m/m.y. over a 10–15 m.y. period. A short time lag between the age of the principal source rocks in the Peninsular Ranges and the age of deposition implies that erosion and rapid forearc sedimentary accumulation rates were driven by a combination of tectonic and magmatic intra-arc thickening. Similar linkages were inferred for the Sierra Nevada–Great Valley arc-forearc system (Linn et al., 1992). The detrital mineral ages and the ages of crosscutting or overprinting events limits the exhumation history of the igneous and metamorphic source rocks, as well as subsequent molasse deposition, to a short time period. This suggests rapid denudation rates, which can be evaluated by comparing discordant cooling ages obtained from different mineral chronometers. Although this approach is applicable in some sedimentary systems, in this case we cannot be certain of the specific geological source or crustal level from which the detrital minerals were derived. Regional orogenic cooling rates can be obtained, however, from the adjacent crystalline basement. U-Pb and 40Ar/39Ar cooling ages from different mineral chronometers in the Nimrod Group yielded a post-kinematic cooling rate of ~10 °C/m.y. for igneous and metamorphic rocks of the middle crust (Goodge and Dallmeyer, 1992, 1996). Using a metamorphic geotherm of ~25 °C/km, this inverts to a denudation rate of ~0.4 mm/yr, comparable in order of magnitude to modern convergent or collision belts (e.g., Harrison et al., 1992). For example, recent exhumation rates of 0.2–1.0 mm/yr are reported from the Himalayan, Alpine, and Andean orogenic belts (Zeitler 1985; Copeland et al., 1987; Burbank and Beck, 1991; Harrison et al., 1992; Gregory-Wodzicki, 2000; Bernet et al., 2001; White et al., 2002). By comparison, exhumation rates of 5–15 mm/yr proposed for the Ross-Delamerian orogen in South Australia (Turner et al., 1996) are extreme. Rapid unroofing in the axial Ross Orogen is consistent with the sharp stratigraphic transition observed within the upper Byrd Group of the Holyoake Range, which reflects severe syntectonic erosion, development of an unconformity on uplifted carbonate, and outpouring of clastic materials into marginal-marine molasse basins (Myrow et al., 2002b). If we estimated cooling rates using zircon and muscovite as detrital mineral chronometers, we would obtain values on the order of 10–30 °C/m.y., which are similar to cooling rates determined for the Nimrod Group and which imply similar denudation rates using the basement geotherm. If the muscovite ages are
partially reset and the detrital sources are actually older, or if the geotherm were cooler, the resulting denudation rates would be even faster. Cooling rates calculated from detrital mineral closure ages are highly dependent on grain-age sampling bias, assumed closure temperature, and determination that the minerals were derived from the same source rocks, among other factors. Having at present only two detrital mineral chronometers of uncertain specific source relationship is therefore insufficient to uniquely define cooling rates, but it is likely that there is a link between the basement and supracrustal successions with respect to uplift, cooling, denudation, and sedimentation. Additional detrital mineral age data (e.g., amphibole, feldspar, apatite, etc.) would provide additional constraints for the inferred denudation rates. CONCLUSIONS We argue that siliciclastic rocks of the upper Byrd Group represent a depositional response to rapid denudation in the developing Ross Orogen. These rocks are interpreted as forearc molasse deposits on the basis of their sedimentary structures, composition, transport direction, and provenance. In the context of regional tectonic relationships, they represent an “unroofing” succession, in which igneous and metamorphic rocks of the Ross magmatic arc, as well as older, structurally shortened passive-margin deposits, were erosionally inverted and deposited in forearc marginal basins. The entire episode of interrelated tectonism, denudation, sedimentation, deformation, and magmatism appears to have lasted for a period of 7–25 m.y. in the late Early Cambrian to earliest Ordovician. Along with evidence of left-oblique transpression along the Ross margin (Goodge et al., 1993a), the timing constraints indicate both erosional and tectonic controls on denudation. Because the short lag time between tectonism and sedimentation, as deduced from the available geochronological constraints, indicates a rapid denudational response to the orogenic process, crustal thickening produced by both magmatic intrusion and structural shortening appears to have been balanced in part by erosional exhumation. ACKNOWLEDGMENTS Field and laboratory work were supported by the National Science Foundation (OPP-9725426 and OPP-9912081). We thank the ANU Electron Microscopy Unit for assistance with CL imaging, and Shane Paxton, John Mya, and Sally Mussett for their excellent mineral separations. We also thank Keiji Miswa (NIPR) for assistance in collecting the SHRIMP U-Pb data. We are grateful for critical reviews by Matt Heizler and John Miller, who provided helpful insight that reshaped our interpretations. REFERENCES CITED Allibone, A., and Wysoczanski, R.J., 2002, Initiation of magmatism during the Cambro-Ordovician Ross Orogeny in southern Victoria Land, Antarctica: Geological Society of America Bulletin, v. 114, p. 1007–1018. Allibone, A.H., Cox, S.C., and Smillie, R.W., 1993, Granitoids of the Dry
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MANUSCRIPT ACCEPTED BY THE SOCIETY OCTOBER 21, 2003
Printed in the USA
Contents Introduction: Detrital thermochronology Matthias Bernet and Cornelia Spiegel
4.
on the inference of thermal history models from apatite fission-track data A synthetic data study Andrew Carter and Kerry Gallagher
Miocene siliciclastic deposits of Naxos Island: Geodynamic and environmental implications for the evolution of the southern Aegean Sea (Greece) J. Kuhlemann, W. Frisch, I. Dunk I, M. Kazmer, and G. Schmied!
5.
Fundamentals of detrital zircon fission-track analysis for provenance and exhumation studies with examples from the European Alps Matthias Bernet, Mark T. Brandon, John I. Garver, and Brandi R. Molitor
Detecting provenance variations and cooling patterns within the western Alpine orogen through 40 Ar~9Ar geochronology on detrital sediments: The Tertiary Piedmont Basin, northwest Italy B. Carrapa, J. Wijbrans, and G. Bertotti
6.
Siliclastic record of rapid denudation in response to convergent-margin orogenesis, Ross orogen, Antarctica John W. Goodge, Paul Myrow, David Phillips, C. Mark Fanning, and lan S. Williams
1. Characterizing the significance of provenance
2.
3.
Toward a comprehensive provenance analysis: A multi-method approach and its implications for the evolution of the Central Alps Cornelia Spiegel, Wolfgang Siebel, Joachim Kuhlemann, and Wolfgang Frisch
ISBN 0-8137-2378-7
9 780813 723785