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 evolution 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 characteristics 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,

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. block (Xu et al., 1992). The plateau margins were subjected to slow, Bulk and quantitative X-ray diffraction (XRD) was used to determine steady exhumation during the early Cenozoic, followed by two pulses sample mineralogy and clay composition, as well as their relative of rapid exhumation, one beginning at 30–25 Ma and a second one at amounts along the YBF, in order to characterize the mineralogical alter- 15–10 Ma that is still ongoing (Wang et al., 2012). The Cenozoic ation that occurred during the deformation. The samples were analyzed Longmen Shan thrust belt is mainly composed of three thrust faults: using a Dmax 12 kW powder diffractometer (CuKα)at40kVand the Wenchuan–Maoxian, Yingxiu–Beichuan and Guanxian–Anxian 100 mA, with a sampling width of 0.02° and a scanning speed of 4° faults, from northwest to southeast (Deng et al., 1994; Jia et al., 2006; (2θ)min−1. Li et al., 2006)(Fig. 1a). These faults have been active throughout the Our goal is to distinguish different fault rocks on the basis of micro- late Quaternary, with slip-rates of up to 1.0–1.5 mm/yr (Densmore structures and their composition, and compare their distribution at the et al., 2007; Li et al., 2006). The hanging wall of the YBF is uplifted at a outcrops with that in the drilling cores, to confirm the internal struc- rate of 0.6–1 mm/yr and it has had a dextral-slip rate of 1 mm/yr since tures of the YBF. the Pleistocene (Deng et al., 1994; Densmore et al., 2007; Li et al., 2006). The YBF, striking N30–55°E and dipping 50–80° to the northwest, 4. Fault rock characterization along the Yingxiu–Beichuan fault zone was the causative fault of the Wenchuan earthquake, producing the lon- gest surface rupture of about 270 km-long (Fu et al., 2011; Li et al., 4.1. Outcrop description 2008a; Xu et al., 2008) or up to 300 km-long (Lin et al., 2012), and is mainly characterized by thrust motion with some dextral strike-slip Rocks associated with faults and shear zones are broadly called fault motion (Fu et al., 2011; Li et al., 2008a, 2009; Liu-Zeng et al., 2010; Xu rocks (Sibson, 1977). Fault rocks, including cohesive and incohesive et al., 2009)(Fig. 1). There are two slip centers located along this surface fault rocks are attributed to cataclasis (Keulen et al., 2007). Here, we fol- rupture zone: the southern part is centered in Bajiaomiao, Hongkou low the Sibson's (1977) fault-rock classification to distinguish fault area (Fig. 2) with a maximum vertical offset of 6.0–6.7 m (Fig. 2b, c), gouge from cataclasite, where gouge is defined as a cataclastic rock while the northern part is centered in Qushan town with a maximum without cohesion while the cohesive rock is called cataclasite. Fault vertical displacement of 11–12 m (Li et al., 2008a). The surface rupture breccia and fault gouge are distinguished based on the volume fraction in Bajiaomiao village formed in the Late Triassic sedimentary rocks of visible rock fragments: N30% in fault breccia and b30% in fault gouge. (Fig. 2a) near the Neoproterozoic Pengguan massif that thrusts over We measured an exposed profile along the eastern tributary of the the sedimentary sequence. In this region, the rupture was clear with Baisha River from the Pengguan complex to the Xujiahe Formation fresh scarps of about 4.3 m-high (Fig. 2d) but was more ambiguous to (Fig. 3). Many faults and small subsidiary faults have developed in this the north. The fault did not slip along the interface between the Sinian area (Wang et al., 2010), and there are various fault rocks with different and Triassic rock layers, but along the weak layers of the charcoal mud- colors varying from light-gray, dark-gray to black. We divide this out- stone in the Triassic rocks (Wang et al., 2009). The Pengguan complex, crop, into 5 distinct units, based on color, amount of subsidiary faults, displaying a clear geomorphic boundary with the Xujiahe Formation fault gouge and fracture density, and the appearance of bedding and (Fig. 2b), mainly consists of granodiorite, plagiogranite, moyite, biotite foliation. granite, tonalite, intermediate-acidic and some mafic–ultramafic intru- sive rocks, volcanics, pyroclastic rocks and green schist phased meta- 4.1.1. Unit 1, cataclasite zone morphic rocks (Chengdu college of Technology, 1996; Li et al., 2002; Fault rocks located in the Pengguan complex are composed of Ma et al., 1996; Sichuan Bureau of Geology, 1975). The Late Triassic protocataclasite, cataclasite and ultracataclasite, and also pseudotachylyte Xujiahe Formation mainly consists of conglomerate, sandstone, silt- (Fig. 4). Protocataclasite and cataclasite are not easy to tell apart just from stone, mixed sandstone and siltstone, mudstone, shale, coal-bearing their grayish appearance (Fig. 4a, d), while the ultracataclasite clearly has mudstone and coal-lines (Sichuan Bureau of Geology, 1975). a different appearance with a slightly darker color and consists of angular The borehole of WFSD-1 (31.14987°N, 103.69125°E) is located in the to sub-angular fragments (Fig. 4b, c, e). Pseudotachylyte is injected into Pengguan complex rocks, in Bajiaomiao village, 385 m west of the anetworkoffractures, with brown-black, flinty appearance, and flow Wenchuan earthquake co-seismic surface rupture (Fig. 2a). WFSD-1 patterns (Fig. 4f). The width of the cataclasite zone is about 70 m in cut through the YBF, reached 1201.15 m-depth, with a total drilling the Pengguan complex which forms a sharp contact (31.14669°N, core length of 1368.29 m (Li et al., 2013). 103.69130°E) with the Xujiahe Formation, striking N62°E with a dip angle of 55° to the southeast. 3. Methods 4.1.2. Unit 2, black fault gouge–breccia zone Field investigations, X-ray diffraction, microstructure analysis Along the downstream direction, from the boundary of the and comparisons between outcrop and drilling cores were used to de- Pengguan complex and Xujiahe Formation to the limit (31.14612°N, termine the internal structure and composition of the YBF in Bajiaomiao 103.69148°E) between dark-gray and light-gray breccia (Fig. 5a, b), a village. much different zone is developed in the Late Triassic Xujiahe Formation, The distribution of fault rocks in the YBF is determined from the out- characterized by thick fault gouge layers and heavily foliated fault brec- crop in Bajiaomiao village near the WFSD-1 drilling site, where the YBF cia. The fault breccia is dark-gray to black (Fig. 5a, b, c), with a heavily 104 H. Wang et al. / Tectonophysics 619–620 (2014) 101–114

WFSD-2 WFSD-1

WFSD-1

Pengguan Complex Surface rupture zone Tianzhuping (Pt3) Village

Xujiahe Formation Fig.3 BaishaB rive r a (T3) i s h a Fig.2b

r i Bajiaomiao v e Village r 200 m (a)

SW NE

WFSD-1 Pengguan Complex3) A (Pt

Xujiahe Formation (T3)

B 6~6.7 m Fig.2c NW B SE Fig.2d Surface rupture zone A Pt3 B (b) T3

SW NE NW SE Slip plane

6~6.7 m Fault breccia

Fault gouge (c) (d)

