Late Permian–early Middle Triassic back-arc basin development in West Qinling, China

Journal of Asian Earth Sciences 87 (2014) 116–129

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Late Permian–early Middle Triassic back-arc basin development in West Qinling, China Lin Li a,⇑, Qingren Meng a, Alex Pullen b,c, Carmala N. Garzione b, Guoli Wu a, Yanling Wang b, Shouxian Ma a, Liang Duan a a b c

Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China Department of Earth and Environmental Sciences, University of Rochester, Rochester, NY 14627, USA Department of Geoscience, University of Arizona, Tucson, AZ 85721, USA

a r t i c l e

i n f o

Article history: Received 21 September 2013 Received in revised form 17 February 2014 Accepted 20 February 2014 Available online 5 March 2014 Keywords: West Qinling Triassic Paleo-Tethys Deep-marine sedimentation Back-arc rift basin Rollback tectonics

a b s t r a c t The Late Permian–early Middle Triassic strata of the northern West Qinling area, northeastern Tibetan Plateau, are composed of sediment gravity flow deposits. Detailed sedimentary facies analysis indicates these strata were deposited in three successive deep-marine environments. The Late Permian–early Early Triassic strata of the Maomaolong Formation and the lowest part of the Longwuhe Formation define a NW–SE trending proximal slope environment. Facies of the Early Triassic strata composing the middle and upper Longwuhe Formation are consistent with deposition in a base-of-slope apron environment, whereas facies of the Middle Triassic Anisian age Gulangdi Formation are more closely associated with a base-of-slope fan depositional environment. The lithofacies and the spatial–temporal changes in paleocurrent data from these strata suggest the opening of a continental margin back-arc basin system during Late Permian to early Middle Triassic time in the northern West Qinling. U–Pb zircon ages for geochemically varied igneous rocks with diabasic through granitic compositions intruded into these deep-marine strata range from 250 to 234 Ma. These observations are consistent with extensional back-arc basin development and rifting between the Permian–Triassic Eastern Kunlun arc and North China block during the continent–continent collision and underthrusting of the South China block northward beneath the Qinling terrane of the North China block. Deep-marine sedimentation ended in the northern West Qinling by the Middle Triassic Ladinian age, but started in the southern West Qinling and Songpan-Ganzi to the south. We attribute these observations to southward directed rollback of Paleo-Tethys oceanic lithosphere, continued attenuation of the West Qinling on the upper plate, local post-rift isostatic compensation in the northern West Qinling area, and continued opening of a back-arc basin in the southern West Qinling and Songpan-Ganzi. Rollback and back-arc basin development during Late Permian to early Middle Triassic time in the West Qinling area explains: the truncated map pattern of the Eastern Kunlun arc, the age difference of deep-marine sediment gravity flow deposits between the Late Permian–early Middle Triassic northern West Qinling and the late Middle Triassic–Late Triassic southern West Qinling and Songpan-Ganzi, and the discontinuous trace of ophiolitic rocks associated with the Anyemaqen-Kunlun suture. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction Rollback or retreat of oceanic lithosphere and back-arc basin development adjacent to continent–continent and arc–continent collision zones has been pervasive in late Cenozoic global tectonics. Examples include: the Western Mediterranean (Lonergan and White, 1997; Malinverno and Ryan, 1986; Royden, 1993); the Hel⇑ Corresponding author. Present address: Department of Earth and Environmental Sciences, University of Rochester, Rochester, NY 14627, USA. E-mail address: [email protected] (L. Li). http://dx.doi.org/10.1016/j.jseaes.2014.02.021 1367-9120/Ó 2014 Elsevier Ltd. All rights reserved.

lenic subduction system of the Eastern Mediterranean (Brun and Sokoutis, 2010; Jolivet and Faccenna, 2000; Le Pichon and Angelier, 1979; Meulenkamp et al., 1988); and the Andaman-Java-SumatraBandit subduction system (Kennett and Cummins, 2005; Richards et al., 2007; Spakman and Hall, 2010). Many factors influence oceanic rollback and back-arc basin develop, including: the horizontal velocity of the subducting plate; the velocity of the upper plate; the velocity of subduction; the velocity of vertical sinking of the slab; slab pull force; and mantle forces acting on the subducting slab. In collisional settings mentioned above, rollback initiated when negatively buoyant oceanic lithosphere adjacent to colliding

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continental or arc-continental landmasses continues to sink at a rate greater than that of the overall rate of convergence between the two plates (Dewey, 1980; Garfunkel et al., 1986; Heuret and Lallemand, 2005; Jolivet and Faccenna, 2000; Molnar and Atwater, 1978; Nur et al., 1993; Royden, 1993; Schellart, 2008). In spite of the pervasiveness of oceanic rollback in modern global tectonic, this geodynamic process is less widely invoked in models describing ancient subduction systems, especially in close proximity to collisional orogens. The Permian–Early Jurassic evolution of the Paleo-Tethys Ocean remains widely debated, most notably regarding the nature of its closure (Chen et al., 1987; Gu, 1994; Nie et al., 1994; Sengör, 1984; Zhou and Graham, 1996). The Songpan-Ganzi complex of the Tibetan Plateau, considered to be ‘the remnant’ of the Paleo-Tethys, consists of one of the most areal extensive and near continuous exposures of marine rocks on Earth with a depositional thickness of 8 km and >370,000 km2 (Fig. 1A) (Nie et al., 1994). The Songpan-Ganzi complex is composed mostly of Middle–Late Triassic sediment gravity flow deposits that have been extensively intruded by Late Triassic–Early Jurassic granitoids (Gradstein et al., 2004; Roger et al., 2004; Weislogel, 2008; Xiao et al., 2007; Zhang et al., 2006). Here we show that the marine rocks in the area of West Qinling, typically mapped as contiguous with the Middle– Late Triassic Songpan-Ganzi marine deposits (Pan et al., 2004; Weislogel et al., 2010), include Late Permian–early Middle Triassic rocks associated with formation of a E–W trending rift between the Eastern Kunlun arc in the west and East Qinling-Dabie orogen in the east (Fig. 1A).

