Mesozoic Sedimentary Basin Development and Tectonic Implication, Northern Yangtze Block, Eastern China: Record of Continent–Continent Collision

中国科技论文在线 http://www.paper.edu.cn

Journal of Asian Earth Sciences 25 (2005) 9–27 www.elsevier.com/locate/jaes

Mesozoic sedimentary basin development and tectonic implication, northern Yangtze Block, eastern China: record of continent–continent collision

Shaofeng Liua,*, Ronald Steelb,1, Guowei Zhangc

aFaculty of Geosciences and Resources, China University of Geosciences, Beijing 100083, China bDepartment of Geology and Geophysics, University of Wyoming, Laramie, WY 82071, USA cDepartment of Geology, Northwest University, Xi’an 710069, China

Received 2 June 2003; revised 10 December 2003; accepted 16 January 2004

Abstract A Mesozoic foreland-basin complex formed along the northern Yangtze plate during subduction of this plate under the Qinling– Dabieshan orogenic belt along the Mianlue suture. As the Yangtze plate moved northwestwards and was obliquely subducted under the Qinling–Dabieshan (Middle–Late Triassic), a flysch foredeep developed in the Diebu–Songpan in the western part of the northern Yangtze plate. During the Late Triassic, a nonmarine molasse basin first formed in the eastern part of the northern Yangtze plate in response to initial collision there. This molasse clastic wedge prograded over the former marine basin and was accompanied by a change from high-sinuosity river systems flowing into basinal lakes, to higher gradient braidplains. Complete oceanic closure along the Mianlue suture during the Middle Jurassic produced a more extensive east–west molasse basin with rivers, deltas and lakes. During Late Jurassic through Early Cretaceous, the depocenter of the nonmarine molasse basin migrated continually from east to west because of intracontinental deformation associated with clockwise rotation of the Yangtze plate relative to the North China plate. In this time interval, the basin was again dominated by fluvial and lake-delta deposition and rivers continued to disperse sediments southwards into the basin. q 2004 Elsevier Ltd. All rights reserved.

Keywords: Foreland basin; Sedimentation; Tectonic implication; Northern Yangtze plate

1. Introduction to two episodes of rifting during late Precambrian and Devonian time (Liu and Zhang, 1999; Zhang et al., 2001). The Qinling–Dabieshan orogenic belt, a major tectonic The Shangdan and Mianlue oceans separated these crustal feature in central China, underwent two major suturing events, fragments from north to south. Closure of the Shangdan ocean and generated two main suture belts during Mesozoic time. led to continent–continent collision between the North China plate and Qinling–Dabieshan plates beginning in late These two belts, referred to as the Shangdan and Mianlue belts, Paleozoic time (Zhang et al., 2001). The development and are exposed in northern Qinling–Dabieshan along the extension of the Mianlue ocean during the Late Paleozoic and Xinyang–Shucheng fault (XSF) and in southern Qinling– the Early Triassic have been documented by Li and Sun Dabieshan along the Xiangfan–Guangji fault (XGF) (Fig. 1). (1996); Li et al. (1996); Lai et al. (1997, 1998); Xu et al. The major continental fragments that make up central (1998); Liu and Zhang (1999); Dong et al. (1999); Meng and China—from north to south the North China plate, Qinling– Zhang (1999, 2000); Zhang et al. (2001).WiththePermian Dabieshan microplate, and Yangtze plate—owe their origins and Early Triassic subduction and consumption of the Shangdan oceanic basin, the northern margin of the Yangtze * Corresponding author. Tel.: þ86-10-8232-3274; fax: þ86-10-8232- plate extended along the Mianlue suture, and was completely 1620. separated by this suture from the Qinling–Dabieshan plate to E-mail addresses: [email protected] (S. Liu), [email protected]. edu (R. Steel). the north. The ophiolites representing the remnants of the 1 Present address: Department of Geological Sciences, The University of palaeo-oceanic crust crop out in the Mianxian-Lueyang Texas at Austin, Austin, TX 78712, USA. and Suixian regions (Li et al., 1996; Lai et al., 1997, 1998;

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Fig. 1. Tectonic framework of the northern Yangtze and adjacent Qinling–Dabieshan orogenic belt, showing the distribution of basins, sutures, major faults, major structural belts and the location of structural sections. The names of major faults and suture belts are: XSF, Xinyang–Shucheng fault (Shangdan suture); XGF, Xiangfan–Guangji fault which buried the Mianlue suture; TLF, Tanlu fault; DYF, Dayong thrust fault; QYF, Qiyueshan thrust fault; HYF, Huayingshan thrust fault. Other abbreviations are: NC, North China plate; QD, Qinling–Dabieshan plate; YZ, Yangtze plate; NYFB, Northern Yangtze fold-thrust belt; JNFB, Jiangnan fold-thrust belt. Inset map shows major faults and sutures of China from Liu et al. (1997): LC, Lancangjiang suture; KM, South Kunlun- Mianlue suture; SD, Shangdan suture; AT, Altyn Tagh Fault; SL, Suolun-Linxi suture; TS, Tianshan suture.

Xu et al., 1998; Dong et al., 1999). A passive continental reconstructing the paleogeography of the regions in different marginal basin was developed along the northern Yangtze stages. The structure, stratigraphic successions and paleogeo- plate (Feng, 1991; Feng et al., 1997; Liu, 1997; Liu and Zhang, graphic evolution of the foreland basin belt allows us to 1999; Zhang et al., 2001). Details of how the Mianlue ocean examine the process of continental collision along the Mianlue closed or how the Yangtze plate collided with the Qinling– suture and the extent of continental consumption of the Dabieshan plate after the Middle Triassic, however, are still northern Yangtze plate. unknown. Long-distance subduction of the northern Yangtze plate under the Qinling-Dabashan plate led to destruction of some parts of the northern Yangtze plate, making analysis of 2. Structural framework of the northern Yangtze plate the collision process difﬁcult. However, the stratigraphy of a peripheral foreland basin belt, developed along the northern The Northern Yangtze plate contains two generalized Yangtze plate south ofthe Mianlue suture belt, provides a basis structural units: the northern Yangtze fold-thrust belt for reconstruction of the details of the collision and its (NYFB) in the north and the Jiangnan fold-thrust belt geodynamics. In this paper, we combine structural constraints (JNFB) in the south. Both were formed during collision with the sedimentary history recorded in the foreland basin between the Yangtze and Qinling–Dabieshan plates along belt to reconstruct the evolution of collision between the the Mianlue suture during the Mesozoic (Figs. 1 and 2). northern Yangtze and Qinling–Dabieshan plates. Theforelandbasinbelt,boundingthesouthernmarginofthe 2.1. The northern Yangtze fold-thrust belt (NYFB) Qinling–Dabieshan plate, now occurs in the Diebu–Songpan Basin, the Sichuan Basin, the Zigui Basin, the Dangyang Basin The NYFB is distributed from southern Dabashan to the and the Southeast Hubei Basin. These basins contain thick Xiangfan and Guangji areas (Fig. 1). The XGF in the fold- sedimentary successions of Late–Middle Triassic through thrust belt is an important boundary between the Qinling– Cretaceous ages, and provide a relatively continuous record of Dabieshan and Yangtze plates (Figs. 1, 2) and locally buries source-area deformation. Regional basin analysis, coupled older deformational features including the Mianlue suture and with structural cross-sections, provides a means of the NYFB under its thrust nappe. Strata in the Late Paleozoic 中国科技论文在线 http://www.paper.edu.cn .Lue l ora fAinErhSine 5(05 9–27 (2005) 25 Sciences Earth Asian of Journal / al. et Liu S.

