Oblique closure of the northeastern Paleo-Tethys in ... · Oblique closure of the northeastern Paleo-Tethys in central China Shaofeng Liu 1, Tao Qian , Wangpeng Li , Guoxing Dou ,

Oblique closure of the northeastern Paleo-Tethysin central ChinaShaofeng Liu1, Tao Qian1, Wangpeng Li1, Guoxing Dou1, and Peng Wu1

1State Key Laboratory of Geological Processes and Mineral Resources and College of Geosciences and Resources, ChinaUniversity of Geosciences (Beijing), Beijing, China

Abstract A branch of the Paleo-Tethys Ocean once separated the north China plate from the south Chinaplate. However, the mode of closure of the northeastern Paleo-Tethys Ocean during the Late Paleozoic toEarly Mesozoic has been debated. One reason for this debate is that the collisional suture zone was later buriedby large-scale thrust faults in the southern Qinling-Dabieshan orogen, which made it difficult to reconstructthe amalgamation of the supercontinent in central China. New regional geologic mapping provides stratigraphicand structural constraints on themechanism of this ocean closure. Our results indicate that dextral transpressionalsuturing in the southern Qinling-Dabieshan foreland fold-thrust belt resulted in the formation of the northernYangtze foreland basin, where the stratigraphy precisely shows the time-transgressive closure of the ocean, andthe orogenic sediments shed over 1000 kmwestward from eastern China to the closing Paleo-Tethys. Therefore,we propose an oblique subduction model to describe the closure of the Paleo-Tethys Ocean. Our findingssuggest that prolonged slab pull during the oblique subduction of the oceanic plate continued to drive deepcontinental subduction, thereby forming high- and ultrahigh-pressure metamorphic rocks and leading tosustained ocean closure.

1. Introduction

The Qinling-Dabieshan orogen formed during the collision between the south China and north China platesalong two north dipping suture zones in central China (Figure 1) [e.g., Zhang et al., 2001; Liu et al., 2005b]. TheShangdan suture developed during the Late Paleozoic, and the Mianlue suture formed to the south during theEarly Mesozoic. These suture zones divide the Qinling-Dabieshan orogen into three plates from north to south,the north China plate, the Qinling-Dabieshan microplate, and the South China plate. The south China plateis composed of the Yangtze Block in the north and the Cathaysia Block in the south [Zhang et al., 2013]. TheMianlue suture zone is a composite tectonic zone that formed from the Qinling-Dabieshan subduction-collisionsuture and was superimposed by Mesozoic and Cenozoic intracontinental structures [Zhang et al., 2001]. Mostportions of this suture zone are cut at a depth below the Qinling-Dabieshan microplate by thrust, and thedevelopment and extension of this zone was documented by Li and Sun [1996], Li et al. [1996], Lai et al. [1997],Xu et al. [1998], Dong et al. [1999], Zhang et al. [2001], and others. Structural melanges that consist of LatePaleozoic-Early Mesozoic ophiolite remnants, island arc volcanic rocks, deepwater and fore-arc sediments,Neoproterozoic basement rocks, and mafic and volcanic fragments separated by ductile shear belts areexposed along this tectonic zone from the western Qinling and its western extension in the East Kunlun orogento the southern Dabieshan [e.g., Zhang et al., 2004]. These melanges provide evidence of the existence of aMianlue oceanic basin between the Qinling-Dabieshan microplate and the south China plate during the LatePaleozoic and Early Triassic, with some more ancient rifting records during the Neoproterozoic locally involvedin theMianxian region [e.g., Zhang et al., 2004; Xu et al., 2013]. Geological and geochemical (including Pb, Sr, andNd isotopic tracers [Zhang et al., 2001]) analyses imply that the oceanic basin was part of the northeasternbranch of the Paleo-Tethys Ocean during the Late Paleozoic. A passive continental marginal basin developedalong the northern Yangtze Block [Liu and Zhang, 1999; Zhang et al., 2001]. Therefore, the Mianlue suturezone represents the closure of the Mianlue Ocean, which separated the previously amalgamated northChina-Qinling Dabieshan plate from the south China plate in the northeastern branch of the Paleo-TethysOcean during the Late Paleozoic [e.g., Zhang et al., 2001; Liu et al., 2005b].

Models have been proposed for describing the mechanics of ocean basin closure and continental suturing,including those involving plate rotation [e.g., Zhao and Coe, 1987; Gilder et al., 1999; Wang et al., 2003],continental indentation [Yin and Nie, 1993], and a closing remnant ocean basin [Zhou and Graham, 1996].

LIU ET AL. ©2015. American Geophysical Union. All Rights Reserved. 1

PUBLICATIONSTectonics

RESEARCH ARTICLE10.1002/2014TC003784

Key Points:• We suggest an oblique subductionmodel for closure of the Paleo-TethysOcean

• Stratigraphy precisely recordstime-transgressive closure of theocean basin

• Dextral transpressional suturingresulted in the formation of theforeland basin

Supporting Information:• Text S1, Figures S1–S6, andTables S1 and S2

Correspondence to:S. F. Liu,[email protected]

Citation:Liu, S. F., T. Qian, W. P. Li, G. X. Dou, andP. Wu (2015), Oblique closure of thenortheastern Paleo-Tethys in centralChina, Tectonics, 34, doi:10.1002/2014TC003784.

Received 19 NOV 2014Accepted 31 JAN 2015Accepted article online 6 FEB 2015

However, none of these models fully explain the mechanisms of the eastward deepening continental subduction,the westward time-transgressive suturing of the ocean, or the westward progradation of the northern Yangtzeforeland basin. The causes of the development and extrusion of the high- and ultrahigh-pressure metamorphicrocks in the Dabieshan have not yet been fully understood. Therefore, a new continental suturing mechanismshould be developed. The structure and stratigraphy of the foreland fold-thrust belt and peripheral forelandbasin, respectively, along the northern Yangtze Block south of the Mianlue suture zone provide a basis forreconstructing the collision details and their geodynamics. In this paper, structural constraints are combinedwith sedimentary history and sediment provenance recorded in the paired foreland fold-thrust belt and forelandbasin to explain how the dextral transpressional deformation in the southern Qinling-Dabieshan forelandfold-thrust belt (SQDB) progressed from east to west, how the northern Yangtze foreland basin diachronouslydeveloped and stepped to the west, and how these events explain the Paleo-Tethys Ocean closure.

2. Methodologies

Structure, stratigraphy, and sediment source areas were examined to model the mechanics of the Paleo-TethysOcean closure. Structural analyses were based on field mapping and interpretation of seismic reflection datato reconstruct the structural framework of the foreland fold-thrust belt (Figure 1 and Figure S1 in the supportinginformation). In addition, measurements and stereographic projections of structural elements were conductedto demonstrate the kinematics of syncollisional fault zones, and 40Ar/39Ar radiometric dating was performedto constrain the structural ages. The northern SQDB is characterized by penetrative, banded, or distributedasymmetric plunging folds in the Upper Sinian through Lower Paleozoic limestone and foliation of the rotatedporphyroclasts in the Neoproterozoic phyllite, which are bounded by strike-slipping thrust faults. We documentedthe orientations of various structural features in the field, including plunge azimuths and dips in the plungefold axes, the porphyroclast B axes, and mineral lineations and dip vectors in the boundary faults. Thestereographic projections of these measured structural elements demonstrated the preferred orientationsand kinematics of the strike-slipping thrust fault zones (Figures 2 and 3). Syntectonically formed mica-quartzschists (S1103106 and S1103017), and foliated phyllite (S1103077) samples were collected from the thrustand shear zones (Figure 1), from which three synkinematic biotite and sericite minerals were separated. Theminerals were subjected to 40Ar/39Ar analysis using a MM-5400 (China University of Geosciences, Beijing) massspectrometer (detailed descriptions of methods are provided in the supporting information [Wang et al.,2011b]). These results are tabulated in the supporting information and are summarized in Table S1 in thesupporting information. The corresponding age spectra of the samples are illustrated in Figure S2 in thesupporting information.