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The Anyemaqen-Kunlun suture and Mianlue suture, with their traces defined by ophiolite fragments, juxtapose the Permian–Triassic Eastern Kunlun arc to the Songpan-Ganzi terrane in the west (Bian et al., 2004; Konstantinovskaia et al., 2003) and the East Qinling orogen to the South China block in the east (Dong et al., 2011; Zhang et al., 1995), respectively (Fig. 1A). It has been suggested that the Anyemaqen-Kunlun suture connects with the Mianlue suture, which together constitutes a huge E–W trending suture zone representing traces of the closure of the Paleo-Tethys Ocean during the early Mesozoic (Meng and Zhang, 2000). However, in the area of the central West Qinling the suture is widely inferred by connecting the Anyemaqen ophiolite in the west with the Mianlue ophiolite in the east (Fig. 1A; Meng et al., 2005). Pullen et al. (2008) explored the idea that near synchronous continent–continent collisions around the Paleo-Tethys realm, as evidenced by the distribution of Middle Triassic age HP–UHP continental affinity rocks, lead to a decreased rate of convergence between the South China block and the Qiangtang terrane with Eurasia, and initiated rollback of the north dipping Paleo-Tethys ocean slab beneath the Eastern Kunlun arc and North China block. The hypothesis explains the apparent truncated map pattern of the Eastern Kunlun arc to the east and the wide geographic distribution of compositionally varied granitoids intruded into the Triassic sediment gravity flow deposits of the Songpan-Ganzi complex. The aim of this paper is to: (1) illustrate the stacking patterns of deep-marine deposits in the northern margin of the West Qinling by detailed facies and facies association analysis; (2) establish the sedimentary architecture and stratigraphic correlation of

Fig. 1. (A) Subdivisions of main tectonic units of China (modified from Meng and Zhang (2000)). (B) Geological map of the West Qinling. The rectangle boxes in (A) and (B) represent the local enlarged areas in (B) and Fig. 2, respectively. Note the Triassic strata of West Qinling are subdivided into the northern and southern divisions (boundary roughly along the Tongren-Xiahe-Hezuo towns), represented by T1–2 and T1–3, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).

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depositional sequences; (3) provide absolute age control on the timing of sedimentation and magmatism; and (4) better characterize the Permian–Triassic tectonic evolution of the West Qinling area within the framework of the Paleo-Tethys Ocean. We describe an upward deepening Late Permian–early Middle Triassic deepmarine succession intruded by 250–234 Ma rocks with gabbro– granite compositions along the north margin of the West Qinling (Fig. 1B). These observations, coupled with paleocurrent data and additional regional observations, suggest that a continental margin rift and back-arc basin formed in this area between Late Permian and early Middle Triassic time. 2. Geologic setting The West Qinling is located near the center of the Qinling-Qilian-Kunlun central orogenic belt of China, incorporated into much of the northeastern Tibetan Plateau (Fig. 1A). This is considered a zone where five distinct tectonic units meet; these include: North China block, South China block, Songpan-Ganzi terrane, Qaidam terrane and Qilian terrane (Fig. 1A; Feng et al., 2003; Ratschbacher et al., 2003; Zhang et al., 2004). The West Qinling is bounded on the north by the Qinghainanshan-Wushan fault (also known as the West Qinling fault), on the south by the Maqin-Lueyang fault, on the west by the Wahongshan-Wenquan right lateral strike-slip fault, and on the east by the Ningshan left lateral strike-slip fault (Fig. 1; Feng et al., 2003; Meng et al., 2005). The West Qinling,

the Qilian terrane, the Kunlun arc, and the Qaidam terrane accreted to the North China block prior to Late Permian time (Yin and Harrison, 2000). Subduction of a north dipping Paleo-Tethys oceanic slab beneath the Kunlun arc and amalgamated North China block began by at least Early Permian time (Yang et al., 1996), although the Permian–Triassic phase of arc magmatism was superimposed on an early Paleozoic arc with Cambrian–Middle Ordovician marine strata (Huang et al., 1996; Jiang et al., 1992). Through detailed stratigraphic study in the West Qinling area, Yin et al. (1992) predicted that the late Paleozoic–early Mesozoic West Qinling was an intercontinental trumpet-like rift with a westward opening between the North China block and the South China block that closed after the collision of these two blocks by Middle Triassic time. However, Zhou and Graham (1996) came to a different conclusion that took into consideration the relationship between the West Qinling and the Songpan-Ganzi terrane to the south. They concluded that the West Qinling was the northeastern most part of the Songpan-Ganzi remnant ocean basin in Triassic time and that the Songpan-Ganzi sediment gravity flow deposits were mainly derived from the east Dabie-Sulu orogen. Additional work has shown that the detritus of the Songpan-Ganzi sediment gravity flow deposits were shed into the basin from all sides, including from the North China block, South China block, Qinling-Dabie orogen, and Yidun arc complex (Bruguier et al., 1997; Ding et al., 2013; Enkelmann et al., 2007; Weislogel et al., 2010, 2006). Other workers have suggested that the Triassic deep marine

