Abstract:

The arcuate tectonic belt along the northeastern margin of the Qinghai-Xizang Plateau represents the leading edge of the plateau’s northeastward expansion. Since the Late Cenozoic, this region has accommodated intense tectonic activity, forming four major active fault zones aligned from south to north: the Haiyuan, Tianjingshan, Yantongshan, and Sanguankou-Niushoushan-Luoshan Faults. Situated between the Tianjingshan and Luoshan Faults, the Yantongshan Fault lies at the triple junction of the Qaidam, Alxa, and Ordos blocks.

While previous studies have extensively focused on the larger, more active Tianjingshan and Luoshan Faults, research on the Yantongshan Fault remains sparse. The Tianjingshan fault zone extends for approximately 240km and comprises the northern Tianjingshan Fault and the southern Miaoshan Piedmont Fault. The 1709 Zhongwei M7½earthquake ruptured this zone, which exhibits a late Quaternary left-lateral slip rate of 0.9~1.1mm/a. In contrast, the Luoshan Fault(~50km long)transitioned from thrusting prior to the mid-Late Pleistocene to right-lateral strike-slip motion in the late Late Pleistocene, with an average slip rate of 2.15mm/a.

Unlike its neighbors, the Yantongshan Fault lacks clear geomorphic expression, making it difficult to identify Holocene-active natural fault scarps. Consequently, direct evidence confirming Holocene activity on this fault remains lacking.

The Yantongshan Fault has a total length exceeding 150km and is naturally divided into southern and northern segments by Yaoshan. The northern segment is approximately 90km long, with about 15km concealed beneath the Zhongning Basin and the remaining 75km exposed at the surface. In this study, the northern segment of the Yantongshan Fault was taken as the research object. This study employs methods such as remote sensing interpretation, geological survey, aerial survey of typical geomorphology, trench excavation, and Optically Stimulated Luminescence(OSL)dating. The main findings are as follows: 1) The northern segment of the Yantongshan Fault has linear fault scarps. It is a low-angle reverse fault, dipping to the southwest with a dip angle of about 30°. 2) The geomorphic scarp near Xiaoyushugou in the northern segment of the Yantongshan Fault corresponds to this fault’s trace. During trench excavation, a new fault was identified and has been shown to be active during the Holocene. 3)OSL dating shows that the trench profile records at least two paleo-earthquake events. The latest earthquake event occurred after(7.99±0.42)ka in the Holocene, and at least one paleo-earthquake event occurred after(63.08±3.18)ka in the late Pleistocene. Considering the stratum erosion, the vertical displacements of the two events are at least 1.3m and 1.0m, respectively, and the lower limits of the earthquake magnitudes can reach 6.9 and 6.8. 4) The northern segment of the Yantongshan Fault may still be active at present. Its deep part, together with the Luoshan Fault, intersects the Tianjingshan Fault and ultimately merges with the Haiyuan Fault at the detachment layer. As a secondary fault associated with the Tianjingshan Fault, affected by the northeastward expansion of the Tibetan plateau, there is an abnormal zone of wave velocity and electromagnetism beneath the northern segment of the Yantongshan Fault, which is closely related to moderate-to-strong earthquakes, indicating that it still has seismic risk. Moreover, the longer the energy accumulation time, the higher the current risk. Referring to the average recurrence interval of earthquakes on the Tianjingshan Fault, which is (4 306±212)a, the latest strong earthquake(Event E1) on the Yantongshan Fault exposed by this trench occurred approximately 8 000a later. The elapsed time approaches or even exceeds the recurrence period of paleo-earthquakes, therefore, the seismic risk of its northern segment is urgent.

Key words: Qinghai-Xizang Plateau, Yantongshan Fault, Holocene, trench, paleo-earthquake, seismic risk

摘要：

晚新生代以来, 青藏高原东北缘弧形构造带的构造活动强烈。弧形构造带内的烟筒山断裂夹持在天景山断裂和罗山断裂之间, 三者均处于柴达木、阿拉善和鄂尔多斯地块交会的特殊位置。然而, 与天景山断裂和罗山断裂相比, 针对烟筒山断裂的研究成果较少, 尤其缺乏其全新世活动的直接证据。文中选取烟筒山断裂北段为研究对象, 通过遥感解译、地质调查、典型场点地貌航测、探槽开挖及释光测年的方法, 获得了以下认识: 1)烟筒山断裂北段全长约90km, 部分地貌陡坎清晰, 剖面上以低角度逆断层为主, 倾向SW, 倾角约30°; 2)烟筒山断裂北段小榆树沟附近的地貌陡坎与断层位置对应, 通过开挖探槽发现一条新生且为全新世活动的断层; 3)释光测年显示, 探槽剖面记录在晚更新世(距今(63.08±3.18)ka)之后至少发生过2次古地震事件, 其中最新一次地震事件发生在全新世(距今(7.99±0.42)ka)之后, 考虑地层剥蚀, 2次古地震造成的垂直断距至少分别为1.3m、1.0m, 震级下限可达6.9级、6.8级; 4)烟筒山断裂作为天景山断裂的伴生次级断层, 其最新一次地震事件的离逝时间接近甚至超过古地震复发周期, 因此, 其北段的地震危险性值得关注。

关键词: 青藏高原, 烟筒山断裂, 全新世, 探槽, 古地震, 地震危险性