A low velocity belt beneath northern and eastern Tibetan Plateau from Pn tomography

Abstract

New Pn tomography on Tibetan Plateau (TP) and neighboring region is conducted with data assembled from various sources. The improved Pn tomographic image delineates several distinct geophysical regimes that are approximately separated by geological boundaries at surface. Dense distribution of the stations in the eastern half of the plateau allows us to infer a 3D view of crustal thickness variation of the region. The most striking feature is the bended low velocity belt (LVB), which extends from west to east beneath Qiangtang block and Songpan Ganzi fold belt (SGFB) in northern TP and then turns consistently southward with SGFB beneath eastern TP and further south to Indo-China. The consistency between geological boundaries on surface and sharp velocity contrast boundaries in uppermost mantle may suggest strong deformation coupling between crust and, at least uppermost mantle, if not whole lithosphere. A north-south striking LVB is also observed cutting through the southern TP, coinciding with the Yadong-Gulu rift at surface. Our tomographic results are generally consistent with dynamic models that involve with underthrusting in the southern TP, shortening in the northern TP, and extrusion to the east and the southeast. Furthermore, our results suggest significant east-west variations in the structure and dynamics. Copyright 2006 by the American Geophysical Union.