Fig. 2. Structural cross sections from northern Yangtze to the adjacent Qinling–Dabieshan Orogenic Belt. (A) Section across the Jiangnan fold-thrust belt, northern Yangtze fold-thrust belt and southern Qinling. (B) Section across the northern Yangtze fold-thrust belt and Dabieshan, modiﬁed from Liu et al. (2003). Location of sections A and B are shown in Fig. 1. Ages of units shown are: Ar–Pt1, Archean to Early Proterozoic; Pt1, Early Proterozoic; Pt2, Middle Proterozoic; Pt3, Late Proterozoic; Pt3 –Pz1, Late Proterozoic to Early Paleozoic; Z, Sinian (equivalent to Vendian); C, Cambrian; O, Ordovician; S, Silurian; D, Devonian; C, Carboniferous; P, Permian; T, Triassic; T1, Early Triassic; T2, Middle Triassic; T3, Late Triassic; T3-J2, Late Triassic to Middle Jurassic; K2-E, Late Cretaceous to Early Tertiary; N–Q, Late Tertiary to Quaternary. 11 中国科技论文在线 http://www.paper.edu.cn

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Mianlue ocean basin and the Mesozoic foreland basin (Feng, sub-belt, a compound fold sub-belt and a thin-skinned fold 1991; Feng et al., 1997; Liu and Zhang, 1999)weredeformed sub-belt (Figs. 1, 2a). The basement uplift is bounded by the into the NYFB. A structural me´lange, consisting of ophiolite Dayong thrust fault (DYF), and the Proterozoic basement remnants, island arc volcanic rocks, deepwater and forearc and the Lower Paleozoic stratigraphy were folded and sediments, and basement rocks separated by ductile shear belts thrusted northwards. The folds in the compound fold sub- were locally developed between Xiangfan and Wuhan cities belt (Figs. 1 and 2a) affect mainly Paleozoic and Lower along the XGF (Lai et al., 1998; Dong et al., 1999). This Mesozoic rocks. The thin-skinned fold sub-belt, bounded by suggests that the older Mianlue suture between the Qinling– the Qiyueshan and Huayingshan thrust faults (QYF and Dabieshan and Yangtze plates is cutoff at depth below the HYF), is characterized by tight anticlines and wide Qinling–Dabieshan plate by the XGF (Zhang et al., 2001). synclines, and affects mainly Upper Paleozoic and Lower The western part of the NYFB from Hanzhong to Xiangfan Mesozoic strata that overlie subsurface detachment faults (Fig. 1) contains several arc-shaped thrust faults cut by the (Liu, 1995; Yan et al., 2000). The JNFB pinches out to the XGF, and almost all the faults dip to the north. These thrust northeast because of NYFB southward thrusting (Fig. 1). faults and folds intersect the XGF at an acute angle at their eastern extent. To the west, northwestern trending folds (parallel to XGF) are superimposed over northeasterly 3. Stratigraphic successions of the foreland basins trending folds (Fig. 1). The eastern part of the NYFB, from Xiangfan to Wuhan and Guangji, is The sedimentary successions of the Mesozoic foreland unconformably overlapped by Cretaceous to Early Tertiary basins suggest that there were four phases of depositional deposits of the Jianghan Basin, and formed during later rifting history associated with ocean closure and development: the (Figs. 1 and 2b). The thrust belt was later overthrust and buried late Middle Triassic and early Late Triassic Phase, the Late by Dabieshan basement near Guangji (Fig. 1). To the south, Triassic Phase, the Early–Middle Jurassic Phase, and the the northward-verging JNFB belt developed (Fig. 1). Late Jurassic and Early Cretaceous Phase (Fig. 3). The ﬁrst of these phases, producing a marine succession, is 2.2. The Jiangnan fold-thrust belt (JNFB) developed in the Diebu–Songpan Basin and the westernmost Sichuan Basin. The Ladinian-Carnian ﬂysch (.10 km The JNFB is present from Daxian to Dayong, to the south thick), Norian marine clastic deposits (about 4000 m thick) of the NYFB (Figs. 1 and 2). The belt is arc-shaped, and in the Diebu–Songpan Basin, and time-equivalent carbon- protrudes to the northwest. From the southeast to the ate ramp (,600 m thick) and clastic and coaly paralic and northwest, the JNFB is divided into a basement uplift shoreline (about 5–800 m) successions of the Tianjinshan,

Fig. 3. Middle Triassic through Lower Cretaceous stratigraphic unit names and foreland basin names across the northern Yangtze region. Vertical ruled pattern represents unconformities. 中国科技论文在线 http://www.paper.edu.cn

S. Liu et al. / Journal of Asian Earth Sciences 25 (2005) 9–27 13

Maantang and Xiaotangzi Formation in the westernmost sinuosity through time. Overlying intervals of rippled and Sichuan Basin have been well documented by Nie et al. thin-bedded mudstones/siltstones with abundant leaf fossils (1994); Xu et al. (1996); Guo et al. (1996), and Yang and and wood fragments are likely to represent levee, floodplain Yang (1997). We focus here on the successions of the Late and floodbasin deposits (e.g. Kraus and Bown, 1993). The Triassic through Early Cretaceous, which are well exposed depositional cycles are capped by three horizontally in the Sichuan, Zigui, Dangyang and Southeast Hubei laminated mudstones with occasional, current ripple- Basins (Fig. 3) and contain a complete record of foreland laminated, fine-grained sandstone beds and thin coal- basin nonmarine molasse accumulation. The age of the bearing shales. The great lateral extent and laminated nature nonmarine deposits is mostly determined from plants and of these mudstones (8–20 m thick) suggest periods of lake nonmarine bivalves (Chen et al., 1996; Guo et al., 1996). development and expansion across the adjacent braided plain (see Remy (1992) for analogous settings). 3.1. The Late Triassic successions Within each of the measured sections, the whole succession coarsens upwards suggesting overall expansion and prograda- During the Late Triassic, two separate systems of tion of the alluvial wedge through time (see Steel (1993) for foreland basins were formed along the northern Yangtze comparable succession). The lower cycles are absent in the plate: the Diebu–Songpan, Sichuan and Zigui Basin eastern sections. In the Daxiakou (S-4 in Fig. 1), for example, System, and the Dangyang and Southeast Hubei Basin only the upper part of the upper cycle is developed (Fig. 4d). System. By the latest Triassic, deposition had ceased in the Paleocurrent indicators (tabular cross strata) suggest paleo- Diebu–Songpan Basin because of thrusting and erosion flow to the south, although the paleocurrent indicators in the (Guo et al., 1996), and the foreland basin deposits were western Sichuan Basin are mainly to the east (Li, 1994). The mostly developed in the Sichuan Basin. There, foreland whole Upper Triassic in the Sichuan and Zigui Basins basin deposits show an evolution from marine to nonmarine. constitutes a succession that evolves from marine (Xu et al., The latest Triassic Xujiahe Formation represents the 1996; Guo et al., 1996) to nonmarine conditions, and shows the nonmarine molasse phase in the Sichuan Basin. In contrast, regressive development of a large-scale clastic wedge that the Dangyang and Southeast Hubei foreland basin System migrated southwards with time. began with a phase of nonmarine molasse in the early Late Triassic, and did not develop any marine phase. 3.1.2. The Dangyang and Southeast Hubei Basins Because of thrust burial of the Southeast Hubei Basin, 3.1.1. The Sichuan and Zigui Basins only the most distal basin deposits are exposed there. The The Sichuan and Zigui Basins are located to the southeast Dangyang Basin (Fig. 1) is better exposed and contains a of Diebu–Songpan Basin. The Norian–Rhaetian Xiao- more complete record of proximal foreland basin molasse tangzi and Xujiahe Formations unconformably overlie the sedimentation. Maantang and Xiaotangzi Formations and lap eastwards The molasse deposits in the Dangyang Basin include the onto the Sichuan Basin. Four continuous stratigraphic Jiuligang and Wanglongtan formations (Fig. 4e), containing sections, each 20–300 m thick, are well exposed along the an abundance of plants and bivalves that date the succession as northern margin of the Sichuan Basin and the eastern Late Triassic (Chen et al., 1996). The basal part of the molasse margin of the Zigui Basin, located in Wenquan (Kaixian succession in the Dangyang Basin rests unconformably upon County; S-1), Nanxi (Yunxian County; S-2), Zhuyuan an older, shallow marine succession (the Badong Formation) (Fengjie County; S-3) and Daxiakou (Xinshan County; S-4) that formed prior to uplift in the Qinling–Dabieshan (Liu and (Fig. 1). These sections provide a representative record of Zhang, 1999). The Badong formation contains fossils Late Triassic sedimentation (Fig. 4). Members 2–7 in the Eumorphotis (Asoella) subillyrica, E. (A.) illyrica, Myophoria Xujiahe Formation consist of three depositional cycles. atites sp. of bivalves of Middle Triassic age (Chen et al., 1996), Each shows a ca 90 m-thick upward-fining succession of but uncertainty as to its upper age boundary still exists (Guo stacked sand bodies, and about 10 m thick, interbedded et al., 1996; Chen et al., 1996). mudstone and thin sandstone beds (Figs. 4a–d). Each main A continuous stratigraphic section exposed northeast of sand body is coarse to medium-grained with massive, plane- Yichang (S-5, Fig. 1) provides a representative record of parallel, and/or low-angle cross-strata. The base of most Late Triassic sedimentation (Fig. 4e). In the Dangyang sandstone beds is a sharp, undulate scour surface with Basin, this Late Triassic succession consists of three mudclasts and tree trunks. We interpret these beds as the depositional units. Above the unconformity between the amalgamated channels and bars of braided streams (see Badong and Jiuligang Formations are channelized con- Schumm, 1977). The thick upward-fining alluvial units are glomerates (1 m) that fine upwards into tabular cross- likely to reflect increased subsidence rate pulses (e.g. Olsen stratified sandstones (16 m). The channel deposits occur et al., 1995) or climatic change and corresponding decrease regionally and probably record the initial sedimentation in sediment supply rates (Schumm, 1968; Blum and associated with the deforming thrust belt. Tornquist, 2000; Muller et al., 2002), either of which Immediately above the basal conglomerate is 70 m of the might have caused the river system to increase its planform Jiuligang Formation (Fig. 4e, 16–86 m). This section 中国科技论文在线 http://www.paper.edu.cn

