JOINT ANALYSIS OF P-TRAVEL-TIMES TELESEISMIC TOMOGRAPHY AND GRAVITY MODELING FOR NORTHERN TIBET

A combined study of delay time tomography analysis and gravity modelli ng was carried out for a profile across the Northern Tibetan plateau. During the summer and fall of 1993, a seismological array was installe d along the Tibetan highway from the Tangula Shan piedmont to the Nan Shan range. A P-wave delay time tomography analysis was conducted, usi ng 1630 P arrival times from 124 earthquakes recorded by more than 6 s tations. The resulting velocity model reveals three main crustal units and a low velocity zone in the mantle. The resulting seismic image wa s used to construct a 2D gravity model, using a linear velocity-densit y relationship, focusing on the important features of the velocity ano malies. A high coherency is observed between low velocity zones and lo w density zones. The crust shows a gathering of thick and thin, low an d high velocity blocks. This crustal structure is similar to the crust al structure known for southern Tibet. It is well correlated with the tectonic map and reveals that the main sutures visible at the surface have a lithospheric prolongation. The very low velocity anomaly (-5% m aximum) of the model is located between the Jinsha suture and the Kunl un fault, with its top at 200 km depth. The size and the amplitude of this deep velocity anomaly suggest a mantle plume. (C) 1998 Elsevier S cience Ltd. All rights reserved.