Initial geometry of western Himalaya and ultrahigh-pressure metamorphic evolution

Ultrahigh-pressure metamorphic rocks on both sides of the western Himalayan syntaxis show different P–T–t paths. The Kaghan unit was metamorphosed under the UHP conditions significantly later (∼46 Ma) than the Tso Morari unit (∼53 Ma), implying that the Tso Morari was subducted earlier (∼57 Ma) than the Kaghan unit (∼52 Ma). The age difference likely reflects the initial shape of Greater India, with the Kaghan unit located greater than 300 km south of the Tso Morari before the collision of two continents. We calculate the dip of the subducting plate using two independent methods. The results show gentle dipping subduction east of the western syntaxis, and steep subduction west of the syntaxis since the time of India–Eurasia collision to the present time. We propose that the steep subduction in the western part is likely related to the proto-Chaman and Karakorum faults along which the Indian plate moved northward. In the eastern part, the overlying Eurasian plate extruded to east, which allowed gentle dipping subduction of the Indian continent. Although the main period of eastward extrusion of the Eurasian continent occurred between 30 and 15 Ma, our results suggest that this was likely taking place since the early India–Asia collision. Using those geometrical constraints, a 3D image of the slab is reconstructed in the western part, showing the sharp bending of the western syntaxis along the proto-Chaman fault. This bending resulted in the warping of the slab surface to form a conical fold with a north-dipping axis located near the western syntaxis.