Gravity Study On Crustal Structure And Material Migration Beneath The Eastern Tibetan Plateau

The eastern Tibetan plateau is located in southwet China, and characterized by complex Cencozoic structures during the Indo-Asian collision, where is one of the areas with occrrring of frequent strong earthquakes. Studies of the crustal structure and deep matter migration progress have important significance to reveal the mechanism of the Tibetan orogen development and tectonic activities.As an effective way, the gravity method has been a long history and widely used to study crustal structure and movement. Through the subsurface material property, gravity and its changes can be linked to seismic, geomagnetic and geodetic data based some relationship, thus the gravity method has a significant advantage. Therefore, the gravity anomaly and gravity change data are employed to study the crustal structure and material migration beneath the eastern Tibetan plateau.The gravity anomaly data derived from the Earth Gravity Model (EGM2008) was used to obtain the isostay, local and regional field, multi-scale source and tectonic lineament. Furthermore, the normalized full gradient of Bouguer gravity anomaly of the three measured profiles was calculated. The result of correlation analysis between topographt and Bouguer gravity anomaly (BGA) indicated that the study area is basically in the isostatic state. Trend analysis, wavelet multi-scale decomposition and gradient analysis of BGA were used to investagate the crustal structure from basic pattern, local deep source and tectonic lineament, and the results showed that:(1) The Longmenshan-Xiaojinhe faults and the region west Nujiang fault can be considered as the boundaries of the Tibetan plateau and its sourrounding blocks in the study area; (2) The subduction of the Burma microplate made the forth-order details, revealed the anomaly of the middle crust, display the multi-order en echelon patterns of positive and negative anomalies from southwest to northeast across in the southwestern study area. The effect of the subduction extend over broad area of the Indo-China block and the Chuan-Dian rhombus block (CDB), and it should be an inducements of clockwise rotation of the CDB; (3) The negative anomaly in the southern Three River resion, northern CDB and northeastern Bayan Har block (BHB) were induced by the Tengchong volcano for the first area and partial melting of lower cust for the latter two area, respectively. The positive anomaly in southern CDB and the Longmenshan area were caused by injected cooling magma and eastward extrusion mass accumulation for obstacle of the Sichuan basin, respectively; (4) The detachement layer between upper and lower crust beneath the BHB stretched upward gradually with deep in the west and shallow in the east. The eastward extension material reached the surface along the detachement layer and caused the Longmenshan uplift. The eastward tilt of the Xianshuihe fault in the middle crust suggested that eastward movement of the CDB was prominenty while clockwise rotation.The topography and geoid data were used to obtain the crust-mantle boundary (Moho) and lithosphere-asthenosphere boundary (LAB) in the eastern Tibetan plateau under the assumption of local isostasy. Forthermore, Bouguer gravity anomaly was used to invert the Moho depth. And then, the crustal density perturbation was obtained by combination of the residual gravity anomaly and P-wave velocity structure of the CRUST1.0 model. The results show that the crust and lithosphere of the Qiangtang block (QB), BHB and the northern CDB are very thick under local isostasy. The Moho morphology derived from the gravity inversion is largely similar to the isostatic Moho, and the depth of the former is 10km shallower than the leatter in the QB, BHB and the northern CDB. The Moho morphology presents a depression structure with about 5km northwest the Longmenshan beneath the BHB, and a dome structure with about 5-7km in the southern CDB. The There is a nearly NS trending belt with the low density anomaly in the upper and middle crust west three Rivers faults, which reveals that this belt is an important channel of the crustal mass southward movement beneath the Tiobetan plateau. The density distribution in the upper and middle crust of the CDB appears feature alternate with high and low anomaly, and it is related to the subduction of the Burma microplate. Restricted by the Sichuan basin, the eastward extending mass of the Tibetan plateau converge under the Longmenshan area, and forms a NE-SW trending high density belt in the upper and middle crust.The gravity change data observed on the surface was used to investigate the crustal matter migration in the eastern Tibetan plateau. Using the accumulated and differential change of the gravity at one year scale during 2010 and 2015, the three-dimention maps of crustal density change (CDC) were obtained based on the methd of compact gravity inversion. The characteristics of the crustal matter migration were analyzed via the maps of crustal density change. The results indicated that the most significant density change located in the upper and middle crust, and the negative CDC was dominant in the BHB and the northern CDB, surggested that the eastward and southeastward expension and stretch of the crust accumulated beneath Longmenshan area restricted by the Sichuan basin. Expansion of the crustal mass in northern CDB more significant than in the Bayan Har block surrgested the difference in movement velocity between two blocks. The occurrence of strong earthquakes released the substances for pregnandy and made the crustal density increace obviously. Meanwhile, the occurrence of strong earthquakes may obstruct or slow the eastward and southeastward migration of the crustal matter.The results of crustal structure and density change were combined to interpret the mechanism of the tectonic activities in the eastern Tibetan plateau. The middle and lower crust flow have differential speed among tectonic blocks under the control of the major faults, and then the movement of the upper crust dragged by crustal flow should also have differential speed. The characteristic features of the deep mass migration are eastward extrusion and expansion beneath the BHB and the CDB and accumulation beneath Longmenshan area. The Wenchuan earthquake and the Lushan earthquake were induced by accumulation of deep eastward movement material at the detachement layer. Furthermore, the Lushan earthquake had different magnitude and source location compared to the Wenchuan earthquake, suggested that the deep material beneath southwest segment of the Longmenshan fault had been southward dragged and eastward pushed by the strong sinstral shear of the Xianshuihe fault. The crustal dome was significant and the current magmatic activity was continuous in the southern CDB.