Fig. 2. Geological map of study area. Dots show locations of measured profiles: Longwuhe section (Lw01–24), Galeng section (Gl01–07), and Baizhuang section (Bz01–15). Different colored dots represent different sedimentary stages: dark blue, stage 1; yellow, stage 2; black, stage 3 (see text in Section 4.3). Marked dots are selected measured profiles shown in Fig. 5. Rectangular box shows location of Fig. 6. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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deposits of West Qinling were deposited in a back-arc basin separated from the main Songpan-Ganzi remnant ocean by the Kunlun arc (Yin and Harrison, 2000), however the timing of back-arc basin development in this scenario is poorly understood. Although the above interpretations of the tectonic setting of Triassic West Qinling are all different, a common feature they share is the assumption of the suture characteristic of the Maqin-Lueyang fault (Fig. 1B), which typically delineates the boundary between the West Qinling and the Songpan-Ganzi terrane. However, ophiolites or volcanic rocks, indicative of ophiolite obduction or ocean basin closure, have yet to be widely documented along the Maqin-Lueyang fault zone. Previous workers have argued that the Maqin-Lueyang fault zone connects with the Mianlue suture to the east and the Anyemaqen-Kunlun suture to the west (Meng et al., 2005; Meng and Zhang, 2000). Therefore, the Maqin-Lueyang fault zone is interpreted by some to be the suture between the North China block and Songpan-Ganzi terrane (Meng and Zhang, 2000; Zhang et al., 2003). Based on the lithostratigraphic difference of Triassic strata, we divided the Triassic strata of West Qinling into two subdivisions: the northern division and the southern division (T1–2 and T1–3 respectively in Fig. 1B). It should be noted that these two subdivisions are only stratigraphic, different from previous tectonic divisions to a North Qinling block and a South Qinling block, which are separated by the Shangdan suture zone (Meng and Zhang, 1999, 2000). The detailed stratigraphic investigation for this study was made in the northern division, however igneous samples reported here come from both subdivided areas (Fig. 1B). The study area is located at the central part of the northern West Qinling, within 50 km south of the Qinghainanshan-Wushan

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fault (Fig. 2). To the north of the Qinghainanshan-Wushan fault, there locates some pre-Cambrian basement rocks and the Xunhua Cenozoic basin, which is dominated by terrestrial fluvio-lacustrine deposits (Fig. 2; BGMRQH, 1991). The W–NW trending Gangcha anticlinorium, locally, exposes the Late Permian–early Middle Triassic deep marine strata reported here, which are unconformably covered by Late Triassic terrestrial strata. Triassic granitoid plutons are widely outcropped in the study area (Fig. 2) and adjacent regions (Fig. 1B).

3. Stratigraphy The Late Permian–early Middle Triassic successions in the northern division of the West Qinling are divided into three formations: the Maomaolong Formation, Longwuhe Formation and Gulangdi Formation. These strata consist largely of deep-marine silici- and calci-clastic sediments produced by a variety of sediment gravity flows and slumps with great spatial–temporal variability (Fig. 3). The Maomaolong Formation is mainly composed of greenschist to sub-greenschist facies metamorphic sandstones, siltstones, shales, thinly-bedded carbonates, as well as massive brecciated carbonates and interlayered conglomerates (Fig. 3). In situ fossils denoting a depositional age were not found in this formation. However, Late Carboniferous to Early–Middle Permian fusulinid, coral, and gastropod fossils, such as Schwagerina sp., Parafusulina sp., Uncinunellina cf. wangenheimi (Pander) (BGMRGS, 1972), included as brecciated clasts provide a maximum depositional age of Late Permian time for this formation. In addition, the Maomaolong

Fig. 3. Generalized lithostratigraphic column for the Late Permian–early Middle Triassic deep marine strata of the Longwuhe, Galeng and Baizhuang sections. The Late Permian strata in the Baizhuang section are based on descriptions of the regional geological survey (BGMRGS, 1972). Also shown are the stratigraphic height of selected measured profiles.

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Formation is conformably overlain by Early Triassic strata (Yin et al., 1992). The lower part of the Early Triassic Longwuhe Formation is mainly pebbly sandstones, sandstones, siltstones, slates, thinlybedded carbonates and massive brecciated carbonates; the majority of the upper part is sandstones and slates, with few pebbly sandstones and massive brecciated carbonates (Fig. 3). Several layers of rhyolitic volcanic breccias, andesitic tuff and gabbro have been observed in this formation. Abundant Early Triassic fossils are well preserved in the slates, among which ammonites and bivalves dominated, such as Claraia bittner, Meekoceras sp., Columbitidae cf. columbites sp. (BGMRGS, 1972). The Gulangdi Formation indicates significant lateral lithological change (Fig. 3). In the west Longwuhe section (Fig. 2), thick bedded sandstones, conglomerates, pebbly sandstones are the main facies, whereas both the grain size and bedding thickness decrease towards the east (Galeng section and Baizhuang section). Fossils preserved in slates are mainly ammonites and bivalves: Hollandites sp., Hollandites cf. voiti, Enoploceras sp., Trematoceras sp., which are typical early Middle Triassic fossil associations (BGMRGS, 1972). The Gulangdi Formation conformably overlies the Early Triassic Longwuhe Formation. No marine strata following the Middle Triassic Anisian stage have been recognized in the northern West Qinling subdivision. Subaerially deposited Late Triassic (Rhaetian) volcanic rocks, including andesites and rhyolites, are interbedded with coal-bearing strata exposed over a wide area in the West Qinling area (BGMRQH, 1991). A notable unconformity has been recognized between these Late Triassic terrestrial sediments and the underling early Middle Triassic deep-marine deposits of the Gulangdi Formation (Fig. 2; BGMRQH, 1991).

4. Sedimentology 4.1. Facies and facies association Facies and facies association analysis are carried out for sedimentary environment interpretation, and are based on detailed measurement of three well exposed sections. From west to east, they are the Longwuhe section, Galeng section and Baizhuang section (for location see Fig. 2, for lithostratigraphy see Fig. 3, for facies description see Fig. 4, and for typical measured sections see Fig. 5). The Longwuhe section is located in the downstream gorge of Longwu River (Fig. 2, Lw01–24). This section includes strata from Late Permian to early Middle Triassic, with a total thickness of more than 10 km (Fig. 3). The Galeng section is located along a local highway (Fig. 2, Gl01–07). This section has limited outcrops including only the middle and upper portions of the early Middle Triassic Gulangdi Formation (Fig. 3). The Baizhuang section is also located in a river valley (Fig. 2, Bz01–15) and includes Early to early Middle Triassic Longwuhe Formation and Gulangdi Formation. Given the huge thickness of these deposits, we did not carry out detailed measurements of entire sections. Rather, we chose typical locations (Lw01–24, Gl01–07, Bz01–15) to record detailed characteristics of individual facies. A few of the measured sections are shown in Fig. 5. Five facies associations are identified in the field: Slope facies association (A1–A3), submarine fan channel facies association (B1–B4), submarine fan proximal facies association (C1–C2), submarine fan lobe facies association (D1–D2), and submarine fan fringe/hemipelagic facies association (E1–E2). Each facies association is composed of several individual facies. Fig. 4 lists detailed descriptions of each facies, interpretation of their origins, as well as their stratigraphic representations and typical field appearances.

