Indentation-induced tearing of a subducting continent: Evidence from the Tan–Lu Fault Zone, East China

Earth-Science Reviews 152 (2016) 14–36

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Indentation-induced tearing of a subducting continent: Evidence from the Tan–Lu Fault Zone, East China Tian Zhao, Guang Zhu ⁎, Shaoze Lin, Haoqian Wang School of Resource and Environmental Engineering, Hefei University of Technology, Hefei 230009, China

a r t i c l e

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Article history: Received 4 March 2015 Received in revised form 4 November 2015 Accepted 5 November 2015 Available online 10 November 2015 Keywords: Tan–Lu Fault Zone sinistral ductile shear zone Dabie–Sulu orogens 40 Ar/39Ar dating collision of the North China and Yangtze plates continent tearing

a b s t r a c t Vertical tearing of oceanic slabs has been well documented at subduction zones. It remains unclear whether a subducting continent can tear vertically. Origin of the continental-scale Tan–Lu Fault Zone (TLFZ) in East China provides an example of the vertical tearing of the subducting continent at the convergent stage. The Dabie and Sulu orogens between the North China Plate (NCP) and the Yangtze Plate (YZP) are left-laterally offset about 400 km along the NE-striking TLFZ, but the fault zone terminates abruptly at the southeastern corner of the Dabie Orogen, suggesting unique origin of the fault zone. We present structural evidence to show that the TLFZ initiated as a sinistral fault zone that is exposed in ductile shear belts just to the east of the Dabie Orogen. Our muscovite 40Ar/39Ar dating results, coupled with existing age data, indicate that the TLFZ formed at 240–230 Ma (Middle Triassic), when the YZP continental crust was subducted beneath the NCP along both the Dabie and Sulu sutures. We also show that SE-directed extrusion of the subducted crust in the Dabie Orogen occurred during 230–209 Ma (Late Triassic) and overprinted contractional deformation in the TLFZ to the southeast of the Dabie Orogen. Syn-collisional folds and thrusts in the YZP, to the east of the TLFZ, exhibit evidence for largescale dragging by the sinistral fault zone, whereas those in the NCP to the west are perpendicular to the TLFZ without obvious evidence for drag. Combining these lines of evidence with published data on the tectonic evolution of the two orogenic belts, we propose an indentation-induced continent-tearing model for the origin of the TLFZ. We suggest that the present southern boundary of the NCP represents its original shape, with a promontory in front of the Dabie Orogen. At the oceanic subduction stage, the overriding NCP promontory led to vertical tearing of the subducting oceanic plate along the TLFZ, which formed the eastern boundary of the promontory. Once the rigid NCP indenter collided with the passive YZP along the Dabie Orogen, the oceanic slab tear propagated into the YZP lithosphere, resulting in long-distance, low-angle subduction of the YZP underneath the NCP indenter and NNE-ward, horizontal motion of the torn YZP east of the TLFZ until the final collision at the Sulu Orogen. Thus, the indentation induced the vertical tearing of the subducting YZP along the TLFZ. © 2015 Elsevier B.V. All rights reserved.

Contents 1. 2. 3.

4.

Introduction . . . . . . . . . . . . . . . . . Geological setting . . . . . . . . . . . . . . . Syn-collisional structures . . . . . . . . . . . 3.1. Northern Zhangbaling TLFZ . . . . . . . 3.2. Southern Zhangbaling TLFZ . . . . . . . 3.3. Lujiang TLFZ . . . . . . . . . . . . . . 3.4. Southeastern margin of the Dabie Orogen 3.5. Southwestern margin of the Dabie Orogen 40 Ar/39Ar dating . . . . . . . . . . . . . . . 4.1. Dating results . . . . . . . . . . . . . 4.2. Interpretation . . . . . . . . . . . . . 4.3. Previous 40Ar/39Ar ages . . . . . . . . .

⁎ Corresponding author. E-mail address: [email protected] (G. Zhu).

http://dx.doi.org/10.1016/j.earscirev.2015.11.003 0012-8252/© 2015 Elsevier B.V. All rights reserved.

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5.

Syn-collisional marginal deformation . . . . . . . . . . . 5.1. Yangtze Plate . . . . . . . . . . . . . . . . . . . 5.2. North China Plate . . . . . . . . . . . . . . . . . 6. Genesis and evolution of the Tan–Lu syn-collisional structures 6.1. Zhangbaling TLFZ . . . . . . . . . . . . . . . . . 6.2. Lujiang TLFZ . . . . . . . . . . . . . . . . . . . 6.3. Southeastern margin of the Dabie Orogen . . . . . . 7. Discussion and conclusions . . . . . . . . . . . . . . . . 7.1. Summary of the syn-collisional TLFZ . . . . . . . . . 7.2. Comments on previous models . . . . . . . . . . . 7.3. Indentation-induced continent tearing model . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . .

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1. Introduction Continental collision can result in the indenting of a rigid continent into a soft continent (England and Houseman, 1986; Houseman and England, 1986, 1993; Robl and Stüwe, 2005; Doglioni et al., 2007). Examples of continental indenters with higher viscosity include the Indian plate in the India–Asia collision zone and the Adriatic plate in the European Alps collision zone (Tapponnier et al., 1982; Ciancaleoni and Marquer, 2008; Garzanti and Malusà, 2008; Schattner, 2010). The Asian and European plates in front of the indenters behaved as soft continents that developed arcuate deformed belts (England and Houseman, 1986; Doglioni et al., 2007; Rosenberg et al., 2007; Dumont et al., 2011; Malusà et al., 2011, 2015). It remains unclear whether an indented continent with lower viscosity can tear vertically during the indentation. Vertically torn oceanic slabs are well documented at convergent plate boundaries (Doglioni et al., 1994, 2001; Lallemand et al., 1997; Govers and Wortel, 2005; Miller et al., 2005; D'Orazio et al., 2007; Rosenbaum et al., 2008; Agostini et al., 2010; Hale et al., 2010; Kennett and Furumura, 2010; Özbakır et al., 2013; de Sigoyer et al., 2014; Karaoğlu and Helvacı, 2014; van Benthem et al., 2014), and are known as Subduction–Transform Edge Propagator (STEP) faults (Govers and Wortel, 2005). Vertical slab tears commonly occur during subduction rollback (Doglioni et al., 1994; Govers and Wortel, 2005; Mason et al., 2010; Özbakır et al., 2013; de Sigoyer et al., 2014; Karaoğlu and Helvacı, 2014), and the key trigger for tearing of the slab is variation in the relative slab motion velocities along the strike of a subduction system (Govers and Wortel, 2005; Rosenbaum et al., 2008; Rosenbaum and Piana Agostinetti, 2015). Passive continent subduction may follow oceanic lithosphere subduction during continent–continent collision, because of the possible pull exerted by the subducted oceanic slab (Riguzzi et al., 2010). Three-dimensional numerical models suggest that a 40-km-thick continental crust adjoining a large ocean can be pulled downwards by the oceanic plate and subducted to depths of N200 km (van Hunen and Allen, 2011). The effective viscosity of average continental lithosphere is estimated to be at least an order of magnitude smaller than that of oceanic lithosphere (Gordon, 2000). It can therefore be inferred that vertical tearing of a subducting oceanic slab may propagate into the attached, subducting continent. The Tan–Lu Fault Zone (TLFZ) in East China provides an opportunity to test this inference. The continental-scale left-lateral TLFZ strikes generally NE–SW for about 2400 km (Fig. 1), but has an unusual southern termination and displacement change along strike. The fault zone separates the Dabie and Sulu orogens by about 400 km, but terminates abruptly at the southeastern corner of the Dabie Orogen. The northern boundary of the North China Plate (NCP, also referred to as the North China Craton) is located ~800 km north of the Sulu Orogen, and is sinistrally displaced only ~ 150 km along the TLFZ (inset in Fig. 1; Zhu et al., 2005). The abrupt termination and discordant displacements exclude the possibility that the TLFZ originated as a post-orogenic strike-slip fault zone, as suggested by Xu et al. (1987); Xu and Zhu (1994) and Leech and

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Webb (2013), because the post-orogenic fault zone would extend further to the south of the Dabir Orogen and show roughly consistent displacements for both the two orogens and the northern boundary of the NCP. It is widely accepted that the TLFZ initiated during the Middle Triassic collision of the NCP and Yangtze Plate (YZP) along the Dabie and Sulu orogens (Li et al., 1993; Rowley et al., 1997; Hacker et al., 2000; Faure et al., 2003), although the mechanism by which it initiated remains controversial. Several syn-collisional models have been proposed, in which the fault is interpreted to be developed either as a syn-exhumation transform fault (Fig. 2a; Okay and Şengör, 1992), an indenter boundary (Fig. 2b; Yin and Nie, 1993), a tear fault triggered by upper crust overthrusting in the Sulu Orogen (Fig. 2c; Li, 1994; Lin, 1995; Chang, 1996), or a rotated suture line during collision between the NCP and YZP (Fig. 2d; Gilder et al., 1999). The fault has also been explained in terms of a post-collisional orocline model (Fig. 2e; Wang et al., 2003) and as a syn-subduction transform fault or slab tear (Fig. 2f; Zhu et al., 2009). Constraining the structural features and timing of the initial TLFZ is crucial for understanding its origin, but neither of these points have been well constrained because the fault is often overprinted by younger structures (Wang, 2006; Zhu et al., 2010) and covered by Early Cretaceous to Paleogene rift basins (Fig. 1; Zhu et al., 2010; G. Zhu et al., 2012). Lin et al. (2005) proposed that no syn-collisional strike-slip ductile shear belt exists in the southern Zhangbaling TLFZ (Fig. 1). In contrast, Zhang et al. (2007) argued that the southern Zhangbaling TLFZ was deformed by the Tan–Lu syn-collisional ductile shear zone along an obliquely convergent zone between the NCP and YZP, whereas the northern Zhangbaling TLFZ represents a 240–235 Ma (40Ar/39Ar muscovite ages) top-to-the-south detachment zone that coupled deeper ductile shear with shallower brittle deformation. The NEstriking sinistral ductile shear belts on the western margin of the Sulu Orogen and the eastern margin of the Dabie Orogen yield 40Ar/39Ar muscovite ages of 221–181 Ma (Zhu et al., 2009), which were interpreted as recording syn-collisional sinistral faulting (Zhu et al., 2009). However, their overprinting on the exhumed ultrahighpressure rocks of the orogenic belts suggest that they developed postexhumation. Structures in the low-grade metamorphic rocks of the Zhangbaling Group, exposed just east of the Dabie Orogen, have received little attention. This work is focused on the southern segment of the TLFZ. We analyzed the TLFZ in five different areas (see location in Fig. 1) and present structural and geochronological evidence to show that the TLFZ originated as a Middle Triassic sinistral fault zone. The initial structures are locally preserved on the western margin of the YZP and have been overprinted by younger deformation. We integrate the structural and geochronological data with constraints on the collisional development of the NCP and YZP, as well as syn-collisional, marginal structures. On this basis, we propose that the TLFZ developed along the lateral boundary of an indenter that was located in the NCP, and that the indentation led to syn-collisional vertical tearing of the subducted YZP.

