Collision leading to multiple-stage large-scale extrusion in the Qinling orogen: Insights from the...

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Gondwana Research xx

Collision leading to multiple-stage large-scale extrusion in the Qinlingorogen: Insights from the Mianlue suture

Sanzhong Li a,⁎, Timothy M. Kusky b, Lu Wang c, Guowei Zhang d, Shaocong Lai d,Xiaochun Liu e, Shuwen Dong e, Guochun Zhao f

a Department of Marine Geology, College of Marine Geoscience, Ocean University of China, No. 238, Songling Road, 266100, Qingdao, Chinab Department of Earth and Atmospheric Sciences, St. Louis University, 3507 Laclede Avenue, St. Louis, MO 63103, USA

c College of Marine Geoscience, Ocean University of China, Qingdao, Chinad Department of Geology, Northwest University, Xi'an, China

e Institute of Geomechanics, Chinese Academy of Geological Sciences, Beijing, Chinaf Department of Earth Science, The University of Hong Kong, Hong Kong, China

Received 1 March 2006; received in revised form 13 November 2006; accepted 13 November 2006

Abstract

The geologic framework of the Phanerozoic Qinling–Dabie orogen was built up through two major suturing events of three blocks. From north tosouth these include the North China craton (including the northQinling block), the Qinling–Dabiemicroblock, and the SouthChina craton (including theBikou block), separated by the Shangdan andMianlue sutures. TheMianlue suture zone contains evidence forMesozoic extrusion tectonics in the formofmajor strike–slip border faults surrounding basement blocks, a Late Paleozoic ophiolite and a ca. 240–200Ma thrust belt that reformed by 200–150Mathrusts during A-type (intracontinental) subduction. The regional map pattern shows that the blocks are surrounded by complexly deformed Devonian toEarlyTriassicmetasandstones andmetapelites, forming a regional-scale block-in-matrixmélange fabric. Five distinct tectonic units have been recognizedin the belt: (1) basement blocks including two types of Precambrian basement, crystalline and transitional; (2) continental margin slices including EarlyPaleozoic strata, and Late Paleozoic fluviodeltaic sedimentary rocks, proximal and distal fan clastics, reflecting the development of a north-facing riftmargin on the edge of the South China plate; (3) out of sequence oceanic crustal slices including strongly deformed postrift, deep-water sedimentaryrocks, sheeted dikes, basalts, and mafic–ultramafic cumulates of a Late Paleozoic ophiolite suite, developing independent of the rift margin in a separatebasin; (4) out-of-sequence island-arc slices; (5) accretionary wedge slices. All the tectonic units were deformed during three geometrically distinctdeformation episodes (D1, D2 andD3 during 240–200Ma). Units 2–4 involved southward thrusting and vertical then southward extrusion of about 20 kmof horizontal displacement above the autochthonous basement during the D1 episode. Thrust slices 20 km south of the Mianlue suture are related to thisvertical extrusion due to the same rock assemblages, ages and kinematics. The D2 and D3 episodes folded all the units in a thick-skinned style about east–west (D2) and west–northwest (D3) axes in the Mianlue suture zone. An early foreland propagating sequence of accretion of Late Paleozoic rocksdeposited above the Yangtze craton is not involved in D1 deformation but is temporally equivalent to the D2 and D3 deformation in the Mianlue suture.Two stages of strike–slip faulting mainly occurred at the end of D2 and D3, respectively. During D2 deformation, the Bikou blockwas obliquely indentedto the ESE into theMianlue suture, rather than being thrust over theMianlue suture from the north as a part of the Qinling–Dabiemicroblock. During D3

deformation, however, the Bikou block was bounded by the south boundary fault of theMianlue suture, and the Yangpingguan fault on the south. Thesefaults are coeval strike–slip faults, but of opposite senses, and accommodated minor southwestward extrusion of the Bikou block into Songpan–Ganzeorogen. The other basement blocks north of theMianlue suture were extruded eastward by about 20 km of lateral displacement, based on the offset of theWudang dome, during the D3 episode due to the northeastward indentation of the Hannan complex of the South China craton. Post-D3 emplacement ofgranite, cutting across the strike–slip faults such as the Mianlue suture, provides a minimum age of 200 Ma for D3 deformation. Therefore, based oninsights from the evolution of the Mianlue suture, the D2 and D3 episodes in the Mianlue suture and its neighbors are not responsible for and associatedwith the two-stage extrusion of the Dabie UHP-HP terranes from the Foping dome to the present erosional surface (more than 350 km).© 2006 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

Keywords: Mianlue suture; Tectonic evolution; Extrusion tectonics; Indentation tectonics; Qinling; Deformation

⁎ Corresponding author. Tel.: +86 532 66781971 (office).E-mail address: [email protected] (S. Li).

1342-937X/$ - see front matter © 2006 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.doi:10.1016/j.gr.2006.11.011

Please cite this article as: Li, S. et al. Collision leading to multiple-stage large-scale extrusion in the Qinling orogen: Insights from the Mianlue suture. GondwanaResearch (2007), doi:10.1016/j.gr.2006.11.011

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1. Introduction

The Qinling–Dabie Orogen marks the irregular suturebetween the North and South China cratons (Fig. 1), andcontains the largest belt of ultrahigh pressure (UHP)metamorphic rocks in the world. It is a major part of the E–W-trending Central China Orogen of central China (Jiang etal., 2000), extending 1500 km westward from the Qinlingrange, through the Kunlun range, and 600 km eastwardthrough the Tongbai–Dabie range, then continues northeast-ward through the Sulu area of the Shandong peninsula into theImjingang or Hongseong–Odesan fold belt of Korea (Fig. 1)(Ratschbacher et al., 2003; Oh and Kusky, in press). Largeamounts of continent–continent convergence have beenaccommodated along the geometrically irregular suture, butthe convergence was diachronous in time and inhomogeneousin space so that the related indentation, extrusion and rotationhas a complex spatial and temporal pattern (Tapponnier et al.,1982; Ratschbacher et al., 1991; Davison et al., 1995; Zhanget al., 1995; Jones et al., 1997; Thompson et al., 1997a,b;Kusunoki and Kimura, 1998; Hacker et al., 2000; Ratschba-cher et al., 2000; Johnston et al., 2000; Beaumont et al., 2001;Xypolias and Koukouvelas, 2001; Hatcher, 2002; Ratschba-cher et al., 2003; Xypolias et al., 2003; Wang et al., 2005).Especially extrusion and indentation tectonics in Asia areevident tectonic phenomena. Consequences of Cenozoic andMesozoic indentation tectonics throughout Eurasia are mainlydriven by active indentors such as the Indian plate into theEurasian plate (Tapponnier et al., 1982) and the South Chinacraton into the North China craton (Yin and Nie, 1993),respectively. Some basement blocks or microblocks betweenor in these orogens may have undergone lateral and/or vertical(upward) extrusion tectonics. For example, Mesozoic andCenozoic convergent tectonics in southeast Asia (Tapponnieret al., 1982; Morley, 2002) included lateral extrusion of southChina (Zhang et al., 1995) and large-scale eastward or upwardextrusion of the Qinling–Tongbai–Dabie belt (Maruyama etal., 1994; Hacker et al., 2000; Ratschbacher et al., 2000;Hacker et al., 2000; Li et al., 2002; Wang et al., 2003). Otherexamples include Late Tertiary tectonic extrusion of theEastern Alps (Frisch et al., 1998), extrusion tectonics ofCentral Anatolia, Turkey (Dirik, 2001), and Neogene–Quaternary lateral extrusion of the southern Apennines.Lateral extrusion and escape tectonics are typically accom-modated along large-scale strike–slip faults or subductionzones, for example, subduction-related extrusion of theWestern Carpathians (Sperner et al., 2002), or Cenozoicextrusion of eastern Asia beside the Tancheng–Lujiang FaultZone (Zhang et al., 2003).

Every possible kind of extrusion tectonics such as eastward,vertical (upward) and southward, has been proposed for theQinling–Dabie Orogen in the last decade (Hacker et al., 2000;Li et al., 2002; Zhang, 2002; Wang et al., 2003; Ratschbacheret al., 2003; Wang et al., 2005), resulting in many controversies.Maruyama et al. (1994) proposed that vertical extrusion isimportant for the exhumation of the UHP terrane in the eastcentral China in the Triassic. Hacker et al. (2000) pointed out

Please cite this article as: Li, S. et al. Collision leading to multiple-stage large-scaleResearch (2007), doi:10.1016/j.gr.2006.11.011

that orogen-parallel eastward extrusion occurred diachronouslyfrom 240 Ma in the east to 225–210 Ma in the west.Ratschbacher et al. (2000) described the Cretaceous andCenozoic unroofing with eastward tectonic escape and Pacificbackarc extension in the Early Cretaceous, and Pacificsubduction in the mid-Cretaceous. Wang et al. (2003) proposedthat the Dabie HP-UHP metamorphic rocks were originallylocated beneath the Foping dome, a dome in the narrowest partof the Qinling orogen (Fig. 1), where it underwent ultrahigh-pressure metamorphism in the Early Triassic. Then it wasextruded eastward to its present-day location.

Since so many different models for extrusion mechanismsand timing have been proposed, we further focus on theMianxian–Lueyang (abbreviated to the Mianlue) suture southof the Foping dome of the Qinling Orogen to provide structuralconstraints on whether or not, and how the extrusion of theDabie UHP terranes from the Foping dome to the present-daylocation is associated with the Mianlue suture (Wang et al.,2003). Furthermore, we discuss an alternative tectonic model inthe Qinling–Dabie Orogen using our detailed field data from theMianlue suture of the Qinling orogen.