Figure 1. Structural map of the southern Qinling-Dabieshan orogen and northern Yangtze Block. The heavy black linesdenote the locations of the structural cross sections, and the heavy purple lines denote the locations of the stratigraphiccross sections. The circled plus symbols denote the sites of the samples for 40Ar/39Ar dating. NC: north China plate (yellow),SC: south China plate (pink), QD: Qinling-Dabieshan microplate (white), YZ: Yangtze Block (pink), CA: Cathaysia Block, F1:Mianxian-Chengkou-Xiangfan fault (which buried theMianlue suture; MCXF), F2: Pingba fault, F3: Chaotianmen-Zhenba-Jingshanthrust fault, F4: Tiexi-Wuxi thrust fault, F5: Shangdan fault (suture), F6: Tanlu fault, F7: Dahe-Qiaoting fault, and F8: Tongchenghefault. Mountains: LM: Longmenshan, HN: Hannan, MC: Micangshan, NDA: North Dabashan, SDA: South Dabashan, DH:Dahongshan, SDE: Southern Dabieshan, and XF: Xuefengshan. Basins: SCB: Sichuan, DYB: Dangyang, and SHB: SoutheastHubei. Locations: SP: Songpan, MX: Mianxian, and CK: Chengkou.

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The stratigraphic framework of the foreland basin along the Mianlue suture was reconstructed using detaileddepositional data from more than 100 measured sections, wells penetrating the Upper Triassic strata(Figure 6, Figures 7a–7c, Figure 8, and Figure 9) and some correlated, previously published data (Figure 7d).Time-stratigraphic correlations were based on the physical continuity of time-parallel beds, including themaximum flooding surfaces within the intervals of mudstone, regionally continuous coal beds, and zones ofbivalves and nonmarine plants [Yin et al., 1992; Chen et al., 1996; Liu et al., 2005b; Ma et al., 2009]. Age controlin these basins was provided from available stratigraphic studies (Figure 5) [Yin et al., 1992; Chen et al., 1996;Ma et al., 2009]. The minimum ages of detrital zircon via U-Pb dating provide further constraints on the timecorrelations of certain stratigraphic sections (Figures 7b–7d).

The source areas of sediments feeding the northern Yangtze foreland basin were evaluated by determiningthe single-zircon ages, trace element compositions, and Hf isotopic compositions through a comparativeanalysis of the source areas and were further constrained using paleogeographic analyses and paleocurrentmeasurements. We collected more than 20 Upper Triassic sandstone samples from the Sichuan andDangyang Basins to investigate the provenance of the basin sediments and the associated unroofing of theQinling-Dabieshan orogen and adjacent source terranes. Laser ablation–inductively coupled plasma–massspectrometry (ICP-MS) was used for U-Pb dating and trace element analysis of more than 1248 single detritalzircons. In situ Lu-Hf isotopic analysis of more than 81 zircons was conducted using a Neptune multicollectorICP-MS, equipped with a 193 nm laser. These analyses were conducted at the State Key Laboratory ofContinental Dynamics at Northwest University in Xi’an, China (Table S2 in the supporting information andFigure 10; for detailed descriptions of methods, see the supporting information). The internal textures(Figure S3 in the supporting information) of the zircons, the Th/U ratios (Figure S4 in the supporting information),and the REE compositions (Figure S5 in the supporting information) were used to determine the origins ofthe igneous or metamorphic rocks. In addition to the age ranges of the zircons (Figure 10), the geochemicalsignatures (e.g., Hf isotopic compositions) of the zircons in the sediments and potential zircon sources(Figure S6 in the supporting information) were compared for the provenance analysis.

3. Time-Transgressive Transpressional Deformation Along the Suture Zone

The structure of the Mianlue suture zone provides important evidence regarding the mechanisms of closure ofthe Mianlue Ocean and the collision between the south China and Qinling-Dabieshan plates during the EarlyMesozoic. Due to the extensive subduction in the northern Yangtze Block under the Qinling-Dabieshan plate andthe south directed overthrust of the Mianxian-Chengkou-Xiangfan fault (MCXF), certain parts of the northernYangtze Block were destroyed, and the Mianlue suture was cut off at a depth below the Qinling-Dabieshanorogen [Zhang et al., 2004]. Consequently, it is difficult to analyze this collision [Liu et al., 2005b]. However,structural analysis of the SQDB helped us reconstruct the collision history along the Mianlue suture, and theresidual syncollisional structures provide a record of the kinematics and timing of the continental suturing.

Figure 2. Dextral transpressional structures in the northern subbelt of the SQDB and adjacent regions. The locations of thestructural cross sections, photograph (shown in Figure 4), and published 40Ar/39Ar dating data [Li et al., 2011, 2007; Yanet al., 2011] are shown in the map. The other symbols are the same as those in Figure 1.

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Figure 3

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3.1. Structural Framework of the Southern Qinling-Dabieshan Foreland Fold-Thrust Belt

The SQDB is divided into northern, southern, and frontal subbelts (Figure 1 and Figure S1 in the supportinginformation) by the MCXF (F1), Chaotianmen-Zhenba-Jingshan fault (F3), and Dahe-Qiaoting fault (F7) in theMicangshan region; the Tiexi-Wuxi fault (F4) in the South Dabashan region; and the Tongchenghe fault (F8)in the southern Dabieshan region (Figure 1). In the north, the MCXF (F1) links the SQDB to the fold-thrustbelt in the Qinling-Dabieshan region. The northern subbelt extends from the regions of Songpan and Hannanto Chengkou and Dahongshan and is overthrusted by the southward North Dabashan and Dabieshan thrustnappes in the Qinling-Dabieshan region [Zhang et al., 2001; Liu et al., 2003, 2005b].

The southern subbelt, which varies structurally along its east-west strike, includes the Micangshan, SouthDabashan, and southern Dabieshan fold-thrust belts. The folds and thrusts in this subbelt mainly involveCambrian to Middle Triassic strata in the north and Late Triassic to Middle Jurassic foreland basin deposits inthe south. The southern Dabieshan fold-thrust belt is unconformably overlapped by the Late Cretaceousto Early Tertiary deposits of the rift basin, which formed later during normal reactivation of the thrust faults atits basement (Figure S1e in the supporting information). The fold-thrust belt that developed below the riftbasin deformed the Triassic to Middle Jurassic strata, which suggests that this belt developed during the LateJurassic and Early Cretaceous [Liu et al., 2003]. The South Dabashan fold-thrust belt, which formed a typicalsouthwest-west protruding arc-shaped structure, deformed the Jurassic to Early Cretaceous strata (Figure S1din the supporting information) and was thrust over the earlier Micangshan belt in the northwest (Figure 1).Rapid pre-Late Cretaceous cooling/denudation in theMicangshan fold-thrust belt was coeval with the thrustingevent [Tian et al., 2012], which affected the deposition in the Late Jurassic to Early Cretaceous foredeep ofthe northwestern Yangtze Block [Liu et al., 2005b]. All of this evidence demonstrates that the Micangshan beltwas primarily formed during the Late Jurassic and Early Cretaceous and that the South Dabashan belt wasactive during the late Early Cretaceous to Late Cretaceous.

The frontal belt in the northwestern Yangtze Block developed a passive roof duplex structurewith a passive roofthrust fault within the Lower-Middle Triassic strata. The duplex thrust induced folding of the overlyingMesozoicstrata (including the Cretaceous) above the roof thrust fault (Figures S1a and S1c in the supporting information).These findings indicate that this part of the fold-thrust belt was active during the Tertiary [Tian et al., 2012].Therefore, the southern and frontal subbelts were primarily overprinted by postcollisional deformation, and thesyncollisional structure was best preserved in the northern subbelt (Figures 2–4).

