青海拉鸡山:一个多阶段抬升的构造窗

拉鸡山断裂带位于祁连山褶皱带内,呈北西-南东向延伸.后者构成青藏高原的东北边缘,由三个主要构造单元组成:北部是一条早古生代的板块缝合带,中部是一个元古代的结晶地块,南部由一套晚古生代到三叠纪的被动大陆边缘沉积物组成.对拉鸡山及其邻区的构造研究结果表明,祁连山褶皱带在古生代加里东期发生过大规模的缩短,北祁连的早古生代蛇绿岩和岛弧火山岩沿祁连山中央冲断层向南,陆内俯冲到中祁连元古界变质杂岩之下.由于发生在晚古生代和晚中生代的陆内变形,位于中祁连之下的北祁连的蛇绿岩和岛弧火山岩发生褶皱,并被抬升到地表.到新生代,由于印度板块和欧亚大陆之间的碰撞和陆内汇聚作用,拉鸡山断裂带再次活动,这些下古生界蛇绿岩和岛弧火山岩通过冲断作用快速抬升,将中祁连地块一分为二.因此,拉鸡山是一个抬升的构造窗,不是一个中祁连结晶地块中的早古生代大陆裂谷.

THE LAJISHAN FAULT BELT IN QINGHAI PROVINCE: A MULTI-STAGED UPLIFTING STRUCTURAL WINDOW

The Qilian Shan fold belt constitutes the northeastern margin of the Tibetan plateau and can be divided into three major tectonic elements, among which the northern and central parts are characterized as a suture zone of Early Paleozoic age and a metamorphic belt of Proterozoic age, respectively, whereas the southern part is composed of a set of deposits formed in passive continental margin with age ranging from Silurian to Triassic . Marked by a high mountain range, the Lajishan fault belt extends within the eastern part of the central Qilian Shan in a NW SE direction. Our recent field studies show that the Qilian Shan orogenic belt was shortened in N S direction in the Middle Paleozoic along the Qilian Shan central thrust fault, along which the ophiolite and island arc volcanic rocks of Early Paleozoic age of the northern Qilian Shan subducted southward for a long distance under the high grade metamorphic rocks of Proterozoic age of the central Qilian. Resulted from intracontinental convergence of the Tibetan plateau occurred in the Late Paleozoic and Late Mesozoic, the ophiolite and volcanic rocks overlain by the high grade metamorphic rocks of the central Qilian Shan were folded and exhumed to the surface along the Lajishan fault belt. By the Late Cenozoic, trigged by collision between the Indian plate and Eurasia, the Lajishan fault belt underwent rapid uplift along its northern and southern boundary thrust faults, divided the central Qilian Shan into two parts. We suggest here that the Early Paleozoic ophiolite and island arc volcanic rocks along the Lajishan fault belt are the allochthon within a structural window, instead of an continental rift, as some previous studies proposed.