Collision Leading to Multiple-Stage Large-Scale Extrusion in the Qinling Orogen: Insights from the Mianlue Suture ⁎ Sanzhong Li A, , Timothy M

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Gondwana Research 12 (2007) 121–143

Collision leading to multiple-stage large-scale extrusion in the Qinling orogen: 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, China b 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, China d Department of Geology, Northwest University, Xi'an, China e Institute of Geomechanics, Chinese Academy of Geological Sciences, Beijing, China f 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 Available online 16 December 2006

Abstract

The geologic framework of the Phanerozoic Qinling–Dabie orogen was built 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 microblock, and the South China craton (including the Bikou block), separated by the Shangdan and Mianlue sutures. The Mianlue suture zone contains evidence for Mesozoic extrusion tectonics in the form of major strike–slip border faults surrounding basement blocks, a Late Paleozoic ophiolite and a ca. 240–200 Ma thrust belt that reformed by 200–150 Ma thrusts during A-type (intracontinental) subduction. The regional map pattern shows that the blocks are surrounded by complexly deformed Devonian to Early Triassic metasandstones and metapelites, forming a regional-scale block-in-matrix mélange fabric. Five distinct tectonic units have been recognized in the belt: (1) basement blocks including two types of Precambrian basement, crystalline and transitional; (2) continental margin slices includingEarly Paleozoic strata, and Late Paleozoic fluviodeltaic sedimentary rocks, proximal and distal fan clastics, reflecting the development of a north-facing rift margin on the edge of the South China plate; (3) out of sequence oceanic crustal slices including strongly deformed postrift, deep-water sedimentary rocks, sheeted dikes, basalts, and mafic–ultramafic cumulates of a Late Paleozoic ophiolite suite, developing independent of the rift margin in a separate basin; (4) out-of-sequence island-arc slices; (5) accretionary wedge slices. All the tectonic units were deformed during three geometrically distinct deformation episodes (D1,D2 and D3 during 240–200 Ma). Units 2–4 involved southward thrusting and vertical then southward extrusion of about 20 km of horizontal displacement above the autochthonous basement during the D1 episode. Thrust slices 20 km south of the Mianlue suture are related to this vertical 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 rocks deposited 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 block was obliquely indented to the ESE into the Mianlue suture, rather than being thrust over the Mianlue suture from the north as a part of the Qinling–Dabie microblock. During D3 deformation, however, the Bikou block was bounded by the south boundary fault of the Mianlue suture, and the Yangpingguan fault on the south. These faults are coeval strike–slip faults, but of opposite senses, and accommodated minor southwestward extrusion of the Bikou block into Songpan–Ganze orogen. The other basement blocks north of the Mianlue suture were extruded eastward by about 20 km of lateral displacement, based on the offset of the Wudang dome, during the D3 episode due to the northeastward indentation of the Hannan complex of the South China craton. Post-D3 emplacement of granite, cutting across the strike–slip faults such as the Mianlue suture, provides a minimum age of 200 Ma for D3 deformation. Therefore, based on insights from the evolution of the Mianlue suture, the D2 and D3 episodes in the Mianlue suture and its neighbors are not responsible for and associated with 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).