Fig. 2. (a) Satellite image of the WFSD-1 drilling site (Google Earth) with the Wenchuan earthquake co-seismic surface rupture zone striking N40°E (red line) in Bajiaomiao village. The white dashed line is the boundary between the Neoproterozoic Pengguan complex (light pink area) and the Late Triassic Xujiahe Formation (light blue area), which does not parallel to the co-seismic surface rupture zone (red line); (b) overview of the surface rupture zone in Bajiaomiao village. The white dashed line is the boundary between the Pengguan complex and the Xujiahe Formation which can be distinguished from topographic features. The red arrows indicate the co-seismic surface rupture zone, the yellow dashed line AB locates the proﬁle displayed in the lower right corner; (c) maximum vertical dislocation of 6–6.7 m in the Bajiaomiao area; (d) a distinct fault scarp with ~10 cm of black fault gouge (yellow dashed line represents the layer of fault gouge), two orientation striations were found on the slip plane. Panel a is from Li et al. (2013).

foliated to scaly fabric (Fig. 5c). The fault gouge is soft, dark-gray, and fo- little foliation, together with many thin gouge layers and gouge veins. liated (Fig. 5d), with a maximum thickness of ~4 m. Small quartz veins The protolith of the fault breccia is grayish sandstone, siltstone and and calcite veins are visible in the foliated breccia (Fig. 5c). The protolith dark-gray coal-bearing shale. As shown in Fig. 5e, thin layers of black of the foliated breccia might be coal-bearing siltstone and shale. This gouge develop in the grayish sandstone fault breccia (Fig. 5g), and black fault gouge and fault breccia zone is about 60 m-thick. clear striations are visible on the surface of the gouge layer. Black gouge veins are reticular in the fault breccia, with a much more ﬁne- 4.1.3. Unit 3, gray fault breccia zone grained matrix in the gray fault breccia than in the dark-gray part This unit forms a sharp contact with the previous one, striking (Fig. 5h). Liqueﬁed breccia (soft sediment deformation) is an indicator N70°E with a dip angle of 80°. It is a much wider zone (~70 m) of ancient earthquakes (Qiao et al., 2012) and appears in this unit like which displays mainly gray fault breccia with a random texture and belts (Fig. 5f). H. Wang et al. / Tectonophysics 619–620 (2014) 101–114 105

NW SE Pengguan Complex (Pt3)

Xujiahe Formation Bajiaomiao new village cataclasite (T3) Unit 1 zone black gouge breccia +zone Unit 2 Fig.4a gray fault breccia zone Fig.5a surface rupture zone

Unit 3 Fig.5d black fault gouge

breccia+ zone Unit 4 dark gray Fig.5e fault breccia Fig.7 Fig.5h trench zone Unit 5

Fig.6a Fig.6d

Fig. 3. Panoramic photograph of fault rocks distribution at the outcrop of the YBF in Bajiaomiao village. Five units were identiﬁed: cataclasite, fault gouge and breccia, gray fault breccia, dark gray fault breccia, black fault gouge and breccia. The red line represents the surface rupture zone.

4.1.4. Unit 4, dark-gray fault breccia zone ﬁne-grained matrix. Some very thin calcite veins are visible in small This ~20 m-wide unit is characterized by thick, weakly deformed subsidiary faults and fractures. sandstone breccia and heavily deformed silt-medium sandstone breccia (Fig. 6a), showing little foliation and many light veins (Fig. 6b, c). The 4.1.5. Unit 5, black fault gouge–breccia zone fault breccia has localized brittle structures bearing evidence of their de- A continuous fault rock series consisting of dark-gray fault breccia, formation. This breccia zone is composed of quartz and feldspar as well black fault gouge and gray fault breccia (Fig. 6d) is exposed near the as minor carbonaceous minerals. Siltstone fragments are cemented by a co-seismic surface rupture zone. Fault breccia is foliated with different

Fig. 4. Field photographs of the YBF at Bajiaomiao village taken in August 2012. (a) An overview photograph of the cataclasite zone (unit 1) in the Pengguan complex. Fault rocks include protocataclasite, cataclasite (b, d, e), ultracataclasite (c, e) and pseudotachylyte (f). 106 H. Wang et al. / Tectonophysics 619–620 (2014) 101–114

SE NWNW

Unit 3 Unit 2 (b) 70°, 80° (SE) gray fault breccia dark-gray fault breccia (a)(a) SESE NWNW N S SESE NWNW

fault breccia + fault gouge veins fault gouge (b)(b) (c)(c) (d)(d) N S

fault breccia fault breccia (f) (g) (e)(e)

N S N fault breccia S S N fault breccia gouge veins fault breccia

liquefied breccia fault breccia fault fault gouge breccia with (f)(f) (g)(g) slickenside (h)(h)

Fig. 5. Field photographs of the YBF at Bajiaomiao village taken in August 2012, showing fault rocks in units 2 and 3. (a) The YBF forms a sharp contact with a stretch striking N70°E and with a dip angle of 80° between gray fault breccia (unit 2) and dark-gray fault breccia (unit 3); (b) close-up of the boundary between units 2 and 3; (c) foliated fault breccia and fault gouge in unit 2, with the white arrow pointing at the small quartz veins; (d) ~4 m-thick foliated fault gouge in unit 2; (e) fault rocks in unit 3 are mainly gray fault breccia, some liqueﬁed breccia developed in the fault breccia (f), thin layer of fault gouge with clear slickenside is displayed (g), and many gouge veins (h) developed in this unit.

tones of gray (Fig. 6e), and small calcite veins are present along the frac- Undeformed sandstone, including some liquefied breccia is exposed tures in the fault breccia. The fault gouge is recognized as a tacky, clay- about 40 m west of the co-seismic surface rupture zone (31.14456°N, rich material, weakly foliated with nearly-ductile behavior when wet, 103.69163°E) in the Xujiahe Formation. It is covered by alluvial deposits and its thickness is about 25 cm. The dark-gray fault breccia is foliated between the sandstone and the surface rupture zone, and fault rocks are while the grayish breccia is not (Fig. 6f). Fault breccia and small fault absent in the footwall. gouge layers are about 20 m-wide and are exposed along the co- seismic surface rupture zone. The thickness of the gouge layers vary from several mm to ~2 cm. 4.2. WFSD-1 drilling core In order to reveal the undisturbed formation, we excavated a trench across the co-seismic surface rupture. As shown in Fig. 7, fault gouges The lithologies of the drilling core from WFSD-1 are reported in Li and gouge veins are well-developed. Thin light-grayish veins and a et al. (2013), from 3 to 575.7 m-depth, and mainly consist of diorite, fine-grained matrix are visible in the fault breccia. The thickest section porphyrite, pyroclastics and volcanic rocks which belong to the of fault gouge exposed in the trench is dark gray, about 30–50 cm- Pengguan complex, while the section from 575.7 to 1201.15 m-depth thick, and the dark gray fault breccia contains small white calcite is dominated by sandstone (including coal-bearing sandstone), siltstone veins. Two major gouge veins are present in the light gray fault breccia, and shale, together with some liquefied breccia in the Xujiahe Forma- an ~10 cm black gouge was present at the margin of the co-seismic tion (Fig. 8a). Fault rocks are seen mostly between 575.7 and 759 m- scarp of the Wenchuan earthquake, and forms a sharp contact between depth (Fig. 8b): from 575.7 to 585.75 m-depth are mainly cataclasite, the scarp and the deposits at a dip angle of 76–78°. and a continuous fault zone composed of fault breccia and fault gouge H. Wang et al. / Tectonophysics 619–620 (2014) 101–114 107