2. Geological background

The geologic framework of the Qinling–Dabie orogen wasbuilt up through two major suturing events of three blocks.From north to south these include the North China craton(including the north Qinling block), the Qinling–Dabie micro-block (including, from west to east, the Qaidam, the WestQinling, South Qinling and Dabie–Sulu blocks), and the SouthChina craton, separated by the Shangxian–Danfeng (abbrevi-ated to the Shangdan) and the Mianlue sutures. The Shangdansuture resulted from Middle Paleozoic closure of the ShangdanOcean and collision of the North China craton and the Qinling–Dabie microblock. The Mianlue suture, however, resulted fromLate Triassic closure of the Mianlue ocean and collision of theQinling–Dabie microblock and the South China craton (Liuet al., 2004; Meng et al., 2005; Zhang et al., 2005).

Rocks in the Qinling orogen record a prolonged history ofcontinental divergence and convergence between blocks. FromLate Neoproterozoic to Early Paleozoic times, sediments fromthe Proto-Tethyan Shangdan Ocean were deposited on a passivemargin in the South Qinling on the northern margin of the SouthChina craton, while in the North Qinling, passive marginsediments were also deposited on the southern margin of theNorth China craton. When the Proto-Tethyan Shangdan Oceansubducted northward during the Ordovician, the North Qinlingevolved into an active continental margin characterized bythick-skinned deformation involving crystalline basement. Wesuggest that this conversion of the margin followed an arcaccretion/collision event, the record of which has been obscuredin the high-grade internal zones of the orogen. Collision of theSouth and North Qinling blocks took place in Middle Paleozoicalong the Shangdan suture. Different isotopic geochronometersto the North Qinling arc system north of the Shangdan suturerecord a Middle Paleozoic tectonothermal event (including an40Ar/39Ar age of 426±2 Ma, a Rb–Sr mineral isochron age of

extrusion in the Qinling orogen: Insights from the Mianlue suture. Gondwana


ig. 1. Simplified structural map showing the Mianlue suture and adjacent parts of the Qinling–Dabie orogen. The Mianlue suture is located in the southern part the Qinling orogen, extending westward from theuixian, Xianhuang, Fangxian, Gaochuan, Shiquan, Mianxian, Lueyang, Kangxian, Pipasi, Nanping to Wenxian counties, and northwesterly through the N-MOR -type Derni ophiolite linked with the A'nyemaqenuture south of the eastern Kunlun orogen (Xu et al., 1996; Chen et al., 2000; Roger et al., 2003; Lai et al., 2004). Abbreviations as follows: CAO—Central Asian oro n; TM—Tarim craton; NCC—North China craton;CO—Central China orogen; WQL western Qinling orogen; EQL—eastern Qinling orogen; TB—Tongbai orogen; WDB—western Dabie orogen (or Hong'an ock); EDB—eastern Dabie orogen; YC—Yangtzeraton (or South China craton); CC—Cathysian craton; SGO—Songpan–Ganze orogen; AHO—Alps–Himalaya orogen; WDD—Wudang dome; FPD—Foping ome.

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411±5 Ma and a Rb–Sr whole rock isochron age of 406±22 Ma) (Sun et al., 2002a). This tectonothermal event isrestricted to the North Qinling block and the southern margin ofNorth China craton (Ratschbacher et al., 2003; Wang et al.,2005), and is not observed in the South Qinling block or thenorthern margin of the South China craton (Sun et al., 2002a).

Rifting occurred along the southern rim of the South Qinlingat the same time as the collision along the Shangdan suture inthe north, and was followed by the opening of the Paleo-Tethyan Mianlue ocean during the Late Paleozoic, resulting inthe separation of the Qinling–Dabie microblock from the SouthChina craton (Meng and Zhang, 2000). The Qaidam, WesternQinling, Southern Qinling and Dabie–Sulu blocks form a stringof Paleozoic crustal fragments between two sutures in theQinling–Dabie orogenic belt. The Songpan–Ganze block(including the Bikou block) south of the Mianlue suturepreserves similar Precambrian crystalline and transitionalbasement and Early Paleozoic sedimentary cover (Zhang,2002). It is shown here that they all could have belonged to asingle unified passive continental margin along the northmargin of the South China craton in Early Paleozoic times.Collision of the Qinling–Dabie microblock and the South Chinacraton began in the Late Triassic along the Mianlue suture

Fig. 2. Simplified structural map showing the Mianlue suture and adjacent parts of theof Fig. 9; C—locality of Fig. 6; D—locality of Fig. 8; E—locality of Fig. 10; The


(Li et al., 1996; Zhang et al., 2002). The Late Triassic collisionalorogeny along the Mianlue suture caused extensive thin-skinned fold-and-thrust deformation, extrusion, rotation, upliftand granitoid plutonism throughout the Qinling, and led to finalamalgamation of the North and South China cratons (Meng andZhang, 2000; Sun et al., 2002a,b).

Most previous models for the Qinling–Dabie orogenrecognized only one suture between the North and SouthChina cratons, and placed this along the Shangdan suture,suggesting that the collision along this zone was Triassic in age.The existence of a second suture, the Mianlue, in the Qinling–Dabie orogen was not anticipated, and little is yet known of itscomposition, geometry and tectonic evolution.

The Mianlue suture is located in the southern part of theQinling orogen (Fig. 1) (Zhang et al., 1996; Xu et al., 1996;Chen et al., 2000; Roger et al., 2003; Lai et al., 2004). It is alarge-scale, southward curved fold-thrust belt, with well-studiedoutcrops located in the Gaochuan–Kangxian segment of thebelt. South of this segment is the South China craton includingthe Bikou block (Fig. 1) and the Hannan complex. However,north of the segment is the Late Paleozoic structurallyimbricated Qinling–Dabie microblock which is separated bythe east–west Shangdan suture from the North China craton

Qinling orogen from Gaochuan to Kangxian. A—locality of Fig. 7; B—localitydirection of second extrusion shown in this figure is the same as that in Fig. 1.



Fig. 3. Schematic section showing an interpretation of distinct tectono-stratigraphic suites and some basement slices and/or blocks in the Mianlue suture and its neighbors.

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Fig. 4. Structural profile across the Mianlue suture and its surroundings (arrows represent the direction of the latest movement); the symbols of faults are the same as those in the Fig. 3; A–B is the location of this profilein the Fig. 5.

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Fig. 5. Major faults and litho-slabs of Mianlue suture and its neighbors; F1—Zhuanyuanbei fault; F2—Kangxian—Lueyang fault; F3—Shuigouyan fault; F4—Majiagou–Henxianhe fault; F5—Sigoukou fault; F6—Zhujiashan—Wujiayin fault; F7—Heyeba (Jiamenzigou) fault; F8—Longmen Shan fault; F9—Muguayuan–Lianghekou fault; F10—Baishuijian fault; F11—Shanglianghe fault; F12—Yuguan fault; F13—Huayinshan fault; A–B is the location of Fig. 4.

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(Fig. 1). The Foping dome is located in the narrowest segmentbetween the two sutures (Figs. 1 and 2). However, other domessuch as the Wudang, Tongbai, Xinxian and North Dabie, arelocated at different positions within the Qinling–Dabie micro-block (Fig. 1).

Volcanic rocks in the Gaochuan–Kangxian segment of theMianlue suture are divided into island-arc type, intra-arc-rifttype, and juvenile-rift type (Li et al., 1996; Lai and Zhang,1996; Lai et al., 1997, 1999, 2000). Tectono-lithological unitsare classified according to the non-Smith stratigraphic system(Du et al., 1998; Li et al., 2001a,b,c,d). Five distinct tectonicunits have been recognized in the belt: (1) basement blocksincluding two types of Precambrian basement, crystalline andtransitional; (2) continental margin slices including Early andLate Paleozoic strata; (3) oceanic crust slices including postrift,deep-water sedimentary rocks, sheeted dykes, basalts, andmafic–ultramafic cumulates of a Late Paleozoic ophiolite suite;(4) island-arc slices; (5) accretionary sediment wedge slices(Fig. 3).The deformation history of this suture zone is poorlyknown, so there are few constraints on which dynamic models


might explain the evolution of this segment of the orogen. Inthis paper we systematically describe the composition,structural architecture, deformation history and related orogenicprocesses of the Mianlue suture based on our ten-year study oflocal and regional field relationships, and other published keygeochronology results. Finally, we test a model to interpret thecollision leading to multiple-stage large-scale extrusion basedon insights from the Mianlue suture of the Qinling orogen.

3. Deformation history of sutures and tectonic units

We have divided rocks and structural data of the suture zoneinto coherent structural domains, typically separated by shearzones. In this section we systematically discuss and compareeach of the structural elements from each of these units, andthen provide a structural analysis of the orogen. The earliest,first-stage of deformation is preserved in the regionallyextensively Devonian–Early Triassic sandstones that form thedeformed matrix to belts of melange in the suture. Later stagesof deformation are preserved in the numerous structural slices,




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blocks and fault belts within the suture. Our analysis ofstructural sequence is based on the sequence of superimposeddeformational fabrics, and comparison of the sequence of eventsfrom slice to slice in the orogen. Analysis of kinematic relationsand orogenic processes has led to the establishment of a newcomprehensive tectonic model of the Mianlue suture.

3.1. The Shangdan suture

The Shangdan suture is an important tectonic belt, locatednorth of the Minalue suture, separating not only the NorthQinling block from the South Qinling block, but also the NorthChina craton from the South China craton (Fig. 1). It ischaracterized by a 2 km wide E–W striking, south-dipping,large-scale mylonite zone that extends continuously for morethan 1000 km (Zhou and Zhang, 1996). This suture containsMiddle Ordovician to Early Silurian radiolaria fossils and 447–357 Ma ophiolitic derived from the Shangdan ocean (Zhanget al., 1996). This suture has a multi-stage deformational historyincluding Late Triassic sinistral–transpressional shearingmarked by S–C fabrics and other kinematic indicators fromearlier deformation, followed by Cretaceous brittle-normalfaulting in younger deformation events (Zhou and Zhang,1996). The deformation regime of the suture is transpressionalwith characteristic flower structures consisting of several active,sinistral strike–slip master faults and oblique-slip thrust faultswith opposing vergence (Zhang et al., 1996).