3.2. Dextral Transpressional Structures

Thewestern part of the northern subbelt in the northern Longmenshan and Hannan regions is characterized bysouth or south-eastward propagating imbricate thrusting of Mesozoic and Neoproterozoic and Early Paleozoicstrata (Figures S1a and S1b in the supporting information). From the northern Longmenshan region to theHannan region, the structural trend changes from northeast to east-northeast. The northwestern part of thissubbelt (i.e., the Bikou basement uplift) is characterized by south or south-southeast vergent folds and thrusts;this part is bounded by the northeast trending Qinchuan fault to the south and the east-west trendingMCXF tothe north and forms a wedge-shaped structural zone that narrows to the east and widens to the west. Thesoutheast portion of this subbelt is characterized by moderately tight folding, closely spaced cleavage, minerallineation, and thrust faults (Figures S1a and S1b in the supporting information). The mineral lineations

Figure 3. Structural cross sections and stereographic projections of the plunging vertical folds and rotated clasticporphyroclasts showing the deformational features of thrusting with an oblique, dextral strike-slip component:(a) Luchiba-Maliu section, (b) Tianba section, (c) Dazhou section showing the site of sample S1103077 for 40Ar/39Ar dating,(d) Zhongtin section, (e) Dangan section, (f) Fangxian section, (g) Zhangji section, and (h) Gaoqiao section. The stereographicprojections: the black circles and black triangles in the stereograms denote the attitudes of planes of shear zones or thrustfaults and mineral lineations (L), and “N” denotes the numbers of measurements of fold hinges or B axes of rotated clasticporphyroclasts. All are equal-angle lower hemisphere projectionswith 10 contour intervals from zero to themaximumdensitymarked as “max” in each stereogram. Plunge azimuths and angles of fold hinges or B axes of rotated clastic porphyroclastsand representative mineral lineations (L) are shown in the stereograms. The directions of strike-slip offsets along faults andshear zones were primarily based on asymmetric plunging folds, rotated clastic porphyroclasts, mineral lineations, and S-Cfabrics. The ages of the units shown are Pt, Proterozoic; Z, Sinian (equivalent to Vendian); Z1, Lower Sinian; Z2, Upper Sinian; Є,Cambrian; O, Ordovician; S, Silurian; D, Devonian; C, Carboniferous; P, Permian; P1, Lower Permian; P2, Upper Permian; T1,Lower Triassic; T3, Upper Triassic; and K2-E, Upper Cretaceous to Tertiary. Photographs at points PhB, C, D, E, F, G, and H areshown in Figure 4. Other symbols are the same as those in Figures 1 and 2.

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developed during the first stage of cleavage surfaces weremeasured and used as indications of dextral strike-slipmovement in the Longmenshan thrust belt (Figure 2) [Yan et al., 2011].

A predominantly dextral strike-slip component accompanying the thrust faulting is demonstrated by themeasurements of hundreds of plunging vertical fold axes (Figures 4b, 4c, and 4e), foliations of rotatedporphyroclasts (Figures 4a, 4d, and 4f), S-C fabrics, and mica-fish structures along the MCXF zone, Zihuang faultzone, and Pingba fault zone in the middle of the northern subbelt in the South Dabashan and adjacent NorthDabashan overthrusts (Figures 2 and 3). The northern subbelt consists of imbricated thrust sheets involvingNeoproterozoic, Early Paleozoic, and Permian to Lower Triassic strata. The main thrust faults are typically

Figure 4. Photographs of (a, d, and f) rotated clastic porphyroclasts, (b, c, e, and h) plunging vertical folds, and (g) S-Cfabrics in shear zones showing deformational features due to thrusting with a dextral strike-slip component. The x-zdirections marked in Figures 4a, 4d, and 4f denote the axes of maximum and minimum strains. Standing persons in Figures4b and 4c and pencils in Figures 4e–4h provide scales. The locations of the photographs are denoted by designations PhA,B, C, D, E, F, G, and H in Figures 2 and 3.

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detached along the base of the Neoproterozoic (Lower Sinian), metamorphic, and conglomerate-sandstonestrata, and the thrust sheets have been tightly folded and faulted (Figure S1 in the supporting information andFigure 3). The northeast or southwest dipping thrust fault zones exposed near Chengkou in South Dabashan(i.e., the MCXF, Zihuang fault, and Pingba fault) display two flower structures (Figure S1d in the supportinginformation and Figures 3a–3c) that merge downward into a single crustal scale fault that dips to the northaccording to seismic reflection data [Dong et al., 2013]. These deformation zones are characterized by ~1–3 kmwide, penetrative-distributed, asymmetric plunging folds involving the Upper Sinian to Lower Paleozoiclimestone and foliations of rotated porphyroclasts in the Neoproterozoic phyllite bound by strike-slipping thrustfaults. The asymmetrical plunging fold hinges and the B axes of the rotated porphyroclasts within the southernregions of each fault zone plunge to the northeast or southeast. In addition, mineral lineations in this regionplunge to the northwest and thosewithin the northern part dip to the northwest (Figures 3a–3e). The asymmetryof the folds and porphyroclasts in the foliations resulted from the orientation between the boundary faults ofthe shear zones. Together, these data are consistent with transpressional deformation with a dextral strike-slipcomponent. This dextral strike-slip component was also identified along the basement-involved thrust faultzones in North Dabashan based on the plunging folds, mineral lineations, porphyroclasts, and S-C fabrics in thefoliations (Figure 3h).

In the eastern MCXF zone, the dextral strike-slip thrust deformation is characterized by ductile shear zones withS-C fabrics, mica-fish, rotated porphyroclasts, and plunging foliation folds (Figure 2, Figures 3f and 3g, andFigures 4g and 4h) in the town of Fangxian and the Dahongshan region. The hinges of the plunging foldsformed by dextral strike-slip deformation plunge to the east and northeast with a dip angle of ~50° in the townof Fangxian and the region of Dahongshan, respectively. The mineral lineations plunge to the west-northwest(WNW) in Fangxian. These features indicate a top-to-the-southeast thrust with strong dextral strike-slipmovement along the MCXF zone. At the eastern end of the MCXF zone in the Dabieshan, the earliest regionaldeformation corresponds to a southeastward compression that occurred during the subduction of the southChina plate below the north China plate and is dominated by southeast verging recumbent folds and thrustingof blueschist units during the Middle Triassic [Li et al., 2011]. Top-to-the-southeast shearing coeval with themetamorphism of Late Permian to Early Triassic blueschist facies was potentially associatedwith the developmentof regional foliation in the Dabieshan complexes and flat-ramp-style thrusts within the Neoproterozoic-Paleozoicsediment cover on the southern margin of Dabieshan [Li et al., 2011]. This deformation was formed by thesame kinematic processes as the dextral strike-slip thrust. Therefore, all evidence from the northern subbeltindicates the occurrence of a type of transpressional deformation with combined thrusting and dextralstrike-slip faulting along the Mianlue suture zone in the SQDB.

3.3. Geochronology of Dextral Transpressional Deformation

The ages of the dextral transpressional deformation in the northern subbelt were constrained by 40Ar/39Arradiometric dating of biotite and sericite and the unconformity. One biotite and two sericite separates wereselected for 40Ar/39Ar dating based on their field and textural relationships. Samples S1103106 (biotite)and S1103017 (sericite) were separated from the syntectonic mica-quartz schist with well-developed foliationand lineation in the Mianlue suture zone and from the southern thrust fault within the Neoproterozoic BikouGroup, respectively (Figure 1). These schists contained quartz, plagioclase, biotite, sericite, and minor clasticfragments. In addition, the synkinematic sericite and fine-grained biotite were mainly developed along thefoliations. Sample S1103077 (sericite) was collected in South Dabashan from Neoproterozoic phyllite withpenetrative foliations of rotated porphyroclasts (Figures 1 and 3c). This phyllite consists of metamorphosedconglomerate and sandstone tillites deformed by the Pingba thrust belt. In this rock, rotated quartz andclastic grains are surrounded by synkinematic sericite and fine-grained biotite and chlorite (Figure 4d).

The 40Ar/39Ar plateau ages of the biotites (S1103106) and sericites (S1103017) separated from the westernpart of the northern subbelt are 201.93 ± 0.71Ma and 207.8 ± 3.22Ma (Figures S2a and S2b in the supportinginformation), respectively. The sericites separated from the mica-quartz schist and the detachment fault inthe northern Longmenshan thrust belt yield well-defined 40Ar/39Ar plateau ages ranging from 197.0 ± 0.9 to183.2 ± 1.4Ma (Figure 2) [Yan et al., 2011]. These ages are also supported by the unconformities in northernLongmenshan and Hannan. A Late Triassic to Middle Jurassic wedge-top basin unconformably overlies theMianlue suture (Figure S1b in the supporting information). Within the northern and southern subbelts innorthern Longmenshan, Early Jurassic conglomerates are tilted and folded and unconformably overlie the

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deformed Silurian to Triassic strata (Figure S1a in the supporting information). These results indicate that thedeformation began during the early Late Triassic in Hannan and during the Late Triassic to Early Jurassic innorthern Longmenshan.

The 40Ar/39Ar preferred ages of sericite in the middle part of the northern subbelt in South Dabashan are245.38 ± 3.26Ma and 191.13 ± 2.59Ma, respectively (Figure S2c in the supporting information). These agespotentially represent the initial syncollisional, strike-slip thrust deformation, and its subsequent postcollisionaloverprinting. In addition, this earlier syncollisional deformation age is strongly supported by our observationthat the Late Triassic conglomerate and sandstone unconformably overlie the deformed Early Triassicstrata (Figure 3c).