W E

fault breccia + 60°, 70° (NW) fault gouge

fault breccia

fault breccia fault breccia

(b)(b) (c)(c) S N

(f) (e) fault breccia fault breccia fault breccia (d)(d) S N S N

fault breccia fault fault breccia gouge

calcite veins fault breccia

(e)(e) (f)(f)

Fig. 6. Field photographs of the YBF at Bajiaomiao village taken in August 2012 showing fault rocks in units 4 and 5. Unit 4 is mainly composed of dark gray fault breccia with little gouge (a, b, c). Unit 5 is characterized by black fault gouge and fault breccia (d), the fault breccia is foliated and calcite veins developed (e), fault gouge in this unit is soft, black, weakly foliated, with a thickness of ~25 cm (f).

with various thicknesses is present between 585.75 and 759 m-depth 759m-depth,wehavefaultgougeandfaultbrecciainsiltstoneandcar- (Li et al., 2013). bonaceous shale (Fig. 8l), that we classified into one group as black fault Below, we give a detailed introduction of the different fault rocks. gouge zone with fault breccia. The fault gouge is black and soft, contain- They can be classified into 5 different groups according to the drilling ing carbonaceous materials. core research, based on their lithology, color, foliation, and fracture characteristics (Fig. 8). The cataclasite zone is present from 575.7 to 4.3. Microstructure 585.75 m-depth, and consists of mainly light-gray to dark-gray in the Pengguan complex, with sub-angular fragments and weak foliation In the analyzed thin sections, protocataclasite, cataclasite, ultra- (Fig. 8c, d, e). From 585.75 to 595.5 m-depth, we report a thick fault cataclasite and pseudotachylyte were recognized in the Pengguan com- zone of black foliated gouge with scattered fragments (Fig. 8e, f) and plex. Protocataclasite is typical of small local cataclasite zones, and some black fault breccia in this black fault gouge zone. The next lower consists of large fractured angular grains, which are in contact with unit is a dark grayish fault breccia zone, which consists of fault breccia al- one another, where small grains forming a fine matrix (b30%) are ternating with narrow fault gouge layers from 595.5 to 693.5 m-depth. only present in the local cataclasite zone (Fig. 9a). Cataclasite is charac- The fault breccia is mostly dark-gray, weakly foliated to foliated, with vis- terized by the random-fabric cataclasite zone from a wide fault, and ible calcite veins (Fig. 8g, h). A gray fault breccia zone is identified be- consists of a small amount of clear large quartz clasts and cloudy feld- tween 693.5 and 749 m-depth, and is mainly composed of gray fault spar clasts, owing to fine-grained alteration products (mainly clay min- breccia without foliation and rare fault gouge (Fig. 8i, j, k). From 749 to erals). The clasts are dominantly angular to sub-angular in shape, poorly 108 H. Wang et al. / Tectonophysics 619–620 (2014) 101–114

125° surface rupture zone

(a) 0 1 m

125°

surface rupture gray gouge veins zone

fault breccia

gouge

hk-11 calcite gouge veins hk-12 veins

hk-14 hk-13 alluvial deposits

(waterish) black gouge dark gray fault breccia (b) fault breccia hk-15 0 1 m

Fig. 7. Photograph (taken in April 2011) of fault rocks in the trench (a) and its sketch (b) showing internal structures of fault core. Fault rocks include gray fault breccia, black fault gouges and gouge veins. The red dashed lines indicate the co-seismic surface rupture zone of the Wenchuan earthquake. The gouge samples, hk-11, hk-12, hk-13, hk-14, hk-15 are for XRD.

sorted,partlyincontactwithoneanotherandpartlysurroundedbya material near the margins of the fault breccia. Fault breccia consists fine matrix which accounts for N50% (Fig. 9b). The grains are smaller of fragments which retain the characteristics of protolith. The grain seg- and less angular while the matrix is finer than that observed in the ments show little displacement or rotation (Fig. 9e). The irregular brec- protocataclasite. Ultracataclasite is typically constitutive of an assem- cias are partly in contact with one another and partly surrounded by a blage of few large grains, sub-angular to sub-rounded feldspar and sub-microscopic matrix which consists of extremely fine rock flour, quartz fragments floating in a well-developed and abundant fine and the small fragments are generally surrounded by fine-grained matrix. Deformation is largely dominated by cataclastic flow of fine- rock flour. Thin layers of black fault gouge viewed from the outcrop de- grained particles in the matrix, and fragments are rarely in contact veloped in this fault zone, the photomicrograph showing that the black with one another. Pseudotachylytes are generally pale-brown to dark- layers are opaque in polarized light with irregular shapes of belts in fault brown in plane polarized light, and dark in crossed polarized light breccia (Fig. 9f). Fault gouge is generally yellowish-brown to dark- (Fig. 9c). The contacts between the veins and the host cataclasite brown in plane polarized light and dark in crossed polarized light. rocks are generally sharp but locally less clear. The veins consist of Fault gouge from the trench, however, shows different features in the fine-grained matrix which exhibits optical characteristics of glass, with photomicrographs. As shown in Fig. 9g, the sample from the thickest sub-rounded to rounded fragments of quartz and feldspar scattered in fault gouge in the trench is strongly foliated with S–C fabrics and asym- the matrix (Fig. 9c). The pseudotachylyte matrix consists primarily of metric rotational augens indicative of sinistral shear. Gouge samples fine-grained fragments which are generally N2–3 μmandaresmaller from the gouge veins display much homogeneous fine-grained materials than cataclasite. with several quartz and feldspar fragments, without foliation (Fig. 9h). Different kinds of fault gouge and fault breccia were recognized in Fig. 9i reveals the feature of fault gouge from sample hk-15, which is fo- the Triassic Xujiahe Formation. There, the fault rocks are predominantly liated with fragments of the host rock. composed of K-feldspar, plagioclase and quartz, together with variable amounts of clay-rich matrix. The microstructure of the foliated fault 4.4. X-ray diffraction analysis breccia with small veins we described in the previous section is presented in Fig. 9d. The irregularly shaped quartz veins have some car- Sixteen fault rocks were sampled at the Bajiaomiao outcrop. The re- bonate at their margin, and are reticulated in the black carbonaceous sults from the X-ray diffraction analysis (XRD) of bulk rocks are H. Wang et al. / Tectonophysics 619–620 (2014) 101–114 109

Lithology Fault rocks WFSD-1 drilling cores (m) (m) 500 cataclasite (c) 583.74 m 584.49 m

(d) cataclasite 584.53 m 585.27 m 550 fault gouge (e) cataclasite Pengguan Complex 585.27 m PSZ 585.75m 586.07 m Unit 1 PSZ Unit 2 (f) black fault gouge gouge with fragments 589.84 m 600 589.04 m 589.21m

(g) foliated fault breccia 616.03m 615.25 m Unit 3 Depth (m) fault breccia 650 (h) 642.88 m 642.14 m Xuji ahe Formation

