关键词: 活动断裂带, 晚更新世, 全新世, 定量地貌学, 断层分段

Abstract: The southern marginal fault zone of the Datong-Yangyuan basin is the largest fault in the basin-range area of NW Beijing-N Shanxi.Heights of late Quaternary fault-scarps and their a-long-striking distribution Pattern can divide the fault zone into two segments,the southwest(SW)segment and the northeast(NE)segment.Between these two segments there have been significant variations in mountain front and piedmont slope tectonic geomorphology.as well as in evolutional history of micro-physiographic stages.Individual segments have different quantitative mountain front geomorphobgical indices.Means of mountain front sinuosity(S).valley widthdepth relief ratio(V_f),and stream-gradient index(K)are 1.93,0.49 and 27.18,respectively,for the SW segment,wherea are1.57,0.39 and 56.61,respectively,for the NE segment.The SW segment piedmont slope is characterized by the landform assesbles of youngest alluvial fan A₃(Q₃²+Q₄¹)inerted into older fan A₂(Q₃²),indicating that an upwarping of the near-fault part of older fan A₃(Q₃²)might have occurred during late Quaternary.On the contrary,the NE segment peidmont slope is characterized by the landform assembles of youngest fan A₃(Q₃²+Q₄¹)overlapped onto oldest fan A₁(Q₃¹),showing that a downwarping might have occurred on the near-fault part of oldest fan A₁(Q₃¹).Before 36 8000a B.P,the NE segment might have experienced an evolutional history of erosion～aggradatin～erosion,while the SW segment minght have had evolutional stages of aggradation～erosion～aggradation.Tectonic geomorphological process rates,fault behavior,and segment boundary charateristics seem to be reasons of such segmented varations in tectonic geomorphology of the fault zone.During late Quaternary,the SW segment has a lower faulting rate relative to a fluvial downcutting rate,which were estimated to be 0.34mm/a and 0.41～0.48mm/a,resepectively.Displacement per event was averaged 0.9m ad recurrence interval was estimated to be 900～1000a.the NE segment has a higher faulting rate relative to a flurial downcutting rate,which were estimated to be 0.99mm/a and 0.22mm/a,respectively.Displacement per event was averaged 3.1m and recurrence interval was estimated to be 1800a.Furthermore,it is the overlapping faults between the SW segment and the NE segment to become the segment boundary,where a tectonic effect of displacement deficit might have occurred,playing and important role in such segmented variations in tectonic geomorphology.

Key words: Active fault zone, Late Pleistocene, Holocene, Quantitative geomorphology, Fault segmentation