3.2. Major faults in the Mianlue suture

The Mianlue suture consists mainly of ophiolitic tectonicmélange disposed in a series of thrust slices separated by brittleand/or ductile faults exhibiting different scales of deformation,ages of formation, structural levels, tectonic settings and origins(Figs. 3–5) (Li et al., 2001a,b,c,d). This suggests that theMianlue suture records prolonged continental divergence andconvergence between blocks. The suture is bounded by theZhuangyuanbei fault (classified as F1, using the nomenclaturewhere F corresponds to the designation of a structure as a fault,and the numerical subscript denotes simply the numerical name

Fig. 6. Cross-section showing deformation of matrix


of the fault and has no significance for the age of the fault) to thenorth and the Kangxian–Lueyang fault (classified as F2) to thesouth (Fig. 4).

(1) The F1 Fault (Fig. 4) is the northern boundary fault of theMianlue suture. It is a regional, east–west striking, severaltens to hundreds of meters-wide and fault zone separatingthe Qinling–Dabie microblock from the Mianlue suture(Li et al., 2001a). Different protolith lithofacies flank thefault, reflecting respectively lower Paleozoic sedimentson the north, middle to upper Paleozoic sediments on thesouth, showing that this fault juxtaposes previouslywidely separated rock units. The fault experienced along-lived deformation history. The first-stage of defor-mation in the fault belt is recorded in EW to WNWstriking transpressional ductile shear zones, preservingmany kinds of mylonitic fabrics. The orientation of themylonitic foliation is parallel to that of the schistosity inthe suture. Many sheared clasts in the shear zone indicatesinistral strike–slip non-coaxial strain in the fault zone forD1 (note: the subscript for deformation episodes, Dx,relates to the relative timing of deformation). These D1

fabrics in the fault zone truncate older fabrics outside thefault zone.The D2 deformation episode included a component ofsinistral dutile strike–slip faulting, reworking the D1 foldsand generating a D2 generation of folds with steeply N–Sdipping axial surfaces, and steeply west plunging hinges.D2 deformation locally resulted in the formation ofstructurally differentiated bands in discrete quartz-richand mica-rich microlithons, implying deformation nearthe ductile–brittle transition (e.g., Borradaile et al., 1982).The strike–slip faulting overprinted and re-orientedfabrics from the thrusting on the flanks of the fault duringthe peak collisional stage. The latest D3 dextral strike–slipfaults with subhorizontal slickenlines are superimposedon the earlier ductile shear zone and crosscut the D2

crenulation cleavage at a small angle. All the abovestructures are offset by the northwest striking, brittlestrike–slip faults. The existence of a steeply dipping

in the suture zone from Jingjiahe to Dengjiaying.




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strike–slip zone (typically within a reverse, hinterland-directed component) located behind a thrust-dominatedwedge is a feature of numerous orogenic belts worldwide(e.g., Shaer and Rogers, 1987). The last faulting event isbrittle normal faulting, resulting in extensional brecciaand triangular fault planes.

(2) The Shuigouyan Fault (classified as F3) separates thenorthern continental margin sedimentary slice from thecentral ophiolitic slice (Figs. 3 and 4). This fault has anarcuate trend in plan view, dips north at angles of 60°–83°in profile. Many structural indicators in mafic, felsic, andcarbonate mylonite and proto-mylonite show top to thesouth–southwest thrusting.

(3) The Qiaozigou–Hengxianhe Fault (classified as F4) is aductile fault separating not only the ophiolite slice fromthe southern continental margin sedimentary slice, butalso the Qiaozigou island-arc igneous slice and theJinjiahe marble slice (Fig. 3). The ophiolite slice is thrustover the southern continental margin sedimentary slicealong a 60–70 m wide shear zone. A good exposure maybe found at Qiaozigou cottage where it exhibits dips 75°–80° to 000°–010°. The fault is composed of carboniferousprotomylonite, felsic mylonite and protomylonite, andmafic mylonite. Asymmetric folds, S–C fabrics and otherkinematic indicators show thrusting with the top to thesouth sense of movement. The shear zone between thelatter two is about 40 km long, 20–100 m wide and dips50°–70° to 010° to 030°, suggesting that major displace-ments may have been accommodated along this zone.

(4) The Sigoukou Fault (classified as F5) is a boundary shearzone. It strikes northwest (100–110°) and east (70–85°)but swings west–northwest toward the west, but maintainsa 50–150mwidth (Fig. 4). This change in strike suggests asinistral sense of shear during the later deformation stage.The fault consists of felsic protomylonite, banded felsicmylonite, phyllite and carbonate mylonite. Reliable shearsense indicators including polycrystalline quartz aggre-gates, shear folds, mica-fish, composite “schistosite–cisaillement (S–C)” planar fabrics, and antithetic micro-

Fig. 7. Cross-section from Shuigouyan to Jinggu


faults in palgioclase grain indicate a top to the south senseof the early thrust movement.

(5) The Zhujiashan–Wujiaying Fault system (classified as F6)developed within the Zhujiashan slice is characterized bynorthwest striking sinistral strike–slip faults (Figs. 3 and 4).The ductile shear zone was 6–10 km long and 20–50 mwide in its early deformation stage, and developed manyquartz veins parallel to the shear foliation. Younger small-scale brittle faults developed within the early ductile shearzone, and accommodated horizontal displacement asexemplified by a 1 km offset of the Majiashan ophiolitebody across the Zhujiashan–Shifanggou fault, a branch ofthe Zhujiashan–Wujiaying Fault system. Lithologically thebrittle zone consists of a few to tens of meters of lenticularcataclastic masses, including breccia and gouge of variablydeformed host rocks that flank the fault.

(6) The Kangxian–Lueyang Fault (classified as F2) is a largeregional-scale fault that forms the southern boundary ofthe Mianlue suture (Li et al., 2001b) (Fig. 4). It iscomposed of early, intensely-deformed mylonite, and laterfault breccia and gouge. The early (D1) deformationrecords ductile thrusting fabrics. However, the latestdeformation produced a nearly east–west striking (095°–105°), steeply dipping (60°–80°) brittle fault zone that isseveral meters to a few tens of meters wide. The fault planeis characterized by smoothly polished steep slickensidesplanes with horizontal slickenlines and striae showing abrittle sinistral strike–slip sense of motion. This fault zoneseparates the Late Paleozoic accretionary complex to thenorth from the Late Paleozoic initial rift and passivecontinental margin formations to the south (Meng et al.,1996). The boulders in the conglomerate have noophiolitic materials nor island-arc-derived materials intheir grains or matrices, but are made of continental-derived materials from the South China craton. Theconglomerates as a whole have many kinds of passivecontinental margin-derived rocks. This indicates that thepebbles of the early stage conglomerates were formedwithin a rift environment near the passive continental

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oli at Guozhen showing fold impososition.



Fig. 8. Structural profile showing folds and nappes from Caimahe, via Lianghekou to Guojiaba.


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margin. Therefore, the fault is an important, long-livedboundary fault, that influenced Late Paleozoic lithofacies.

3.3. Major faults south of the Mianlue suture

Two major faults are closely related to the evolution of theMianlue suture.

(1) The Heyeba Fault (F7) is a 350–400 m wide, nearly east–west striking steeply dipping structure located within theTapo slice (Fig. 4). Tectonites in the fault zone arecomposed of mylonitic carbonate, carbonate mylonite,protomylonite and strongly deformed conglomerate.Kinematic indicators including asymmetric folds, S–Cfabrics, asymmetric porphyroclasts and en echelonboulders, show that the D1 deformation involved top-to-the south–southwest ductile thrusting. The earlier S1schistosity was folded by the D2 brittle–ductile dextralshearing to form reclined folds.

(2) The Longmen Shan fault system (F8) (Figs. 1, 2 and 4) is aboundary fault zone in the inner part of the South Chinacraton, separating the Bikou block on the west from theHannan complex on the east (Fig. 1). The fault zone con-sists of the Longmen Shan fold-thrust belt formed duringthe Late Triassic Indosinian orogeny, spanning the timeperiod c. 227–206Ma (Yong et al., 2003) and some strike–slip faults such as the sinistral Yangpingguan transpres-sional fault. The fault zone plays an important role inaccommodating the deformation in the Kangxian–Gao-chuan segment of the suture and controlling the sedimen-tary lithofacies of the Sichuan basin (Figs. 1 and 3). To thewest, no lower Paleozoic strata except the upper PaleozoicTabo Group and Lueyang Formation covered the Bikoublock (Fig. 3). However, less upper Paleozoic strataremained in the fault zone, suggesting that the fault zoneformed during the Hercynian and controlled the lithofaciesdistribution. The present-day recorded deformations in-clude the early ductile thrust zone, the subsequent brittle–


ductile sinistral strike–slip zone and the last dextral brittlestrike–slip fault caused by the Indosinian orogeny andsubsequent movements.

3.4. Deformation of matrix in the mélange

The regional-scale structure of the suture zone shows manyblocks and thrust sheets that are surrounded by a complexlydeformed assemblage of Devonian to Early Triassic sandstonesand mudstones. These underwent intense ductile shearing atlower to middle metamorphic grade to form phyllite, schist, andcarbonate mylonite (Li et al., 2001a,b,c,d). The structuralrelationship of the blocks to thesemetasandstones andmetapelitesis that of blocks-in-mélanges, similar to other convergent marginmélanges in other orogens (Bradley and Kusky, 1986). Thematrix of this mélange records earlier deformation fabrics thanobserved in the blocks. The first deformation event in themélangematrix, D1, was the most intense, producing the main bedding-parallel penetrative foliation in the matrix of the belt. Structurallysome marble blocks are also incorporated within the matrix.Therefore, the matrix appears, in map view, as an east–westtrending anastomosing envelope of intensely deformed rocks thatrecord early deformation events.

