Analysis of Structural Deformation in the North Dabashan Thrust Belt

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International Geology Review Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tigr20 Analysis of structural deformation in the North Dabashan thrust belt, South Qinling, central China Wangpeng Liab, Shaofeng Liuab, Tao Qianab, Guoxing Douab & Tangjun Gaoc a School of Earth Science and Resources, China University of Geosciences, Beijing, China b State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing, China c Research Institute of Exploration Southern Company, SINOPEC, Chengdu, China Published online: 10 Jul 2014.

To cite this article: Wangpeng Li, Shaofeng Liu, Tao Qian, Guoxing Dou & Tangjun Gao (2014) Analysis of structural deformation in the North Dabashan thrust belt, South Qinling, central China, International Geology Review, 56:10, 1276-1294, DOI: 10.1080/00206814.2014.935966 To link to this article: http://dx.doi.org/10.1080/00206814.2014.935966

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Analysis of structural deformation in the North Dabashan thrust belt, South Qinling, central China Wangpeng Lia,b, Shaofeng Liua,b*, Tao Qiana,b, Guoxing Doua,b and Tangjun Gaoc aSchool of Earth Science and Resources, China University of Geosciences, Beijing, China; bState Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing, China; cResearch Institute of Exploration Southern Company, SINOPEC, Chengdu, China (Received 17 March 2014; accepted 14 June 2014)

The North Dabashan thrust belt, which is located in South Qinling, is bounded by the Ankang fault on the north and the Chengkou–Fangxian fault on the south. The North Dabashan thrust belt experienced multiple stages of structural deformation that were controlled by three palaeostress fields. The first structural event (Middle Triassic) involved NNW–SSE shortening and resulted in the formation of numerous dextral strike-slip structures along the entire Chengkou–Fangxian fault zone and within the North Dabashan thrust belt, which suggests that the South China Block moved to the NW and was obliquely subducted under the North China Block. The second structural event (Late Triassic–Early Jurassic) involved NE– SW shortening that formed NW–SE-trending structures in the North Dabashan thrust belt. The third structural event (Late Jurassic–Early Cretaceous) involved ENE–WSW or nearly E–W shortening and resulted in additional thrusting of the North Dabashan thrust belt to the WSW and formation of the WSW-convex Chengkou–Fangxian fault zone, which has an oroclinal shape. Owing to the pinning of the Hannan massif and Shennongjia massif culminations, numerous sinistral strike- slip structures developed along the eastern Chengkou–Fangxian fault zone and were superimposed over the early dextral strike-slip structures. Keywords: Dabashan belt; collisional orogeny; intra-continental orogeny; strike-slip structure; palaeostress fields

1. Introduction South Dabashan arcuate belt by the Chengkou–Fangxian The Qinling–Dabie orogen is a complex orogenic belt in fault (Figure 1; Zhang et al. 2001; Dong et al. 2008). The central China (Zhang et al. 1996, 2001; Meng and Zhang belt's typical arc-shaped geometry and formation mechan- 1999). Previous studies have demonstrated that the ism make this area a natural laboratory to investigate Qinling–Dabie orogen was generated by the northward tectonic deformation related to a continental collision. subduction of the Shangdan oceanic crust and the subse- The style of structural deformation, formation mechanism, quent collision between the North China Block (NC) and and tectonic evolution of the South Dabashan arcuate belt the South China Block (SC) along the Shangdan suture have been investigated extensively in recent years (He (Enkin et al. 1992;Liet al. 1993; Okay et al. 1993; Nie et al. 1997; Liu et al. 2003, 2006, 2010; Dong et al. et al. 1994; Ames et al. 1996; Hacker et al. 1998; Zhai 2005, 2006, 2008; Wang et al. 2006; Shen et al. 2007; et al. 1998; Liu and Zhang 1999; Zhang et al. 2001, Shi et al. 2007, 2012; Cheng and Yang, 2009;Huet al. 2003). However, the Mianlue suture, which is located 2009; Zhang et al. 2009, 2011; Zhang and Dong 2009;Wu ﬁ Downloaded by [Monash University Library] at 19:21 24 September 2014 between the SC and Qinling–Dabie micro-block (QD), et al. 2010; Liu and Zhang 2013). However, insuf cient has been recognized based on geological, geochemical, research has been performed on the North Dabashan thrust geophysical, and geochronological studies (Zhang et al. belt (He et al. 1999;Liet al. 2012, 2013;Shiet al. 2012). 1995a, 2001, 2003; Liu and Zhang 1999; Liu et al. 2001). In addition, the debate about the mode of the collision Later, a model including ‘three blocks (NC, QD, and SC) between the NC and SC is ongoing and has been high- Ş with two sutures (Shangdan suture and Mianlue suture)’ lighted in several tectonic models. Okay and Celal engör was proposed; this model represents the regional tectonic (1992) proposed a face-to-face collision model between framework of the Qinling–Dabie orogen (Zhang et al. NC and SC. An indentation model was subsequently pro- 1995a, 1996, 2001, 2003). posed by Yin and Nie (1993) and Zhang (2002). The The Dabashan thrust belt, which is a typical SW-con- former believed that the collision between NC and SC vex arcuate structural belt, is located at the edges of the began with the indentation of the northeastern SC into collision zone (Mianlue suture zone) between the QD and the eastern NC, whereas the latter proposed that the colli- SC and is divided into the North Dabashan thrust belt and sion occurred by the indentation of the southeastern NC

*Corresponding author. Email: [email protected]

Figure 1. (A) Structural outline map of Dabashan and its adjacent areas. (B) The main tectonic divisions of the Qinling orogen and the location of Figure 1A. (C) The locations of the Qinling orogen in China and Figure 1B. F1, Baihe–Shiyan fault; F2, Ankang fault; F3, Hanwang–Liushuidian fault; F4, Hongchunba fault; F5, Gaoqiao fault; F6, Gantan fault; F7, Lujiaping fault; F8, Chengkou–Fangxian fault; F9, Xinglongchang fault; F10, Pingba fault; F11, Xujia–Yangri fault; F12, Zhenba–Jimingsi fault; F13, Tiexi–Wuxi blind fault; F14, Xinhua–Xingshan fault; F15, Mianlue suture; F16, Xixiang fault; A, Wudang nappe; B, Shennongjia massif; C, Pingli dome; D, Manpoling dome; E, Fenghuangshan dome; F, Hannan massif; G, Gaochuan terrane; H, Micangshan massif; I, Ankang nappe; J, Ziyang nappe; K, Gaoqiao nappe; L, Gaotan nappe; M, Lujiaping nappe. Two strike-slip zones developed along the Chengkou–Fangxian fault zone: the dextral strike-slip zone developed along the entire Chengkou–Fangxian fault; the sinistral strike-slip zone developed along the eastern Chengkou–Fangxian fault. The former was superimposed by the latter along the eastern segment of the Chengkou–Fangxian fault.

