康西瓦断裂晚第四纪左旋走滑速率的光释光定年研究

The Tibetan Plateau has experienced intensive crustal shortening and tectonic deformation during the Cenozoic. As a result, a series of strike-slip faults formed along the northern margin of the Tibetan Plateau. The faults play important roles in the intra-continental deformation by distributing deformation since the convergence of India and Eurasian continental plates. Quantification the late Quaternary strike-slip rates of major bounding faults is not only the keys to understand the activity of the strike-slip faults at the northern margin of the Tibetan Plateau, but also helps to evaluate the seismic hazards in the associated areas. At present, there are still different opinions on the long-term slip rate of the associated faults at the northern margin of the Tibetan Plateau, impeded by the correct estimation of the offsets and ages of the displaced geological units produced by the associated faults. In the study, we focus on the Kangxiwar Fault at northwestern Tibetan Plateau. The optical dating technique is applied to date the displaced geologic units which were produced by the left-lateral movements along the Kangxiwar Fault through the Karakax valley. Additionally, the offsets of the displaced geologic units along the Kangxiwar Fault are evaluated by using the satellite image analysis and the field geodetic measurements. The late Quaternary left-lateral slip rate along the Kangxiwar Fault is estimated using the optical dating ages and the offsets.

青藏高原在新生代经历了强烈的地壳缩短和构造变形。受其影响，青藏高原北缘发育了一系列的大型走滑断裂。这些断裂对调节印度、亚欧板块碰撞作用产生的陆内构造变形起着非常重要的作用。对这些断裂晚第四纪以来走滑速率的研究，对评估青藏高原北缘相关区域的断裂活动性和地震危害性具有重要科学和现实意义。目前，对青藏高原北缘相关区域不同断裂地质时期的走滑速率, 不同学者之间仍存在较大争议。产生争议的主要原因之一是与相应的错断地质体错距和年代测定的精度有关。本项目以青藏高原西北缘康西瓦左旋走滑断裂为研究对象，通过光释光定年技术对喀拉喀什河谷段沿康西瓦断裂发育的错断地质体进行年代约束。同时，在卫星影像解译基础上，结合野外详细的定量观测，对相应错断地质体的水平位错量进行准确测定，进而估算康西瓦断裂晚第四纪不同时段以来的左旋走滑速率。

康西瓦断裂（又称喀拉喀什断裂）为青藏高原西北缘的一条大型左旋走滑断裂。该断裂和其东边的阿尔金断裂构成了青藏高原北部和塔里木盆地的重要地质边界。目前，不同学者对康西瓦断裂晚第四纪平均走滑速率仍存在争议。对该断裂晚第四纪不同时期以来走滑速率的准确厘定，对理解陆内构造变形的地球动力学机制以及评估青藏高原北缘相关区域的断裂活动性和地震危害性具有重要科学和现实意义。通过野外调查和高分辨率卫星影像，发现喀拉喀什河谷段沿康西瓦断裂发育一系列以左旋错断为特征的地质地貌，如错断山脊、错断水系、错断河流阶地和拉分盆地等。本项目研究重点围绕两个地点的错断地质体（分别称为A、B和C）。（A）喀拉喀什河谷段在新藏公路里程碑415km 附近发育的一处错断洪积扇。该洪积扇上发育一个小型拉分盆地。基于拉分盆地演化两种简单模式，分别利用拉分盆地长边（450±45m）和斜边（650±65m）限定对应冲洪积扇的水平位错位量和错位量的上限值，通过光释光定年技术得出该冲洪积扇的形成年代≥52.4±3.4 ka。因此，分别估算出康西瓦断裂晚第四纪以来的平均左旋走滑速率为≤8.6±1.0 mm/a和< 12.4mm/a。（B）该地点冲洪积扇上发育的错断阶地，以陡坎T2/T1为错断标志，其水平位错为260±26 m，而对应阶地的放弃年龄为30.9±2.1ka，估算出康西瓦断裂晚第四纪平均左旋走滑速率为8.4±1.0 mm/a。（C）三十里营房东侧地区塔尔萨依吉勒尕河下游东侧四级河流阶地，通过卫星影像分析，发现这四级河流阶地分别被左旋位错了~60 m，~235 m，~260 m和~365 m。光释光测年结果表明这四级河流阶地平台分别形成于距今 14.5±1.0 ka, 21.2±1.2 ka, 28.5±1.7 ka 和39.7±2.5 ka。结合这四级河流阶地的左旋错断位距和阶地面形成年代，估算出康西瓦断裂晚第四纪左旋走滑速率为7.8±1.6 mm/a。因此，围绕喀拉喀什河谷段两个地点三处错断地质体得出的康西瓦断裂晚第四纪平均走滑速率和现代卫星系统观测到数据较为一致。因此，康西瓦断裂在青藏高原西北缘为调节印度-欧亚板块的碰撞起着非常重要的作用。