滇西南新生代走滑断裂运动学、年代学、及对青藏高原东南部块体运动的意义

KINEMATICS AND DATING OF CENOZOIC STRIKE-SLIP FAULTS IN THE TENGCHONG AREA, WEST YUNNAN: IMPLICATIONS FOR THE BLOCK MOVEMENT IN THE SOUTHEASTERN TIBET PLATEAU

依据走滑断裂的运动学和年代学 ,确认滇西腾冲地区新生代大型走滑断裂带变形作用的三个阶段 :1 )始新世初 (54— 56Ma) ,在槟榔江两岸出露的与新特提斯俯冲和两大陆碰撞相关的左旋走滑 -逆冲断裂 ,由此推断腾冲地块西缘南北向展布格局是两大陆碰撞后发生顺时针旋转达 90°的结果。2 )渐新世 -中新世 ,腾冲地块东缘的高黎贡右旋走滑断裂和西缘的那邦右旋走滑断裂存在两个走滑活动的峰期 :2 4 - 1 9Ma和 1 1 - 1 4 Ma,早期与 Tapponnier模式中挤出块体东边界红河 -哀牢山左旋走滑断裂活动的时限相一致 ,指示高黎贡和那邦右旋走滑断裂在此时期是挤出的印支地块的西边界 ;晚期与安达曼海的扩张、缅甸境内实皆断裂的右旋活动相一致 ,可能是此期地块再次发生挤出的结果。3)中新世末 ,约 5- 8Ma间两大陆的进一步会聚 ,引起了腾冲地区岩石圈结构的重要变化 ,腾冲地块发生了向南的挤出和顺时针的旋转 ,促成了一系列与此前右旋走滑相关的盆地的折返和南北向凹陷盆地的形成 ,制约了腾冲火山岩的喷发和整个地区的快速抬升。腾冲地块及其周缘新生代断裂带多阶段运动的转换对揭示青藏高原东南部块体运动型式具有重要的研究意义。

The Tengchong area, situated in southwest China, is an important field workshop to study the continental dynamics and the collision effects between the India and Eurasia plates. There are three stages of strike slip shearing movements during the Cenozoic. The early Binglangjiang River transpressional shear zone is a left lateral strike slip fault. The ages of the syntectonic dykes yield about 54—56Ma that is the same as the age of collision between the India and Eurasia plates. Although now the transpressional shearing zone trends south to north, when the fault occurred, it must extend in west east direction according to the direction of the movement of India plate. It can be concluded that the western boundary of the Tengchong block trends south to north is the result of the block clockwise rotation and used to be in west east direction at the beginning of Cenozoic era. The second stage is characterized by dextral strike slip shearing deformation about 19—24Ma and 11—14Ma determined by mineral K Ar method. On one hand, the Gaoligong fault, together with the Nabang fault, is the west boundary of the extrusion Indochina block during the collision of two plates and the uplift of Tibet instead of Sagaing fault in the central Burma. On the other hand, because of the India plate moved northwords, the Gaoligong and Nabang faults, the same as the Sagaing fault, are also the response to the opening of Andeman Sea. The third stage have being occurred since 5—8Ma B.P. The structural stress field changed rapidly to be left lateral shearing instead of dextral shearing. The basins, formed during Oligocene due to the dextral shearing movement, stopped their evolution with the strata folded. The new stretch basins formed and the famous Tengchong lavas erupted and the steep normal fault formed responding to the crust uplifting. All evidences show that the Tengchong block extrudes southword and rotates clockwise, in the meantime an mantle upwelling take the place of the low lithosphere due to the delamination. We can rebuild up the movement pattern as following. During the subduction and collision between two continents, responding to the oblique stress, there occurred the Binglangjiang transpressional fault as an example. A rapid clockwise rotation of all blocks and shearing zones developped between about 55 and 24 Ma B.P., followed by the extrusion of the Indochina block from 24 to 19 Ma B.P. The movement of Gaoligong and Nabang faults during 11—15 Ma is the result of the opening of Andeman Sea. About 5—8 Ma B.P., the Tengchong block began to extrude southwords and rotate clockwise.