青藏高原地球动力学初探

青藏高原是当今世界上最高、最大和最年轻的高原。青藏高原的地球动力学研究是一个复杂而饶有兴趣的问题。本文试图从高原的范围、地球物理特征、地震活动、构造演化和新构造运动等方面探讨其地球动力学过程。 从新构造分析,笔者着重指出,青藏高原是欧亚大陆板块中最活动的一个块体,它在造就中国乃至东亚新构造运动的格局方面,是一个十分重要的因素。

A PRELIMINARY STUDY ON GEODYNAMICS OF QINOHAI-XIZANQ (TIBET) PLATEAU

Qinghaiizang plateau, the highest, largest and youngest plateau in the world, has an average elevation about 4,500m and crustal thickness about 50 km, covers an area about 3,300,000km2, and intensively uplifts during the later Cenozoic Era. The geodynamics of the plateau is one of the most interesting problems for the earth scientists. Recently, the author took part in the compiling work of the Atlas of the Lithospheric Geodynamics of China and managed in field investigations in the plateau and surrounding areas. This paper will briefly discuss the geodynamics according to the realm, geophysic features, seismicities, tectonic evolution and neotecto-nics. Qinghaiizang plateau is bounded 'by mountain ranges, the Himalaya on the south, the Karakorun on the west, the Altun Tagh, Kunlun and Qilian on the north and the Longmen and Hengduan on the east. Strictly speaking, the Pamir plateau is a component of it. Moreover Qing-haiizang plateau is an unified body, confining the Siwalik foredeep on the south margin, the Kunlun foredeep and Hexi corridor depression on the north margin, and the Longmen foredeep on the east margin. These fore-deeps indicate that the whole plateau has been uplifting. The map of the bouguer gravity anomalies ( fig. 2 ) and the distribution of epicenters of earthquakes (fig. 3 ) clearly show the outline of the plateau. Results of the seismic reflection Profiling and electromagnetic prospecting reveal that the crustal structure is complex in the Qinghaiizang plateau, which consists of multilayer velocity features. There are low velocity layers in the crust and upper mantle. The Qualue of the cnist is very low and is less than the half of the groble average (Feng et al., 1985) . The geothermal anomalies are high too. The heat flow value reaches 4.245 HFU at Yamzhoumco in southern Xizang (Wei et al. ,1985). The crustal density is low (2.84 g/cm3) , while the density of the upper mantle is remarkably high (3.40?.65g/cm3) (Ma et al., 1987). Based on the calculation of seismic data,the lithospheric thickness is about 100?60km. Thus, the geophysic characteristics of the Qinghaiizang plateau may be summarized as a relatively soft lithosphere with a light but thick crust and a dense but thin upper mantle. These are important in the geodynamic evolution of the plateau. The Qinghaiizang plateau is one of the most intensive seismic regions in China. There have been 345 earthquakes with M^ 6 from 192 A. D. to 1986, i. e. 11 earthquakes with M 8 , 53 earthquakes with M=7.9? and 281 earthquakes with M=6.9 ?6. Most of the earthquakes are shallow ones, but some intermediate earthquakes occur along the Yarlung Zanbo fault and the west Kunlun Mountian. Gathering the seismic foci depths, we plotted a profile of seismic focal depth of the Qinghaiizang plateau (fig. 4) . It shows that; ( 1 )There is a dominant seismic layer at 33 km in deep and the seismic foci are shallower in the Qianton area. ( 2 ) A northward seismic zone which declines from the south foot of the Himalaya extends to the plateau about 200 km in deep. From the viewpoint of seismic focal depth, we suggest that the seismic focal plan is a"V"type in the Qinghai?Xizang plateau, and that the Tarim block wedges from the north in forming the plateau, besides the Indian plate subducted from the south. The tectonic evolution of the Qinghaiizang plateau is virtually considered as a history with opening of the Gandwanaland and closing of the Tethyan Ocean. Before Mesozoic, this region evidently belonged to the platform of the northern margin of the Gandwana. After Mesozoic, a series of opening and closing events were continuously happened, which formed aseries of arcuate erogenic zones (fig. 5 ) . .Finally, the Gangdise trough was closed and created the current framework of the Plateau, since the Indian plate collided with the Eurasian plate (about 40 Ma) . According to the intensity and the styles on neotectonics, 5 active blocks may be subdivided in the Qinghaiizang plateau, i. e. the Himalayan block, the Xizang block, the Gansuinghai block, the west Kunlu