阿尔金断裂带康西瓦段晚第四纪以来的左旋滑移速率及其大地震复发周期的探讨

在高分辨率Ikonos卫星影像（1m分辨率）分析基础上,结合野外考察和定量测量,详细研究了阿尔金断裂带西段康西瓦段三十里营房地区晚第四纪以来的变形特征,在三十里营房东侧塔尔萨依吉勒尕河下游地区,断裂切割了一系列发育完好的冲积扇和阶地。6级不同阶地陡坎和邻近冲积扇面上冲沟的左旋位错量分别为251±4m,250±5m,198±4m,22±2m,12±1m和约6m。T2表面的放弃年龄约10.9±0.2ka（10Be）所限定的位错量22²00m,得到左旋滑移速率为2¹8mm/a;而T4阶地的最大累积位错可能达500m,暗示的左旋滑移速率约4⁵mm/a。最近一次大地震造成的最新地貌左旋水平位错量约6m,该地震同震地表破裂带沿喀拉喀什河谷延伸长达100km,估算为Mw7.4地震,约12m的位错量可能是公元975¹020年（AMS14C）以来最近两次大地震的累积同震地表位错,约6m的特征滑移量暗示该地段发生类似约Mw7.4地震的复发周期约370⁵00 a。这些结果表明,在青藏高原北缘,阿尔金断裂带西段为大型的左旋走滑断裂,它吸收了印度/欧亚大陆碰撞产生的较大部分应变,并使高原西部物质向东运移。

LATE QUATERNARY LEFT-SLIP RATE AND LARGE EARTHQUAKE RECURRENCE TIME ALONG THE KANGXIWAR(OR KARAKAX)SEGMENT OF THE ALTYN TAGH FAULT,NORTHERN TIBET

We describe the Late Quaternary deformation in the Sanshiliyingfang area of the Karakax（or Kangxiwar）segment of western Altyn Tagh fault,on the basis of highresolution Ikonos satellite images analysis,complemented with field survey and quantitative measurements.In the downstream area of the Taersayigilega River to the east of Sanshiliyingfang,the fault cuts a set of 8 well developed alluvial fans and inset terraces,from which the main levels（T0 to T5）are distributed at the regional scale throughout the Karakax valley.The upper T4～T5 levels belong to the penultimate climatic optimum（120～140ka¹⁰Be age）,the intermediate T3 levels to the last glacial period,and T2 levels to the post-glacial period（15～10ka¹⁰Be age）,the youngest T1 and T0 levels spanning the Late Holocene to present（7～0ka¹⁰Be age）.Left-lateral offsets of six distinct terrace risers and of rill channels incised in the adjacent fan have values of 251±4m,250±5m,198±4m,22±2m,12±1m and ca.6m.The offset of surface T2 abandoned at about 10.9±0.2ka（¹⁰Be age）ranges from 22m to 200m,and constrains the slip-rate to be between 2mm/a and 18mm/a.In addition the upper T4 surface total cumulative offset may reach 500m,implying a left-slip rate of about 4～5mm/a.The latest surface left-lateral horizontal offset caused by the most recent large seismic event is ca.6m.The co-seismic surface ruptures reach a length of about 100km along the Karakax segment,implying the occurrence of a magnitude Mw 7.4 earthquake.The offsets of ca.12m may be the cumulative offsets of the latest two earthquakes in the last 975～1020A.D.（AMS14C age）,with a characteristic slip of ca.6m and a recurrence time of 370～500 years for large magnitude earthquake.These results suggest that the western branch of the Altyn Tagh fault is the largest left-lateral strike-slip fault at the rim of Northern Tibet,that accommodates a large part of strain due to the India-Eurasia collision and induces the eastward movement of material from Western Tibet.