4.2. Facies architecture 4.2.1. Longwuhe section (Lw01–Lw24) The Late Permian Maomaolong Formation in the Longwuhe section occurs as a NW–SE striking, 1000 m thick strata, sandwiched in the Early Triassic Longwuhe Formation with faults as boundaries (Fig. 2). This formation is mainly comprised of the slope facies association dominated by slump related facies A1, A2 and A3, and hemipelagic facies association dominated by E2 (Fig. 5, Lw01). Breccias (A3) are very widely distributed, with the thickest layer reaching about 300 m. The breccias are made up nearly completely of very poorly sorted (centimeters to meters in size) light gray shallow water carbonates with Late Carboniferous to Early– Middle Permian fusulinid and coral fossils, though mud rip-up clasts and quarts grains can also be observed. The upper part of this formation is much more fine-grained, with mudstones (E2) interbedded with slump related chaotic mudstones (A1). The depositional processes of Early Triassic Longwuhe Formation is multi-phased (Fig. 3). Different facies associations dominate at different stratigraphic heights. The lowest part of this formation reflects continuation of the slump-related slope facies association, mainly A1 and A2, as the dominant deposits. Syndepositional folds (A2) and slump related tabular clasts (A1) are very common and can be used to constrain paleo-slopes. The remaining parts of the Longwuhe Formation are dominated by different submarine fan facies associations. Stratigraphically higher, there are: (1) coarsegrained submarine fan channel facies association dominated by B4 (Fig. 5, Lw06); (2) proximal fan facies association dominated by C1; (3) slump related facies A1 and A2; (4) medium-grained fan lobe facies association dominated by D1 (Fig. 5, Lw12), but facies B4, C1 and A2 also outcrop occasionally; (5) fine-grained hemipelagic facies association dominated by E1 and E2. The upper most potion of the Longwuhe Formation includes a thick layer (50 m) of breccias (A3). The early Middle Triassic Gulangdi Formation is primarily made up of coarse-grained submarine fan channel facies association (B1, B2, B3, and B4), fan proximal facies association (C1) and hemipelagic facies association (E1 and E2) (Fig. 5, Lw 17, 24). There are three layers (several tens to more than a hundred meters thick) of breccias caused by slumps (Lw14, 18, 20). These breccias were previously considered to be clast-supported conglomerates (BGMRGS, 1972). Although the gravels are well-rounded, there are abundant large, angular mudstone and sandstone clasts of tens of centimeters in size that show internal deformation, which suggests these deposits are more likely to be slumped breccias. At least four consecutive channel fill cycles (each more than 200 m thick) were identified in the upper portion of the Gulangdi Formation. An interesting phenomenon to note is the change in breccias through time, not only in the frequency of occurrence, but also the thickness and composition of clasts. Breccias in the Late Permian are very thick (up to 300 m), and the composition is dominated by carbonates. However, in the Triassic, the occurrence of breccia layers decreased, and the single layer thickness also became smaller (no more than 50 m). The most remarkable change is the change in compositions: in Early Triassic, the percentage of carbonates declined, and sandstones, metamorphic rocks and granitoids began to appear and increase; and during early Middle Triassic, the clasts were dominated by well rounded gravels, reworked mudstones and sandstones, with some metamorphic and granitic gravels. 4.2.2. Galeng section (Gl01–Gl07) The Galeng section only outcrops the early Middle Triassic Gulangdi Formation, which has rather uniform sequences. The whole section is comprised of submarine fan lobe facies

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Fig. 4. Summary of the facies of the Late Permian–early Middle Triassic sediment gravity flow deposits in the northern West Qinling. A1–A3, slope facies associations; B1–B4, submarine fan channel facies associations; C1–C2, submarine fan proximal facies associations; D1–D2, submarine fan lobe facies associations; E1–E2, submarine fan fringe or hemipelagic facies associations. (See above-mentioned references for further information).

association dominated by normally graded sandstone (D1) and interbedded sandstone/siltstone and mudstone (D2) (Fig. 5, Gl05). A layer of more than 10 m thick amalgamated pebbly sandstone with scours about 15–30 cm deep and 50–70 cm long

occurs at site Gl03. Within the scours, cross beddings and lag deposits with gravels were observed. This amalgamated pebbly sandstone is interpreted as deposits in the sub-channels of submarine fan lobes.

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Fig. 5. Selected measured profiles in the Longwuhe section (Lw01, 06, 12, 17 and 24), Galeng section (Gl05), and Baizhuang section (Bz03, 06 and 13). See Fig. 2 for profile locations and Fig. 3 for stratigraphic height.

4.2.3. Baizhuang section (Bz01–Bz15) In the Baizhuang section, Late Permian strata was not observed because of the intrusion of Mesozoic granitoid (Fig. 2). However, according to the description of regional geologic map (BGMRGS, 1972) for the type section of the Late Permian Maomaolong Formation, which is outcropped only about 6 km to the southeast of our measured section, the Late Permian strata of this area is dominated by slope facies association A1, A2 and A3, similar as the Late Permian depositions in the Longwuhe section. The lowest part of the Early Triassic Longwuhe Formation is still dominated by slope facies association A1 and A2 (Fig. 5, Bz03), with a few layers of A3. Medium- to thinly-bedded normallygraded sandstones (D1) are commonly observed to be interbedded with this facies association in this section. However, the interiors of the sandstone beds are disturbed and contain exotic materials: carbonates, reworked sandstones (2–15 cm in length), and mud rip-up clasts. Some reworked sandstones are also normally graded, and distinct squeezing traces around those slump clasts are discernible. The remaining parts of this formation are dominated by various submarine fan facies associations, including channel facies association dominated by B4, fan proximal facies association C1 and C2, fan lobe facies association D1 and D2 (Fig. 5, Bz06), and fan fringe facies association E1 and E2. The Longwuhe Formation in both the Longwuhe and Baizhuang sections is composed of similar facies associations, but the sediment grain size is much smaller in the Baizhuang section, and facies associations change more frequently. The early Middle Triassic Gulangdi Formation in this section is nearly all fine-grained deposits, dominated by submarine fan fringe/hemipelagic facies association E1 and E2 (Fig. 5, Bz13).

