FORELAND NORMAL-FAULT CONTROL ON NORTHWEST HIMALAYAN THRUST FRONT DEVELOPMENT

In the Trans-Indus Ranges along the western part of the northwest Hima layan thrust front, unconformities, changes in paleocurrent directions , and locally derived conglomerates in synorogenic foreland basin depo sits provide evidence for major local deformation at greater than or e qual to 3.5 Ma. The tectonic history of the Trans-Indus Ranges has pre viously been described in terms of a single episode of major thrusting at less than or equal to 1 Ma, thus our work implies that there were two distinct phases of deformation. Tn conjunction with published evid ence in the Salt Range to the east for two phases of deformation (simi lar to 6 to 5 Ma, and similar to 2.5 Ma to present), this study demons trates that these two phases of deformation are regionally significant and probably correlative along the entire present-day NW Himalayan th rust front. Reconstruction of possible source areas for the locally de rived conglomerates shows that the earlier deformation is probably rel ated to normal Faulting. These results suggest that the tectonic evolu tion of the area along the present-day thrust front is characterized b y (I) latest Miocene to early Pliocene formation of north dipping norm al fault zones (total throw greater than or equal to 600 m) within the foreland basin, related to syn-orogenic flexure of the Indian plate, and (2) late Pliocene to early Pleistocene initiation of south directe d thrusting along the, present-day thrust front, related to outward gr owth of the NW Himalayan thrust wedge. The location of the present-day thrust front appears to be controlled by north dipping normal faults and monoclines that formed during the earlier deformation and subseque ntly localized structural ramps during later thrusting.