Passive Sources Seismic Observation In Deep Crustal Structure Of The Eastern Margin Of The Tibetan Plateau

In order to improve the characterization of crustal heterogeneity and quantitative analyses of the velocities and viscous flow in the deep crust, we analyzed data from a regional network of about50temporary broadband seismometers and40permanent seismic stations of CDSN (Chinese Digital Seismological Network).Our research has made three main discoveries, as follow:1. From0-10km, Vs is lower in the Sichuan Basin because the basin has huge thickness of sediments that have low velocity characteristics. In contrast, the mountain ranges in the eastern margin of the Tibetan plateau show higher velocity. From10-40km the Sichuan Basin shows higher Vs velocity in the mid and lower crust, and in contrast, the eastern margin of the Tibetan Plateau shows lower velocity. From-50-60km the Sichuan Basin has been extends to the upper mantle that shows high velocity. Still in this depth range low velocities are found in the lower crustal portion of the eastern margin of the Tibetan Plateau. At80km depth, upper mantle is present in both region, but the velocity distribution indicates the obvious difference. In the eastern margin of Tibetan Plateau the low velocity layer still is found in the northern part. According to the distribution and pattern of low velocity zone in each layer, we can infer the presence of viscous crustal flow which drives the crustal material around the Theologically strong Sichuan Basin. Throughout the Alpine-Himalaya orogenic belt, continental crust deformation and crustal thickening have resulted in active orogenesis. These blocks behave as rigid obstacles to ongoing deformation. In the eastern margin of the Tibetan Plateau, this obstacle is the strong, rigid, Sichuan Basin. It caused the crustal thickening and elevated topography in the margin region. This has been proposed to be the direct result by weak of lower crustal material being evacuated from beneath the central Tibetan Plateau. 2. The results suggest the presence of complex heterogeneity in the crust beneath the eastern margin of the Tibetan Plateau and the Sichuan Basin.The most prominent features are the strong and rigid crust and upper mantle beneath the Sichuan Basin which indicates the normal value of Poisson’s ratios generally decrease from0.28to0.24.The normal-to-high Poisson’s ratios (0.24-0.34) were found in the northern part of the eastern margin of the Tibetan Plateau, and highest values (0.3-0.35) found in the central part of the eastern margin of the Tibetan Plateau. In the southern part of the eastern margin of the Tibetan Plateau the Poisson’s ratio exhibits the variation from0.23to0.34.The variation of the Poisson’s ratio in the northern and southern parts of the eastern margin of the Tibetan Plateau indicate that the higher heterogeneity in the crustal composition are consistent within a mafic/ultramafic lower crust and upper mantle. The highest value of Poisson’s ratios (0.3-0.35) were found in the central part of the eastern margin of the Tibetan Plateau may indicate the presence of partial melt in the mid and lower crust.This region just located the place which has thickest crust and lowest velocity zone, and locally, the existence of partial melt in the eastern margin of the Tibetan Plateau.3. According to the distribution of low velocity zones in each layer we can infer that the viscous crustal flow drives the crustal material around the obstacle of the strong and rigid Sichuan Basin. The crustal flow is not growing anywhere, and most of the flow direction is along the main active faults. The flow direction coincides with the observed GPS velocity field around the eastern margin of the Tibetan Plateau relative to the Sichuan Basin and Yangtze Craton. The effective mid and lower crustal thicknesses have been estimated to be between the20and40km, namely, the crustal viscous flow distribution is dispersedly at the depth of20km even extending to60km heterogeneously. Estimates of the viscosities the flow range from1017-1020Pa s and occur in the crust over length scales of tens to hundreds of kilometers and possibly over more than1000km, with flow durations on the order of106-107yr (Cook et al.2008; Clark et al.2000). Flow within the crust is driven by horizontal pressure gradients from the central Tibetan Plateau, due to topographic gradients and variations in crustal thickness. When the viscous flows in the mid and lower crust hits the obstruction of the strong rigid Sichuan Basin then flow delamination occurs. These flows are divided into two or more branches with different directions. The upper part of viscous flow upwells to produce enough pressure to intrude the upper crust, thereby uplifting the mountain ranges and high peaks along the eastern margin of the Tibetan Plateau. In contrast, the lower, down-welling part of viscous flow produces the pressure to intrude the lower crust and upper mantle to deepen the Moho boundary causing the crust to thicken.