关键词: 分段模型, 晚第四纪, 中条山断裂, 山西

Abstract: The Zhongtiaoshan Fault, southern Shanxi is an active fault in the southeastern part of the active fault system around Ordos Massif, being the boundary between the Yuncheng Faulted basin and the Zhongtiaoshan Faulted Mountain. The recent study about fault activity indicates that it has a lower level of activity and a relatively long recurrence interval of faulting. On the basis of this understanding, a further study is carried out on the segmentation of the Zhongtiaoshan Fault, while the late Quaternary segmentation model and its quantitative parameters are provided in the light of evidence from tectonic discontinuity, tectonic geomorphology, trench paleoseismology, and fault kinematics. The Zhongtiaoshan Fault runs from the northern piedmont to the western piedmont of the Zhongtiaoshan Mountains, with a length of 137km, a general strike of NE-NEE, and a dip angle of 58°～75°, being a high angle normal fault. On one hand, the northern piedmont part of the fault exhibits a big curve convex southeast; and on the other hand, the western piedmont part of the fault intersects with its northern piedmont part at an angle of 120°. By these two major tectonic discontinuities, the Zhongtiaoshan Fault can be divided into three geometric segments from northeast to southwest: the Xiaxian segment, the Xiezhou segment, and the Hanyang segment. The major tectonic discontinuities persistently control the Cenozoic sedimentation in the Yuncheng Faulted Basin. The Xiezhou segment of the Yuncheng Faulted Basin was the depocenter not only of Tertiary system but also of Quaternary system. Morphometric analysis has indicated that the elevation profile of divides and geomorphic characteristics of drainage system in the Zhongtiaoshan Mountains, such as length and gradient index of stream, as well as evolution of its mountain front, have apparent segmented variations, showing an along strike tectonic inhomogeneity caused by long term faulting. The quantitative parameters, such as long term slip rate, displacement per event, age of the last event, and average recurrence interval obtained from the studies of trench paleoseismology and fault kinematics are indicative of segmented differences in late Quaternary fault behavior of the Zhongtiaoshan Fault. In summary, the Zhongtiaoshan Fault is a typical fault exhibiting relatively long recurrence intervals. The surface rupturing events revealed in the trenches correspond to strong earthquakes of magnitude 7.7～8.1, which were separated by a quiescence period of 12,000 years for the Xiezhou segment, and 50,000～85,000 years for the Xiaxian and Hanyang segments. For the Zhongtiaoshan Fault, the only segment that was active during the Holocene epoch is the Xiezhou segment, which has the fastest long term slip rate, the biggest displacement per event, and the shortest average recurrence interval among the three segments. The Holocene activity of this segment might be attributed to the growth and strong activity of the secondary faults of the Zhongtiaoshan Fault since at latest the middle Pleistocene, inducing the frequent rupturing on the main fault. The occurrence of such a type of fault as the Zhongtiaoshan Fault with a relatively long recurrence interval(10⁴ years)in the active fault system around the Ordos Massif implies the inhomogeneity of the movement of the Ordos Massif as a whole. The segmentation model and its quantitative parameters proposed in this paper may provide basic materials for seismic risk evaluation.

Key words: segmentation model, late Quaternary, Zhongtiaoshan Fault, Shanxi Province