into the SC. Eide (1993) proposed that the SC rotated Dabashan thrust belt (Dong et al. 2006, 2008, 2010; Liu clockwise relative to the NC and first made contact with et al. 2006;Huet al. 2009, 2012). The North Dabashan Downloaded by [Monash University Library] at 19:21 24 September 2014 the eastern part of the NC and later with the western part. thrust belt underwent a different tectonic evolution from Several researchers have accepted the rotation collision that of the South Dabashan arcuate belt, which resulted in model based on palaeomagnetic and sedimentologic stu- the syn-collisional structures between the NC and SC dies (Gilder and Courtillot 1997; Yokoyama et al. 2001; being retained in the North Dabashan thrust belt and the Wang et al. 2003; Meng et al. 2005; Zhang et al. 2007). collision zone in addition to the structures related to the However, another group of researchers proposed that the intra-continental deformation. The deformational struc- SC moved to the NW and was obliquely subducted under tures of the North Dabashan thrust belt are significant for the NC based on tectonic and sedimentologic studies of understanding the tectonic evolution of the Dabashan belt the Dabie orogen and Tanlu fault (Gilder et al. 1999; Liu and the collisional processes between NC and SC. They et al. 2005a; Zhu et al. 2009). The initial formation of the may also represent an opportunity to examine a continen- North Dabashan thrust belt is generally considered to have tal collision and intra-continental deformation. been related to the Middle–Late Triassic collision of the This paper presents our detailed field investigations, QD and SC (He et al. 1997; Sun 2002; Wang et al. 2004), microscopic observations, and structural analysis of the whereas the South Dabashan arcuate belt was derived North Dabashan thrust belt and provides a synthesis of mainly from the intra-continental thrusting of the North the structural framework of the region, including the 1278 L. Wangpeng et al.

structural geometry, deformational sequence, and stress belts (Figure 1); it has a distinct stratigraphy and structure, ﬁelds. Additionally, a new model is proposed to explain including a Devonian–Carboniferous marine sedimentary the tectonic evolution of the Dabashan thrust belt. sequence, N–S-trending high angle dextral strike-slip faults, and sheath folds that are different from those in the North and South Dabashan belts (Zhang et al. 2001;Li 2. Geological background et al. 2007;Huet al. 2008, 2012). The Wudang nappe, The Qinling–Dabie orogen underwent a multistage oro- which was thrust southward onto Cambrian units, is genic evolution, including: (1) extension of the Shangdan located east of the North Dabashan thrust belt (Zhang ocean between the NC and SC during the Neoproterozoic– et al. 2001; Shi et al. 2012). The North Dabashan thrust Early Ordovician, (2) northward subduction of the belt consists of Meso–Neoproterozoic metamorphic rocks Shangdan ocean during the Early Ordovician–Silurian, and early Palaeozoic strata (Zhang et al. 2001; Dong et al. (3) closure of the Shangdan suture accompanied by the 2008;Huet al. 2012). The Neoproterozoic strata include extension of the Mianlue ocean during the Devonian– dolomite, sandstone, mudstone, and tuff, and the Middle Triassic, and (4) closure of the Mianlue ocean Cambrian sequence is composed mainly of biocalcarenite, and a collisional orogeny during the Middle–Late dolomitic limestone, argillaceous limestone, sandy slate, Triassic (Zhang et al. 1996, 2001; Liu and Zhang 2008). and siliceous rocks. The Ordovician sediments are com- The South Qinling, which includes the North Dabashan posed primarily of sandy slate, siliceous slate, dololithite, thrust belt, is located south of the Shangdan suture and and limestone, and the Silurian rocks are dominated by was a passive continental margin during the early feldspathic quartz sandstone, silty slate, carbonaceous Palaeozoic that contained Meso–Neoproterozoic meta- slate, and trachyte. Studies of sedimentation, the geochem- morphic volcanic-sedimentary basement rocks (Wudang ical characteristics of the volcanic rocks and their isotope Group and Yaolinghe Group) and Neoproterozoic to chronology, and the thrust tectonic deformation have Middle Triassic sedimentary layers. In contrast, most of shown that the North Dabashan thrust belt has undergone the Yangtze area does not contain Meso–Neoproterozoic three stages of tectonic evolution since the late volcanic sequences, and the Upper Yangtze contains no Proterozoic: extension (∈–D2), tectonic inversion (T2), Devonian–Carboniferous units, which are extensive in the and thrusting (T2–J1) (He et al. 1999). South Qinling. The QD separated from the northern margin of the SC with the extension of the Mianlue ocean during the late Palaeozoic (Zhang et al. 1996, 2001). 3. Deformational structures Therefore, the QD and South Qinling were situated 3.1. Main thrust faults between the Shangdan suture to the north and the Mianlue suture to the south. The QD collided with the Series of NW-trending thrust faults and thrust sheets are NC along the Shangdan suture in the north, and the SC present in the North Dabashan thrust belt and terminate – collided with the NC–QD along the Mianlue suture in the against the Chengkou Fangxian fault. From north to south, resulting in a complete orogeny during the early south, these faults include the Ankang thrust fault, Mesozoic (Zhang et al. 1996, 2001). Numerous Hongchunba thrust fault, Gaoqiao thrust fault, Gaotan thrust – geochemical and geochronological studies have shown fault, Lujiaping thrust fault, and Chengkou Fangxian fault – – that the Mianlue suture formed due to the closure of the (Figure 2A B and C D; Zhang et al. 2001). Mianlue ocean, and this suture was subsequently over-

Downloaded by [Monash University Library] at 19:21 24 September 2014 printed by the SW-vergent overthrusting of the Chengkou–Fangxian fault in the Dabashan area (Zhang 3.1.1. Ankang thrust fault et al. 2001, 2003; Dong et al. 2011a, 2011b). The east- The NW–SE-trending Ankang fault is located at the northward extension of the Mianlue suture was tectonically ern boundary of the North Dabashan thrust belt and sepa- buried by the North Dabashan thrust belt (Zhang et al. rates the Ankang–Wudang nappe to the north from the 1995b;Huet al. 2012; Liu and Zhang 2013). The Pingli–Ziyang nappe to the south (Figure 1). The western Chengkou–Fangxian fault is regarded as the surface out- Ankang fault curves near Shiquan County, where it trends crop of the Mianlue suture, along which the North NNW and merges with the Mianlue suture. A 15 to 30 m- Dabashan thrust belt was superimposed on the South wide, NE- to NEE-dipping zone of intense deformation is Dabashan arcuate belt and Mianlue suture (Zhang et al. located west of Shiquan County. Numerous striations and 1995b, 2001; Dong et al. 2011a, 2011b;Huet al. 2012). drag folds resulting from ductile shearing developed in the The North Dabashan thrust belt is bounded by the deformation zone and indicate the dextral strike-slip char- NW-trending Ankang fault to the north and the SW-con- acter of the Ankang fault. Li et al. (2013)recognizedtwo vex Chengkou–Fangxian fault to the south ( Figure 1). The ductile shear zones that are characterized by dextral strike- Gaochuan terrane is located west of the North Dabashan slip shearing in the metamorphic rocks along the Ankang thrust belt and lies between the North and South Dabashan fault. The eastern segment of the Ankang fault, which is the International Geology Review 1279

