Pn uppermost mantle tomography of Central Tibet: Implication for mechanisms of N-S rifts and conjugate faults

Abstract

The north-south convergence between the Indian and Eurasian continents since Cenozoic has produced the Tibetan plateau, and east-west extension structures have also developed since late Cenozoic. Whether these extensional structures are limited to the crust or extend to the deep lithosphere remains controversial. The uppermost mantle serves as a link between the crust and deep lithosphere, the velocity structure of which provides an important constraint on exploring the mechanism of extensional structures. Based on Pn traveltime data recorded by the SANDWICH network in central Tibet, we use the interstation traveltime difference method to obtain the uppermost mantle velocity structure. Our results show significant velocity differences for the uppermost mantle beneath the conjugated strike-slip faults and rifts, which indicates their different formation mechanisms. The high Pn velocity beneath the conjugated strike-slip faults support an eastward horizontal shear at the base of the upper crust rather than the lower crust has contributed to the development of the conjugated strike-slip faults. The low Pn velocity beneath the rifts in Lhasa may provide evidence for the rifts cutting through the entire crust and reflect the connection between the formation of the rifts and deep tectonic activities. The most prominent features in our results are the low Pn velocity zones beneath the Tangra Yum Co Rift (TYR), the Pumqu-Xianza Rift (PXR) and the zone between the two rifts, and we explain the low Pn velocity as the top of the mantle wedge upon the subducting Indian lithospheric slab.