柴达木块体内部都兰南断裂晚第四纪活动特征

The Late Quaternary Activity Characteristics of the Dulan South Fault in the Qaidam Block

青藏高原是新生代期间印度与欧亚板块持续强烈陆陆碰撞作用下形成的陆内活动造山带,发育了复杂的活动断裂系统,并成为东亚显著的陆内强震活动区。已有学者对高原活动断裂的研究多集中于地块边界带上,缺少对块体内部变形的研究。近期在开展青海省海西州都兰县察汗乌苏镇地震小区划工作中,调查发现在柴达木地块东南部的都兰次级断块内部存在明显的晚第四纪活断层−都兰南断裂。通过对都兰南断裂开展详细的野外地质调查、高分辨率遥感影像解译和无人机低空摄影精细测量等,得到该断裂的构造地貌特征、空间几何展布及运动特性,并通过开挖探槽和地质测年等,对其最新活动时代及滑动速率等进行初步约束。研究结果表明,该断裂为全长约43 km、全新世活动的左旋走滑断裂,并在其东段存在长约6 km的地表破裂带。在该断裂东段,地表的晚第四纪累积左旋位移达(14.5±1.8)m,西段的左旋走滑量为(6.7±0.8)m,初步估算其东段的水平走滑速率为1.56~1.9 mm/a,西段的水平走滑速率为0.9~1.16 mm/a。该断裂的发现及全新世活动的厘定表明,青藏高原内部活动构造变形样式复杂,断块内部通常存在不同程度的弥散变形。因此,断块内部的强震危险性不容忽视。该活动断裂的发现为认识都兰次级断块内部变形样式、应变分配等提供了参考,为都兰地区地震危险性的认知提供了支撑,对防御和减轻区域地震灾害风险具有一定指导意义。

The Qinghai Tibet Plateau is an intracontinental active orogenic belt formed under the continuous strong continental collision between India and Eurasia during the Cenozoic era.It has developed an extremely complex active fault system and has become an extremely significant intracontinental strong earthquake activity area in East Asia.Previous studies on active faults in the plateau have focused more on the boundary zone of the block,but paid less attention to the internal deformation of the block.Recently,during the seismic zoning of Chahanwusu Town,Dulan County,Haixi Prefecture,Qinghai Province,it was found that there was an obvious late Quaternary active fault-Dulan South fault in the Dulan secondary fault block in the southeast of Qaidam block.Through detailed field geological survey,high-resolution remote sensing image interpretation and low altitude photography fine measurement of the Dulan South fault,the structural and geomorphic characteristics,spatial geometric distribution and movement characteristics of the fault are obtained.The latest activity age and sliding rate are preliminarily constrained by excavation of trench and geological dating.The results show that the fault is a 43 km long,Holocene left lateral strike slip fault,and there is a 6 km long surface fracture zone at its east end.In the east section of the fault,the accumulated left lateral displacement of the surface in the late Quaternary is(14.5±1.8)m,and the left lateral strike slip of the west section is(6.7±0.8)m.It is preliminarily estimated that the horizontal strike slip rate in the east section is about 1.56~1.9 mm/a,and that in the west section is about 0.9~1.16 mm/a.The discovery of the fault and the determination of Holocene activity indicate that the active tectonic deformation style in the Qinghai Tibet Plateau is very complex,and there is usually dispersion deformation in the fault block with different degrees.Therefore,the risk of strong earthquakes in the fault block cannot be ignored.The new discovery of this active fault provides an important basis for understanding the internal deformation style and strain distribution of Dulan sub-fault block,provides strong support for the recognition of seismic risk in Dulan area,and has a certain guiding significance for the prevention and mitigation of regional seismic disaster risk.