DYNAMICS OF EAST-WEST EXTENSION IN THE WESTERN REGION OF THE INDO–ASIAN COLLISION ZONE

Models of Late Cenozoic east-west extension of the Indo-Asian collision zone are highly variable, and have broad implications for understanding the dynamics of continental deformation. We present a north-south transect across the western region of the High Himalayan thrust wedge and Southern Tibet, documenting the geometry and kinematics for active structures accommodating east-west extension. Our field observations, thermochronometric data, and thermokinematic and geodynamic modeling results suggest that from south to north, oblique collision and underthrusting of India exerts strong controls on the dynamics of east-directed extension. In the High Himalaya, the deformation patterns based on field and remote sensing observations are consistent with extensional forcing due to variably-oblique shear tractions on the Main Himalayan Thrust, resulting in a strain partitioned region in the western Himalaya accomodated by the Karakoram fault to the northwest and the Western Nepal Fault Zone to the southeast cutting obliquely across the thrust wedge. In a distinctly different structural regime north of the Karakoram Fault and Indus-Yarlung Suture zone, we observe a northward-propagating kinematic wave of rapid cooling in the footwalls of normal faults accomodating E-W extension that may be linked to buoyancy stresses resulting from the northward underthrusting of India. Convective removal of the mantle, or extrusion tectonics may have operated earlier, but presently are not as important in the development of active structures along the southern and central regions of the Indo-Asian collision zone. Taken together, these observations highlight the roles of coeval crustal extension and shortening, as well as orogen-parallel strike-slip faulting.