东昆仑断裂带东段分支断裂--阿万仓断裂晚第四纪构造活动特征

THE TECTONIC ACTIVITY CHARACTERISTICS OF AWANCANG FAULT IN THE LATE QUATERNARY,THE SUB-STRAND OF THE EASTERN KUNLUN FAULT

文中从几何结构特征、断裂长期滑动速率和古地震复发特征3个方面对阿万仓断裂进行了研究。详细的遥感解译和野外调查结果表明:1)阿万仓断裂作为东昆仑断裂带东段(玛沁—玛曲段)的分支断裂,和东昆仑断裂一样也是1条全新世活动断裂,性质为左旋走滑兼逆断,总长约200km。西北段由2条总体走向310°,相距约16km近平行的次级断层组成,向SE方向合为1条断裂。2)在阿万仓断裂上发现大约15km长的古地震地表破裂带,表现为断层陡坎、断塞塘、地裂缝、断层沟槽等典型断错微地貌现象。3)经航、卫片解译,野外现场调查,断错地貌测量和样品测试,得到该断裂晚第四纪以来的平均左旋水平滑动速率为3mm/a,垂直滑动速率约0.07mm/a。4)通过对断错最新地貌面的测年和探槽剖面分析,认为阿万仓断裂带存在4次古地震事件,属原地复发型,最新1次事件是在(850±30)a BP以后发生的。5)阿万仓断裂左旋滑动速率与东昆仑断裂带玛沁—玛曲段递减的滑动速率量值相当,它的存在和发现可以很好地解释东昆仑断裂带东段(玛沁—玛曲段)滑动速率递减的特征。东昆仑活动断裂带中东段滑动速率逐渐递减,与东昆仑活动断裂带中东段帚状散开的几何结构有关,其中的阿万仓断裂是东昆仑断裂带东延过程中的重要分支断裂,吸收了东昆仑断裂带东延的应变分配。

It is well known that the slip rate of Kunlun Fault descends at the east segment,but little known about the Awancang Fault and its role in strain partitioning with Kunlun Fault. Whether the sub-strand（ Awancang Fault） can rupture simultaneously with Kunlun Fault remains unknown. Based on field investigations,aerial-photo morphological analysis,topographic surveys and14 C dating of alluvial surfaces,we used displaced terrace risers to estimate geological slip rates along the Awancang Fault,which lies on the western margin of the Ruoergai Basin and the eastern edge of the Tibetan plateau,the results indicate that the slip rate is 3mm / a in the middle Holocene,similar to the reduced value of the Kunlun Fault. The fault consists of two segments with strike N50° W,located at distance about16 km,and converged to single stand to the SE direction. Our results demonstrate that the Awancang fault zone is predominantly left-lateral with a small amount of northeast-verging thrust component. The slip rates decrease sharply about 4mm / a from west to east between the intersection zone of the Awancang Fault and Kunlun Fault. Together with our previous trenching results on the Kunlun Fault,the comparison with slip rates at the Kunlun fault zone suggests that the Awancang fault zone has an important role in strain partitioning for east extension of Kunlun Fault in eastern Tibet. At the same time,the 15 km long surface rupture zone of the southeast segment was found at the Awancang Fault.By dating the latest faulted geomorphologic surface,the last event may be since the 1 766± 54 Cal a BP. Through analysis of the trench,there are four paleoearthquake events identified recurring in situ on the Awancang Fault and the latest event is since（ 850± 30） a BP. The slip rate of the Awancang Fault is almost equivalent to the descending value of the eastern part of the east Kunlun Fault,which can well explain the slip rate decreasing of the eastern part of the east Kunlun Fault（ the Maqin-Maqu segment） and the characteristics of the structure dynamics of the eastern edge of the Tibet Plateau. The falling slip rate gradient of the eastern Kunlun Fault corresponds to the geometric characteristic. It is the Awancang Fault,the strand of the East Kunlun Fault that accommodates the strain distribution of the eastward extension of the east Kunlun Fault. This study is helpful to seismic hazard assessment and understanding the deformation mechanism in eastern Tibet.