Modeling the evolution of continental subduction processes in the Pamir–Hindu Kush region

Several geological and geophysical studies suggest the presence of two converging subduction zones in the western syntaxis of the India–Eurasia collision zone, with steep northward subduction of Indian lithosphere beneath the Hindu Kush and southward subduction of Asian lithosphere under the Pamir. We investigate the geometry and the timing of these subduction processes. Seismic tomography images are used to constrain the geometry of the two slabs under the Pamir and the Hindu Kush. By measuring the tomographically inferred length of the Indian slab under the Hindu Kush region and by comparison with paleomagnetic reconstructions, we estimate that the process of Indian slab break-off most likely occurred at the early stage of the collision, at ca. 44–48 Ma. We infer that after slab break-off, western India continued its northward motion until its northern boundary reached the present Hindu Kush region, where it began to subduct. We estimate an age for the initiation of subduction in this region of about 8 Ma. We apply 2D thermo-kinematic and rheological numerical modeling to compute the temperature distribution and brittle field along two vertical sections across the Hindu Kush and Pamir regions. For the Hindu Kush we introduce parameters reproducing fast and near vertical subduction, whereas in the Pamir region we use the results from published studies to reproduce slower subduction. Modeling results indicate that faster subduction under the Hindu Kush results in a deeper brittle region, which is consistent with observed deeper seismicity than in the Pamir region.