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Fig. 1. Structural map of the southern segment of the Tan–Lu Fault Zone and syn-collisional marginal structures (see location in the inset). The localities of igneous rocks with a Yangtze Plate signature are from Miao et al. (1997); Zhang et al. (2002); Zhang and Sun (2002); Zhou et al. (2003); W. L. Xu et al. (2004); Xu et al. (2005, 2009); Y. G. Xu et al. (2004); Liu et al. (2009); Yang et al. (2010); Zhang et al. (2010) and Li et al. (2014).

2. Geological setting The southern segment of the TLFZ lies between the Dabie Orogen and YZP, the NCP and YZP, and the NCP and Sulu Orogen, from south to north (Fig. 1). The middle segment cuts the interior of the NCP whereas the northern segment extends as two branches in the Central Asia Orogen (see insert in Fig. 1). The NCP consists of high-grade basement of Archean to Paleoproterozoic age, capped with a widespread

Mesoproterozoic–early Permian marine cover (Zhai and Santosh, 2011). The northern margin of the NCP collided with the Siberian– Mongolian blocks in the late Permian, forming the Central Asia Orogen (Wilde et al., 2003; Xiao et al., 2003) and leading to the cessation of cover sedimentation and the onset of shortening in the northern NCP (Lin et al., 2013). At the end of the Middle Triassic, the southern NCP experienced an episode of shortening known as the Indosinian Movement, when it collided with the YZP (R. X. Zhu et al., 2012). Meso-Cenozoic

T. Zhao et al. / Earth-Science Reviews 152 (2016) 14–36

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Fig. 2. Existing tectonic models for the origin of the Tan–Lu Fault Zone (see text for details).

terrestrial deposits occur in various basins, including rift basins of Cretaceous to Paleogene (G. Zhu et al., 2012), within the NCP. The YZP, which is the northern part of the South China Plate, consists of N820 Ma low-grade Proterozoic basement (Zheng et al., 2008; Zhao and Cawood, 2012) that is overlain by a 7–10-km-thick Neoproterozoic to Middle Triassic marine cover (Zhu et al., 2009; Zhao et al., 2014a). No angular unconformity has been recognized within the northern YZP cover, and the cover sequence terminates at an angular unconformity that developed between Middle Triassic marine and Upper Triassic terrestrial strata during the Indosinian Movement (Dong et al., 1994). Late Triassic to Middle Jurassic terrestrial deposits that are locally preserved in the northern YZP have been considered to record foreland deposition during the development of the Dabie and Sulu orogens (Zhu et al., 2009). The YZP contains local Cretaceous–Paleogene rifted basins, such as the Subei basin that is located south of the Sulu Orogen (Fig. 1), as well as Early Cretaceous plutons. The WNW-trending Dabie Orogen is bounded by the Feizhong Suture (Hacker et al., 2006) to the north and the Xiangfang–Guangji Fault to the south (Fig. 1). From south to north, the orogenic belt consists of a highpressure (HP) greenschist facies unit, HP amphibolite facies unit, HP eclogite facies unit, ultrahigh-pressure (UHP) eclogite facies unit, the North Dabie unit, and the northern Huaiyang unit (Hacker et al., 2000). Eclogites are thus exposed on the upper plate side of the orogenic wedge, behind lower pressure metamorphic rocks as observed in other well studied (U)HP orogenic belts (e.g., Malusà et al., 2011). The NEtrending Sulu Orogen is bounded by the Jiashan–Xiangshui Fault to the south and the Wulian Suture to the north, and is composed of a HP greenschist facies unit in the south and an UHP eclogite unit in the north (Liu and Xu, 2004), thus showing the same metamorphic polarity of the Dabie Orogen. Geochronological data show that the continent– continent collision of the NCP and YZP along the Dabie and Sulu orogens took place in the Middle Triassic, as indicated by peak UHP metamorphism caused by the subduction of continental crust, and that subsequent crustal exhumation in Late Triassic (e.g., Li et al., 1993, 2000; Ames et al., 1996; Rowley et al., 1997; Hacker et al., 2000, 2006; Liu and Xu, 2004; Zheng, 2012). The YZP has been clearly shown to have

subducted underneath the NCP along both the Dabie and Sulu sutures, and metamorphic rocks in both orogens comprise exhumed YZP crust (e.g., Hacker et al., 2000, 2006; Faure et al., 1999, 2003; Liu and Xu, 2004; Zheng, 2012). Previous studies (e.g., Webb et al., 1999; Hacker et al., 2000; Faure et al., 2003) demonstrated that the subducting crust slabs along both the Dabie and Sulu orogens were extruded generally towards the SE during the exhumation whereas ductile fabrics related to the exhumation in the interiors of the orogens mostly exhibit a top-tothe-NW sense of shear. The WNW-striking Xiaotian–Mozitan Fault and the NE-striking Wulian Suture are considered to represent the northern boundaries of crustal exhumation in the Dabie and Sulu orogens, respectively (Fig. 1; Ames et al., 1996; Rowley et al., 1997; Hacker et al., 2000, 2006; Faure et al., 2003). Many Early Cretaceous intermediate–acid plutons intruded the orogenic belts during regional extension (Zhao et al., 2005; Wang et al., 2011). The North Dabie unit represents an Early Cretaceous cordillera-type metamorphic core complex (Fig. 1; Ratschbacher et al., 2000; Wang et al., 2011). After its inception, the TLFZ behaved as a sinistral fault in the Late Jurassic (Wang, 2006; Zhu et al., 2010), probably continuing into the earliest Cretaceous (Zhu et al., 2005). The resulting ductile shear belts are exposed along the northern part of the eastern edge of the Dabie Orogen, the southern part of the Zhangaling uplift belt, and the northern part of the western edge of the Sulu Orogen (Fig. 1). Other parts of the TLFZ exhibit brittle strike-slip faulting (Zhu et al., 2010). Early Cretaceous to Paleogene extensional reactivation of the TLFZ led to the development of terrestrial rifted basins along the fault zone (Zhang et al., 2003; Zhang and Dong, 2008; Mercier et al., 2007; Zhu et al., 2010; G. Zhu et al., 2012). The basins along the southern segment include, from south to north, the Qianshan basin, located to the east of the Dabie Orogen, the Hefei basin, located to the west of the Zhangbaling uplift belt, and the Jiashan graben and two separate Yishu grabens that lie to the west of the Sulu Orogen (Fig. 1). The Hefei basin is located on the southern NCP and was a terrestrial depression during the Early–Middle Jurassic. Its subsidence, related to exhumation of the Dabie Orogen (Zhu et al., 2009), occurred prior to the Cretaceous– Paleogene rifting (Zhu et al., 2010).

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3. Syn-collisional structures Late Jurassic sinistral faulting gave rise to NE-striking sinistral ductile shear belts that have been exhumed in the northern parts of the western edge of the Sulu Orogen and the eastern edge of the Dabie Orogen; in each case, brittle counterparts are exhumed to the south of the ductile shear belts (Fig. 1; Zhu et al., 2005, 2010; Zhao et al., 2014a). Exhumation of the ductile shear belts is mainly controlled by later normal faulting (Zhu et al., 2010). It is inferred from our field investigations that the syn-collisional structures of the TLFZ that developed immediately west of the Sulu Orogen were covered by Cretaceous graben fill or Quaternary deposits. However, the Tan–Lu syn-collisional structures are locally preserved in the Zhangbaling TLFZ, and in exposures of the Zhangbaling Group immediately east of the Dabie Orogen (Fig. 1). Structures in the southeastern Dabie Orogen, neighboring the TLFZ, were also investigated in this study. 3.1. Northern Zhangbaling TLFZ The NNE-trending Zhangbaling TLFZ is an uplift belt containing exhumed metamorphic rocks of the YZP. It extends for ~150 km and is located between the Yangtze fold-and-thrust belt to the east, and the Hefei basin overlying the NCP to the west. The uplift belt forms the footwall of the Cretaceous–Paleogene Hefei rifted basin, and its western edge is cut by a basin-bounding normal fault that strikes NNE–SSW and dips steeply to the WNW (G. Zhu et al., 2012).The northern Zhangbaling TLFZ exposes metamorphosed Zhangbaling Group along the western margin of the YZP (Fig. 3a). The Early Cretaceous Guandian, Wawuliu, and Wawuxue granite plutons (Niu et al., 2002; Ma and Xue, 2011), from north to south, are exhumed along the western margin of the northern Zhangbaling TLFZ (Fig. 3a). To the east, the Zhangbaling Group is overlain by Sinian (late Neoproterozoic) to Ordovician cover sediments of the YZP, which strike NE–SW, generally young eastwards, and are locally overlain by Early Cretaceous red beds or volcanic rocks (Xie et al., 2007, 2009). The Zhangbaling Group consists of a basal white mica schist, middle quartz–plagioclase schist, and upper blue-amphibole-bearing schist (Zhang et al., 2007). Their protoliths are dominated by volcanic rocks, such as rhyolite, andesite, and tuff, as well as clastic rocks (Liou et al., 1992; Zhang et al., 2007; Zhao et al., 2014a). The distribution of the lithological units defines a N–S trending open antiform, with lower Sinian cover rocks overlying the core and eastern edge of the structure (Fig. 3a, c). The lower Sinian rocks are dominated by phyllite and meta-sandstones whereas unmetamorphosed limestones predominate in the upper Sinian–Ordovician sequence. Mineral assemblages indicate middle greenschist facies metamorphism for the Zhangbaling Group, lower greenschist facies metamorphism for the lower Sinian rocks, and no metamorphism for the upper Sinian–Ordovician strata (Liou et al., 1992; Jing et al., 1996; Zhang et al., 2007). These observations indicate a gradual decrease in metamorphic grade from the bottom to the top of the sequence. LA–ICP–MS zircon U–Pb dating of two metavolcanic rock samples from the Zhangbaling Group (Fig. 3a) yielded protolith ages of 754 Ma and 753 Ma (Zhao et al., 2014a), which demonstrate that the Zhangbaling Group is lower Sinian in age and belongs to the lowest part of the YZP cover (Li et al., 2003; Zheng et al., 2008; Zhao and Cawood, 2012). The exposed sequence, from the basal Zhangbaling Group to the Ordovician strata in the northern Zhangbaling TLFZ, represents a successive lower cover sequence deposited on the YZP. The low-grade Zhangbaling Group has been extensively ductilely deformed. The competent rocks in the group, such as the meta-volcanic rocks and meta-sandstones, have been sheared into protomylonite, mylonite (Fig. 4a, b), and locally ultramylonite. The mylonitic foliation or schistosity in the group is gently dipping to flat-lying (Fig. 4a), with variable dip angles of b30° (Fig. 3b). A mineral elongation lineation is defined by the orientation of feldspar or elongate quartz grains. The lineation plunges gently and trends NNE–SSW in the southern section of the

