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

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