Quantitative Geometry of Thrust and Fold Belt Structures (AAPG Methods in Exploration 6)

Quantitative Geometry of Thrust and Fold Belt Structures Peter B. Jones International Tectonic Consultants (ITC) Ltd. Calgary, Alberta

Published by The American Association of Petroleum Geologists Tulsa, Oklahoma, U.S.A.

Quantitative Geometry of Thrust and Fold Belt Structures Peter B. Jones International Tectonic Consultants (ITC) Ltd. Calgary, Alberta Principles of step-faulting are simulated in computer-synthesized and balanced structural cross sections of faulted and folded terranes. Isolation of geometry from mechanics and chronology allows a review of kinematics and chronology of deformation in layered rocks. Blind faults and duplex structures are geometric consequences of stepfaulting, and are responsible for disparities in crustal shortening between superposed tectonostratigraphic units. These disparities are greatest in fold belts, which overlie belts of blind thrusts. Failure to recognize blind thrusts leads to incorrect estimates of both timing and amount of deformation. Geometric modeling demonstrates the intuitive nature of some widely accepted geologic assumptions. Geometric considerations alone suggest that faults and folds, which can be represented as products of faulting, form in response to a balance between differential tangential and vertical stresses. Where tangential stress provides the driving force, distribution of overburden load appears to control location of thrust ramps. Forward modeling by computer generates balanced cross sections at all stages of growth of a given structure, and shows that purportedly balanced cross sections may include impossible intermediate stages. Forward modeling also reveals geologic questions that must be answered before more sophisticated computer modeling programs can be written.

BIOGRAPHY Following 25 years in petroleum and minerals exploration, Peter Jones founded International Tectonic Consultants Ltd. in 1980 as a geological consulting company specializing in exploration of areas of complex geological structure. Educated in Great Britain and the U.S.A., Dr. Jones has worked on deformed belts in Europe, Asia and the Americas. Author of more than 25 papers on structural geology and tectonics, he has given seminars in structural geology to oil company exploration staffs around the world, and has recently returned from a lecture tour of Chinese universities and research institutes. For the past six years he has been involved in the development of computer techniques for simulating the formation of geological structures.

most hand-balanced cross sections are geometric transforms in which a deformed stage is balanced with respect to an originally undeformed stage. This can be done even Exploration for petroleum requires much data, and the though it may be geometrically impossible to reach the principles with which to interpret them. Because well and deformed state from the undeformed one along the fault seismic data are extremely expensive in fold and thrust belts, paths specified. In contrast, forward modeling by computer application of simple geometric principles can multiply the simulates progressive movement along fault planes, so that effectiveness of geological and geophysical interpretation. each balanced cross section includes within it an infinite Geometry of layered rocks, expressed by maps, cross sec- number of balanced cross sections of intermediate stages. tions, and seismic profiles, is the basis for interpreting the All balancing involves approximations. This follows geological structure. from the fundamental geometric fact that it is not possible Rigorous geometric modeling can simulate many thrust to transform a plane figure with a given area and perimeter and fold belt structures; but even where it cannot, hitherto into another figure with the same area and perimeter withunconsidered factors may be highlighted. For example, the out passing through intermediate stages in which one or impossibility of synthesizing a balanced cross section to fit both parameters must change. Any balanced cross section the data may lead to the recognition of posr-tectonic gravity must involve distortion of one or more of the following: slides within a deformed belt. area, bed length, or fault cutoff. In drawing a balanced As more deep seismic-reflection data become available, cross section, a geologist judges (perhaps unwittingly) the thin-skinned style of deformation is becoming more which distortions to accept. important in interpreting geological structures on all scales. In the models used in this paper, cross-sectional area is In thin-skinned deformation, faults are listric, flattening constant. The computer program uses a grid containing a into a basal decollement. Depending on the scale involved, specified number of cells (usually 150,000), like the squares this base may be anything from an incompetent shale to the on a sheet of graph paper, and maintains that number durbase of the lithosphere. Geometric principles of thin- ing deformation so that the resultant cross section is areally skinned deformation in layered sedimentary rocks are out- balanced. The logic might be called thepack-of-cards logic, lined by Rich (1934), Douglas (1950), Fox (1959), with each card represented by a column of cells (Figure 2a). Dahlstrom (1969a,b; 1970), Bally et al. (1966), Royse et al. Lines representing stratigraphic boundaries are drawn on (1975), Elliott (1976), Boyer and Elliott (1982), Suppe the edge of the pack. Fault planes slice through the pack of (1983), and Laubscher (1985). This paper owes much to cards along or oblique to the bases of layers representing those authors. stratigraphic units. The user specifies the positions of Figure 1 is a sketch cross section through a typical thrust faults, and the amount and sense (compression or extenand fold belt. Structures include folds, thrusts, and listric sion) of movement (Figure 2b). Relative movement occurs normal faults, rooted in a basal decollement or detachment along fault planes and, on a very much smaller scale, zone, and overlain (in some cases) by an upper detachment between adjacent "cards" (which need not be vertical), like zone, all of which can be simulated by the Thrustbelt™ sys- cleavage planes. The pack-of-cards model maintains contem, a computer program for forward modeling of bal- stant vertical thicknesses and cross-sectional area. Stratianced cross sections, developed by Helmut Linsser. Models graphic thicknesses in a dipping sequence are reduced in generated by this program are described by Jones (1982, proportion to the cosine of the dip angle, while the bed 1984), and Jones and Linsser (1984). Charlesworth and length increases inversely with the cosine of the dip angle. Gagnon (1985) describe what appears to be a similar model- The distortion is systematic and can be corrected manually ing program. Most of the illustrations that follow were cre- if required. ated by the Thrustbelt Program. The program incorporates the following geologic criteria: 1. Thrust faults are emplaced in sequence from higher to Balancing Cross Sections by Computer lower, by progressive deformation of an undisturbed footwall (Elliott, 1976). They cut up-section in their direction of movement and may also follow bedding Balancing is an essential process in constructing cross secplanes. Normal faults cut down-section in their directions through folded and faulted terranes. A balanced cross tion of movement, or follow bedding planes. section is not necessarily correct, but an unbalanced one is 2. Faults do not cut earlier faults but may merge with wrong. them and transfer their movement to them. Cross-sectional area and bed length of each stratigraphic These definitions are dependent on the dip of fault planes unit, as well as fault offsets, are adjusted in balancing, so that the deformed structure can be restored back to an relative to bedding, and are independent of the absolute dip assumed undisturbed condition without gain or loss of of the fault planes. Sub-horizontal faults that cut steeply material (cross-sectional area). Offsets of boundaries across dipping strata are often referred to as thrusts, but as Perry a given fault should be consistent in all units. Balancing (1978) shows, they are extensional relative to the strata they does not mean that shortening must be the same at all levels cut. Using the thrust-fault symbol to designate these extenor in all deformed stratigraphic units; indeed one of the con- sional faults on geologic maps causes many problems in clusions of this paper is that equality and synchroneity of structural interpretation. shortening are extremely unlikely. Each computer run generates the entire cross section in a Computer balancing of cross sections does nothing that higher-to-lower sequence of fault emplacement. A lowercannot be done manually using pencil and paper. However, to-higher sequence can be simulated by successive computer

INTRODUCTION

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Figure 1. Typical thrust belt, showing thrust and listric normal faults, duplex structures, underthrust foreland margin. All these structures can be created by the forward modeling program described in the text. runs in which each new fault is added behind the preceding one. This is common operational practice. Because each fault is deformed by its successor, it is easier to model the last-formed fault first. Folding of layered rocks involves a mixture of concentric, similar, and kink-band styles. The pack-of-cards model generates quasi-similar folds in which flanks thin and bed lengths extend. This is a workable approximation in most cases and is systematic and reproducible. According to W. B. Perry (personal communication, October, 1983), "The idealized concentric fold with a constant radius of curvature (to an inflection point) is almost never present." There are many questions to be resolved with regard to geometry and kinematics before more sophisticated modeling becomes feasible.

