| About 70Ma ago,the Indian plate began to move northward continuously,resulting in the closure of the Tethys Ocean about 55-50Ma ago,and the contact,collision and extrusion between Indian and the Eurasian plate.The Tibetan Plateau as a whole rose upward,forming the largest plateau on Earth so far and the highest mountain range on the southern margin--the Himalayas.The convergence of Indo-Eurasia plate is as high as 40%-60%(Tapponnier and Molnar,1976;Tapponnier et al.,1981,1982),The plateau shortened horizontally and thickened vertically,and the topographic relief reached 3000-5000m,forming a 40-70km thick crust.Under the north-south compression,the collision front formed two structural knots at the eastern and western ends of the Himalayan arc mountain system.It stretches for more than 2400km from the Nanche Barwa syntaxis in the east to the Nanga Parbat syntaxis in northeast Pakistan in the west.The crust and mantle materials in the central and eastern parts of the plateau then flow eastward and westward.In the central and eastern parts of the plateau,they rotate around the eastern Himalayan syntaxis and escape to the southeast.Scientists have carried out extensive research on the dynamic process of the deep coupling between the Eurasian plate and the Indian plate.Especially in recent years,with the progress of monitoring instruments and the improvement of transportation conditions,a large number of geophysical monitoring methods have obtained the finer structure of the crust and achieved a large number of exciting results.These large-scale complex tectonic movements are closely related to the material structure,state and migration in the deep mantle.In other words,the deep dynamic mechanism of tectonic changes has been the focus of Earth science for a long time,and the study of the geological structure and mantle morphology is a very important part of the exploration of the deep dynamic mechanism.In this paper,the Great bend in the eastern part of the Himalayan arcuate system and its surroundings are selected as the research area,and the deep mantle P-wave velocity structure in the eastern Himalayan syntaxis and surrounding area is retrieved as the means.The inversion results and a large number of existing research results are combined to explore the scientific problems related to the subduction of the Indian plate in the upper mantle in this area.In order to improve the accuracy of mantle structure inversion in the eastern Himalayan syntaxis and surrounding area,we collected the data of 17 international and domestic scientific survey stations,permernent networks and stations,and our own portable seismic network set up for the Nanga Bawa syntaxis.Several sets of tempraray network data from the northeastern part of the Indian plate were added to increase the data coverage on the Indian side.Using the teleseismic data with the epicentre distance of 30°～90°,strict data screening was carried out with the rationality of travel time,the travel time caused by the crustal material and structure was corrected,152689 effective seismic rays were selected,and the P-wave velocity structure of 70～800km underground in the eastern Himalayan syntaxis and surrounding area was obtained by using the 3D P-wave tomography method.The resolution of most areas with a depth of 70～800km in the study area can reach 80km through the checkboard and recovery experiment.According to the structural characteristics,the inversion results of the whole region can be divided into three parts:the central and eastern Tibetan Plateau region,the great bend region of the eastern Himalayas and the Indo-Myanmar arc region.Combined with the previous research results,the following discussions are carried out respectively:Central and eastern Tibetan Plateau:The front of the Indian lithosphere subducted in the central and eastern part of the plateau(85°E-95°E)and the changes of the subduction angles and morphology from west to east are emphatically discussed.The Indian plate tore in the central part of the plateau and carried out uneven northward subduction with certain angle differences in different parts.The subduction angles were lower in the central to western part and deeper in the central and eastern part.The Indian lithospheric plate subducted at a high angle in the middle of the plateau may have delaminated in the upper mantle at a depth of 200～300km below the Lhasa block(92°～95°E),and the plate droped into MTZ,leading to mantle upwelling in the front.The eastern Himalayan sytaxis area:The Indian plate in the eastern Himalayan sytaxis area may have plate gap,and the asthenosphere mantle may migrate southeast to the volcanic area under the interaction between the complex mantle flow in the upper mantle and the corner flow caused by the subduction of the Indian plate to the north and east.The NE-marching Indian lithosphere front may not exceed the Jinsha suture zone.The northward subduction in the great bend area(93°～98°)of the eastern part of the Tibetan Plateau presents a large angle.The Indo-Myanmar Arc and the southeastern margin of the Tibetan Plateau:The depth and extension of the Indian plate subduction along the Indo-Burma Arc to the east and the origin of the Cenozoic volcanoes are discussed.During the eastward subduction of the Indian plate under the Indo-Myanmar arc,the continental lithosphere and oceanic lithosphere of the Indian plate were fractured at a depth of 200～300km in the upper mantle under the Indo-Myanmar arc(94°～96°E).The mantle of the Indian plate formed a series of volcanoes along the range of the Indo-Myanmar arc,and some blocks broke and droped into the mantle transition zone at～96°E.The subduction angle of the subducted Indian plate changes during its southward migration along the IndoMyanmar arc.The subduction angle decreases by about 10° from the east Himalayan sytaxis to the south of the Tengchong volcanic area.The low-velocity mantle material in the east Himalayan sytaxis area of the western Indo-Myanmar arc may supplement the material source of Tengchong volcano through the window of the subducted Indian plate. |
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