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1. Introduction that orogen-parallel eastward extrusion occurred diachronously from 240 Ma in the east to 225–210 Ma in the west. The Qinling–Dabie Orogen marks the irregular suture Ratschbacher et al. (2000) described the Cretaceous and between the North and South China cratons (Fig. 1), and Cenozoic unroofing with eastward tectonic escape and Pacific contains the largest belt of ultrahigh pressure (UHP) backarc extension in the Early Cretaceous, and Pacific metamorphic rocks in the world. It is a major part of the E– subduction in the mid-Cretaceous. Wang et al. (2003) proposed W-trending Central China Orogen of central China (Jiang et that the Dabie HP-UHP metamorphic rocks were originally al., 2000), extending 1500 km westward from the Qinling located beneath the Foping dome, a dome in the narrowest part range, through the Kunlun range, and 600 km eastward of the Qinling orogen (Fig. 1), where it underwent ultrahigh- through the Tongbai–Dabie range, then continues northeast- pressure metamorphism in the Early Triassic. Then it was ward through the Sulu area of the Shandong peninsula into the extruded eastward to its present-day location. Imjingang or Hongseong–Odesan fold belt of Korea (Fig. 1) Since so many different models for extrusion mechanisms (Ratschbacher et al., 2003; Oh and Kusky, in press). Large and timing have been proposed, we further focus on the amounts of continent–continent convergence have been Mianxian–Lueyang (abbreviated to the Mianlue) suture south accommodated along the geometrically irregular suture, but of the Foping dome of the Qinling Orogen to provide structural the convergence was diachronous in time and inhomogeneous constraints on whether or not, and how the extrusion of the in space so that the related indentation, extrusion and rotation Dabie UHP terranes from the Foping dome to the present-day has a complex spatial and temporal pattern (Tapponnier et al., location is associated with the Mianlue suture (Wang et al., 1982; Ratschbacher et al., 1991; Davison et al., 1995; Zhang 2003). Furthermore, we discuss an alternative tectonic model in et al., 1995; Jones et al., 1997; Thompson et al., 1997a,b; the Qinling–Dabie Orogen using our detailed field data from the Kusunoki and Kimura, 1998; Hacker et al., 2000; Ratschba- Mianlue suture of the Qinling orogen. cher et al., 2000; Johnston et al., 2000; Beaumont et al., 2001; Xypolias and Koukouvelas, 2001; Hatcher, 2002; Ratschba- 2. Geological background cher et al., 2003; Xypolias et al., 2003; Wang et al., 2005). Especially extrusion and indentation tectonics in Asia are The geologic framework of the Qinling–Dabie orogen was evident tectonic phenomena. Consequences of Cenozoic and built up through two major suturing events of three blocks. Mesozoic indentation tectonics throughout Eurasia are mainly From north to south these include the North China craton driven by active indentors such as the Indian plate into the (including the north Qinling block), the Qinling–Dabie micro- Eurasian plate (Tapponnier et al., 1982) and the South China block (including, from west to east, the Qaidam, the West craton into the North China craton (Yin and Nie, 1993), Qinling, South Qinling and Dabie–Sulu blocks), and the South respectively. Some basement blocks or microblocks between China craton, separated by the Shangxian–Danfeng (abbrevi- or in these orogens may have undergone lateral and/or vertical ated to the Shangdan) and the Mianlue sutures. The Shangdan (upward) extrusion tectonics. For example, Mesozoic and suture resulted from Middle Paleozoic closure of the Shangdan Cenozoic convergent tectonics in southeast Asia (Tapponnier Ocean and collision of the North China craton and the Qinling– et al., 1982; Morley, 2002) included lateral extrusion of south Dabie microblock. The Mianlue suture, however, resulted from China (Zhang et al., 1995) and large-scale eastward or upward Late Triassic closure of the Mianlue ocean and collision of the extrusion of the Qinling–Tongbai–Dabie belt (Maruyama et Qinling–Dabie microblock and the South China craton (Liu al., 1994; Hacker et al., 2000; Ratschbacher et al., 2000; et al., 2004; Meng et al., 2005; Zhang et al., 2005). Hacker et al., 2000; Li et al., 2002; Wang et al., 2003). Other Rocks in the Qinling orogen record a prolonged history of examples include Late Tertiary tectonic extrusion of the continental divergence and convergence between blocks. From Eastern Alps (Frisch et al., 1998), extrusion tectonics of Late Neoproterozoic to Early Paleozoic times, sediments from Central Anatolia, Turkey (Dirik, 2001), and Neogene– the Proto-Tethyan Shangdan Ocean were deposited on a passive Quaternary lateral extrusion of the southern Apennines. margin in the South Qinling on the northern margin of the South Lateral extrusion and escape tectonics are typically accom- China craton, while in the North Qinling, passive margin modated along large-scale strike–slip faults or subduction sediments were also deposited on the southern margin of the zones, for example, subduction-related extrusion of the North China craton. When the Proto-Tethyan Shangdan Ocean Western Carpathians (Sperner et al., 2002), or Cenozoic subducted northward during the Ordovician, the North Qinling extrusion of eastern Asia beside the Tancheng–Lujiang Fault evolved into an active continental margin characterized by Zone (Zhang et al., 2003). thick-skinned deformation involving crystalline basement. We Every possible kind of extrusion tectonics such as eastward, suggest that this conversion of the margin followed an arc vertical (upward) and southward, has been proposed for the accretion/collision event, the record of which has been obscured Qinling–Dabie Orogen in the last decade (Hacker et al., 2000; in the high-grade internal zones of the orogen. Collision of the Li et al., 2002; Zhang, 2002; Wang et al., 2003; Ratschbacher South and North Qinling blocks took place in Middle Paleozoic et al., 2003; Wang et al., 2005), resulting in many controversies. along the Shangdan suture. Different isotopic geochronometers Maruyama et al. (1994) proposed that vertical extrusion is to the North Qinling arc system north of the Shangdan suture important for the exhumation of the UHP terrane in the east record a Middle Paleozoic tectonothermal event (including an central China in the Triassic. Hacker et al. (2000) pointed out 40Ar/39Ar age of 426±2 Ma, a Rb–Sr mineral isochron age of 中国科技论文在线 http://www.paper.edu.cn .L ta./Gnwn eerh1 20)121 (2007) 12 Research Gondwana / al. et Li S. – 143

Fig. 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 of the Qinling orogen, extending westward from the Suixian, Xianhuang, Fangxian, Gaochuan, Shiquan, Mianxian, Lueyang, Kangxian, Pipasi, Nanping to Wenxian counties, and northwesterly through the N-MORB-type Derni ophiolite linked with the A'nyemaqen suture 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 orogen; TM—Tarim craton; NCC—North China craton; CCO—Central China orogen; WQL western Qinling orogen; EQL—eastern Qinling orogen; TB—Tongbai orogen; WDB—western Dabie orogen (or Hong'an block); EDB—eastern Dabie orogen; YC—Yangtze craton (or South China craton); CC—Cathysian craton; SGO—Songpan–Ganze orogen; AHO—Alps–Himalaya orogen; WDD—Wudang dome; FPD—Foping dome. 123 中国科技论文在线 http://www.paper.edu.cn

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411±5 Ma and a Rb–Sr whole rock isochron age of 406± (Li et al., 1996; Zhang et al., 2002). The Late Triassic collisional 22 Ma) (Sun et al., 2002a). This tectonothermal event is orogeny along the Mianlue suture caused extensive thin- restricted to the North Qinling block and the southern margin of skinned fold-and-thrust deformation, extrusion, rotation, uplift North China craton (Ratschbacher et al., 2003; Wang et al., and granitoid plutonism throughout the Qinling, and led to final 2005), and is not observed in the South Qinling block or the amalgamation of the North and South China cratons (Meng and northern margin of the South China craton (Sun et al., 2002a). Zhang, 2000; Sun et al., 2002a,b). Rifting occurred along the southern rim of the South Qinling Most previous models for the Qinling–Dabie orogen at the same time as the collision along the Shangdan suture in recognized only one suture between the North and South the north, and was followed by the opening of the Paleo- China cratons, and placed this along the Shangdan suture, Tethyan Mianlue ocean during the Late Paleozoic, resulting in suggesting that the collision along this zone was Triassic in age. the separation of the Qinling–Dabie microblock from the South The existence of a second suture, the Mianlue, in the Qinling– China craton (Meng and Zhang, 2000). The Qaidam, Western Dabie orogen was not anticipated, and little is yet known of its Qinling, Southern Qinling and Dabie–Sulu blocks form a string composition, geometry and tectonic evolution. of Paleozoic crustal fragments between two sutures in the The Mianlue suture is located in the southern part of the Qinling–Dabie orogenic belt. The Songpan–Ganze block Qinling orogen (Fig. 1)(Zhang et al., 1996; Xu et al., 1996; (including the Bikou block) south of the Mianlue suture Chen et al., 2000; Roger et al., 2003; Lai et al., 2004). It is a preserves similar Precambrian crystalline and transitional large-scale, southward curved fold-thrust belt, with well-studied basement and Early Paleozoic sedimentary cover (Zhang, outcrops located in the Gaochuan–Kangxian segment of the 2002). It is shown here that they all could have belonged to a belt. South of this segment is the South China craton including single unified passive continental margin along the north the Bikou block (Fig. 1) and the Hannan complex. However, margin of the South China craton in Early Paleozoic times. north of the segment is the Late Paleozoic structurally Collision of the Qinling–Dabie microblock and the South China imbricated Qinling–Dabie microblock which is separated by craton began in the Late Triassic along the Mianlue suture the east–west Shangdan suture from the North China craton

