Crustal structure of central Tibet as derived from project INDEPTH wide-angle seismic data

In the summer of 1998, project INDEPTH recorded a 400km long NNW-SSE wide-angle seismic profile in central Tibet, from the Lhasa terrane across the Banggong-Nujiang suture (BNS) at about 89.5°E and into the Qiangtang terrane. Analysis of the P-wave data reveals that (1) the crustal thickness is 65+/-5km beneath the line; (2) there is no 20km step in the Moho in the vicinity of the BNS, as has been suggested to exist along-strike to the east based on prior fan profiling; (3) a thick high-velocity lower crustal layer is evident along the length of the profile (20-35km thick, 6.5-7.3kms^-1) and (4) in contrast to the southern Lhasa terrane, there is no obvious evidence of a mid-crustal low-velocity layer in the P-wave data, although the data do not negate the possibility of such a layer of modest proportions.

Combining the results from the INDEPTH III wide-angle profile with other seismic results allows a cross-section of Moho depths to be constructed across Tibet. This cross-section shows that crustal thickness tends to decrease from south to north, with values of 70-80km south of the middle of the Lhasa terrane, 60-70km in the northern part of the Lhasa terrane and the Qiangtang terrane, and less than 60km in the Qaidam basin. The overall northward thinning of the crust evident in the combined seismic observations, coupled with the essentially uniform surface elevation of the plateau south of the Qaidam basin, is supportive of the inference that northern Tibet until the Qaidam basin is underlain by somewhat thinner crust, which is isostatically supported by relatively low-density, hot upper mantle with respect to southern Tibet.