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THRUST FAULTS Effects of Spacing of Incipient Thrust Faults Relative to Their Displacements The profile in Figure 3b is typical of many thrust belts. The surface geology features an irregular spacing and local concentration of faults, which a geologist might interpret as evidence for an underlying zone of weakness or basement buttress. However, the irregular thrust spacing in this model is due entirely to varied slipping of a uniformly spaced set of incipient thrusts (3a). While basement warping and faulting may have a significant influence in localized thrust ramping (Wiltscho and Eastman, 1983), basement warping and faulting may not be applicable to this style of thrusting, which involves large numbers of faults having no immediate connection with basement. In areas such as the southern Alberta foothills, where this structural style predominates (Ollerenshaw, 1978), it seems more likely that thrust ramping in the sedimentary cover is ultimately controlled by the distribution of overburden in the hanging wall sequence (Royse et al., 1975). This overburden may govern the distribution of high fluid pressure at the base of the overthrust mass (Gretener, 1972). Controlling ramp location by differential overburden loading is a feature of thrust emplacement at the bases of deltas and submarine fans (Evamy et al., 1978). Differential loading in those environments is caused by sedimentation (Mandl and Crans, 1981), and is

Figure 2. (a) Undisturbed section with incipient fault represented by "pack of cards" 1000 cards long, with stratigraphic units drawn on edge of pack, sliced along line of fault, (b) After displacement of upper part of pack along fault plane. Thicknesses of tilted beds in hanging wall are reduced in proportion to the cosine of their angle of dip. This model is also described as a vertical shear model. Oblique shear can be simulated by tilting the model.

unrelated to the underlying section. Although a more competent section may be involved in thrust and fold belt deformation, differential loading because of faulting is potentially much greater. Displacement Transfer (En Echelon Faults) Closely spaced incipient thrusts interact with each other. Figure 4 is a set of cross sections parallel to each other along the strike of a thrust belt. Aggregate slip is the same in all profiles and slip is progressively transferred from fault A to fault B in successive profiles along strike. In Profile 1, thrust A moves to create a ramp anticline. Along strike, as fault B develops and A dies out, the ramp anticline and the culmination of the surface structure reach maximum amplitude in Profile 4 where both A and B are only partly developed. At that stage, thrust A has a slip equal to the spacing between it and thrust B, measured along a bedding plane. This causes the leading edge of a given stratigraphic unit in

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Figure 5. (a) Incipient step thrusts, regularly spaced, (b) After movement. Slip of each thrust equals spacing divided by three, (c) After movement. Slip of each thrust equals spacing divided by two. Where slip matches spacing between incipient thrusts, separate ramp anticlines are stacked beneath each other.

the "A" sheet to lie directly above the leading edge of the same unit in the underlying " B " sheet, thus enhancing the earlier ramp anticline so that it has greater amplitude and smaller wavelength. Step Faults Fault planes that are parallel or subparallel to bedding surfaces for long distances (in the dip direction before cutting obliquely through other strata) have a stepped profile consisting of ramps and flats. The different angles of fault planes relative to bedding reflect variations in the competence and rigidity of the different stratigraphic units. Movement along fault planes, whether as thrusts or normal faults, generates ramp anticlines or fault-bend folds (Suppe, 1983). If the steps are widely spaced relative to the slip of individual thrust faults, a series of discrete ramp anticlines is formed (Figure 5a). As the spacing decreases or slip increases, the ramp anticlines merge. When slip equals spacing (Figure 5b,c), all ramp anticlines are stacked on top of each other, each one increasing the amplitude of the anticline overlying it. When slip exceeds spacing, this stacking relationship does not change significantly. Thus, given a layered sequence that is varied enough to generate step

faults, anticlines in thrusted terranes are geometrically inevitable. Thrust sheets commonly stack vertically. If ramping were basement-controlled, a regularly spaced arrangement of stacked thrusts would require a corresponding regularity of basement features in the footwall in order for each thrust to develop its ramp precisely beneath the ramp of the overlying thrust. It seems more reasonable that the hanging wall sequence is the main factor that controls the position for ramping of successive thrusts.

DUPLEX STRUCTURES In the previous models, the deeper parts of thrust belts were discussed and the shallower section removed by erosion was ignored. Do such thrusts reach the surface and become erosion thrusts? Do they steepen upward and die out in some shallower part of the section, as indicated by Boyer and Elliott (1982)? If so, how is their slip accommodated? There is good evidence that thrust faults do not reach the erosional surface, but are blind, merging with a preexisting, overlying bedding-plane thrust or upper detachment

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Figure 6. Generation of folds above thrust duplex. Top figure, a, shows the sequence before thrusting; bottom figure, b, shows postthrust configuration. Faults are numbered in order of emplacement. Duplex requires a long sector of bedding plane slip for Fault 1, although its slip may be small. Resultant slip equals initial slip of Fault 1 plus sum of the slips of 2, 3, 4, and 5.

zone forming a duplex structure (Thompson, 1981; Jones, 1982, 1984, 1985; Charlesworth and Gagnon, 1985). Figure 6 shows how a fault duplex structure develops by emplacement of a deeper thrust beneath an existing bedding-plane thrust. In this duplex, displacement is progressively transferred from the lower to the upper detachment, which folds in response to the intervening faulting. The presence of an upper detachment means that each thrust fault beneath it generates a ramp anticline where it flattens along the detachment surface. In this way, folds are generated above blind thrusts and may also be generated above blind normal faults (Gibbs, 1984; Jones, 1985). Dahlstrom (1970) uses the terms roof thrust and floor thrust for the upper and lower detachments, respectively. However, the term detachment is preferable because detachments are not necessarily discrete faults, but may be zones of complex deformation. Also, upper and lower detachments have been described in a similar context for fold duplex structures (Dahlstrom, 1969a). Boyer and Elliott (1982) state that the roof thrust of a duplex is initiated as a major thrust. This is not necessarily true. The only requirement for a roof thrust is that it must follow a bedding plane far enough that subsequent thrusts generated beneath it rise and merge with it, a condition that is independent of the amount of initial slip. Transfer of displacements from the underlying thrusts ensures that the slip along a roof thrust is not less than the sum of the displacements of the underlying thrusts that merge with it. In the Canadian Cordillera, the Lewis thrust is the lower detachment of a duplex capped by the Mount Crandell thrust in the Waterton area (Douglas, 1952; Boyer and Elliot, 1982), and by the Tombstone thrust a few kilometers to the northwest (Fermor, 1986). Thirty km further west, the

Lewis thrust forms the upper detachment for the Sage Creek duplex (Figure 7). Duplexes can stack on top of each other, and one thrust or horizon of contortion may function both as an upper and a lower detachment. Scale can vary greatly. Cooper et al. (1983) describe a duplex that is a few meters in thickness. Teal (1983) shows a number of duplexes stacked on top of each other at the eastern edge of the Canadian Cordillera, to a thickness of several thousand meters. Theoretical models of duplex structures by Boyer and Elliott (1982) and Charlesworth and Gagnon (1985) illustrate upper and lower detachments that are parallel to each other after fault movement. Geometrically, this condition demands a very precise and regularly recurring relationship between thrust spacing and thrust slip. Where this requirement is not met, the roof thrust and its hanging wall are folded by differential uplift. In practice, there are many examples of undeformed roof thrusts. Fault duplexes described by Dahlstrom (1970), Cooper et al. (1983), and Fermor (1987) include upper and lower detachments that are more or less parallel to each other. It appears that there is a particular combination of load, rigidity, and other aspects of the overlying sequence that prevents the thrusting within the duplex from deforming the overlying sequence. The same is true for fold duplexes in which the sequence above the folded section is essentially undeformed (see "Ideal Concentric Fold" in Figure 19). Hanging Wall "Drag Fold" The hanging walls of major overthrusts commonly include large folds. In some tectonic environments, these folds are a function of the slip and propagation rates of the

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Figure 7. Sage Creek duplex, Canadian Rockies. Lewis thrust forms the upper detachment. Flathead fault is a listric normal fault, downthrown to the west. Sediments in the half-graben are Eocene to Oligocene (from Bally et al., 1966).

