Abstract:

Kouquan Fault is located in the north part of Shanxi graben which controls the west edge of Datong Basin.Two M6 1/2 earthquakes happened in the west side of basin in historical time,and there has been a concern about the future hazard of the fault.However,previous researches on Kouquan Fault were limited only in several points,especially,there was lack of measurements and dating data.Based on the 1 :50000 geological mapping of Kouquan Fault,the paper investigates the late Quaternary faulted landforms of its middle part(the part between Shangshenquan village and Yangjiayao village),combining with remote sensing interpretation of Spot image and field validating of the study area,and finally obtains the late Quaternary dip-slip rates of this fault.Five stratiform landforms can be found from piedmont to riverbed.The topmost part(the fifth geomorphic surface)is piedmont erosion surface which might be the planation surface of Tangxian period; the fourth geomorphic surface,which formed in the end of the middle Pleistocene to the early stage of late Pleistocene,consists of T₃ terrace of big rivers and diluvium mesas developed on piedmont; the third and the second geomorphic surface can be found in valleys and are represented as T₂ and T₁ terrace,respectively.Diluvium mesas of the same period formed in the end of the late Pleistocene and the middle stage of Holocene are distributed in different parts in front of mountains.The first geomorphic surface is flood plain and modern alluvial fans at mountain front.According to OSL dating and radiocarbon dating of different terraces,we obtained the ages below: T₃,no less than 70ka; T₂,about 33ka; T₁,4～8ka.The characteristic of faulted landform of the research area is different due to the fault activity of different parts.In the segment with intense faulting,the fault trace is obvious,and we could see fault scarps and triangular facets in the field,low river terraces such as T₁ and T₂ had been faulted; In the segments with less activity,the fault trace is unclear,the older fault scarps have gentle slope due to river erosion and reverse slope,and there is no evidence of faulted low terraces.Based on faulted landforms of the different terraces,we divide the middle part of Kouquan Fault into three sections by Baipo village and Chanfang village.At south of Baipo village,the diluvium mesas corresponding to the period of T₃ were faulted,but there is no evidence found dislocating the younger geomorphic surface.This indicates that this part has not been active since Holocene; T₂ and the older terraces were faulted between Baipo village and Chanfang village; the evidence of offset of T₁ terrace could be seen at the north of Chanfang village,especially in the part between Xiaoyukou village and Louzikou village.In the section north of Chanfang village,the fault throw of T₁ terrace is 50cm in Dayukou village,over 3m in the part between Xiaoyukou village and Louzikou village,and 25～30cm in the part between Shijing village and Ermaokou village; the fault throw of T₂ terrace is 5.7m in the part between Chanfang village and Dayukou village,over 17.5m in the part between Xiaoyukou village and Louzikou village,and 13m in the part between Shijing village and Ermaokou village. We calculated the slip rates combining with the fault throw of T₂ terrace at different sites,and the results are as follows: >0.53mm/a between Xiaoyukou village and Louzikou village,0.4mm/a between Shijing village and Emaokou village and 0.17mm/a between Chanfang village and Dayukou village.These maybe indicate that the late Quaternary activity of the fault was centered on the part between Xiaoyukou village and Louzikou village,and became weaker towards both sides.

Key words: Kouquan Fault, stratiform landforms, faulted landform, slip rate

摘要：

通过口泉断裂1／5万地质填图,调查了口泉断裂中段(上神泉至杨家窑段)晚第四纪以来断错地貌特征。从山麓麓原面到山间沟谷河床一共可以划分出5级层状地貌面,最顶部(第5级地貌面)为山麓剥蚀面,推测为形成于新近纪的唐县期夷平面; 第4级地貌面由大河间T₃阶地以及山麓地带发育的同期洪积台地组合而成,形成于中更新世末、晚更新世初; 第3级、第2级地貌面在山区只见于山间河谷,分别代表着T₂及T₁阶地,山前不同部位展布T₂及T₁同期洪积扇,分别形成于晚更新世末、全新世中期; 第1级地貌面为河漫滩及山前现代洪积扇。口泉断裂中段晚第四纪活动以小峪口至楼子口段最为明显,T₁阶地断距超过3m,T₂阶地断距不＜17.5m。根据测年、T₂阶地断距与断代,得出小峪口至楼子口段滑动速率不＜0.53mm/a,石井至鹅毛口段滑动速率为0.4mm/a,大峪口至禅房段滑动速率为0.17mm/a。这可能表明口泉断裂中段晚第四纪活动中心位于小峪口至楼子口一带,向两侧活动强度减弱。

关键词: 口泉断裂, 层状地貌, 断错地貌, 滑动速率