VARIATIONS OF CRUSTAL THICKNESS AND VP/VS ACROSS MONGOLIAN FOLDBELT, THE BAIKAL RIFT, AND THE SIBERIAN PLATFORM: OBSERVATIONS AND INTERPRETATION

Spatial variation of crustal thickness (H) and Vp/Vs is investigated by stacking P-to-S converted waves and their multiples recorded at about 30 portable seismic stations along a 1280 km profile across the Mongolian foldbelt (MF), the Baikal rift (BR), and the Siberian platform (SP). We found that beneath the MF and BR, the thickness is about 45 km which is about 10 km thicker than the global average, and beneath the SP, it decreases abruptly to about 37 km in an area from the northern boundary of the BR to about 200 km north of it on the SP. It gradually increases to about 44 km in the area 200 - 400 km from BR. The mean value of Vp/Vs in the SP is about 1.70 which is slightly higher than that in the IASP91 earth model (1.73), and in the BR and MF it is about 1.82.

In order to interpret the observations, we measured crustal thickness beneath permanent seismic stations in Asia using the same procedure as what we used for the profile, and found a linear decrease of crustal thickness as a function of distance from the India/Tibet collision zone. This relationship, complemented by geological observations about the tectonic history and evolution of the study area, supports the hypothesis that the crustal thickening beneath the MF and BR is the result of horizontal shortening caused by the continental collision. The higher-than-normal Vp/Vs could be the result of a small amount of partial melt in the crust, which reduces both Vp and Vs and especially the latter. The relatively thin crust in the 200-km-wide zone beneath the southernmost part of the SP could be the result of eclogitization at the bottom of the crust, as a result of the ancient northward subduction of the Amurian microplate. The eclogitization might have created a high velocity rock layer of several kilometers thick at the bottom of the crust, and thus resulted to a "seismic Moho" which is several kilometers shallower than a "petrological Moho".