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Abstract

Over the last 20 years project INDEPTH has helped to image the underthrusting Indian continental lithosphere, crustal low velocity zones, and large changes in seismic anisotropy that correspond with the major tectonic blocks that make up the plateau. INDEPTH-IV differed with the prior INDPEPTH phases by having a 2-D seismic array as well as with resolution profiles. This 2D array combined with other 2-D arrays across the eastern edge of the plateau have helped to give a new high resolution three dimensional picture of crustal and upper mantle structure in this important region. Current velocity models of the Tibetan Plateau have had uniform structure parallel to the Himalaya and major east-west suture zones; however, our new images have significant orogen-parallel variability in upper-mantle seismic velocities. In general the upper mantle, from the Main Frontal Thrust to central Tibet around 33°-34°N, exhibits laterally variable P- and S-wave velocity anomalies extending to at least 250 km in depth. Significant low velocity anomalies are observed beneath southern Tibet, elongated in a north-south direction and extending to at least 150 km depth, possibly deeper. These slow velocity anomalies can be interpreted as evidence for fragmentation of the UICL as it is forced beneath the overriding Tibetan lithosphere. In contrast, the upper mantle of the northern Qiangtang terrane and Songpan-Ganzi terrane shows a homogeneous low velocity zone with no sign of a southward subduction of the Asian continental lithosphere. We also find consistent results between the velocity and attenuation tomography. QLg and QPg models show a high seismic attenuation zone along the Kunlun belt. Similarly, a high velocity and high Q block are observed in southeastern Tibet around the eastern Bangong-Nujiang Suture and Eastern Himalaya Syntaxis. There is a significant azimuthal anisotropy of seismic Q. The isotropic seismic Q, suggested as an estimate of Q of the crust, is low in northern Qiangtang and the Songpan-Ganzi. The high-Q directions parallel major fault planes in eastern Tibet, and correlate with the fast-direction of shear wave splitting and Rayleigh wave azimuthal anisotropy, suggesting coherent deformation between the crust and upper mantle in this region