Subsidence history of the north Indian continental margin, Zanskar-Ladakh Himalaya, NW India

Detailed geological field mapping has allowed the restoration of two full stratigraphic sections through the highly deformed Mesozoic and Early Tertiary fold and thrust belt of the north Indian continental margin. The two sections, which are representative of a proximal and a distal facies on the margin, have been backstripped using standard techniques. Profiles of the tectonic subsidence and uplift through the precollisional history of the margin have been constructed and compared with the predictions of simple thermal and mechanical models. The pre-collisional history can be explained by a thermal model with an initial age of rifting of 270 Ma and a stretching factor, beta of c. 1.2. This model accounts for the general exponential decrease in the backstripped tectonic subsidence. The model fails, however, to completely explain the subsidence and uplift history of the margin since the late Cretaceous. The history during this time is characterized by uplift at the most proximal location and an increase in subsidence at the most distal location. We attribute these differential vertical movements to flexural loading of the north Indian margin by obduction of the Spontang ophiolite. The best fit model is one in which a 70 kin wide wedge-shaped load, tapering from 10 to 0 kin thick, is emplaced on rifted lithosphere with an elastic thickness, T-e, of 5-10 km. These results, which are in accord with the late Cretaceous timing of obduction and the structure of the Spontang ophiolite, provide new constraints on the T-e structure of extended continental lithosphere 120-150 Ma after a rifting event.