(i) fault breccia gouge veins 698.94 m 699.74 m

700 (j) fault breccia 726.40 m 725.60 m Unit 4 fault breccia (k) 745.65 m 746.32 m 750 Unit 5 (l) fault gouge 758.71 m 758.15 m

mostly cataclasite dark-colored diorite, porphyrite sandstone host rock unidentified fine sandstone fault breccia cataclasite volcanics mixed sandstone (pyroclastics, liquefied breccia siltstone, shale fault gouge host rock and siltstone/shale 800 (a) (b) volcanic rocks)

Fig. 8. Fault rocks in WFSD-1 drilling cores. Lithology column (a) and fault rocks distribution (b) of WFSD-1 from 500 to 800 m-depth. Fault rocks are classiﬁed into ﬁve different zones: cataclasite zone (unit 1), light-gray to dark-gray, weakly foliated cataclasite mainly present in the Pengguan complex at depths between 575.7 and 585.75 m (c, d, e); black fault gouge zone (unit 2) from 585.75 to 598 m-depth, the fault gouge is foliated with some scattered fragments (e, f); fault gouge and breccia zone (unit 3) from 598 to 698 m-depth, characterized by many small gouge layers and weakly foliated to foliated fault breccia (g, h); fault breccia zone (unit 4) from 698 to 749 m-depth, mainly consists of fault breccia (i, j, k), fault gouge rarely displays in this zone; fault gouge and breccia zone (unit 5) from 749 to 759 m-depth, characterized by black fault gouge containing coarse materials (l). The red line shows the Principal Slip Zone (PSZ) of the Wenchuan earthquake (b, f).

presented in Table 1.TheXRDproﬁle (Fig. 10) yields mineral composi- The XRD proﬁle of fault gouge from the trench (Fig. 10c) shows that tions of mainly quartz, albite and clay minerals. Two pseudotachylyte hk-11 and hk-12 have the same composition, with 43% quartz, 3% albite samples, hk-1 and hk-2 (Fig. 10a), obtained from the Pengguan com- and 54% clay minerals, and the other three samples have similar peaks. plex, are characterized by the appearance of ankerite, with quartz (54%, 56%), albite (26%, 16%) and clay minerals (11%, 22%) and a 5. Discussion mixed layer of illite–smectite (I/S), and hk-2 contains 1% hematite. The foliated fault gouge hk-3 from unit 2 shows a high clay mineral con- 5.1. Internal structure of the Yingxiu–Beichuan fault zone tent (66%), which includes a mixed layer of illite–smectite (I/S) (72%), kaolinite (14%), chlorite (10%), and pyrophyllite (4%), while the fault Two typical kinds of fault zone structures are generally acknowl- gouge without foliation (hk-4) contains 28% clay minerals, with 2% edged (Faulkner et al., 2010), one has a single high strain fault core smectite and 31% illite. The kaolinite content decreases from 14% to 1% (Chester and Logan, 1986), and the other is a combined fracture zone while chlorite decreases from 10% to 3%, and pyrophyllite is absent. with multiple high strain fault cores (Faulkner and Rutter, 2003). Fault gouge and breccias from unit 3 (Fig. 10b) show similar composi- Depending on the degree of deformation, fault zones can be divided tions of quartz (65–75%), albite (6–15%), microcline (3–5%) and clay into fault core and damage zone (Spray, 1995). The fault core is gener- minerals (8–16%) including a mixed layer of illite–smectite (I/S) (37– ally presented as a slip plane, composed of clay-rich fault gouge, 53%), illite (18–27%), and chlorite (22–39%). cataclasite or ultracataclasite (or a combination of both), while the 110 H. Wang et al. / Tectonophysics 619–620 (2014) 101–114

cataclasite pt qz qz qz cracks qz

pl pl qz ultracataclasite (a) 2mm (b) 2mm (c) 2mm

fault breccia black material

black material qz qz qz fault breccia (d) 2mm(e) 2mm (f) 2mm

qz+pl crack crack

fragments qz+pl C’ (g) S 2mm (h) 2mm (i) 2mm

Fig. 9. Photomicrographs (optical images in polarized light) of fault rocks from the outcrop. qz—quartz, pl—plagioclase, pt—pseudotachylyte, S—compressive plane, C′—secondary shear plane. (a) Protocataclasite (from unit 1), with small local cataclasite zone (yellow dashed line). (b) Cataclasite (from unit 1). (c) Ultracataclasite and pseudotachylyte (from unit 1). (d) Quartz veins (from unit 2). (e, f) Fault breccia (from unit 3). (g) Fault gouge (from the trench), strongly foliated with S–C fabrics and asymmetric rotational augens. (h) Gouge vein (from the trench), without foliation, much homogeneous ﬁne-grained materials with several quartz and feldspar fragments. (i) Fault gouge (from the trench), foliated with fragments of host rock. damage zone is generally composed of fractures over a wide range of growth and connection between smaller faults (Childs et al., 2009; length scales and subsidiary faults. The fault zones of thrust faults are Peacock and Sanderson, 1991; Walsh and Watterson, 1988; Walsh generally wide, while the fault core is relatively narrow, with the Princi- et al., 2002). pal Slip Zone (PSZ) being only a few mm to tens of cm-thick (Sibson, The microstructure and XRD analysis show the characteristics of the 2003). It is generally acknowledged that the width of the fault zone con- fault rocks. Based on the fault rock distribution at the Bajiaomiao out- sists of fault gouge, cataclasite, fault breccia and fractured surrounding crop (Fig. 11), the YBF consists of 5 different fault rock units: cataclasite rocks. Many authors have suggested that larger faults result from the zone, black fault gouge–breccia zone, gray fault breccia zone, dark-gray

Table 1 Mineralogy of fault rocks from the outcrop in the Bajiaomiao village.

Sample Lithology and location Mineralogy

hk-1 Pseudotachylyte, unit 1 Quartz 54%, albite 26%, ankerite 8%, clay minerals 11% (I/S, 100%) hk-2 Pseudotachylyte, unit 1 Quartz 56%, albite 16%, ankerite 4%,hematite 1%, clay minerals 22% (I/S, 100%) hk-3 Black fault gouge, unit 2 Quartz 34%, clay minerals 66% (I/S, 72%, kaolinite 14%, chlorite 10%, pyrophyllite 4%) hk-4 Black fault breccia, unit 2 Quartz 68%, microcline 4%, clay minerals 28% (smectite 2%, I/S 63%, illite 31%, kaolinite 1%, chlorite 3%) hk-5 Gray fault breccia, unit 3 Quartz 65%, albite 14%, microcline 5%, calcite 6%, dolomite 2%, clay minerals 8% (smectite 2%, I/S 37%, illite 26%, chlorite 35%) hk-6 Black gouge, next to hk-5, unit 3 Quartz 69%, albite 6%, microcline 5%, dolomite 3%, clay minerals 16% (I/S 41%, illite 23%, chlorite 36%) hk-7 Black gouge with slickenside, unit 3 Quartz 66%, albite 15%, microcline 3%, calcite 1%, dolomite 2%, clay minerals 12% (smectite 7%, I/S 44%, illite 27%, chlorite 22%) hk-8 Gray fault breccia, unit 3 Quartz 75%, albite 11%, microcline 3%, calcite 1%, dolomite 3%,clay minerals 8% (smectite 2%, I/S 53%, illite 18%, chlorite 27%) hk-9 Black gouge, next to hk-7, unit 3 Quartz 75%, albite 8%, microcline 3%, glaucophane 1%, dolomite 3%, clay minerals 10% (smectite 8%, I/S 47%, illite20%,chlorite25%) hk-10 Black gouge veins, unit 3 Quartz 71%, albite 13%, microcline 3%, calcite 2%, dolomite 1%, clay minerals 10% (I/S 37%, illite 24%, chlorite 39%) hk-11 Black fault gouge (trench), unit 5 Quartz 43%, albite 3%, clay minerals 54% (smectite 1%, I/S 61%, illite 22%, chlorite 16%) hk-12 Black fault gouge (trench), unit 5 Quartz 43%, albite 3%, clay minerals 54% (smectite 1%, I/S 71%, illite 15%, chlorite 13%) hk-13 Gouge vein (trench), unit 5 Quartz 37%, albite 3%, calcite 10%, dolomite 4%, clay minerals 46% (I/S 69%, illite 20%, chlorite 11%) hk-14 Gouge vein (trench), unit 5 Quartz 40%, albite 4%, calcite 15%, dolomite 11%, clay minerals 30% (I/S 62%, illite 22%, chlorite 16%) hk-15 Black fault gouge (trench), unit 5 Quartz 41%, albite 4%, calcite 7%, dolomite 2%, clay minerals 46% (I/S 67%, illite 16%, chlorite 17%) hk-16 Black fault gouge, unit 5 Quartz 74%, albite 4%, hematite 1%, clay minerals 20% (smectite 3%, I/S 80%, illite 13%, kaolinite 2%, chlorite 2%) H. Wang et al. / Tectonophysics 619–620 (2014) 101–114 111