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Fig. 4. Stratigraphic/sedimentary sections through Upper Triassic successions, measured in northern Yangtze. (A) Section (S-1) measured in Wenquan, Kaixian County, northern Sichuan Basin; (B) Section (S-2) measured in Nanxi, Yunyang County, northern Sichuan Basin; (C) Section (S-3) measured in Zhuyuan, Fengjie County, northern Sichuan Basin; (D) Section (S-4) measured in Daxiakou, Xinshan County, Zigui Basin; (E) Section (S-5) measured along the road between the towns of Jiuligang and Xiaoping in Yuanan County, Dangyang Basin (modified from Liu et al., 2003). The locations of sections of S-1, S- 2, S-3, S-4, S-5 are shown on Fig. 1. Depositional phases are described in text. Paleocurrent directions (n, number of measurements) taken from foreset orientations. consists mostly of bioturbated siltstone with carbonate rippled and laminated siltstone, abundant coaly shale, and coal nodules. However, there are intervals of lenticular sand- seams. This succession is interpreted to record meandering stone and conglomerate. The overall lithologic similarity stream/flood-plain sedimentation (see similar examples by with the underlying Badong Formation, widely thought to Miall, 1996). The coarser sand bodies represent channels, be tidal in origin, possibly suggests a similar depositional channel belts and amalgamated channel belts whereas the finer setting. units likely are overbank deposits, as well as swamp and The second depositional phase is represented by a section floodplain lake deposits. These lithofacies are organized into a about 370 m thick consisting of the upper Jiuligang Formation series offining upward units, 30 m thick, on average (Fig. 4e), (Fig. 4e, 86–456 m). This succession consists of a series of that are interpreted in a similar manner as done above in the upward-fining units, each of which typically begins with a Sichuan and Zigui Basins. scoured channel base overlain by 5–10 m of fine-grained The third phase of sedimentation is found in the sandstone with well developed tabular cross strata or, in Wanglongtan Formation (Fig. 4e) that is composed mainly places, lateral accretion bedding. These sand bodies are of nested channel-filling sandstones. The unit is 845 m thick lenticular and 20–60 m wide. Overlying the sand bodies is an (Fig. 4e, 456–1301 m). Nested channel deposits are com- interval of rippled and thin-bedded mudstone containing posed, on a scale of several meters, of massive or plan-parallel abundant leaf fossils and wood fragments. Within the laminated medium sandstones overlain by trough and planar mudstones are thin beds (,1 m thick) of fine sandstone, cross stratified sandstones of similar grain size. We interpret 中国科技论文在线 http://www.paper.edu.cn

S. Liu et al. / Journal of Asian Earth Sciences 25 (2005) 9–27 15 these deposits to represent channel fill overlain by medial or intervals of lake formation on the braidplain. Some marine transverse bars in braided rivers (see Olsen et al., 1995 for fossils, Bakevelloides subhekiensis, Mytilus tenuiformis, analogous examples). Capping the sandstones are fine to very Waagenoperna cf. triangularis, are found in the Wanglong- fine-grained, thin-bedded, sandstone intervals that are up to tan Formation, suggesting that the Dangyang foreland basin 0.5 m thick. Scour surfaces primarily define channel bases and underwent ‘short times’ of marine transgressions, and was margins, although the section also contains through-going probably occasionally connected with the southwestern erosion surfaces whose lateral extent is much greater than marine embayment during the Late Triassic (Zhang, 1981). individual channels. These larger-scale scours are typically found every 30–50 m throughout the section i.e. spaced much 3.2. The Early–Middle Jurassic successions farther apart than channel flow depths (5 m on average based on thickness of fining-upward sequences), and probably There was a significant change in the nonmarine basinal represent times of base-level fall and degradation in the setting by Early–Middle Jurassic times, as the region system (see also Steel, 1993). Sitting on the large-scale scour became a single, integrated molasse foreland basin system surfaces in places are rip-up clasts of mudstone and coaly from Guangyuan, Yichang to Guangji (Fig. 1). Depositional shale. facies belts were distributed in an E–W direction, south of We interpret these deposits as the product of multi- Qinling–Dabieshan. The whole thickness of the Early– channel braided streams (Miall, 1978, 1996; Liu and Yang, Middle Jurassic is about 6000 m near the town of Nanxi in 2000). Paleocurrent indicators (planar cross strata and Yunxian County (S-2 in Fig. 1). S-6 in the Dangyang Basin trough axes) suggest paleoflow was to the west-southwest. and S-4 in the Zigui Basin have the best exposures of the The braided stream deposits are interrupted up by three, Early Jurassic (Figs. 1 and 5a), and the Middle Jurassic (Fig. 12–50 m-thick intervals of mudstone which may represent 5b) respectively.

