天山内部走滑断裂晚第四纪活动特征研究--以开都河断裂为例

THE LATE QUATERNARY ACTIVITY CHARACTERISTICS OF THE STRIKE-SLIP FAULTS IN THE TIANSHAN OROGENIC BELT: A CASE STUDY OF THE KAIDUHE FAULT

天山是典型的陆内再生造山带,其内部构造变形样式复杂且多样。其中尤为显著的是多条长度长、线性良好的NW-SE和ENE-WSW的走滑断裂切割山体内部,并表现出线性笔直的槽谷型地貌景观。现今天山内部的多次强震活动与这类走滑断裂关系密切,因此精细化厘定这些断裂的活动速率和活动特征对于认识现今天山内部变形方式和变形过程具有重要的意义。文中对南天山内部1条NW-SE向右旋走滑断裂——开都河断裂的活动特征进行研究。开都河断裂起自大尤鲁都斯盆地南缘并向SE延伸,切穿南天山焉耆盆地内部,在盆地西南部形成线性连续且笔直的陡坎地貌,尤其是断层在穿过中晚更新世2期洪积扇时断错了扇体上一系列不同时代的河流冲沟,形成从3～248m不等的右旋位错量。通过利用多视角摄影测量技术沿断裂对近12km线性构造地貌进行了条带状航拍,建立了分辨率高达0. 25m的数字地形数据。精确测量了22个典型右旋位错值,发现其位错峰值主要集中在3. 5m、7. 0m、11. 8m和14. 5m,总结认为开都河断裂单次地震右旋走滑位移量大致在3～4m之间。此外,利用原地宇宙成因核素深度剖面法测定断错洪积扇面的暴露年龄约为235. 7ka。结合所测得的冲沟最大的右旋位移量248m,得到开都河断裂晚更新世以来最小右旋走滑速率约1mm/a。通过与天山内部其余活动构造变形速率进行对比,认为开都河断裂在调节天山内部构造变形中起到重要作用,应是天山内部主要的变形构造和应力应变积累区,未来具有发生强震的危险性。

As the most active intracontinental orogenic belt in the world,the Tianshan orogenic belt has complex and diverse internal structural deformation patterns,and among them,the particularly striking is the linear straight U-type valley landscapes which cut inside the mountains by multiple NW-SE and ENE-WSW strike-slip faults.Many of the modern strong earthquakes in Tianshan orogenic belt are closely related to these strike-slip faults.Therefore,it is important to elaborate the activity characteristics of these faults to understand the deformation process inside the Tianshan Mountains belt.This paper focuses on one of the NW-SE right-lateral strike-slip fault(the Kaiduhe Fault),which lies inside the southeastern Tianshan.Typical offset landforms and scarp lineaments on the western segment of the Kaiduhe Fault can be used to study the activity characteristics and strike-slip rate.In particular,the fault cuts through the late Quaternary alluvial fans and a series of river gullies were right-laterally faulted,producing dextral offsets ranging from 3 to 248m.A digital elevation model(DEM)with resolution of 0.25m was established by using multi-angle photogrammetry technique to stripe about 12km linear tectonic landforms along the Kaiduhe Fault.Geological and geomorphic mapping in DEM with 22 high-resolution dextral offset measurements reveals that the dextral offsets can be divide into four groups of 3.5m,7.0m,11.8m and 14.5m.It is presumed from the approximately uniformly-spaced offsets that the coseismic offset was 3~4m.In addition,the exposure age of an older alluvial fan surface was about 235.7ka by in situ 10 Be terrestrial cosmogenic nuclide method.Combining the exposure ages and the maximum dextral offset of 248m,we found that the strike-slip rate of the Kaiduhe Fault is about 1mm/a.It is found by this study that the Kaiduhe Fault plays an important role in regulating SN compression deformation within Tianshan Mountains,and it should also be the main stress-strain accumulation area which has the risk of occurrence of strong earthquake.