Fig. 2. Simplified structural map showing the Mianlue suture and adjacent parts of the Qinling orogen from Gaochuan to Kangxian. A—locality of Fig. 7;B—locality of Fig. 9;C—locality of Fig. 6;D—locality of Fig. 8;E—locality of Fig. 10; The direction of second extrusion shown in this figure is the same as that in Fig. 1. 中国科技论文在线 http://www.paper.edu.cn .L ta./Gnwn eerh1 20)121 (2007) 12 Research Gondwana / al. et Li S. – 143

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. 125 中国科技论文在线 http://www.paper.edu.cn 126 .L ta./Gnwn eerh1 20)121 (2007) 12 Research Gondwana / al. et Li S. –

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 profile 143 in the Fig. 5. 中国科技论文在线 http://www.paper.edu.cn

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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.

(Fig. 1). The Foping dome is located in the narrowest segment might explain the evolution of this segment of the orogen. In between the two sutures (Figs. 1 and 2). However, other domes this paper we systematically describe the composition, such as the Wudang, Tongbai, Xinxian and North Dabie, are structural architecture, deformation history and related orogenic located at different positions within the Qinling–Dabie micro- processes of the Mianlue suture based on our ten-year study of block (Fig. 1). local and regional field relationships, and other published key Volcanic rocks in the Gaochuan–Kangxian segment of the geochronology results. Finally, we test a model to interpret the Mianlue suture are divided into island-arc type, intra-arc-rift collision leading to multiple-stage large-scale extrusion based type, and juvenile-rift type (Li et al., 1996; Lai and Zhang, on insights from the Mianlue suture of the Qinling orogen. 1996; Lai et al., 1997, 1999, 2000). Tectono-lithological units are classified according to the non-Smith stratigraphic system 3. Deformation history of sutures and tectonic units (Du et al., 1998; Li et al., 2001a,b,c,d). Five distinct tectonic units have been recognized in the belt: (1) basement blocks We have divided rocks and structural data of the suture zone including two types of Precambrian basement, crystalline and into coherent structural domains, typically separated by shear transitional; (2) continental margin slices including Early and zones. In this section we systematically discuss and compare Late Paleozoic strata; (3) oceanic crust slices including postrift, each of the structural elements from each of these units, and deep-water sedimentary rocks, sheeted dykes, basalts, and then provide a structural analysis of the orogen. The earliest, mafic–ultramafic cumulates of a Late Paleozoic ophiolite suite; first-stage of deformation is preserved in the regionally (4) island-arc slices; (5) accretionary sediment wedge slices extensively Devonian–Early Triassic sandstones that form the (Fig. 3).The deformation history of this suture zone is poorly deformed matrix to belts of melange in the suture. Later stages known, so there are few constraints on which dynamic models of deformation are preserved in the numerous structural slices, 中国科技论文在线 http://www.paper.edu.cn

128 S. Li et al. / Gondwana Research 12 (2007) 121–143 blocks and fault belts within the suture. Our analysis of of the fault and has no significance for the age of the fault) to the structural sequence is based on the sequence of superimposed north and the Kangxian–Lueyang fault (classified as F2) to the deformational fabrics, and comparison of the sequence of events south (Fig. 4). from slice to slice in the orogen. Analysis of kinematic relations and orogenic processes has led to the establishment of a new (1) The F1 Fault (Fig. 4) is the northern boundary fault of the comprehensive tectonic model of the Mianlue suture. Mianlue suture. It is a regional, east–west striking, several tens to hundreds of meters-wide and fault zone separating 3.1. The Shangdan suture the Qinling–Dabie microblock from the Mianlue suture (Li et al., 2001a). Different protolith lithofacies flank the The Shangdan suture is an important tectonic belt, located fault, reflecting respectively lower Paleozoic sediments north of the Minalue suture, separating not only the North on the north, middle to upper Paleozoic sediments on the Qinling block from the South Qinling block, but also the North south, showing that this fault juxtaposes previously China craton from the South China craton (Fig. 1). It is widely separated rock units. The fault experienced a characterized by a 2 km wide E–W striking, south-dipping, long-lived deformation history. The first-stage of defor- large-scale mylonite zone that extends continuously for more mation in the fault belt is recorded in EW to WNW than 1000 km (Zhou and Zhang, 1996). This suture contains striking transpressional ductile shear zones, preserving Middle Ordovician to Early Silurian radiolaria fossils and 447– many kinds of mylonitic fabrics. The orientation of the 357 Ma ophiolitic derived from the Shangdan ocean (Zhang mylonitic foliation is parallel to that of the schistosity in et al., 1996). This suture has a multi-stage deformational history the suture. Many sheared clasts in the shear zone indicate including Late Triassic sinistral–transpressional shearing sinistral strike–slip non-coaxial strain in the fault zone for marked by S–C fabrics and other kinematic indicators from D1 (note: the subscript for deformation episodes, Dx, earlier deformation, followed by Cretaceous brittle-normal relates to the relative timing of deformation). These D1 faulting in younger deformation events (Zhou and Zhang, fabrics in the fault zone truncate older fabrics outside the 1996). The deformation regime of the suture is transpressional fault zone. with characteristic flower structures consisting of several active, The D2 deformation episode included a component of sinistral strike–slip master faults and oblique-slip thrust faults sinistral dutile strike–slip faulting, reworking the D1 folds with opposing vergence (Zhang et al., 1996). and generating a D2 generation of folds with steeply N–S dipping axial surfaces, and steeply west plunging hinges. 3.2. Major faults in the Mianlue suture D2 deformation locally resulted in the formation of structurally differentiated bands in discrete quartz-rich The Mianlue suture consists mainly of ophiolitic tectonic and mica-rich microlithons, implying deformation near mélange disposed in a series of thrust slices separated by brittle the ductile–brittle transition (e.g., Borradaile et al., 1982). and/or ductile faults exhibiting different scales of deformation, The strike–slip faulting overprinted and re-oriented ages of formation, structural levels, tectonic settings and origins fabrics from the thrusting on the flanks of the fault during (Figs. 3–5)(Li et al., 2001a,b,c,d). This suggests that the the peak collisional stage. The latest D3 dextral strike–slip Mianlue suture records prolonged continental divergence and faults with subhorizontal slickenlines are superimposed convergence between blocks. The suture is bounded by the on the earlier ductile shear zone and crosscut the D2 Zhuangyuanbei fault (classified as F1, using the nomenclature crenulation cleavage at a small angle. All the above where F corresponds to the designation of a structure as a fault, structures are offset by the northwest striking, brittle and the numerical subscript denotes simply the numerical name strike–slip faults. The existence of a steeply dipping

