TECTONICS, VOL. 16, NO. 2, PAGES , APRIL 1997

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1 TECTONICS, VOL. 16, NO. 2, PAGES , APRIL 1997 Simultaneous development of noncylindrical folds, frontal ramps, and transfer faults in a compressional regime: Experimental investigations of Himalayan Examples Ashok Kumar Dubey Wadia Institute of Himalayan Geology, Debra Dun, India Abstract. Experiments were performed with modeling clay models to understand the simultaneous development of folds, frontal ramps, and transfer faults/oblique ramps in layered sequences. The models were deformed by layer parallel contraction. At a late stage of deformation, the models were subjected to a maximum contraction parallel to the layering and perpendicular to the early axis of maximum contraction to study the effect of the superposed deformation. The experimental results reveal that a transfer fault joining two frontal thrust ramps acts as a shear zone and controls the orientation of fold hinge lines in the adjacent region. Hence two orientations of fold hinge lines may be observed: (1) oblique to the axis of maximum compression in the vicinity of the transfer fault and (2) normal to the axis and parallel to the frontal ramps away from the transfer fault. During the longitudinal fold propagation, the two hinge lines may coalesce to form a fold curvature opposite to the tectonic transport direction. The curvature varies along the transfer fault depending on the shear zone geometry. Additional fold hinge line curvatures may result during the superposed deformation both in the early and the superposed fold hinge lines. All these curvatures produce a systematic pattern which can be analyzed with the help of the experimental results. The fold hinge line orientation may also be used to establish an oblique ramp in field as demonstrated by examples from the Lesser Himalayas, India. Introduction The geological maps of the Himalayas display strike-slip faults, with varying intensities of distribution and displacements [e.g., Valdiya, 1980; Sinha, 1989]. These faults have been described as youngest in the tectonic history of the region as they appear to displace the prominent Himalayan thrusts. Many of these faults do not involve any controversy regarding the time of their initiation, but a closer look at some of them (e.g., Figure 1) reveals that (1) they are confined to offset thrust segments; that is, the displacement along the fault is equal to the length of the fault, a condition most improbable for strike-slip faults [Freund, 1974; Dubey, 1980]; (2) they do not displace stratigraphic horizons, but the stratigraphic horizons follow the trend of the fault; (3) a single-fold hinge line is not traceable across the fault; and (4) the sense of fold hinge line curvature in the vicinity of the faults is opposite to the tectonic transport' direction, and the amount of curvature varies in different folds, implying that the folds have not been displaced by the same amount and in proportion to the depicted Copyright! 997 by the American Geophysical Union. Paper number 96TC /97/96TC-0223! $12.00 fault displacement. The above featuresuggest that these fau are most likely oblique ramps (gentle dipping) or transfer fau (steep dipping) [Ramsay and Huber, 1987, Figure 23.46] or transfer zones [Dahlstrom,1969]. These faults may have formed as normal transfer faults during a tensional regime in the region [Gibbs, 1984, 1990a, b). Evidence for a tensional regime in the Himalayas comes from the study of preorogenie marie magmatism and the subsidence history of the Tethys basin sediments [Bhat, 1987 and references therein; Bhat and Le Fort, 1992]. These studies have shown that the region has undergone repeated rif ing episodes from the Late Archaean until just before the Tertiary compressional phase. The rift phase resulted in the development of a number of listrio normal faults. The parallel arrangement of stratigraphic formations and the faults, even where the faults have a marked curvature in trend, suggests that these faults are preorogenie and their pattern has controlled the shape of the depositional basin. These faults were later reactivated as thrusts during the Himalayan orogeny [Dubey and Bhat, 1986], and folding and thrusting took phee simultaneously [Dubey and Bhat, 1991]. Since oblique ramp and transfer faults have not previously been reported from the Himalayas, the orientations of fold structures adjacent to oblique ramps have been gathered from other orogenic belts. This shows that fold hinge lines in the vieinky of an oblique ramp are either parallel to or at an angle with the trend of the oblique ramp, whereas hinge lines adjacento frontal ramps are parallel to the frontal ramp and normal to the tectonic transport direction [e.g. Pfi er, 1981; Butler, 1982; Coward, 1984, 1988; Apotria, 1990; Gibbs, l 90al. The possibility of transfer faults and oblique ramps in the Himalayas and the above variety of angular relationships between oblique ramps and fold hinge lines in different orogenie belts prompted the present modeling study. Modeling clay models were deformed to study the effect of oblique ramps and thrustransfer faults on the hinge line orientation of multilayer folds. In these experiments, the faults were preeut in the material to simulate preexisting faults that were rejuvenated as thrusts during the Himalayan contractional phase. Experiments Five layers of modeling clay each mm thick formed the multilayer packet which was enclosed in a matrix of modeling clay. The layer surfaces were coated with tale powder to allow flexural slip during deformation. The external dimensions 0f the models ranged from 15 x 11 x 6.5 em to!5 x 12 x 7.5 cm. A fractur either planar or listrie with frontal and/or transfer fault (i.e., steep dip) or oblique ramp (i.e., gentle dip) 336