Zhangbaling TLFZ, N–S in the central section, and NNW–SSE in the northern section (Fig. 3b). Small, tight, recumbent folds with axes parallel to the mineral elongation lineation (Fig. 4b) are common in the Zhangbaling Group. Various shear sense indicators, including S–C fabrics (Fig. 4a), rotated feldspar porphyroclasts, and deformed quartz-filled tension gashes (Fig. 4b) all demonstrate a generally top-to-the-south sense of shear. Our observations of fabrics and shear sense in the Zhangbaling Group are consistent with those reported by Zhang et al. (2007). Microscope observations provide further evidence for mylonitization in the competent rocks of the Zhangbaling Group, and for the top-to-thesouth sense of shear. Mylonites in the Zhangbaling Group show evidence of widespread dynamic recrystallization of quartz by both bulging (BLG) and subgrain rotation (SGR). Feldspars in the mylonites occur as porphyroclasts that are free of dynamic recrystallization and that are elongate, reorientated, show undulatory extinction, and locally contain brittle fractures. These microstructures suggest deformation temperatures of ~ 350–400 °C (Stipp et al., 2002; Mancktelow and Pennacchioni, 2004), which is consistent with temperature estimates based on quartz c-axis analyses in the same area (Zhang et al., 2007). The Zhangbaling Group exhibits a progression from early ductile to later brittle–ductile deformation. Evidence for the later brittle–ductile deformation takes the form of numerous quartz-filled tension gashes in competent rocks, as well as kink bands, kink folds (Fig. 4c), and crenulation lineations that overprint the earlier foliation in the schist. The later quartz-filled tension gashes, kink bands, axial planes of kink folds, and crenulation lineations are all roughly perpendicular to the earlier mineral elongation lineation. Progressive rotation of the quartz-filled tension gashes and asymmetric kink folds also indicates a generally top-to-the-south sense of shear, suggesting that kinematics remained consistent during the transition from earlier ductile deformation to later brittle–ductile deformation. Our field observations demonstrate that the lower Sinian low-grade rocks show similar fabrics and shear sense to those in the Zhangbaling Group, but the foliations in the lower Sinian rocks predominantly dip gently to the southeast (Fig. 3a). A brittle–ductile detachment zone, with fabrics and shear sense that are similar to those in the lower Sinian rocks, appears between the lower and upper Sinian rocks. In contrast to the gently dipping ductile fabrics in the low-grade Zhangbaling Group and lower Sinian rocks of the northern Zhangbaling TLFZ, structures in the overlying unmetamorphosed upper Sinian–Ordovician strata are characterized by NE-trending tight and locally overturned folds that verge to the southeast (Fig. 3c). Associated NW-dipping thrusts appear locally in the Cambrian–Ordovician strata. 3.2. Southern Zhangbaling TLFZ Exposures in the southern Zhangbaling TLFZ are dominated by the high-grade Feidong Complex, and the Zhangbaling Group is locally preserved on its eastern and southern margins (Fig. 5a). The Feidong Complex is composed of meta-igneous rocks, such as biotite– plagioclase gneiss, hornblende–plagioclase gneiss, plagioclase amphibolite, amphibolite and granitic gneiss, and meta-sedimentary rocks such as marble and biotite schist. Previous studies suggest that the complex underwent metamorphism in the lower to intermediate amphibolite facies (Jing et al., 1991, 1996; Zhang et al., 2007; Shi et al., 2009). LA–ICP–MS zircon U–Pb dating for five orthogneiss samples from the Feidong Complex (Fig. 5a) yielded protolith ages of 745–800 Ma (Zhao et al., 2014a), suggesting that the complex belongs to the lowest part of the YZP cover rocks. Previous studies have demonstrated that the Feidong Complex was deformed by a series of NNE-striking sinistral ductile shear belts (Zhu et al., 2005; Figs. 4d and 5a) that were active during the Late Jurassic (Zhu et al., 2010). Hornblende and biotite samples from mylonites in the shear belts (Fig. 5a) yielded 40Ar/39 Ar cooling ages ranging from 144 Ma to 119 Ma (Zhu et al., 2005). Ductile shear belts within the northern section of the southern Zhangbaling TLFZ dip steeply to the

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SE and have gently SSW-plunging lineations. Shear on these belts has resulted in almost total transposition of earlier fabrics into the strikeslip fabrics (Fig. 5b; Zhu et al., 2005). In contrast, the earlier ductile fabrics in the southern section are preserved between the NNE-striking sinistral shear belts, and are only partially transposed at upper levels (Fig. 5b; Zhu et al., 2005). The earlier foliation in the southern section has been folded along E- to ENE-trending axes, with a preferential limb dip to the SSE. The associated mineral lineation has a variable plunge, but trends mainly ENE–WSW. The shear sense for these earlier fabrics is varied, but is predominantly top-to-the-WSW. In the field, the attitudes of the earlier fabrics can be seen to rotate towards those of the later fabrics when approaching the ductile shear belts. The sense of drag is consistent with later sinistral ductile shear. No sinistral ductile shear belts have been found in the low-grade deformed Zhangbaling Group on the eastern and southern margins of the southern ZLTFZ. Fabrics in the Zhangbaling Group typically consist of a NE-striking folded foliation and a NE–SW trending mineral lineation (Fig. 5b). A top-to-the-SW sense of shear is apparent in the Zhangbaling Group. 3.3. Lujiang TLFZ The Lujiang TLFZ lies south of the Zhangbaling TLFZ, at the NE-striking boundary between the NCP covered by the Hefei basin to the west, and the YZP to the east (Fig. 1). Within the YZP, Sinian to Early Triassic marine strata are tightly folded along NE-trending axes and are cut by several fold-parallel thrusts. Jurassic terrestrial clastic rocks and Early Cretaceous volcanic rocks and plutons occur locally in the area (Fig. 6a). The Zhangbaling Group is locally exposed in the Lujiang TLFZ (Fig. 6a), where it has a similar lithology and low-grade metamorphism to the Zhangbaling Group of the northern Zhangbaling TLFZ. LA–ICP–MS zircon U–Pb dating of three meta-volcanic rock samples from the Zhangbaling Group (Fig. 6a) yielded protolith ages of 749–751 Ma (Zhao et al., 2014b), which again suggests that they belong to the lower cover of the YZP. Exposures of the Zhangbaling Group in the Lujiang TLFZ have been ductilely deformed and widely mylonitized (Fig. 4e). The mylonitic foliation in the Zhangbaling Group strikes predominantly NE–SW and dips steeply to the SE (Fig. 6b). The mineral elongation lineation in the Zhangbaling Group trends NE–SW and generally plunges gently to the NE. The foliation is locally folded by a later deformation phase, which leads to local variation of the fabric attitudes. Shear sense indicators in the Zhangbaling Group, such as S–C fabrics, asymmetrical folds, and deformed quartz-filled tension gashes (Fig. 4e), all demonstrate sinistral shearing. Ductile deformation in the low-grade Zhangbaling Group typically exhibits the same kinematics as brittle–ductile deformation (Fig. 4e). Microstructures in the Zhangbaling Group show similarities to those in the northern Zhangbaling TLFZ, such as the widespread dynamic recrystallization of quartz in mylonites, which suggests a deformation temperature of 350–400 °C. Structures in the Zhangbaling Group in the Lujiang TLFZ therefore seem to represent steeply SEdipping, sinistral ductile shear belts (Fig. 6a). 3.4. Southeastern margin of the Dabie Orogen On the eastern margin of the Dabie Orogen, the Late Jurassic TLFZ changes from a northern NE-striking sinistral ductile shear belt to a southern zone of brittle faulting (Fig. 1; Zhu et al., 2005, 2010). Structures related to the syn-collisional history of the TLFZ are developed in the Zhangbaling Group, to the east of the southern section (Fig. 7a). East of the Zhangbaling Group outcrop area, the Yangtze fold-andthrust belt is covered by unmetamorphosed Paleozoic strata. The