Fig. 6 shows a structural profile from Jinjiahe–Dengjiaying(location C in Fig. 2). In Fig. 6, small D2 folds fold the S1schistosity. Their asymmetry implies southward movementduring ductile thrusting. The whole sequence was folded by D3

folds, then the deformed matrix was tectonically overthrust bysouth-vergent klippen of Cambrian to Ordovician marble blocksthat contain the same phosphorite as the Cambrian strata of theSouth China craton.

Fig. 7 illustrates a cross-section through the metapeliticmatrix of the mélange from Shuigouyan–Jingouli (near thetown of Guozhen, profile A on Fig. 2). D2 folds are tight and theaxial planar crenulation cleavage, S2, is strongly developed. D3

folds are relatively open and have a well-developed axial planarcleavage. The S1 schistosity is folded by D2 and D3 folds whichare superimposed forming a coaxial fold interference pattern.




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Both D2 and D3 fold interference probably resulted from thesuperposition of two discrete phases of folding, involving asignificant shift in the strain field from east to west between thetwo phases of folding. In this cross-section, the brittle thrusting

Fig. 9. Characteristic structures of different slices in the Mianlue suture. (A) F1 tight foXixiang area. (C) F2 fold of S1 schistosity in the Qiaozigou slice in Qiaozigou river.shaped crenulations S2 in the Gaochuan slice in Qiaozigou river. (F) crenulation in theto the dark bedding S0 in the Gaochuan slice in Gaochuan area. (H) F3 open fold sup8 cm.


is relatively weaker than that shown in the Jinjiahe–Dengjiay-ing cross-section (Fig. 6).

In general, the syn-D3 thrusting in the eastern part of thesuture is coeval with folding in the western part of the suture.

ld of bedding S0 in Xixiang area. (B) Southward thrusting of the Xixiang slice in(D) Crenulation cleavage S2 in the Qiaozigou slice in Qiaozigou river. (E) fan-Gaochuan slice in Gaochuan area. (G) crenulation S2 and schistosity S1 parallelerimposed on F2 tight fold in the Gaochuan slice in Gaochuan area. Scale pen is




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This reflects the switch of N–S-directed D2 contractionperpendicular to the suture toward northeast-oriented D3

contraction oblique to the suture.The Zhangyagou slice (Fig. 3) is composed of tectonic-

sedimentary mélange that underwent intense deformation andinvolved different scales of brecciation of metasandstone andmetacarbonate. These sedimentary breccias are displaced intosmall scale lenticles aligned parallel to schistosity. The D2

deformational event produced asymmetric folds accompaniedby sinistral strike–slip faults.

Fig. 8 illustrates a structural cross-section from Guojiaba–Lianghekou–Caimahe (location D in Fig. 2). In this area someIndosinian HP granulite blocks were involved in intensedeformation (Li et al., 2000; Zhang et al., 2002). The D2

deformational event produced steeply plunging folds. The foldaxes have steep but variably plunging axes between Leigong-shanmountain and the town of Lianghekou, and thrust planes aretypically also steep with an obvious oblique-slip component.Generally these folds are closer to having attitudes consistentwith formation during strike–slip faulting, with steeply plungingaxes. In the northern part of this cross-section, the circa 200 Mapost-kinematic Guangtoushan granitic pluton (Zhang et al.,1996) intruded the Late Paleozoic strata, and cuts the fold axesand the suture. Many enclaves in the pluton are tremolitic marblewith the same preferred orientation including a north-plunginglineation, reflecting that their orientation was not changed by thegranitic magma emplacement. The lineation records top-to-thesouth thrust movement. In addition, there are some rootless foldspreserved in the mafic to ultramafic enclaves in the granite.

Many more-coherent slices and blocks are enclosed withinmélange defined by the thrust faults, ductile shear zones and thepenetratively-deformed mélange matrix described above, form-ing an anastomosing structural slice shear system (Fig. 4) (Liet al., 2001a,b,c,d). The obvious diversity in structuralorientation and style, stratigraphy, magmatism and metamor-phism between these slices and blocks is consistent with otherrecent research on different tectonic lithofacies, structuralanalysis and volcanic geochemistry of the suture (Meng et al.,1996; Lai and Zhang, 1996; Lai et al., 1997, 1999, 2000, 2003;Li et al., 2003). Tectono-stratigraphic units in this study area canbe divided into four first-order units: the Qinling–Dabiemicroblock (or the South Qinling block), the Mianlue suture,the Bikou block and the South China craton (Li et al., 2001a,b,c,d). The thrust slices related to the Mianlue suture can be furthergrouped into several related slice groups (Li et al., 2001a,b,c,d):the basement slice group, the northern slice group related to theSouth China craton, the oceanic crust slice group, the oceanicisland arc slice group, the continental island arc slice group, thecollisional sedimentary wedge slice group and the southern slicegroup related to the Qinling–Dabie microblock. Every slicegroup contains many independent slices or small isolatedblocks, for example, there are more than 200 uncoherent slicesand blocks in the Kangxian to Gaochuan segement of theMianlue suture (Chen et al., 1997). In various first-order units,the slices have different types of structural associations in space,which includes three dominant structural types: an imbricatedthrust association in the Qinling–Dabie microblock, a strike–


slip-dominated parallel-aligned association, and a complicatedanastomosing association related to complex deformation. Thelatter formed within the Mianlue suture, reflecting more intenseor complex south-directed contraction and significant strainpartitioning with a dominantly dip–slip component in the southand strike–slip component in the north.

3.5. Major thrust slices and their deformations

Tectonic slices in the Mianlue suture can be divided into twoprincipal types based on their present-day distribution: internalthrust slices in the north, and external thrust slices in the south.The latter includes two types of slices, autochthonous andallochthonous.

3.5.1. External (southern) thrust slicesExternal thrust slices are mainly located in the Hannan

complex and the Bikou block south of theMianlue suture (Fig. 2).In the Hannan complex, rocks previously known as the

Xixiang “Group” include two parts: one is the SanlangpuFormation (Fig. 3), consisting of undeformed conglomeratesunconformably overlying lower formations; the other is a suiteof volcano-sedimentary rocks belonging to the Wangjiahe,Sunjiahe and Dashigou Formations (Fig. 3). The volcanic rockassociation of the Xixiang “Group” exhibits distinctivegeochemical characteristics indicating a volcanic-arc origin(Lai et al., 2003). Basalts, andesites and rhyolites in the this areashow remarkable depletion of Nb and Ta (Lai et al., 2000). TheBaimianxia basalt association of the Xixang “Group” might berelated to partial melting of a depleted mantle source andformed in an ocean island-arc (immature island-arc) tectonicsetting (Lai et al., 2000). In this case the Sunjiahe, Dashigouvolcanic-sedimentary rock series should be allochthonous (Laiet al., 2000, 2003). The region developed into an activecontinental margin in the Devonian–Carboniferous period, as aconsequence of oceanic crust subduction of the Mianlue oceanicbasin and /or an intrarc rift (Lai et al., 2000, 2003). This tectonicallochthon should have a close relationship with and have beeninitially connected with the Mianlue suture (Lai et al., 2003)because the same rock assemblage is preserved in the Mianluesuture north of the Xixiang “Group”. However, the deformationof the latter has the same sequence as that of the slices andmatrix in the suture. Genetically, the intrafolial south-vergentfolds and other kinematic indicators indicate that the Xixiang“Group” has been thrust as an exotic terrane far from the north(the vertically lined range near the Xixiang County in Fig. 2).Therefore, the Mianlue suture is thus a northern root zone of theXixiang slice (Fig. 3), of which the frontal thrust belt was thrustsouthward at horizontal displacement of about 20 km over theHannan complex, i.e. the basin margin, and isolated as a klippeby uplift, vertical exhumation and erosion of the northern part ofthis complex. The basal shear zone of the klippe is a duplexstructure (Fig. 9A). However, most of the internal strata of theslice preserve no fabrics related to this early stage ofdeformation. Weak bedding-parallel continuous cleavage andfolds are only observed in thin sections from several soft layersof the Xixiang slice (Fig. 9B). The deformation of the internal



Fig. 10. The earlier deformation and the later folding-thrusting and their superposition on the suture (from Qiaozigou profile).


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strata of the slice reflects two-stages of contractional foldingafter its displacement. The first-stage fold axes are east–westtrending, whereas the second are nearly north–south trending.The latter folds were superposed over the former to form basinand dome interference patterns. No foliation formed during thetwo fold generations because of shallow structural levels. Insummary, the Xixiang slice belongs to an exotic slice verticallyextruded out of the Mianlue suture before the strongcontractional deformation. In terms of the present-day gentledecollement plane, the Xixiang slice as a whole is a thin-skinned thrust terrane with the weak early deformation and thelater two-stage foldings of bedding S0. Subsequent tectonism isa top-to-the south brittle thrusting. Therefore, the intensity ofcompression of the Xixiang slice increases towards the north,where they come into the Mianlue suture.

The Tabo slice (Fig. 3) is located in the northern part of theBikou block (Figs. 3, 4 and 5). Boulders in the conglomerate ofthe Tapo Group have some materials derived from the Bikoublock. This indicates that the slice is an external allochthonousslice. The earlier deformation in this slice (D1) is dominated bygentle folding of vertical structural transposition fabrics, ofwhich structural patterns, the orientation of tectonic lineationsand others are equivalent to those of the second-stagedeformation of the other slices and the matrix in the suture.The regional D3 deformation has influenced this area to form thesecond-stage deformation in this slice. Meanwhile, somelithofacies-controlled normal faults inverted to form south-vergent thrust faults that rework the uncomformity between the

Fig. 11. Structural profile from Zhongba to Houliu. The diversity in structural orientatincluding structural superposition and competency contrast between slices or blocks.deep crustal deformation zones adjacent to shallow-crustal deformation zones.


