Transitional Moho and low velocity crust under eastern border of the Tibetan Plateau

Abstract:

The lateral expansion and uplift of the Tibetan Plateau causes devastating earthquakes, but is poorly understood. In particular, the links between regional variations in surface motion and the deeper structure of the plateau are still unclear. The plateau may deform either by movement of rigid crustal blocks along large strike-slip faults, by continuous deformation, or by the eastward flow of a channel of viscous crustal rocks. Recently, high resolution seismic imaging, by using seismic data recorded with an array of about 300 seismographs in western Sichuan, China, has identified zones of weak rocks in the deep crust, which is interpreted as crustal flow channels. They also found gradual crust-mantle transition as commonly found in tectonically active area. Here we further identify the upper and lower boundary of the transitional Moho under the eastern edge of the Tibetan Plateau using the same data set. The upper layer of the boundary is relatively clear and thicker than normal Moho discontinuity. The lower boundary is commonly visible under the Chuan-dian block despite it is normally weaker. Combined with the widely spread low-velocity middle and lower crust and upper-most mantle, this may indicate the geodynamic process in the region of the Eastern Tibetan Plateau is not solely limited in the crust, the whole lithosphere could have involved in the building of the plateau and the collision between the Tibetan Plateau and the Yangzi block.