Fig. 5. Stratigraphic/sedimentary sections through the Lower Jurassic unit (A, S-6) measured along a road between the towns of Guodiken and Gaodaochang in Jinmen City, Dangyang Basin (location S-6 on Fig. 1) (modiﬁed from Liu et al., 2003) and through the Jurassic unit (B, S-4) measured in Daxiakou, Xinshan County, Zigui Basin (location S-4 on Fig. 1). See explanation in Fig. 4. 中国科技论文在线 http://www.paper.edu.cn

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S-6, the Early Jurassic section (Figs. 1 and 5a), is composed mainly of mudstone passing upward to an upward-coarsen- of the Tongzhuyuan Formation. The base of the formation ing sandstone succession (Fig. 5b, 250–1100 m). The everywhere contains thick conglomerates that fine upwards, mudstones are parallel- or ripple laminated or are simply reaching up to 40 m thick. Each sand body within this massive, and the siltstone intervals developed massive beds succession is massive or cross-stratified and conglomeratic in and ripple lamination, interpreted to be lake deposits. its lower part, and ripple- or, horizontally laminated and very Nonmarine bivalves occur in the middle part of the fine grained in its upper part. We interpret this motif as succession and represent a maximum flood surface that is meandering channel deposits (see also Allen, 1978 for synchronous over the whole foreland basin (Fig. 5b). The analogous deposits). The next 250 m of the section contains upper part of the Qianfuyan and the Shaximiao Formations two depositional cycles that coarsen and thicken upwards (Fig. consist of several coarsening-upward depositional cycles. 5a, 40–290 m marks). In their lower parts, these cycles Each one is about 100–300 m in thickness. Mudstones and contain mudstones with bivalves. Within the mudstones about 0.5 m thick massive or ripple siltstone intervals are are repeated intervals of fine-grained sandstone, typically developed at the lower part of the cycles. Two kinds 0.5–1 m thick, composed of massive, plane-parallel and of lithofacies are developed at the upper part. One is about ripple-laminated. These sandstone beds are capped by a 1– 20–80 m thick, and contains vertically stacked, laterally 2 m interval of lenticular medium to fine-grained sandstone extensive, medium to coarse-grained sand bodies with with planar and trough cross strata. These sandy intervals are massive, parallel laminated or cross stratified beds, in which interpreted to be distributary mouth-bar deposits of lake scour contacts and lag deposits are developed. The other is margins, overlain by distributary channels. At the top of each an approximately 15 m thick lenticular coarse sandstone cycle is an interval several meters thick of medium to fine- with cross bedded strata and basal scour surface. The upper grained sandstone. This interval is composed of sand bodies Shaximiao Formation contains up to 90 m thick, vertically with scoured basal surfaces and massive, cross-stratified and stacked sand bodies with massive bedding, large trough plane-parallel laminated sandstones. Mudstones between the cross bedding and tabular bedding, along with abundant sand bodies contain coal, shale and oxidized paleosol horizons wood fragments. We interpret these lithofacies as lake suggesting that the entire interval was deposited on an alluvial mudstones containing distributary mouth bar intervals, or deltaic plain (Ritts et al., 2001). Overall, this interval of delta-front sheets and (subaqueous) channels, and braided deltaic deposits likely formed along the shores of isolated, channel plain, respectively, which make up braided small lakes that were typical in this part of China in the Early channel-delta systems (McPherson et al., 1987). The sheet Jurassic (Chen et al., 1996). sand bodies of delta fronts were deposited when the large The last phase of basin filling in the Dangyang foreland braided channel flowed into the lake (Liu and Yang, 2000). consists of a thick interval (460 m) of channel-filling sandstone that is conglomeratic towards the base (Fig. 5a, 3.3. The Late Jurassic–Early Cretaceous successions 290–750 m). Channels are most abundant in the lower 120 m and upper 50 m of the interval. The middle part is South of Dabieshan, nonmarine depocenters were fine-grained, dominantly mudstone, with some isolated focused in the Zigui and Sichuan Basins during the Late fining-upward channel sand bodies. The mudstones contain Jurassic–Early Cretaceous. Both Late Jurassic and Early locally abundant bioturbation and leaf fossils. Sand bodies Cretaceous successions are developed in the Sichuan Basin, are composed of massive and cross-stratified, medium to but only the lower part of the Late Jurassic remains in the fine-grained sandstone. Paleoflow, based on tabular cross Zigui Basin. sets, was to the south-southeast (Fig. 5a). In the Zigui Basin, the Late Jurassic consists of the In contrast to the deposits of the Dangyang Basin, the Suining and Penglaizhen Formations. The Suining For- Zigui and northern Sichuan Basins were mainly filled with mation (830 m thick) unconformably covers the Middle sandstone deposits (about 250 m in thickness in S-4, 150 m Jurassic Shaximiao Formation with an erosion surface at the in thickness in S-1), called the Baitianba Formation. The base, and consists of six upward-coarsening sequences (Fig. sand bodies are composed of massive and cross-stratified, 5b, 3060–3890 m). The lower three sequences contain medium to fine-grained sandstone with a basal scour horizontal or ripple-laminated mudstones. Sub-meter scale, surface. We interpreted them as meandering channel tabular cross bedded strata or massive, rippled siltstone deposits. Within the middle of the succession, a thin intervals occur in the lower part, and about 2–5 m thick mudstone (0.5–1 m thick) with bivalve fossils suggests the massive, parallel-laminated, and ripple-laminated lenticular occurrence of a lake, probably corresponding to the fine-grained sand bodies occur in the upper part (Fig. 5b). deposition of a lacustrine delta in the Dangyang Basin. We interpret the lower part as lacustrine deposits with some Middle Jurassic successions are the Qianfuyan and mouth-bar intervals, and the upper part as representing Shaximiao formations (Fig. 5b). In the Zigui Basin the subaqueous channel deposition. The upper three sequences entire Middle Jurassic interval reaches about 2810 m thick have similar characteristics to the lower three. Besides the (Fig. 5b, 250–3060 m). The Qianfuyan Formation, 850 m in lacustrine, mouth-bar and subaqueous channel deposits, thickness and overlapping the Early Jurassic rocks, consists there are 40–100 m thick, stacked tabular sand bodies 中国科技论文在线 http://www.paper.edu.cn

S. Liu et al. / Journal of Asian Earth Sciences 25 (2005) 9–27 17 interpreted as delta-front sheet deposits (Liu and Yang, (mostly medium grained), we believe that compositional 2000). These deposits are massive bedded, or contain plane- percents counted by the traditional method are useful as parallel lamination and tabular cross strata from base to top source-area indicators, but use of the Gazzi-Dickinson within a single unit. We interpret these sequences in the method will result in higher percentages of Q, F, and lower Suining Formation to be braid-delta deposits (McPherson L values, especially lower plutonic rock fragments, et al., 1987). compared to the traditional method. Samples collected The Penglaizhen Formation (above the Suining For- from the Dangyang Basin come from two measured sections mation) is more than 1060 m thick (Fig. 5b, 3890–4950 m), (S-5, S-6), and samples of the Zigui and northern Sichuan and can be divided into two parts. The lower part is Basins from S-4 and S-2. composed dominantly of massive sandstones whereas the upper part shows trough and planar cross-strata, coarse- 4.1. Ternary plots for sandstone compositional data medium grained sandstone. Capping the sandstones are fine to very fine-grained, thin bedded, sandstone intervals with As a whole, Late Triassic–Jurassic sandstones from the mudstone. Scour surfaces define channel margins. The northern Yangtze are lithic rich (about 15%) and dominated upper part contains much more mudstone and siltstone by chert and sedimentary-rock fragments. Most samples fall above the coarse-medium sandstone, resulting in upward- in the area of recycled orogen provenances in Qm–F–Lt fining sequences. We interpret these deposits to represent and Qt–F–L ternary diagrams, and some of the Middle braided channel and flood-plain deposition (Miall, 1978, Jurassic samples fall in the areas of the arc orogen sources 1996; Liu and Yang, 2000). Paleocurrent indicators (planar and mixed orogenic sands in the Qp–Lv–Ls ternary cross beds and trough axes) suggest paleoflow to the south. diagram (see Fig. 6 in the Zigui Basin as an example). We The Early Cretaceous is mainly distributed to the west of the think that the source areas of the Late Triassic and Jurassic Zigui Basin, and consists of fluvial and alluvial fan systems foreland basins are mostly the foreland fold and thrust belt (Guo et al., 1996). and the consumed island-arc belt of the Mianlue ocean.