Fig. 6. Cross-section showing deformation of matrix in the suture zone from Jingjiahe to Dengjiaying. 中国科技论文在线 http://www.paper.edu.cn

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strike–slip zone (typically within a reverse, hinterland- faults in palgioclase grain indicate a top to the south sense directed component) located behind a thrust-dominated of the early thrust movement. wedge is a feature of numerous orogenic belts worldwide (5) The Zhujiashan–Wujiaying Fault system (classified as F6) (e.g., Shaer and Rogers, 1987). The last faulting event is developed within the Zhujiashan slice is characterized by brittle normal faulting, resulting in extensional breccia northwest striking sinistral strike–slip faults (Figs. 3 and 4). and triangular fault planes. The ductile shear zone was 6–10 km long and 20–50 m (2) The Shuigouyan Fault (classified as F3) separates the wide in its early deformation stage, and developed many northern continental margin sedimentary slice from the quartz veins parallel to the shear foliation. Younger small- central ophiolitic slice (Figs. 3 and 4). This fault has an scale brittle faults developed within the early ductile shear arcuate trend in plan view, dips north at angles of 60°–83° zone, and accommodated horizontal displacement as in profile. Many structural indicators in mafic, felsic, and exemplified by a 1 km offset of the Majiashan ophiolite carbonate mylonite and proto-mylonite show top to the body across the Zhujiashan–Shifanggou fault, a branch of south–southwest thrusting. the Zhujiashan–Wujiaying Fault system. Lithologically the (3) The Qiaozigou–Hengxianhe Fault (classified as F4)isa brittle zone consists of a few to tens of meters of lenticular ductile fault separating not only the ophiolite slice from cataclastic masses, including breccia and gouge of variably the southern continental margin sedimentary slice, but deformed host rocks that flank the fault. also the Qiaozigou island-arc igneous slice and the (6) The Kangxian–Lueyang Fault (classified as F2) is a large Jinjiahe marble slice (Fig. 3). The ophiolite slice is thrust regional-scale fault that forms the southern boundary of over the southern continental margin sedimentary slice the Mianlue suture (Li et al., 2001b)(Fig. 4). It is along a 60–70 m wide shear zone. A good exposure may composed of early, intensely-deformed mylonite, and later be found at Qiaozigou cottage where it exhibits dips 75°– fault breccia and gouge. The early (D1) deformation 80° to 000°–010°. The fault is composed of carboniferous records ductile thrusting fabrics. However, the latest protomylonite, felsic mylonite and protomylonite, and deformation produced a nearly east–west striking (095°– mafic mylonite. Asymmetric folds, S–C fabrics and other 105°), steeply dipping (60°–80°) brittle fault zone that is kinematic indicators show thrusting with the top to the several meters to a few tens of meters wide. The fault plane south sense of movement. The shear zone between the is characterized by smoothly polished steep slickensides latter two is about 40 km long, 20–100 m wide and dips planes with horizontal slickenlines and striae showing a 50°–70° to 010° to 030°, suggesting that major displace- brittle sinistral strike–slip sense of motion. This fault zone ments may have been accommodated along this zone. separates the Late Paleozoic accretionary complex to the (4) The Sigoukou Fault (classified as F5) is a boundary shear north from the Late Paleozoic initial rift and passive zone. It strikes northwest (100–110°) and east (70–85°) continental margin formations to the south (Meng et al., but swings west–northwest toward the west, but maintains 1996). The boulders in the conglomerate have no a50–150 m width (Fig. 4). This change in strike suggests a ophiolitic materials nor island-arc-derived materials in sinistral sense of shear during the later deformation stage. their grains or matrices, but are made of continental- The fault consists of felsic protomylonite, banded felsic derived materials from the South China craton. The mylonite, phyllite and carbonate mylonite. Reliable shear conglomerates as a whole have many kinds of passive sense indicators including polycrystalline quartz aggre- continental margin-derived rocks. This indicates that the gates, shear folds, mica-fish, composite “schistosite– pebbles of the early stage conglomerates were formed cisaillement (S–C)” planar fabrics, and antithetic micro- within a rift environment near the passive continental

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Fig. 7. Cross-section from Shuigouyan to Jingguoli at Guozhen showing fold impososition. 中国科技论文在线 http://www.paper.edu.cn