Figure 2. Structural cross sections and photomicrographs showing the ductile fabrics of the mylonitic rocks taken from the main fracture zone in the Dabashan thrust belt. (a–b and c–d) Locations of sections a–b and c–d are shown in Figure 1;(A–C) Locations of sections A–C are shown in Figure 2;(D–H) show σ porphyroclast, domino structure, σ porphyroclast, quartz ﬁsh, and σ porphyroclast, respectively. Am, amphibole; Bt, biotite; Cal, calcite; Qtz, quartz. Abbreviations for stratigraphic units are: Zayx, Yunxi Group; Zayl, Yaolinghe Group; Z1n, Nantuo Formation of the lower Neoproterozoic; Zbdn, Dengying Formation of the upper Neoproterozoic; ∈, Cambrian; ∈1, Lower Cambrian; ∈-O, Cambrian to Ordovician; (∈-O)dh, Donghe Group; ∈-S, Cambrian to Silurian; O-S, Ordovician to S, Silurian; P, Permian; T1, Lower Triassic; T1d–T1j, Daye to Jialingjiang Formations of the Triassic; T2, Middle Triassic; T1–2, Lower to Middle Triassic; T3–J1–2, Upper Triassic to Middle Jurassic; J1–2, Lower to Middle Jurassic; K2–E, Upper Cretaceous to Paleogene.

main slip plane of the Wudang nappe, links with the 3.1.2. Hongchunba thrust fault – Chengkou Fangxian fault near Fangxian County (Figure The NW-trending Hongchunba fault is located in the cen- 1). Along the thrust front, the Neoproterozoic Wudang tral North Dabashan thrust belt in the eastern part of Group rocks (Ling et al. 2002) are thrust onto the Langao County. The western end of this fault was cut by Cambrian rocks (Zhang et al. 2001;Donget al. 2008) the Chengkou–Fangxian fault at a high angle north of the along a detachment layer within the greenschist facies vol- town of Lianghekou, and the east end terminates against Downloaded by [Monash University Library] at 19:21 24 September 2014 canic and sedimentary rocks of the Wudang Group (Shi et al. the Chengkou–Fangxian fault east of the town of Fengxi 2012). (Figure 1). – The Shiquan Ankang segment of the Ankang fault A 150 m-wide ductile shear zone is present in the divided a single dome into the Manpoling dome to calcic slate of the Ordovician Erdaoqiao Formation north the north and the Fenghuangshan dome to the south of the town of Xiangyang in Ziyang County. This shear (Figure 1). The Manpoling dome was thrust onto the zone is characterized by S-C fabrics (Figure 3B) and Fenghuangshan dome, which transposed the layers of the asymmetric sub-vertical folds with hinges that plunge Yaolinghe and Yunxi Groups into cleavages (Figure 3A). steeply SE (e.g. 110°∠50°, 114°∠56°, 125°∠48°), indi- The NE-dipping cleavage indicates that the Ankang fault cating SW-trending shear and dextral strike-slip on the was thrust to the SW. Additionally, NE-dipping axial- Hongchunba fault (Figure 4E). Furthermore, microscopic plane cleavages are common in the Palaeozoic strata structures within the mylonitic rocks sampled from a frac- throughout the entire North Dabashan thrust belt and are ture zone near the town of Xiangyang also contain dextral – coeval with the NW SE-trending folds. They structurally strike-slip features (Figure 2D). Therefore, the transposed the bedding of the strata in many locations Hongchunba fault experienced dextral strike-slip and within the North Dabashan thrust belt (Li et al. 2013). 1280 L. Wangpeng et al.

Figure 3. Structural features of the main faults in the North Dabashan thrust belt. (A) Cleavage replaces bedding in the lower Downloaded by [Monash University Library] at 19:21 24 September 2014 Neoproterozoic volcanic clastic rocks for the tectonic slices (southwest of Ankang City); (B) S-C fabrics in the Silurian sandy slate showing the southwestward thrusting (north of Xiangyang Town); (C) A rotational Granitic gravel porphyroclast in the lower Neoproterozoic till conglomerate showing the dextral strike-slip feature (southeast of Zhenba County); (D) Asymmetric sub-vertical folds in the Cambrian limestone showing the dextral shear (south of Maliu Town); (E) Asymmetric folds in the Cambrian limestone showing the northeastward thrusting (south of Maliu Town); (F) Asymmetric sub-vertical folds in the Cambrian siliceous rock (northwest of Zhongting Town); (G) S-C fabrics in the Silurian sandy slate showing the dextral strike-slip feature (east of Fangxian County); (H) A rotational calcite porphyroclast in the Silurian sandy slate showing the sinistral strike-slip feature (southeast of Fangxian County).

thrust displacements. The Gaotan and Lujiaping faults curves to strike nearly N–S. Both its west end (west of the contain similar structural features. town of Lianghekou in Xixiang County) and east end (southern Zhenping County) were cut or terminated against the Chengkou–Fangxian fault. Previous studies 3.1.3. Gaoqiao thrust fault proposed that similar to most thrust faults in the North The Gaoqiao fault developed within the Palaeozoic strata Dabashan thrust belt, the Gaoqiao fault had SW displace- (Figures 1 and 2A–B). The main part of this fault strikes ments on a NE-dipping fault plane (Zhang et al. 2001; NW–SE, parallel to the regional structure, but its west end Dong et al. 2008, 2010, 2011a, 2011b;Huet al. 2009). International Geology Review 1281

Figure 4. Stereographic plots of the structural elements indicating the dextral strike-slip feature in the North Dabashan thrust belt and along the Chengkou–Fangxian fault zone. Based on the analysis of the occurrence of strata, fault, and kinematics direction in the corresponding location, the slip plane in stereonet is a comprehensive inference.