2 DUBEY: FOLDS, FRONTAL RAMPS, AND TRANSFER FAULTS 337 INDEX I,, I / RISHIKE H Bansi S Subothu Fins. Tal Fro. K rol Fro. Blaini Fro. i..' Nagthat - Elerinag F'rns, Rishikesh o 5 Km 11' Damtho Group RarnGarh Group Almora Group Figure 1. Geological maps illustrating curvature of fold hinge lines adjacent to a fault [after Valdiya, 1980]. The sense of relative movement is author's interpretation. (a) A part of the Garhwal Lesser Himalayas and (b) a part of the Kumaun Lesser Himalayas. geometry was cut prior to deformation. In some experiments, relationship with the thrust surface were similar to those the fracture was confined to the basement (i.e., lower modeling described by Dubey and Bhat [1986] and Dubey and Paul ½hy matrix), in some, it extended into the multilayers, and in [!993]. Hence the main object of the following description is to the rest, it cut through the entire model including the upper provide the details of the simultaneous development of matrix. noncylindrical folds and faults on the layer surface and new The models were deformed in a biaxial press described by observations on the profile surface. Dubey and Cobbold [1977]. The maximum compression was parallel to the initially horizontalayering, but the models First Series of Experiments underwent deformation under two sets of boundary conditions. In the first one, the maximum extension was along the vertical A homogeneous block of modeling clay was cut for frontal axis of the press under the plane strain boundary condition; ramp and transfer fault geometries prior to deformation. Three that is, the model was confined perpendicular to the maximum stages in the deformation of the model are shown in Figure 2. compression direction. In the second one, in addition to the The displacement along the fault initiated at the onset of vertical axis, extension was also allowed along the layering deformation. The horizontal component of displacement is normal to the axis of maximum compression under the general visible by the displacement of the grid lines. A few fix points strain boundary condition [Hobbs et al., 1976, p. 28]. At a late on the top perspex plate revealed that the relative displacement stage of deformation, the models were subjected to a new took place both in the hanging wall and the footwall since the compression direction perpendicular to the early axis of model was compressed equally from both the sides. The maximum compression under the general strain condition to maximum thrust displacement in the hanging wall took place study the behavior of the transfer fault during superposed along the leading frontal ramp but with a characteristic deformation and hinge line curvature of superposed folds. variation along its strike, that is, a gradual decrease in In both series of experiments, the geometrical shape displacement toward the transfer fault. The amount of thrust modifications in cross-sectional fold profiles and their displacement was of slightly lower magnitude around the

3 338 DUBEY: FOLDS, FRONTAL RAMPS, AND TRANSFER FAULTS _I r I B- Figure 2. Three stages in the deformation of a modeling clay model deformed under plane strain boundary condition. (a) 0% shortening- (b) 29% shortening; and (c) 31% shortening during thearly deformation and 24% shortening during the superposed deformation.

4 DUBEY: FOLDS, FRONTAL RAMPS, AND TRANSFER FAULTS 339 A Figure 3. A crossection of the model shown in Figure 2. (a) Geometry and displacement along the thrust fault after 31% shortening and (b) the same section after 31% shortening during the early deformation and 24% shortening during the superposed deformation. Note a decrease of fault displacement as a result of reversal of displacement (i.e., normal faulting) during superposed deformation