Figure 8. Hanging wall fold formed by stacking of ramp anticlines within a duplex. Rhomb-shaped "horse" or "schuppen" between thrusts 1 and 2 doubles the amplitude of the final ramp anticline. This geometry also produces the out-of-the-syncline thrust that typically lies in front of a hanging wall fold. Computer simulation shows geometry (a) before thrusting, with the positions of subsequent thrust planes; (b) after the first thrusting event; (c) after movement of second thrust. Note the effect the horse has on the final configuration. underlying thrust (Williams and Chapman, 1983). It is important not to confuse folds formed in that manner with hanging-wall folds formed as a result of duplex development. Figure 8 illustrates a method of generating such folds through formation of a duplex, a process that may be more relevant to the deformation of layered sedimentary rocks.

The footwall sector that matches the bedding plane fault at X in the hanging wall occurs (Figure 8c), not in the undisturbed footwall at Y, but within the overthrust structure itself, at Z, where it forms the upper surface of the horse. In this model, the lower, younger fault (2) merges with the upper fault (1) at or close to the upper ramp of fault 1. This

coincidence happens often enough to suggest that it is controlled by influences above, not below, the level of deformation. Figure 9a is a cross section of the Livingstone thrust, southern Alberta foothills, Canada (Douglas, 1950). Figure 9b is similar to the restoration of the Livingstone thrust by Douglas (1950), except that the Gap fault has been included. North of the line of cross section, the Gap fault is folded

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over the crest of the Livingstone anticline, passing up through the Paleozoic and flattening in the overlying Jurassic Fernie Shale (Douglas, 1950). It appears that the Gap fault follows the Fernie bedding eastward, merging with the Livingstone thrust where it also flattens in the Fernie Shale section. In Figure 9b, both the Gap and Livingstone faults are elements of a fault duplex. In this way, the process of step-faulting can also account for creation of the hanging

RECONSTRUCTED FAULT PATHS —

Livingstone thrust Gap fault

Figure 9. (a) The Livingstone thrust, Alberta foothills, (b) Reconstruction of the profile of the incipient Livingstone thrust. This reconstruction is modified from the original (Douglas, 1950) by inclusion of the Gap fault, which creates the Livingstone Range anticline.

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Figure 10. Computer simulation of evolution of the Powell Valley anticline as a thrust duplex, (a) Undisturbed section before thrusting, showing positions of future thrust faults, (b) After emplacement of Wallen Valley thrust, (c) After emplacement of (proto-) Pine Mountain thrust, (d) After emplacement of Bales thrust, showing present configuration after erosion. Present Pine Mountain thrust, where it emerges northwest of the Middlesboro syncline, incorporates the combined displacements of the Wallen Valley, proto-Pine Mountain and Bales thrusts of the Powell Valley duplex, giving a total slip of about 35 km. Modified from Harris and Milici, cross section F-F', 1977.

9 wall anticline that forms the Livingstone Range, something that the original interpretation by Douglas (1950) failed to do. The same model can be applied to the Pine Mountain overthrust and Powell Valley anticline in the Appalachians. The Wallen Valley thrust (Figure 10) is analogous to the Gap fault (Figure 9). In this interpretation, slip on the Pine Mountain thrust west of the Middlesboro syncline is the

sum of slips of (from bottom to top) the Bales thrust, the Pine Mountain thrust exposed in fensters in the Powell Valley anticline, the Wallen Valley thrust, and possibly more. This gives a minimum displacement of 35 km for the resultant Pine Mountain thrust.

Unbalanced Cross Section, Utah-Wyoming Thrust Belt

Absaroka thrust

The cross section in Figure 1 la appears to be unbalanced. In this diagram, simplified from a computer-synthesized cross section, offset (X) of a Mesozoic marker bed by the Absaroka fault is much greater than the offset (Y) of the top of the Paleozoic. The cause of the anomaly is indicated by the profile of the next major fault to the west, the Crawford thrust (Figure 1 lb), which includes a dog-leg where its fault plane follows the bedding of a Mesozoic rock unit. Length of the dog-leg is equal to the difference in offset (X-Y) between the Mesozoic and Paleozoic markers, showing that Absaroka Crawford protothrust thrust after its initial emplacement, the Crawford thrust was offset Absaroka thrust by that amount along a bedding-plane thrust which extended several kilometers eastward to become the upper sector of the Absaroka thrust. Later movement, rooted in the Paleozoic, formed a duplex having the "protoAbsaroka" thrust in the Mesozoic section as its upper Figure 11. (a) Anomalous offsets across Absaroka thrust, Wyoming thrust belt. Slip X is much greater than slip Y. (b) Dog-leg in detachment. The geometry and kinematics are essentially Crawford thrust profile indicates Proto-Absaroka bedding-plane the same as in Figures 8, 9, and 10. The preceding models showed how individual folds were thrust with slip of (X-Y). This thrust forms upper detachment of duplex in which second stage of Absaroka thrust generates addi- generated above blind thrusts. On a larger scale, an entire tional slip. Total slip equals X (not to scale). fold belt may overlie a belt of blind thrusts (Figure 12). In

Figure 12. Computer-synthesized model of a fold belt overlying a thrust belt. Slip of thrusts is balanced by shallow back-thrust rooted in the upper detachment. In a molasse sequence, the back-thrust may be difficult or impossible to detect. Estimates of crustal shortening on the basis of fold shortening would be too low.

10 1982). Similar antithetic thrust faults also occur in the Kirthar and Sulaiman thrust belts of Pakistan, as well as in the southern Taiwan thrust belt (Banks and Warburton, 1986). If a fold belt is deeply eroded to the level of the underlying thrust belt, the mountain-facing flank of a foreland syncline, or lower limb of a frontal monocline, is the only remnant of the original fold belt. This results in a characteristic structure which Gordy et al. (1977) called a triangle zone; Butler (1982) subsequently applied the term to a totally different type of structure. Triangle zones as Gordy described them occur along the outer margins of fold and thrust belts around the world, wherever there is a foreland syncline

this model, cumulative slip of the blind thrusts is accommodated by an antithetic thrust dipping in the opposite direction. It is easy to underestimate the slip of such antithetic faults where they affect a molasse sequence in which fault repetitions are hard to detect (Figure 13). According to W. J. Hennessey (personal communication, September, 1985) the section overlying the upper detachment in the west flank of the foreland syncline in southern Alberta is much thicker than the equivalent section in the east flank, thickened by foreland-dipping antithetic thrusts similar to those in Figure 13. Antithetic thrust faults are well exposed in the Alberta foothills in the Athabasca River valley (Irish, 1962; Jones,

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Figure 13. Seismic profile through eastern margin of Canadian Cordillera, northern Alberta Foothills. Upper detachment is indicated by the dashed line. It follows a shale unit which rarely outcrops. The east-dipping thrusts above it are almost impossible to identify from well logs or outcrops. Without well control, this type of structure could easily be interpreted as a vertical uplift. Without seismic data, faults penetrated by the well might be interpreted as rising almost to the surface.

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Figure 14. Grease Creek structure, a typical triangle zone or frontal fold at the east edge of the Canadian Cordillera. Faults are exposed where erosion cuts below the upper detachment, giving the impression that faults are confined to the anticlinal core. This cross section is apparently unbalanced in the sense that there is greater shortening due to thrusting than to folding. It is not known whether there are east-dipping back-thrusts in the eastern part of the profile.

Figure 15. Computer-synthesized model of a triangle zone. Compare with Figure 14. It is a duplex structure in which the hanging wall of the upper detachment is autochthonous. The fold generated above the upper detachment migrates as it grows (from Jones, 1982).

11 (Jones, 1982). Because each thrust, at the time of its emplacement, also marked the outer margin of the deformed belt, remnants of triangle zones also occur within deformed belts, although these remnants are less well preserved. Figure 14 is a typical cross section through the triangle zone at the east margin of the Alberta foothills. In this cross section, as in the computer-synthesized model (Figure 15), shortening of the faulted sequence is much greater than shortening of the overlying folded sequence. In northeastern British Columbia, crustal shortening of the exposed section of folded Mesozoic rocks of the foothills belt is 10 km less than shortening in the underlying thrust-faulted Paleozoic section (McMechan, 1985). MeMechan describes the entire foothills belt as a "low-taper triangle zone." In some deformed belts, this type of imbalance is accommodated at least in part by back-thrusts rooted in the upper detachment

(Figures 12 and 13). In other areas such thrusts are unknown.

