Lithologic characteristics and fault zone structure revealed by No. 2 hole cores of the Wenchuan Earthquake Fault Zone Scientific Drilling(WFSD-2)

With the WFSD-2 drill hole as the object, the authors studied core logging, petrology and structural geology. The results show that the cores can be divided into six segments, which are in downward succession the Pengguan complex (0~599.31m), the second Member of the Xujiahe Formation (599.31~1211.49m), the Pengguan complex (1211.49~1679.51m), the third Member of the Xujiahe Formation (1679.51~1715.48m), the Pengguan complex (1715.48~2081.47m) and the fourth Member of the Xujiahe Formation (2081.47~2283.56m). The Pengguan complex mainly consists of granite and volcanic rocks, and the Xujiahe Formation comprises Triassic sedimentary rocks of sandstone, slitstone, mudstone, shale, coal beds (streaks) and conglomerate. Three sections of the Pengguan complex and three sections of the Xujiahe Formation occur in alternate repetition in the WFSD-2, indicating that the Longmenshan tectonic belt is composed of a series of thrust lithologic sheets. There are a lot of fault-related rocks in the WFSD-2 cores, such as fault breccia, cataclasites and fault gouge, reflecting the brittle deformation. Based on a statistical analysis of the fault-related rocks, the authors detected twenty fault zones in the WFSD-2, indicating that many earthquakes like 2008 Wenchuan Earthquake occurred in Longmenshan area. Of the nine primary fault zones of FZ600, FZ720, FZ782, FZ817, FZ922, FZ951, FZ1449, FZ1681, FZ2082 and FZ1681, the FZ1681 is the biggest primary fault zone in the whole WFSD-2. According to the association characteristics of fault-related rocks, the fault zones can be divided into symmetric and asymmetric fault zones, with fault gouge as the center. According to the dip angles of the trench and WSFD-1 primary slip zone (PSZ), the PSZ in the WFSD-2 should occur within one of the primary fault zones of FZ1134, FZ1681 and FZ1681, which all have fault gouge and reasonable geometrical interpretation among rupture zones in WFSD-1 and WFSD-2. An integrated analysis of four thrust rock segments, twenty fault zones and high breakage density shows that the Longmenshan area must have experienced powerful tectonic activities.