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Internal Structure of the Wenchuan Earthquake Fault Zone.Pdf Tectonophysics 619–620 (2014) 101–114 Contents lists available at ScienceDirect Tectonophysics journal homepage: www.elsevier.com/locate/tecto Internal structure of the Wenchuan earthquake fault zone, revealed by surface outcrop and WFSD-1 drilling core investigation Huan Wang a,b, Haibing Li a,b,⁎, Jialiang Si a,b, Zhiming Sun c, Yao Huang d a Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China b State Key Laboratory of Continental Tectonics and Dynamics, Beijing 100037, China c Institute of Geomechanics, Chinese Academy of Geological Sciences, Beijing 100081, China d No. 6 Brigade of Jiangsu Geology & Mineral Resources Bureau, Lianyungang, Jiangsu 222300, China article info abstract Article history: Fault zones record a series of faulting events that have occurred under different physical conditions during their Received 27 February 2013 evolution. Therefore, it is essential to understand the internal structures of fault zones in order to better under- Received in revised form 15 August 2013 stand the mechanical behavior of faults. The internal structure of the Wenchuan earthquake fault zone that Accepted 21 August 2013 prevailed at the Bajiaomiao outcrop and in the WFSD-1drilling cores, located along the southern segment of Available online 30 August 2013 the Yingxiu–Beichuan surface rupture in the Hongkou area, is described in details in this paper. Based on field surveys, X-ray diffraction analysis, microstructure and analysis of the drilling cores, an ~240 m-wide fault zone Keywords: fi – Wenchuan earthquake fault zone was con rmed as the Yingxiu Beichuan fault zone (YBF) at the Bajiaomiao outcrop, corresponding to the Yingxiu–Beichuan fault zone (YBF) ~100 m fault zone in the WFSD-1 drilling cores. Fault rocks, including fault breccia, fault gouge and cataclasite Fault rocks were identified in both the outcrop and drilling cores, while pseudotachylyte was only present at the outcrop. Fault zone structure Two different types of gouge veins, formed by thermal pressurization and fluidization respectively, are observed WFSD-1 in this area. The YBF possesses the characteristics of a multiple core model, and consists of 5 different fault rock units. From top to bottom, these are cataclasite zone, black fault gouge–breccia zone, gray fault breccia zone, dark-gray fault breccia zone and black fault gouge–breccia zone. Outcrop investigation and drilling core research show that the slip zone of the Wenchuan earthquake does not completely follow the ancient fault slip zone. The Wenchuan earthquake fault is a high angle thrust fault which crosses the YBF obliquely. The multi-layered fault rocks displayed in the research area might indicate that the YBF comes from the long-term fault activity and evo- lution over the last ~15–10 Ma. © 2013 Elsevier B.V. All rights reserved. 1. Introduction the ruptured YBF and Guanxian–Anxian fault zones as the Wenchuan earthquake fault zone. Fault zones record a series of faulting events that occurred under Shortly after the Wenchuan earthquake struck, detailed field surveys different physical conditions during their evolution (Chester, 1995; were carried out by many research groups, and valuable data were Chester and Chester, 1998; Chester et al., 1993), which can then help gathered along the two surface ruptures. Previous research studied on us clarify the mechanical behavior of the crust (Wibberley et al., surface deformation and the seismic mechanism of the Wenchuan 2008). To better understand the earthquake process from nucleation earthquake, as well as the deep structure of the Longmen Shan fault and propagation to arrest, it is essential to assess the internal structures zone (Lin, 2011; Liu-Zeng et al., 2010; Verberne et al., 2010; Wang of the fault zones, because they convey significant information on fault et al., 2009). As the YBF was the causative fault of the Wenchuan earth- behavior and mechanical properties (Biegel and Sammis, 2004; quake, displaying large amounts of thrust and dextral slip (Xu et al., Faulkner and Rutter, 2003; Faulkner et al., 2008). 2009), numerous works have focused on this same section, located The 2008 Wenchuan earthquake (Mw7.9) occurred at the eastern along the southern segment of the YBF surface rupture in Bajiaomiao margin of the Tibetan Plateau in Sichuan, China, producing 270 km village, Hongkou town. This outcrop exposes a distinct fault plane and 80 km co-seismic surface ruptures along the Yingxiu–Beichuan (strike: N40–50°E, dip angle: 70–80°) with two striation orientations fault (hereafter YBF) and Guanxian–Anxian fault, respectively (Fu and a vertical displacement of 2–6m(Jia et al., 2010; Li et al., 2008a; et al., 2011; Li et al., 2008a; Xu et al., 2008)(Fig. 1). Here, we define Lin et al., 2009; Xu et al., 2008). The structures and fault rock character- istics of the southern segment of the YBF were studied at this outcrop (e.g. Lin et al., 2010; Togo et al., 2011; Wang et al., 2010). Wang et al. ⁎ Corresponding author at: Institute of Geology, Chinese Academy of Geological (2010) reported a 120 m-wide YBF with fault gouge, fault breccia, Sciences, No. 26, Baiwanzhuang Road, Beijing 100037, China. Tel.: +86 10 68990581; fax: +86 10 68994781. cataclasite and pseudotachylyte, while Togo et al. (2011) reported a E-mail address: [email protected] (H. Li). much narrower zone of 30–40 m with fault breccia and fault gouge, 0040-1951/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tecto.2013.08.029 102 H. Wang et al. / Tectonophysics 619–620 (2014) 101–114 103°30’ 103°40’ 103°50’ 104°00’ S 31°20’ Wenchuan-Maoxian fault 0 5 10km P NP T3 Yingxiu-Beichuan fault Pengguan Complex C Bailu P Xiaoyudong Guanxian-Anxian fault WFSD-1 31°10’ P Q NP Tongji Minjiang River Bajiaomiao Hongkou T3 Sichuan D Shenxigou J Basin Xiang’e Jianjiang River 103 o E 104 o E 105 o E YingxiuYingxiu Songpan Qingchuan P Pingwu Baisha River T3 EasternK Tibet Pengzhou o Dujiangyan Q 32 N 31°00’ Beichuan Minjiang(Guanxian) River Maoxian J K F2 F3 N Wenchuan F1 (b) E Longmen Shan Mianyang WFSD-1 Sichuan Basin Lower Triassic o Q Quaternary T3 Volcanics 31 N (Xujiahe Formation) Neoproterozoic (NP) Dujiangyan N Neogene Gran. and (Pengguan complex) P Permian met. rocks Fig.1b Paleogene C Carboniferous Klippen 0 km 50 E Thrust fault Chengdu (a) K Cretacious D Devonian Normal fault Fault Strike and dip Surface Epcenter of the WFSD J Jurassic S Silurian River of bedding rupture zone Wenchuan earthquake drilling site Fig. 1. Geological structures of the Longmen Shan area and WFSD drilling site location. (a) Tectonic sketch of the Longmen Shan area, F1 is the Maoxian–Wenchuan fault, F2 is the YBF and F3 is the Guanxian–Anxian fault; (b) geological structure of the Hongkou area and WFSD-1 drilling site. and they did not find any cataclasite and pseudotachylyte in the changes during their exhumation (Zoback et al., 2007). Therefore, it is Pengguan complex. These papers present preliminary results at this very important to compare the fault zones at the surface with the dril- outcrop, but the lithologies, microstructures, mineralogical changes, ling core analysis extracted at depth, which potentially preserve a fault activity and history, fault mechanism and internal structure of more accurate record of internal structure and associated composition. the YBF are not clear yet and need to be further investigated. The first borehole of the Wenchuan earthquake Fault Scientific Drilling Fortunately, a much wider and complete section is visible on the (WFSD-1) project lies on the Pengguan complex rocks, penetrates the same side of the river near Bajiaomiao village, due to the fact that sever- YBF and reaches a final depth of 1201.15 m (Li et al., 2013). The drilling al debris flows occurred soon after the Wenchuan earthquake, causing core helps us examine fault zone properties and compare them to out- sediments to be washed away along the valley, and providing a good crop observations. opportunity to study the fault zone structures and composition along In this paper, we focus mainly on the southern part of the YBF in the southern segment of the YBF. Bajiaomiao village, and compare its internal structure with borehole Rocks exposed at the surface may be affected by weathering pro- data obtained from WFSD-1. The objective of this paper is intended to cesses, and have undergone structural, mineralogical and geochemical delineate the internal structure and fault rock types of the YBF mainly H. Wang et al. / Tectonophysics 619–620 (2014) 101–114 103 based on field surveys at the Bajiaomiao outcrop and comparisons to the is known to be exposed. We systematically sampled sandstone, fault results obtained from the WFSD-1 drilling cores; as well as to illustrate gouge, fault breccia and cataclasite along the Baisha River valley, docu- the connection between the slip zone of the Wenchuan earthquake fault mented surface outcrops along the YBF, and collected fault gouge and and the YBF, and to interpret the multi-type fault rocks and their tecton- gouge veins from a trench across the surface rupture zone. The charac- ic implications. teristics of fault rocks are ascertained by visually examining the outcrop and core samples. 2. Tectonic setting Several thin sections of the samples were prepared to characterize the microstructure deformation and alteration related to faulting. Ob- The Longmen Shan thrust belt (about 500 km long and 30–50 km servations of thin sections from fault rock samples were made using wide) lies at the eastern margin of the Tibetan Plateau, just west of the an Olympus BX51 optical microscope at various magnifications (1.25, Sichuan Basin. This is the topographic boundary of eastern and western 2, 4, 10, 20, and 50×) at the State Key Laboratory of Continental Tecton- China, between the Songpan–Ganzi terrane (a Triassic orogenic belt) ics and Dynamics of the Institute of Geology, Chinese Academy of Geo- and the Sichuan Mesozoic–Cenozoic foreland basin on the Yangtze logical Sciences in Beijing.
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