Relationships Between Fold and Thrust Belts The preceding examples suggest the generalized model of a fold belt overlying a thrust belt shown in Figure 16. Each erosional level exposes a different structural style, giving different impressions of the amount of crustal shortening involved. Only in the most deeply eroded version, in which all thrusts are exposed, could crustal shortening be correctly calculated from surface geology. At higher erosional levels, the calculated amount of crustal shortening may be too low. Thrusting and/or back-thrusting that occurs within the fold belt may be difficult to recognize.

Foreland syncline

Foreland syncline

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Foreland syncline

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Figure 16. (a) Fold belt overlying thrust belt, showing changes in structural style with different erosion levels. Diagrammatic (modified from Jones, 1982). (b) Level 1: Fold belt exposed, foreland syncline present. Examples: British Columbia foothills, Canada; southern Oman fold belt. Level 2: Fold belt with narrow faulted anticlines (style "ejectif" of Dahlstrom, 1970). Examples: Folded Molasse, Switzerland-Germany, Central Alberta foothills. Level 3: Thrust Belt with underthrust foreland margin. Examples: Southern Alberta foothills (see also Fig. 19). Level 4: Thrust belt with overthrust foreland margin, no foreland syncline. Example: Utah Wyoming overthrust belt, U.S.A.?.

12

STRUCTURAL EVOLUTION OF DEFORMED BELTS BY BLIND THRUSTING The foreland margin of the southern Canadian Cordillera is formed by blind thrusts merging with an upper detachment (Gordy and Frey, 1975; Jones, 1982). In the normal higher-to-lower sequence of thrusting (Elliott, 1976), each thrust, at the time of its emplacement, cuts undisturbed foot wall at the leading edge of the thrust belt. There is no obvious reason why intermediate leading edges of a deformed belt should behave any differently from the final one, which suggests that an entire thrust and fold belt can be formed by this mechanism (Jones, 1982; 1984a; CharlesworthandGagnon, 1985). In contrast, Butler (1985) proposes that blind thrusting at a foreland margin marks the "last gasp" of tectonic activity, and Morley (1986) relates underthrusting to the rate of decrease of stress at the end of the period of deformation. Figure 17 shows that a large part of the southern Canadian Cordillera, comprising both the foothills and eastern Rockies, can be modeled with blind thrusts beneath a continuous upper detachment at all stages. Strong evidence for this model lies in the fact that the Belly River Formation, which underlies the upper detachment zone at the outer edge of the foothills, occurs in the footwalls of almost all thrusts exposed in that part of the foothills as well as in the footwalls of the McConnell and Lewis thrusts of the eastern Rockies, regardless of which formations comprise the hanging walls. This consistency shows that successive thrusts emplaced at the outer edge of the foothills flattened along a common upper detachment. The 14-km slip assigned to the McConnell thrust in Figure 17 is the minimum required to generate the model from the data in the original cross section (Gordy and Frey, 1975); the actual displacement is about 40 km (Elliott, 1976). The western extent and thickness of the Tertiary section before thrusting is unknown. Upper Cretaceous sediments occur in the southeastern Canadian Rockies, more than 50 km west of the leading edge of the McConnell thrust, and may have been 150 km further west before thrusting (Norris, 1964). The overlying Tertiary may also have extended far to the west of the present edge of the Cordillera. The Canadian Cordillera is not unique. If it evolved through blind thrusting, the same mechanism may be applicable to thrust and fold belts elsewhere. If underthrusting is observed only at the foreland margins of thrust belts, it is because the margin is the only part of the thrust belt where the upper detachment and overlying fold belt have not been removed by erosion. Perry and Sando (1982), among others, describe thrusts that reach the surface and move along it (erosion thrusts), but such thrusts may represent a small minority. Within a thrust or fold belt generated by blind thrusts, the only faults to reach the surface during movement are the upper detachment itself and antithetic thrusts (or back-thrusts) rooted in it, which dip in the opposite direction to the underlying blind thrusts. The monocline resulting from blind thrusting can gener-

ate a steep dip slope with great topographic relief, both during and after its outward migration across the foreland (Figure 17). This configuration, combined with a shallow upper detachment zone, provides ideal conditions for epidermal gravity sliding similar to that described by Van Bemmelen (1954) and many others, most recently by Hauge (1985) for the Heart Mountain detachment, and Schultz (1986) in the Appalachians. Van Bemmelen attributed the topographic relief to basement uplift, but topographic relief generated by blind thrusting should be equally effective. In western Canada, no special property of the craton, its basement, or its stratigraphy, appears to set a limit to thrusting. Abundant seismic profiles of the Canadian Cordillera (Bally et al., 1966) show that the basement surface has a relatively uniform westward tilt, unbroken by faulting. The base of thrusting is relatively high in the Phanerozoic cover. There are no abrupt lateral changes in stratigraphy. It appears that the outer limit of thrusting marks the extent of thrusting at the time when the applied stress fell below a critical level.

GENERATION OF FOLDS

Concentric Folds Most folds in layered rocks can be described geometrically as the products of a finite number of faults. Flow of incompetent rocks can be simulated in two dimensions by modeling normal and thrust fault movement simultaneously (Figure 18). The style of folding depends upon the nature of the rocks involved. In the concentric style of folding (Figure 19a), geometry demands the existence of both lower and upper detachment zones (Dahlstrom, 1969a), each of which is a fault duplex. If these detachments involve incompetent material such as evaporites, folding is accommodated by material flow from synclinal troughs to anticlinal crests in the lower detachment, and from anticlinal crests to synclinal troughs in the upper zone of detachment. Shortening in the folded sequence may match the shortening derived from flow in the detachment zones. However, if the material in the detachment zones is too competent to flow, then the upper and lower detachment zones are filled by thrust duplexes. In this case, the aggregate shortening in the thrust duplexes is much greater than the shortening within the folded sequence. In Figure 19b, differential horizontal movement (horizontal distance X-X' between the uppermost and lowermost units is several times greater than the shortening of the folded sequence. Each fold in this model resembles the Grease Creek structure (Figure 14) or an inversion of it, and each was built up in the same manner by step thrusting between detachments. For this cross section to be balanced, slip in the upper faulted section may occur in the manner shown by Figure 12. Displacement in the lower fault duplex may also be transferred to the upper fault duplex by lowangle thrusts within the middle folded sequence. Such

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folds are much greater than depths calculated for tight folds. Similarly, when applied to the De Sitter (1964) model of fold evolution, this method of depth-to-detachment calculation gives different depth values for each growth stage. The method also gives incorrect results when applied to the type of fold illustrated by Figures 14 and 15, in which the depth of the basal detachment is accurately defined. Failure of depth-to-detachment calculations results from the unwarranted assumption that a fold is a closed system in which material is neither added nor removed during deformation. Figures 20 and 21a are good examples of cross sections that are balanced with respect to an undeformed state, although their intermediate stages are physically impossible. Clearly, folds do not form in the manner shown by Figure 21a. Some folds certainly are formed by ramp anticlines; in other cases the slip/propagation-rate dislocation model of Williams and Chapman (1983) may be more appropriate. Both models involve large amounts of differential horizontal movement. Figure 22 shows theoretical variations in profiles of a growing fault-bend fold or ramp anticline. In this model, the fold changes shape and migrates laterally during growth. Norris (1971) and others suggest that material movement Calculation of Depth to Detachment, and its Implications along the structural strike is the reason for obviously unacfor the Kinematics of Folding ceptable results obtained from depth-to-detachment calculations. However, considering the large amount of The geometry of concentric folding requires an upper and differential movement in the dip direction that is required to a lower detachment zone (Dahlstrom, 1969a). Figure 20 generate folds of the type described, even more movement is shows a commonly used method of calculating the distance required to generate a fold profile in the strike direction. In to either (usually the lower) detachment zone. This method any case, any direction along which there has been a compogives inconsistent results when applied to adjacent folds in nent of movement cannot, by definition, be the structural areas where the basal detachment zone follows one strati- strike. graphic unit over a large area. Calculated depths for gentle The problems with depth-to-detachment calculations

thrusts could be very difficult to detect in the field. McMechan and Thompson (1985) remapped the northern Alberta and northeastern British Columbia foothills showing that many folds include previously unrecognized low-angle thrusts. Because of the large amount of thrusting needed to generate folds, crustal shortening in fold belts is seriously underestimated if it is based on fold shortening alone. McMechan (1985) describes a cross section through the British Columbia foothills where shortening of the shallow, folded section is much less than that of the the underlying thrust belt. In the Fernie basin, southern Canadian Rockies, subhorizontal Cretaceous beds (no shortening) overlie tight anticlines in Paleozoic carbonates (moderate shortening), while the intervening synclines are filled by thrust-repeated Jurassic shales (much shortening) in a pattern resembling the upper half of Figure 19b (Dahlstrom, 1969a). Local and regional imbalance of crustal shortening between different structural levels is normal. Attempts to balance cross sections by assuming uniform shortening, especially short cross sections such as those through the edges of Rocky Mountain foreland uplifts, may introduce more problems than they solve.

