关键词: 正断层, 阶区, 连接方式, 口泉断裂, 山西裂谷系

Abstract:

The overlapping area between the ends of adjacent fault segments is known as a fault stepover. The normal fault stepover has two endmember connection modes, i.e., soft-link mode and hard-link mode. The soft-link stepover's border faults are connected through a relay ramp, and the border faults' displacements are transmitted through the bending deformation of the relay ramp. The hard-link stepover's border faults are connected through a breaching fault, and the border faults' displacements are transmitted through the faulting deformation of the breaching fault. Distinguishing the current connection mode of a normal fault stepover can shed light on the evolution stage of the normal fault. It can also indicate the potential earthquake rupture pattern in the stepover, which is important for evaluating the seismic hazard of engineering sites within the stepover. The straightforward technique to distinguish the current connection mode of a normal fault stepover is to determine whether an active breaching fault exists within the stepover. However, in many cases, due to the small amount of accumulated offset and human modification of the breaching fault, it is always hard to observe fault scarps in the field even though the fault stepover is deforming under the hard-link mode.
The Shanxi Rift System is a prominent intracontinental rift zone in East Asia. It comprises a series of left-stepping en échelon grabens bounded by high-angle normal faults. It is distributed in an S-shaped geometry with a narrow, NNE-trending zone in the middle and two broad, NEE-trending extensional zones in the north and south. The Shanxi Rift System is one of the strong earthquake-prone regions in China. Since 780 BC, the Shanxi Rift System has hosted three M8 earthquakes, five M7-7娻 earthquakes, and a series of M6-7 earthquakes. The Kouquan fault is the western border fault of the Datong Basin in the northern part of the Shanxi Rift System. A stepover is developed near the Chanfang village on the Kouquan fault, which we named the Chanfang stepover.
In this study, we use a combination of the tectonic geomorphological investigation in the field, high-resolution topographic data analysis, and Ground Penetrating Radar(GPR)surveying to study the current connection mode of the Chanfang stepover. Three fault outcrops on the border faults of the Chanfang stepover are investigated. The outcrop D1 is in an alluvial fan covered by loess on the southwestern boundary fault of the Chanfang stepover. Two branch faults are present at this outcrop. One offsets a bedrock surface and the alluvial fan's gravel layer. The other is the boundary between a gravel layer and the loess, where imbricated gravel can be observed. The fault outcrop D2 is also on the southwestern boundary fault of the Chanfang stepover. The fault at the outcrop D2 offsets a gravel layer and the vertical offset of the top of the gravel layer is approximately 2m. The fault outcrop D3 is located on the northeastern boundary fault of the Chanfang stepover. At the outcrop D3, the fault separates the gneiss of the Archean Jining Group from the loess. Based on the Chinese GF-7 satellite stereo imagery, we obtain the high-resolution digital elevation model(DEM)covering the Chanfang stepover and identify two levels of geomorphic surfaces, i.e., T1 and T2. The surface T1 is an alluvial fan, mainly developed in the piedmont areas. The surface T2 is an erosion surface distributed in the bedrock mountain. To quantify the deformation pattern within the Chanfang stepover, we construct a series of topographic cross-sections on the surface T1 and find a gentle geomorphic scarp within the stepover. We conduct two GPR surveying lines across the Chanfang stepover. On the GPR images, we identify two known faults, F₁ and F₃, that previous researchers have mapped and a buried fault, F₂, that has not been constrained previously.
The Fault F₂ observed by the GPR is consistent with the geomorphic scarp constrained on the DEM, suggesting that a breaching fault exists in the Chanfang stepover. The existence of the Chanfang breaching fault indicates that the connection mode of the Chanfang stepover is the hard-link mode. We thus infer that the future earthquakes on the Chanfang stepover may cause concentrated surface ruptures on the breaching fault. This study shows that the combination of the tectonic geomorphologic investigation in the field, high-resolution topographic data analysis, and the GPR survey can effectively locate near-surface, slow, active normal faults. This comprehensive technique can be used for the connection mode of a normal fault stepover.

Key words: normal fault, stepover, link mode, Kouquan Fault, Shanxi Rift System