Zhangbaling Group in this area has a similar lithology and low-grade metamorphism to that of the Zhangbaling Group in the northern Zhangbaling and Lujiang TLFZ. LA–ICP–MS zircon U–Pb dating of four meta-volcanic rock samples from the Zhangbaling Group in this area yielded protolith ages of 751–748 Ma (Fig. 7a; Zhao et al., 2014b), which are consistent with the lower part of the YZP cover. An abrupt change in metamorphic grade, from high-grade rocks in the Dabie Orogen to low-grade rocks in the adjacent Zhangbaling Group, also demonstrates that the exposed Zhangbaling Group is not exhumed from the Dabie Orogen. The Zhangbaling Group to the southeast of the Dabie Orogen is exposed along a curved 1–2-km-wide belt that consists of NE-trending northern and southern sections, and an E–W-trending central section. The rocks have been ductilely deformed throughout, and metavolcanic rocks are commonly mylonitized (Fig. 4f). The mylonitic foliation and schistosity generally strikes parallel to the curvature of the deformation belt shown by the outcrop trend of the Zhangbaling Group (Fig. 7a). In the NE-trending sections, the foliation dips steeply in the northern section and moderately to steeply in the southern section, although both sections have variable dips (Fig. 7b). In the E–W trending central section, the foliation dips moderately to steeply southwards. The foliation is commonly parallel to compositional layering in the Zhangbaling Group, and small intrafolial rootless folds with stretching-lineation-parallel axes are common. The mineral elongation lineation on the dominant NE-striking foliation has a variable trend and plunge. The lineation in the northern and southern sections mostly trends NE–SW to nearly E–W and plunges gently to moderately; in the central section it trends NW–SE to nearly E–W and plunges gently to moderately (Fig. 7b). The NE–SW striking foliation and lineations in the Zhangbaling Group exhibit either sinistral or dextral shear sense, but a sinistral shear sense is more common for the steep fabrics. Microscope observations reveal that competent rocks of the Zhangbaling Group in this area have been mostly deformed into protomylonite, mylonite, and locally ultramylonite, whereas the incompetent Zhangbaling schists show only weak mylonitization. Quartz in the mylonites shows widespread dynamic recrystallization by BLG + SGR, whereas feldspar is elongate, shows undulose extinction, and locally contains brittle fracturing without dynamic recrystallization. Muscovite in the mylonites occurs as oriented porphyroclastic grains and as fine matrix grains. These microstructures in the mylonites suggest a deformation temperature of 350–400 °C. To understand the deformation of the Zhangbaling Group in detail, we used a U-stage to measure the quartz c-axes for 10 mylonite samples. Their quartz c-axis plots (Fig. 8) are dominated by near-periphery, double point maxima, indicating dominantly basal plane slip (Passchier and Trouw, 2005). A few of the plots show evidence for weak prism plane slip during quartz deformation. The quartz c-axis patterns demonstrate that deformation occurred under low-grade (greenschist facies) conditions (Passchier and Trouw, 2005), which is consistent with the deformation temperatures estimated from recrystallization mechanisms. The asymmetry of the c-axis plots with respect to the foliation (XY plane) indicates an either sinistral or dextral shear sense (Fig. 8). A progression from ductile to brittle–ductile deformation can be also recognized in the Zhangbaling Group. Evidence for the brittle–ductile deformation is provided by numerous quartz-filled tension gashes, kink bands, kink folds, and crenulation lineations on schistosity planes (Fig. 4g). All of these features are approximately perpendicular to the earlier lineation. The ductile and brittle–ductile deformation show the same shear sense at each locality. Field observations (Fig. 4h), and the orientation of foliation (Fig. 7a, b), demonstrate that the foliation, together with the transposed layering in the Zhangbaling Group, was deformed into E- to NE-trending folds by a

Fig. 3. Structural map of the northern Zhangbaling Tan–Lu Fault Zone (see location in Fig. 1). (a) Distribution of lithological units. 40Ar/39Ar muscovite ages from the Zhangbaling Group are from Zhang et al. (2007), zircon U–Pb ages from the Zhangbaling Group are from Zhao et al. (2014a) and ages of Early Cretaceous igneous rocks are from Niu et al. (2002) and Xie et al. (2007). (b) Lower-hemisphere equal-angle stereograms showing the orientations of ductile fabrics, including measurements by Zhang et al. (2007). (c) Representative cross-section.

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Fig. 5. Geology of the southern Zhangbaling TLFZ (see location in Fig. 1). (a) Structural map showing structures and 40Ar/39Ar hornblende and biotite ages from Zhu et al. (2005) and zircon U–Pb ages from Zhao et al. (2014a). (b) Lower-hemisphere equal-angle stereograms showing the orientations of fabrics.

later event. The trends of the fold axes change with the trend of the eastern margin of the Dabie Orogen (Fig. 7a). The brittle–ductile structures were also deformed during this phase of folding.

The Zhangbaling Group is in fault contact with the Yangtze fold-andthrust belt to the east along a west- to north-dipping thrust (Fig. 7d). The marine sedimentary cover in the Yangtze fold-and-thrust belt is

Fig. 4. Outcrop photographs of the Tan–Lu Fault Zone and its marginal structures. (a) Flat-lying mylonite belt in the Zhangbaling Group in the northern Zhangbaling TLFZ (site DT902) with S–C fabrics and two generations of tension gashes indicating top-to-the-south sense of shear. (b) Folded quartz veins in the mylonitized Zhangbaling Group within the northern Zhangbaling TLFZ, indicating top-to-the-south sense of shear (site DT912). (c) Kink folds in the mylonitized Zhangbaling Group in the northern Zhangbaling TLFZ, showing top-to-the-south sense of shear (site DT907). (d) The NE-striking and steeply dipping Late Jurassic sinistral Tan–Lu ductile shear belt exposed in the Feidong Complex of the southern Zhangbaling TLFZ (site DT326). (e) Photograph taken looking down on a NE-trending steep mylonite belt in the Zhangbaling Group, in the Lujiang TLFZ. Quartz vein boundins indicate a sinistral shear sense (site DT931). (f) Photograph taken looking down on a NE-trending steep mylonite belt exposed in the Zhangbaling Group to the southeast of the Dabie Orogen. S-C fabrics indicate a sinistral shear sense (site DT457). (g) The Zhangbaling muscovite schist in the TLFZ to the southeast of the Dabie Orogen (site DT443), showing a crenulation lineation and quartz-filled tension gash perpendicular to a mineral lineation. (h) D2 folding of the Zhangbaling schist in the TLFZ, to the southeast of the Dabie Orogen (site DT448). Their sites are shown in Figs. 3, 5, 6, and 8.

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Fig. 6. Geology of the Lujiang TLFZ (see location in Fig. 1). (a) Structural map showing 40Ar/39Ar muscovite (Chen et al., 2000) and zircon U–Pb ages (Zhao et al., 2014b). (b) Lower-hemisphere equal-angle stereogram showing the orientations of fabrics.

tightly folded along NE–SW to E–W axes that are parallel to the axes of folds in the Zhangbaling Group. These marginal folds are unconformably overlain by Late Triassic to Cretaceous terrestrial sedimentary rocks (Zhu et al., 2009). The high-grade rocks of the HP amphibolite facies Susong Complex of the Dabie Orogen are dominated by muscovite-bearing granitic gneiss, marble, and amphibolite. The complex is thrust over the Zhangbaling Group along a curved, ductile shear belt that dips to the NW to NNW (Fig. 7d). The top-to-the-SE sense of shear on the thrust coincides with the slab extrusion direction of the Dabie Orogen during the exhumation (Hacker et al., 2000; Faure et al., 2003). Our observations show that ductile fabrics in the Susong Complex are dominated by a nearly E–W-striking foliation that dips predominately to the south, and a SE-plunging mineral lineation (Fig. 7c). Intrafolial folds, with axes that are sub-parallel to the mineral lineation, are common within the complex. The fabrics always show top-to-the-NW sense of shear, which Hacker et al. (2000) interpreted as overturned structures that formed in a SE-directed extrusion channel. Our analysis of quartz caxis fabrics in four samples from the Susong Complex (Fig. 8) further confirms the top-to-the-NW sense of shear. The c-axis plots show

both near-periphery and central maxima, suggesting that both basal and prism plane slip systems co-existed during quartz deformation. These c-axis plot patterns indicate N500 °C conditions for deformation of the Susong Complex (Passchier and Trouw, 2005). 3.5. Southwestern margin of the Dabie Orogen We investigated structures along the southwestern margin of the Dabie Orogen to elucidate the structural relationship between the TLFZ and the Dabie Orogen. The Dabie Orogen is in contact with the YZP along a NE-dipping brittle thrust belt (the younger Xiangfan– Guangji Fault; Fig. 9a, d). Wang et al. (2012) interpreted the thrust belt as being synchronous with the Late Jurassic sinistral faulting of the TLFZ. A foreland fold-and-thrust belt in the YZP, south of the Dabie Orogen, folds the Sinian to Middle Triassic marine cover beds into tight NW-trending folds (Wang et al., 2012). Low-grade rocks, also called the Zhangbaling Group, are exposed along the southwestern margin of the Dabie Orogen, i.e. the HP greenschist facies unit. Outcrops of the Zhangbaling Group show similar lithologies to those found along the TLFZ. Zircon from one meta-volcanic rock sample from the

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Fig. 7. Geology of the TLFZ to the southeast of the Dabie orogenic belt (see location in Fig. 1). (a) Structural map showing zircon U–Pb ages (Zhao et al., 2014b). Note the foliation trajectory in the Zhangbaling Group, which is based on numerous measurements of the foliation attitudes. Lower-hemisphere equal-angle stereograms are shown for fabrics within the Zhangbaling Group (b) and within the HP amphibolite facies rocks of the Dabie Orogen (c). (d) Representative cross-section through the northern section.

Zhangbaling Group (Fig. 9a) yielded a LA–ICP–MS U–Pb protolith age of 750 Ma (Zhao et al., 2014b), suggesting that this rock represents the metamorphosed lower cover of the YZP in the Dabie Orogen. Earlier structures of the Xiangfan–Guangji Fault occurs as ~ 2-kmwide ductile shear zone in the Zhangbaling Group along the southwestern edge of the Dabie Orogen (Fig. 9a, d). Most rocks of the Zhangbaling Group in the shear zone are mylonites or protomylonites. The mylonitic foliation dips steeply to the NNE or SSW (Fig. 9b) at typical angles of 50°–80°. The mineral elongation lineation plunges at 5°–30° and trends NW–SE (Fig. 9b). Outcrop shear-sense indicators, including S–C fabrics, rotated feldspar porphyroclasts, and asymmetric folds, demonstrate dextral shearing within the shear zone. Microstructures and quartz caxis fabrics (Fig. 9c) of the Zhangbaling Group in the Xiangfan– Guangji shear zone are similar to those along the TLFZ, indicating deformation temperatures of ~350–400 °C for the shear zone and confirming the occurrence of dextral slip.

In contrast to the steep foliation of the Xiangfan–Guangji shear zone, the Zhangbaling Group located outside the shear zone has a gentle to flat-lying foliation with a variable dip (Fig. 9b, d). The mineral elongation lineation in the Zhangbaling Group outside of the shear zone trends NW–SE, and usually plunges gently to the SE (Fig. 9b). A variety of shear sense indicators in the Zhangbaling Group demonstrate a top-to-theNW sense of shear. We note that the kinematics in the Zhangbaling Group is similar to the crustal exhumation kinematics of the northern parts of the Dabie Orogen (Hacker et al., 2000; Faure et al., 2003), including the Susong Complex just to the north of the Zhangbaling Group (Fig. 9a). 4. 40Ar/39Ar dating To date the deformation of the TLFZ, we collected 10 mylonite samples from the Zhangbaling Group in the TLFZ, to the southeast of the

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Fig. 8. Lower-hemisphere equal-area stereograms showing quartz c-axis fabrics. The upper four plots are for high-grade rocks from the Susong Complex in the Dabie Orogen. The lower 10 plots are for mylonites with a NE- to ENE-striking foliation and NE-plunging lineation in the Tan–Lu Fault Zone to the southeast of the Dabie Orogen. Sample locations are shown, along with their 40Ar/39Ar muscovite ages obtained in this study.