However, detailed ﬁeld investigations have indicated that lineations (L1) within the North Dabashan thrust belt the Gaoqiao fault was a backthrust with a SW-dipping (Shi et al. 2012), which suggests that thrusting on the fault plane (Li et al. 2012). brittle faults occurred after the ductile deformation. In the western part of the Gaoqiao fault, an approxi- Similarly, the eastern part of the Gaoqiao fault (south mately 70 m-wide fractured zone in the sandy slate of the of the town of Taohe in Langao County) is a 60 m-wide Ordovician Gaoqiao Formation is located north of the SW-dipping zone of ductile deformation. Several distinct town of Gaotan (Figure 2A and B) and is marked by a shear planes with similar dips (e.g. 232°∠65°, 226°∠60°, series of asymmetric folds, whose southern limbs have 218°∠57°) are well exposed in this zone and indicate NE lower dip angles and are longer than the northern limbs. thrusting. The kinematic features of the folds with steeply Numerous SW-dipping cleavages (e.g. 196°∠68°, NW-plunging hinges in the footwall also indicate a com- 205°∠63°) formed in the footwall (Figure 2B). These ponent of dextral strike-slip (Figure 4H). Downloaded by [Monash University Library] at 19:21 24 September 2014 structures indicate that the Gaoqiao fault thrust to the NE, which is opposite to the propagation direction of the North Dabashan thrust belt, and is a backthrust. Numerous 3.2. Strike-slip zone along the Chengkou–Fangxian tight asymmetric folds developed adjacent to the fault thrust fault plane; both of their limbs dip to the SW, and their hinges The Chengkou–Fangxian fault, which is located along the plunge steeply NW, indicating the dextral strike-slip thrust northern margin of the SC, is an arcuate fault zone (Zhang displacement of this fault (Figures 2A and 4D). The et al. 2001). This fault zone is generally considered to be microscopic structures within the mylonitic rocks of the the boundary fault between the North and South Dabashan fracture zone also demonstrate the fault has a component belts, and between the QD and SC. The Pingba thrust fault of dextral strike-slip (Figure 2F). The older folds were is part of the Chengkou–Fangxian fault zone and repre- truncated by the Gaoqiao fault and other associated faults sents its southwestern margin (Figures 1 and 2A–B). The in this fracture zone; this type of relationship is found in Chengkou–Fangxian fault can be divided into three seg- many other locations throughout the North Dabashan ments based on its strike direction and deformation style: thrust belt (Li et al. 2013). The striations on fault planes the western segment, which strikes N–S with the undulat- generally plunge steeply and overprint the ductile ing curve in the northern part; the middle segment, which 1282 L. Wangpeng et al.

strikes NW–SE, parallel to the thrust faults in the North positive ﬂower structure (Figure 2A–B). The hinges of the Dabashan thrust belt; and the eastern segment, which folds plunge steeply to the NW–NNW (Figure 3D), and strikes nearly E–W(Figure 1). the SW limbs of the largely asymmetric folds have lower dip angles and are longer than the NE limbs, which have higher dip angles (Figure 3E). Domino structures – 3.2.1. Western segment of the Chengkou Fangxian (Figure 2E) and similar microscopic structures that indi- thrust fault cate dextral strike-slip motion are present in the mylonitic The fault plane of the western segment of the Chengkou– rocks north of the town of Maliu. These geological fea- Fangxian fault dips steeply to the E. The deformed strata tures indicate that the Chengkou–Fangxian fault under- in the hanging wall include the Neoproterozoic Yaolinghe went dextral strike-slip thrusting in the Maliu–uDazh and Nantuo Groups and the early Palaeozoic Donghe area (Figure 4V and W). Group; the footwall consists of the Devonian The Zhongting segment of the Chengkou–Fangxian Panlongshan Formation, Permian Guojiayan Formation, fault in Wanyuan County contains an imbricate thrust and Triassic Daye and Jialingjiang formations. The shear that consists of several parallel faults with SW displace- zone that cuts through the Nantuo Formation contains ment. Northwest of the town of Zhongting, the till con- numerous rotated gravels of granite and limestone glomerates of the Neoproterozoic Nantuo Formation in the (Figure 3C). The statistics of the measured rotation axes hanging wall are thrust onto the siliceous rocks of the of the rotated gravels indicate that the dominant directions Cambrian Lujiaping Formation in the footwall on a steeply of the rotation axes plunge steeply to the SE–SSE NE-dipping fault plane (e.g. 51°∠72°). A 40 m-wide zone (Figure 4Z). A series of asymmetric sub-vertical folds of ductile deformation along the thrust fault in the silic- (near the town of Guanyin) with hinges that plunge steeply eous rocks of the Cambrian Lujiaping Formation is char- SE or SSE are well exposed adjacent to the shear zone acterized by numerous rotated calcite lenticles and (Figure 4Y). These geological features indicate that the asymmetric sub-vertical folds with NEE-plunging hinges western Chengkou–Fangxian fault was thrust to the west (Figure 3F), which also indicate dextral strike-slip thrust- with a component of dextral strike-slip (Figure 4Y and Z). ing (Figure 4S). The NE-dipping thrust faults exhibit SW This type of dextral strike-slip thrust displacement was displacements, and several ductile deformation zones that also demonstrated by the N–S-trending high-angle dextral indicate dextral strike-slip thrusting are also well exposed strike-slip faults and sheath folds in the Gaochuan terrane adjacent to the thrust faults in the Chengkou, Xiuqi, and bounded by the western Chengkou–Fangxian fault on the Gaoguan segments. east (Hu et al. 2008, 2012).

– 3.2.2. Middle segment of the Chengkou–Fangxian thrust 3.2.3. Eastern segment of the Chengkou Fangxian fault thrust fault – The middle segment of the Chengkou–Fangxian fault zone Near the town of Dongan, the strike of the Chengkou – is located between the towns of Maliu (Ziyang County) in Fangxian fault changes from NW SE in the middle seg- – the NW and Dongan (Chengkou County) in the SE. The ment into nearly E W in the eastern segment. The east fault strikes NW–SE, parallel to the thrust faults in the ends of several parallel faults in the North Dabashan thrust – North Dabashan thrust belt, and the boundary fault planes belt were cut or terminated by the eastern Chengkou Fangxian fault (Figure 1). The deformed strata in the

Downloaded by [Monash University Library] at 19:21 24 September 2014 dip to the NE (Figure 1). The shear belt within the fault zone, which dips steeply to the NE and SW, is well hanging wall include the Neoproterozoic and the Early developed in the till conglomerate of the Neoproterozoic Palaeozoic (Cambrian and Silurian) units, whereas the Nantuo Formation in the hanging wall. Abundant rotated strata in the footwall consist mainly of Cambrian, granite gravels within the shear zone, which have rotation Ordovician, and Silurian rocks. axes that plunge at high angles to the SE and NW, indicate The town of Dongan is located at the intersection of – dextral strike-slip thrusting on the fault (Figure 4R and T). the middle and eastern segments of the Chengkou Thrusts with directions that are opposite to the kine- Fangxian fault. To the north, an approximately 25 m- matic direction in the Dabashan area are conﬁned to the wide fracture zone crops out in the tuff slate of the middle segment of the Chengkou–Fangxian fault and are Neoproterozoic Yaolinghe Group. Rotated gravels with well developed between Maliu and Dazhu. In this region, rotation axes that plunge steeply to the SE are distributed the deformation zone of the Chengkou–Fangxian fault is throughout this ductile shear zone and indicate that it is a characterized by approximately 3 km-wide penetrative and dextral strike-slip thrust (Figure 4O). To the south, asym- distributed asymmetric sub-vertical folds. The axial planes metric overturned folds with N-dipping axial planes are of the folds and the fault planes mainly dip steeply to the well exposed in the Cambrian limestone and indicate – SW, and the boundary thrust dips to the NE, forming a southward thrusting of the Chengkou Fangxian fault. International Geology Review 1283