The high- and ultrahigh-pressuremetamorphism in Dabieshan was constrained at 244–236Ma and ~230–220Ma[e.g., Ames et al., 1993; Li et al., 2000; Liu et al., 2004], and the age of the early, top-to-southeast, upward extrusionof high- and ultrahigh-pressure rocks ranged from 241 to 231Ma (Figure 2) [Li et al., 2011]. Thus, thesepublished data indicate Middle Triassic ages for the continental collision in the eastern part of the northernsubbelt [Liu et al., 2003].

4. Transition From a Closing Ocean to a Progressively Developed Foreland Basin

The northern Yangtze foreland basin belt, which bounds the southern margin of the Qinling-Dabieshanorogen, is now present in the Sichuan, Dangyang, and Southeast Hubei Basins (Figure 1). These basinscontain thick sedimentary successions of Middle and Upper Triassic strata and provide a relatively continuousrecord of source-area deformation. The stratigraphic successions and paleogeographic evolution of theforeland basin belt and its associated Songpan Ocean Basin to the west, which developed along the northernYangtze Block south of the Mianlue suture belt, provide detailed sedimentologic evidence regarding thegeodynamics of the continental collision.

4.1. Stratigraphic Successions of the Foreland Basin

The sedimentary successions of the Early Mesozoic basins suggest that two phases of deposition wereassociated with ocean closure and foreland basin development, with one phase spanning the late MiddleTriassic through the early Late Triassic and one spanning the late Middle Triassic (to the east) through LateTriassic (Figure 5). The first phase, which is primarily represented in the Songpan Basin and westernmostSichuan Basin, produced a marine succession. The Ladinian to Early Norian flysch (>10 km thick) in theSongpan Basin and the time-equivalent carbonate ramp (<600m thick) and clastic and coaly paralic andshoreline (~5–800m) successions of the Tianjinshan and Maantang Formations in the westernmost SichuanBasin were well documented by Nie et al. [1994], Xu et al. [1996], Guo et al. [1996], Zhou and Graham [1996],Yang and Yang [1997], Meng et al. [2007], andWeislogel et al. [2010]. The late Middle Triassic and Late Triassicphases are exposed in the Sichuan, Dangyang, and Southeast Hubei Basins and contain extensive records ofmarine and nonmarine foreland basin molasse accumulation [Liu et al., 2005b]. The diachronous boundary

Figure 5. Middle Triassic through Late Triassic stratigraphic units and their correlation across the northern Yangtze region.The vertically ruled pattern represents unconformities. Mbr., Member and Fm., Formation.

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between these two phases was identified from our detailed measurements and the time correlations of theEarly Mesozoic sedimentary successions, which recorded the transition from the closing ocean basin with asouthern passive continental margin to the foreland basin stepping from east to west (Figure 5).

4.2. Westward Stepping Closure of the Ocean Basin and Diachronous Initial Developmentof the Foreland Basin

Geological and paleomagnetic analyses suggested that the Mianlue Ocean, which separated the north Chinaand south China plates, was wide in the west, narrow in the east, and connected to the Paleo-Tethys during theLate Paleozoic [Lai et al., 1995; Zhao and Coe, 1987]. The deposition of the Lower Triassic and lower MiddleTriassic successions in northern Songpan was characterized by the development of shallow-marine carbonateduring the Early Triassic to the early Middle Triassic and a rapid shift to deep-marine flysch sediments beginningin the Ladinian [Meng et al., 2007]. From the Ladinian Stage of the Middle Triassic to the Early Norian Stageof the Late Triassic, an ocean basin was present in the Songpan region that resulted in the deposition of a thickturbidite sequence [Zhou and Graham, 1996;Meng et al., 2007] (Figures 6a and 6b). However, the Ladinian andCarnian sequences in the westernmost Sichuan Basin were composed of carbonate ramps and tidal-influencedshallow-marine facies. Furthermore, no depositional records were found farther to the southeast (Figures 6and 8) [Liu et al., 2005b]. This basin system pinches out toward the east due to the long-distance overthrustingof the MCXF. However, shallow-marine and terrestrial molasse deposits of the same age are exposed farthereast (Figure 6).

In front of the northern subbelt, the lateMiddle Triassic Huangmaqing Formation deposits [Li and Jiang, 1997] inthe basin southeast of Dabieshan (Figure 5) are composed of ~1300–2000m thick, very fine grained sandstone,and sandymudstone with intervals of lenticular limestone and fine-grained and pebbly sandstone that becomecourser upward. The lower mudstone is horizontally laminated, and the very fine grained sandstone displaysrippled or parallel stratification. The interbedded fine-grained sand body in the upper part is cross or troughcross and parallel stratified. The entire succession contains bivalve fossils and trace fossils. We interpret theselithofacies as originating from a coastal plain and shallow marine delta [Legler et al., 2014]. Here the deposition

Figure 6. Tectonic paleogeographic maps of the northern Yangtze Block: (a) Carnian Stage, (b) Early Norian Stage,(c) Middle Norian Stage, (d) Early Rhaetian Stage, and (e) Late Rhaetian Stage of the Late Triassic. Paleocurrent directionsshown in the rose diagrams with an arrow in the north were taken from cross beds, imbricated gravels, and flute orientations.Tilt corrections were applied for strata with dip angles greater than 10°. The thicknesses (fine black dotted lines (m)) of theXiaotangzi Formation, Member 2 of the Xujiahe Formation, Member 4 of the Xujiahe Formation, and Members 5, 6, and 7of the Xujiahe Formation in the Sichuan Basin are shown in Figures 6b–6e, respectively. Other symbols are the same asthose in Figure 1.

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was differentiated from the lower part of the Jiuligang Formation in a previously published section of theDangyang Basin (Figure 7d) [Liu et al., 2005b]. This part of the succession rests unconformably on an older,shallow-marine succession (the Badong Formation), which indicates that it predates the uplift in theQinling-Dabieshan orogen [Liu and Zhang, 1999]. Channelized conglomerates (1m) exist at its base thatbecome finer upward and transition into tabular cross-stratified sandstones (16m) [Liu et al., 2005b] with aminimal detrital zircon U-Pb age date of 242Ma. Thus, these sandstones were likely deposited during theLadinian Stage (Table S2 in the supporting information and Figure 10). The basal conglomerate is overlaid by70m of the Jiuligang Formation, which primarily consists of bioturbated calcareous siltstone of tidal origin withcarbonate nodules and intervals of lenticular sandstone and conglomerate [Liu et al., 2005b]. The overlyingstratigraphy of the early Late Triassic Juligang Formation above the late Middle Triassic succession wasinterpreted as a meandering stream and deltaic deposits by Liu et al. [2005b].

Several incomplete stratigraphic sections exposed in South Dabashan (Figure 3c) provide a representativerecord of Late Triassic sedimentation within the northern subbelt. The Pingba section near the town ofChengkou (Figure 7c) consists of a ~150m thick succession of stacked sand bodies and interbedded veryfine grained sandstone and mudstone with cobbled to pebbly conglomerate at the base. Each main sandbody is coarse to medium-grained with massive, low angle, and/or trough cross bedding. The base of mostsandstone beds is a sharp, undulated scour surface with mudclasts, pebbles, and tree trunks. We interpretthese beds as amalgamated channels [Schumm, 1977]. The interbedded intervals of rippled, thin-bedded,very fine grained sandstone and mudstone with abundant leaf fossils and wood fragments in this area likelyrepresent levee and floodplain deposits [e.g., Kraus and Bown, 1993]. The minimum U-Pb detrital zircon ages

Figure 7. Regional Middle Triassic through Upper Triassic stratigraphy across section S-A along the northernmargin of the northern Yangtze foreland basin based onmeasured sections in (a) Yangjiayan, (b) Mianxian, and (c) Pingba, and revised section in (d) Jiuligang [Liu et al., 2005b]. The ages shown in the (b) Mianxian, (c) Pingba,and (d) Jiuligang sections indicate the minimum ages of U-Pb detrital zircon dating from the samples marked beside the sections (Table S2 in the supportinginformation and Figure 10). M, mudstone; S, sandstone; G, gravel. Fm., Formation and Mbr., Member. The location of section S-A is shown in Figure 1.

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of 220–228Ma and 203–206Ma of the samples from the section (Table S2 in the supporting information andFigure 7c) indicate that the sandstones were deposited during the Norian and Rhaetian Stages.

