Abstract:

Active anticlines, as first-order structural elements within foreland thrust systems, play a fundamental role in seismic hazard assessment. Surface fault expression and deformation style directly constrain estimates of seismic potential. The Kash anticline thrust system, located within the Pamir-South Tianshan convergent zone, hosts Holocene-active deformation. Its intense and persistent tectonic activity poses a significant seismic threat to Kashgar City. However, fault-related studies remain limited, and key issues—including fault trace distribution on both limbs, structural segmentation, and slip rates—remain unresolved.

This study applies a multi-scalar methodological framework to systematically investigate the fault-related deformation of the southwestern segment of the Kashgar anticline: 1)High-resolution satellite imagery, UAV-derived orthophoto mosaics, and digital elevation models(DEMs), combined with detailed field investigations, were employed to identify and interpret fault traces; 2)Paleoseismic trenching was conducted at representative geomorphic sites, resulting in the excavation of three trenches(TC1-TC3)at two key locations(Shuijing and Haitang), directly revealing fault geometries and deformational features; 3)At trench TC4, optically stimulated luminescence(OSL)dating was performed on eolian sand lenses located within the overlying(XJ24-49)and underlying(XJ24-50)strata across the fault tip to constrain the timing of fault activity; 4)UAV-based high-resolution DEMs were used to systematically measure fault scarp heights, and cumulative offset probability density(COPD)curves were generated and compared with trenching results.

The integrated interpretation of satellite image analysis, UAV photogrammetry, field surveys, trench excavations, and scarp morphology analysis delineates the spatial distribution, segmentation characteristics, and paleoseismic deformation history of the northern limb of the Kashgar anticline. A nearly east-west-trending fault scarp extends for ~20km along the northern limb. Multiple Holocene alluvial fan surfaces are displaced by the fault, with cumulative vertical displacements ranging from 0.13m to 3m. The fault system can be subdivided into western, central, and eastern segments based on geomorphic and structural features, demarcated by the Chakmak River and Ake Expressway. In the western segment(~15.7km), located along the mountain front, north-directed thrust scarps dominate. Trenching at TC1 and TC2 reveals north-vergent reverse faults reaching the ground surface, with no overlying sediments, indicating that the most recent surface-rupturing event is geologically recent. The central segment(~4.7km), situated across T2 and T3 terrace surfaces, shows well-developed scarps produced by both north-verging thrusts and south-verging backthrusts. Trench TC3 exposes such a fault pair, and the most recent earthquake event is capped by only ~0.3m of sediment, suggesting an occurrence within the past century. In the eastern segment, low-angle north-dipping faults are observed in a large quarry exposure, but historical imagery shows no surface faulting. TC4 reveals a buried fault with no surface rupture. OSL dating of sediments overlying the fault tip yields an age of(1.0±0.9)ka, while the deformed underlying unit dates to(8.6±2.9)ka, suggesting a Holocene-active blind fault. The COPD curves derived from cumulative offset measurements in the western and central segments were compared with deformation revealed in trenches TC1 and TC3. The first prominent peak in the COPD likely represents cumulative effects from multiple seismic events rather than a single coseismic displacement.

By integrating tectonic geomorphology, trench observations, chronological constraints, statistical analysis of fault scarps, and historical seismicity—especially the 1902 Artux earthquake rupture zone—the following conclusions are drawn: 1)The northern limb of the Kashgar anticline exhibits clear segmentation: the western segment is dominated by north-verging thrust faults; the central segment is characterized by both forethrust and backthrust structures; and the eastern segment is defined by a buried Holocene-active fault; 2)It is inferred that the northern limb of the Kashgar anticline likely participated in the 1902 Artux earthquake, generating a series of fault scarps at the surface; 3)COPD curves alone may be insufficient to reliably resolve individual paleoearthquake events, especially when the first major peak reflects cumulative displacements from multiple events. Additional constraints, including trenching and chronometric data, are essential to refine paleoearthquake interpretations.

Key words: Pamir, active anticline, active fault, reverse fault, paleoearthquake

摘要：

活动背斜是前陆冲断带重要的构造类型之一, 其活动性及断层出露程度, 均会影响其地震危险性分析。喀什逆断裂-背斜带是帕米尔与南天山会聚带上发育的一条全新世活动构造, 是影响喀什市区地震安全的主要构造之一。通过卫星影像解译、 野外调查和探槽开挖等工作, 在喀什背斜北翼发现了长约20km, 呈近EW向展布的断层陡坎。断层在喀什背斜北翼断错多期洪积扇, 形成了高0.2~3.2m的陡坎。在恰克马克河以西, 主要以断层向N逆冲形成的陡坎为主, 总体长约15.7km; 恰克马克河至阿喀高速, 断层向N逆冲和向S反冲形成的陡坎均较为发育, 长约4.7km; 阿喀高速以东, 断层未断错至地表。文中通过对断层陡坎的系统测量和统计分析, 结合探槽古地震结果, 发现累积位移概率密度(COPD)曲线中的首个显著峰包含了多期古地震事件。此外, 根据1902年阿图什地震等震线及近20a以来的地震分布, 结合地表断错特征及探槽剖面揭示的断层最新活动特征, 推测最新一次地震事件很可能为1902年阿图什M8¼地震。

关键词: 帕米尔, 活动背斜, 活动断层, 逆冲断层, 古地震