Timing and Rates of Quaternary Normal Faulting in Central Tibet

In Summer 1998, a 400 km long seismic refraction profile (INDEPTH III) was shot in Central Tibet. Apart from recording the signals of 11 shots along the line, the stations also recorded a number of local earthquakes more or less in the extension of the line. Record sections and travel time data from both shots and earthquakes were compiled and interpreted jointly. The following peculiarities are observable: (1) All southern shot points and earthquakes exhibit a sudden offset or time delay in the travel time curves for crustal phases (Pg ) at about the latitude of the Bangong-NuijangSuture (BNS). (2) Strong Pn branches, but no PM P are visible in the earthquake records. (3) Neither Pn nor sub-critical PM P are observed in the shot records but some post-critical PM P phases can be identified. The first observation can be explained by a vertical lowvelocity zone near the BNS with a steep northerly dip and reaching to at least 35 km depth. The other two observations indicate the presence of a seismic velocity gradient between 55 and 70 km depth. A gradient zone rather than a first order boundary at the Moho was also observed in Southern Tibet and beneath the Altiplano in the Andes and was predicted by previous experimental investigations of the gabbro-eclogite transformation for areas with strongly thickened crust such as Tibet. In contrast to southern Tibet, the travel time data do not show any evidence of a mid-crustal layer of low P velocities; P velocities in the lower crust are low.