West of South Dabashan, the Xiaotangzi Formation of the early Norian Stage, unconformably rests upon theMaantang Formation of the Carnian Stage or the Likoupo Formation of the Middle Triassic, and mainly consistsof two or three depositional cycles (Figures 7a and 7b). Each depositional cycle exhibits a coarsening-upwardsuccession of interbedded fine-grained sandstone and mudstone with ripple, horizontal bedding, mud drapes,cross-bedding, and penecontemporaneous deformations. The mudstone contains trace fossils, such asTeichichnus and Chondrites, and bivalve fossils, such as Yunnanophorus boulei, Myophoriopis nuculiformis, andHeminajas forulota. We interpret these beds as tidally influenced delta deposits [Aschoff and Steel, 2011; Legleret al., 2014]. Minimum detrital zircon U-Pb age dates of 216–229Ma were obtained from the samples fromthe base of the Mianxian section, which further defined this succession as belonging to the Norian Stage of theLate Triassic (Table S2 in the supporting information and Figure 7b).

In summary, the depositional section along the northern Yangtze Block south of the MCXF indicates that thebasin system includes a closing oceanic depozone with a southeastern carbonate ramp in the west and initialdevelopment of a foreland depozone in the east (Figures 6a and 6b). The oceanic depozone was part ofthe Mianlue Ocean Basin, and the foreland depozone was part of the northern Yangtze foreland basin. Thisforeland basin was initiated in the southern Dabieshan during the Ladinian Stage of the Middle Triassic anddiachronously migrated to the town of Mianxian and the western Sichuan Basin in the Early Norian Stagesof the Late Triassic, passing through South Dabashan. Paleocurrent indicators (cross bedding) indicatepredominantly westward and southwestward paleoflow in the east and southward and southwestwardpaleoflow in the west (Figures 6a and 6b).

4.3. Westward Progradation of the Foreland Basin

During the late Norian Stage of the Late Triassic, the strata in the Songpan region began experiencingdeformation. The ocean basin in the Songpan region and western Sichuan gradually closed and evolved toform a foreland fold-thrust belt and foreland basin system. The northern Yangtze foreland basin laterallymigrated to western Sichuan (Figure 6c).4.3.1. Middle Norian Through Early Rhaetian StageTheWanglongtan Formation, which filled the eastern part of the northern Yangtze foreland basin (Figure 7d), isprimarily composed of an ~850m thick sequence of nested channel-fill sandstones. On a scale of severalmeters, these channel deposits consist of massive or plane-parallel laminated medium sandstones overlain bytrough and planar cross-stratified sandstones with similar grain sizes. The entire succession was interpretedto be a deposit from multichannel braided streams by Liu et al. [2005b].

In the west, the stratigraphic sections exposed north of the Sichuan Basin provide a representative record ofMiddle Norian to Early Rhaetian deposition (Figures 7 and 8). The succession at Yangjiayan consists of Members2, 3, and 4 of the Xujiahe Formation (Figure 7a). Member 2 is primarily composed of ~10 to 30m thick, stacked,medium-grained sand bodies and ~0.5 to 2m thick interbedded mudstone beds. Two lithofacies weredeveloped in the sand bodies. One lithofacies is approximately 20–30m thick and contains vertically stacked,laterally extensive sand bodies with massive, parallel laminated or low-angle cross-stratified beds, in whichsharp, undulated scour contacts with mudclasts and tree trunks and lag deposits are developed. The otherlithofacies is approximately 15m thick and consists of lenticular coarse sandstone with cross-bedded strata anda basal scour surface. Themudstone beds contain ripple bedding and trace fossils. We interpreted these beds asdelta-front sheets with grain-flow and subaqueous channel deposits. Together with the underlying fine-grainedfacies assemblages in the uppermost portion of the Xiaotangzi Formation (Figures 7a and 7b and Figures 8eand 8f), the entire succession is composed of a braided-channel delta sequence that becomes coarser upward(seeMcPherson et al. [1987] for analogous examples). Member 3 of the Yangjiayan section (Figure 7a) primarilyconsists of interbedded 10 to 30 cm thick fine-grained sandstones, 1 to 5m thick mudstones, and very finegrained siltstone beds. In addition, Member 3 contains ~50 to 70m thickmedium-grained sandstone bed in themiddle. The mudstone and siltstone are horizontally laminated and contain bivalve fossils. The fine-grainedsandstone intervals developed massive beds and ripple lamination and are interpreted as lake or predeltadeposits. The medium-grained sandstone contains vertically stacked, laterally extensive sand bodies withmassive, parallel-laminated, or cross-stratified beds and penecontemporaneous deformed beds, where scourcontacts and lag deposits are developed. This sandstone is interpreted as a delta-front sheet deposits within the

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braided channel-delta systems (see Liu and Yang [2000] for analogous examples). Massive and cross-stratifiedsandstone compose most of the lower part of Member 4. The upper part of this member features verticallystacked conglomerate beds with ~0.5m thick calcareous sandstone and limestone intervals. The polymicticconglomerate beds are 0.5 to 3m thick, are dominated by carbonate gravel clasts (up to 85% of the volume),and display massive or crude cross stratification. The very fine grained siltstone and mudstone beds are minorand display scouring by the overlying sandstone and conglomeratic beds. We interpreted these lithofacies assubaqueous sandy and gravelly debris-flow, channel, and plug-flow deposits that developed on a delta front.Members 3 and 4 collectively constitute another braided-channel-delta sequence [McPherson et al., 1987; Liuet al., 2005b]. Member 4 is unconformably overlapped in places by Lower Jurassic Baitianba conglomerates,with a hiatus of the upper part of the Xujiahe Formation.

Therefore, during the Middle Norian to the Early Rhaetian Stages of the Late Triassic, the northern Yangtzeforeland basin was primarily filled with nonmarine deposits, including those of the braided-channel plains inthe east, braided deltas in the west, and lacustrine deposits in the middle of the Sichuan Basin, whichprograded westward (Figures 6c and 6d and Figures 7–9). Paleocurrent indicators (cross bedding, imbricatedgravels, and flute orientations) indicate a predominantly westward paleoflow, although smaller transversesystems also flowed in from the southeast and northeast. The zone of maximum foredeep subsidence, asindicated by isopach patterns, crossed the area later occupied by the Longmenshan thrust belt, whereas thesmaller, backbulge depozones were primarily located farther to the southeast in the center of the present-day Sichuan Basin, where the lake deposits were present.4.3.2. Late Rhaetian StageDuring the Late Rhaetian Stage of the Late Triassic, the northern Yangtze foreland basin expanded southward,and the lake deposits continuously withdrew westward. The braided-channel deposits continuously accumulatedin the eastern part of the foreland basin (Figure 6e). To the south, the Late Rhaetian succession occurs, whichcorresponds with the upper part of the Wanglongtan Formation, is ~50–100m thick, and fines upward. Thebase of this succession contains pebbly coarse to medium-grained sandstone beds, and its upper part consists

Figure 8. Regional Upper Triassic stratigraphy across section S-B in the Sichuan Basin, based on well logs [Ma et al., 2009].The other symbols are the same as those in Figure 7. The location of section S-B is shown in Figure 1.

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of a series of upward fining units. Each of these units typically begins with a scoured channel base and isoverlain by 5–15m of fine-grained sandstone with well-developed tabular cross beds or, in places, lateralaccretionary bedding. These sand bodies are lenticular and 50–80mwide. An interval of rippling or horizontallylaminated very fine grained sandstone and mudstone containing leaf fossils overlie the sand bodies. Weinterpret this interval as representing deposition by meandering channels (see also Liu and Yang [2000] foranalogous deposits) (Figure 6e).

In the west, the Late Rhaetian succession exposed in the Sichuan Basin is composed of Members 5 through 7of the Xujiahe Formation (Figures 8 and 9). Members 5 and 7 primarily consist of interbedded thin sandstoneand mudstone beds. The sandstone beds are fine to very fine grained and exhibit ripple bedding, and therippled and laminated mudstone contains abundant coaly shale beds and coal seams (Figures 9j and 9m).Member 6 is primarily composed of medium and coarse-grained sandstone, which is primarily medium tothickly bedded, massive to planar, and cross stratified. The beds fine upward and display erosional bases withmudstone and coaly shale clasts (Figure 9m). This succession is interpreted as a record of braided-channeland flood-plain deposition [Olsen et al., 1995].