4.2. Lithic petrofacies and source-area analyses 4. Provenance analysis of the foreland basins The results for lithic composition distribution in Zigui The unroofing history recorded by sandstone compo- (S-4) and northern Sichuan (S-2) Basins are shown in Fig. 7, sitions in basin stratigraphy provides a useful indicator of and for petrofacies in different sub-basins are shown in Fig. the uplift and erosional evolution of the adjacent orogenic 8. The percentage of each petrofacies in Fig. 8a is belt (Hendrix, 2000; Hendrix et al., 1996). The deposits in normalized percent of the total lithic content in each the basins of the northern Yangtze are dominated by stratigraphic unit. The petrofacies consist of groups of sandstones whose compositions were determined as fol- lithologies that are found together in different source regions lows. Hand samples were collected in the field. Five (Fig. 8b), thus defining distinctive provenance. Source areas samples were collected at intervals of approximately 50 m are designated with roman numerals (e.g. I, II, etc.) in Fig. 8. where exposure was good and sandstone grain size was Lithic Petrofacies I contains granite and diorite clasts. The medium-coarse-grained. In areas of poor exposure and/or source for Lithic Petrofacies I probably is Late Triassic small grain size, fewer samples were collected. Thin granite and diorite, locally exposed along the southern sections were cut and 500 framework grains were counted margin of the Qinling–Dabieshan (Li and Sun, 1996; Sun per slide. Two methods were used for point-counting. One is et al., 2000), Petrofacies II contains medium- to low-grade the Gazzi-Dickinson method, which minimizes the influ- metamorphic rocks including quartzites, quartz-schists, ence of grain size on modal composition (Ingersoll et al., schists and phyllites. The specific difference between the 1984). The point-count results were plotted on Qt–F–L, lithic fragments of schists and phyllites is that the former Qm–F–Lt and Qp–Lv–Ls ternary diagrams to provide contain muscovite and quartz, whereas the latter contain insight on the tectonic setting of the source areas (Fig. 6). muscovite and sericite due to different metamorphism. The Since the Qinling–Dabieshan has a wide range of rock types source area for Petrofacies II includes Middle Proterozoic exposed, lithic grain composition is the most useful rocks distributed along the southern Qinling–Dabieshan. indicator of source area. We therefore used the traditional However, there is never very much Petrofacies II material method of point-counting and then showed only the results found, implying that this source area was not well exposed for lithic composition (Figs. 7 and 8), which typically during deposition. Petrofacies III includes mainly sericitic represents about 15% of all the grains counted. However, sedimentary fragments and slates possibly derived from other studies have demonstrated that lithic grain abundance Silurian rocks of the Qinling–Dabieshan plate. The is, in part, a function of grain size (Suttner et al., 1985). difference between both fragments is that the sericites in Although we show the results as percentages, we realize that sedimentary fragments have different extinction directions to some degree these reflect sandstone grain size. in thin-section, but in slate fragments have uniform Nonetheless, given the general consistency of grain size extinction direction. Both are distinguished from phyllites 中国科技论文在线 http://www.paper.edu.cn

18 S. Liu et al. / Journal of Asian Earth Sciences 25 (2005) 9–27

Fig. 6. Ternary plots for Late Triassic and Jurassic sandstone compositional data in Daxiakou, Xinshan County (S-4). Provenance ﬁelds are from Dickinson and

Suczek (1979). Qm, monocrystalline quartz; F, plagioclase þ potassium feldspar; Lt, total lithic fragments, including Lv (volcanic-rock fragments), Lcarbonate (carbonate-rock fragments), Ls þ Lm (sedimentary- and metamorphic-rock fragments combined), Qp (polycrystalline quartz, including chert), and unidentified grains. Qt, Total quartz; L, Total lithic fragments except polycrystalline quartz (Qp) (Hendrix et al., 1996). in thin-section by micro-fold structure in phyllite fragments. Samples collected from the northern Sichuan Basin (S- Petrofacies IV includes intermediate to siliceous volcanic 2), the Zigui Basin (S-4) and Dangyang Basin (S-5, S-6) rocks and volcanic tuff probably derived from the arc indicate an abundance of fine-grained sedimentary and low- volcanics. A possible source area along the southern side of grade metasedimentary rocks (Petrofacies III). The source Qinling–Dabieshan is the presently-buried rocks of the for these lithologies may be either marine, Silurian fine- Mianlue suture. Petrofacies V and VI contain abundant grained units that are now only locally exposed within the chert and limestone fragments, respectively. These grain southernmost Qinling–Dabieshan plate (Zhang et al., 1992) types most likely were derived from Paleozoic to Triassic or from more distal flysch deposits of late Middle–Late marine sedimentary units in the Mianlue suture. Triassic age found along the southwestern Qinling Mianlue 中国科技论文在线 http://www.paper.edu.cn

S. Liu et al. / Journal of Asian Earth Sciences 25 (2005) 9–27 19

Fig. 7. Summary of lithic fragment data in Daxiakou, Xinshan County, Zigui Basin (S-4) (A), and Nanxi, Yunyang County, northern Sichuan Basin (S-2) (B). The location of the sections is shown in Fig. 1, and location of some samples in the sections is shown in Figs. 4 and 5. suture (the Diebu–Songpan area) (Zhang et al., 1992)(Fig. Qinling–Dabieshan. However, much farther west, along 8). The scarcity of medium-grade metamorphic rocks the XGF fault in western Qinling, there are limited (Petrofacies II) from the Middle Proterozoic basement of occurrences of arc volcanics (Lai and Zhang, 2000)of the Qinling–Dabieshan core indicates that these rocks were similar composition, suggesting there were arc rocks of the barely exposed during the Late Triassic through Jurassic Mianlue suture exposed to the north of the Zigui and (Fig. 8a). There is an abundance of chert fragments northern Sichuan Basins, now mostly covered by thrust (Petrofacies V) in the late Triassic through Jurassic deposits nappes (Fig. 8b). Late Triassic granite and diorite are locally of the basins (Fig. 8a). There are two possible sources for exposed along the southern margin of the Qinling– these grains. There are abundant chert beds intercalated with Dabieshan (Li and Sun, 1996; Sun et al., 2000), and they limestones of Permian to Early Triassic age exposed in the supplied sedimentary source for the northern Yangtze basin NYFB. However, if this was the source, we would expect to during Jurassic time. Also in the Zigui Basin, limestone find at least some detrital limestone grains in the deposits, clasts (Petrofacies VI) began to appear in abundance by Late especially conglomerates, as well. There is no such Jurassic time (Figs. 7 and 8). The most obvious source area evidence. In addition, uplift of the Paleozoic rocks by the for this composition is the late Paleozoic shelf limestones thrust belt did not take place until Late Jurassic time, after now widely exposed across the NYFB (Fig. 8b). deposition of the Late Triassic through the Middle Jurassic The petrofacies data from Triassic through Jurassic strata deposits as suggested by abundant limestone lithic sands along the southern flank of the Qinling–Dabieshan suggest and pebbles in the Penglaizhen Formation in the Zigui the relative timing of uplift of the mountain. Source areas Basin. An alternate source area may be the Mianlue suture for the Late Triassic foreland basins were the sedimentary belt, now buried by the XGF (Fig. 8b). Although rocks of cover of the southern Qinling–Dabieshan and/or early the suture are no longer exposed, it is possible that deep- Mesozoic flysch basin deposits. No high-grade meta- water cherts (seafloor of the Mianlue ocean) may have been morphic rocks from deeper levels were exposed. The present. We note that in some detrital chert pebbles in the Mianlue suture belt may also have provided some of the Nanxi (S-2), we have found radiolaria, suggesting a deep- detritus to this area, particularly chert fragments. Beginning water source. in Middle Jurassic time (J2), a volcanic arc source area Middle Jurassic deposits exposed in the Zigui Basin and provided some detritus along the southern foreland basin the northern Sichuan Basin also contain andesitic and belt (the Zigui and northeastern Sichuan Basins). It is not siliceous volcanic grains, and volcanic tuff (Petrofacies IV), clear where, or even if, the volcanic arc was active prior to and granite and diorite clasts (I) (Figs. 7 and 8a). There is no this time, although regional tectonics imply there should present source for any extrusive rocks in the eastern have been an arc forming. By Late Jurassic time, limestones 中国科技论文在线 http://www.paper.edu.cn 20 .Lue l ora fAinErhSine 5(05 9–27 (2005) 25 Sciences Earth Asian of Journal / al. et Liu S.