Dabie Orogen, for 40Ar/39Ar muscovite dating (Fig. 8). We collected a further four samples from the HP amphibolite facies Susong Complex (Fig. 8), and four from the Xiangfan–Guangji shear zone (Fig. 9a) for 40 Ar/39Ar muscovite dating, to investigate the structural relationship between the TLFZ and the Dabie Orogen. The muscovite in the samples was separated using standard techniques (Wang, 2006) and irradiated in the B4 position of the Swimming Pool Reactor, located at the Institute of Atomic Energy, Chinese Academy of Sciences, Beijing, China. The monitor used in this study was the 132.7 ± 1.2 Ma Fangshan standard biotite (ZBH-25), which has a potassium content of 7.6%. 40Ar/39Ar analyses of the samples were performed on a mass spectrometer (MM-1200B) in the Ar–Ar Laboratory at the Institute of Geology, Chinese Academy of Geological Sciences, Beijing. Measured isotopic ratios were corrected for mass discrimination, atmospheric Ar, and blank and irradiation-induced mass interference. The correction factors for interfering isotopes produced during irradiation, as determined by analyses of irradiated K2SO4 and CaF4 pure salts, are as follows: (36Ar/37Ar)Ca = 0.000240 ± 0.000015, (40Ar/39Ar)K = 0.004782 ± 0.00034, and (39Ar/37Ar)Ca = 0.000806 ± 0.000040. The blanks of the m/e = 40, m/e = 39, m/e = 37, and m/ e = 36 are less than 6 × 10−15 mol, 4 × 10−16 mol, 8 × 10−17 mol, and 2 × 10−17 mol, respectively. The decay constant used is λ = 5.543 × 10−10 a−1 (Steiger and Jäger, 1977). The 40Ar/39Ar analytical data are listed in Table A.1. Their total gas ages (TGA), plateau ages (Tp),

and inverse isochron ages (Ti) were defined and calculated using ISOPLOTS3.0 (Ludwig, 2003), and obtained age spectra are shown in Fig. 10. All errors are quoted at the 1σ level. 4.1. Dating results Ten muscovite samples were dated from mylonites in the TLFZ, southeast of the Dabie Orogen (Fig. 8). One muscovite sample (DT455) did not yield a plateau age (Fig. 10). Its variable age spectra are indicative of partial resetting caused by a later thermal event. Its weighted mean age of 209.7 ± 2.3 Ma for higher temperature steps, total gas age (TGA) of 192.3 ± 1.9 Ma, and inverse isochron age of 212.3 ± 1.8 Ma (Fig. 10; Table A.1) are not interpreted in this paper. The remaining nine muscovite samples yield plateau ages that are within error of their inverse isochron ages, suggesting that they are reliable. These samples give plateau ages of 235.5 ± 2.4 Ma (DT457), 235.4 ± 1.9 Ma (DT446), 234.9 ± 2.1 Ma (DT437), 234.2 ± 2.3 Ma (DT448), 230.0 ± 2.3 Ma (DT443), 229.5 ± 2.0 Ma (DT454), 225.1 ± 0.6 Ma (DT458), 218.2 ± 0.8 Ma (DT436), and 217.0 ± 2.0 Ma (DT445) (Fig. 10). The nine plateau ages can be treated as two groups, consisting of an older group of 236–230 Ma for six samples and a younger group of 225–217 Ma for three samples. Four muscovite samples, taken from mylonites in the Xiangfan– Guangji ductile shear zone (Fig. 9a), yield plateau and inverse isochron

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Fig. 9. Geology of the Xiangfan–Guangji Fault on the SW margin of the Dabie Orogen, and its relationship to the TLFZ (see location in Fig. 1). (a) Structural map showing the location of zircon U–Pb age (Zhao et al., 2014b) and 40Ar/39Ar muscovite ages from this study. Lithological legend follows Fig. 7. (b) Lower-hemisphere equal-angle stereograms show the orientations of fabrics within the HP greenschist facies unit and the Xiangfan–Guangji shear zone. (c) Quartz c-axis fabrics in shear zone mylonites. (d) Representative cross-section for the XiangfanGuangji Fault.

ages that are consistent with each other (Fig. 10; Table A.1). Their plateau ages of 230.0 ± 0.8 Ma (DT257), 227.6 ± 0.3 Ma (DT297), 226.5 ± 0.8 Ma (DT306), and 223.4 ± 1.8 Ma (DT305) are reliable and geologically meaningful. The four muscovite samples collected from the Susong Complex, in the HP amphibolite facies unit of the Dabie Orogen, close to the TLFZ (Fig. 8), also yield plateau ages that are within error of their inverse isochron ages (Fig. 10; Table A.1). These plateau ages are 219.0 ± 0.7 Ma (DT423), 214.1 ± 0.7 Ma (DT453), 211.4 ± 0.6 Ma (DT442), and

209.8 ± 0.6 Ma (DT426), and are treated as being geologically meaningful. 4.2. Interpretation A closure temperature range of muscovite is 350 ± 50 °C (Dunlap, 1997), but could be up to 450 °C in some cases (Baldwin et al., 1993). The deformation temperature that we estimate from the microstructures and quartz c-axis fabrics (Figs. 8 and 9) of the TLFZ and Xiangfan–Guangji

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shear zone mylonites is 350–400 °C, which is close to the closure temperature of muscovite. Muscovite could recrystallize below its closure temperature in a ductile shear zone (Villa, 1998, 2006) so that deformation could be reliably dated by 40Ar/39Ar ages. We therefore infer that the 40 Ar/39Ar muscovite ages from the TLFZ and the Xiangfan–Guangji shear zone represent the ages of deformation or of immediately postdeformation cooling. The progression from ductile (N 300 °C) to brittle– ductile (b 300 °C) deformation of quartz grains that we observed in the dated mylonites suggests that most parts of the two shear zones cooled to below the muscovite closure temperature at the end of the deformation. This supports the use of muscovite ages to date the deformation of the mylonites.

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Nine muscovite samples from the TLFZ to the southeast of the Dabie Orogen yielded 40Ar/39Ar ages of 236 Ma to 217 Ma (Fig. 8). The older group of 236–230 Ma (six dated samples) is interpreted as representing the timing of deformation of the TLFZ, whereas the younger group of 225–217 Ma (three samples) is interpreted as indicating the time of cooling of the fault zone related to a later event. The two age groups correspond to the two deformation events experienced by the TLFZ to the southeast of the Dabie Orogen. The thrusting, folding and associated exhumation during the younger event could account for the local muscovite cooling ages of 225–217 Ma in the TLFZ. It is also possible that the younger event was associated with local muscovite recrystallization below its closure temperature (Villa, 1998, 2006), and is therefore

Fig. 10. Muscovite age spectra for mylonites from the Tan–Lu and the Xiangfan–Guangji shear zones, and the Susong Complex.

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dated by the younger muscovite ages. As mentioned previously, continental crust of the YZP subducted along the Dabie suture (Feidong Suture in Fig. 1) in the Middle Triassic and was subsequently exhumed in the Late Triassic (Li et al., 1993, 2000; Ames et al., 1996; Rowley et al., 1997; Hacker et al., 2000, 2006; Liu and Xu, 2004; Zheng, 2012). We therefore suggest that the 236–230 Ma (Middle Triassic) deformation of the TLFZ was synchronous with the subduction of continental crust along the Dabie suture, whereas the 225–217 Ma (Late Triassic) deformation coincides with the crust exhumation. We interpret the 230–223 Ma 40Ar/39Ar muscovite ages, which we obtained from the low-grade mylonites of the Xiangfan–Guangji shear zone, to be representative of the age of deformation age of the shear zone. This shear zone bounds the southwestern edge of the Dabie Orogen, which is composed of deeply exhumed crust, and acted as the boundary of lateral exhumation of the Dabie Orogen. The 230–223 Ma ages correspond to the earlier period of exhumation when low-grade rocks of the Dabie Orogen were first transported into the brittle regime (Hacker et al., 2000; Faure et al., 2003). The dating results provide further evidence for the onset of crustal exhumation in the Dabie Orogen at 230 Ma. The Susong Complex in the HP amphibolite facies unit of the Dabie Orogen experienced high-grade metamorphism at temperatures of N500 °C (Hacker et al., 2000, 2006; Zheng, 2012), which is higher than the closure temperature of muscovite. The 219–209 Ma 40Ar/39Ar muscovite ages obtained from the Susong Complex are therefore interpreted as cooling ages that record the time at which the Susong Complex cooled through the muscovite closure temperature during the crustal exhumation at the Dabie Orogen. Our dating results from the Susong Complex and the Xiangfan– Guangji shear zone indicate that subducted crust along the Dabie suture was exhumed between 230 Ma and 209 Ma (Late Triassic). These data support our interpretation that the 236–230 Ma deformation of the TLFZ predates the crustal exhumation at the Dabie Orogen (Liou et al., 2009; Li et al., 2010; Liu et al., 2010), whereas the 225–217 Ma deformation is synchronous with that exhumation. 4.3. Previous 40Ar/39Ar ages Zhang et al. (2007) obtained 40Ar/39Ar ages of 240–229 Ma for six muscovite samples from the low-grade Zhangbaling Group in the northern Zhangbaling TLFZ (Fig. 3a). Based on the correlation of age with grain size fractions within a given sample, they inferred that ductile deformation in the northern Zhangbaling TLFZ occurred at ~240–235 Ma. These data indicate that the ductile deformation in the northern Zhnagbaling TLFZ was also synchronous with the subduction of continental crust along the Dabie and Sulu sutures. Chen et al. (2000) reported a 40Ar/39Ar muscovite plateau age of 238.7 ± 1.2 Ma from the low-grade Zhangbaling Group in the Lujiang TLFZ (Fig. 6a). They interpreted the age as representing sinistral deformation along the TLFZ. This age is consistent with age data obtained both north and south of the Lujiang area, and indicates that ductile deformation along the Lujiang TLFZ occurred during collision of the NCP with the YZP. 5. Syn-collisional marginal deformation The continent–continent collision along the Dabie and Sulu orogens caused shortening deformation in both the NCP and YZP. The syncollisional structures around the TLFZ are therefore important in terms of understanding the origin of the fault zone. 5.1. Yangtze Plate Collision of the NCP and YZP led to the termination of marine sedimentation and to foreland folding and thrusting in the northern YZP (Zhu et al., 2009). The angular unconformity that separates Upper