Tabo Group and the Bikou block. The fold axes of the TaboGroup and the Lueyang Formation are dominated by east–westtrends, at a large angle with the northeast trending Bikou Groupin the Bikou block (Pei, 1989). Zeng et al. (2001) suggested thatthe Bikou block extruded southwestward during the Indosinianand Yanshanian periods. Wang et al. (2001) also considered thesouthwestward extrusion of the Bikou block during theIndosinian period by considering of scissor-type contraction ofthe Mianlue suture and exhumation of UHP metamorphic rocks.However, the northern marginal structure of the basement of theBikou block has been strongly reworked by sinistral strike–slipfaulting along the southern border fault of the Mianlue suture.The parallelism between their foliations of these two thrustsshows that the strike–slip faulting is very intense, while thethrust shortening and the intensity of the north margin of theblock are less than those of the Longmen Shan thrust tectonicbelt. That is to say, the lateral displacement component of thenorth margin of the block is larger than its horizontaldisplacement component. This indicates that the primaryposition of the Bikou block should not be far away from thesouth border of the Mianlue ocean, or the Bikou block may nothave migrated and indented toward the north far from the south(Li et al., 2001a), but be transported from the west (Li et al.,2002). Moreover, conglomerate of the Tabo Group formed in theinitial rift indicates that the Bikou block did not escapesouthward from the Mianlue ocean proposed by Wang et al.(2003). Furthermore, the southeast part of the Bikou block wasoffset by the early Yanshanian Yangpingguan strike–slip fault

ion and style between these slices has probably been caused by numerous factorsLate strike–slip fault systems have also played a significant role in bringing up




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after major deformation and formation of the suture because thestrike–slip fault crosscuts the suture and is intruded by the200 Ma Guangtoushan granite (Zhang et al., 1996). Taking thefollowing data into account, for example, the intense shorteningof the middle Ordovician and upper Triassic strata in theLongmen Shan thrust tectonic belt, the north–south trendingfolding in the Songpan–Ganze orogen and the sinistral strike–slip nature of the early Yanshannian Yangpingguan andQingchuan–Maowen faults, we propose that the primarygeographic position of the Bikou block is more westerly thanits present-day location, consisting of a south branch of triple riftsystem, of which the north failed arm is called as the “Gonghe”graben filled in the thick Triassic between the Qaidam block andthe west Qinling block (Zhang et al., 2004a,b). When theMianlue suture and the Longmen Shan thrust-fold belt couldnot be compressed further, the Bikou block was extrudedsouthwestward.

3.5.2. Internal (northern) thrust slicesThe internal thrust slices are located north of the external

thrust slices. The representative slices are characterized bydifferent rock formations.

The Sanchazi slice strikes through the town of Sanchazi(Figs. 4 and 5) and is comprised of island-arc-type volcanics,ophiolites, metagabbro, chert, radiolarian chert and otheroceanic assemblages (Lai and Zhang, 1996; Feng et al., 1996)that underwent strong D1 deformation, especially the lenticularand serpentinized harzburgites and lherzolites. The schistosity,S1, is sharply crenulated and deflected by E–W-trending foldsof the D2 deformation. D2 folds are tighter in the metavolcanicsand metapelites, and more gentle in the metagabbro andmetachert (Li et al., 2001a,b,c,d). 3T-type phengite, stilpnome-lane and other HP/LT metamorphic mineral assemblages arepresent in the S1 and the crenulation cleavage S2 of the arc andoceanic assemblages (Li, 1998). Therefore, the two-stages ofdeformation were related to the subduction or earlier collisionproducing the HP/LT metamorphic rocks. However, D1 in theharzburgite and lherzolite is not associated with the HP/LTassemblages, and the early fabrics in the rocks may beassociated with HT mantle flow and be older than the D1 inthe shallow-crustal rocks. The D3 deformation formed a seriesof north-dipping, top-to-the south brittle–ductile thrust faultsalong lithological boundaries.

To the east, the Qiaozigou slice in Fig. 10 (the profilelocation is A in Fig. 2) preserves a series of meter-to kilometer-scale, south-vergent, asymmetric folds deformed by the S1schistosity, showing that some folding pre-dates the formationof the first foliation. The axial plane of the S2 crenulationcleavage is predominantly north dipping and east–west striking(Fig. 9C, D). Locally, axial planar cleavage developed duringfolding has a convergent fan-shaped arrangement about theaxial surface (Fig. 9E). A coaxial double zig–zag interferencepattern is generated by a set of D2 upright folds superimposedon previous D1 folds (Fig. 9C). The subsequent deformation is aseries of near east–west trending, upright ductile shear zonesdeveloped along the lithological interfaces, in which the S2crenulation cleavage is strongly deformed into shear band


cleavage or cleavage lamellae to produce large amounts of shearzone-parallel structurally differentiated quartz veins, and locallyreorients the D2 drag folds by sinistral strike–slip ductileshearing. Meanwhile, in many outcrops of the Qinling–Dabieorogen, the orientations of the D2 fold axes are variable,typically curved into vertical by later deformation. This phe-nomenon is often associated with strike–slip faults. Therefore,the later deformation of the D2 fabrics occurred during strike–slip adjustment among various blocks or slices along the uprightductile shear zones, related to oblique contraction in the suture.The latest D2 deformation is characterized by brittle–ductile orbrittle, south vergent thrust faults controlling the distribution ofsome blocks of marble overlying closely a suite of black cherts.However, these cherts are different from the deep-oceanic chertsin the Sanchazi slice, and thought to have formed in a deep-water continental margin environment (Sheng et al., 1997).

Samples from a basalt and gabbro section of the Paleo-Tethyan (approximately 350 Ma; Xu et al., 2002a,b) Mianluenorthern ophiolites in the Mianlue suture display sub-paralleland relatively smooth depleted incompatible trace elementpatterns and have high εNd (350 Ma) (8.1–11.3) and low Pb206/Pb204(350 Ma) (16.90–17.25) (Xu et al., 2002a,b). They arecompositionally similar to MORB, particularly to those fromthe Carlsberg Ridge and Indian Ocean Ridge Triple Junction.They also have the Dupal isotopic anomaly, characteristic ofbasalts from the southern hemisphere. Although the Mian–Luenorthern ophiolites are presently in the northern hemisphere,paleomagnetic data suggests that they formed in the southernhemisphere in the region of the DUPAL anomaly (Xu et al.,2002a,b).

To the east, the Gaochuan slice in Fig. 11 (the profile locationis E seen in Fig. 2) is dominantly composed of Carboniferousand Permian strata. Except for the intense top-to-the westthrusting in the boundary of the slice, the large-scale internalstructural framework comprises two stages of superimposedfolds, forming a basin and dome interference pattern of N–Selongate domes that deform the S1 schistosity. Regional analysissuggests that the trend of S2 is dominated by the near east–westor north–northwest trends (Fig. 9F, G). In some stronglydeformed zones, the D2 folds form a series of meter-scale tightisoclinal folds. The D2 folds with axial-planar crenulationcleavage and domainal spaced cleavage marked as S2 arerelatively tighter that the D3 folds. Some D3 gentle open or kinkfolds deformed S2 cleavage and produced the axial plane, S3, ofkink bands of N–S regional trend (Figs. 9H and 10). Wepropose below a genetic interpretation of intense N–S directedcompression of the D2 folds and structural adjustmentassociated with oblique compression to the Mianlue suture ofthe N–S trending D3 folds.

4. Deformation sequence of the Mianlue suture andorogenic process

The deformation sequence and orogenic processes in theMianlue suture can be divided into three stages based on thedeformation characteristics of the “matrix” and slices in thesuture, structural analysis, and differences in tectonic




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lithofacies. Excluding the pre-Paleozoic tectonic evolutionarystages in the basement blocks, these include subduction-relateddeformation, the main collision-dominated deformation and thelate intracontinental blocks-adjustment deformation.

4.1. Relative and absolute timing of deformation

Some papers report isotopic ages of igneous and metamorphicrocks from the Qinling and Kunlun regions related to the Mianluesuture. Important metamorphic ages of the Mianlue suture andassociated granitoid plutons are well constrained (Li et al., 1996;Sun et al., 2002a,b; Zhang et al., 2002). Li et al. (1996) obtained aSm–Ndwhole-rock isochron age of 242±21Ma and a 40Ar/39Arage of 220–230 Ma from phyllite, and interpreted these Triassicages to reflect the initial regional metamorphic age. Granulitesfrom the basement slices in the suture yielded a Sm–Nd isochronage of 206±55Ma and a biotite Ar40/Ar39 plateau age of 199.7±1.7 Ma, interpreted to be the peak and retrograde metamorphicage, respectively (Li et al., 2000; Zhang et al., 2002), consistentwith or close to those of the Dabie ultrahigh-pressure and high-pressure metamorphic rocks (Zhang et al., 1996; Hacker et al.,2000; Ratschbacher et al., 2000, 2003). Therefore, the formationand uplift of the granulites were related to the subduction of theSouth China craton beneath the Qinling–Dabie microblock, andthe collision and final amalgamation of the South andNorth Chinacratons in the Indosinian orogeny (Zhang et al., 2002). Sun et al.(2002b) gave a range in age from 220±1 and 205±1 Ma for sixgranitoid bodies in the ca. 400-km-long granitoid belt in the SouthQinling using single and multigrain zircon U–Pb dating,supporting the idea that the collision between the North andSouth China cratons along the Qinling–Dabie orogenic belthappened in the Triassic.