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

margin. Therefore, the fault is an important, long-lived ductile sinistral strike–slip zone and the last dextral brittle boundary fault, that influenced Late Paleozoic lithofacies. strike–slip fault caused by the Indosinian orogeny and subsequent movements. 3.3. Major faults south of the Mianlue suture 3.4. Deformation of matrix in the mélange Two major faults are closely related to the evolution of the Mianlue suture. The regional-scale structure of the suture zone shows many blocks and thrust sheets that are surrounded by a complexly (1) The Heyeba Fault (F7) is a 350–400 m wide, nearly east– deformed assemblage of Devonian to Early Triassic sandstones west striking steeply dipping structure located within the and mudstones. These underwent intense ductile shearing at Tapo slice (Fig. 4). Tectonites in the fault zone are lower to middle metamorphic grade to form phyllite, schist, and composed of mylonitic carbonate, carbonate mylonite, carbonate mylonite (Li et al., 2001a,b,c,d). The structural protomylonite and strongly deformed conglomerate. relationship of the blocks to these metasandstones and metapelites Kinematic indicators including asymmetric folds, S–C is that of blocks-in-mélanges, similar to other convergent margin fabrics, asymmetric porphyroclasts and en echelon mélanges in other orogens (Bradley and Kusky, 1986). The boulders, show that the D1 deformation involved top-to- matrix of this mélange records earlier deformation fabrics than the south–southwest ductile thrusting. The earlier S1 observed in the blocks. The first deformation event in the mélange schistosity was folded by the D2 brittle–ductile dextral matrix, D1, was the most intense, producing the main bedding- shearing to form reclined folds. parallel penetrative foliation in the matrix of the belt. Structurally (2) The Longmen Shan fault system (F8)(Figs. 1, 2 and 4)isa some marble blocks are also incorporated within the matrix. boundary fault zone in the inner part of the South China Therefore, the matrix appears, in map view, as an east–west craton, separating the Bikou block on the west from the trending anastomosing envelope of intensely deformed rocks that Hannan complex on the east (Fig. 1). The fault zone con- record early deformation events. sists of the Longmen Shan fold-thrust belt formed during Fig. 6 shows a structural profile from Jinjiahe–Dengjiaying the Late Triassic Indosinian orogeny, spanning the time (location C in Fig. 2). In Fig. 6, small D2 folds fold the S1 period c. 227–206 Ma (Yong et al., 2003) and some strike– schistosity. Their asymmetry implies southward movement slip faults such as the sinistral Yangpingguan transpres- during ductile thrusting. The whole sequence was folded by D3 sional fault. The fault zone plays an important role in folds, then the deformed matrix was tectonically overthrust by accommodating the deformation in the Kangxian–Gao- south-vergent klippen of Cambrian to Ordovician marble blocks chuan segment of the suture and controlling the sedimen- that contain the same phosphorite as the Cambrian strata of the tary lithofacies of the Sichuan basin (Figs. 1 and 3). To the South China craton. west, no lower Paleozoic strata except the upper Paleozoic Fig. 7 illustrates a cross-section through the metapelitic Tabo Group and Lueyang Formation covered the Bikou matrix of the mélange from Shuigouyan–Jingouli (near the block (Fig. 3). However, less upper Paleozoic strata town of Guozhen, profile A on Fig. 2). D2 folds are tight and the remained in the fault zone, suggesting that the fault zone axial planar crenulation cleavage, S2, is strongly developed. D3 formed during the Hercynian and controlled the lithofacies folds are relatively open and have a well-developed axial planar distribution. The present-day recorded deformations in- cleavage. The S1 schistosity is folded by D2 and D3 folds which clude the early ductile thrust zone, the subsequent brittle– are superimposed forming a coaxial fold interference pattern. 中国科技论文在线 http://www.paper.edu.cn

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Both D2 and D3 fold interference probably resulted from the is relatively weaker than that shown in the Jinjiahe–Dengjiay- superposition of two discrete phases of folding, involving a ing cross-section (Fig. 6). significant shift in the strain field from east to west between the In general, the syn-D3 thrusting in the eastern part of the two phases of folding. In this cross-section, the brittle thrusting suture is coeval with folding in the western part of the suture.

Fig. 9. Characteristic structures of different slices in the Mianlue suture. (A) F1 tight fold of bedding S0 in Xixiang area. (B) Southward thrusting of the Xixiang slice in Xixiang area. (C) F2 fold of S1 schistosity in the Qiaozigou slice in Qiaozigou river. (D) Crenulation cleavage S2 in the Qiaozigou slice in Qiaozigou river. (E) fan- shaped crenulations S2 in the Gaochuan slice in Qiaozigou river. (F) crenulation in the Gaochuan slice in Gaochuan area. (G) crenulation S2 and schistosity S1 parallel to the dark bedding S0 in the Gaochuan slice in Gaochuan area. (H) F3 open fold superimposed on F2 tight fold in the Gaochuan slice in Gaochuan area. Scale pen is 8 cm. 中国科技论文在线 http://www.paper.edu.cn