The detailed ﬁeld investigations indicated that in con- and superposed by folds with NW-plunging hinges trast to the western and middle segments, both dextral and (Figure 5B), which indicate dextral and sinistral strike-slip sinistral strike-slip indicators are present throughout the features, respectively. The dextral strike-slip indicators eastern segment of the Chengkou–Fangxian fault, and the formed prior to the sinistral strike-slip structures. latter are superposed on the former. The sinistral strike-slip To the east of the town of Fengxi in Zhuxi County, a indicators are largely located from west of the town of typical sinistral strike-slip zone with a width of approxi- Zhongbao in Zhenping County to Fangxian County. For mately 100 m is well exposed in the limestone, siliceous example, to the south of Zhongbao, two types of asym- rocks, and mudstone of the Cambrian Shuijintuo metric sub-vertical folds with hinges that plunge in different Formation. The fault zone contains penetrative and dis- directions were recognized in the dolomite of the tributed tightly asymmetric sub-vertical folds. The folded Neoproterozoic Dengying Formation. Sub-vertical folds strata generally dip to the NNW–N, and the shearing with ENE–SE-plunging hinges (Figure 4N) were folded fracture planes within the zone dip to the N–NNE. The Downloaded by [Monash University Library] at 19:21 24 September 2014

Figure 5. Stereographic plots of the structural elements indicating the sinistral strike-slip feature along the eastern segment of the Chengkou–Fangxian fault zone. Based on the analysis of the occurrence of strata, fault, and kinematics direction in the corresponding location, the slip plane in stereonet is a comprehensive inference. 1284 L. Wangpeng et al.

kinematic indicators of the chevron folds and tightly sub- The northern sub-zone trends NW–SE and mainly vertical folds, including hinges that mainly plunge steeply consists of Meso–Neoproterozoic and early Palaeozoic to the NW–NNW (Figure 5C and K) and microscopic units. It includes the Fenghuangshan (described above) structures (Figure 2G) along the narrow shear fractures, and Pingli domes, which contain outcrops of the basement demonstrate sinistral strike-slip thrusting on the rocks, namely, the Yunxi and Yaolinghe Groups. The Chengkou–Fangxian fault. Kinematic indicators that indi- Pingli dome was thrusted southward onto the Gaotan cate dextral and sinistral strike-slip motion, such as rotated nappe, which bent the eastern segment of the gravels (Figure 2H), S-C fabrics (Figure 3G), and rota- Hongchunba fault. tional clasts (Figure 3H), are also present in other parts of The southern sub-zone, which is mainly composed of the eastern segment of the Chengkou–Fangxian fault. the Neoproterozoic and early Palaeozoic rocks, is bounded The folds with high plunge angles could have formed by the Hongchunba fault on the north and the concave by two processes. One is the superposition of two phases Chengkou–Fangxian fault on the south and is composed of folding; in this case, an earlier horizontal fold was of SW-vergent imbricate stacked thrusts (Shi et al. 2012; subsequently deformed by a vertical lateral fold. The Li et al. 2013) and a few NE-vergent backthrusts. other process is strike-slip shear. Based on our observa- Numerous overturned folds indicate SW-directed move- tions, objective screening, and elimination of the influence ment. For example, a large-scale asymmetric overturned of superposition, the steeply plunging to sub-vertical folds fold is well exposed to the south of the town of Gaotan identified in our fieldwork are interpreted to be strike-slip (Figure 6A). Upright folds gradually become more com- features. In addition, the distributions of the high-angle mon as the distance from the collision front increases in folds and other indicators that indicate strike-slip motion the southern sub-zone. During propagation of the south- have typical zonations and universality. No field evidence westward thrusting, a series of backthrusts and folds for the superposition of folds with nearly vertical hinges formed because of the obstruction from the frontier of has been recorded in the Dabashan area; thus, the forma- the North Dabashan thrust belt (Li et al. 2012) and the tion of the folds was attributed to strike-slip shear. continuously deep subduction of the SC under the NC Therefore, based on a series of detailed field investigations (Dong et al. 2013). The Maliu–Dazhu area of the collision and structural analysis, we recognized kinematic features front contains SW-vergent thrusts with clear backthrust along the Chengkou–Fangxian fault zone that indicate two structures and asymmetric sub-vertical folds. The strata types of deformation (Figure 1): dextral strike-slip thrust in this area are the most deformed in the study region. deformation and thrust deformation of the entire fault zone This area represents the top of the Dabashan arcuate (Figure 4) and sinistral strike-slip deformation mostly in tectonic belt. the eastern segment (Figure 5). The dextral strike-slip shear zone, including the shear zone in the Gaochuan terrane (Hu et al. 2008, 2012; Liu et al. 2011), is wider 4. Structural evolution in the west than in the east and extends eastward to Fangxian County. The sinistral strike-slip shear zone, 4.1. Deformational sequence which begins west of the town of Zhongbao and extends The timing of the convergence between the NC and SC eastward to Fangxian County, modified and overprinted and deforming sequence of the North Dabashan thrust belt the previous dextral strike-slip kinematic indicators to the is controversial (Dong et al. 2011a; Shi et al. 2012). Many east of Zhongbao. The sinistral strike-slip zone along the studies have shown that the SC began colliding with the –

Downloaded by [Monash University Library] at 19:21 24 September 2014 eastern segment of the Chengkou Fangxian fault zone NC in the Middle Triassic (Lin et al. 1985; Zhao and Coe becomes wider toward the east. 1987; Huang and Opdyke 1991; Enkin et al. 1992; Yang et al. 1992; Gilder and Courtillot 1997; Yokoyama et al. 2001; Meng et al. 2005;Tanet al. 2007). The tilted Upper Triassic conglomerates and sandstone unconformably 3.3. Deformational style overlie the Lower Triassic limestone in the town of The magnitude of the deformation of the North Dabashan Pingba in Chengkou County (Figure 7A, D, and E). The thrust belt is generally considered to be more than to the Lower Triassic limestone below the angular unconformity south (Dong et al. 2008;Liet al. 2012). The ages of the was folded to steep dips (Figure 7A and E), and most of uplift events within the Dabashan became younger from the axial planes of the folds dip to the NNW. A similar NE to SW, indicating the increasing inﬂuence of extrusion angular unconformity also crops out in the town of Dazhu thrusting, folding, and uplifting (Shen et al. 2007). Based in Wanyuan County (Figure 7B, F, and G); the underlying on the deformational features and rock fabrics, we further limestone was tightly folded with fold hinges that plunge divided the North Dabashan thrust belt into the northern steeply to the NE or ENE (Figure 7G), which indicates and southern sub-zones, which are separated by the that it is a dextral strike-slip thrust feature. Therefore, we Hongchunba fault (Figure 1). interpret that the dextral strike-slip folds and thrusts that International Geology Review 1285

Figure 6. Structural features and stress analysis in the North Dabashan thrust belt. (A) Asymmetric overturned fold in the Cambrian Downloaded by [Monash University Library] at 19:21 24 September 2014 limestone showing the southwestward thrusting (south of Gaotan Town); (B) two stages of deformation in the Middle–Upper Silurian quartz sandstone: early NNW–SSE shortening was superposed by late NE–SW shortening (south of Xiangyang Town); (C) two stages of deformation in the Lower Cambrian limestone: early NE–SW shortening was superposed by late ENE–WSW shortening (north of Zhongting Town); (D) cleavages transposing bedding in the Cambrian limestone (south of Gaotan Town); (E) shear structure in the Cambrian limestone showing southwestward thrusting (north of Zhenping County); (F) thrust fault in the Middle–Upper Silurian quartz sandstone showing the southwestward thrusting (south of Xiangyang Town); (G) overturned folds in the Cambrian limestone showing the ENE–WSW shortening (north of Taohe Town); (H) gentle fold in the Cambrian limestone showing the ENE–WSW shortening (west of Xuanwo Town).