Triassic terrestrial strata in the northern YZP from the Middle Triassic marine cover (Dong et al., 1994; Faure et al., 1999; Schmid et al., 1999; Zhu et al., 2009) indicates that the foreland shortening deformation during the Indosinian Movement occurred at the end of the Middle Triassic when continental crust was being subducted along the Dabie and Sulu sutures (e.g., Li et al., 1993, 2000; Ames et al., 1996; Rowley et al., 1997; Hacker et al., 2000, 2006; Liu and Xu, 2004; Zheng, 2012). Previous studies, based on interpretations of seismic reflection profiles and borehole data in the Subei basin (Hu and Zhu, 2013), and on field observations of the exposed Sinian to Middle Triassic marine cover (Zhu et al., 2009; Wang et al., 2012; Hu and Zhu, 2013), have shown that the sedimentary cover in the northern YZP was extensively deformed by tight folding and thrusting parallel to the fold axes. The fold axes to the east of the TLFZ trend NE to ENE and their axial planes verge to the SE to ESE, while the associated thrusts dip mostly to the NW to NNW (Fig. 1; Zhu et al., 2009; Hu and Zhu, 2013). The trends of the fold axes east of the TLFZ changes from ENE–WSW to NE–SW as they approach the TLFZ (Fig. 1), which has been interpreted as the effect of drag caused by synchronous sinistral faulting on the TLFZ (Zhu et al., 2009). This interpretation is supported by the absence of such a fold change in strata younger than Middle Triassic (Zhu et al., 2009; Wang et al., 2012; Hu and Zhu, 2013). In contrast, the fold axes south of the Dabie Orogen trend WNW–ESE within a few tens of kilometers of the Dabie Orogen, but become more E–W-trending with increasing distance from the Dabie Orogen (Fig. 1; Zhu et al., 2009; Wang et al., 2012). Arcuate folds around the southeastern corner of the Dabie Orogen have axes that change trend, when moving from west to east, from NW–SE and E– W, to NE–SW (Fig. 1).

5.2. North China Plate Collision of the NCP with the YZP shortened the Mesoproterozoic to early Permian marine cover in the southern NCP (Zhu et al., 2009). Analysis and interpretation of seismic reflection profiles and borehole data from the Hefei basin (Zhao et al., 2000), combined with field observations of the exposed cover (Zhu et al., 2009), have demonstrated that the syn-collisional structures west of the TLFZ that underlie the Hefei basin and expose on its northern margin consist of tight WNWtrending folds and fold-parallel SSW-dipping thrusts (Fig. 1). These structures are parallel to the trend of the Dabie Orogen, and show no evidence for large-scale dragging by the TLFZ. To the west of the TLFZ and NW of the Sulu Orogen, the exposed cover in the area between Tancheng and Jinan is deformed by open NW–SE-trending folds (Fig. 1). South of this fold system, the arcuate Xu–Huai thrust system forms a salient, similar to those observed in the Apennines foredeep (Mariotti and Doglioni, 2000) and Rocky Mountains (Boyer, 1995), in the marine sedimentary cover west of the Sulu Orogen. The thrust system is considered to have developed during collision between the NCP and YZP (Shu et al., 1994; Zhu et al., 2009). It consists of a series of generally west-verging thrusts and tight folds (Shu et al., 1994; Zhu et al., 2009) that contrast with the NNE-directed thrusting to the south and the NE–SW shortening to the north (Fig. 1). For this reason, the Xu– Huai thrust system has been considered as a marker for the original northwestern corner of the Sulu Orogen during the Middle Triassic (Zhu et al., 2009). This implies that the Dabie and Sulu orogens had a primary separation of ~ 250 km during the collision and were further separated by ~150 km of sinistral displacement during Late Jurassic activity on the TLFZ. High-grade basement rocks in the Jiaobei uplift belt of the NCP, which is located to the east of the TLFZ and north of the Sulu Orogen, contain syn-collisional ductile fabrics with a top-to-the-NW sense of shear (Fig. 1; Zhu and Xu, 1994; Faure et al., 1999, 2003). The Neoproterozoic sedimentary cover that is locally preserved in the belt contains tight NE–SW-trending folds (Zhu and Xu, 1994) that are parallel to the Sulu Orogen.

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The drag of syn-collisional structures by the TLFZ is minor on the NCP, especially compared with the large-scale drag seen in the YZP, which suggests that the NCP is more rigid than the YZP. This view is widely accepted, based on the high-grade basement and numerous strong earthquakes in the NCP compared with the low-grade basement and absence of strong earthquakes in the YZP (Wan and Zeng, 2002, and references therein). 6. Genesis and evolution of the Tan–Lu syn-collisional structures Our structural and geochronological data from the TLFZ, together with the style and geometry of marginal structures, provide evidence for the genesis and evolution of the syn-collisional fault zone. The different parts of the TLFZ have evolved in distinct ways, which has led to obvious differences in the preserved structures. 6.1. Zhangbaling TLFZ The flat-lying to gently dipping foliation in the Zhangbaling Group and lower Sinian rocks in the northern Zhnagbaling TLFZ suggests that the two sets of low-grade rocks were deformed by a flat-lying ductile detachment zone with a top-to-the-SSW sense of shear (Fig. 11a). Previous muscovite 40Ar/39Ar dating indicates that the detachment zone

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was active during 240–235 Ma (Zhang et al., 2007), which is coeval with the subduction of continental crust along both the Dabie and Sulu sutures. Amphibolite facies metamorphic rocks in the Feidong Complex of the southern Zhnagbaling TLFZ are locally overlain by the greenschist facies Zhangbaling Group. This demonstrates that the Feidong Complex, which is the lowest part of the YZP cover, originally underlay the Zhangbaling Group. The complex's earlier fabrics, which are preserved between the later sinistral ductile shear belts, were ductilely deformed into ENE- to NE-trending folds during Late Jurassic sinistral shear on the TLFZ (Figs. 5a, 11c). The preserved attitudes of the earlier fabrics suggest that they were originally flat-lying, with similar fabrics to those in the northern Zhnagbaling TLFZ, and thus also represent a ductile detachment zone below the Zhangbaling Group (Fig. 11a). The unmetamorphosed lower Sinian–Ordovician strata in the northern Zhnagbaling TLFZ, which are in contact with the underlying lower Sinian low-grade rocks along a brittle–ductile detachment belt, display tight NE-trending folds that developed in a brittle regime. These folds are part of the Yangtze foreland fold-and-thrust belt that formed at the end of the Middle Triassic (Dong et al., 1994; Faure et al., 1999; Schmid et al., 1999; Zhu et al., 2009) and are therefore synkinematic with the underlying detachment zone. The syn-collisional structures exposed in the Zhnagbaling TLFZ therefore consist of a ductile top-to-the-

Fig. 11. Model of the structural evolution of the Zhangbaling part of the Tan–Lu Fault Zone.

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SSW detachment zone overlain by a fold belt that developed in a brittle regime (Fig. 11a). The detachment zone in the northern Zhnagbaling TLFZ evolved from an earlier ductile to later brittle–ductile deformation, with the same shear sense. The kinematic consistency between the earlier and later structures indicates that they were produced by progressive deformation during one phase of the syn-collisional deformation event. The later brittle–ductile deformation caused the overprinting of steep quartz-filled tension gashes, kink bands, and folds on earlier ductile fabrics (Fig. 11b). Tectonic exhumation and cooling of the shorteninginduced detachment zone during the syn-collisional deformation, similar to a hanging wall of a thrust fault, can account for the evolution from ductile to brittle–ductile deformation. Late Jurassic shear on the TLFZ produced NNE-striking sinistral brittle faults in the Zhangbaling Group along the western edge of the northern Zhnagbaling TLFZ, at the same time as it produced ductile shear belts in the Feidong Complex in the southern Zhnagbaling TLFZ (Fig. 11c). These structures demonstrate that brittle deformation of the Zhangbaling Group coincided with ductile deformation of the Feidong Complex during Late Jurassic slip on the TLFZ. The NE–SW folds in both the Feidong Complex and Zhangbaling Group in the southern Zhnagbaling TLFZ (Fig. 5) were possibly produced during the Late Jurassic faulting. West-dipping normal faults reactivated a NNE-striking fault of the TLFZ during the Cretaceous, displacing the western edge of the Zhnagbaling TLFZ and controlling the development of the Heifei rift basin (Fig. 11d; Zhu et al., 2010; G. Zhu et al., 2012). The footwall rocks of the Zhnagbaling TLFZ were exhumed and intruded by Early Cretaceous granites (Niu et al., 2002; Ma and Xue, 2011). The early Cretaceous intrusion caused enhanced domal exhumation of the northern Zhnagbaling TLFZ (Figs. 3, 11d), which probably accounts for the northward-changing orientation of the mineral lineations, from the original NNE-SSW in the south, to N–S in the central section, and to NNW–SSE in the northern section of the northern Zhnagbaling TLFZ. No outcrops of the Tan–Lu syn-collisional sinistral shear zone are found in the Zhnagbaling TLFZ, but its influence can be documented from the syn-collisional structures described above. The trend of syncollisional folds axes in the brittle upper crust changes from ENE– WSW to NE–SW with increasing proximity to the Zhnagbaling TLFZ (Fig. 1), indicating sinistral drag by the TLFZ. The shear direction of a

mid-crustal ductile detachment zone should be perpendicular to foreland fold axes in the brittle upper crust. However, the top-to-the-SSW shear sense of the mid-crustal detachment zone in the northern Zhnagbaling TLFZ is sub-parallel to the NE–SW fold axes of the upper Sinian–Ordovician cover strata (Fig. 3a), which demonstrates the influence of the NNE-striking TLFZ on both the kinematics of the detachment zone and fold orientation. Existing physical experiments (Tikoff and Peterson, 1998) demonstrate that fold hinges in a transpressional zone rotate parallel to the long axis of the horizontal finite strain ellipse. We therefore infer that the syn-collisional NNE-striking Tan–Lu sinistral shear zone is located immediately west of the Zhnagbaling TLFZ, where it is buried by the Jurassic to Tertiary fill of the Hefei basin (Fig. 3a, c). This implies that the Zhnagbaling TLFZ represents the eastern wall of the syn-collisional sinistral TLFZ (Fig. 11a) rather than an attachment zone overlying the Tan–Lu sinistral ductile shear zone as proposed by Zhang et al. (2007). This interpretation is compatible with observations of the stratigraphic succession and the gradual decrease in metamorphic grade from the Zhangbaling Group to the Ordovician strata in the northern Zhnagbaling TLFZ. The syn-collisional folds and thrusts in the YZP, east of the TLFZ, verge predominantly to the SE (Zhu et al., 2009; Hu and Zhu, 2013), suggesting that the continent–continent collision at the Sulu Orogen exerted a SE-ward push, which caused a top-to-the-SE sense of shear along the mid-crustal detachment zone (Fig. 12). The synchronous sinistral faulting along the NNE-striking TLFZ dragged nearby uppercrustal folds into a NE–SW trend and re-oriented the shear sense of the TLFZ-proximal detachment zone towards top-to-the-SSW due to lateral boubary role of the TLFZ in kinematics of the detachment zone (Fig. 12). 6.2. Lujiang TLFZ Fabrics in the Zhangbaling Group in the Lujiang TLFZ demonstrate the presence of the NNE-striking, steeply SE-dipping Tan–Lu sinistral ductile shear zone (Fig. 6). The 40Ar/39Ar muscovite age of 239 Ma obtained by Chen et al. (2000) indicates that the sinistral shear zone formed during collision between the NCP and YZP, and was coeval with slip on the detachment zone that is exposed in the northern Zhnagbaling TLFZ. To the east, the ductile shear zone is in contact with unmetamorphosed, dominantly Paleozoic marine cover rocks (Fig. 6a)

Fig. 12. Block diagram showing the syn-subduction Tan–Lu Fault and its marginal structures.

