Study On Characteristics Of Lithology, Texture And Structure Of Yingxiu-Beichuan Fault

Introduction of the author: Zhang Wei，born in May.，1985，was under the guidanceof Prof. Li Yong from Sep.2009at Chengdu University of Technology and Prof.Haibing Li from june2010at Institute of Geology,Chinese Academy of GeologicalSciences. He was awarded Master Degree of Structure Geology in June,2012.Wenchuan Earthquake Fault Zone Scientific Drilling (WFSD) is a national specialtechnological program, which is co-organized by Science&Technology Ministry,Ministry of Land and Resources and China Earthquake Administration. WFSDprogram plans to carry out several scientific drilling holes in the hanging-wall of theco-seismic surface rupture zones, the Yingxiu-Beichuan and Anxian-Guanxian faultzones, caused by the2008Wenchuan earthquake. WFSD-2is located in thehanging-wall of the Yingxiu-Beichuan fault zone at Bajiaomiao, Hongkou, Dujiangyan,Sichuan province. The hole went through the Yingxiu-Beichuan fault with the finaldepth of2283.56m. This paper focuses on the WFSD-2by the studies of field corecataloging, petrology, structural geology. The results show that the cores can dividedinto six segments (or four rock assembled segments), including the Pengguan Complex(0~599.31m, rock assembled segment I), the second member of the Xujiahe Formation(599.31~1211.49m), the Pengguan Complex (1211.49~1679.51m, rock assembledsegment II with the former segment), the third member of the Xujiahe Formation(1679.51~1715.48m), the Pengguan Complex (1715.48~2081.47m, rock assembledsegment III with the former segment) and the fourth member of the Xujiahe Formation(2081.47~2283.56m, rock assembled segment IV), from top to bottom, respectively.The Pengguan Complex mainly consists of granite and volcanics, and the XujiaheFormation is Triassic sedimentation rocks with sandstone, slitstone, mudstone, shale,coal beds (lines) and conglomerate. Three sections of the Pengguan Complex and threesections of the Xujiahe Formation alternately repeat in the WFSD-2, which indicatedthat the Longmenshan tectonic belt is composed of a series of thrust lithologic sheets.There are a lot of fault-related rocks in the WFSD-2cores, such as fault breccia,cataclasites and fault gouge, reflecting the brittle deformation. Based on the statisticalanalysis to the fault-related rocks, twenty fault zones were detected in the WFSD-2,indicating many earthquakes like2008Wenchuan earthquake occurred in the LongmenShan region. Among the nine primary fault zones of FZ600, FZ720, FZ782, FZ817,FZ922, FZ951, FZ1449, FZ1681, FZ2082and FZ1681, the FZ1681is the biggest primary fault zone in the whole WFSD-2. According to the assembled characteristics offault-related rocks, fault zones can be divided into symmetric or asymmetric fault zone,with the center of fault gouge, or not. Basing on the dip angles of trench and WSFD-1primary slip zone (PSZ), lithological profile, temperature measurement andcomprehensive well log, the PSZ in the WFSD-2should be within one of the primaryfault zone among FZ1449and FZ1681, which all have fault gouge and reasonablegeometrical interpretation among rupture zone, WFSD-1and WFSD-2. Synthesisanalysis with four thrust rock segments, twenty fault zones, high breakage density, theLongmen Shan experienced powerful tectonics. ATM profile of WFSD-1,WFSD-2and WFSD research results reveal Pengguan complex may be rootless, and this couldbe about to change the long-standing understanding of the tectonic framework ofLongmen range.