Fig. 8. (A) Summary of lithic petrofacies in the northern Sichuan Basin (S-2), Zigui Basin (S-4) and Dangyang Basin (S-5, S-6). Data for northern Sichuan and Zigui Basins from Fig. 7 and, for Dangyang Basin at S-5 and S-6 from Liu et al. (2003). (B) Simpliﬁed map of source areas within Qinling–Dabieshan (from Fig. 1). Dashed line shows trace of the buried Mianlue suture that contains source rocks for petrofacies IV and V. 中国科技论文在线 http://www.paper.edu.cn .Lue l ora fAinErhSine 5(05 9–27 (2005) 25 Sciences Earth Asian of Journal / al. et Liu S.

Fig. 9. Tectonic paleogeographic maps of the northern Yangtze and adjacent regions for (A) Carnian and Early Norian stages of Late Triassic. The thickness of Maantang and Xiaotangzi Formations in Sichuan Basin and Jiuligang Formation in Dangyang Basin are shown; (B) Late Norian and Rhaetian stages of the Late Triassic. The thickness of the Xujiahe Formation in Sichuan Basin and the Wanglongtan Formation in Dangyang Basin are shown; (C) Middle Jurassic. A continuous basin belt, called the northern Yangtze Basin, was formed in the southern front of Qinling–Dabieshan. The thickness of the upper part of the Shaximiao Formation in Sichuan Basin is shown; (D) Late Jurassic. The depositional center migrated to the Zigui and Sichuan Basins. The thickness of the Upper Jurassic in Sichuan Basin is shown; (E) Early Cretaceous. The depositional center migrated to the Sichuan Basin. The thickness of the Lower Cretaceous in Sichuan Basin is shown. Position of units not palinspastically restored. Area of each map is almost identical to that shown in Fig. 1. The data of the stratigraphic thickness in Sichuan Basin is from Guo et al. (1996). 21 中国科技论文在线 http://www.paper.edu.cn

22 S. Liu et al. / Journal of Asian Earth Sciences 25 (2005) 9–27 of Early Triassic and Late Paleozoic ages were being the early Norian stage of the Late Triassic, the foredeep unroofed along the NYFB and incorporated farther south depozone in the Diebu–Songpan filled with coarse-median into the evolving foreland basin. The lithic grains of the grained marine molasse deposits (Yang and Yang, 1997), volcanic rocks, granites, and limestones first appeared in the while the eastern margin depozone in the westernmost Middle and Late Jurassic, respectively, (Fig. 7), showing that Sichuan Basin filled with paralic and shoreline sandstones, at least two major thrust episodes took place after the Late mudstones and coaly shales. At the late Norian stage, the Triassic. stratigraphy in the Diebu–Songpan Basin began to be deformed.

5. Paleogeographic reconstructions of the foreland 5.1. The Late Triassic foreland basin systems basin belt During the Late Triassic, two nonmarine foreland basin Based on the structure and distribution of lithofacies in the systems were developed along the northern Yangtze plate: foreland basin belt, in addition to the composition of clastic the Sichuan foreland basin system and the Dangyang- strata south of the Mianlue suture, we have interpreted the Southeast Hubei foreland basin system. During the late paleogeographic history of the area (Fig. 9). The sequence Norian and the Rhetian stages, the early formed Diebu– stratigraphic units and their architectural styles in each stage Songpan Basin and Longmenshan belt were deformed and document the sedimentary response to the collision process thrusted southeastward. At this time, the foredeep depocenter and development of the Mianlue suture. migrated to the eastern side of the Longmenshan thrust belt. Tectonic and paleogeographic analyses suggest that the From west to east braided fan delta, lacustrine, and fluvial Mianlue ocean was wide in the west and narrow in the east, plain deposits accumulated during the late Norian and and connected with the western ocean during Late Paleozoic Rhaetian stages (Li, 1994)(Fig. 9b) in the Sichuan foreland time (Lai et al., 1995). Platform carbonate rocks formed basin system. Braided-channel deposits, mostly derived from during the early Middle Triassic, and the foreland flysch was the northeast according to the paleocurrent data (Figs. 4a–c), deposited during the late Middle Triassic in the Diebu– accumulated at the northeastern margin of the Sichuan Songpan. The flysch belt disappears eastwards, overlapped foreland basin. The thickness of Members 2–7 of the Xujiahe by the long distance movement of the XGF. The 10–12 km Formation on the northern margin of the basin varies from thick flysch belt is considered to be a foredeep depozone of 300 to 20 m from west to east. The depositional system of the the marine foreland basin system and extended continuously whole basin thins eastwards (Fig. 4). The eastern depositional to the east under the southern Qinling–Dabieshan thrust margin migrated from Wanxian to Enshi during the late nappe. The Sichuan Basin, including the Longmenshan thrust Norian and Rhetian stages (Fig. 9b). The depocenter was belt, was at the lateral margin of the foredeep depozone, and located in the western side of the basin, and the western underwent uplift in the Ladian stage of the Middle Triassic. Longmenshan thrust belt caused loading subsidence. The Shallow-water carbonate rocks developed in this area. The northern margin, southern Qinling, supplied sediments to the depositional belts extend to the NE and are also overlapped basin, but did not cause much of a tectonic load that by Qinling–Dabieshan thrusted nappes in the northern side. contributed to subsidence in the foreland basin. The Dangyang and Southeastern Hubei Basins were mostly During Late Triassic time, the foreland basin began to uplifted and probably do not preserve a depositional record. develop from Yichang to Guangji, it was the Dangyang- To summarize, the depositional section from the western Southeast Hubei foreland basin system. Palaeocurrent and Songpan to the Dangyang Basin shows that the marine composition data indicate that sediments derived from the foreland basin system includes a flysch foredeep depozone in orogen were shed southward into the basin. At this time, the Diebu–Songpan, a Carbonate ramp depozone and a regional source area included only the shallow, supracrustal, sedi- uplift in the Sichuan Basin in the Ladian stage of the Middle mentary part of the Dabieshan core zone, including uplifted Triassic. oceanic sedimentary rocks of the Mianlue suture. Isopachs of During the Carnian stage of the Late Triassic, the flysch the Wanglongtan Formation (late Late Triassic; Fig. 9b) foredeep was still located in Songpan-Diabu (Fig. 9a). show a gradual decrease in thickness towards the south, with However, no depositional record is found east of the Sichuan a maximum thickness in excess of 1 km, delineating the Basin, and some part of the Middle Triassic Leipokou Fm geometry of the foreland basin. Lithofacies coarsen towards was eroded away. Hence, we infer the presence of a regional the mountain front, where braided streams dominated. unconformity between Middle Triassic and Late Triassic Elsewhere fluvial deposits fill the exposed parts of the strata (Li, 1994). At the western margin of the Sichuan Basin, foreland basin. Paleogeographic reconstruction shows that including the Longmenshan thrust belt, are shallow water the eastern part of the foreland basin during this time was carbonates and shorezone sandstones. Thus, the foreland subsequently overthrust and is not exposed. The western part basin system includes a western foredeep, a western of the basin does not connect with the Sichuan Basin system, carbonate ramp, and an uplift on the east side of the Sichuan as both show clear differences in terms of depositional Basin. The whole system further extends to the NE. During environment and thickness. 中国科技论文在线 http://www.paper.edu.cn

