Simultaneous development of noncylindrical folds, frontal ramps, and transfer faults in a compressional regime: Experimental investigations of Himalayan examples

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.