Slump-related slope facies was also observed in this formation, but are usually only several meters thick and outcrop as lenses.

4.3. Facies evolution Comparing the Longwuhe, Galeng and Baizhuang sections, the Late Permian to early Middle Triassic deep-marine depositions of the northern division of the West Qinling can be divided into three sedimentary stages, each with a distinct stratigraphic architecture (Fig. 3, also see Fig. 9B–D). The first stage is from Late Permian to early Early Triassic time. The depositions of the Longwuhe and Baizhuang sections are both dominated by slope facies association (A1–A3) and submarine fan fringe/hemipelagic facies association (E1, E2) (Figs. 3 and 4). These facies associations represent a submarine slope and/or slope base environment. The second stage includes the remaining Early Triassic time. Both of the Longwuhe and Baizhuang sections are made of various submarine fan facies associations, including fan channel facies association (B1–B4), fan proximal facies association (C1–C2), fan lobe facies association (D1–D2) and fan fringe/hemipelagic facies association (E1–E2). Slump-related facies (A1–A3) occur, but are not common. The grain size difference between these two sections is probably associated with the diverse lithology of source areas. The similar facies associations of the Longwuhe Formation in both sections indicate they were probably deposited in distinct small lobes adjacent to the base of slope. We interpret this as a baseof-slope apron system regionally (Reading and Richards, 1994) or alternatively called coalescing fans (Mattern, 2005).

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for the evolution of stratigraphic architecture (see Section 7.1 and Fig. 9 for discussions).

5. Paleocurrent directions

Fig. 6. Summary of paleocurrent indicator orientation rose diagrams collected from the Longwuhe section. ‘‘n = number’’ means the total number of paleocurrent data collected at one site. See Fig. 2 for abbreviations and legend.

The early Middle Triassic Gulangdi Formation is the third stage. The deposits in the west Longwuhe section are characterized by coarse-grained facies associations (submarine fan channel facies association B1–B4, and proximal fan facies association C1–C2) with poor lateral extent; in the middle Galeng section, medium- to thickly-bedded normally-graded sandstones (submarine fan lobe facies association D1–D2) are widely developed; while the Baizhuang section in the west is nearly completely made up of finegrained facies (submarine fan fringe/hemipelagic facies association E1–E2) (Fig. 3). We interpret the early Middle Triassic Gulangdi Formation, from west to east, to be a base-of-slope fan system. The Longwuhe section represents the channel and proximal fan, the Galeng section as the middle fan lobe, and the Baizhuang section as the distal fan fringe. The stratigraphic architecture change with time can be attributed to several factors, both tectonic or intrinsic, such as the variation of basin shape through time driven by tectonic or intrinsic thermal evolution, or the availability and type of sediment supply controlled tectonic activity or sea level change (Romans et al., 2011). For the northern West Qinling Triassic basin, we suggest that the change of basin shape is the primary controlling factor

Abundant paleocurrent indicators occur throughout the sediment gravity flow deposits of the Longwuhe section, including clast imbrications, flute casts, tabular cross-beddings, imbricated mud rip-up clasts, syn-depositional fold axes and tabular slumps. The latter two categories are slump related, and the orientation of fold axial planar surfaces and slumped clasts can be used to infer paleo-slope direction. Paleocurrent data were not obtained from the Galeng and Baizhuang section due to their fine grain size and absence of clearly defined paleocurrent indicators. Four to ten paleocurrent measurements were made within individual bed as a set, and sets of measurements within a particular site were made whenever possible. A summary of all sites measurements within the Longwuhe section are plotted on rose diagrams in map view on Fig. 6, where ‘‘n’’ represents the number of total measurements for one site. Slump-related paleocurrent indicators, indicating paleo-slope, were mainly observed in the lower part of the Longwuhe section (Fig. 6, site 1, 2, 5). The axes of W–NW verging asymmetrical syn-depositional folds and imbricated slumped clasts at the lowest portion of the Early Triassic strata indicate that there was a northerly paleo-slope in the two most southern sites (Fig. 6, site 1, 2). However, imbricated tabular mud rip-up clasts at the upper portion of the Early Triassic strata demonstrate a southerly paleo-slope (Fig. 6, site 5). During the Late Permian to early Middle Triassic time, the West Qinling region was the southern continental margin of the North China block (Yin and Harrison, 2000; Zhou and Graham, 1996), and the southerly paleo-flow indicators are consistent with this continental margin paleogeographic setting. However, the northerly paleocurrent indicators indicate that there was a northerly opposing paleo-slope that developed on the northern margin of West Qinling during the Early Triassic. It is, thus, inferred that a normal fault-controlled rift basin developed in the northern part of West Qinling during the Early Triassic, most probably extending from the Late Permian through the early Middle Triassic. Additional paleocurrent indicators in the upper portion of the Early Triassic and early Middle Triassic strata (tabular cross-beddings and clast imbrications; sites 3–4, 6–9 in Fig. 6) all give southerly flow directions, indicating that terrestrial clasts from the northern North China block were the main sediment sources to the rift basin that were transported as submarine sediment gravity flows. Detrital zircon provenance studies of the West Qinling area confirmed that the North China block was a dominant sediment source for the West Qinling Triassic strata (Ding et al., 2013; Weislogel et al., 2010). We speculate that the lack of northward paleocurrent indicators during the early Middle Triassic Gulangdi Formation in these more northern sites of West Qinling relates to the observation that the southern side of the rift basin was carbonate platform, and did not have significant sediment supply (see Fig. 9D). This rift model can also explain the changes in breccia compositions (see Section 4.2.1). The pre-Late Permian shallow water carbonate that developed on the West Qinling continental shelf formed a rift shoulder and was the dominant source and location for slumping during the Late Permian and Early Triassic rift basin stage. Breccias of this time are mainly composed of carbonate materials. Increasing amounts of basement metamorphic rocks and granitoids in the early Middle Triassic breccias indicate an unroofing sequence as carbonate shelf rocks were exhausted. The decreasing frequency of breccias can indicate the decreasing activity of the bounding faults of the rift.

