西南天山迈丹断裂阿合奇段左旋走滑的证据及其最新活动

Evidence of Left-lateral Strike-slip along the Aheqi Segment of Maidan Fault in Southwest Tianshan Mountains, China and its Latest Activity

摘要:

柯坪推覆构造的根部断裂记录到的地震活动相对较弱,以至于多数学者认为该断裂晚第四纪以来活动性不强。笔者根据遥感影像解译和野外调查得到迈丹断裂的几何展布,确认F₃阿合奇段为最新地表破裂带,并通过一系列河流阶地的左旋位移测量确定其晚更新世以来有过走滑活动。结合地貌测量和探槽开挖得到断层垂直错距,探槽揭示的古地震事件发生在距今(1.76±0.22) ka之后,根据现场考察获得的活动构造定量数据,依据不同震级与地表破裂关系式推算出该次古地震震级为7.5级。研究成果可能对区域活动断裂的研究以及区域活动构造图像的完整性提供基础资料,同时最新地表破裂证据的发现可能有助于更新认识该断裂的危险性。

Abstract:

Maidan Fault is the boundary fault of the Tianshan Mountains and Tarim Basin.The geometric and historical seismic distributions revealed relatively weak seismic activity at the foot of Kalpin nappe;thus, the majority of scholars have reported that Maidan Fault has undergone low activity since the late Quaternary.On the basis of remote sensing image interpretation and field investigation, we obtain the geometric distribution of Maidan Fault.The fault distribution map of Aheqi Segment shows that the fault zone, with a length of approximately 60 km, is mainly composed of three approximately parallel NE faults;F₃ is confirmed as its freshest surface fracture zone.At present, the fracture feature is still in dispute.Some researchers have reported that Maidan Fault was caused mainly by strike slip, whereas others have stated that no evidence exists for the westward continuation of the Maidan fault zone across the Pamirs.We believe that strike-slip activity created a series of river terraces with sinistral displacement during the late Pleistocene.Through topography measurement and trench exploration, we obtain the fault's offset.A cross section of the trench across Maidan Fault shows that a paleoearthquake occurred before (1.76±0.22) ka.We used different regression equations and quantitative date of active faults to determine that the magnitude this earthquake was 7.5.This research can provide new information on regional active faults.The regional active tectonic images and the discovery of the new surface rupture zone may help us to recognize the danger of this fault.