青藏高原中部的东西向扩张构造运动

EW-trending extensional motions in central Tibetan Plateau

系统分析了1933～2003年间青藏高原及其周缘发生的745个中、强地震的震源机制解,研究了高原地壳构造运动及其动力学特征.结果表明,大量正断层型地震集中发生在青藏高原中部海拔4 000 m以上的地区,其中许多地震是纯正断层型地震.震源机制结果显示,该区正断层型地震的断层走向多为南北方向,断层位错矢量的水平分量均位于近东西方向,这表明青藏高原高海拔地区存在着近东西方向的扩张构造运动.地震震源应力场的研究结果表明,在高原中部高海拔地区,E-W向或WNW-ESE向的水平扩张作用控制着该区的地壳应力场.青藏高原高海拔地区近东西方向的扩张构造运动是该区引张应力场的作用结果,其动力学原因可能与持续隆升的高原自重增大引起的重力崩塌及其周边区域构造应力状况有关.而青藏高原周缘地区,除了东部边缘外,南部的喜马拉雅山前沿以及青藏高原的北部、西部边缘所发生的绝大部分地震都是逆断层型或走滑逆断层型地震.在青藏高原周缘地区,北东或者北北东方向水平挤压的构造应力场为优势应力场.在中国西部的大范围内,主压应力P轴水平分量位于NE-SW方向,形成了一个广域的NE-SW方向的挤压应力场.青藏高原及其周缘应力场特征表明,印度板块的北上运动以及它与欧亚板块之间的碰撞所形成的挤压应力场是高原强烈隆起的直接原因.在青藏高原中南部形成了近东西向引张应力场为主的区域,并以东西向扩张构造运动部分释放其应力积累.研究高原高海拔地区的引张应力场和近东西向扩张构造运动的特征,对于认识青藏高原强烈隆起的地球动力学过程与机制,有着重要的理论意义.

In the present paper, the authors analyzed the earthquake mechanism solutions （M4.6-8.5） during 1933-2003 for the study of the extensional tectonic movement at the surface of the Tibetan Plateau and its relative dynamics. The results show that a lot of normal fault type earthquakes are concentrated in the central part of the Tibetan Plateau with the elevation greater than 4 000 m, Many earthquakes are pure normal fault events. Their focal mechanism solutions show that the strike directions of faults are almost in the N-S direction. The dislocation slip vectors of the normal fault type events have large components in the E-W direction, suggesting the extensional motions in nearly E-W direction in the high elevation area of the plateau. The tensional forces in the E-W or WNW- ESE directions are predominant in this region, and may result in the above extensional tectonic motions. The dynamics for extensional motions can be attributed to the gravitational collapse of the rising plateau and the stress of its surrounding areas. Around the Tibetan Plateau, earthquakes caused by reverse and strike-sllp faulting are predominant on the north and west margins （except for the east margin）, and the Himalayan Mountains. Horizontal stress field in nearly NE or NNE direction dominates the tectonic motions along all margins. The tectonic forces from the relative movement between the Indo-Australian and Eurasian plates cause P-axes to line up in the NE-SW direction in the western China.