5 34O DUBEY: FOLDS, FRONTAL RAMPS, AND TRANSFER FAULTS transfer fault. The variation in displacement has also resulted in a minor rotation of the hanging wall in a clockwise direction (note the grid lines in Figure 2b). The footwall grid lines demonstrate a normal drag [Hobbs et al., 1976, pp ], that is, curvature toward the tectonic transport direction. At the onset of superposedeformation, reversal of strikestip displacement became apparent. The earlier right-lateral horizontal component of displacement was modified to a leftlateral horizontal component. In the cross-sectional profile (Figure 3), the reversal of fault displacement was accompanied by a dip-slip displacement in a normal fault sense (Figure 3b). Thus the early thrust fault was reactivated as a normal fault during the superpose deformation. The deformation of multilayer models showed that displacement along the thrust was concomitant with formation of noncylindrical folds (Figure 4). Most of the fold hinge lines were parallel to the strike of the frontal ramps and normal to the transport direction. HoweYer, some of the folds in the vicinity of the transfer fault were oblique to the transfer fault and the transport direction. The folds propagated longitudinally (i.e., extended theix hinge lines) during amplification and linked with other folds along the hinge fines [Dubey and CobboM, 1977]. The initial variation in the orientation of folds resulted in formation of folds, the hinge fines of which were curved opposite to the transport direction, near the transfer fault [cf. Dubey, 1980]. The curvature was more prominent at the junction of frontal ramp and transfer fault (e.g., Figure 4). The orientation of the fold hinge line became constant during the later stages of deformation. The transfer fault gradually acquired a sinusoidal shape with a prominent restraining in the middle. Second Series of Experiments The second set of experiments were carried out under th general strain boundary condition. Consequently, disphceme of hanging wall out of the tectonic transport plane bas a significant feature of the experiments [Apotria et al., 1991] (Figure 5), that is, the oblique slip displacement was observed simultaneously along the frontal ramp and the transfer fau Points A, B, and C are marked on the same grid lines to visualize the displacement along the transfer fault. The total fault displacement was observed to be larger than the previot experiment performed under the plane strain boundary condition (Figure 2). The inhibition of the trailing frontal ramp movement by the transfer fault was apparent in the formation of a right-lateral shear zone at the junction of the two ramp structures (Figure 5a). A larger component of vertical displacement was observed along the leading frontal ramp. Similar to the first series of experiments, this model also showed a variation of displacement along the frontal ramp and a larger displacement away from the transfer fault. This resulted in the formation of extension and shear fractures in the hanging wall. The extension fractures were normal to the transport direction, and they displayed a decreasing width toward the transfer fault. The shear zone geometry was typical of a fight-lateral shear zone (Figure 5a). HW Figure 4. Simultaneous development of folding and thrusting the modeling clay multilayer model deformed under plane strain boundary condition after 19% shortening. HW is hanging wall. The initial fault geometry was similar to Figure 2a. The noncylindrical fold shown by an arrow displays a prominent curvature of hinge line near the transfer fault.

6 DUBEY: FOLDS, FRONTAL RAMPS, AND TRANSFER FAULTS 341. I b Figure 5. Two stages in the deformation of a modeling clay model deformed under general strain boundary condition. (a) 29 % shortening and (b) 31% shortening during the early deformation and 15% shortening during the superposedeformation. The initial fault geometry was similar to Figure 2a.