TwoKokoxili granitoids along the Kunlun suture, as a westernsegment of the Mianlue suture, which separates the Bayan Har–Songpan Ganze orogen from the Tarim and Qaidam blocks, yieldU–Pb zircon emplacement ages of 217±10 and 207±3 Ma (LateTriassic) and Rb–Sr isochron cooling ages of 195±3 and 190±3 Ma (Early Jurassic). The geochemical signatures of thesegranitoids suggest that they are related to subduction continuinginto the Late Triassic. Combining isotopic dating with structuralevidence on subduction polarity and paleomagnetic reconstruc-tions, Roger et al. (2003) proposed that the Kunlun and Qinlingblock boundaries, which were distinct in the Permian, subse-quently formed a continuous, Late Triassic, northward subductingplate margin. Three samples from the Jinsha suture that separatesthe Songpan Ganze orogen from the Qiangtang blocks, aleucocratic granite, an orthogneiss and a paragneiss, yield U–Pbzircon dates of 206±7 and 204±1Ma, and a U–Pbmonazite dateof 244±4 Ma, respectively (Roger et al., 2003). The existence ofcoeval magmatism in the Jinsha, Kunlun and Mianlue suturessuggests that the three subduction zones were simultaneouslyactive. Therefore, the evolution of the Mianlue suture is closelyrelated to these other western sutures and blocks.

According to the above geochronological data, the collisionalong Qinling–Dabie between the South and North Chinacratons is not documented to be diachronous or to becharacterized by a proposed younging and westward migration


(Yin and Nie, 1993; Zhang et al., 1996; Hacker et al., 2000;Ratschbacher et al., 2000, 2003). In fact, the range of Triassicages in Kunlun and Qinling correspond to those in Dabie.However, the subduction in the east is possible to be earlier thanthat in the west (Yin and Nie, 1993; Zhang et al., 1996; Hackeret al., 2000; Ratschbacher et al., 2000, 2003; Roger et al., 2003).Furthermore, there does appear to be a diachronous trend on amore regional scale, with the collision occurring earlier in theKorean segment of the orogen than in the Qinling–Dabie orogen(Oh and Kusky, in press).

4.2. Structural history

4.2.1. Pre-collisional tectonic evolutionary stage and subduc-tion-related deformation in the suture

4.2.1.1. Pre-collisional tectonic evolutionary stage. Based onthe sedimentary formations and their ages, the pre-collisionaltectonic evolutionary stage can be subdivided into the followingthree detailed stages (Li et al., 2001a,b,c,d).

4.2.1.1.1. Early Devonian to Early Carboniferous rifting toform a small ocean. This initial rifting is dated by largeamounts of Early Devonian fossils in the Tabo Group of theTabo slice and Early Carboniferous radiolarians in the chertsclosely associated with the ophiolite of the Sanchazi slice (Fenget al., 1996; Zhang et al., 1996). Therefore, the oceanic crustmust be Carboniferous. A string of relict ophiolites perhapsreflects spatially a string of small oceans, termed the Huashan–Mianlue–A'nyemaqen small ocean (Lai and Zhang, 1996; Laiet al., 1997, 1999, 2000, 2004), similar to the present-day SouthChina Sea–Sulu–Sulawesi. Alternatively, these ophiolitescould represent closure of a major ocean with ophiolites onlybeing obducted and preserved in isolated locations, much like inthe Alpine–Himalayan system.

4.2.1.1.2. Early Carboniferous to Late Permian contem-poraneous spreading and subduction. The oceanic volcanicrocks in the Mianlue ocean were being subducted andmetamorphosed to produce metamorphic distinctive subduc-tion-related HP/LT metamorphic minerals along S1 before pre-Early Triassic, dated by Li et al. (1996).

On the southern continental margin of the ocean, in theGaochuan range, the carbonates make an upward transition tocalcalaceous mudstone, black mudstone and cherts recordingthe evolution from the Devonian gentle slope, transitionalupwards to the Carboniferous marginal plateau and the Permianstable deep-water platform (Meng et al., 1996). However, in theQinling–Dabie microblock north of the Mianlue ocean,intrusion of Permian subduction-type granites at 285 Masuggests that subducted had begun by the Permian (Zhanget al., 1996). Thus, both spreading and subduction coexisted inthe Early Caboniferous to Late Permian.

4.2.1.1.3. Early to Middle Triassic subduction without newoceanic crust generation. Li (1998) calculated that thetermination of ocean spreading was about Early Triassicbased on the ocean's width and spreading rates identifiedusing geochemical methods. Moreover, deep-sea cherts andradiolaria characteristic of marine crust are not observed in the




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Triassic strata (Feng et al., 1996). This is to say, no oceanic crustis known to have formed during the Triassic. In the range fromWudu County to Mianxian County, the metamorphic ages ofSm–Nd and 40Ar/39Ar of 242±21 Ma and 220–230 Ma areobtained from the subduction-related slices with HP/LT mineralassemblage aligned along S1 (Li et al., 1996; Jiang et al., 2000).Therefore, the entire subduction began in the Early Triassic.

4.2.1.2. Pre-collisional subduction–accretion related deforma-tion and its sequence. Previous research mainly focusedon the collisional deformation and did not pay much attention tothe D1 structural association. In many slices and blocks withinthe Mianlue suture, there are many intrafolial folds of bedding.The bedding-parallel schistosity is locally composed ofphengite, stiplnomelane and other minerals formed under low-temperature and high-pressure metamorphic conditions consis-tent with metamorphism above a subducting slab, reflecting thatthey are products of subduction. These bedding-parallel foldsare restricted to the meta-cherts and marble strata. However,they are rarely observed in the easily-deformed volcanic layersbecause of complete replacement in the penetrative, S1-parallelcompositional bands. These bedding-parallel folds in the meta-cherts and marble are asymmetric, tight and isoclinal. Shearsense indicators show south vergent, north-dipping, low-angleand nearly bedding-parallel thrust movement. These bedding-parallel folds resulted from a piggyback sequence of accretionof late Paleozoic rocks above a basal decollement. During thesame deformation stage, the materials in the suture experiencedstrong tectonic mixing. Furthermore, oblique subductionproduced boudinage of the competent layers along strike and

Fig. 12. Simplified map showing the relation between the Mianlue suture and the strikand Zhang (1997); ⑥, ⑦ from Li (1998); and this study). Except for stereodiagram


the juxtaposition of slices of various composition and nature,leading to out-of-sequence strata. This kind of intense bedding-parallel transposition resulted in the same deformationalsequence and style of the slices and the matrix except forthose induced by local stress fields. This style of deformation isreminiscent of subduction accretion complexes elsewhere in theworld (e.g. Kusky et al., 1997a,b, 2003).

Every thrust slice within the main suture belt preserves“bedding-parallel” intrafolial folds and ductile shear zones,produced in the toe of the accretionary wedge at the frontalmargin of the subduction zone during low angle subduction ofoceanic crust (Li, 1998; Li et al., 2001b). The easily-deformedvolcanic rock layers preserve some small-scale weakly-deformedlenticles, while the strongly deformed layers comprise thepresent-day schistose “matrix” in the suture. Subduction relateduplift of the overriding plate resulted in the gravitationalinstability of the carbonates, cherts, sandstones and mudstonesover the low-angle south-dipping plane, which slumped into theforedeep and trench. Southward propagation of the decollementbetween the basement and the cover formed the consistentsouthward vergence of asymmetric bedding-parallel folds, ductilesliding, and stacking. The abundant geochronological data on thesubduction implies that the subduction-related deformationoccurred in the Early Triassic (Li et al., 1996; Jiang et al., 2000).

4.2.2. Geometry and kinematics of syn-collisional deformationSubduction-related structures are overprinted by three

generations of structures related to the collisional (D2–D3) andpost-collisional (D4) stages of tectono-thermal development andorogenesis: (1) thermal heating and prograde metamorphism (Li

e–slip faults (①② from He et al. (1997);③,④ from Li (1998);⑤ from Zhong, others are plan view maps; stress field map is in the upper-right corner.




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et al., 2003), leading to the development of a ductile basal shearzone; (2) out-of-sequence thrusting at thermal peak conditionsinvolving the incorporation of basement slices into the thrustbelt; and (3) post-thermal peak out-of-sequence thrusting in theMianlue Suture. Post-kinematic emplacement of the Guang-toushan granite provides a minimum age of 200 Ma for stage 2(Zhang et al., 1996). The crystalline basement blocks north ofthe Mianlue suture were extruded laterally (eastward orwestward) out of the belt during the D2–D3 episodes. The D2

and D3 episodes folded the thrust belt and its footwall basementin a thick-skinned fashion about east–northeast (D2) and west–northwest (D3) axes. During the collisional stage (D2–D3), allstrata except for parts of the accretionary prism (Li et al., 1999)underwent intense deformation. These strata are the middleTriasssic and pre-Triassic. During the early collisional stage, D2

deformation was dominated by the formation of folds withvertical axial surfaces and brittle–ductile thrusting. The D3

deformation is dominated by buckling and brittle thrusting. Theirregular nature of the plate boundary is thought to have led to theformation of various structural styles during collision alongthe suture. In total, the two folding episodes are responsible for theisolation of the thrust belt in a klippe, for the regional west–eastplunge and for the second-stage extrusion of the HPmetamorphicrocks from the middle crust to the present erosional surface.

For the entire Mianlue suture of the Qinling orogenic belt,the collisional event is diachronous from earlier in the easternsegments (such as Late Permian to Early Triassic of the Dabie–Sulu segment (Yin and Nie, 1993), and corrlelatives in Korea(Oh and Kusky, in press), to later in the western segments (suchas Late Triassic of the A'nyemaqen in the west Qinling orogenicbelt (Xu et al., 1996)). In terms of the Gaochuan–Kangxiansegment, the strong Late Triassic collision produced somecollision-type granites of 209 Ma–219 Ma (Li et al., 1996; Sunet al., 2002b). Meanwhile, the upper Triassic strata that flank thesuture are nonmarine sediments characteristic of intracontinen-tal evolution. An east–west striking, foreland basin also formedat the same time (Liu and Zhang, 1999). The position ofsuccessive basin margins can be related to a thrust structure tothe north. This localized deformation must have terminatedbefore 200 Ma because the Guangtoushan pluton emplaced intothe suture is undeformed.