Paleogeographic analysis suggests that the northern part of the foreland basin was filled with a braid plain,shallow lake, and deltaic deposit during the Late Rhaetian Stage (Late Triassic). By contrast, eastern-derivedfluvial plain deposits were dominant in the south (Figure 6e). The braided-channel plain deposits continuouslyprograded westward, and the lake deposition laterally migrated to the southwestern Sichuan Basin. Paleocurrentindicators (planar cross bedding and trough axes) suggested that the paleoflow was to the west-southwest.However, the paleocurrent indicators in the southern part of the foreland basin were primarily to the northwestor north. Beginning in the Middle Norian, the area transitioned from a foreland to a wedge-top depozone nearthe towns of Mianxian and Chengkou as the northern subbelt stepped southward (Figure 6).

Thus, the lithofacies and subsidence patterns indicated a transition from a closing Mianlue Ocean to aforeland basin that progressed from east to west during the Triassic. In this foreland basin system, thedeposits prograded westward, and the depocenters migrated west or southwest. This lateral depositionalmigration contrasts the forward progradation models of typical foreland basins [DeCelles and Giles, 1996].

5. Basin Sediment Provenance and Its Dispersal Pattern

Single-grain U/Pb dating of detrital zircons is a potent tool in provenance studies because it can fingerprintsource areas with distinctive zircon age populations [Gehrels et al., 1995; Amidon et al., 2005]. The zircon age

Figure 9. Regional Upper Triassic stratigraphy across section S-C in the Sichuan Basin based on measured sections and well logs [Ma et al., 2009]. The location ofsection S-C is shown in Figure 1. The other symbols are the same as those shown in Figure 7.

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populations recorded in the basin sediments provide a useful indicator of the source-area exhumation anderosion and dispersal patterns of the corresponding synorogenic sediments. Single detrital U-Pb zircondating of the northern Yangtze foreland basin sediments and its use in provenance analysis indicate theunroofing of the Qinling-Dabieshan orogen and adjacent source terranes and their sediment contributions tothe basin in response to the continental collision.

5.1. Detrital Zircon U-Pb Ages and Trace Elements

A total of 1248 single detrital U-Pb zircon age dates frommore than 20 samples of the Upper Triassic sandstonesare listed in Table S2 in the supporting information (for detailed descriptions of our sampling and analyticalmethods, see the supporting information). These analyses yielded the age dates of fivemajor zircon populations,the Permian-Triassic, Paleozoic, Neoproterozoic, Late Paleoproterozoic, and Early Paleoproterozoic (Figure 10).The Permian-Triassic zircon population is in the age range of 200–310Ma. Nearly all of the analyzed zircons in

Figure 10. Probability density plots and histograms of detrital zircon ages from the Triassic (~242–201Ma) samples. Thesample locations are shown in Figure 6. Higher-resolution plots of Permian-Triassic ages are shown to the right of the figure.

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this population exhibit Th/U ratios of 1.0–0.3, and their cathodoluminescence (CL) images primarily show clearoscillatory zoning (Figures S3 and S4 in the supporting information). Three zircons exhibit Th/U ratios of lessthan 0.07 with an unzoned (uniform) or irregular internal structure (Figure S3j (08) in the supporting information).The former (the majority) are typical grains of igneous origin, whereas the latter (the three distinct grains)are characteristic metamorphic grains [Corfu et al., 2003]. Trace elements in the 24 detrital zircon grainsfrom samples S110380, S110355, and S070655 are characterized by distinct enrichment in heavy rare earthelements (HREEs) ((Lu/Gd)N = 10–44), positive Ce anomalies (δCe= 1.26–87.82), and negative Eu anomalies(δEu = 0.01–0.69) (Figure S5a in the supporting information). The corresponding Th/U ratios of these zirconsvary from 0.14 to 0.7. These features further indicate that the Permian-Triassic zircon population is mostly ofigneous origin. The zircon grain dated at 219Ma (S08103174; Th/U ratio = 0.04) has a lower REE value, arelatively flat HREE pattern, a positive Ce anomaly, and no negative Eu anomaly, whichmatch those of zirconswith very low Th/U ratios of metamorphic origin [Rubatto, 2002].

The Paleozoic, Neoproterozoic, Late Paleoproterozoic, and Early Paleoproterozoic detrital zircon grains range inage from 400Ma to 490Ma, 650 to 900Ma, 1640 to 1960Ma, and 2200 to 2600Ma, respectively. Similarly, theCL images of these zircon populations show that typical igneous oscillatory zoning dominates these groups,with most of the Th/U ratios exceeding 0.2. By contrast, approximately 24 grains display sector/planar zoned orunzoned internal structures (Figures S3d (059), S3f (02 and 07), S3g (S110920-65 and S110308-58), S3h (09),and S3j (56) in the supporting information). Of these grains, 5 grains ranging in age from 466 to 415Ma haveTh/U ratios of <0.054, 3 grains ranging in age from 900 to 1022Ma have Th/U ratios of 0.052–0.07, 13 grainsranging in age from 1529 to 1981Ma have Th/U ratios of <0.079, and 3 grains ranging in age from 2270 to2600Ma have Th/U ratios of 0.032–0.069 (Figure S4 in the supporting information). The grains of these fourgroups from samples S110355 and S070655 have variable but generally high chondrite-normalized REEconcentrations, steep HREE patterns, and large negative Eu anomalies. The chondrite-normalized patternsof the zircons from 2200–2600Ma, 400–500Ma, 690–900Ma, and 1640–1960Ma exhibit large negative Euanomalies and are enriched in HREEs (Figure S5 in the supporting information). The δEu values of thesegroups (i.e., 0.11–0.56, 0.06–0.62, 0.02–0.35, and 0.01–0.22, respectively) exhibit a general decreasing trend.These four groups of grains are dominated by zircons of igneous origin, as indicated by their internal CLimages (Figure S3 in the supporting information). In contrast, a few zircons (shown in Figure S5 in thesupporting information) with ages of 429Ma (sample SC110301, Th/U=0.027), 413Ma (sample S070655,Th/U=0.55), 900Ma (sample S1109020, Th/U=0.05), 1947–1886Ma (samples S110355, Th/U=0.07; SC110301,Th/U=0.07; and S081031, Th/U = 0.03), and 2403Ma (sample S110355, Th/U = 4.49) yield lower REE values.In addition, their normalized REE patterns display lower negative Eu anomalies (δEu=0.15–0.62) (Figures S5 inthe supporting information). These zircon grains (e.g., those in Figures S3c (021 and 024), S3f (02 and 07),and S3g (65) in the supporting information) consistently exhibit sector and planar-zoned internal structures,which indicate the metamorphic origins of the grains.

5.2. Potential Source Areas of Detrital Zircons

From the Middle Neoproterozoic to the Early Triassic, the Yangtze Block was a stable marine depositionalenvironment [Zhang et al., 2001]. Continental collision between the north China and south China plates alongthe Qinling-Dabieshan orogen resulted in a major regression across the Yangtze Block, the initiation ofterrestrial conditions in the northern Yangtze foreland basin, and a completely changed pattern of sedimentdispersal and provenance. The paleogeographic reconstruction of the foreland basin and the publisheddetrital zircon dating of the sediments [e.g., Enkelmann et al., 2007; Weislogel et al., 2010; Yang et al., 2010;She et al., 2012] indicate that the U/Pb zircon ages can be correlated with discrete source areas in theQinling-Dabieshan ranges in the north, the Yangtze Block, and the Cathaysia Block in the south.

