Upper Mantle Shear Wave Velocity Structure of Southeastern China: Seismic Evidence for Magma Activities in the Late Mesozoic to the Cenozoic

The Fujian-Taiwan Strait-Taiwan region, as the active eastern margin of Eurasia since the Mesozoic, has experienced the subduction and retreat of the Paleo-Pacific plate from the Middle Mesozoic, the breakup of the continental margin in the early Cenozoic, and then the collision with the Philippine Sea plate in the late Cenozoic. To better understand the tectonic evolution process, a high-resolution 3-D shear wave velocity model of the crust and upper mantle was obtained from surface wave tomography through a direct inversion method. Intermediate- to long-period Rayleigh wave dispersion curves, extracted from teleseismic surface wave with the traditional two-station method, were combined with the short-period dispersion curves obtained from ambient noise cross correlation to obtain broader band surface wave data. Two groups of large-scale low-velocity bodies are found to be located in the upper mantle of Fujian and the Taiwan Strait and are inferred to be closely related to the magmatic activities caused by the Mesozoic subduction and Cenozoic extension, respectively. The low-velocity anomalies in Fujian are concentrated in the west Cathaysia block in deep regions and split into small bodies in the uppermost mantle, while the low-velocity body in the strait is divided into northern and southern parts, corresponding to the two-stage Cenozoic extension. The velocity gradient was also calculated to estimate the lithospheric thickness, which is only 60-80 km, and the lithosphere-asthenosphere boundary also features strongly horizontal variations and discontinuities. Our results provide important seismological constraints to investigate the geological and tectonic processes of this region.