A synthesis of the structural profiles described above revealsthat the collision-related deformation in the suture can besummarized as follows: 1) the early collision-related deforma-tion is related to N–S directed contraction including regionalfolding forming the E–W trending, horizontal fold axes. The D2

fold axial planes form a crenulation cleavage S2, locallyexhibiting growth high-pressure minerals such as phengite,stiplnomelane and others. 2) the later collision-related deforma-tion is dominated by south-directed brittle–ductile thrusting.The irregular subducted plate boundary led to the variation of dipangles and the curved shape of thrust planes and fold symmetry.For example, we infer that the subducted plate boundary hadgentle dips from the town of Guozhen to west of Wenxian.However, it was steep east of Youshui village (Fig. 2). In moreeasterly regions, it was also gentle from the Dabasha area to theeasterly Chengkou–Fangxian area (Fig. 1). We consider that this


boundary effect continued until the later intracontinentaladjustment (Yin and Nie, 1993; Li et al., 2001a,b,c,d). 3) Theeast–west striking foreland formed during the south-directedbrittle–ductile thrusting (Dong et al., 2005). Then, 4) the dextraland/or sinistral strike–slip faults were superimposed locally onthe suture, locally re-orienting fold axes (B2) from shallow tosteep plunging near the strike–slip faults. The sinistral strike–slip faults formed after the D2 folds.

4.2.3. Intracontinental adjustment of post-collisional deforma-tion (post-Late Triassic)

The suture was intruded by the 200 Ma Guangtoushangranite (Fig. 2), truncating all fabrics in the suture. This showsthat the ocean had closed by the Late Triassic leading to thecollision of the South China craton with the Qinling–Dabiemicroblock and deposition of nonmarine sedimentary forma-tions (Liu and Zhang, 1999; Yong et al., 2003; Meng et al.,2005). Therefore, all younger deformations are related tointracontinental processes such as continued convergence andcrustal thickening, A-type subduction, post-orogenic extension-al collapse, strike–slip adjustments of intracontinental blocks,and syn- to post-collisional foreland basin formation and relateddeformation.

In the south Dabashan area (Fig. 1), post-Triassic peakorogenesis led to the filling and deformation of the syn-collisional foreland basin, and subsequent Late Jurassic to EarlyCreataceous filling of the basin, driven by oblique intraconti-nental A-type subduction. Late Jurassic to Early Cretaceouspost-collisional deformation deformed the basin again, resultingin the formation of a large-scale arcute thrust-fold belt in theDabashan segment of the suture. This process also resulted inthe main east–northeast trending structural line of the Paleozoicstrata over the north Hannan complex west of the Dabashanarcute thrust-fold belt. Meanwhile, structural analysis of foldsfrom the Zhenba area of the south Hannan block indicates thatthe earlier structures exhibited a near east–west striking foldpattern consistent with the results of deformation analysis forpeak syn-collisional structures. This suggests that the present-day north–northwest striking arcute fold belt is controlled bythe indentation of the competent Hannan complex with theQinling microblock during oblique intracontinental subduction,leading to the rotation of the strata in this area. Until this event,fold axes in lower Paleozoic strata in the Fangxian range east ofthe Dabashan arcute belt (Fig. 1), remained at a small angle tothe major thrust fault of the Dabashan arcute belt. At this stage,the Dabashan arcuate belt was cut by a conjugate set of NW-striking dextral and NE-striking sinistral strike–slip faults(Fig. 12). Metamorphic (Li et al., 2003) and structuralobservations suggest that rocks of the middle level under theFoping dome extruded eastward about 20 km due to the offsetof the Wudang dome at the same time or soon after theindentation of the Hannan complex.

In general, the locations where promontories on one margincaused indented locations on the opposing margin exhibithigher-grade metamorphism than the embayment or non-indented areas. For instance, the Mianlue suture from Changbain Lueyang County to Youshui village in the east (Fig. 2) has




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higher grade metamorphism than nearby regions, suggestingthat the lower crust was uplifted to shallower levels (Li, 1998;Li et al., 2003). In the segment of Kangxian County to LueyangCounty, the suture is offset by a series of NW-striking brittlestrike–slip faults. Based on S–C fabrics, en echelon quartzveins (Fig. 12) and striae, it is deduced that the earlier strike–slip faulting was mainly sinistral, and later ones became locallydextral. Regional paleo-stress field analysis shows that the laterdextral strike–slip faulting may be related to the NE-directedoblique shortening, the regional NW-directed extension and thelocal stress fields are associated with block extrusion (Zenget al., 2001; Wang et al., 2001; Zhang et al., 1996). Meanwhile,

Fig. 13. A model showing deformation dynamics in


thrusting mainly occurred in the Dabashan arcute belt range,perhaps being a composite structure formed after the later stageof the oblique indentation, and the strike–slip faulting issecondary. However, strike–slip faulting is characteristic of theQinling microblock. This indentation also produced the lastuplift of the Foping and Madao domes (Fig. 12).

4.2.3.1. Strike–slip faulting, indentation, extrusion and obliqueintracontinental subduction (J1–K1). Closure of the MinalueOcean between Kangxian County and Gaochuan Town led topeak deformation and metamorphic conditions in the SouthChina Craton and the Qinling Microblock beginning in Late

the Mianlue suture zone (see text for details).



Fig. 13 (continued).


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Triassic. The collision affected the entire Qinling microblockdue to its soft basement. Major Chevron type folds are welldeveloped, especially in the Foping area, and overprinted byduplex thrusting. The steep collisional boundary geometry andflexural rebound of the overriding plate of both the north Ordosblock and the Hannan complex resulted in the disappearance ofthe foreland basin on the northern margin of the Hannancomplex. In the Carnian (Late Triassic), the Dabashan forelandbasin was a nonmarine foreland molasse basin in anintracontinental collision stage. Within the microblock, thepre-existing faults are typically preserved as brittle structures.Most are WNW dextral strike–slip faults, whereas othersinclude ENE-and NNE-striking sinistral strike–slip faults. Bothfault sets accommodated the movement between different sub-blocks of the microblock. Pull-apart basins along these faultbelts were filled by lower Jurassic to lower Cretaceoussedimentary and (volcanic) rocks. The stress-field analysis(Fig. 12) indicates NE-ENE directed oblique A-type subductionof the South China craton under the microblock showing that itwas in an oblique intracontinental subduction stage, consistentwith the formation of major strike–slip faults.

At shallow structural levels, the ophiolitic blocks crop outalong the bulged border of the competent Hannan complex andforeland basin. However, the foreland basin sequence ispreserved poorly in this area. The geophysical profile revealsthat the present-day geometry of the suture shows some steepercollisional faults resulting in the poorly developed lower anglethrusts from the late stages of orogenesis. The indentation of theHannan complex into the Qinling microblock led to a loweringof the angle of thrusting in the suture, the development ofstrike–slip faults on the flank of the Hannan complex, andobduction of ophiolites and detachment of the earlier thrustedXixiang slice to the root belt of the suture. Therefore, this


segment of the suture basically represents the primary locationof closure of the Mianlue ocean. However, in the subsequentoblique intracontinental subduction, the Yangpingguan sinsitralstrike–slip fault west of the Hannan complex perhaps shows thethrust fault exhumed from deep levels. Additionally, the ESEdipping, Huayingshan dextral strike–slip fault, probably showsgently ENE dipping thrust fault relationships (Zhang et al.,1996). This resulted in the near E–W compression acting on thestrata of the Gaochuan slice, the Donghe slice and the Wudangdome (Figs. 1 and 2) to form the third generation of NNW orNNE striking gentle folds that were superimposed on the secondgeneration of WNW striking folds.

4.2.3.2. Foreland thrust tectonics in the north margin of theSouth China craton (T3–K1). The foreland deformation isclosely related to collisional processes rather than the oceanicsubduction process (Dong et al., 2005). A Mesozoic foreland-basin complex formed along the northern South China cratonduring A-type subduction of the South China plate under theQinling–Dabie orogenic belt along the Mianlue suture. As theSouth China craton moved northwestwards and was obliquelysubducted under the Qinling–Dabie (Middle–Late Triassic), aflysch-filled foredeep developed in the Diebu–Songpan in thewestern part of the northern South China craton (Zhang et al.,1996, 2004a,b). During the Late Triassic, a nonmarine molassebasin first formed in the eastern part of the northern South Chinacraton in response to initial collision there (Liu et al., 2005;Meng et al., 2005). This clastic wedge prograded over theformer marine basin and was accompanied by a change fromhigh-sinuosity river systems flowing into basinal lakes, tohigher gradient braidplains. Complete oceanic closure along theMianlue suture during the Middle Jurassic produced a moreextensive east–west molasse basin with rivers, deltas and lakes




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(Liu et al., 2005). The Dabashan foreland fold-thrust belt in thenorthern margin of the South China craton, closely related to theMianlue suture, is a result of normal and oblique subduction ofthe South China craton beneath the Qinling–Dabie microblockin the Indosinian and Yanshanian periods.

The southern Dabashan thin-skinned foreland fold-thrustbelts south of the Chengkou–Fangxian fault, presently has asouthwestward curvature, and can be divided into a thrust belt inthe south, merging north into a thrust-fold belt and frontal foldbelt (He et al., 1997; Dong et al., 2005). The sedimentarysuccessions of the Mesozoic foreland basins suggest that therewere three phases of depositional history associated with oceanclosure and development: the Late Triassic phase coeval withsyn-orogenic collision, the Early–Middle Jurassic phaseequated with intense post-Late Triassic buckling of the basinbasement, and in which the middle Jurassic Mianxian Groupunderwent the intense compressive deformation, and the LateJurassic and Early Cretaceous phase in which the LowerCretaceous deposits were thrust over the Upper Cretaceous inthe Sichuan basin (Liu et al., 2005).