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Fig. 7. Concordia plots of U–Pb ages for zircons in igneous rock samples 12AP09, 11, 15, 16, and 17 (for locations, see Fig. 1B). Uncertainties are at the 2r level. Also shown are CL imagines of selected analyzed zircon grains from samples 12AP09, 11, 15 and 16.

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Fig. 8. Late Permian–Late Triassic lithostratigraphic evolution of the northern and southern West Qinling (modified from Meng et al. (2007)). Also shown are the lithostratigraphies of the northern and southern parts of Songpan-Ganzi. See text for detailed interpretations and discussions. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).

6. U–Pb Geochronology Zircon crystals were separated from igneous rock samples using standard mineral separation techniques. They were mounted in epoxy, polished to section the zircon crystals for SEM imaging and to avoid common Pb-loss domains on the outer 3–4 lm of the crystals. Mounts were imaged on a Hitachi 3400N SEM to generate high-resolution BSE and cathodoluminescence images for navigation purposes to avoid inclusions, and metamict and inherited cores. U, Th, and Pb isotopes were measured on a Nu Plasma HR multicollector inductively coupled plasma–mass spectrometer coupled to a Photon Machines Analyte G2 excimer 193 laser following the methods outlined in Gehrels et al. (2008). Ablation of all samples was accomplished using 30 lm spot diameter. 238U, 232 Th, and 208Pb–206Pb were measured using Faraday detectors, whereas 204Pb measured and mass 202 was monitored on ion counters. Common Pb corrections were made by using measured 204 Pb and assuming an initial Pb composition from Stacey and Kramers (1975). Sri Lankan zircon standard (564 Ma) was used to correct for isotope fractionation, whereas R33 zircon standard

(419 Ma; Black et al., 2004) was treated as an unknown and used to monitor the fractionation correction. Uncertainties reported here include random and systematic errors, and are reported at the 2r level. We report ages here as weighted mean ages of multiple analyses and as concordia ages (Ludwig, 2003). 6.1. U–Pb Results U–Pb dating of zircon crystals from intrusive and extrusive igneous rocks in the West Qinling area were used to better constrain the timing of deposition. Zircon crystals separated from sample 12AP09 (Fig. 1B), a diorite intruded into the middle Gulangdi Formation exposed in the Longwuhe section, yield a weighted mean 206Pb/238U age of 241.7 ± 4.9 Ma and concordia age of 241.6 ± 3.9 Ma (Fig. 7). Another sample of diorite (12AP17) intruded into the upper Gulangdi Formation yields a weighted mean age 206Pb/238U age of 234.9 ± 5.7 Ma (2r; Fig. 7). These ages and interpreted intrusive relationship suggest most of the Gulangdi Formation (T2) was deposited prior to the Ladinian.

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Fig. 9. Simplified model for the Early Permian–early Middle Triassic tectono-sedimentary evolution of the West Qinling. Note the southward migration of trench from location B1 to B4 due to rollback. See text for detailed explanations.

Samples 12AP15 and 12AP16 were collected from the southern division of West Qinling (Fig. 1B). 12AP15 is an andesite in a conformable depositional contact with the Middle Triassic strata (BGMRQH, 1973). Zircon crystals from this sample yield a weighted mean age of 241.4 ± 3.6 Ma and concordia age of 240.8 ± 3.3 Ma (Fig. 7). We interpret this 241 Ma to date the timing of deposition of these strata. A diorite sample (12AP16) intruded into the Middle Triassic and in a nonconformable contact with the Late Triassic unit yields a weighted mean 206Pb/238U age 241.5 ± 3.5 Ma and concordia age of 241.5 ± 3.3 Ma (Fig. 7). This U–Pb zircon age with the cross-cutting relationship reported here provides a minimum depositional age for the Middle Triassic rocks in the southern division of West Qinling of 241 Ma. Sample 12AP11, a diabase intruded into the Early Triassic Longwuhe Formation yields a weighted mean 206Pb/238U age of 242.3 ± 3.8 Ma (Fig. 7). Based on the igneous zoning observed in SEM CL images (Fig. 7), concordance of individual analyses, and U/Th ratios, we interpret this age to approximate the timing of crystallization of this pluton. Wang et al. (2010) reported a U–Pb zircon age of 250.1 ± 2.2 Ma for a gabbro in the same area (Fig. 2). Given the similarities in U–Pb zircon ages, lithologies, and location, we speculate that the diabase of sample 12AP11 and gabbro intrusion reported in Wang et al. (2010) are part of poorly documented phase of Early–early Middle Triassic mafic magmatism within the West Qinling area.

7. Discussions 7.1. Tectonic background of the West Qinling during Permian and Triassic Paleocurrent indicators and intrusive mafic rocks suggest the development of a rift basin on the northern edge of the West Qinling area during Late Permian to early Middle Triassic time. The gabbro and diabase compositions, and the observation of adjacent deep-water siliciclastic rocks indicate that the E–W trending rift