40000 (a) Qz (b) 35000 30000 Qz Alb 40000 25000 Ank Il Qz Qz Qz Il Il Il 20000 hk-1 35000 Qz 15000 hk-2 Il Chl

10000 hk-3 Counts Per Second 30000 5000 Qz hk-4 0 0 5 10 15 20 25 30 35 40 45 25000 Alb Cal Qz 2θ(deg. CuKa) Qz Qz hk-5 Il Il Alb Chl Dol Chl Il 12000 20000 (c) Qz hk-6

10000 Cal hk-7 Chl Qz Dol 15000 Counts Per Second Il Il Qz 8000 Il Chl Alb Il Qz Qz Chl Il hk-14 hk-8 6000 10000 hk-12 hk-9 4000 hk-11 hk-10 5000 Counts Per Second 2000 hk-13 hk-16

0 hk-15 0 0 5 10 15 20 25 30 35 40 45 0 5 10 15 20 25 30 35 40 45 2θ (deg. CuKa) 2θ(deg. CuKa)

Fig. 10. X-ray diffraction (XRD) profiles of fault rocks from the Hongkou outcrop. XRD profile of fault rocks from units 1 and 2 (a) and from units 3 and 5 (hk-16) (b). (c) XRD profile of fault gouges from the trench (unit 5).

fault breccia zone and black fault gouge–breccia zone. The YBF strikes data, the average trend of foliations is N305° and the average dip N55–65°E and is ~240 m-thick on the basis of fault rocks distribution. angle is 65–71° (Li et al., 2013). We consider this fault zone as the YBF The cataclasite zone is about 70 m-thick in the Pengguan complex, in the drilling core, with a length of at least 183.3 m in the core, while while it is ~170 m-thick in the southern part, consisting of weakly foli- the real thickness is about 100 m due to the hole inclination of 11° (Li ated fault gouge and breccia alternating with blocks of less deformed et al., 2013). This fault zone can be divided into 5 different units on fine-grained sandstone and siltstone in the Xujiahe Formation. the basis of lithology, color, foliation, and fracture characteristics, Two types of gouge veins have been found at many locations along which are similar to those of the surface outcrop. the YBF: gouge veins injected into the fault breccia, and network of Generally, we consider that the fault gouge is the core of the fault. gouge veins in the fault breccia. The injected gouge veins are clearly vis- From the outcrop and drilling core research, many fault gouge layers ible in the trench with their thicknesses ranging from a few mm to sev- with various thicknesses (mm to several meters) are distributed in the eral cm (Fig. 7). The reticulate gouge veins are generally b1cm-thick. YBF, indicating that the YBF possesses the characteristics of a multiple The photomicrographs show different structures for the gouge veins. core model. The total extent of the fault zone at the surface outcrop is The injection veins as shown in Fig. 9h, demonstrate the characteristics ~240 m wide, while the real thickness in the drilling core corresponds of gouge without foliation. The gouge vein composition is very similar to to about 100 m. the gouge layer of the slip plane (Fig. 10c), indicating that the gouge Comparing the fault rock distribution at the outcrop and in the dril- veins might have formed during the same episodes of activity as that ling core, the 5 similar units show slight differences in width. on the slip plane, which might be caused by thermal pressurization Pseudotachylyte is present at the outcrop but not identified in the dril- (Pei et al., 2010). While the reticulate gouge veins we observed at the ling core. The PSZ of the Wenchuan earthquake was reported at 589.21– microscale level display fluidization-type features, diagnostic evidence 589.22 m-depth with ~1 cm-thick fresh fault gouge in WFSD-1 (Li et al., for shear is absent at the optical scale, suggesting that these textures 2013). Based on fault rock distribution in WFSD-1 drilling core (Fig. 8b), are related to local fluidization after fault activity. fault gouge mostly developed in the footwall of the Wenchuan earth- Although previous studies have reported the YBF with a thickness of quake fault, but at the outcrop, where the co-seismic surface rupture 30–40 m (Togo et al., 2011)or120m(Wang et al., 2010), new results of is visible, fault gouge mainly appears in the hanging wall of the our research show that the YBF is ~240 m-thick. That is because some Wenchuan earthquake fault and sandstone was present in the footwall segments of the fault zone were covered in alluvium, with debris instead of fault rocks. flows following the Wenchuan earthquake that washed away the sediments along the valley, a much wider and complete section became vis- 5.2. Relationship between the Yingxiu–Beichuan fault zone structure and ible, and revealing an integrated YBF structure. the Wenchuan earthquake slip zone Fault rocks ~183.3 m-thick are recognized in the WFSD-1 drilling core from 575.7 to 759 m-depth, including fault gouge, fault breccia According to the detailed research at the outcrop in Bajiaomiao, the and cataclasite, while pseudotachylyte is not confirmed. The boundary ~240 m-wide fault zone consists of cataclasite, fault gouge, and fault between the Neoproterozoic Pengguan complex and the Late Triassic breccia. The fault zone, which lies in the Neoproterozoic Pengguan com- Xujiahe Formation is located at 585.75 m-depth, based on logging plex and Late Triassic Xujiahe Formation, is regarded as the YBF, with an 112 H. Wang et al. / Tectonophysics 619–620 (2014) 101–114

WFSD-1 Legend Pengguan Complex Fault gouge (Pt3) Xujiahe Formation Surface rupture (T3) zone

Cataclasite zone Foliation

Black gouge Strike and dip + breccia zone 62° angle of foliation Gray fault breccia Road zone Pengguan Complex Dark gray fault River breccia zone (Pt3) Sandstone

~65°

67° 60°

70° ~40° 62°

~70m Surface rupture zone Unit 1 80° 80° 65° 80° Yingxiu-Beichuan fault zone 25° ~60m Building Unit 2 72°

