阿尔金断裂系西段——康西瓦断裂的 晚第四纪构造地貌特征研究*

阿尔金断裂带是青藏高原北部的一条大型左旋走滑断裂带,近EW向延伸2000多公里, 它构成了青藏高原与塔里木盆地之间的重要地质边界。康西瓦断裂位于阿尔金断裂带西段, 呈WNW-ESE向延伸约 700km。文章在高分辨率卫星遥感图像(印度遥感卫星5.8m分辨率)和数字高程地形模型(DEM)数据分析的基础上,并结合野外构造地貌考察观测,对康西瓦断裂的第四纪构造活动及其地貌特征进行了初步研究。沿断裂带发育的系统错断水系、错断冲积扇、挤压脊、走滑拉分盆地等典型构造地貌特征表明,该断裂晚第四纪经历了强烈的左旋走滑活动。同时,研究还揭示沿康西瓦断裂发育了一条长约80km的地表地震破裂带,最大同震左旋水平错位为4m,估算产生该地表破裂带的地震是一矩震级为M_w7.3的大地震。 另外,文章根据不同年代地表地貌特征的左旋错位距离,估算出康西瓦断裂晚第四纪以来的长期走滑速率为8~12mm/a,远低于早期估算的20~30mm/a,但是与阿尔金断裂带中、东段的地质估算结果9±2mm/a及GPS测量结果9±4mm/a接近。

LATE QUATERNARY TECTONO-GEOMORPHIC FEATURES ALONG THE KANGXIWAR FAULT, ALTYN TAGH FAULT SYSTEM, NORTHERN TIBET

The Tibetan Plateau is one of the most remarkable topographic features on Earth, and has widely taken to be the classic example of continent-continent collision. In the northern Tibet, the Altyn Tagh fault (ATF) system over 2000-km-long is the longest left-lateral strike-slip fault in the Asia, which accommodates sinistral motion between the Tibetan Plateau and the Tarim block within the India-Eurasia collision zone. The Kangxiwar Fault, extending over a length of 700km, is located on in northwestern Tibet as the western segment of the ATF. Our goal of this study is to document the Late Quaternary geomorphic features along the Kangxiwar Fault, Western Kunlun Mountains based on the analyses of high-resolution satellite images (Landsat ETM, IRS and SRTM DEM data) as well as field observations. We also discussed the lateral slip-rate and the relationship between growth of tectono-geomorphic features and repeated large seismic events. We identified an 80-km-long surface rupture zone produced by the most recent large seismic event with a maximum 4m of coseismic displacement. Consequently, this earthquake might be of moment magnitude (M_w) 7.3. Geomorphic features such as systematic offset streams, dis￣placed fluvial fans and terrace risers, pull-apart grabens, and pressure ridges or push-ups are well developed along the surface rupture zone. These well-preserved geomorphic features are related to long-term geomorphic growth as well as the dry and cold alpine climate. In addition, we inferred that a long-term slip rate is 8~12mm/a since 55±5kaB.P. according to these displaced geomorphic features and their apparent dating age, which is far lower than the high slip rate (20~30mm/a) as the extrusion model expected, but it is close to the result estimated from geologic (9±2mm/a) and geodetic measurements (9±4mm/a) in the central and eastern segments of the ATF.