basin was initiated in Late Permian time and probably developed into deep water oceanic rift basin by Early Triassic beginning with the emplacement of mafic intrusive rocks into the Early Triassic Longwuhe Formation, and rifting continued until to the early Middle Triassic. This inference is consist with the evolution of the sedimentary architecture: from a Late Permian slope setting dominated by north-directed paleocurrents (Fig. 6, site 1–2), to an Early Triassic base-of-slope apron system with both north- and southdirected paleocurrents (Fig. 6, site 3–6). Followed in early Middle Triassic time by a base-of-slope fan system dominated by southdirected paleocurrents (Fig. 6, site 7–9), potentially indicating a gradually spreading of the basin floor. The opening and spreading of a rift basin in the northern part of the West Qinling during Late Permian to early Middle Triassic was probably caused by crustal attenuation in the back-arc. Although there is no direct evidence for the existence of an island arc at the southern boundary of West Qinling, north of the Maqin-Lueyang fault, there are abundant basaltic rocks interbdded in the Early– Middle Triassic successions at the Zeku and Henan area to the south of studied area (Fig. 1B; BGMRQH, 1973). These interbedded volcanic rocks could imply an island arc during the Early–Middle Triassic time. We interpret the back-arc spreading as a result of the rollback of subduction of the Paleo-Tethys oceanic lithosphere under the West Qinling of North China block. In this scenario, the inferred island arc to the south of the northern West Qinling would have migrated to the south, possibly for hundreds of kilometers (e.g. Yidun arc; Pullen et al., 2008). It is therefore postulated that the Late Permian–Middle Triassic deep-marine deposition of the northern West Qinling took place in a back-arc rift basin that may have evolved into an actively spreading back-arc ocean basin.

7.2. Stratigraphic evolution in the West Qinling and Songpan-Ganzi during Permian and Triassic Although the sediment gravity flow deposits in the West Qinling and Songpan-Ganzi area are commonly known as very thick,

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very monotonous flyshoid type deposits and individual formations are difficult to distinguish, sedimentary successions are distinguishable on a regional scale of 10s–100s of km (Fig. 8). The sedimentary architecture of Late Permian–early Middle Triassic deep-marine sediment gravity flow deposits in the northern division of the West Qinling can be distinguished from the southern division of West Qinling and Songpan-Ganzi based on lithology and depositional ages (Fig. 8). In the southern West Qinling, the Late Permian to early Middle Triassic strata are dominated by platform carbonate deposits; while, deep-marine sediment gravity flow depositions define the late Middle Triassic to Late Triassic Norian Stage strata. The lithostratigraphy of the northern part of Songpan-Ganzi is strikingly similar to the southern West Qinling (Fig. 8), however the southern Songpan-Ganzi can be distinguished from the southern West Qinling and northern Songpan-Ganzi both in terms of lithostratigraphy and timing of facies appearance (Meng et al., 2007). During Late Permian time, submarine volcanic rocks were deposited in the southern Songpan-Ganzi. The Early to early Middle Triassic were dominated by deep-marine carbonate depositions. Deep-marine sediment gravity flow deposits began to accumulate in the southern Songpan-Ganzi from the Ladinian stage and lasted to at least the Rhaetian stage. The onset of deep marine gravity flow deposition began much earlier in the northern West Qinling area and lasted longer in the southern Songpan-Ganzi area. 7.3. Implication for the closure of the eastern Paleo-Tethys Ocean The Tethys realm reflects a series of oceanic basins that closed between Gondwana and Eurasia. Deposits associated with the Paleo-Tethys were recognized within Paleozoic strata exposed in an intercontinental setting in central Eurasia today (Sengör, 1987; Stampfli et al., 2002). The Paleo-Tethys was thought to have closed during early Mesozoic time (Sengör, 1987; Sengör et al., 1988), although possibly diachronous, younging towards the west (Pan et al., 1990). The final stage of the eastern Paleo-Tethys Ocean closure occurred during Triassic time and was marked by the continent–continent collision between the South China block and Qiangtang terrane of Gondwana affinity with the North China block and Qaidam-Qilian block with a Eurasia affinity (Roger et al., 2010). The nature of the closure of the eastern Paleo-Tethys Ocean is still debated. End-member hypotheses include: (1) simple non-extensional back-arc basin (Gu, 1994); (2) intracontinental rift basin (Chang, 2000); (3) remnant ocean basin (Zhou and Graham, 1996); and (4) Mediterranean-style back-arc basin (Pullen et al., 2008). The all four hypotheses assume that the Paleo-Tethys oceanic lithosphere subducted beneath the Eastern Kunlun arc terrane and an arc-trench system existed on the southern margin of what is now considered the Anyemaqen-Kunlun suture because of the extensive Permian–Triassic phase of arc magmatism associated with the Eastern Kunlun arc and the trace of the AnyemaqenKunlun suture eastward to the Qinling-Dabie orogen (i.e. Mianlue suture) (Fig. 1A). In addition to the northward subduction of Paleo-Tethys oceanic lithosphere beneath the Eastern Kunlun arc terrane, the Anyemaqen-Kunlun suture is widely thought to have also accommodated the subduction of Paleo-Tethys beneath the amalgamated terranes of the North China block (Meng and Zhang, 2000; Yang et al., 1992). If valid, this continuous subduction boundary during Permian–Triassic time would have crossed the southern West Qinling area. This, widely assumed hypothesis, predicts a geographic separation of the West Qinling and the SongpanGanzi basin and an eastern continuation of Kunlun arc past its apparent (present-day) truncation at 99°E. The first three hypotheses mentioned above do not address the apparent lack of a Kunlun-like arc in West Qinling and the continuation of sedimen-

