Abstract:To study the S-wave velocity （VS） structure of the middle and lower crust beneath the eastern margin of the Tibetan Plateau and viscous flow in the deep crust,we adopted data from a regional network of approximately 50 temporary broadband seismometers and 10 permanent seismic stations of the Chinese Digital Seismological Network （CDSN） to invert the detailed structure of the crust and the upper mantle in the eastern margin of the plateau and in Sichuan Basin by using the linear inversion method.We determined that from 0 km to 10 km,VS is lower in the basin because it contains a huge thickness of sediments characterized by low velocity.In contrast,the regions in the eastern margin of the plateau show a higher velocity.From 10 km to 40 km,the basin shows higher VS in the middle and lower crust.In contrast,the eastern margin of the plateau shows lower velocity; the average velocity of the middle crust is 3.0～3.4 km/s.From 50 km to 60 km,the basin extends to the upper mantle,which shows high velocity.Low velocity still occurs in this depth range in the lower crustal portion of the eastern margin of the plateau at an average velocity of 4.0～4.5 km/s.The prominent features of the VS profiles indicate that the majority of low-velocity zones appear in the middle crust at 20 km to 40 km and that small low-velocity zones occur in the upper crust at 10 km to 20 km and in the lower crust at 40 km to 60 km.In addition,only a portion of the low-velocity zone occurs in the southern region of eastern margin of the plateau.The crust varies in thickness from 50 km to 66 km in the eastern margin of the plateau and from 40 km to 44 km in the basin.The gradient of the Moho discontinuity is highest in the boundary along the eastern margin of the plateau toward the basin.The crustal flow does not increase,and most of the flow direction occurs along the main active faults.The effective middle and lower crustal thicknesses have been estimated to be between the 20 km and 40 km; the crustal viscous flow is generally heterogeneously dispersed at a depth of 20 km but extends to 60 km in some areas.The dominant flow direction is along the main active faults.When the viscous flows in the middle and lower crust were obstructed by the strong,rigid Sichuan Basin,they divided into two or more branches with different directions.The upper part of viscous flow upwelled to produce sufficient pressure for intrusion into the upper crust,thereby uplifting the mountain ranges and high peaks along the eastern margin of the Tibetan Plateau.In contrast,the lower downwelling part of viscous flow produced sufficient pressure for intrusion into the lower crust and upper mantle to deepen the Moho discontinuity,which caused the crust to thicken.