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Figure 19. (a) The ideal concentric fold. If spaces above and below Unit B are filled through flow of incompetent beds, relative movement between A and C may equal shortening due to folding in B, as shown. However, if spaces are filled by thrust repetition of competent material, differential horizontal movement may exceed shortening in unit B (from Dahlstrom, 1970). (b) Folds with overlying and underlying thrust duplex. X-X' is the amount of differential horizontal movement required to generate the folds in Unit B. Note large amount of thrust slip and relative movement between units A and C, very small amount of shortening in Unit B. Low angle thrusts may carry slip of lower thrusts through Unit B into Unit C.

suggest that folds, except perhaps those with an evaporite or other mobile core, do not form by simple compression but require a large amount of differential horizontal movement. Folding in disturbed belts by stacking blind thrusts beneath an upper detachment may be the most important mode of generation in layered rocks. Folds generated this way form in the same order as the thrusts underlying them, and migrate during their growth. If thrusts are emplaced in sequence, the folds they form are generated in the same sequence. Concentric Folding and Interstratal Slip The geometry of concentric folding requires interstratal slip (Norris, 1971). If interstratal slip is regarded as a form of bedding-plane faulting, then it is likely that it requires both a well-developed stratification, and a fluid pressure environment that allows movement along planes of stratification. This may be the reason why perfect concentric fold-

ing is rare. Interstratal slip may be bidirectional (Figures 23a and 23b). However, the differential unidirectional movements shown in Figure 23c may be more appropriate to the deformation of layered rocks. Unidirectional interstratal slip is a normal process in the formation of a ramp anticline, where an upper stratum is offset further than is a lower one (Figure 24a). This model resembles Elliott's (1976), except that it simulates a finite number of thick layers with interstratal slip and a minimum of internal strain, whereas Elliott's model uses the virtually idealized condition of an infinite number of infinitesimally thin layers. The model can be modified (Figure 24c) to represent a sequence in which the thin-bedded, shalier units (source and cap rocks) deform concentrically, whereas the more massive units (reservoir rocks) deform through fracturing (Figure 24b,c). Although interstratal slip on a small scale occurs in concentric folds, the Absaroka thrust model (Figure 11) shows that interstratal slip of several kilometers can also take place. The kinematics of interstratal slip on both the regional and macroscopic scales appear to be identical.

16 SHORTENING

Only the amount of interstratal slip differs, and this may be a function of time. These examples show that concentric folding, thrust faulting, and duplex formation all depend on differing scales of bedding-plane slip. Conversely, the style of folding and deformation in response to an applied stress appears to be governed by the capability of the deformed section to slip along bedding planes (which is a function of lithology and fluid pressure environment). This applied stress is clearly not uniform, nor is it necessarily applied to the entire deforming section simultaneously, but rather it is applied differentially in space and time. The influence of overburden loading suggests that the applied Figure 20. Method of calculation of depth to lower detachment of stress is a complex and variable combination of differential concentric folds (from Dahlstrom, 1970). It assumes that Area B is horizontal and vertical stresses. equal to Area A. Then, depth to detachment = Area B divided by (L2 - LI). However, if this method is applied to folds such as the one in Figure 14, where the depth of the lower detachment is THRUSTING AND FOLDING known, it gives values that are many times too large. Similarly, if TIME AND SEQUENCE applied to adjacent folds in a fold belt, it gives larger results for gentle folds than for tight folds. The method fails because it assumes that the system is a closed one, with no net material addition or removal of material (no change in cross-sectional area), Sequence of Thrust Fault Emplacement during growth. More important, the failure indicates that folds do not form by the simple compression assumed in this model. Minor thrusts rooted in a major thrust are sometimes interpreted to be imbrications within the hanging wall of an existing overthrust sheet, and are believed to be emplaced in

Figure 22. Computer simulation of growing fault-bend fold. Numbers show crests at successive stages of growth.

Figure 21. (a) Formation of a concentric fold according to DeSitter (1964). If folds were formed in this way, depthto-detachment calculations (Figure 20) should give identical results for all stages of Figure 23. Interstratal slip in concentric folds, (a) Undisturbed secfold growth, (b) In fact, results for stages 2 and 3 and 4 are all dif- tion, (b) Bi-directional interstratal slip, (c) Unidirectional interstratal slip. ferent, showing that folds form in some other manner.

17 a lower-to-higher sequence (Figure 25a) (Douglas, 1950; Dahlstrom, 1970). Imbrications in the footwall of a major thrust (Figure 25b), however, are believed to be emplaced in a higher-to-lower sequence. Although intuitively it appears that the hanging wall imbrications formed after emplacement of the basal thrust, there is no geometric basis for this assumption. Figure 26 shows two computer-generated models with the same hanging wall and footwall imbrications as in Figure 25. Both models in Figure 26 were generated in a left-to-right sequence by progressive faulting of an undisturbed footwall (Figure 26c). The only difference between them is that the sequence for hanging wall imbrications (Fig. 26a) was made by three minor faults followed by a major one, and the footwall imbrications sequence (Fig. 26b) by one major fault followed by three minor ones. If hanging wall imbrications formed in a sequence different from that for footwall imbrications, the normal transition along strike from major into minor faults and vice versa (i.e., displacement transfer) would be impossible. If both sets of imbrications are emplaced in the same sequence, faults in Figure 25a can pass along strike into those in Figure 25b through displacement transfer. The term imbrication is itself misleading, since it implies the break up of an originally unbroken thrust sheet following its emplacement. The relationships shown in Figure 25 can be modeled by a left-to-right (higher-to-lower) or a right-to-

interstratal slip concentric folding

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left (lower-to-higher) sequence of thrust emplacement. The geometry is not diagnostic of a particular sequence of fault emplacement. The same geometric relationships can also be misleading on a regional scale. Because the Darby and Prospect thrusts in the Wyoming thrust belt share a common ramp, Dixon (1982) concludes that the Prospect thrust was emplaced before the Darby, which crops out west of Prospect and overlies it. This sequence is the opposite of the commonly accepted west-to-east sequence of thrust emplacement for the Utah-Wyoming thrust belt. Computer modeling (Jones, 1984b) shows that the relationships can be created just as easily with the regional west-to-east sequence of thrust emplacement. Quantitative modeling does not disprove the temporal relationships that Dixon cites, but shows that geometry alone may not be useful in determining the chronologic sequence of fault emplacement. Geometric relationships that appear to indicate a certain chronology of thrust emplacement may be the result of nothing more than a randomly occurring relationship between spacing of incipient thrusts, and their displacements. The models suggest that it is unnecessary and misleading to classify thrusts into fore-limb and back-limb thrusts (imbrications). The classification is based on the positions of thrust faults relative to the crest of a fold. Where the fold itself is formed by later, underlying thrusting, the location

a. Sequence of imbrication in the hanging wall.

