Southern Tibet: its deep seismic structure and some tectonic implications

Abstract

An international seismic investigation in southern Tibet, the indepth (International Depth Profiling of Tibet and the Himalayas) II project, was conducted in 1994. The German contribution, gedepth (German Depth Profiling of Tibet and the Himalayas), included broad-band seismometers and 30 mobile seismic stations. These stations were situated in the area of the Yarlung Tsangpo Suture and recorded signals up to a distance of 250 km from shots along the indepth II reflection line. Velocity information from an offset-dependent travel time diagram revealed a 35 km thick (sialic) upper crust with V P below 6.3 km/s, including a low-velocity-channel between 15 and 20 km depth. V P-values in the lower crust are not well constrained, but some events with an 8 km/s velocity were recorded from a depth of around 70 km. They are only observed between 240 and 250 km distance. This short offset range does not allow a decision on whether they are P n or P MP, or diving waves from a velocity gradient zone at the base of the crust. Several observations argue for a velocity gradient zone, compatible with the presence of a smooth gabbro-eclogite transition. Also a temperature model of southern Tibet is presented, which is based on certain boundaries of heat flow models, P- and S- velocities, surface waves, and seismicity. Based on our velocity and temperature models, an assessment on the petrological and rheological structure for the evolution of the Tibetan plateau was made. Among the four most viable models we prefer the “Hydraulic Pump” model, supported by crustal “Escape” and mantle “Underthrusting”.