developed along the Chengkou–Fangxian fault zone The NW–SE-trending folds and SW-vergent thrust (Figure 4) were related to the continental collision in the faults have been conﬁrmed by several geologists (Dong late Middle Triassic and were syn-collisional structures. et al. 2008, 2010; Zhang et al. 2010;Huet al. 2012; Shi The ENE–WSW-trending folds that are widely exposed in et al. 2012). The superposition of two stages of folds is the North Dabashan thrust belt are interpreted to be the well exposed in the Silurian Wuxiahe Formation to the ﬁrst episode of deformation (D1). south of the town of Xiangyang in Ziyang County 1286 L. Wangpeng et al.

Figure 7. Structural features of three angular unconformities (A. Pingba section, B. Dazhu section, C. Hongchunba section) in the North Dabashan thrust belt. (A and B) provide three pieces of evidence: (i) an angular unconformity between the Upper Triassic conglomerate Downloaded by [Monash University Library] at 19:21 24 September 2014 and Lower Triassic limestone record a deformational event (continental collision between the NC and SC); (ii) NNW–SSE compressive stress ﬁeld driven by the continental collision event; (iii) numerous dextral strike-slip structures along the Chengkou–Fangxian fault zone driven by the continental collision event; (C) provides two pieces of evidence: (i) an angular unconformity between the Middle–Lower Jurassic conglomerate and Silurian mudstone record a deformational event; (ii) the large-scale thrust of the North Dabashan thrust belt from N to S happened after the Middle Jurassic at least; (D) an angular unconformity (yellow line) deﬁnes contacts between the Upper Triassic conglomerate and Lower Triassic limestone; (E) asymmetric sub-vertical folds in the Lower Triassic limestone underlies the unconformity; (F) the Upper Triassic conglomerate rests on the unconformity; (G) the Lower Triassic limestone underlies the unconformity; (H) the Lower–Middle Jurassic conglomerate rests on the unconformity and is also overthrusted by the Silurian quartz sandstone; (I) the Silurian mudstone underlies the unconformity.

(Figure 6B). Based on the ﬁeld observations and the axial latter was formed by NE–SW-oriented shortening. To the plane of the fold restoration, the early ENE–WSW-trend- north of Xiangyang, a series of asymmetric folds, the north ing folds (D1) were deformed by the NW–SE-trending limbs of which have shallower dip angles and are longer folds. The former, which has steeply SW–WSW-plunging than the south limbs, are present in the Ordovician hinges, indicates dextral strike-slip deformation, and the Erdaoqiao Formation (Figure 2C). These folds, which International Geology Review 1287

have NNW-dipping axial planes and ENE-plunging Thus, we suggest that D3 might have occurred in the Late hinges, and the microscopic structures within these folded Jurassic–Early Cretaceous. rocks also indicate dextral strike-slip deformation (Figure 2D). In addition, these folds were overprinted by a pene- fi trative NE-dipping cleavage that was clearly not related to 4.2. Palaeostress elds the folding and formed during a different stage of defor- The palaeostress fields in the South Dabashan foreland mation (Figure 2C). Therefore, we suggest that the ENE– belt have been investigated extensively (Sun and Lu WSW-trending structures (D1) formed earlier than the 1993; Liu et al. 2005b;Liet al. 2005, 2009, 2013; Dong NW–SE-trending structures. The NW–SE-trending struc- et al. 2006, 2010;Shiet al. 2007; Zhang et al. 2010;Hu tures developed widely throughout the North Dabashan et al. 2012) and experimentally simulated (Wu et al. 2009; thrust belt and represent the second episode of deforma- Zhang et al. 2010; Wang et al. 2011). However, the tion (D2). The Lower–Middle Jurassic conglomerates palaeostresses in the North Dabashan thrust belt have not unconformably overlie the Silurian units in the been studied systematically (Dong et al. 2006, 2010; Hongchunba area (Figure 7C, H, and I) and are over- Zhang et al. 2010;Liet al. 2013). Here, we present a thrusted by the Hongchunba fault (Figure 7H; Geological regional dataset that includes measured axial planes, clea- Survey of Sichuan Province 1973). Based on these obser- vages, and fault-slip data from the entire North Dabashan vations, we infer that the SW-vergent thrusting of the thrust belt. Using the inversion techniques of Angelier North Dabashan thrust belt might have occurred during (1984), we identified three regional palaeostress fields the latest Triassic, and another deformational event that were associated with the formation of the North occurred after the Middle Jurassic. Although we could Dabashan thrust belt. not clearly differentiate these two structural events, we At the exposures of angular unconformities in Pingba suggest that the D2 structures formed in the latest Triassic. (Figure 7A and D) and Dazhu (Figure 7B, F, and G), the The North Dabashan thrust belt has been considered to folds with NNW-dipping axial planes and ENE-plunging be a single SW-vergent thrust sequence in the past (Zhang hinges were located in the strata below the unconformities et al. 2001; Dong et al. 2008, 2010;Huet al. 2009, 2012; but were not present in the overlying strata. Detailed field Shi et al. 2012); however, our fieldwork indicates that observations revealed that numerous folds with NNW- NNW–SSE-trending folds (nearly E–W shortening) also dipping axial planes are present throughout the North developed throughout the North Dabashan thrust belt. Dabashan thrust belt and indicate NNW–SSE shortening These folds, which represent the third episode of deforma- (Figure 8). Additionally, most of the dextral strike-slip tion (D3), formed after the NW–SE-trending deformation. thrusts and shear zones (Figure 4) were also formed by In addition, the folds (D3) with NNW-plunging hinges that the NNW–SSE compressive stress field, and penetrative developed along the eastern segment of the Chengkou– cleavages that dip to the NNW can also be recognized in Fangxian fault indicate sinistral strike-slip deformation. the field outcrops (Figure 6D). This shortening event was For example, an approximately 40 m-wide shear zone is related to the continental collision between the NC and SC present in the limestone of the Cambrian Lujiaping in the Middle Triassic. Formation to the north of the town of Bashan in The structural analysis of numerous outcrop-scale Chengkou County. The superposition of two stages of folds indicates that their hinges trend NW or SE and that folds is well exposed in this shear zone (Figure 6C). The their axial planes dip NE or SW, which indicates NE–SW- vertically plunging folds (D3) formed by sinistral strike- vergent shortening (Figure 9). Besides the folds, other