S. Liu et al. / Journal of Asian Earth Sciences 25 (2005) 9–27 23

5.2. The Early–Middle Jurassic foreland basin system subduction and collision of continental crust in the plate- tectonic system. At the beginning of the Mianlue oceanic The Jurassic was an important time for foreland basin closure, the Yangtze plate moved northwestwards, but the evolution to the south of the Qinling–Dabieshan. The basin North China plate moved southwards (Liu, 1997). There framework is quite different from that of the Late Triassic. was an ocean centered in the Songpan area, extending During the Early Jurassic, fluvial and lake deposits eastwards to Qinling–Dabieshan. Here the eastern part of accumulated in the basins, and some fluvial conglomerate the ocean is referred to as the Mianlue ocean (Fig. 10a)(Liu, was deposited along the northwestern margin of the basin 1997). The closure of the Mianlue ocean marks the final and over the NYFB. The thickness of this succession is collision of the North China and the Yangtze plates. As the about 100–600 m. Paleocurrents are mainly from north to Qinling–Dabieshan area thrusted southwards, the early south, but there is no obvious foredeep subsidence. Middle basin deposits along the Mianlue belt were covered and Jurassic braided stream and braid-delta deposits are found modified. From the remnant depositional records, we can along the northern areas of the basin and lacustrine deposits analyze the geodynamic process of the basin-mountain fill the distal part of the basin (Fig. 9c). The northeastern system. Sichuan, Zigui and Dangyang Basins were filled with the same depositional system, so it is likely that all three basins belonged to the same foreland belt. The overall coarsening- upward stratigraphy is seen most clearly in outcrops and 6.1. The Triassic depositional records of oblique subduction some well logs. Total sediment accumulation during this of the northern Yangtze plate under the Qinling–Dabieshan time was about 1.05 km in the Southeastern Hubei Basin, and about 3 km in the Zigui and northern Sichuan Basins, The early flysch (Ladinian-Carnian) foreland basin is decreasing southwards. The foredeep of the basin is located mainly distributed in the western and northern parts of the in front of the NYFB, and deposits of this age show a well- Sichuan Basin. The foredeep was located in the Diebu– developed asymmetry typical of foreland basin geometry Songpan area because of the northwestward subduction of (Fig. 9c). the Yangtze. As the movement direction of the Yangtze block was at an acute angle to the extension of the Qinling– 5.3. Late Jurassic through Early Cretaceous foreland basin Dabieshan block, we interpret the flysch foredeep to have system extended and narrowed eastwards (Yin and Nie, 1993)(Fig. 10b). It was cut and covered by long-distance thrusts of the The basin depocenter at this time began to move to Qinling–Dabieshan at later stages. the west of Dangyang Basin and into the Zigui and With the compression and movement of the Yangtze northern Sichuan Basins. Within the Dangyang Basin, blocks toward Diebu–Songpan during the Norian through small basin remnants of the Early Cretaceous are found Rhaetian stages, the early flysch deposits in the foredeep near Yichang. Thrusting continued along the south flank were deformed, and a proto-Longmenshan thrust belt was of the Dabieshan, including the JNFB, and possibly formed. The foredeep migrated to the western Sichuan continued motion on the XGF. The Late Triassic through Basin, and deposited the Xujiahe Formation (Fig. 10c). Middle Jurassic strata of the Dangyang Basin were As the thrust belt expanded, the depositional stratigraphy involved in the deformation. overlapped southeastwards. The depositional records in During the Late Jurassic, the depocenter of the basin the northern margin of the Sichuan Basin provide migrated to the Zigui Basin (Fig. 9d). About 2000 m of important information on the tectonics of the Mianlue strata, deposited by braid-delta and braided stream systems, filled the Zigui Basin. The paleocurrents are mainly toward belt. In this area, coarse-clastic braided channel deposi- the SSW, SW and south. During the Early Cretaceous, the tional systems developed, with a source area from the Zigui Basin uplifted because of southward thrusting of the Dabashan. There was no foredeep subsidence at this NYFB and northwestward thrusting of the JNFB. time. The foredeep formed from the Carnian stage in the The depocenter moved to the west of Wenquan in the Dangyang Basin, and thick Late Triassic molasse was northern Sichuan Basin, and the foredeep of the foreland deposited. The Sichuan Basin was disconnected from the basin, located to the west of the town of Wanyuan, infilled Dangyang Basin by the Huangling dome. From these with about 400–1200 m of fluvial deposits (Fig. 9e). data, we consider that a thrust load formed first in the northern Dangyang and Southeastern Hubei Basins, and uplift (but no thrust) was developed in the northern 6. Geodynamics of the basin-mountain Sichuan Basin. The basin fill at this time was still system—discussion dominated by materials from supracrustal source areas or the early formed flysch foreland basin deposits (?), The formation and further development of the foreland indicating that the basement core in the southern basin marks the closing of the oceanic basin, and initial Qinling–Dabieshan was not yet uplifted to the surface. 中国科技论文在线 http://www.paper.edu.cn

24 S. Liu et al. / Journal of Asian Earth Sciences 25 (2005) 9–27

Fig. 10. Interpreted maps showing the evolution of closure and collision along the Mianlue suture from Late Paleozoic through EarlyCretaceous time. The geological and paleomagnetic data show that the Yangtze plate subducted and intracontinentally shortened clockwise relative to the North China plate from the Middle Triassic through Early Cretaceous, along the Mianlue suture (Zhao et al., 1996; Yokoyama et al., 2001). Here we assumed that the North China-Qinling–Dabieshan plate was relatively stable, and the Yangtze plate rotated clockwise about 3 degrees at every stage. The widths of the Mianlue ocean, foreland basin and forelandthrustbeltin different stages are not correctly restored. (A) Late Paleozoic,showing the opened Qinling–Dabieshan and Yangtze plates and the separating Mianlue ocean, that was widest in the west and narrowed eastwards. At this time, the Tanlu fault probably was a transform fault across the spreading ridge. The relative location of the Kunlun and Songpan districts are shown in this ﬁgure. (B) Ladinian stage of the Middle Triassic, showing the beginning of oblique plate collision and formation of a ﬂysch foreland basin along the Mianlue suture belt. (C) Rhaetian stage of the Late Triassic, showing continuedoblique collision across the Mianlue suture. The development of the Molasse foreland basin south of Dabieshan and east of Longmenshan is shown. (D) Middle Jurassic, showing face-to-face collision of the Qinling–Dabieshan and Yangtze plates and formation of a continuous northern Yangtze foreland basin. (E) Late Jurassic, showing oblique intracontinental shortening between NYFB and JNFB, and westward migration of the northern Yangtze foreland basin. (F) Early Cretaceous, showing continuous oblique intracontinental shortening between NYFB and JNFB, and westward migration of the northern Yangtze foreland basin. YZ, Yangtze plate; QD, Qinling–Dabieshan plate; NC, North China plate. XGF, Xiangfan–Guangji fault; TLF, Tanlu strike-slip fault; MLS, Mianlue suture; SDS, Shangdan suture. 中国科技论文在线 http://www.paper.edu.cn

