Crustal Footprint of Mantle Upwelling and Plate Amalgamation Revealed by Ambient Noise Tomography in Northern Vietnam and the Northern South China Sea

摘要

A high-resolution crustal and uppermost mantle velocity model of northern Vietnam and the northern South China Sea (SCS) is essential for understanding the evolution of the SCS and its relationship to the Hainan plume, which is still controversial. Based on ambient seismic noise analysis, we processed ambient noise data from 65 broadband stations in southwestern China and northern Vietnam to obtain the crustal and uppermost mantle, S-wave velocity model, through the direct surface-wave tomography method, with a lateral resolution of 100-150 km. Our results show distinct lateral velocity variation at shallow crustal depths, such as significant low velocity in the Beibu Gulf Basin and the Yinggehai Basin and relatively higher velocity in Hainan Island. We suggest that the low velocity of the lower crust and uppermost mantle beneath the Yinggehai Basin indicates the present upwelling of upper mantle material. However, the observed high-velocity upheaval feature beneath the Leizhou Peninsula may represent the ancient magmatic underplating, which has a relation to the Hainan plume. Moreover, our results demonstrate different crustal structures of the two flanks of the Red River Fault, that is, in the middle and lower crust, the S-wave velocity in the region north of the Red River Fault is higher than that in the south. This may be associated with the collision and amalgamation of the South China and Indochina Blocks. In addition, our results also indicate a crustal footprint of the amalgamation of the Yangtze and Cathaysia Blocks.