青藏高原东北缘岩石圈三维密度结构

3Ddensity structure of lithosphere beneath northeastern margin of the Tibetan Plateau

综合重力观测资料和地震波走时资料反演了青藏高原东北缘岩石圈三维密度结构，并对该区岩石圈结构及动力学特征进行了讨论．首先利用收集到的P波近震和远震走时数据进行地震层析成像，得到研究区岩石圈三维P波速度结构．然后利用速度-密度经验关系式，将速度扰动转化为密度扰动建立研究区三维初始密度模型．最后利用分离的布格重力异常反演得到了岩石圈三维密度结构．反演结果表明：青藏高原东北缘地壳密度结构特征有利于地震孕育发生和地壳物质侧向流动；地壳内，密度异常等值线走向与地表断裂走向基本一致，进入地幔后，密度异常等值线走向发生了顺时针旋转，这表明青藏高原东北缘地壳和地幔具有不同的构造运动模式，暗示该区可能发生了壳幔解耦；80～100km深度上，P波速度异常较密度异常明显偏低，推测该区可能发生了部分熔融或者岩石含水量的增加；印度板块俯冲和周围坚硬块体阻挡联合作用，使得青藏高原东北缘形成了强大的区域构造应力场，并导致深部软流圈热物质上涌，为该区壳幔解耦、部分熔融和P波涑唐降低俐浩了条件．

We obtained the beneath northeastern margin three of the dimensional velocity and density structures of Tibetan Plateau （TP） using gravity and seismic lithosphere data, and discussed its geodynamic implication. Firstly, P wave velocity structures are determined by seismic tomography using P wave travel time data, and then converted to initial density model for the following gravity inversion. Subsequently, the density structures in lithosphere are inverted by Bouguer gravity anomaly. Our density model shows that the crustal density structures are critical for the occurrence of earthquakes as well as the lower crustal flow. The trend of density isopleths is coherent with that of surface fractures in the crust. However, it presents a clockwise rotation in the mantle. This phenomenon may indicate the decoupling between curst and mantle In 80-100 km depth, P wave velocity anomalies are significantly lower than density anomalies. This may demonstrate the partial melting and increase of rock water Content. The combined effects of the subduction of Indian plate and the obstruction force from Ordos and Alashan blocks made a strong tectonic squeezing, inducing the asthenosphere upwelling, and providing conditions for the crustal-mantle decoupling, partial melting, and reduction of the P velocity.