Crustal and upper mantle structure of southeast Tibet from Rayleigh wave tomography

摘要

We have applied the two-plane wave inversion technique to fundamental mode Rayleigh waves in southeast Tibet. Phase velocity variations are obtained at 15 periods from 20 to 143 s and are used to construct three-dimensional shear wave velocity structure. A low-velocity zone is imaged in the middle crust of southeast Tibet, in which velocity perturbation varies from -4% to 3%. The slow area might be associated with high temperature and partial melt and can flow ductilely, while the high-velocity region could be relatively cold and mechanically strong enough to pass crustal strain, implying that coherent lithospheric deformation is possible with the presence of complicated crustal flow. Strong negative anomalies in the shallow crust are imaged at 92°E, correlating with the location of a north-south rift zone. A fast mantle lid to a depth of -120 km is present beneath most of the study area. The lithosphere is significantly slower in the central part of the study area than in the west and east. To the east of 96°E, a subvertical high-velocity zone is imaged at 120-240 km depths under the Bangong-Nujiang Suture (BNS), which is more like the subduction of the Asian lithosphere. We interpret the high-velocity anomalies to the south of the BNS as the subducted Indian lithosphere and those to the north as the Asian lithosphere. A vertical low-velocity column is observed in the central part of the region from the lower crust to the midupper mantle, which is probably in part responsible for the rift at 92°E. The slow anomaly is likely associated with asthenosphere upwelling following the opening of a slab window that could have developed when the slab was torn apart vertically because of the significant change of subducting direction at the eastern syntaxis.