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that are different from the sedimentary succession observed in the detachment zone and the overlying fold belt of the northern Zhnagbaling TLFZ. It is therefore suggested that the TLFZ acted as a sinistral fault during collision of the NCP with the YZP. 6.3. Southeastern margin of the Dabie Orogen Two phases of deformation can be recognized in the Zhangbaling Group to the southeast of the Dabie Orogen. The first phase (D1) produced a mylonitic foliation or schistosity, a mineral elongation lineation, and intrafolial or rootless folds in a ductile regime, as well as brittle– ductile structures including quartz-filled tension gashes, kink bands, and kink folds. The second phase of deformation (D2) led to folding and bending of the foliation, re-orientation of the D1 lineation, and the development of thrusts flanking the exposed Zhangbaling Group (Fig. 7a, b, d). Our 40Ar/39Ar muscovite ages indicate that D1 occurred during 236–230 Ma and was coeval with the subduction of continental crust along the Dabie suture; D2 occurred during 225–217 Ma and was synchronous with crustal exhumation. Based on the predominantly NE–SW-trending mineral lineation, the commonly sinistral sense of shear, and the steep mylonitic foliation in the Zhangbaling Group, as well as the marginal structures on the YZP side and the evidence for synchronous sinistral faulting in the Lujiang TLFZ, we consider that the first phase of deformation resulted from

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sinistral ductile shearing along the steeply SE-dipping TLFZ (Fig. 13a). The progressive exhumation that was associated with the sinistral shearing caused later brittle–ductile deformation (Fig. 13b). Several aspects of the second phase of structures in the Zhangbaling Group, including the boundary thrusts, suggest that they resulted from SE-directed crust slab extrusion in the Dabie Orogen during the exhumation (Fig. 7a, b, d). Our structural and geochronological data show kinematic and temporal consistency between the slab extrusion in the Dabie Orogen and the D2 event in the TLFZ. During D2, the high-grade rocks of the Dabie Orogen were extruded SE-wards over the low-grade Zhangbaling Group, which were in turn thrust over unmetamorphosed Cambrian–Ordovician strata. The Dabie Orogen boundary and the corresponding curvature of the Zhangbaling Group deformation belt clearly defines an arcuate thrust belt (Fig. 7a). Fold axes in the marine cover to the east of the Zhangbaling Group maintain approximate parallelism change with the arcuate thrust belt, indicating that they were influenced by the SE-directed thrusting (Fig. 7a). Bending of the central section of the exposed Zhangbaling Group, into an ENE-WSW orientation, resulted in approximately E–W-trending D2 folds and re-orientation of the D1 foliation and lineation (Fig. 7a, b). NE-trending D2 folds predominate to the north and south of the central section of the Zhangbaling Group, and the change is reflected in variable attitudes of the D1 foliation and lineation. Outer layer stretching and inner layer flattening (Ramsay and Huber, 1987) during the D2 folding, especially within the less competent

Fig. 13. Model of the structural evolution of the Tan–Lu Fault Zone on the southeastern margin of the Dabie Orogen.

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schist, could have further scattered the attitudes of the D1 lineation (Fig. 13c). It is demonstrated therefore that the Tan-Lu sinistral shear belt (D1) just to the southeast of the Dabie Orogen was folded and bended during the D2 event, leading to the coexistence of an either sinistral or dextral shear sense in the folded shear belt. 7. Discussion and conclusions 7.1. Summary of the syn-collisional TLFZ Structural and geochronological evidence suggests that the TLFZ initiated as a sinistral fault zone at 240–230 Ma (Middle Triassic), when continental crust was subducted along the Dabie and Sulu sutures during collision between the NCP and YZP. During the Late Triassic, the SEdirected extrusion of the subducting crust slab at the Dabie Orogen imposed a NW–SE contractional overprint on the TLFZ to the southeast of the Dabie Orogen. The syn-collisional Tan–Lu ductile shear zone marks the western edge of the YZP, and slip at the plate boundary resulted in ductile deformation and up to middle-amphibolite facies metamorphism in the lower cover of the YZP. Syn-collisional folding and thrusting in the brittle upper crust, and the kinematics of the mid-crustal ductile detachment zone in the nearby YZP, were strongly affected by the TLFZ, and provide further evidence for the fault zone's sinistral kinematics. In contrast, the syn-collisional folds and thrusts in the NCP west of the TLFZ are parallel to the Dabie Orogen and shown no evidence for large-scale dragging phenomena related to the TLFZ. Comparison of the marginal structures on both sides of the TLFZ indicates that the NCP is rigid compared with the soft YZP. As proposed by Zhu et al. (2009), the location and geometry of the arcuate Xu–Huai thrust system suggest that the Dabie and Sulu orogens were separated by ~250 km along the sinistral TLFZ during the continent–continent collision. 7.2. Comments on previous models The YZP has been shown to have subducted beneath the NCP along both the Dabie and Sulu sutures (Ames et al., 1996; Rowley et al., 1997; Hacker et al., 2000, 2006; Faure et al., 2003), and the initiation of the TLFZ pre-dated the crustal exhumation of the two orogens. These observations are inconsistent with the syn-exhumation transform fault model (Okay and Şengör, 1992) in which the sinistral TLFZ transferred the exhumation motion between the opposite-polarity Dabie and Sulu orogens (Fig. 2a). The syn-collisional TLFZ was treated as a thrust zone in the rotated suture line model (Fig. 2d; Gilder et al., 1999) and in the postcollisional orocline model (Fig. 2e; Wang et al., 2003). However, the sinistral nature of the syn-collisional TLFZ and its marginal structures are not consistent with both of these models. The tear fault model proposed by Li (1994) suggested that the upper crust of the YZP detached from its lower crust and was thrust over the NCP for N 400 km during collision along the Sulu Orogen, and that this led to synchronous tear faulting along the TLFZ (Fig. 2c). This model has difficulty in explaining the large-scale exhumation of YZP-affinity UHP and HP rocks in the Sulu Orogen (Fig. 1; Hacker et al., 2006; Zheng, 2012; Zhou et al., 2012) and the tectonic evolution of the Sulu Orogen from continental subduction to exhumation. A deep reflection seismic profile across the Sulu Orogen (Yang, 2002, 2009) does not show such wedge structures. According to the indenter boundary model (Yin and Nie, 1993), a promontory on the YZP indented the NCP during collision at the Sulu Orogen, pushing the Sulu Orogen northward by ~ 400 km (Fig. 2b). The indentation was bounded by the sinistral TLFZ to the west, and the dextral Honam shear zone on the Korean Peninsula to the east. It would be expected in the indenter boundary model that more intense collision at the Sulu Orogen would lead to a larger scale of the Sulu Orogen and more intense shortening of the NCP to the north in comparison with the Dabie Orogen. However, the Sulu Orogen is not of a larger

scale than the Dabie Orogen, nor does it contain a greater volume of UHP rocks than the Dabie Orogen (Fig. 1). Furthermore, the NCP to the north of the Sulu Orogen does not show more intense shortening than its counterpart to the north of the Dabie Orogen (Zhu and Xu, 1994; Faure et al., 2003; Zhu et al., 2009). Overall, the syn-collisional structures are incompatible with the 400 km (in the indenter boundary model) or 250 km of shortening (this study) required by the model. The marginal structures on the NCP west of the TLFZ exhibit no largescale dragging or bending phenomena, which differs from the arcuate structures that are typically found along the indented margins of other indented continents, such as the Asian and European plates of the Himalayan and Alpine orogens, respectively (Rosenberg et al., 2007; England and Houseman, 1986; Houseman and England, 1986, 1993; Dumont et al., 2011). As previously mentioned, the distribution of marginal structures, metamorphic grade of basement, and earthquake activity (Wan and Zeng, 2002) all demonstrate that the NCP is more rigid than the YZP, and thus could not have been indented by 400 km by the softer YZP, as proposed by Yin and Nie (1993). The syn-subduction transform fault model (Fig. 2f) regarded the TLFZ as a product of vertical, sinistral tearing of the subducted YZP due to differential subduction velocities (Zhu et al., 2009). The Dabie and Sulu orogens were sinistrally offset by the tearing, and the NCP north of the Sulu Orogen was pushed northwards for ~250 km in a similar way to the indentation boundary model. The same factors that argue against the indenter boundary model also hinder the syn-subduction transform fault model. 7.3. Indentation-induced continent tearing model Here, we propose an indentation-induced continent tearing model to account for the origin, nature, geometry, and kinematic history of the TLFZ and its marginal structures (Fig. 14). The shape of the present southern margin of the NCP is considered to represent its original geometry, with a promontory in front of the future Dabie Orogen and a NNEtrending lateral boundary at the location of the future TLFZ (Fig. 14a). A similar configuration has been inferred for the Adriatic upper plate in the European Alps (Channell et al., 1979; Malusà et al., 2015). The NCP promontory behaved as a rigid indenter and resulted in pre-collisional vertical tearing of the closing Paleo-Tethyan oceanic plate along the lateral boundary. Once the YZP collided with the indenter, the vertical tear in the oceanic plate propagated into the passive YZP (Fig. 14b, c). The eastern boundary of the NCP promontory (the site of the TLFZ) underwent a transition from oblique subduction of the oceanic plate to oblique collision with the YZP continental crust. We infer that subduction of the YZP continental crust along the Dabie suture lasted until continental subduction was complete at the Sulu suture. The indentation was accommodated by greater continental subduction along the Dabie suture rather than by extreme shortening in the foreland south of the Dabie Orogen. The foreland shortening of the YZP passive margin began first in the region to the south of the Dabie Orogen, then spread along the TLFZ to the crust south of the Sulu Orogen (Fig. 14). Following break-off of the oceanic slab (Hacker et al., 2000), the SE-directed crust extrusion of the Dabie Orogen during the exhumation stage overprinted the shortening deformation of the TLFZ to the southeast of the Dabie Orogen (Fig. 14d). The nature and timing of activity on the syn-collisional TLFZ are consistent with the continent tearing model. The arcuate marginal structures of the YZP, around the intersection of the Dabie Orogen and TLFZ (Figs. 1 and 14), are consistent with indentation of the YZP by the NCP. Apart from the arcuate Xu–Huai thrust system that resulted from the corner collision of the Sulu Orogen (Fig. 14a; Zhu et al., 2009), the absence of large-scale dragging or bending structures in the NCP, west of the TLFZ, is consistent with its role as an overriding rigid indenter. Previous dating results show that the Zhangbaling Group and Feidong Complex in the TLFZ belong to the lowest part of the YZP cover (Zhao et al., 2014a, 2014b). However, they experienced greenschist to

