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Lithospheric S-wave velocity structure of west China and neighboring areas from surface wave tomography

HUANG Zhong-Xian;LI Hong-Yi;XU Yi;Institute of Crustal Dynamics,China Earthquake Administration;School of Geophysics and Information Technology,China University of Geosciences;Institute of Geology and Geophysics,Chinese Academy of Sciences;  
We used Rayleigh wave dispersion data and the tomographic method to study the lithospheric S-wave structure in west China and neighboring areas(20°N—55°N,65°E—110°E).The results indicate three active tectonic regions characterized by low crust/upper mantle velocities,i.e.,the west Mongolia-Baikal region,Tibetan plateau,and Indo-China region.The lithosphere beneath west Mongolia is about 80 km thick,where the low velocity layer in uppermantle extends down to 300 km depth,indicating the existence of thermal activity originating from deep mantle.The upper mantle low velocity zone in the Myanmar back-arc region reaches200 km depth,which is apparently related to the thermal/chemical activities above the eastward subducting India plate.The crust of the Tibetan plateau is as thick as 70km;in its marginal area the thickness exceeds 50 km and varies rapidly across the plateau margin,which accords with the flat top and steep edge character of the plateau landform.The average mid-lower crust velocity of the Tibetan plateau is remarkably lower than normal continental crust;in the middle crust(depth20~40 km)a low velocity layer with velocity reversal exists in the entire plateau.These characteristics indicate that the mid-lower crust deformation of the plateau is mainly plastic thickening and lateral flow under the northward compression of India plate.On the other hand the upper mantle structure shows distinctly different characteristics from the crust.Low velocity exists in the top of upper mantle beneath the plateau proper and the west Sichuan—Yunnan region,and the scope of this low velocity diminishes rapidly with depth:it vanishes below 100 km in west Sichuan-Yunnan,and is basically absent below 150 km in the entire plateau.The upper mantle structure of the Tibetan plateau varies significantly in EW direction.In the KarakorumPamir-Hindu Kush region west of 84°E the northward subduction of India and the southward subduction of Asia caused magnificent high velocities in the upper mantle.Between 84°E and 94°E the velocity at the top of upper mantle is low,and a slab-like high velocity exists in the depth range 150~220km,which probably is the subducting India lithosphere with its front reaching beneath the Kunlun-Bayankala mountains.In the region north of the eastern Himalayan syntaxis the velocity in upper mantle is obviously high,which may hamper the eastward movement of upper mantle material.In the west Sichuan-Yunnan region the lithosphere thickness is similar to that of the Yangtze craton,about 180 km,but the velocity in the top of upper mantle is low.These phenomena indicate that the deformation/tectonic movement of the lithospheric mantle in the Tibetan plateau is fundamentally different from its crust.
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