Downloaded by [Monash University Library] at 19:21 24 September 2014 slip deformation were superimposed on the folds (D2) measured kinematic indicators include cleavages, shear with moderately SE-plunging hinges. The NNW–SSE- structures, and faults that developed throughout the trending folds within the North Dabashan thrust belt North Dabashan thrust belt. For example, cleavage repla- and along the eastern segment of the Chengkou– cement structures are exposed in the Fenghuangshan Fangxian fault zone should represent the D3 folds, and (Figure 3A) and Pingli domes and in some deformed the latter indicates the sinistral strike-slip thrusts (Figure strata. These cleavages generally dip to the NE, which 5). The deformation of the remaining Lower–Middle indicates NE–SW shortening. S-C fabrics (Figure 3B) Jurassic conglomerates (Figure 7C) and the additional and other shear structures (Figure 6E) typically indicate thrusting (Figure 7H) in the North Dabashan thrust belt shearing toward the SW. The brittle faults (Figure 6F) indicate that the D3 structural event initiated after the trend NW–SE and dip moderately to steeply to the NE Middle Jurassic and caused the additional thrusting. The or SW. All of these structural indicators were formed by Upper Cretaceous–Paleogene conglomerates and sand- the NE–SW compressive stress ﬁeld (Figure 9). Analysis stone unconformably overlie the lower units along the of the superposition relationship (Figures 2C and 6B) Ankang fault in the Ankang, Zhushan, and Fangxian suggests that NE–SW shortening (Figure 9) began in the areas (Figure 1;Shiet al. 2012), which indicates that the latest Late Triassic after the episode of NNW–SSE short- D3 phase might have continued into the Early Cretaceous. ening (Figure 8). 1288 L. Wangpeng et al.

Figure 8. NNW–SSE compressive stress ﬁeld in the North Dabashan thrust belt is revealed by the analysis of structural deformation. The stereonets (lower hemisphere) on the map give foliation (P) and lineation (L) data of speciﬁc outcrops, foliations (P) display the limbs of folds, and lineations (L) display the hinges of folds (B-type lineation). Opposing arrows are trends of the maximum principal stress (σ1).

Furthermore, the numerous folds with ENE-dipping shortening. The D3 NNW–SSE-trending folds formed in axial planes in the North Dabashan thrust belt indicate the Late Jurassic–Early Cretaceous and were controlled by ENE–WSW (nearly E–W) shortening (Figure 10). For the ENE–WSW (nearly E–W) compressive stress ﬁeld. example, to the north of Taohe (Langao County), a series Both the D2 and D3 folds and thrusts were responses to of overturned folds with ENE-dipping axial planes are the intra-continental deformation in the South Qinling. well exposed in the limestone of the Cambrian Heishuihe Formation (Figure 6G) and indicate SWW-vergent thrusting and ENE–WSW shortening. Gentle folds with NNW– 4.3. Structural deformation mechanism and SSE-trending axial planes (Figure 6H) are also common in evolutionary model – the North Dabashan thrust belt and indicate ENE WSW –

Downloaded by [Monash University Library] at 19:21 24 September 2014 Numerous NW SE-trending intermediate-basic and alkali shortening. Superposition relationships (Figure 6C) indi- dikes are present in the North Dabashan thrust belt and cate that the ENE–WSW shortening (Figure 10) began in terminate against the Chengkou–Fangxian fault zone. the Late Jurassic–Early Cretaceous after the episode of These dikes record early Palaeozoic rifting in the northern NE–SW shortening (Figure 9). This stress field also margin of the SC and indicate that the North Dabashan induced deformation of the sinistral strike-slip thrust region experienced a stage of tectonic extension during the along the eastern segment of the Chengkou–Fangxian early Palaeozoic (Luo and Duan 2001; Wang et al. 2009; fault zone (Figure 5). Zou et al. 2011). The Middle Triassic continental collision In summary, three stages of superposed folding and between the NC and SC drastically changed the dynamics thrusting have been recognized within the North Dabashan of the North Dabashan area (He et al. 1999; Zhang et al. thrust belt. The D1 folds, which have ENE–WSW-trend- 2001). New seismic data indicate that eclogitization of the ing axial traces, were formed by the Middle Triassic con- dominantly mafic crust on the northern margin of the SC tinental collision and were controlled by the NNW–SSE might have facilitated the prolonged continental conver- compressive stress field. The NW–SE-trending folds and gence (Dong et al. 2013). After the closure of the Mianlue thrusts in D2 were deformed in the latest Late Triassic and ocean, the continued continent–continent convergence earliest Early Jurassic and were controlled by the NE–SW resulted in the formation of the North Dabashan thrust International Geology Review 1289

Figure 9. NE–SW compressive stress ﬁeld in the North Dabashan thrust belt is revealed by the analysis of structural deformation. The stereonets (lower hemisphere) on the map give foliation (P) and lineation (L) data of speciﬁc outcrops, foliations (P) display the limbs of folds excluding the foliations in station number C that display the cleavage, and lineations (L) display the hinges of folds (B-type lineation). Opposing arrows are trends of the maximum principal stress (σ1). Downloaded by [Monash University Library] at 19:21 24 September 2014

Figure 10. ENE–WSW (nearly E–W) compressive stress ﬁeld in the North Dabashan thrust belt is revealed by the analysis of structural deformation. The stereonets (lower hemisphere) on the map give foliation (P) and lineation (L) data of speciﬁc outcrops, foliations (P) display the limbs of folds, and lineations (L) display the hinges of folds (B-type lineation). Opposing arrows are trends of the maximum principal stress (σ1). 1290 L. Wangpeng et al.