76°

~70m Unit 3 45° 75~80° 75° Xujiahe Formation

70° ~20m (T3) Unit 4

~20 m

Unit 5 65° Abandoned channel

~46° 50 m Bajiaomiao new village

Fig. 11. Sketch of fault rocks distribution along the river valley at Bajiaomiao outcrop. The Wenchuan earthquake surface rupture zone has an average strike of N40°E, intersects at about 15–25° angle with the YBF which has an average strike of N55–65°E.

average strike of N55–65°E, while the Wenchuan earthquake surface thrust fault, which does not entirely follow the ancient earthquake slip rupture zone exposed in Bajiaomiao has an average strike of N40°E, as zone but crosses the YBF obliquely. shown in Fig. 11. Some fault gouge was also found northeast of the surface rupture zone. This attests that the Wenchuan earthquake fault slip 5.3. Asymmetric distribution of fault rocks and tectonic implication zone intersects the YBF at an angle of about 15–25° (Fig. 11). In the drilling core, about 183.3 m of fault zone was conﬁrmed as Major fault zones are commonly rooted in the middle to the lower being the YBF from 575.7 to 759 m-depth, the PSZ of the Wenchuan crust (Coward, 1984; Ramsay, 1980). Fault rocks from different depths earthquake is reported at a depth of 589.21–589.22 m (Li et al., 2013). in the crust will record different deformation mechanisms. Fault zones This position in the drilling core, in addition to the coseismic fault at commonly exhibit a complex overprinting of fault rocks, recording the surface, yields a dip angle of the fault of 62–65° to the northwest changes in deformation mechanisms during progressive deformation (Li et al., 2013). From the surface outcrop, the PSZ of the Wenchuan or deformation during exhumation. Many major ancient fault zones earthquake lies at the bottom of the YBF (Fig. 11), but from the drilling are exposed at the surface with fault rocks documenting well the history core, the PSZ lies in the upper part of the YBF (Li et al., 2013), which also of fault activity and erosion levels. The common juxtaposition of fault illustrates that the Wenchuan earthquake fault intersects the YBF. rocks with different characteristics across major dip-slip fault zones in- These outcrop investigations, combined with the drilling core re- dicates that the rate of displacement and shearing far exceeded the rate search, suggest that the Wenchuan earthquake fault is a high angle of thermal equilibrium across the fault zone (Schmid and Handy, 1991). H. Wang et al. / Tectonophysics 619–620 (2014) 101–114 113

The YBF is the main fault in the Longmen Shan fault zone, located in the entirely follow the ancient earthquake slip zone but crosses the geomorphically steep boundary of the Longmen Shan (Li et al., 2002). YBF obliquely. The YBF is a steep thrust fault that places crystalline basement of the (3) Based on previous thermochronology research, the Longmen Pengguan complex against Triassic rocks of passive margin affinity Shan was uplifted rapidly since at least 15–10 Ma, while our cal- (Wang et al., 2012), playing an important role on the Longmen Shan up- culation shows that it takes 22.5–7.5 Ma for cataclasite to reach lift history. the surface from 10 to 15 km-depth. Therefore, these multi- Fault rocks exposed in the YBF, including fault gouge, fault breccia, layered fault rocks displayed in the research area indicate that cataclasite and pseudotachylyte, show brittle deformation characteris- the 100 to 240 m-wide YBF might have been formed by the tics. Pseudotachylyte is widely acknowledged as providing important long-term fault activity and evolution from about 15–10 Ma to geologic evidence of brittle faulting, frictional melting, and presumed present. seismic activity along ancient, now exhumed, faults (Sibson, 1975; Spray, 1995). Cataclasite was produced by an active fault at a deeper Acknowledgments depth (~10–15 km) than fault gouge and fault breccia (~1–4km) (Sibson, 1977). The fault rocks exhibit an asymmetrical distribution in The authors would like to thank the Wenchuan Earthquake Fault this area. Cataclasite and pseudotachylyte formed at great depths, and Scientific Drilling Centre for their help. We thank T. Shimamoto, Lu are now mainly exposed in the Pengguan complex. They can be consid- Yao and Yu Wang for helping us during field work, M.L. Chevalier for ered as the oldest fault rocks in the YBF, while fault breccia and fault suggesting improvements to our manuscript, and anonymous re- gouge are distributed in the Late Triassic sedimentary rocks. A clear geo- viewers for giving many useful comments and suggestions. The present morphic limit (Fig. 2b) between the Pengguan complex and the Xujiahe work was supported by the “Wenchuan Earthquake Fault Scientific Formation (with the former being steeper and higher than the latter), Drilling” of the National Science and Technology Planning Project, and which might be caused by the different erosion rates between volcanic the National Natural Science Foundation of China (41330211). and sedimentary rocks. Fault rocks are the direct products of the rapid Longmen Shan uplift, References with the Pengguan complex uplift directly reflecting the uplift of the Longmen Shan. Many layers of fault gouge developed in the YBF with Biegel, R.L., Sammis, C.G., 2004. Relating fault mechanics to fault zone structure. Adv. – thicknesses varying from 1 mm to several meters, viewed from both Geophys. 47, 65 111. Burchfiel, B.C., Royden, L.H., Van der Hilst, R.D., Hager, B.H., Chen, Z., King, R.W., Li, C., Lü, J., the outcrop and drilling cores. This multi-layered fault gouge and micro- Yao, H., Kirby, E., 2008. A geological and geophysical context for the Wenchuan earth- scale fractures reflect multiple episodes of fault slip. Cataclasite forms at quake of 12 May 2008, Sichuan, People's Republic of China. GSA Today 18 (7). http:// depths of ~10–15 km (Sibson, 1977), so if we consider the recurrence dx.doi.org/10.1130/GSATG18A.1. – Chengdu College of Technology, 1996. Geologic Map of Dujiangyan (Scale 1:50,000) (in period to be 3000 6000 years for large events such as the Wenchuan Chinese). earthquake (Burchfiel et al., 2008; Li et al., 2008b; Zhang et al., Chester, F.M., 1995. A rheologic model for wet crust applied to strike-slip faults. 