Southern Tibet: its deep seismic structure and some tectonic implications

An international seismic investigation in southern Tibet, the INDEPTH (Inte rnational Depth Profiling of Tibet and the Himalayas) II project, was condu cted in 1994. The German contribution, GEDEPTH (German Depth Profiling of T ibet and the Himalayas), included broad-band seismometers and 30 mobile sei smic stations. These stations were situated in the area of the Yarlung Tsan gpo Suture and recorded signals up to a distance of 250 km from shots along the INDEPTH II reflection line. Velocity information from an offset-depend ent 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 dep th. 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 a re only observed between 240 and 250 km distance. This short offset range d oes not allow a decision on whether they are P-n or PMP, or diving waves fr om a velocity gradient zone at the base of the crust. Several observations argue for a velocity gradient zone, compatible with the presence of a smoot h gabbro-eclogite transition. Also a temperature model of southern Tibet is presented, which is based on certain boundaries of heat flow models, P- an d S- velocities, surface waves, and seismicity. Based on our velocity and t emperature models, an assessment on the petrological and rheological struct ure for the evolution of the Tibetan plateau was made. Among the four most viable models we prefer the "Hydraulic Pump" model, supported by crustal "E scape" and mantle "Underthrusting". (C) 2001 Elsevier Science Ltd. All righ ts reserved.