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This reflects the switch of N–S-directed D2 contraction slip-dominated parallel-aligned association, and a complicated perpendicular to the suture toward northeast-oriented D3 anastomosing association related to complex deformation. The contraction oblique to the suture. latter formed within the Mianlue suture, reflecting more intense The Zhangyagou slice (Fig. 3) is composed of tectonic- or complex south-directed contraction and significant strain sedimentary mélange that underwent intense deformation and partitioning with a dominantly dip–slip component in the south involved different scales of brecciation of metasandstone and and strike–slip component in the north. metacarbonate. These sedimentary breccias are displaced into small scale lenticles aligned parallel to schistosity. The D2 3.5. Major thrust slices and their deformations deformational event produced asymmetric folds accompanied by sinistral strike–slip faults. Tectonic slices in the Mianlue suture can be divided into two Fig. 8 illustrates a structural cross-section from Guojiaba– principal types based on their present-day distribution: internal Lianghekou–Caimahe (location D in Fig. 2). In this area some thrust slices in the north, and external thrust slices in the south. Indosinian HP granulite blocks were involved in intense The latter includes two types of slices, autochthonous and deformation (Li et al., 2000; Zhang et al., 2002). The D2 allochthonous. deformational event produced steeply plunging folds. The fold axes have steep but variably plunging axes between Leigong- 3.5.1. External (southern) thrust slices shan mountain and the town of Lianghekou, and thrust planes are External thrust slices are mainly located in the Hannan typically also steep with an obvious oblique-slip component. complex and the Bikou block south of the Mianlue suture (Fig. 2). Generally these folds are closer to having attitudes consistent In the Hannan complex, rocks previously known as the with formation during strike–slip faulting, with steeply plunging Xixiang “Group” include two parts: one is the Sanlangpu axes. In the northern part of this cross-section, the circa 200 Ma Formation (Fig. 3), consisting of undeformed conglomerates post-kinematic Guangtoushan granitic pluton (Zhang et al., unconformably overlying lower formations; the other is a suite 1996) intruded the Late Paleozoic strata, and cuts the fold axes of volcano-sedimentary rocks belonging to the Wangjiahe, and the suture. Many enclaves in the pluton are tremolitic marble Sunjiahe and Dashigou Formations (Fig. 3). The volcanic rock with the same preferred orientation including a north-plunging association of the Xixiang “Group” exhibits distinctive lineation, reflecting that their orientation was not changed by the geochemical characteristics indicating a volcanic-arc origin granitic magma emplacement. The lineation records top-to-the (Lai et al., 2003). Basalts, andesites and rhyolites in the this area south thrust movement. In addition, there are some rootless folds show remarkable depletion of Nb and Ta (Lai et al., 2000). The preserved in the mafic to ultramafic enclaves in the granite. Baimianxia basalt association of the Xixang “Group” might be Many more-coherent slices and blocks are enclosed within related to partial melting of a depleted mantle source and mélange defined by the thrust faults, ductile shear zones and the formed in an ocean island-arc (immature island-arc) tectonic penetratively-deformed mélange matrix described above, form- setting (Lai et al., 2000). In this case the Sunjiahe, Dashigou ing an anastomosing structural slice shear system (Fig. 4)(Li volcanic-sedimentary rock series should be allochthonous (Lai et al., 2001a,b,c,d). The obvious diversity in structural et al., 2000, 2003). The region developed into an active orientation and style, stratigraphy, magmatism and metamor- continental margin in the Devonian–Carboniferous period, as a phism between these slices and blocks is consistent with other consequence of oceanic crust subduction of the Mianlue oceanic recent research on different tectonic lithofacies, structural basin and /or an intrarc rift (Lai et al., 2000, 2003). This tectonic analysis and volcanic geochemistry of the suture (Meng et al., allochthon should have a close relationship with and have been 1996; Lai and Zhang, 1996; Lai et al., 1997, 1999, 2000, 2003; initially connected with the Mianlue suture (Lai et al., 2003) Li et al., 2003). Tectono-stratigraphic units in this study area can because the same rock assemblage is preserved in the Mianlue be divided into four first-order units: the Qinling–Dabie suture north of the Xixiang “Group”. However, the deformation microblock (or the South Qinling block), the Mianlue suture, of the latter has the same sequence as that of the slices and the Bikou block and the South China craton (Li et al., 2001a,b,c, matrix in the suture. Genetically, the intrafolial south-vergent d). The thrust slices related to the Mianlue suture can be further folds and other kinematic indicators indicate that the Xixiang grouped into several related slice groups (Li et al., 2001a,b,c,d): “Group” has been thrust as an exotic terrane far from the north the basement slice group, the northern slice group related to the (the vertically lined range near the Xixiang County in Fig. 2). South China craton, the oceanic crust slice group, the oceanic Therefore, the Mianlue suture is thus a northern root zone of the island arc slice group, the continental island arc slice group, the Xixiang slice (Fig. 3), of which the frontal thrust belt was thrust collisional sedimentary wedge slice group and the southern slice southward at horizontal displacement of about 20 km over the group related to the Qinling–Dabie microblock. Every slice Hannan complex, i.e. the basin margin, and isolated as a klippe group contains many independent slices or small isolated by uplift, vertical exhumation and erosion of the northern part of blocks, for example, there are more than 200 uncoherent slices this complex. The basal shear zone of the klippe is a duplex and blocks in the Kangxian to Gaochuan segement of the structure (Fig. 9A). However, most of the internal strata of the Mianlue suture (Chen et al., 1997). In various first-order units, slice preserve no fabrics related to this early stage of the slices have different types of structural associations in space, deformation. Weak bedding-parallel continuous cleavage and which includes three dominant structural types: an imbricated folds are only observed in thin sections from several soft layers thrust association in the Qinling–Dabie microblock, a strike– of the Xixiang slice (Fig. 9B). The deformation of the internal 中国科技论文在线 http://www.paper.edu.cn

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

strata of the slice reflects two-stages of contractional folding Tabo Group and the Bikou block. The fold axes of the Tabo after its displacement. The first-stage fold axes are east–west Group and the Lueyang Formation are dominated by east–west trending, whereas the second are nearly north–south trending. trends, at a large angle with the northeast trending Bikou Group The latter folds were superposed over the former to form basin in the Bikou block (Pei, 1989). Zeng et al. (2001) suggested that and dome interference patterns. No foliation formed during the the Bikou block extruded southwestward during the Indosinian two fold generations because of shallow structural levels. In and Yanshanian periods. Wang et al. (2001) also considered the summary, the Xixiang slice belongs to an exotic slice vertically southwestward extrusion of the Bikou block during the extruded out of the Mianlue suture before the strong Indosinian period by considering of scissor-type contraction of contractional deformation. In terms of the present-day gentle the Mianlue suture and exhumation of UHP metamorphic rocks. decollement plane, the Xixiang slice as a whole is a thin- However, the northern marginal structure of the basement of the skinned thrust terrane with the weak early deformation and the Bikou block has been strongly reworked by sinistral strike–slip later two-stage foldings of bedding S0. Subsequent tectonism is faulting along the southern border fault of the Mianlue suture. a top-to-the south brittle thrusting. Therefore, the intensity of The parallelism between their foliations of these two thrusts compression of the Xixiang slice increases towards the north, shows that the strike–slip faulting is very intense, while the where they come into the Mianlue suture. thrust shortening and the intensity of the north margin of the The Tabo slice (Fig. 3) is located in the northern part of the block are less than those of the Longmen Shan thrust tectonic Bikou block (Figs. 3, 4 and 5). Boulders in the conglomerate of belt. That is to say, the lateral displacement component of the the Tapo Group have some materials derived from the Bikou north margin of the block is larger than its horizontal block. This indicates that the slice is an external allochthonous displacement component. This indicates that the primary slice. The earlier deformation in this slice (D1) is dominated by position of the Bikou block should not be far away from the gentle folding of vertical structural transposition fabrics, of south border of the Mianlue ocean, or the Bikou block may not which structural patterns, the orientation of tectonic lineations have migrated and indented toward the north far from the south and others are equivalent to those of the second-stage (Li et al., 2001a), but be transported from the west (Li et al., deformation of the other slices and the matrix in the suture. 2002). Moreover, conglomerate of the Tabo Group formed in the The regional D3 deformation has influenced this area to form the initial rift indicates that the Bikou block did not escape second-stage deformation in this slice. Meanwhile, some southward from the Mianlue ocean proposed by Wang et al. lithofacies-controlled normal faults inverted to form south- (2003). Furthermore, the southeast part of the Bikou block was vergent thrust faults that rework the uncomformity between the offset by the early Yanshanian Yangpingguan strike–slip fault