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Figure 24. (a) Interstratal slip required for concentric folding in ramp anticline, (b) Massive unit cannot fold concentrically due to lack of slip surfaces and adjusts to the top of the ramp by fracturing and small-scale faulting, (c) Alternation of massive and thinbedded units creates an idealized hydrocarbon trap above a ramp anticline. Shalier units (possible source and cap rocks) deform concentrically and retain seal, while massive unit (carbonate, sandstone, or basement) develops a fractured reservoir at its leading edge.

b. Sequence of imbrication in the footwall. Figure 25. "Leading-edge" imbrications of a major thrust sheet (from Dahlstrom, 1970). Note reverse sequence of development.

18

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of its crest is unrelated to emplacement of either the forelimb or back-limb thrusts. Once formed, any thrust may eventually be transported to a fore-limb or back-limb location that has no connection with its original position and mode of emplacement. Thrust faults commonly pass along strike from a back-limb position through a fore-limb position and return to a back-limb position. This would not be possible if back-limb and fore-limb thrusts formed in opposite chronological sequences. Minor sub-horizontal faults in the steep limbs of anticlines and foreland uplifts are sometimes incorrectly classified as fore-limb thrusts. However, they are actually high-angle extensional faults with respect to the steep fold limbs they cut (Perry, 1978). Many problems in structural interpretation arise from miscorrelation of these extensional faults with thrusts. Anomalous Structures Not all structures can be modeled as the products of step faulting. The overturned synclines found in the footwalls of some thrusts cannot be formed through step-faulting (Butler, 1985). Similarly, although some hanging-wall drag folds originate by duplex formation, it is clear that many owe their origin to other kinematic processes (Williams and Chapman, 1983), beyond the scope of this discussion.

Figure 27. (a) Map and cross section of a plunging step thrust. The cross section is commonly illustrated but the surface configuration is rarely shown on geological maps, (b) Structure commonly shown on geological maps and interpreted on seismic profiles. Although anomalous and difficult to relate to the step-fault in the lower part of the cross section, faults are commonly shown dying out upward or along strike in this manner.

In addition to structures whose geometry clearly can or cannot be modeled by step-faulting processes, there are some interesting structures that should be rare, yet are apparently common; and other structures that should be common, but are apparently rare. The following examples of both of these types of structures suggest that geologic data interpretation is not as objective as it appears to be. Thrust Faults That Die Out Along Strike and Up-Section In the block diagram of a plunging ramp anticline (Figure 27a), competent unit B is thrust over incompetent unit C. The cross section shows the normal configuration of a step fault. It is not, to the writer's knowledge, controversial, and is easily modeled. If the cross section is acceptable, the corresponding plan should be a common feature of geologic maps. However, it is very rare. In contrast, cross sections similar to Figure 27b frequently are seen in interpretations of seismic records where

19 of B. Each stage shown is equally probable, as well as the infinite number of other intermediate stages not shown. The geometry shown in Figure 28 occurs only at stage C during the emplacement of Fault B. Although this is no more likely to occur than at any other stage, it is very commonly illustrated, but the infinite number of other possible interpretations, including stages B, D, and E, are rarely illustrated. Whereas there may be proven examples of Stage C geomeFigure 28. Frequently illustrated relationship between thrust try, it appears that it is an interpretation that is applied far faults. Very precise adjustment of the slip of fault B with respect to more frequently than the probability of its occurrence sugthe spacing between it and fault A is necessary to model this rela- gests. If proven, this relationship may indicate that A is later than B, but this interpretation should not be accepted withtionship in a normal sequence of thrust emplacement. out other supporting evidence. a fault is shown following the narrow zone of poor data caused by interference between reflections from the horizontal beds and the adjacent dipping beds. Usually there is no offset of seismic markers across the zone. It is impossible to model this structure. If the lower part of the step fault is modeled with the appropriate amount of offset of B, then unit C in the hanging wall is offset by the same amount and steeply tilted. The plan shown in 27b is common on geologic maps. Almost always, the position of the fault is known only where it follows B, the competent unit, which outcrops. The upper sector is invariably inferred, because C (being incompetent) provides no outcrop. Thus there is usually no geological basis for this type of interpretation, which is geometrically impossible. It appears that such interpretations stem from the belief that thrust faults die out into folds. One must ask how many plunging ramp anticlines have been misinterpreted as faults passing along strike into folds? Figure 27a represents a model that is mechanically and geometrically simple yet rarely illustrated, while 27b shows commonly accepted surface and subsurface interpretations that are mechanically difficult, geometrically impossible or, at best, incomplete. These should not be accepted uncritically. Where such structures are proven (not interpreted), they require further study to determine how the geology appears to defy simple geometry. The difference between the two interpretative styles is very important in hydrocarbon exploration. It bears directly on size, location, and correlation of faults, and extent of overthrust reservoirs. Reinterpreting old geological and geophysical data in the light of these criteria can be a significant first step in developing new plays in mature exploration areas.

Extent of Basal Detachment in Overthrust Belts The structure depicted in Figure 30 is impossible, but is frequently illustrated in the geologic literature. When this cross section is restored to its undeformed state, a process aptly described by Dixon (1982) as "filling the container," parts are missing. The long sector of bedding-plane slip in the hanging wall of thrust B demands a corresponding length of footwall in the same unit. This means that thrust A cannot be rooted in the lower stippled unit, where shown. Instead, the base of its ramp must lie beyond the left end of the diagram. On a larger scale (where, if thin-skinned deformation of the outer part of a thrust belt results in an aggregate shortening of, say, 50 km), the basal decollement must extend a further 50 km into the deformed belt from the leading edge of the cross-hatched unit (Figure 31a). Cross sections like Figure 31a are geometrically and geologically impossible in a normal sequence of thrust emplacement, but are frequently illustrated, without comment, in the geological literature. Figure 31b, on the other hand, is geometrically possible. This geometry is confirmed by seismic profiles of the southern Canadian Rockies, where the Canadian Shield extends undisturbed far into the interior of the Cordillera (Bally et al., 1966). Similarly, the COCORP seismic traverse through the Appalachians (Cook et al., 1979) showed that the lowangle thrusting characteristic of the Valley and Ridge province extends eastward beneath the Blue Ridge and Piedmont metamorphic and crystalline belts. These seismic surveys discovered what was geometrically inevitable, given the known extent of thrusting in the outer parts of each thrust belt. This logic can be applied on both local and regional scales to deformed belts where subsurface data are limited, and may increase the effectiveness of seismic programs.

Divergent Thrust Faults Figure 28 illustrates a geometric relationship commonly found in geologic cross sections. Fault B projects horizontally from Fault A. Although it is easy to model this in the normal higher-to-lower sequence of thrust emplacement, the amount of slip of B must be very precisely adjusted for the ramp of A, carried in its hanging wall, to reach the position shown. Successive stages in the geometry of a pair of thrusts are shown in Figure 29, in which Fault A, following its emplacement, is carried piggy-back in the hanging wall

DISCUSSION Factors Controlling the Development of Thrust Ramps In thrust belts where faults are widely spaced, basement influence may be an important factor in determining the ramp locations (Wiltscho & Eastman, 1983), but it does not seem appropriate in deformed belts where there are very

20

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large numbers of closely spaced thrust faults, having no local connection to basement. Variations in stratigraphy can also account for ramping. As a glide horizon pinches out or changes facies, the basal slippage plane is transferred to a higher incompetent unit (Gretener, 1972; Harris and Milici, 1977). Only one ramp can be formed by each transfer, leaving the majority of ramps unaccounted for. Thrust sheets frequently occur in stacks, which require that each thrust carry its predecessor, piggy-back, by the the precise amount required for stacking (e.g., Figure 16). It is unlikely that this requirement is fulfilled only by chance. It appears more probable that the starting and stopping position of each thrust sheet is governed by a combination of overburden thicknesses, fluid pressures, and other internal characteris-

tics of the overthrust mass, rather than by underlying influences. Direction of Thrust Fault Propagation Wiltschko and Eastman (1983) state without elaboration that thrusts propagate downward. A form of downward propagation in stages is demonstrated by the ' 'unbalanced cross section" model of the Absaroka thrust (Figure 11), which moved first as a bedding plane detachment within the Mesozoic section, and secondly as a bedding plane detachment in the Paleozoic section which climbed up-section to join the earlier thrust, a process involving interstratal slip on

