青藏高原及其邻近地区的地球物理场特征与大陆板块构造

本文搜集了现在已知的关于青藏高原地区的各种地球物理场特征,即:该区的地壳与上地幔构造,地磁场要素的分布,航空磁测的结果,古地磁极移轨迹,重力异常与均衡补偿,地热活动与温泉分布,地震活动以及深地震探测等研究结果,来探讨它与大陆板块构造的关系。 研究的初步结果表明,印度洋板块与欧亚板块交接地带的北界为雅鲁藏布江,南界为恒河平原的北缘。喜马拉雅地带为这两大板块碰撞与挤压的过渡带,其宽度约300公里左右。这一地带的大、小地震绝大部分是浅源地震,只在弧形山系和东西弧顶及其转折部位有中源地震。在这一过渡带内水热活动剧烈,重力也不均衡。 雅鲁藏布江以北到当雄一带,地壳厚度为70—73公里,喜马拉雅地区则为68—45公里左右,并向南翘起。地壳由多层介质组成,在下地壳中存在着低速层。断层面解表现为向南逆冲,主压应力轴基本上为南北向和北东向,且与震源深度相关。现在构造活动与地震活动似均逐渐向南移到主边界大断层一带。 在雅鲁藏布江以北,小震震源深度向南递加,而在恒河平原以北,则向北递加。此外,在上述两个地区均有零星的中源地震发生。因此,喜马拉雅地带的南北两侧有相向“俯冲”之势。在兴都库什地区,中源地震震源面北倾;在帕米尔地带,中源地震震源面南倾。因此,震源面构成了“V”字

CHARACTERISTICS OF THE GEOPHYSICAL FIELDS AND PLATE TECTONICS OF THE QINGHAI-XIZANG PLATEAU AND ITS NEIGHBOURING REGIONS

In this paper,we have collected all the avtilable data that are pertinent to the study of this region,such as the crust and upper mantle structures in the Qinghai-Xizang region,the distributions of the geomagnetic field elements,the results of aero-magnetic surveys,the palaeomagnetic pole wandering paths,gravity anomalies,geo-thermal activities,earthquake activities,fault plane solutions and other results of geophysical interests,and tries to discover the relationships between these features of the geophyaical field and plate tectonics.The preliminary results indicate that the northern boundary of the zone where Indian plate and Eurasian plate are in contact is Yarlung Zangbo River and the south-ern boundary is in the northern edge of Ganga plain. The Himalayan zone is a transitional belt whose width is about 300 km or so. Most of the large and small earthquakes occurring in this zone are of shallow foci. Only in the arcuate mountain system and at the vertices of the east and west area as well as at the turning points of the range that earthquakes of medium depths occurred. In this transitional zone,hot water activity is very strong and the gravity field is not in isostatic equilibrium.From the north of Yarlung Zangbo River to Damxung region,the crust is 70-73 km thick,but in the Himalayan region,the crustal thickness decreases to about 68-45 km and is upwarping towards the south. The crust is multilayered and there exists a low velocity layer in the lower crust. The fault plane solution shows that the fault is thrusting towards the south,the principal compressional stress axis being in general directing either N-S or NE and relating to the focal depth. It seems that the recent tectonic and earthquake activities gradually move towards the south reaching the great fault of the main boundary.In the north of Yarlung Zangbo,the focal depths of small earthquakes increase to-wards the south,but in the north of the Ganga plain it increases to the north. More-over,in the two places mentioned above some earthquakes of medium depths occur.Therefore,there are thrusting faults both on the southern and on the northern sides of Himalayan zone. In the Hindukush region,the plane of the foci of the medium deep earthquakes dip towards to north in the Pamir zone; they dip towards the south. So the plane of the foci forma "V" shaped region.The stress and energy of the tectonic motion in the plateau and its adjacent regions have not been exhausted at all,and had direct influence on the earthquake activities and structures of the provinces of Sichuan,Shaanxi,Gansu and Ningxia.