Pn Anisotropic Tomography of Hainan Island and Surrounding Areas: New Insights Into the Hainan Mantle Plume

Abstract

A high-resolution velocity and anisotropy model of the uppermost mantle under the Hainan Island and surrounding areas is obtained by a tomographic inversion of 8192 Pn arrival-time data hand-picked from high-quality seismograms recorded at a total of 137 seismic stations of our recently deployed HAVESArray portable seismic network and Chinese provincial stations. Our new model shows obvious velocity and anisotropic structural features related to the distribution of Quaternary volcanic rocks on the surface. Extensive low Pn velocity anomalies are detected under the Weizhou Island, northern Hainan Island, southern Leizhou Peninsula, southern South China, and around the Pearl River Mouth Basin (PRMB), whereas high Pn velocity anomalies are imaged under the PRMB and areas of 108°–109°E and between the Baise-Hepu fault and the Wuchuan-Sihui fault. The Pn fast propagation directions exhibit a radiated pattern around the PRMB. Integrated with previous global and regional tomographic models, our present results indicate that the Hainan plume is upwelling around the PRMB in the upper mantle, and then plume-related flow is split into several branches under the volcanoes in the northern Hainan Island, southern Leizhou Peninsula and Weizhou Island. This flow pattern could be associated with the westward deep subduction of the Philippine Sea and Pacific plates and eastward deep subduction of the Indian plate. Our results provide new insights into the formation mechanism of the Hainan-Leizhou and Weizhou volcanoes.