belt. Based on the lithology and mechanical properties of NNW–SSE compressive stress field (Figure 11A). The the stratigraphy, four major detachment layers were iden- Chengkou–Fangxian fault, which is parallel to the faults tified in the North Dabashan thrust belt: the Proterozoic to the north, had been a crucial boundary fault before the metamorphic basement, lower Neoproterozoic, lower collision between NC and SC (Figure 11A). After the early Cambrian, and lower Silurian (Bureau of Geology and oblique collision, the SC rotated clockwise relative to the Mineral Resources of Sichuan 1991; Bureau of Geology NC during the Early Jurassic so that the continental colli- and Mineral Resources of Shaanxi 1991; Dong et al. sion took place across all of the northern Yangtze and 2013). The nappes in the North Dabashan area were thrust formed the extensive foredeep subsidence belt from east to the SW along the detachment layers; for instance, the to west along the northern Yangtze Block (Liu et al. North Dabashan thrust belt was thrust to the SW along the 2005a). The sustainability of the continental collision Chengkou–Fangxian fault onto the north margin of the resulted in the large-scale thrusts of the North Dabashan South Dabashan arcuate belt along the lower thrust belt. This thrust event occurred in the latest Late Neoproterozoic detachment layer. The North Dabashan Triassic in response to the intra-continental collision oro- region is characterized by SW-vergent imbricate thrust geny in the Qinling orogen. The SW-vergent thrusting of sheets with several backthrusts within the soft layers of the North Dabashan area was controlled by the NE–SW the Cambrian and Ordovician units; the backthrusts dip to compressive stress field, which formed numerous NW– the NE and form positive flower structures. For example, SE-trending structures. During the SW-vergent thrusting, the Gaoqiao fault dips to the SW and forms a positive the Chengkou–Fangxian fault zone was not only the pri- flower structure with the NE-dipping thrusts to the south. mary detachment surface but also the leading edge of the In the Chengkou–Fangxian fault zone (Figure 2A–B), only North Dabashan nappe. Owing to the pinning of the the Maliu–Dazhu area exhibits a small-scale positive Hannan massif (to the west) and the Shennongjia massif flower structure with SW-dipping backthrusts (Geological (to the east) culminations, the originally linear Chengkou– Survey of Shaanxi Province 1966). Therefore, the Maliu– Fangxian fault zone began bending into an arcuate fault Dazhu area should represent top of the thrust front of the zone (Figure 11B). During the Late Jurassic–Early North Dabashan thrust belt. Cretaceous, the orientation of the stress field in the North Based on a collisional orogenic model, if the boundary Dabashan region changed gradually from NE to ENE fault is arcuate, the internal tectonic structures should also during intra-continental orogeny. Additional thrust in the be bent (Macedo and Marshak 1999). In contrast, the thrust North Dabashan thrust belt was controlled by the ENE– faults in the North Dabashan thrust belt generally strike NW WSW compressive stress field. The deformation of the and terminate against the arcuate Chengkou–Fangxian fault Dabashan seems to be proceeded by an oblique indenta- zone; however, the thrust faults in the South Dabashan tion of the Hannan massif into the Qinling orogenic belt arcuate belt are parallel to the Chengkou–Fangxian fault (Li et al. 2013). Due to the synthetical effects of the ENE– zone and form an arc (Figure 1). Therefore, the geological WSW compressive stress field and the pinning of the features indicate that the thrust faults within the North Shennongjia massif culmination, the eastern Chengkou– Dabashan thrust belt and Chengkou–Fangxian fault zone Fangxian fault was dragged to the WSW, which formed underwent different kinematic histories, and the former may the sinistral strike-slip structures (Figure 11C). The be the result of reactivation of earlier normal faults (He Chengkou–Fangxian fault zone was deformed into a et al. 1999). Zhang et al. (2010) and Wang et al. (2011) WSW-convex arcuate fault zone due to the pinning of used sandbox models with varying boundary conditions to both of its ends and the detachment layer that developed

Downloaded by [Monash University Library] at 19:21 24 September 2014 demonstrate that the thrust faults were originally exten- in the Lower Neoproterozoic strata. At the same time, the sional faults that formed during the early Palaeozoic and NW-trending thrust faults within the North Dabashan were reactivated as thrust faults in the early Mesozoic. thrust belt were truncated by the arcuate Chengkou– The Early–Middle Triassic Mianlue suture represents Fangxian fault zone (Figure 11C). We suggest that the the final amalgamation of the NC and SC (Zhang et al. final arcuate Chengkou–Fangxian fault zone formed later 1996; Meng and Zhang 2000). The unconformity between than the NW-trending faults to the north, resulting in the the Upper Triassic conglomerates and sandstone and the truncated relationship between the faults. Finally, the Lower Triassic limestone in the Pingba (Figure 7A, D, and Dabashan thrust belt evolved into a SWW-convex arcuate E) and Dazhu areas (Figure 7B, F, and G) along the tectonic belt with a significantly shorter western segment Chengkou–Fangxian fault zone indicates that the final than its eastern segment (Figure 11C). amalgamation of the NC and SC in the Dabashan area occurred during the Middle Triassic. During this period, the SC moved to the NW and was obliquely subducted 5. Conclusions under the NC (Figure 11A). Numerous dextral strike-slip (1) The northern sub-zone of the North Dabashan thrust syn-collision structures in the North Dabashan thrust belt belt is characterized by two Meso–Neoproterozoic base- and Chengkou–Fangxian fault zone were controlled by the ments units that include the Fenghuangshan and Pingli International Geology Review 1291

Figure 11. Block kinematic models for the tectonic evolution of the Dabashan thrust belt and Chengkou–Fangxian fault zone. (A) (i) the Middle Trassic period, during which the Chengkou–Fangxian fault zone acted as the main decollement surface of the North Dabashan thrust belt, was an early orocline geometry boundary fault; (ii) numerous dextral strike-slip syn-collision structures in the North Dabashan thrust belt and Chengkou–Fangxian fault zone indicating that the SC moved northwestwards and was obliquely subducted under the NC; (B and C) the Late Trassic–Early Cretaceous during which the intra-continent orogeny of the Dabashan fell into two stages: (i) (B) the Late Trassic–Early Jurassic period during which the North Dabashan thrust belt happened to thrust toward to the SW, and created numerous NW–SE-trending folds within the North Dabashan thrust belt; (ii) (C) the Late Jurassic–Early Cretaceous during which the North Dabashan thrust belt thrust further. Owing to the pinning of the Hannan massif and Shennongjia massif culminations, the deformation of North Dabashan thrust belt is dominated by ENE–WSW orientation stress, which created numerous sinistral strike-slip thrusts developed along the eastern Chengkou–Fangxian fault zone, and nearly S–N-trending folds developed along the western Chengkou–Fangxian fault zone.

domes and contain SW-vergent imbricate thrust sheets. zones with dextral and sinistral strike-slip properties The southern sub-zone consists of SW-vergent thrusts were recognized along the Chengkou–Fangxian fault and backthrusts that form positive flower structures. zone. The dextral strike-slip motion along the entire fault (2) During the Middle Triassic, the SC moved to the zone was caused by the collision between the NC and SC, NW and was obliquely subducted under the NC. Numerous and the sinistral strike-slip motion, which occurred only dextral strike-slip syn-collision structures formed in the along the eastern part of the fault zone, was related to the Downloaded by [Monash University Library] at 19:21 24 September 2014 North Dabashan thrust belt and Chengkou–Fangxian fault intra-continental orogeny. The dextral deformation was zone and were controlled by the NNW–SSE-compressive superimposed and deformed by the sinistral deformation. stress field. During the Late Triassic–Early Jurassic, the North Dabashan thrust belt generally behaved as a SW- vergent thrust system that was controlled by the NE–SW Acknowledgements fi – compressive stress eld. During the Late Jurassic Early The authors thank the Editor-in-Chief Dr Robert J. Stern, Cretaceous, sinistral strike-slip thrusts formed in the eastern Professor Sanzhong Li, and one anonymous reviewer for their Chengkou–Fangxian fault zone in response to a EME– constructive comments and suggestions; all improved the manu- WSW (nearly E–W) compressive stress field. Due to the script significantly. This research benefited from the participation fi pinning of its west and east ends during thrusting in the of Daixin Chen, Hua Zhu, and Wanfeng Du in the eldwork. North Dabashan region, the originally linear Chengkou– Fangxian fault zone gradually bent into a SWW-convex Funding arcuate fault zone. (3) Based on detailed field investigations, structural Financial support was provided by the National Natural Science Foundation of China [grant number 41030318], [grant number analysis, and microscopic observations, two strike-slip 91114203]; and Specialized Research Fund for the Doctoral 1292 L. Wangpeng et al.

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