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a regional scale. This suggests a hypothetical model of thrust emplacement (Figure 32) in which thrust faults propagate by developing minor interstratal slip and generating duplex structures at progressively deeper detachment surfaces, until a lithology and fluid pressure environment is reached that allows large amounts of movement along the basal thrust plane. Such movement occurs by means of the environment's ability to retain an overpressured condition for a much longer period. Interstratal slip is a form of thrusting, and presumably requires similar overpressured conditions for movement. The longer the high fluid pressure is maintained, the greater the amount to thrust, or interstratal slip. This, in turn, depends inversely on the rate of dissipation of high fluid pressure. Dissipation is slowest in the basal detachment zone, where movement is continuous, while overlying thrusts progressively lose their lubricating fluids and lock successively further out towards the craton, behind the active frontal thrust. Gibbs (1984) outlines a similar mechanism of normal fault formation. Interstratal Slip and Folding Style There is a continuum in bedding-plane movement, ranging from macroscopic interstratal slip to major bedding-

Figure 32. Hypothetical mode of downward propagation of a thrust fault. Minor bedding-plane slip develops along progressively deeper horizons (stages b through g), until a horizon is reached that will retain high fluid pressures long enough to allow significant fault movement, (h).

plane thrusting measured in kilometers. Interstratal slip can generate duplex structures on a large scale or within the shaly interbeds between sandstones or carbonates, distorting the parallelism between beds that is a feature of concentric folding and turning it into quasi-similar folding. Norris (1971) describes folds in layered sediments in which more competent beds retain their thickness in folding, while the incompetent interbeds vary considerably in thickness. Depending upon whether those thickness variations are because of thickening by thrusting alone or a combination

22 of fold and thrust belts. If this is true, deformation in some mountain belts may be much younger than ages presently assigned to them. For mechanical reasons, the locus of an upper detachment zone is commonly a marine shale. Perhaps this is why Kauffman (1984) correlates thrusting episodes with periods of marine shale deposition. The geometrical feasibility of modeling a major part of the Canadian Rockies through the process of blind thrusting suggests that other thrust and fold belts are formed in the same way (Jones, 1984a; Charlesworth and Gagnon, 1985). Butler (1985) does not share this interpretation and suggests that when blind thrusts wedge the foreland margin upward beneath an upper detachment they form a phenomenon restricted to the "last gasp" of orogenic movement. Morley (1986) suggests that the difference between overthrust and underthrust foreland margins reflects differences in the rate of thrusting deceleration. However, no evidence suggests that the last movement of the thrust belt should differ from preceding ones, and much evidence suggests that it Sequence in Folding is similar. In the case of the Canadian Cordillera this evidence is displayed not only at the final foreland margin, but Most folds can be represented as the products of a finite at preceding ones. Also, the huge differences of shortening number of faults. If those faults are emplaced in a chrono- between suprajacent structural-stratigraphic packages logical order, the folds they generate are formed in the same described by McMechan (1985) and Thompson (1981) in the order. Thrusting sequence has been discussed by many foothills of British Columbia show that blind thrusting authors, and there is general agreement that thrusts are occurred throughout deformation, not merely at the final emplaced in sequence from higher to lower, by progressive stage. deformation of an undisturbed footwall (Elliott, 1976). Blind normal faults may also be recognized as natural Folds can be generated above thrusts by ramping and components of certain structural regimes. Gibbs (1984) duplex formation involving large-scale differential horizon- describes duplex structures in extensional basins involving tal movement. Folds formed in this way migrate during blind normal faults. Similar faults may also exist in fold and growth. Fold formation by simple compression above a thrust belts. In the southern Canadian Rockies, the Flatbasal detachment is geometrically impossible, except where head and other listric normal faults cut the Paleozoic and evaporite or similarly incompetent rock forms the basal Proterozoic along the east side of the Fernie basin. detachment zone and fills the anticlinal core. The classical Although mapped as separate faults, they are in line with compression-box method of simulating the formation of each other and extend up to—but not through—Lower Crefolds and thrust faults is misleading in space and time. It taceous sandstones and coal measures, and terminate in produces equal and simultaneous shortening at all levels Jurassic shales. There is no unconformity in the sequence, above a basal detachment, where differential and sequential and the consistency of this relationship belies coincidence, movement is more appropriate. suggesting that these faults are different sectors of a single Flathead fault (Jones, 1985) that flattens upward into the Jurassic Fernie shale. This unit had previously been identiBlind Thrusts in Orogenic Belts fied as the upper detachment for folds in that area (Dahlstrom, 1969a). Recognition of blind thrusts beneath upper detachments can change the interpretation of deformation age in fold and thrust belts. The age of thrusting attributed to a Blind Thrusts and Mechanics of Thrusting deformed belt bounded by a foreland syncline is incorrect when the upper detachment zone is interpreted as an unconHubbert and Rubey (1959) applied Terzhagi's (1950) rock formity. In the southern Alberta foothills, the upper detach- mechanics principles to the mechanics of overthrusting, and ment zone follows an Upper Cretaceous marine shale. This demonstrated that an overthrust mass can be supported and shale was thought to contain an unconformity (Douglas, lubricated by formation fluids confined under near1950; Elliott, 1976) dating the youngest thrusting as Late lithostatic pressures at its base. Gretener (1972) and others Cretaceous. However, no section is missing. Recognition of point out that this model fails to account for the toe of the the upper detachment relates folding of the Paleocene of the overthrust mass; that is, for the sector of the thrust sheet foreland syncline to the underlying thrusting, which is of that moves up a ramp and reaches the syntectonic erosion Eocene age (Bally et al., 1966). Similar errors in interpreta- surface, where it must lose its ability to retain the high fluid tion may confuse the tectonic history of relatively unex- pressures that thrusting requires. plored thrust belts, as well as others that were explored and This problem is largely eliminated in a kinematic model drilled for hydrocarbons long before folded faults, blind involving blind thrusts. In a blind thrust model, thrusting thrusts, and duplexes were recognized as normal elements can occur within an envelope of high fluid pressure instead of thickening with thinning, which can be represented by a combination of thrusting and normal faulting, thickness variation within a fold can represent a large amount of differential movement between competent beds. Dahlstrom (1970) describes chevron folds in which interstratal fold and fault duplex structures fill the thickened fold crests and troughs. If interstratal slip is a geometric requirement of concentric folding, it follows that concentric folding is possible only in well-bedded rocks, whereas massive units are folded in other styles and are more prone to fracturing during deformation. This process of folding and fracturing can generate hydrocarbon traps in which a thrusted reservoir contains fracture permeability and porosity in massive units, while thinly bedded and shalier units that represent source and/or cap rocks fold through interstratal slip and retain their ability to seal.

23 Syntectonic erosion surface

Active upper detachment - fluid leakage

Figure 33. Hypothetical fluid pressure regime beneath a fold belt with buried upper detachment. Shaded area between upper and lower detachments is envelope of high fluid pressures.

of being confined to a basal decollement zone. In such an environment, both the interstratal slip that permits concentric folding, and large-scale thrust movements can be facilitated at different crustal levels. The model includes a relatively efficient seal for retention of high fluid pressure. During blind thrusting, the only active fault plane reaching the erosional surface is either the upper detachment, which may be progressively exposed by erosion, or an antithetic splay fault rooted in it. The upper detachment is the most likely thrust to become an erosion thrust, which may account for the thickness (100 + m) and contorted nature of upper detachment zone exposures along the outer margin of the Canadian Cordillera (Jones, 1982), totally unlike the knife-sharp contacts on fault planes of the underlying westdipping blind thrusts. If the overlying fold belt is not breached, a large part of a thrust belt may be deforming within an envelope whose upper seal is the shale of the upper detachment zone (Figure 33). This situation is capable of retaining high fluid pressures throughout a thick sedimentary section, not only within its basal detachment unit. Where syntectonic erosion causes loss of fluid pressure and deactivation of fault planes near the surface, the deactivation takes place behind the moving thrust sheet rather than at its leading edge (Figure 34). The leading edge and basal decollement are areas where high fluid pressures should permit continued thrusting in response to the applied horizontal and vertical stresses. Blind Thrusting and Gravity Sliding Van Bemmelen (1954) describes large surficial gravity slides and suggests that thrusts form by gliding down the slopes of an uplifted crustal welt or "geotumor." His hypothesis fell into disuse partly because of skepticism about the role of vertical uplift in orogenesis. Recognition of the phenomenon of blind thrusting calls for a revival of Van Bemmelen's hypothesis. Blind thrusting can create the structural and topographic relief needed for superficial gravity sliding at the leading edge of an advancing thrust belt (Figure 34). In addition to creating the relief for gravity gliding, blind thrusting can also form a steep-dip slope of competent strata overlying incompetent strata (the upper detachment zone), an ideal setting for large gravity-driven blocks to break away down the frontal monocline of an advancing belt of blind thrusts (e.g., Figure 17).