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tation between the southern West Qinling and northern SongpanGanzi (Section 7.2, Fig. 8), nor the Permian–Middle Triassic rifting posited here. The possibility of rifting and back-arc basin development raised here would not adequately be addressed by the alternatives to the extensional back-arc model advanced by Pullen et al. (2008). We do, however, note that a modest refinement in the timing back-arc extension would be necessary to fit the observations presented here. 7.4. A model for the Tectono-sedimentary evolution of the West Qinling during Permian and Triassic Based on the sedimentary architecture and paleocurrent data that we present here, and additional regional observations, we propose a model that illustrates the tectono-sedimentary evolution of the West Qinling from Early Permian to early Middle Triassic time (Fig. 9). This model is consistent with the current documented observations of the Paleo-Tethys realm for that time period. During the Early and Middle Permian, possibly including Carboniferous time, the northern part of the West Qinling likely composed part of the southern passive margin of the North China block. This zone was characterized by a continental shelf dominated by shallow-marine platform carbonate sedimentation (Fig. 9A) (Yang and Wang, 1995). During this time, along the southern margin of the West Qinling, the Paleo-Tethys oceanic lithosphere began subducting northward beneath the West Qinling (Mattern and Schneider, 2000; Xiao et al., 2002; Yang et al., 1996; Yin and Harrison, 2000). As early as Late Permian time, the northward subducting PaleoTethys oceanic slab experienced rollback, probably induced by decreased convergence between the South China and North China blocks. The Paleo-Tethys oceanic slab subducting northward beneath the North China block and accreted terranes (e.g., Qinling) began the process of rollback of the South China block towards the south. As continental affinity rocks of the South China block entered the subduction zone beneath the North China block, overall convergence between the two continental blocks decreased. However, the rollback process, driven by the subducted anchor of the Paleo-Tethys slab, initiated. Corner flow in the mantle induced above the subducting and the retreating slab induced extensional stress in the back-arc region of the overriding plate (West Qinling) which led the northern most part of the West Qinling (Northern division in Fig. 1B) to evolve into a back-arc rift basin system. Syn-depositional slumps and debris flows, with both northward and southward paleodirections, are observable along the inferred basin margins in Late Permian–early Early Triassic strata (Fig. 9B). The slump-related breccias are largely Carboniferous to Early–Middle Permian carbonates that deposited on the continental shelf platform (BGMRGS, 1972; Cao et al., 1995; Yang and Wang, 1995). Pullen et al. (2008) considered the rollback of the Paleo-Tethys oceanic lithosphere to have started in the Middle Triassic. However, we extend the initiation of rollback to the Late Permian. This observation may be more consistent with the time at which continental rocks of the South China block would have entered the subduction zone beneath the North China block to reach depths associated with UHP metamorphism by Early–Middle Triassic time (Cheng et al., 2011; Hacker et al., 1998, 2000, 2004; Liu et al., 2012). During the middle and late Early Triassic, the rift basin continued to widen. Extensive attenuation of the overriding plate may have resulted in sea-floor spreading or strongly attenuated continental lithosphere. Emplacement and eruption of the 250 Ma gabbro (Wang et al., 2010) and 242 Ma diabase (this study) exposed in the Longwuhe section are consistent with this attenuation and possible sea-floor spreading. Slump-related deposition decreased

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and a base-of-slope apron system began in the Early Triassic (Fig. 9C). The number of carbonate breccias beds also decreased, whereas the number of metamorphic and igneous clasts increased with time. This is interpreted to indicate the gradual exhumation of the carbonate platform surrounding the rift basin and the exposure of basement rocks by means of thermal isostatic compensation resulting in rift shoulder uplift (Fig. 9C). The largest expansion of the back-arc basin occurred during the Anisian time. Large-scale base-of-slope fan system formed throughout the northern division of West Qinling during this period. During the Anisian, the Longwuhe area was dominated by submarine fan channel and proximal deposition, whereas to the southeast, the Galeng area was dominated by submarine middle fan lobe deposition, and the Baizhuang area by distal submarine fan fringe deposition (Fig. 9D).

8. Conclusions According to detailed sedimentological and paleocurrent study, it is postulated that the West Qinling in the Late Permian to early Middle Triassic time was a rift basin that developed on the southern passive continental margin of the North China block, and evolved into an early ocean stage. Three distinct stratigraphic architectures were developed along with the opening process of the rift basin. The opening of the rift basin might have been caused by the rollback of the Paleo-Tethys oceanic lithosphere, that subducted beneath the West Qinling. Thus the West Qinling Late Permian to early Middle Triassic was an extensional back-arc rift basin. U–Pb zircon ages for geochemically varied igneous rocks with diabasic through granitic compositions intruded into these deepmarine strata are in the range of 250–234 Ma. The ages 250 Ma (Wang et al., 2010) to 242 Ma for mafic rocks in West Qinling are consistent with the rift related sea-floor spreading posited here, and provide a minimum depositional age for the Middle Triassic deposits in the West Qinling to be 234 Ma. The tectono-sedimentary evolution of the West Qinling from Early Permian to Late Triassic can be divided into six stages: (1) a stage of passive continental margin deposition (Early–Middle Permian); (2) an early stage rifting (Late Permian to early Early Triassic), when sedimentation was dominated by slope slumps and breccias in the northern division of West Qinling, and platform carbonates in the southern division; (3) a stage of continued rifting (middle–late Early Triassic), when a base-of-slope apron system developed in the northern division of the West Qinling and platform carbonate deposition continued in the southern division; (4) a late stage of rifting (early Middle Triassic Anisian stage), when new oceanic crust appeared and a base-of-slope fan system formed in the northern West Qinling while the rift basin started to migrate to the south; (5) a stage of rifting basin migration (late Middle to Late Triassic), when deep-marine deposition ended in the northern division of West Qinling, and started in the southern division along with the southern migration of rifting; and (6) a final stage of uplift and deformation (late Late Triassic), marking the closure of the Paleo-Tethys Ocean and the final collision between the South China, North China, and Qiangtang blocks. This final stage ended the marine deposition across the whole West Qinling region and resulted in the uplift and deformation of the deep-marine sediments in the back-arc rift basin. This tectono-sedimentary evolution of the West Qinling is consistent with a Mediterranean-style back-arc basin hypothesis for the Sonpan-Ganzi complex and provides a unifying framework for understanding the opening and ultimate closure of this oceanic basin.

Acknowledgements This work was supported by China Natural Science Foundation (40830314) and the Knowledge Innovation Program of the Chinese Academy of Sciences (KZCXZ-YW-Q05-02) to Meng, and by a U.S. National Science Foundation grant to Pullen and Garzione (EAR1118525). We would like to thank Jianmin Hu from Chinese Academy of Geosciences for help in the field and Clayton Loehn at the University of Arizona for SEM imaging. The associate editor Michel Faure and an anonymous reviewer are thanked for their comments and suggestions, which help clarify many parts of this paper.

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