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Fig. 14. Indentation-induced continent tearing model for the origin of the Tan–Lu Fault Zone.

amphibolite facies metamorphism during the syn-collisional faulting along the TLFZ, suggesting a higher temperature condition probably as a thermo-mechanical consequence of vertical tearing of the YZP lithosphere along the fault zone. Syn-collisional magmatism along the TLFZ, a possible consequence of the YZP tearing, has not been reported from previous studies, but it was also absent in the Dabie and Sulu orogens as well as the overriding NCP (Leech and Webb, 2013; Wu and Zheng, 2013). Syn-tearing subduction could have led to long-distance, low-angle subduction of the YZP underneath the NCP indenter (Fig. 14c). Geochemical studies and zircon U–Pb dating of Jurassic–Cretaceous igneous rocks (Zhang et al., 2002; Zhang and Sun, 2002; W. L. Xu et al., 2004; Xu et al., 2005, 2009; Y. G. Xu et al., 2004; Yang et al., 2010, 2012; Li et al., 2014) have recorded the signature of subducted YZP on the western side of the TLFZ (Fig. 1). The YZP signature, which includes an EMIItype isotopic signature for lithospheric mantle and 700–800 Ma zircon ages (Zheng, 2012), extends northwards to rocks that are located west of the present Sulu suture (Wulian Suture), about 420 km north of the Dabie suture (Feidong Suture). Similar studies have also provided evidence for subducted YZP between the Sulu Orogen and Bohai Bay, on the eastern side of the TLFZ (Miao et al., 1997; Zhou et al., 2003; Liu et al., 2009; Zhang et al., 2010). Our structural data, especially from the Lujiang TLFZ (Fig. 6b), demonstrate that the syn-collisional TLFZ dips steeply to the southeast (Figs. 11 and 13). Geophysical data have shown that the TLFZ cuts across the entire lithosphere of the NCP almost vertically (Yang, 2002, 2009; Li et al., 2012; Zhao et al., 2012). These suggest that the subducted YZP underneath the NCP west of the TLFZ was from the subduction along the Dabie suture rather than the oblique

subduction of the YZP along the TLFZ. The long-distance, low-angle subduction of the YZP beneath the NCP indenter is in favor of our continent tearing model. Long-distance subduction of continental lithosphere has been recognized in other collision belts, including the India–Eurasia collision, where the Indian plate has subducted beneath the Eurasian plate for about 500–700 km north of the suture (Jiménez-Munt et al., 2008; Li et al., 2008; He et al., 2010; Nunn et al., 2014). As demonstrated by Doglioni et al. (2007), the low-angle subduction is in accordance with NNE-directed motion of both the oceanic plate and YZP as well as extensive outcrops of UHP rocks in the Dabie Orogen. The collision of the NCP with the YZP at the Dabie Orogen is supposed to have occurred before collision at the Sulu Orogen in our continent tearing model (Fig. 14a, b). Although the Dabie and Sulu orogens show similar exhumation kinematics (Fig. 1; Leech and Webb, 2013), recent, precise dating of zircon, titanite and garnet, combined with their mineral inclusions, trace element features and Lu-Hf compositions, reveals that subduction-related prograde HP to UHP metamorphism, prior to the peak UHP metamorphism, occurred during 257–242 Ma at the Dabie suture (Wawrzenitz et al., 2006; Cheng et al., 2010; Gao et al., 2011; Liu et al., 2011; Zheng, 2012; Wu and Zheng, 2013) and 247–244 Ma at the Sulu suture (Liu and Xu, 2004; D. Y. Liu et al., 2006; F. L. Liu et al., 2006; Zheng et al., 2009), suggesting that the YZP subduction began as much as 10 Ma earlier at the Dabie suture than at the Sulu suture (Leech and Webb, 2013). The dating results show that retrograde metamorphism associated with exhumation happened during 220–214 Ma at the Dabie Orogen and 219–202 Ma at the Sulu Orogen (Leech and Webb, 2013, and references therein), indicating longer duration of the exhumation at the Sulu Orogen. These different

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subduction and exhumation histories for the Dabie and Sulu orogens support our indentation proposal. The indentation-induced continent tearing model provides the most reasonable explanation for the abrupt termination of the syn-collisional TLFZ at the southeastern corner of the Dabie Orogen (Fig. 14). The arcuate Xu–Huai thrust system (Fig. 1) suggests that the Dabie and Sulu orogens were only separated along the TLFZ by ~250 km during the collision. The TLFZ was reactivated as an intracontinental sinistral fault during the Late Jurassic (Wang, 2006; Zhu et al., 2010), when it propagated northwards into Northeast China (Fig. 1) and offset both the northern NCP boundary and the Dabie–Sulu belts by ~ 150 km (Zhu et al., 2009, 2010). The Late Jurassic sinistral faulting dissipated southwards along the eastern margin of the Dabie Orogen and was transferred into thrusting along the southern edge of the Dabie Orogen (Xu et al., 1987; Wang et al., 2012). This history provides a satisfactory explanation for the apparent incompatibility between the ~ 400 km displacement of the Dabie–Sulu orogens and ~ 150 km displacement of the northern NCP boundary. Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.earscirev.2015.11.003. Acknowledgments This study was funded by the National Natural Science Foundation of China (grant numbers: 41472186, 41072162 and 91414301). Chen Wen and Zhang Yan at the Ar–Ar Laboratory, Institute of Geology, Chinese Academy of Geological Sciences provided support and advice during 40 Ar/39Ar analyses. We are grateful to Carlo Doglioni, Marco G. Malusà, Laura Webb for their detailed, constructive reviews, which improved the paper greatly. References Agostini, S., Doglioni, C., Innocenti, F., Manetti, P., Tonarini, S., 2010. On the geodynamics of the Aegean rift. Tectonophysics 488, 7–21. Ames, L., Zhou, G., Xiong, B., 1996. Geochronology and isotopic character of ultrahighpressure metamorphism with implications for collision of the Sino-Korean and Yangtze cratons, central China. Tectonics 15 (2), 472–489. Baldwin, S.L., Lister, G.S., Hill, E., Foster, D.A., McDougall, I., 1993. Thermochronologic constraints on the tectonic evolution of active metamorphic core complexes, D'entrecasteaux islands, Papua New Guinea. Tectonics 12 (3), 611–628. Boyer, S.E., 1995. Sedimentary basin taper as a factor controlling the geometry and advance of thrust belts. Am. J. Sci. 295, 1220–1254. Chang, E.Z., 1996. Collision orogen between north and south China and its eastern extension in the Korean Peninsula. J. SE Asian Earth Sci. 13 (3), 267–277. Channell, J.E.T., d'Argenio, B., Horvath, F., 1979. Adria, the African promontory, in Mesozoic Mediterranean palaeogeography. Earth Sci. Rev. 15 (3), 213–292. Chen, X.H., Wang, X.F., Zhang, Q., Chen, B.L., Chen, Z.L., Harrison, T.M., Yin, A., 2000. Geochronologic study on the formation and evolution of Tan-Lu fault. J. Changchun Uni. Sci. Technol. 30, 215–220 (in Chinese with English abstract). Cheng, H., DuFrane, S.A., Vervoort, J.D., Nakamura, E., Zheng, Y.F., Zhou, Z., 2010. Protracted oceanic subduction prior to continental subduction: new Lu–Hf and Sm– Nd geochronology of oceanic-type high-pressure eclogites in the western Dabie orogeny. Am. Mineral. 95, 1214–1223. Ciancaleoni, L., Marquer, D., 2008. Late Oligocene to early Miocene lateral extrusion at the eastern border of the Lepontine dome of the central Alps (Bergell and Insubric areas, eastern central Alps). Tectonics 27 (4), TC4008. http://dx.doi.org/10.1029/ 2007TC002196. de Sigoyer, J., Vanderhaeghe, O., Duchêne, S., Billerot, A., 2014. Generation and emplacement of Triassic granitoids within the Songpan Ganze accretionary-orogenic wedge in a context of slab retreat accommodated by tear faulting, eastern Tibetan plateau, China. J. Asia Earth Sci. 88, 192–216. Doglioni, C., Mongelli, F., Pieri, P., 1994. The Puglia uplift (SE-Italy): an anomaly in the foreland of the Apenninic subduction due to buckling of a thick continental lithosphere. Tectonics 13 (5), 1309–1321. Doglioni, C., Innocenti, F., Mariotti, G., 2001. Why Mt. Etna? Terra Nova 13 (1), 25–31. Doglioni, C., Carminati, E., Cuffaro, M., Scrocca, D., 2007. Subduction kinematics and dynamic constraints. Earth Sci. Rev. 83. Dong, S.W., Fang, J.S., Li, Y., Zhu, H.J., 1994. Middle Triassic-Middle Jurassic sedimentary facies and Indosinian movement in the Lower Yangtze region. Geogr. Rev. 40 (2), 111–119 (in Chinese with English abstract). D'Orazio, M., Innocenti, F., Tonarini, S., Doglioni, C., 2007. Carbonatites in a subduction system: The Pleistocene alvikites from Mt. Vulture (southern Italy). Lithos 98. Dumont, T., Simon-Labric, T., Authemayou, C., Heymes, T., 2011. Lateral termination of the north‐directed Alpine orogeny and onset of westward escape in the Western Alpine

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