2008)withanaverageverticaloffsetof~4m(Xu et al., 2009), it takes J. Geophys. Res. 100, 13033–13044. – Chester, F.M., Chester, J.S., 1998. Ultracataclasite structure and friction processes of the 22.5 7.5 Ma for cataclasite to reach the surface. Thermochronology re- Punchbowl fault, San Andreas system, California. Tectonophysics 295, 199–221. search showed that the Longmen Shan was uplifted rapidly since at Chester, F.M., Logan, J.M., 1986. Implications for mechanical properties of brittle faults least 15–10 Ma (Clark et al., 2005; Godard et al., 2009; Kirby et al., from observations of the Punchbowl fault zone, California. Pure Appl. Geophys. 124, 79–106. 2002; Ouimet et al., 2010; Wang et al., 2012). Therefore, the fault Chester, F.M., Evans, J.P., Biegel, R.L., 1993. Internal structure and weakening mechanisms rocks distributed in the Pengguan complex of the YBF might have of San Andreas Fault. J. Geophys. Res. 98, 771–786. formed 15–10 Ma ago due to fault activity, i.e. long-term fault move- Childs, C., Manzocchi, T., Walsh, J.J., Bonson, C.G., Nicol, A., Schopfer, M.P.J., 2009. Ageo- – metric model of fault zone and fault rock thickness variations. J. Struct. Geol. 31 ments and evolution from about 15 10 Ma to present formed the (2), 117–127. 100- to 240 m-wide YBF. Clark, M.K., House, M.A., Royden, L.H., Whipple, K.X., Burchfiel, B.C., Zhang, X., Tang, W., 2005. Late Cenozoic uplift of southeastern Tibet. Geology 33, 525–529. 6. Conclusions Coward, M.P., 1984. Major shear zones in the Precambrian crust: examples from NW Scot- land and southern Africa and their significance. Precambrian Tectonics Illustrated. 207–235. The internal structure of the YBF, which preserves a suite of fault Deng, Q.D., Chen, S.F., Zhao, X.L., 1994. Tectonics, seismicity, and dynamics of the – rocks showing evidence for fault activity and deformation, is described Longmen Shan Mountains and its adjacent regions. Seismol. Geol. 16 (4), 389 403 (in Chinese with English abstract). based on the analysis of an outcrop in Bajiaomiao and on the drilling Densmore, A.L., Ellis, M.A., Li, Y., Zhou, R.J., Hancock, G.S., Richardson, N., 2007. Active tec- cores of WFSD-1, yielding the following results and conclusions: tonics of the Beichuan and Pengguan faults at the eastern margin of the Tibetan Pla- teau. Tectonics 26, TC4005. http://dx.doi.org/10.1029/2006TC001987. fl fi Faulkner, D.R., Rutter, E.H., 2003. The effect of temperature, the nature of the pore uid, (1) The YBF de ned by the subsidiary faults and fault rocks is about and subyield differential stress on the permeability of phyllosilicate-rich fault 240 m-wide, as measured at the outcrop while its real width is gouge. J. Geophys. Res. Solid Earth 108 (B5). about 100 m in the WFSD-1 drilling core. Multiple gouge zones Faulkner, D.R., Mitchell, T.M., Rutter, E.H., Cembrano, J., 2008. On the structure and mechanical properties of large strike-slip faults. Geological Society, London, Special Pub- are observed indicating a multiple core model. The fault zone lications 299, 139–150. consists of 5 different units based on the characteristics of fault Faulkner, D.R., Jackson, C.A.L., Lunn, R.J., Schlische, R.W., Shipton, Z.K., Wibberley, C.A.J., rocks: cataclasite zone, black fault gouge–breccia zone, gray Withjack, M.O., 2010. A review of recent developments concerning the structure, mechanics and fluid flow properties of fault zones. J. Struct. Geol. 32, 1557–1575. fault breccia zone, dark-gray fault breccia zone, and black fault Fu, B.H., Shi, P.L., Guo, H.D., Okuyama, S., Ninomiya, Y., Wright, S., 2011. Surface deforma- gouge–breccia zone. Two types of gouge veins are observed in tion related to the 2008 Wenchuan earthquake, and mountain building of the this area, one injected into the fault breccia formed by thermal Longmen Shan, eastern Tibetan Plateau. J. Asian Earth Sci. 40 (4), 805–824. pressurization during fault activity, while the network of gouge Godard, V., Pik, R., Lavé, J., Cattin, R., Tibari, B., Sigoyer, J., Pubellier, M., Zhu, J., 2009. Late Cenozoic evolution of the central Longmen Shan, eastern Tibet: insight from (U–Th)/ veins were caused by fluidization after fault activity. He thermochronometry. Tectonics 28 (5), TC5009. (2) From the outcrop investigation, the earthquake surface rupture Jia, D., Wei, G., Chen, Z., Li, B., Zeng, Q., Yang, G., 2006. Longmen Shan fold-thrust belt and its relation to the western Sichuan Basin in central China: new insights from hydro- zone strikes N40°E and dips N75°W at the bottom of the fault – – carbon exploration. AAPG Bull. 90 (9), 1425 1447. zone and intersects the YBF at an angle of about 15 25°. Based Jia, D., Li, Y.Q., Lin, A.M., Wang, M.M., Chen, W., Wu, X.J., Ren, Z.K., Zhao, Y., Luo, L., 2010. on the WFSD-1 drilling core research, the PSZ of the Wenchuan Structural model of 2008 Mw 7.9 Wenchuan earthquake in the rejuvenated Longmen earthquake has a dip angle of 62–65° to the northwest in the Shan thrust belt, China. Tectonophysics 491 (1–4), 174–184. Keulen, N., Heilbronner, R., Stünitz, H., Boullier, A.M., Ito, H., 2007. Grain size distributions upper part of the YBF. These results suggest that the Wenchuan of fault rocks: a comparison between experimentally and naturally deformed granit- earthquake fault is a high angle thrust fault, which does not oids. J. Struct. Geol. 29, 1282–1300. 114 H. Wang et al. / Tectonophysics 619–620 (2014) 101–114

