P wave Velocity Structure in the Crust and Upper Mantle Beneath Northeastern South China Sea and Surrounding Regions

Abstract

With the local and teleseismic P wave arrival times from Chinese and International Seismological Centre (ISC) stations, the tomographic inversion was conducted for the three-dimensional velocity structure of the crust and upper mantle beneath the northeastern South China Sea (SCS) and surrounding regions. The tectonic discrepancies and deep geodynamic characters were analyzed based on the tomographic image. High-velocity anomalies at the uppermost mantle beneath the northeastern SCS reveal the extension of the lithospheric mantle from the South China continent to the marginal sea. The cold, thick lithosphere beneath the northeastern SCS inferred from high-velocity anomalies suggests that no magma was intruded from the upper mantle. Therefore, it can be inferred that the rifting of South China continent may only occur inside the crust rather than extend down to the upper mantle. The continental margin of SCS indicates the tectonic nature of passive non-volcanic continental margin. Low-velocity anomalies at the central sub-basin suggest the upwelling of hot materials from the mantle. The high-velocity anomalies in the South China continent and Taiwan region and the low-velocity anomalies at Okinawa Trough delineate the different tectonics of the crust. The former ones belong to the normal crust of Eurasia continents or the thickened crust induced by the collision between the Philippine Sea plate and Eurasian plate, whereas the latter one belongs to the thinned crust induced by back-arc spreading. The Binhai Fault belt represents the suture zone of South China continent and SCS, which separates the high velocities located in its two sides. The low velocities at the upper mantle beneath the marine of South China continent reveal the deep-driven magmatism. Velocity anomalies at Luzon, Manila Trench and eastern Lunzon Trench are all related to their tectonic natures. Low velocities at the upper mantle from southern Taiwan to northern Luzon depict two volcanic chains: western volcanic chain and eastern volcanic chain, whereas the high velocity belt near the north Luzon ridge suggests the subducted continental slab.