S. Liu et al. / Journal of Asian Earth Sciences 25 (2005) 9–27 25

6.2. The Early–Middle Jurassic depositional records of convex-shaped (Fig. 10a). In the beginning, the Tanlu collision between the northern Yangtze and Qinling– fault probably was a transform fault cutting across the ocean Dabieshan plates spreading ridge. The collision along the Mianlue ocean began at During the Early Jurassic, the foredeep was maintained Dabieshan, causing deep subduction of the Dabieshan and in the Dangyang and Southeastern Hubei Basins and was Yangtze plates and forming ultra-high pressure meta- filled with a thick upward-coarsening succession (Fig. 5a). morphic rocks. Later the Dabieshan thrust southwards, However, there was no subsidence center along the northern and overlapped the Yangtze plate for more than 100 km. margin of the Sichuan Basin. The successions deposited in During the collision of these two plates, the Tanlu transform both the northern Sichuan and Dangyang Basins are clearly fault extended to the northern continental plate, and was different, with dominantly fluvial deposits in the former changed into a counter-clockwise strike–slip fault. During basin and lacustrine delta and fluvial deposits in the latter. the collision, the Yangtze plate obliquely subducted and During the Middle Jurassic, an extensive foredeep sub- rotated clockwise relative to the North China plate, from the sidence belt from east to west was formed along the northern Middle Triassic to the Middle Jurassic. Then oblique Yangtze plate, and filled with a 1000–3000 m thick intracontinental deformation continued until the Early succession of braided delta deposits (Fig. 10d). The island Cretaceous along the boundary of the two plates. These arc volcanic rocks along the Mianlue suture were eroded conclusions are consistent with the paleomagnetic studies in and supplied sediments to the foreland basin. We interpret the North China and Yangtze plates (Yokoyama et al., 2001; that continental collision continued in the northern Dan- Zhao et al., 1996). gyang and Southeastern Hubei Basins in Early Jurassic time, and then took place across all of northern Yangtze as the Yangtze plate rotated clockwise relative to North China in 7. Conclusions the Middle Jurassic. The integration of the foreland basin system marks the complete amalgamation of the Yangtze The foreland basin complex, bounding the southern and North China plates, as evidenced by paleomagnetic margin of the Qinling–Dabieshan plate, records a pro- results (Zhao et al., 1996). longed period of continental collision along the Mianlue suture and the extent of continental consumption of the 6.3. The Late Jurassic–Early Cretaceous depositional northern Yangtze plate during Mesozoic time. The time and records of intracontinental deformation along the northern style of tectonism in the region is critically aided by Yangtze plate interpretation of preserved basin deposits. The basin filling history, along with constraints placed by regional structural During Late Jurassic through the Early Cretaceous, the and petrologic studies, provides the basis for paleogeo- foreland basin continually migrated from the east to the west graphic reconstruction. because of intracontinental deformation. The NYFB thrust Depositional systems and paleogeography of the foreland southwards and the JNFB expanded northwestwards, the basin complex indicate that the Mianlue suture, the two belts joining together and the early foreland basin in boundary between the Yangtze plate and the Qinling– turn became involved in deformation from the Yichang to Dabieshan Block, was created by east-to-west closure. As Guangji in the Late Jurassic. The depocenter migrated to the the Yangtze plate moved northwestwards and obliquely Zigui Basin, and more than 2000m of molasses deposits subducted under the Qinling–Dabieshan from late Middle accumulated (Fig. 10e). The NYFB was eroded and Triassic to Late Triassic, a flysch foredeep developed in the supplied sediments to the basin. Limestone lithic grains Diebu–Songpan in the western part of the northern Yangtze appeared first in the Late Jurassic stratigraphy. During the plate. This foredeep is not present along strike to the east Early Cretaceous, the JNFB and the NYFB continually probably because of subsequent thrust telescoping within expanded northwestwards and southwards, respectively, the Qinling–Dabieshan orogenic belt. During the Late and converged in Daxian to the west of the Zigui Basin (Fig. Triassic, a continental molasse foredeep first formed from 9e). The subsidence center migrated to the northwestern the Carnian stage in the eastern part of the northern Yangtze Sichuan Basin (Figs. 9e and 10f). plate in response to initial collision. Sources areas for the Late Triassic foreland basins were the sedimentary cover of 6.4. Geodynamics of continental deep subduction the southern Qinling–Dabieshan, and the Mianlue suture in the Dabieshan belt may also have provided some of the detritus to the basins. As seen in the paleogeographic and geodynamic maps, During the Middle Jurassic, an extensive east–west shortening in the Dabieshan was nearly continuous from depocenter formed along the northern Yangtze plate, and Middle Triassic into Cretaceous time. At the beginning, the filled with a 1000–3000 m thick succession of braided-delta northern boundary of the Yangtze plate was concave shaped deposits. The island arc volcanic rocks along the Mianlue and the North China-Qinling–Dabieshan plate was suture were eroded and supplied sediments to the foreland 中国科技论文在线 http://www.paper.edu.cn

26 S. Liu et al. / Journal of Asian Earth Sciences 25 (2005) 9–27

basin. The development of an integrated basin marks the Ingersoll, R.V., Bullard, T.F., Ford, R.L., Grimm, J.P., Pickle, J.D., Sares, complete closure along the Mianlue suture as the Yangtze S.W., 1984. The effect of grain size on detrital modes: a test of the plate rotated clockwise relative to North China during the Gazzi-Dickinson point-counting method. Journal of Sedimentary Petrology 54, 103–116. Early–Middle Jurassic. During Late Jurassic through Early Kraus, M., Bown, T.M., 1993. Short-term sediment accumulation rates Cretaceous time, the foreland basin continually migrated determined from Eocene alluvial paleosols. Geology 21, 743–746. westwards from Zigui Basin to the northwestern Sichuan Lai, S., Zhang, G., 2000. Identification of the island-arc magmatism zone in Basin due to intracontinental deformation. The NYFB thrust the Lianghe–Raofeng–Wuliba area, south Qinling and its tectonic southwards and the JNFB expanded northwestwards, the significance. Science in China (series D) 43, 69–81. Lai, X., Yang, F., Yin, H., 1995. Reconstruction of the Triassic paleo-ocean two belts joining together and the early foreland basin in in Qinling. Earth Science–Journal of China University of Geosciences turn becoming involved in deformation. The NYFB was 20, 648–656.(in Chinese with English abstract). eroded and supplied sediments to the basin. Limestone lithic Lai, S., Zhang, G., Yang, Y., Cheng, J., 1997. Petrology and geochemistry grains appeared first in the Late Jurassic stratigraphy. features of metamorphic volcanic rocks in Mianxian-Lueyang suture zone, South Qinling. Acta Petrologica Sinica 13, 563–573. Lai, S., Zhang, G., Dong, Y., 1998. Geochemical features and its tectonic significance of the meta-basalt in Zhoujiawan area, Mianlue suture Acknowledgements zone, Qinling-Dabie Mountains, Hubei Province. Journal of Miner- alogy and Petrology 18, 1–8. We thank Dag Nummedal, Paul Heller, Siji Huang, Mary Li, Y., 1994. 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