Kirby,E.,Reiners,P.W.,Krol,M.A.,Whipple,K.X.,Hodges,K.V.,Farley,K.A.,Tang,W.Q.,Chen, Ramsay, J.G., 1980. Shear zone geometry: a review. J. Struct. Geol. 2 (1–2), 83–99. Z.L., 2002. Late Cenozoic evolution of the eastern margin of the Tibetan Plateau: infer- Sibson, R.H., 1975. Generation of pseudotachylyte by ancient seismic faulting. Geophys. ences from 40Ar/39Ar and (U–Th)/He thermochronology. Tectonics 21 (1), TC1246. J. R. Astron. Soc. 43 (3), 775–794. Li, Y., Hou, Z.J., Si, G.Y., Densmore, A.L., Zhou, R.C., Ellis, M.A., Li, Y.Z., Liang, X.Z., 2002. Sibson, R.H., 1977. Fault rocks and fault mechanisms. J. Geol. Soc. 133 (3), 191–213. Cenozoic tectonic sequence and tectonic events at the eastern margin of the Sibson, R.H., 2003. Thickness of the seismic slip zone. Bull. Seismol. Soc. Am. 93 (3), Qinghai–Tibet plateau. Geol. China 29 (1), 30–36 (in Chinese with English abstract). 1169–1178. Li, Y., Zhou, R.J., Densmore, A.L., Ellis, M.A., 2006. Geomorphic evidence for the late Sichuan Bureau of Geology, 1975. Geologic Map of Guanxian (Scale 1:200,000) (in Chinese). Cenozoic strike-slipping and thrusting in Longmen Mountain at the eastern margin Spray, J.G., 1995. Pseudotachylyte controversy: fact or friction? Geology 1119–1122. of the Tibetan Plateau. Quat. Sci. 26 (1), 40–51 (in Chinese with English abstract). http://dx.doi.org/10.1130/0091-7613. Li, H.B., Fu, X.F., Van der Word, J., Si, J.L., Wang, Z.X., Hou, L.W., Qiu, Z.L., Li, N., Wu, F.Y., Xu, Togo, T., Shimamoto, T., Ma, S.L., Wen, X.Z., He, H.L., 2011. Internal structure of Z.Q., Tapponnier, P., 2008a. Co-seisimic surface rupture and dextral-slip oblique Longmenshan fault zone at Hongkou outcrop, Sichuan, China, that caused the 2008 thrusting of the Ms 8.0 Wenchuan earthquake. Acta Geol. Sin. 82 (12), 1623–1643 Wenchuan earthquake. Earthq. Sci. 24, 249–265. (in Chinese with English abstract). Verberne, B.A., He, C.R., Spiers, C., 2010. Frictional properties of sedimentary rocks and Li, H.B., Si, J.L., Pan, J.W., Qiu, Z.L., Sun, Z.M., Pei, J.L., 2008b. Deformation feature of active natural fault gouge from the Longmen Shan fault zone, Sichuan, China. Bull. Seismol. fault and recurrence intervals estimation of large earthquake. Geol. Bull. China 27 Soc. Am. 100 (5B), 2767–2790. (12), 19–42 (in Chinese with English abstract). Walsh, J.J., Watterson, J., 1988. Analysis of the relationship between displacements and di- Li, H.B., Si, J.L., Fu, X.F., Qiu, Z.L., Li, N., Van der Woerd, J., Pei, J.L., Wang, Z.X., Hou, L.W., Wu, mensions of faults. J. Struct. Geol. 10 (3), 239–247. F.Y., 2009. Co-seismic rupture and maximum displacement of the 2008 Wenchuan Walsh, J.J., Nicol, A., Childs, C., 2002. An alternative model for the growth of faults. J. Struct. earthquake and its tectonic implications. Quat. Sci. 29 (3), 387–402 (in Chinese Geol. 24 (11), 1669–1675. with English abstract). Wang, P., Fu, B.H., Zhang, B., Kong, P., Wang, G., 2009. Relationship between surface rup- Li, H.B., Wang, H., Xu, Z.Q., Si, J.L., Pei, J.L., Li, T.F., Huang, Y., Song, S.R., Kuo, L.W., Sun, Z.M., tures and lithologic characteristics of Wenchuan Ms 8.0 earthquake. Chin. J. Geophys. Chevalier, M.L., Liu, D.L., 2013. Characteristics of the fault-related rocks, fault zones 52 (1), 131–139 (in Chinese with English abstract). and the principal slip zone in the Wenchuan Earthquake Fault ScientificDrillingPro- Wang, H., Li, H.B., Pei, J.L., Li, T.F., Huang, Y., Zhao, Z.D., 2010. Structural and lithologic char- ject Hole-1 (WFSD-1). Tectonophysics 584, 23–42. acteristics of the Wenchuan earthquake fault zone and its relationship with seismic Lin, A.M., 2011. Seismic slip recorded by fluidized ultracataclastic veins formed in a coseismic activity. Quat. Sci. 30 (4), 767–777 (in Chinese with English abstract). shear zone during the 2008 Mw 7.9 Wenchuan earthquake. Geology 39 (6), 547–550. Wang, E.Q., Kirby, E., Furlong, K.P., van Soest, M., Xu, G., Shi, X., Kamp, P.J.J., Hodges, K.V., Lin, A.M., Ren, Z.K., Jia, D., Wu, X.J., 2009. Co-seismic thrusting rupture and slip distribu- 2012. Two-phase growth of high topography in eastern Tibet during the Cenozoic. tion produced by the 2008 Mw 7.9 Wenchuan earthquake, China. Tectonophysics Nat. Geosci. 5, 640–645. http://dx.doi.org/10.1038/ngeo1538. 471 (3–4), 203–215. Schmid, S.M., Handy, M.R., 1991. Towards a genetic classification of fault rocks: geological Lin, A.M., Ren, Z.K., Kumahara, Y., 2010. Structural analysis of the coseismic shear zone of usage and tectonophysical implications. Controversies in Modern Geology. Academic the 2008 Mw7.9 Wenchuan earthquake, China. J. Struct. Geol. 32 (6), 781–791. Press, London 339–361. Lin, A.M., Rao, G., Yan, B., 2012. Field evidence of rupture of the Qingchuan Fault during Structure of fault zones: implications for mechanical and fluid-flow properties. In: the 2008 Mw7.9 Wenchuan earthquake, northeastern segment of the Longmen Wibberley, C.A.J., Kurz, W., Imber, J., Holdsworth, R.E., Collettini, C. (Eds.), Geological Shan Thrust Belt, China. Tectonophysics 522–523, 243–252. Society, London, Special Publication 299, 139–150. Liu-Zeng,J.,Wen,L.,Sun,J.,Zhang,Z.H.,Hu,G.Y.,Xing,X.C.,Zeng,L.S.,Xu,Q.,2010.Surficial Xu, Z.Q., Hou, L.W., Wang, Z.X., Fu, X.F., Wang, D.K., 1992. Orogenic Processes of the slip and rupture geometry on the Beichuan Fault near Hongkou during the Mw 7.9 Songpan Ganze Orogenic Belt of China.Geological Press, Beijing 1–190 (in Chinese). Wenchuanearthquake,China.Bull.Seismol.Soc.Am.100(5B),2615–2650. Xu, X.W., Wen, X.Z., Ye, J.Q., Ma, B.Q., Chen, J., Zhou, R.J., He, H.L., Tian, Q.J., He, Y.L., Wang, Ma, Y.W., Wang, G.Q., Hu, X.W., 1996. Tectonic deformation of Pengguan complex as a Z.C., Sun, Z.M., Feng, X.J., Yu, G.H., Chen, L.C., Chen, G.h, Yu, S.E., Ran, Y.K., Li, X.G., Li, nappe. Acta Geol. Sichuan 16 (2), 110–114 (in Chinese with English abstract). C.X., Li, C.X., An, Y.F., 2008. The Ms 8.0 Wenchuan earthquake surface ruptures and Ouimet, W., Whipple, K., Royden, L., Reiners, P., Hodges, K., Pringle, M., 2010. Regional in- its seismogenic structure. Seismol. Geol. 30 (3), 597–629. cision of the eastern margin of the Tibetan Plateau. Lithosphere 2 (1), 50–63. Xu, X.W., Wen, X.Z., Yu, G.H., Chen, G.H., Klinger, Y., Hubbard, J., Shaw, J.H., 2009. Peacock, D.C.P., Sanderson, D.J., 1991. Displacements, segment linkage and relay ramps in Coseismic reverse- and oblique-slip surface faulting generated by the 2008 Mw 7.9 normal fault zones. J. Struct. Geol. 13 (6), 721–733. Wenchuan earthquake, China. Geology 37 (6), 515–518. Pei, J.L., Li, H.B., Sun, Z.M., Wang, H., Si, J.L., 2010. Fault slip in the Wenchuan earthquake Zhang, P.Z., Xu, X.W., Wen, X.Z., Ran, Y.K., 2008. Slip rates and recurrence intervals of the fault zone—information from fault rocks with high magnetic susceptibility. Quat. Sci. Longmen Shan active fault zone, and tectonic implications for the mechanism of the 30, 759–767 (in Chinese with English abstract). May 12 Wenchuan earthquake, 2008, Sichuan. China. Chinese J. Geophys. 51 (4), Qiao, X.F., Guo, X.P., Li, H.B., Gou, Z.H., Su, D.C., Tang, Z.M., Zhang, W., Yang, G., 2012. Soft- 1066–1073 (in Chinese with English abstract). sediment deformation in the Late Triassic and the Indosinian tectonic movement in Zoback, M.D., Hickman, S., Ellsworth, W., 2007. The role of fault zone drilling. Geophysics Longmenshan. Acta Geol. Sin. 86 (1), 132–156 (in Chinese with English abstract). 4, 649–674.