天山是典型的陆内再生造山带,研究其现今内部断裂的变形特征和活动速率对于认识整个天山造山带的应变分配方式和变形过程具有重要意义.现今天山活动构造的研究大部分集中在天山两侧向盆地扩展的前缘部分,然而对于天山内部活动构造的定量化研究并不多见.该研究聚焦于南天山与其内部山间盆地之间的边界断裂——焉耆盆地北缘断裂,通过野外地质调查可将该断裂分为东西2段,其中东段逆冲断错了一系列山前洪积扇,形成了线性明显的陡坎地貌.通过利用高精度差分GPS 对23组断层陡坎的测量,发现其垂向位移大致可分为1.9m、2.4m和3.0m 3组,推测单次地震的同震位移量为0.5~0.6m.其中保存于3.0m左右陡坎的地貌面为区域性地貌面,通过利用原地宇宙成因核素测定该地貌面的暴露年龄约为5ka,这与博斯腾湖沉积物所记录到冷暖气候交替的时间段相符,说明气候的冷暖变化控制了南天山前地貌面的形成和废弃.结合断层陡坎高度及地貌面年龄可得焉耆盆地北缘断裂东段5ka 以来的倾滑速率为0.6~0.7mm/a,SN 向的地壳缩短速率约为0.4mm/a,垂向滑动速率约为0.5mm/a.依据地震矩计算公式评估焉耆盆地北缘具有发生7.5级强震的可能性.该研究为认识现今天山的变形过程和变形方式提供定量化的数据支持,对于理解天山内部的强震发生地点和地震危险性具有重要的现实意义.%The Tian Shan Mountains is an active orogen in the continent.Previous studies on its tectonic deformation focus on the expanding fronts to basins on either side,while little work has been done on its interiors.This work studied the north-edge fault of the Yanqi Basin on the southeastern flank of Tian Shan.Typical offset landforms,and lineaments of scarps on the eastern segment of this fault were used to constrain the vertical displacement and shortening rates.Geological and geomorphic mapping in conjunction with high-resolution GPS differential measurement reveals that the vertical offsets can be divided into three groups of 1.9m,2.4m and 3.0m,and the coseismic vertical offset was estimated as 0.5 ~0.6m.In situ10Be terrestrial cosmogenic nuclide dating of three big boulders capping the regional geomorphic surface that preserved 3.0m vertical offset suggests that the surfaces were exposed at ~5 ka.Meanwhile,the lacustrine sediments from Bosten Lake within the Yanqi Basin suggest climate change during cooling-warming transitions was also at ~5 ka.The climate,therefore, controlled creation and abandonment of geomorphic surfaces in southern piedmont of Tian Shan. Combining the exposure ages and vertical offsets,we inferred that the east section of the north-edge fault in the Yanqi Basin has a dip slip rate 0.6 ~0.7mm/a,~0.5mm/a of vertical slip and~0.4mm/a of shortening since 5 ka.Based on calculation of earthquake moment,we estimated that this fault is capable of generating M7.5 earthquakes in the future.This study provides new data for further understanding tectonic deformation of Tian Shan and is useful in seismic hazard assessment of this area.
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