The Permian-Triassic zircon population corresponds to the ages of the high- and ultrahigh-pressure metamorphicbelt of the Dabieshan to the east [e.g., Li et al., 2000; Liu et al., 2010; Li et al., 2011; Liu and Zhang, 2013]. The CLand trace element data, however, indicate that most of the population is of magmatic origin (Figures S3–S5 inthe supporting information) and is correlated with the documented multistage arc magmatic events alongthe Mianlue suture zone in the western part of the southern Qinling orogen [Ping et al., 2013; Qin et al., 2013].The peaks at ~224Ma and 234–241Ma in the Permian-Triassic zircon population are consistent with Triassiccontinental collision and subduction along the Mianlue suture zone [Zhang et al., 2001]. The detrital zirconsyield negative εHf(t) values (�25.1 to �0.52) with Hf model ages ranging from 1.32 to 2.85Ga, which are

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identical to those of the Triassic granitoid plutons in the western Qinling orogen (Figure S6 in the supportinginformation). These results do not support the suggestion that the sedimentation that occurred during theclosing ocean basin in the Songpan region was derived from the uplift and erosion of the high- andultrahigh-pressure Dabieshan terrane [Yin and Nie, 1993; Zhou and Graham, 1996; Nie et al., 1994]. The agesof the turbidites exceed those of the well-established exhumation ages of the source area [Li et al., 2000;Enkelmann et al., 2007; Weislogel et al., 2010; Liu et al., 2010; Li et al., 2011; Liu and Zhang, 2013], and thedetrital zircons of igneous origin are not found in the Qinling-Dabieshan core zone. Instead, we interpretthis zircon population as an influx of materials derived from the erosion of the Mianlue suture zone.Structural mapping and seismic interpretation indicate that the long-distance overthrust of the MCXFcovered the parts of the northern subbelt and its adjacent Qinling-Dabieshan microplate (Figure S1 in thesupporting information). The Mianlue zone formed in the region of the Qinling-Dabieshan orogen duringthe Late Paleozoic and Early Triassic but is now largely covered by later thrust faults [Liu et al., 2003; Zhanget al., 2004]. In addition, only a portion of the arc igneous rocks remained in the Mianlue suture zone inthe western part of Qinling-Dabieshan orogen north of the Hannan region.

The Paleozoic detrital zircon signature is consistent with the distribution of Paleozoic volcanic rocks, granitoids,and supraultrahigh-pressure rocks [Weislogel et al., 2010] along the southern Qinling-Dabieshan orogen, whichwere potentially exhumed by the thrusting. The peak age of ~437–466Ma in this zircon population mostlycorresponds with the Early Paleozoic rift-related magmatism. However, a few of the zircons are consistent withthe metamorphism of Early Paleozoic, high-grade, granulite facies in the Qinling-Dabieshan orogen [Zhanget al., 2001; Wang et al., 2011a]. The zircons from the sediments and the southern Qinling-Dabieshan orogenhave consistent Hf isotope compositions with εHf(t) values between ~10 and 10 and Hf model ages of between0.7 and 2.1Ga [Wang et al., 2011a; Shi et al., 2013]. The Neoproterozoic detrital zircon population with apeak age of ~720–880Ma and the εHf(t) values of between �15.5 and 5.2 in the foreland sequenceoverlaps the timing and Hf isotope compositions of the widespread arc and rift-related magmatism andsedimentation within the Qinling-Dabieshan orogen in the northern Yangtze Block (Figure S6 in thesupporting information). This evidence suggests that this population was derived from that area [Zhanget al., 2006; Liu et al., 2008].

In contrast, the detrital zircon population of Late Paleoproterozoic age (~1.85Ga) is correlated with thewell-documented series of thermal events that occurred between 1.9 and 1.8 Ga in the south China plate,which is also consistent with the U-Pb age patterns and Hf isotope systematics of the detrital zircons from thebasement rocks of the Cathaysia Block within the south China plate [Greentree et al., 2006; Yang et al., 2010;She et al., 2012]. Zircons with ages of 1.9–1.8 Ga and ~1.86–1.87 from the granitoids and Precambrianbasement metapelitic rocks, respectively, were reported in the eastern Cathaysia Block, with Hf continentalmodel ages ranging from 3.0 to 2.5 Ga [e.g., Wan et al., 2007; Yu et al., 2009; Zhao et al., 2014]. These ageranges are identical to those of the Late Paleoproterozoic zircons described in this study (Figure S6 in thesupporting information). The ages of the Paleoproterozoic zircons indicate their derivation from basementrocks or from recycled materials on the south China plate. The predominant westward and northwestwardpaleocurrents are consistent with a provenance from the east or southeast. Lastly, the Neoarchean and EarlyPaleoproterozoic zircon populations correspond with magmatic events identified in the Kongling Complexand Huangtuling granulites from the Yangtze Block, both of which have identical Hf isotope compositions(Figure S6 in the supporting information) [Zhang et al., 2006; Liu et al., 2008]. The source of this zircon groupwas likely located south or southeast of the northern Yangtze foreland basin [She et al., 2012].

Moreover, detrital zircon populations from the foreland basin fill are very similar to those present in theTriassic turbidite samples in the Songpan Basin [Enkelmann et al., 2007;Weislogel et al., 2010], suggesting thateither the basins shared similar source areas or the foreland basin deposits were partially derived fromrecycling of the earlier turbidite sequence because it was deformed by and incorporated into the SQDB as theocean basin closed and the Mianlue suture stepped westward [Graham et al., 1975].

5.3. Evolution of Basin Provenance

The spectra of detrital zircon ages appear to be endemic to various parts of the foreland basin. These agespectra are distributed among the various subbasins within the foreland basin system, which indicates thatnot all of the sediments were derived from a single sediment source to the east (Figure 10). The wedge-topdepozone, which is barely exposed in the northern subbelt, contains zircon populations of the Middle-Late

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Triassic and Neoproterozoic age ranges, which, together with the southward to south-westward paleocurrentdata, indicate that they were derived from the Mianlue suture zone. This suture zone and the depocenter areprimarily covered by the later Dabashan and Dabieshan thrust nappes [Liu et al., 2003].

In the foredeep depozone, the multiple zircon age populations and the various paleocurrent data indicate aninflux from the north to northeast and east. This observation indicates an influx of materials derived from thenearby Mianlue suture zone, the southern Qinling-Dabieshan orogen farther east yielding Paleozoic ages,the northern Yangtze Block providing Neoproterozoic basement ages, and the south China plate to thesoutheast likely providing the Late Paleoproterozoic zircon ages (Figure 10). This detritus was potentiallypartially derived from the reworking of the turbidite deposits that were exposed in the Mianlue suture zone.Samples from the lower part of the Jiuligang Formation (Ladinian and Carnian) in the Dangyang Basin arecharacterized by major Paleozoic, Neoproterozoic, and Permian-Triassic zircon populations, indicating thesouthern Qinling-Dabieshan orogen, northern Yangtze Block, and Mianlue suture sources. The samples fromthe Xiaotangzi Formation (Early Norian) in the western Sichuan Basin exhibit the same characteristics as thesamples from the Jiuligang Formation, except for a decrease in the influx of Paleozoic and Neoproterozoiczircons. After the Middle Norian, the foredeep deposits experienced an increase in the influx of the LatePaleoproterozoic zircons and a decrease in the influx of the Permian-Triassic and Paleozoic zircons. The samplesfrom the upper portions of the Wanglongtan Formation in the Dangyang Basin are characterized by uniquemajor zircon populations dating ~1850Ma (Figure 10). This observation suggests that increased infilling andlongitudinally bypassing of sediments derived from the south China plate occurred during the Late Triassic.

The backbulge depozone deposits contain a significant Early Paleoproterozoic zircon population derivedfrom the south China plate that is consistent with northward or northwestward paleocurrent indicators.Therefore, the northern Yangtze foreland basin was filled by a major prograding river-deltaic system that wasfed longitudinally along the basin from the eastern source areas, from the SQDB to the north and from thesouth China plate to the south.

6. Discussion

Based on the syncollisional structural deformation in the SQDB, Triassic sedimentation in the northernYangtze foreland basin, and basin detrital zircon provenance, we have interpreted the history of the forelandfold-thrust belt and foreland basin, as well as the continental collision mechanisms, as shown in Figure 11.

6.1. Evolution of the Mountain-Basin System

The reconstructed dextral strike-slip thrust belt and the northern Yangtze foreland basin system alongthe Mianlue suture indicate that the south China plate was obliquely subducted to the northwest under thenorth China plate in the Early Mesozoic (Figure 11). We interpret this structural evidence to indicate that the

Figure 11. Paleogeographic/paleotectonic reconstructions showing the mechanism of the diachronous closure of thenortheastern Paleo-Tethys Ocean and the development of the Mianlue suture during the Middle and Late Triassic.(a) Ladinian Stage, (b) Early Norian Stage, (c) Middle Norian Stage, and (d) Late Rhaetian Stage. The other symbols are thesame as those in Figures 1 and 6.

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Mianlue suture resulted from an oblique collision between the north China-Qinling Dabieshan and south Chinaplates by northwest-southeast compression, where the subducting plate underwent suture-normal andsuture-parallel motions. This oblique motion resulted in a combination of thrusting and dextral strike-slipfaulting, which migrated westward from the Middle Triassic in southern Dabieshan to the Late Triassic inwestern Longmenshan as the underthrusting continued. Meanwhile, the ocean basin retreated as the forelandbasin progressed westward and the SQDB sutured westward with time. The lithofacies and patterns ofsubsidence and sediment dispersal indicate that a transition occurred from a closing Mianlue Ocean to aforeland basin during the Triassic. This transition progressed from east to west as the northern Yangtze wassubducted beneath the north China plate and the SQDB migrated to the west, in a manner similar to a zipper.