Active thrust & lower detachment - high fluid pressure

Figure 34. Fluid pressure regime in thrust belt with exposed upper detachment. Shallow upper detachment may provide a gliding horizon for epidermal gravity sliding down foreland-facing dip slope.

Because they depend on uplift, these gravity slides will be generated out of sequence with respect to underlying blind thrusts. Typically, bedding is parallel to the topographic slope. Gravity structures of this kind contain many features similar to thrust sheets, and can therefore be interpreted in ways that are problematic to the interpretation of the deeper structure. Movements Along Structural Strike Norris (1971) suggests that movement of material along strike is a possible reason why depth-to-detachment calculations fail. However, folds formed by thrusting are the products of a shear couple involving large amounts of differential horizontal movement. Movement of material along strike would require a component of differential movement along structural strike, which, by definition, is without relative movement. It follows that movement of material along the strike of linear deformed belts is insignificant. The Universality of Duplex Structures Profiles of thrust and normal faults commonly include alternating steeply dipping and gently dipping sectors. Movements along such faults deform the package of strata and included faults in the hanging wall. These faults, known as step faults, generate anticlines in the hanging wall as a result of fault movement. Duplex structures, which occur on all scales, also generate anticlinal folds because all faults involved in them are step faults that flatten at the roof and floor of each duplex. Step faulting and duplex formation, which includes blind thrusting, naturally result in arcuate structures (anticlines) that are convex upward. Many of the previous cross sections (e.g., Figures 8, 10, 17), resemble strike-slip faulted terranes in plan. Figure 35 illustrates the similarity between a thrust duplex (Figure 35a) and what might be called a strike-slip duplex (Figure 35b). Notwithstanding differences in direction of fault movement and sequences of fault emplacement, the similarities between Figure 35a and 35b are striking. Horses between thrusts in Figures 10 and 11 are also very similar to horsts

24

Figure 35. (a) Profile of thrust duplex in the Canadian Rockies (Fermor, 1987). (b) Plan of strike-slip fault duplex, California (Dibblee, 1977). and pull-apart basins (Crowell, 1974). It is interesting to speculate that because thrusting (by stacking of lenticular horses) almost inevitably causes arcuate structures (anticlines) in cross section which are convex upward, the horizontal juxtaposition of lenticular terranes by strike-slip faults must also cause structures that are arcuate in plan. In plan, Circum-Pacific strike-slip faulting combined with subduction zones (analog of the upper detachment zone) should generate arcuate structures that are convex oceanward. Time Although this discussion is concerned almost entirely with the geometry of geological structures, formation of such structures requires time. Duration of movement is the principal difference between major and minor faults, and between small-scale interstratal slip and major beddingplane thrusts. The fold belt that is the British Columbia foothills passes along strike into the thrust belt that is the Alberta foothills. Each deformed belt has about the same width. If crustal shortening is calculated on the basis of folding alone, it appears that the British Columbia foothills have much less shortening than the Alberta foothills. Does this mean that (a) the duration of deformation in the British Columbia foothills was less than in the Alberta foothills? (b) deformation was slower in the British Columbia foothills? or (c) the deformation and crustal shortening are similar in both areas, and the apparent differences are due to the differences in levels of erosion? The models shown in this paper suggest that (c) is the most likely alternative. The work of Thompson (1981) and McMechan (1985) for the British Columbia foothills supports this suggestion.

CONCLUSIONS 1. Step faulting is a primary mechanism of folding in layered rocks. 2. Location of ramps in a thrust belt may be governed by

(a) irregularities in the footwall; (b) stratigraphic variations causing changes in the level of basal decollements; or (c) differential overburden load within the hanging wall of the overthrust body. Neither (a) nor (b) are common enough to account for the hundreds of thrusts which may occur in any thrust belt. Differential overburden loading (c) occurs throughout the growth of a fold or thrust belt. As a result of overburden buildup by thrusting, and reduction by erosion, this differential loading is continuously renewed. 3. Some geometric relationships between thrust faults which appear to indicate a reversed sequence of thrust emplacement, can be formed in a normal sequence. 4. Folds in layered rocks may be formed through differential horizontal movement. Others are formed in response to thrusting in an underlying thrust belt. 5. Although pressure-box models of deformation and depth-to-detachment calculations for folds appear intuitively correct, both of them assume a constant-volume, insitu mode of formation that is unjustified. The assumption that the applied stress is equal and simultaneous at all levels of deformation is also unjustified. 6. A thrust belt may be the erosional remnant of a fold belt. 7. Concentric folding is possible only where two conditions are met: (a) a rock lithology permits interstratal slip; (b) confined-formation-fluid pressures are high enough to lubricate such movement. 8. Successive thrusts are normally emplaced in a higherto-lower sequence, by progressive cutting of an undisturbed footwall section. Individual thrusts may form from the top down by relatively minor movement along bedding planes at progressively deeper levels, until a unit is reached that retains high fluid pressures for a long enough period for significant fault movement to take place. This is duplex formation on a small scale, and implies varied amounts of stress and strain at different levels and at different times. 9. Major and minor faults do not differ in mechanics or sequence of emplacement, they differ only in duration of fault movement. 10. Blind thrusting beneath an upper detachment may be

25 the most important process in the formation of thrust belts and the fold belts that overlie them. Shallow upper detachment zones also provide an ideal setting for shallow, out-ofsequence gravity gliding down the face of the advancing foreland monocline and down the trailing edge of a growing mountain belt. 11. The blind thrusting model reduces problems inherent in the classical theory of thrusting along zones of high fluid pressure, because it involves an envelope of high pressures, not just a basal zone. Questions about rock deformation processes posed by simple rigorous geometric models show how much is assumed and how little is actually known. Before rock mechanics can be used in computer models of thrust and fold belts, more needs to be understood. Computer synthesis of the geometry of geological structures may solve important problems in the delineation of hydrocarbonbearing structures in thrust and fold belts. In addition to balanced cross sections (which can reduce the number of possible interpretations), quantitative modeling stimulates questions that might not otherwise be asked. Computer modeling shows that bedding-plane movement is an extremely important factor in the deformation of layered rocks, and demonstrates that the simple step-fault model has applications that extend far beyond its original geologic context. Rigorous computer synthesis of this model shows the applications and defines the limitations of the model. Clearly much more sophisticated programs are necessary, but there are many problems to be overcome. Development of steeply dipping and overturned beds and faults requires a system of modeling concentric folds, but how far up- and down-section should they go? Figures 20 and 21 show that the limits of this style of folding are not geometrically predictable. It is not yet possible to model more than one phase of deformation, mainly because it is difficult to predict the path of a fault through material that has already been deformed. The direct applications of computer modeling to petroleum exploration in deformed belts are obvious. Less tangible, but perhaps more important, is the probability that exposure to quantitative computer modeling will cause earth scientists to develop a more critical and creative approach to the interpretation of structural geological data.

ACKNOWLEDGMENTS The writer is President of International Tectonic Consultants, Ltd.; 715-4th Street, NW; Calgary, Alberta, T2N 1P3; Canada. Manuscript was received by the Association, July 3, 1986. The writer thanks W. B. Hennessey and J. K. Lentin, for reviewing early drafts of the manuscript. He also thanks W. J. Perry, Jr., in particular who suggested many improvements to the manuscript originally submitted.

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