Fig. 11. Structural profile from Zhongba to Houliu. The diversity in structural orientation and style between these slices has probably been caused by numerous factors including structural superposition and competency contrast between slices or blocks. Late strike–slip fault systems have also played a significant role in bringing up deep crustal deformation zones adjacent to shallow-crustal deformation zones. 中国科技论文在线 http://www.paper.edu.cn

134 S. Li et al. / Gondwana Research 12 (2007) 121–143 after major deformation and formation of the suture because the cleavage or cleavage lamellae to produce large amounts of shear strike–slip fault crosscuts the suture and is intruded by the zone-parallel structurally differentiated quartz veins, and locally 200 Ma Guangtoushan granite (Zhang et al., 1996). Taking the reorients the D2 drag folds by sinistral strike–slip ductile following data into account, for example, the intense shortening shearing. Meanwhile, in many outcrops of the Qinling–Dabie of the middle Ordovician and upper Triassic strata in the orogen, the orientations of the D2 fold axes are variable, Longmen Shan thrust tectonic belt, the north–south trending typically curved into vertical by later deformation. This phe- folding in the Songpan–Ganze orogen and the sinistral strike– nomenon is often associated with strike–slip faults. Therefore, slip nature of the early Yanshannian Yangpingguan and the later deformation of the D2 fabrics occurred during strike– Qingchuan–Maowen faults, we propose that the primary slip adjustment among various blocks or slices along the upright geographic position of the Bikou block is more westerly than ductile shear zones, related to oblique contraction in the suture. its present-day location, consisting of a south branch of triple rift The latest D2 deformation is characterized by brittle–ductile or system, of which the north failed arm is called as the “Gonghe” brittle, south vergent thrust faults controlling the distribution of graben filled in the thick Triassic between the Qaidam block and some blocks of marble overlying closely a suite of black cherts. the west Qinling block (Zhang et al., 2004a,b). When the However, these cherts are different from the deep-oceanic cherts Mianlue suture and the Longmen Shan thrust-fold belt could in the Sanchazi slice, and thought to have formed in a deep- not be compressed further, the Bikou block was extruded water continental margin environment (Sheng et al., 1997). southwestward. Samples from a basalt and gabbro section of the Paleo- Tethyan (approximately 350 Ma; Xu et al., 2002a,b) Mianlue 3.5.2. Internal (northern) thrust slices northern ophiolites in the Mianlue suture display sub-parallel The internal thrust slices are located north of the external and relatively smooth depleted incompatible trace element 206 thrust slices. The representative slices are characterized by patterns and have high εNd (350 Ma) (8.1–11.3) and low Pb / different rock formations. Pb204(350 Ma) (16.90–17.25) (Xu et al., 2002a,b). They are The Sanchazi slice strikes through the town of Sanchazi compositionally similar to MORB, particularly to those from (Figs. 4 and 5) and is comprised of island-arc-type volcanics, the Carlsberg Ridge and Indian Ocean Ridge Triple Junction. ophiolites, metagabbro, chert, radiolarian chert and other They also have the Dupal isotopic anomaly, characteristic of oceanic assemblages (Lai and Zhang, 1996; Feng et al., 1996) basalts from the southern hemisphere. Although the Mian–Lue that underwent strong D1 deformation, especially the lenticular northern ophiolites are presently in the northern hemisphere, and serpentinized harzburgites and lherzolites. The schistosity, paleomagnetic data suggests that they formed in the southern S1, is sharply crenulated and deflected by E–W-trending folds hemisphere in the region of the DUPAL anomaly (Xu et al., of the D2 deformation. D2 folds are tighter in the metavolcanics 2002a,b). and metapelites, and more gentle in the metagabbro and To the east, the Gaochuan slice in Fig. 11 (the profile location metachert (Li et al., 2001a,b,c,d). 3T-type phengite, stilpnome- is E seen in Fig. 2) is dominantly composed of Carboniferous lane and other HP/LT metamorphic mineral assemblages are and Permian strata. Except for the intense top-to-the west present in the S1 and the crenulation cleavage S2 of the arc and thrusting in the boundary of the slice, the large-scale internal oceanic assemblages (Li, 1998). Therefore, the two-stages of structural framework comprises two stages of superimposed deformation were related to the subduction or earlier collision folds, forming a basin and dome interference pattern of N–S producing the HP/LT metamorphic rocks. However, D1 in the elongate domes that deform the S1 schistosity. Regional analysis harzburgite and lherzolite is not associated with the HP/LT suggests that the trend of S2 is dominated by the near east–west assemblages, and the early fabrics in the rocks may be or north–northwest trends (Fig. 9F, G). In some strongly associated with HT mantle flow and be older than the D1 in deformed zones, the D2 folds form a series of meter-scale tight the shallow-crustal rocks. The D3 deformation formed a series isoclinal folds. The D2 folds with axial-planar crenulation of north-dipping, top-to-the south brittle–ductile thrust faults cleavage and domainal spaced cleavage marked as S2 are along lithological boundaries. relatively tighter that the D3 folds. Some D3 gentle open or kink To the east, the Qiaozigou slice in Fig. 10 (the profile folds deformed S2 cleavage and produced the axial plane, S3,of location is A in Fig. 2) preserves a series of meter-to kilometer- kink bands of N–S regional trend (Figs. 9H and 10). We scale, south-vergent, asymmetric folds deformed by the S1 propose below a genetic interpretation of intense N–S directed schistosity, showing that some folding pre-dates the formation compression of the D2 folds and structural adjustment of the first foliation. The axial plane of the S2 crenulation associated with oblique compression to the Mianlue suture of cleavage is predominantly north dipping and east–west striking the N–S trending D3 folds. (Fig. 9C, D). Locally, axial planar cleavage developed during folding has a convergent fan-shaped arrangement about the 4. Deformation sequence of the Mianlue suture and axial surface (Fig. 9E). A coaxial double zig–zag interference orogenic process pattern is generated by a set of D2 upright folds superimposed on previous D1 folds (Fig. 9C). The subsequent deformation is a The deformation sequence and orogenic processes in the series of near east–west trending, upright ductile shear zones Mianlue suture can be divided into three stages based on the developed along the lithological interfaces, in which the S2 deformation characteristics of the “matrix” and slices in the crenulation cleavage is strongly deformed into shear band suture, structural analysis, and differences in tectonic 中国科技论文在线 http://www.paper.edu.cn