The Ladinian marine molasse foreland basin was formed in Southeast Hubei in response to the initialthrusting in southern Dabieshan, and the Mianlue ocean basin potentially extended across southernDabashan at the same time. During the Ladinian to Carnian, the Songpan turbidite depocenter in the MianlueOcean basin extended farther east than today and collapsed during the initial oblique continental collisionbefore being overlapped by the foreland basin. As the suture lengthened westward, the foreland basinsuccessively progressed from east of South Dabashan to the west. During the Early Norian, the south Chinaplate continued to collide with the north China plate as the Mianlue Ocean closed and the Mianlue suturezone lengthened. Following this closure, the foreland basin in front of the SQDB overlapped and migratedwest. Collectively, the early Late Triassic detrital zircon age distribution and the paleocurrent indicators inthe basins demonstrate that the sediment source was primarily from the southern Qinling-Dabieshan orogento the northeast, and the Paleozoic blanket rocks and Mianlue suture were largely exposed during suturing inthe eastern part of the orogen. Most of the detrital zircon data from the foreland basin filling units andthe corresponding paleocurrent patterns indicate a provenance from the deforming Mianlue suture beltlocated east of the northern Yangtze foreland basin. As this suture belt closed, the source areas steppedwestward toward the evolving foreland basin.

By the Middle Norian, the Mianlue ocean basin had completely closed, and the foreland basin contained adeltaic system that prograded and enlarged westward into an epicontinental sea opening to the west [Maet al., 2009]. The foredeep depozonemigrated from the Dangyang Basin to thewestern Sichuan Basin (Figure 6).Furthermore, continued shortening along the suture, which was manifested as the northern subbelt ofthe SQDB, caused the foreland basin to continue widening and step to the south during the Middle Norianto Late Rhaetian. The northern Yangtze foreland basin developed from east to west as the Mianlue suturewas transformed into the SQDB over time. The hiatus below the Lower Jurassic in the northwestern part of theSichuan Basin was likely related to erosion due to postthrusting rebound [Liu et al., 2005a]. All of thestratigraphic and lithologic evidence, including the paleocurrent data and detrital zircon age distributions,indicate that detritus shed from the orogenic highlands that were developed along and beside the collisionsuture were sought in the closing ocean basin and molasse foreland basin, primarily along the tectonicstrike from east to west [Graham et al., 1975]. The south China plate in the east, which formed a highlanddue to continental collision, supplied considerable sediment longitudinally through a deltaic system into theforeland basin as it was subsequently subducted and incorporated into the Qinling-Dabieshan orogen as thecollision suture lengthened.

6.2. Oblique Collision Involving the Northeastern Paleo-Tethys

The collision between the north China and south China plates due to the closure of the Paleo-Tethys and itsrelationship with the formation of the Dabieshan high- and ultrahigh-pressure rocks has been debated.Yin and Nie [1993] suggested that a promontory existed along the northeast margin of the south Chinaplate and that this region acted as a rigid indentor that penetrated into the north China plate during thecollision. Although this model may explain the presence of ultrahigh-pressure rocks in the Sulu orogenicbelt far to the northeast of the Qinling-Dabieshan orogen, its inferred sense of convergence toward thenorth-northeast is in conflict with our evidence, which indicates a significant northwestward collisionbetween the south China and north China plates along the Mianlue suture.

The rotational collision model in which the south China plate was subducted beneath the north China platearound an Euler pole in the northeastern part of the south China plate [e.g., Gilder et al., 1999] is generallyconsistent with the westward zippering of the south China and north China plates. However, this modelsuggests sinistral strike-slip faulting along the closing suture, which is the opposite sense of displacement

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indicated by the field observations. In addition, this model is inconsistent with the time of formation of theultrahigh-pressure rocks in the Sulu and Dabieshan regions and suggests little or no subduction betweenthe south China and north China plates in both regions during the Middle Triassic due to rotation. Finally, thetime of their demonstrated rotation likely occurred well after the closure of the Paleo-Tethys.

Based on our observations, we generally agree with the model of a closing remnant ocean basin [e.g., Zhouand Graham, 1996]. However, we suggest oblique closure rather than rotational closure of the Paleo-Tethys.Therefore, we propose a model of oblique collision between the south China and north China plates with anorthwestward continental collision that began earliest along the eastern Qinling-Dabieshan orogen (Figure 11).Early, long-duration subduction of the south China plate beneath the Dabieshan orogen provided the impetusfor the development and subsequent extrusion of the ultrahigh-pressure rocks in that area [e.g., Hacker et al.,2000] but not elsewhere. Regionally, the northeastern Paleo-Tethys was bounded by the north China and southChina plates [Metcalfe, 2006]. The closure of the northeastern Paleo-Tethys was driven by the northwestwardsubduction of the south China plate beneath the north China plate. Our findings indicate that continued slabsubduction and pulling (lasting more than 30Myr) during the oblique ocean closure drove deep continentalunderthrusting at the end of the initial collision, thereby forming ultrahigh-pressure rocks and leading tosustained ocean closure and continental suturing.

The rotational continental collision model introduced by Bottrill et al. [2014] explained the lateral variationand asynchronous onset of the collision and its effects on the burial and exhumation of subducted continentalcrust and high/ultrahigh-pressure rocks in the Norwegian Caledonides. The Qinling-Dabieshan orogen,including the Sulu orogenic belt to the east, underwent a similar asynchronous onset of collision beginning inthe east and propagating westward. However, during the continental collision in the Middle-Late Triassic,the Qinling-Dabieshan orogen continued to compress to the northwest. No extension (or decrease in subductionvelocity) occurred at the eastern end of the Dabieshan after the initial collision. Therefore, the collision inthe Dabieshan was oblique and caused by continuous northwestward continental subduction without adecrease in velocity at the point of initial collision, in turn causing a rotation of the subducting plate about therotational pole.

Oblique subduction has also been observed elsewhere.Malatesta et al. [2013] observed that gradual variationsin the size of the accretionary wedge and the amount of sediment in the trench along the subduction zone areexplained by the lateral tectonicmigration of sediments, which is driven by the obliquity of the subduction. Themodel of Malatesta et al. could be an analogue of the oblique collision of the northeastern Paleo-Tethys,although their model involved an oceanic plate subduction setting without continental collision. In the earlycollision zone in the Dabieshan, the sediments that were deposited in the foredeep were first buried andoverthrusted and then migrated laterally parallel to the suture zone. Lateral migration of the sedimentscontinued until they reached the western Songpan Ocean and the later-developed western foreland basin.The size of the foreland thrust belt-basin system increased along the suture moving forward. This tectonicprocess was coupled with the transport of sediments due to the topographic gradient that occurred along theforeland basin axis, which was driven by the suture-parallel motion of the subducting continental plate. Thesimilarity of both oblique oceanic subduction and continental collision suggests that the lateral migration ofsediments, a parallel motion of the subducting plate in the subduction zone, and gradual variations in thesize of the accretionary wedge (or, in the case of collision, the thrust belt and foreland basin system) mightbe the primary indicators of oblique convergence. Discriminating these tectonic behaviors can help usbetter understand the differences between the oblique and rotational convergence and the other active platemargin dynamics.

7. Conclusions

The new regional scale stratigraphic and structural data along the Mianlue suture indicate that westwarddextral transpressional suturing from the Middle Triassic to the Late Triassic in the SQDB resulted in theformation of the northern Yangtze foreland basin following the westward stepping closure of the ocean andthat the orogenic sediments were transported more than 1000 km westward from eastern China to theclosing Paleo-Tethys. These results suggest an oblique subduction model for the closure of the northeasternPaleo-Tethys Ocean. This collision mechanism supports the notions that the early and long-lived subductionof the south China plate beneath the north China plate provided the impetus for the development and

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subsequent extrusion of the ultrahigh-pressure rocks in the Dabieshan orogeny and that continued slabpulling during the oblique ocean closure drove deep continental underthrusting and led to sustained oceanclosure. The lateral migration of sediments, the suture-parallel motion of the subducting plate, and gradualvariations in the size of the mountain-basin system may be primary indicators of oblique convergence.Discriminating between these tectonic behaviors can improve our understanding of these active platemargin dynamics.

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