白龙江断裂西段晚第四纪构造活动特征研究

Characteristics of the Late-quaternary Activity of the Western Segment of Bailongjiang Fault

白龙江断裂位于青藏高原东缘,东昆仑断裂的北侧。白龙江断裂西段的展布及其活动性对研究东昆仑断裂速率向东衰减的机制有重要科学意义。本文根据遥感解译、地质地貌调查、探槽和地质剖面研究白龙江断裂西段的几何展布和断层活动性。白龙江断裂西段分为3支,包括郎木寺断裂、阿米塘断裂和热尔-旺藏断裂组成的中支、崩巴村断裂构成的南支和下山-迭部断裂构成的北支。这些断裂皆为左旋走滑断层,其中下山-迭部断裂、郎木寺断裂和热尔-旺藏断裂为全新世断层,崩巴村断裂为晚更新世断层。白龙江断裂西段沿袭了前第四纪断裂展布,其分支的几何形态上属于一种典型的尾端构造样式:同向分支断层。研究结果表明白龙江断裂地表上未与东昆仑断裂直接相接,而在深部与之相接,可能分解吸收了东昆仑断裂的部分滑动速率,使得塔藏断裂的滑动速率低于其西侧的玛曲断裂。

Bailongjiang fault lies parallel and north to the Eastern Kunlun fault in east margin of the Qinghai-Xizang Plateau.The geometry,location and activity of the western segment of the Bailongjiang fault are crucial to understanding the falling slip rate gradient of the eastern Kunlun fault.Three branch faults were found at the western end of the Bailongjiang Faults,including the Central fault,the Southern fault and the Northern fault,revealed by our remote-sensing,field survey and trenching.The Central fault is composed by three segments,the Langmusi fault,the Amitang fault and the Reer-Wanzang fault from the west to the east.The Southern fault and Northern fault are named as the Bengbacun fault and Xiashan-Diebu fault respectively.These faults were active left-lateral strike-slip faults in the late quaternary.The Langmusi fault,the Reer-Wanzang fault and the Xiashan-Diebu fault were active in the Holocene,the Bengbacun fault was active in the late-Pleistocene,and the Amitang fault was supposed to be active from the middle-Pleistocene to late-Pleistocene.In the late quanternary,the activities of the Bailongjiang fault at its western end occurred partially along the pre-Quaternary faults.The geometries of the branch faults show a pattern of the Synthetic branch faults,a typical tip geometry for strike-slip faults.They link forming splay as shear fractures,with the same sense of slip as the master fault.Our results demonstrate the Bailongjiang fault doesn’t connect to the Eastern Kunlun fault on the surface,but is combined with it in deep level.This pattern of faults connection suggests partial slip rate of the Eastern Kunlun fault was absorbed by the Bailongjiang fault in the north,causing the slip rate of the Tazang fault lower than the Maqu fault in the west.