西藏东部构造格架的初步分析

<正> 近几年,我们参加西藏地质局组织领导的一些野外地质调查工作,工作地区涉及拉萨、那曲、山南等地区。本文只是根据近年实际工作,并参考和引用西藏地质局、青海地质局、四川地质局、中国科学院青藏高原综合考察队等兄弟单位的工作成果,试图对西藏东部地区的地质与大地构造轮廓作一梗概分析和介绍。

PRELIMINARY ANALYSIS OF THE TECTONIC FRAMEWORK OF EASTERN XIZANG (TIBET)

1) A new stratigraphic sequence in erected in eastern Xizang (Tibet). Several of the stratigraphic units such as the Danbarize Group (Middle Tria-ssic), Lhozhag Group (Upper Triassic), Guoqu Group(Upper Jurassic), Yulang-baijia Group (Upper Jurassic), Shadui Group (Lower and Upper Cretaceous)and Yamzhog Yumco Group (Upper Cretaceous) are renamed and discussedin this paper. 2) A new division of sedimentary regions is made. To the south of theYarlung Zangbo plate suture, there is the North Himalaya Sedimentary Region, while to the north of the suture the Tanggula-Kangdese Sedimentary Region is further subdivided into Tanggula and Kangdese Subregions. 3) Matching to the division of sedimentary regions, the igneous rocks are also characterised by zonal distribution. For instance from the southern slope of the Tanggula Mts. southward up to the northern Himalayas, the South Tanggula Granite Belt, the Nyainqentanglha Granite Belt and the North Himalayan Granite Belt are encountered successively. Ultrabasic rocks occur chiefly along the Nu River and the Yarlungzangbo River, thus constituting two distinct belts. Volcanics to the north of the Yarlungzangbo River are mainly of acid-intermediate composition, while those to the south-basic intermediate. 4) Based on the characteristics of the sedimentary regions and the magmatic belts just mentioned, Estern Xizang may be geotectonically divided into three fold systems, i.e., the Tanggula(southern slope), the Kangdese and the Himalaya. 5) The Variscan folded basement of the Tanggula and the Kangdese fold systems in the middle of Triassic appeared to be a southward dipping continental slope to the north of the "main" Tethys, lying beneath the sea that might be named as the North tethys. The center of sedimentation, as well as the location of the related tectonic cycles within the extend of the said continental slope, seemed to have shifted southward during the Mesozoic times, resulting in the successive formation of the Tanggula and the Kangdese fold systems in the same direction. 6) Basing upon the preliminary knowledge of geological and geophysical data, the existence of the Yarlungzangbo suture, its formation age and subduction direction are also discussed.