Abstract:

Bending-moment fault and flexural-slip fault are two types of fold-related faults in compressional tectonic environments. Historical earthquake records suggest that both fault types may be active simultaneously, with their fault scarps providing crucial insights into strong seismic events. In the northern region of the Guman anticline, located at the foothills of the West Kunlun Mountains, numerous prominent bending-moment normal fault scarps have developed, reaching heights between 0.5m and 16.0m. This study focuses on a fault scarp segment approximately 5.4km long and 4.2km wide. A digital elevation model(DEM)with a 0.2m resolution was generated using drone photogrammetry. A total of 739 cross-fault scarp profiles were extracted, providing key parameters such as scarp height, slope, displacement continuity, and cumulative displacement trends. Data analysis yielded the following findings:
(1)In the study area, dense bending-moment normal faults align along the active anticline axis, dipping toward the axial plane at angle of 70°~80°, as observed in a trench. Among these faults, more than a dozen dip northward, whereas only 1-2 dip southward, forming asymmetric grabens. This asymmetry may be attributed to the overall northward tilt of the strata and the differing limb structures of the underlying anticline. These faults divide the terrace surfaces into multiple rectangular blocks, 380~650m wide. The blocks exhibit outward tilting relative to the fold axis, with those cut by north-dipping faults tilting southward and those cut by south-dipping faults tilting northward. The degree of tilting and fault displacement is closely related to the thickness of the underlying anticlinal strata and the extent of stratal bending.
(2)Displacement profiles along the faults reveal a step-like decrease in displacement as terrace surfaces become progressively younger, with maximum slope profiles displaying similar trends. This pattern suggests long-term fault activity. Cumulative displacement data confirm this trend, with displacement values of(54.5±3.3)m for terrace T3c and(19.5±1.1)m for terrace T1c. The total displacement of T3c is approximately 2.8 times that of T1c, and displacement ratios across different terraces range from 1.5 to 5.5. Higher ratios indicate greater displacement accumulation on older terraces, suggesting an earlier onset of fault activity. These displacement rankings imply that an initial framework of faults developed in the region, followed by subsequent fault intrusion. Notably, Fault F₈ exhibits a displacement ratio of 5.5, forming a(1.0±0.3)m high fault scarp on the young T1b terrace, indicating that even the earliest-formed faults remain active.
(3)Seismic reflection profiles reveal that the south flank of the Guman anticline dips 3°~6° northward, while the north flank dips 12°~14° northward. The underlying blind thrust exhibits a lower flat-ramp-upper flat geometry. However, bending-moment normal faults are not visible in the seismic reflection data, suggesting that they are secondary structures associated with anticline deformation. The fault zone aligns with the anticline’s fault-bend axis, indicating ongoing activity in the anticline zone. The bending-moment normal faults are rootless, meaning they are not primary seismogenic faults. Instead, they primarily develop in poorly layered strata and are largely independent of the kinematics of fold growth. Their formation is closely tied to the degree of strata bending and the thickness of overlying beds. Despite their shallow nature, the bending-moment normal faults exhibit long-term activity, providing evidence that the underlying anticline remains active. These findings support the interpretation of the Guman anticline as an active fault-bend fold.

Key words: Guman anticline, fold-related faults, bending-moment normal faults, fault scarps, compressional tectonic environment

摘要：

弯矩断层和弯滑断层是挤压构造环境下常见的2种褶皱相关断层。历史地震表明这些断层会同步活动, 其地表陡坎蕴含着强震活动信息。西昆仑山前固满背斜带北部发育了众多壮观的弯矩正断层陡坎, 坎高0.5~16.0m。文中以一段长约5.4km、 宽约4.2km的断层陡坎带为研究对象, 通过无人机摄影测量, 获取了该区域0.2m分辨率的数字高程模型(DEM), 并提取了739条跨断层陡坎地形剖面, 计算了陡坎的高度、 坡度参数, 获得了沿断层位移和最大坡度的连续剖面及累积位移剖面。分析表明: 1)弯矩正断层沿着背斜轴大体平行展布, 将阶地面切成了多个条形地块。这些地块在断层活动过程中向背斜轴外侧掀斜, 其掀斜程度和断层位移量受下伏背斜地层厚度和地层弯曲程度控制。研究区坡向N的弯矩正断层陡坎有十几条, 仅在最北侧发育了1、 2条坡向S的陡坎, 形成不对称的地堑, 这可能与研究区地层整体向N倾斜和下伏背斜两翼不对称发育有关。2)单条、 分组和累积断层位移剖面沿走向随阶地变年轻呈现“台阶”状降低, 最大坡度与位移剖面变化的特征相似, 表明研究区的弯矩正断层是长期活动的。不同阶地面上的累积位移量比值暗示, 断层带活动可能先形成了框架断层, 后穿插了一些新生断层。3)研究区的弯矩正断层为浅地表的次级断层, 无根但长期活动, 指示了下伏褶皱的背形断弯也是活动的, 支持固满背斜为活动断弯褶皱的观点。

关键词: 固满背斜, 褶皱相关断层, 弯矩正断层, 断层陡坎, 挤压构造环境