Insight into the origin of the Tengchong intraplate volcano and seismotectonics in southwest China from local and teleseismic data

摘要

A high-resolution tomographic image of the crust and upper mantle under Yunnan Province in southwest China was determined by using a large number of teleseismic data measured precisely from digital seismograms as well as local- earthquake arrival times recorded by the dense Yunnan seismic network. The grid spacing of the tomographic model is —70 km x 78 km in the horizontal directions and 4-70 km in depth. Our resulting model shows a clear low-velocity (low-V, <-0.3% with respect to the average one-dimensional velocity model) column extending from the surface down to about 400-km depth under the active Tengchong volcano and some high-velocity (high-V) anomalies existing in the mantle transition zone. Furthermore, the low-V anomaly extends horizontally toward the northeast at —250- to 400-km depth. Combining our tomographic results with geochemical and geological investigations, we infer that the Tengchong volcano is a rift-related volcano caused by the subduction and dehydration of the Indian plate as well as corner flow in the mantle wedge, though the subducted slab is a continental plate. Our results show that the upwelling flow under Tengchong originates at —400-km depth. A prominent low-V anomaly along the Red River fault zone extends down to the upper mantle (-100-km depth), reflecting that the Red River fault zone may have cut through the crust to the upper mantle. The two Dayao earthquakes (M 6.1 and M 6.2) in 2003 occurred on the margin of high-V anomalies and are underlain by a prominent low-V anomaly in the lower crust and upper mantle, suggesting that the Dayao earthquakes may be associated with fluids released from the dehydration of the subducted Indian slab. These results are significantly improved compared with previous studies and provide new seismic constraints on the dynamic processes of the India-Asia collision.