7 342 DUBEY: FOLDS, FRONTAL RAMPS, AND TRANSFER FAULTS The curvature of footwall grid lines near the leading frontal ramp revealed a normal drag which gradually decreased toward the trailing frontal ramp. The overall rotation pattern of the footwall was antieloelcwise in contrasto the hanging wall where the rotation was clockwise. In the cross-sectional profile, the thrust displacement was 4.5 ½m at 31% shortening, whereas in a previous experiment (Figure 2) the displacement was 3.0 ½m at the same amount of shortening. In one of the deformed multilayer models, the fold hinge line curvature was observed all along the length of a transfer fault (Figure 6). The maximum fold curvature was near the leading frontal ramp with a gradual decrease in curvature toward the trailing ramp. During the early deformation, the horizontal component of disp!aeement along the transfer fault (D) showed a nearly uniform value at different points along its length (Figure 7). However, during the superposedeformation, the displacement at different points followed different tracks, indicating that during the superposedeformation, the fault behaved more like a strike-slip fault due to its geometry and orientation with respect to the axis of maximum compression [ef. Freund, 1974; Dubey, 1980]. All the experiments revealed that the later superposed deformation resulted in an increase in fold curvature both in the hanging wall and the footwall. The folds developed sequentially during the early and the superposedeformations. Consequently, a variety of angular relations could be observed between the fold hinge lines and the transfer fault. At a late stage of deformation, a conjugate set of strike-slip faults developed on the layers, and a further increase in deformation led to an increase in displacement along these faults. Discussion A transfer fault and an oblique ramp are differentiated on the basis of their dip angles. Both of them show oblique slip displacement (with a vertical and a horizontal component of displacement), but a transfer fault is steep or vertical and an oblique ramp has a gentle dip. The present experiments demonstrate that despite a variation in their dip, both of the fault types inhibit motion along a frontal ramp fault and their simultaneous development with folds produces similar interference patterns. The development of frontal ramps and transfer faults requires a boundary condition that satisfies both thrust as well as strike-slip faulting. Hence deformation of models under the general strain boundary condition demonstrated a larger thrust as well as strike-slip displacement as compared to the experiments performed under the plane strain boundary condkion. The geometric features of folds and faults and the mode of fold propagation remained similar in the two series of experiments. The general strain boundary condition led to displacement out of the tectonic transport plane, resulting in strike-slip displacement along the frontal ramp. The component of horizontal translation along a transfer fault displays different patterns in different experiments. It may be uniform along the strike of the transfer fault or large at the leading frontal ramp. The amount of displacement appears to be dependent on the following: (1) frequency, distribution, and amplification of folds both in the hanging wall and the footwall and (2) amount of relative displacement between the hanging wall and the footwall. The models had a limited extent; hence a possible decrease of the strike-slip component along the frontal ramp away from the transfer fault could not be observed in the experiments. However, a gradual increase the amount of thrust displacement away from the transfer fault was dearly visible [Apotria et al. 1991]. The transfer fault may also act as a broad shear zone terminating into frontal ramps [of. Ramsay and Allison, 1979]. The walls of the shear zone may be parallel (Figure 8a)0 taper toward the trailing frontal ramp (Figure 8b). When walls of the shear zone are parallel, the noncylindrical f0h demonstrate a uniform curvature opposite to the teeto transport direction all along the length of the transfer fault both in the hanging wall and the footwall. However, when the w of the shear zone taper toward the tra'fiing frontal ramp, the amount of fold curvature adjacent to the transfer fault may vary, and the maximum curvature is likely to occur near the junction of the transfer fault and the leading frontal ramp. The sense of fold hinge line curvature is similar to normal fault drag, but the described curvature is characteristic of noncylindrical folds propagating toward a transfer fault. The normal fault drag results in folds which are later cut by a fault; hence the same fold hinge line cannot be traced on the side of the fault. The fold hinge line curvatures that may occur in the vicinity of a transfer fault are summarized in Figure 9. Fold hinge orthogonal to the axis of maximum compression and paralld to the frontal ramp may result at a distance away from the transfer fault (Figure 9a, fold a). A noncylindrical f01d propagating toward the transfer fault may show a curvature toward the tectonic transport direction adjacent to the transfer fault (Figure 9a, fold b)(in a tapering shear zone). An opposite fold curvature may form at the junction of a transfer fault and frontal ramp (Figure 9a, fold d) as a result of the linking of two fold complexes: (1) parallel to the frontal ramp (i.e., orthogonal to the axis of maximum compression) and (2) oblique to the frontal ramp (formed in a transfer fault shear zone). In the footwall region, fold curvature in the tectonic transport direction may form as a result of either normal drag along the transfer fault or transfer fault shear zone (Figure folds e and f). Noncylindrical fold propagation toward the transfer. fault produces curvature opposite to the regional tectonic transport direction (Figure 9a, fold h). At late stages of deformation, the transfer fault may rotate away from the axis of maximum compression and acquire a restraining bend in the middle. The curvature of the fault induces an extra curvature of the adjacent fold hinge lines on either side of the fault (Figure 9a, folds e and' g). The experimental results demonstrate that a small variation in the orientation of an oblique ramp does not change the described interference pattern. However, when the oblique ramp made a large initial angle (70 o or greater) with the axis of maximum eompressi0n, the fold hinge lines developarallel to the trend of the oblique ramp in the adjoining region. During the superposed deformation, most of the fold curvatures were the result of orthogonal linking of the early and the superposed folds [Dubey and Paul, 1993]. Apart from the fold interference patterns, superposed fold curvatures resulted from propagation of noncylindrical superposed folds toward a transfer fault (Figure 9b, fold a). The transfer fault showed a rotation of the fault sarfaee away from the axis of maximum compression. The rotation of the fault also resulted in a rotation of folds in an adjoining region on both sides of the fault (Figure 9b, fold b). A conjugate set of strike-slip faults developed at a late stage of superposed deformation (Figure

8 DUBEY: FOLDS, FRONTAL RAMPS, AND TRANSFER FAULTS 343 r r r b Figure 6. Two stages in the deformation of a multilayer model. (a) 6% shortening and (b) 23% shortening during the early deformation and 13 % during the superposed deformation. HW is hanging wall. 9b, location c). The strike-slip faults either terminated into a adjacent noncylindrical folds also led to curvature of the fold shear zone or terminated by bending toward the receding side hinge lines (Figure 9b, fold d). or splay faulting [Freund, 1974; Dubey, 1980]. Simultaneous The above discussion reveals that there is a systematic development of a conjugate set of strike- slip faults and relationship between the transfer fault and the early and