The kinematics of the southern Dabashan thrust-fold beltindicates that in the Late Triassic, the stair-shaped thrustingsouth of the Chengkou–Fangxian fault at first initiated in thedeep crust. In both the Huijunba–Wuliba area at the southernShaanxi province and the Heping–Wuxi area at the Bashancurvature, the structural lines show the WNW or near E–Wstrikes, consistent with those in the suture. This resulted fromthe N–S compression during the head-on collision of the SouthChina craton to the Qinling microblock in the syn-collisionaldeformational episode. However, the Longmen Shan thrust beltis probably related to the eastward extrusion and resistance ofthe Songpan–Gangze block including Bikou block. From theJurassic to Early Cretaceous, the southern Dabashan thrust beltpropagated forward from deep to upper levels, and from thenorth root to the south thrust front. The deep-to-shallowthrusting perhaps is related to the synchronous, horizontallyeastward 20 km extrusion of the Foping midcrust due to theoffset of the Wudang dome (Li, 1998). By the Late Jurassic, thedeformation propagated to the present-day frontal location(Dong et al., 2005). At the same time, there are a few reversethrust and out-of-sequence faults developed in the intensedeformational domain that accommodated the displacementbetween the different blocks (He et al., 1997). Both the faultstrikes and the fold axes in these blocks are dragged into a NNWdirection and the faults are dextral strike–slip faults (Fig. 12)(He et al., 1997). These result from the irregular shape of thepassive margin, oblique subduction of the South China cratonand the foreland thrusting and propagation at the Bashansegment, synchronously. During Late Jurassic through EarlyCretaceous, the depocenter of the nonmarine molasse basinmigrated continually from east to west because of intraconti-nental deformation associated with clockwise rotation of theSouth China craton relative to the North China craton. In thistime interval, the basin was again dominated by fluvial andlake-delta deposition and rivers continued to disperse sedimentssouthwards into the basin (Liu et al., 2005). During Jurassicthrough Early Cretaceous times, the Longmen Shan fold-thrust


belt continuously migrated eastward to the frontal thrust zone, atthe same time, this molassic clastic wedge prograded eastwardand the earlier Jurassic molasse was folded. This wedge wascannibalized forming new Cretaceous–Tertiary molasse depos-its, resulting in the migration of the foredeep from northwest tosoutheast. Similar foredeep migration in other foreland basinshas been equated with rates of plate convergence (e.g., Bradleyand Kusky, 1986).

On the other hand, the Jurassic, Cretaceous and Tertiarydepocenters also migrated southwestward along the Longmeng-shan fold-thrust belt (Liu et al., 1990; Yong et al., 2003), reflectingthat the thrust belt has not only compressive components but alsoa sinistral strike–slip component. These transpressive character-istics are also shown in en echelon folds and depocentermigration. All these reflect that the Jurassic to Early Cretaceous,oblique, intracontinental indentation of the Hannan complex intothe Qinling microblock resulted in the extrusion of the Bikoublock and theDonghe slice ofwhich the north border is the Shiyanfault and the south border is the Bashan arc-shaped fault (Li, 1998;Dong et al., 2005). The Longmen Shan and Shiyan faults formedas sinistral faults. However, the Zhuangyuanbei and Bashan faultswest of the Guangtoushan pluton initiated as dextral faults. It isnoted that the Bikou block is also indented northward, but lessthan the Hannan complex, showing the relative and southwest-ward escape effects (Fig. 12). The middle Jurassic in the Mianluesuture north of Mianxian County underwent steep northwardthrusting in the Late Jurassic.

5. Collision leading to multiple-stage large-scale extrusion:discussion

The Qinling–Dabie orogenic belt is a zone of long-livedshortening with multistage extrusion events that formed duringMesozoic times in central China. Based on all the characteristicsin the Kangxian to Gaochuan and neighboring segments of thesuture, especially the kinematics, sedimentary rock associa-tions, and igneous rock distributions, we propose a three-stagestructural-tectonic model (Li, 1998; Li et al., 2002, 2003).

The first stage involves opening and spreading of an oceanbasin from the Early Devonian to Late Permian (Feng et al.,1996; Meng et al., 1996; Zhang et al., 1996; Lai et al., 1997,1999, 2003, 2004; Fig. 13A). Many lines of evidence suggestthat the Bikou block may have been located south of the“Gonghe” graben, comprising a triple RRR junction togetherwith the Mianlue ocean (Zhang et al., 2004a,b). This area maybe the initial RRR location of opening of the Mianlue Ocean.At the same time, the Qinling–Dabie microblock movednorthward and the northern Shangdan Ocean shrunk into arelict sea basin.

The second stage involves subduction of the Mianlueoceanic crust and subsequent collision of the South and NorthChina cratons during shrinking of the ocean basin andshortening of associated deposits (Fig. 13B). From EarlyPermian to Middle Triassic, subduction and subduction-relateddeformation occurred between the Shangdan suture and theMianlue subduction zone north of the ocean. South of the ocean,however, new extension formed a wider ocean in the west than




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the east. At the same time, the Bikou block moved eastward andthe upper and middle Trassic strata in the present-day Songpan–Ganze orogen west of the Bikou block formed, indicating theclosure of the Longmen Shan seaway and the westwardmigration of the sediments (Yong et al., 2003). Until LateTriassic, two plates were converging and colliding, which led tothe formation of a foreland basin in the south. However, noforeland basin developed north of the Hannan complex due toits competence (Yong et al., 2003). Therefore, the Xixiang slicewas thrust southwestward (with vertical extrusion) as a weakly-deformed thrust slice from the southwest ramp of the Dabashathrust during the earlier thrusting stage.

During the third stage, block adjustment of post-collisionaldeformation was under the oblique intracontinental deep-subduction regime (Dong et al., 2005; Fig. 13C). During LateTriassic to Early Cretaceous, the dynamics of the Pacific and otherplates on the Pacific realm had more and more influence on theCentral China orogen (Hacker et al., 2000; Ratschbacher et al.,2003; Zhang et al., 2004a,b). The blocks in the Qinling–Dabieorogen and its neighbors accommodated various stress fields in thedifferent tectonic parts. Highly variable structural styles charac-terize this period. The last deformation in the Longmen Shanorogen and theDabashan thrusts indicates that the intracontinentalsubduction of the northeast part of the South China craton beneaththe Qinling–Dabie microblock was related to the delamination ofthe subducted Mianlue oceanic slab (Li, 1998).

Based on the above description of the major faults, suturesand the deformation history, the Mianlue suture and itsneighboring blocks involved southward thrusting and south-ward and upward-oblique extrusion of about 20 km above theautochthonous basement during the D1 episode. The D2 and D3

episodes folded all the units in a thick-skinned fashion abouteast–west (D2) and west–northwest (D3) axes in the Mianluesuture. An early foreland propagating sequence of accretion oflate Paleozoic rocks above the Yangtze craton is not involved inD1 deformation but in the equivalent D2 and D3 deformation ofthe Mianlue suture. Two stages of strike–slip faulting mainlyoccurred at the end of D2 and D3, respectively. During D2

deformation, the Bikou block was obliquely indented ESE-directedly into the Mianlue suture, rather than escaping acrossthe Mianlue suture from north of the Mianlue suture as a part ofthe Qinling–Dabie microblock. During D3 deformation,however, it was bounded by the south boundary fault of theMianlue suture and the Yangpingguan fault of coeval, butopposite sense strike–slip faulting, and was extruded south-westward, at a small scale based on the shortening amountsouthwest of the Bikou block, into Songpan–Ganze orogen(Fig. 13C). The other basement blocks north of the Mianluesuture were extruded eastward, at a large scale of about 20 kmlateral displacement based on the offset of the Wudang dome(Fig. 1), during the D3 episode due to the northeastwardindentation of the Hannan complex of the South China craton.Post-D3 emplacement of granite, cutting across the strike–slipfaults such as the Mianlue suture, provides a minimum age of200 Ma for D3 deformation (Zhang et al., 1996).

Liu et al. (2003) pointed out that this prolonged history ofshortening across two nearby (about 100 km apart) sutures


played some roles in the exhumation of ultrahigh-pressurerocks from beneath the Dabie (Maruyama et al., 1994; Hackeret al., 2000; Xu et al., 2002a,b; Ratschbacher et al., 2003;Faure et al., 2003; Tang et al., 2003; Zhang et al., 2004a,b;Liu et al., 2004, 2005). Wang et al. (2003) even suggestedthat the Dabie HP-UHP metamorphic rocks were originallylocated beneath the Foping dome, then it was extrudedeastward to its present-day location. Here our structural datadon't support a linkage between the multiple-stage (earlysouthward/vertical and late eastward/lateral), large-scale (bothabout 20 km) extrusion in the Qinling orogen to theexhumation of UHP-HP metamorphic rock in the Dabieorogenic belt. In summary, based on insights from theevolution of the Mianlue suture, the D2 and D3 episodes inthe Mianlue suture and adjoining areas are not responsible forand associated with the two-stage extrusion of the DabieUHP-HP terranes from the narrowest part of the Qinlingorogen to the present erosional surface (more than 350 kmaway from the Foping dome, Wang et al., 2003).

Acknowledgements

This study was funded by a China NSFC grants 40472098,40002015 and 49732080, the Major State Basic ResearchDevelopment Program of China (TG1999075505), GeologicalInvestigation Project of ChinaGeological Survey (200013000169),and also supported by US NSF grant EAR-02-07886 awardedto T. Kusky. We thank Prof. Wang Tao and an anonymousreviewer for their constructive review and suggestions.

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Collision leading to multiple-stage large-scale extrusion in the Qinling orogen: Insights from the...

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