CRUST AND UPPER MANTLE TOMOGRAPHY IN TIBET USING SURFACE-WAVES

We have obtained maps of lateral variations of structure in the crust and lithosphere in Tibet and surrounding regions by inversion of single station measurements of fundamental mode Rayleigh wave phase velocities in the period range 25-100 sec, for earthquakes located in central Asia and observed at teleseismic digital stations in Eurasia. Phase velocity maps are obtained using a tomographic method without a priori constraints on region boundaries. The lateral variations of S velocity reveal a symmetric pattern on each side of crustal low velocity zone centered on the Chang Tang region (north central Tibet). This zone is bounded by rather steep velocity gradients to the south and east, about 250 and 400 km north and west of the frontal thrusts of the Himalayas and Lungmen Shan, respectively. The northern boundary of the zone crosses the northern limit of the Tibet plateau (Altyn Tagh and Kunlun) extending into the southwest Tarim and Qaidam basins. The low crustal velocities are consistent with a thick, hot crust in the central part of Tibet, consistent with Quaternary volcanism in Chang Tang. The steep gradients towards higher velocities in the south and in the north are compatible with the documented continental subduction in the south, and suggest that a similar situation might exist in the north.