9 344 DUBEY: FOLDS, FRONTAL RAMPS, AND TRANSFER FAULTS 3.0 /2 E2. o I Eorly deformation I + e 0.!9 Supurposed deformation Figure 7. Total model strain (1 +e) versus component of strike-slip displacement in centimeters along the transfer fault. The locations of measurements are shown in Figure 6a. displacement along the frontal ramps in the vicinity of the transfer fault. 2. The noncylindrical folds in the vicinity of a transfer fault are characterized by a systematic curvature of fold hinge lines toward or opposite to the tectonic transport direction, depending on the location of the fold with reference to the transfer fault and transfer fault shear zone geometry. Since the curvature occurs in different directions, the fold orientation can only be used to deduce the sense of relative movement along a transfer fault after a careful analysis. 3. The initial fold hinge line curvature may increase with an increase in shortening accompanied by sequential initiation and propagation of new folds. Hence a large variation in anguhr relationship between folds and an adjacentransfer fault can be observed. 4. A study of orientation of fold hinge lines across a fault can be used to distinguish between a transfer fault and a hter strike-slip fault. superposed fold hinge lines. This relationship needs to be analyzed prior to the deduction of relative displacement along a transfer fault. The superpose deformation resulted in a reversal of fault displacement. The displacement remained as oblique slip, but the right-lateral displacement along the transfer fault changed to left-lateral displacement. The frontal thrust ramp also displayed a component of left-lateral displacement. In sharp contrast to the geometry of strike-slip faults, the frontal ramp part of the fault had a gentle dip. The gentle dip led to a normal dip-slip displacement along the frontal thrust ramp during the left-lateral displacement along the transfer fault (Figure 10). Thus fault reactivation and reversal of fault displacement as normal fauk took place along the earlier frontal thrust ramp. The present model highlights a special condition where the normal dip-slip displacement a result of left-lateral displacement along a transfer fault. This kind of fault reactivation takes place only in the vicinity of a transfer fault (or an oblique ramp). Away from the transfer fault, the superposedeformation along a frontal ramp is likely to result in strike-slip displacement along the ramp and folding of the thrust. The reactivation of earlier thrust fault as normal fault has been described from the Satluj valley (Himachal Pradesh) at Karoham and at Aicpa [Gururajah and Islam, 1991]. It is suggested that the reactivation of the faults may have occurred by a similar mechanism. The present study does not negate the existence of gravity-driven normal faults which may have developed at a late stage of deformation in the Himalayas [Herren, 1987; Burg et al., 1984; Royden and Burchfiel, 1987; $teck et al., 1993] but suggests that normal faults could generate in frontal ramps during superposed folding adjacent to a transfer fault/oblique ramp. Conclusions Figure 8. Shear zones and associated strains developed in the The following conclusions can be drawn from the study: vicinity of a transfer fault during compressional regime. (a)!. A transfer fault joining two frontal ramps acts like a shear Shear zone with parallel walls and (b) shear zone with walls zone, the geometry of which depends on the relative fault tapering toward' th "trailing frontal ramp. HW, hanging wall; displacement along the transfer fault and the strike-slip FW, footwall.

10 DUBEY: FOLDS, FRONTAL RAMPS, AND TRANSFER FAULTS 345 Figure 9. Fold interference patterns produced by simultaneous development of noncylindrical folds and faults. The thick arrows represent the axis of maximum compression. The solid and dashed lines represent antiformal and synformal hinge lines, respectively. (a) early deformation and (b) superposed deformation. HW, hanging wall; FW, footwall. A B Figure 10. Reversal of fault displacement during superposed deformation. (a) Initial configuration of a fault showing frontal ramp and transfer fault geometries. (b) Thrust displacement along the fault during a compressional regime. The thick arrows represent the axis of maximum compression. The transfer fault has a horizontal and a vertical component of displacement. (c) A change in the axis of maximum compression during superposed deformation. Since the frontal ramps have a gentle dip, reversal of displacement along the transfer fault has resulted in normal fault di.splacement along the frontal ramps.

11 346 DUBEY: FOLDS, FRONTAL RAMPS, AND TRANSFER FAULTS 5. The effect of a later superposedeformation is to produce extra curvatures of the fold hinge lines by fold interference or fold-fault interference, reversal of displacement along the transfer fault, and strike-slip displacement along the early frontal thrust ramp. The fold hinge curvature, though observed in various directions, displays a characteristic pattern with reference to the adjacentransfer fault. 6. A superposedeformation in regions of frontal and transfer fault geometries may lead to r_eversal of fault displacement with the resulthat an early thrust fault may reactivate as a normal fault. Acknowledgments. The manuscript was reviewed by V. Thakur and M. I. Bhat. Useful discussions were made with Sangode. 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