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lithofacies. Excluding the pre-Paleozoic tectonic evolutionary (Yin and Nie, 1993; Zhang et al., 1996; Hacker et al., 2000; stages in the basement blocks, these include subduction-related Ratschbacher et al., 2000, 2003). In fact, the range of Triassic deformation, the main collision-dominated deformation and the ages in Kunlun and Qinling correspond to those in Dabie. late intracontinental blocks-adjustment deformation. However, the subduction in the east is possible to be earlier than that in the west (Yin and Nie, 1993; Zhang et al., 1996; Hacker 4.1. Relative and absolute timing of deformation et al., 2000; Ratschbacher et al., 2000, 2003; Roger et al., 2003). Furthermore, there does appear to be a diachronous trend on a Some papers report isotopic ages of igneous and metamorphic more regional scale, with the collision occurring earlier in the rocks from the Qinling and Kunlun regions related to the Mianlue Korean segment of the orogen than in the Qinling–Dabie orogen suture. Important metamorphic ages of the Mianlue suture and (Oh and Kusky, in press). associated granitoid plutons are well constrained (Li et al., 1996; Sun et al., 2002a,b; Zhang et al., 2002). Li et al. (1996) obtained a 4.2. Structural history Sm–Nd whole-rock isochron age of 242±21 Ma and a 40Ar/39Ar age of 220–230 Ma from phyllite, and interpreted these Triassic 4.2.1. Pre-collisional tectonic evolutionary stage and subduc- ages to reflect the initial regional metamorphic age. Granulites tion-related deformation in the suture from the basement slices in the suture yielded a Sm–Nd isochron age of 206±55 Ma and a biotite Ar40/Ar39 plateau age of 199.7± 4.2.1.1. Pre-collisional tectonic evolutionary stage. Based on 1.7 Ma, interpreted to be the peak and retrograde metamorphic the sedimentary formations and their ages, the pre-collisional age, respectively (Li et al., 2000; Zhang et al., 2002), consistent tectonic evolutionary stage can be subdivided into the following with or close to those of the Dabie ultrahigh-pressure and high- three detailed stages (Li et al., 2001a,b,c,d). pressure metamorphic rocks (Zhang et al., 1996; Hacker et al., 4.2.1.1.1. Early Devonian to Early Carboniferous rifting to 2000; Ratschbacher et al., 2000, 2003). Therefore, the formation form a small ocean. This initial rifting is dated by large and uplift of the granulites were related to the subduction of the amounts of Early Devonian fossils in the Tabo Group of the South China craton beneath the Qinling–Dabie microblock, and Tabo slice and Early Carboniferous radiolarians in the cherts the collision and final amalgamation of the South and North China closely associated with the ophiolite of the Sanchazi slice (Feng cratons in the Indosinian orogeny (Zhang et al., 2002). Sun et al. et al., 1996; Zhang et al., 1996). Therefore, the oceanic crust (2002b) gave a range in age from 220±1 and 205±1 Ma for six must be Carboniferous. A string of relict ophiolites perhaps granitoid bodies in the ca. 400-km-long granitoid belt in the South reflects spatially a string of small oceans, termed the Huashan– Qinling using single and multigrain zircon U–Pb dating, Mianlue–A'nyemaqen small ocean (Lai and Zhang, 1996; Lai supporting the idea that the collision between the North and et al., 1997, 1999, 2000, 2004), similar to the present-day South South China cratons along the Qinling–Dabie orogenic belt China Sea–Sulu–Sulawesi. Alternatively, these ophiolites happened in the Triassic. could represent closure of a major ocean with ophiolites only Two Kokoxili granitoids along the Kunlun suture, as a western being obducted and preserved in isolated locations, much like in segment of the Mianlue suture, which separates the Bayan Har– the Alpine–Himalayan system. Songpan Ganze orogen from the Tarim and Qaidam blocks, yield 4.2.1.1.2. Early Carboniferous to Late Permian contem- U–Pb zircon emplacement ages of 217±10 and 207±3 Ma (Late poraneous spreading and subduction. The oceanic volcanic Triassic) and Rb–Sr isochron cooling ages of 195±3 and 190± rocks in the Mianlue ocean were being subducted and 3 Ma (Early Jurassic). The geochemical signatures of these metamorphosed to produce metamorphic distinctive subduc- granitoids suggest that they are related to subduction continuing tion-related HP/LT metamorphic minerals along S1 before pre- into the Late Triassic. Combining isotopic dating with structural Early Triassic, dated by Li et al. (1996). evidence on subduction polarity and paleomagnetic reconstruc- On the southern continental margin of the ocean, in the tions, Roger et al. (2003) proposed that the Kunlun and Qinling Gaochuan range, the carbonates make an upward transition to block boundaries, which were distinct in the Permian, subse- calcalaceous mudstone, black mudstone and cherts recording quently formed a continuous, Late Triassic, northward subducting the evolution from the Devonian gentle slope, transitional plate margin. Three samples from the Jinsha suture that separates upwards to the Carboniferous marginal plateau and the Permian the Songpan Ganze orogen from the Qiangtang blocks, a stable deep-water platform (Meng et al., 1996). However, in the leucocratic granite, an orthogneiss and a paragneiss, yield U–Pb Qinling–Dabie microblock north of the Mianlue ocean, zircon dates of 206±7 and 204±1 Ma, and a U–Pb monazite date intrusion of Permian subduction-type granites at 285 Ma of 244±4 Ma, respectively (Roger et al., 2003). The existence of suggests that subducted had begun by the Permian (Zhang coeval magmatism in the Jinsha, Kunlun and Mianlue sutures et al., 1996). Thus, both spreading and subduction coexisted in suggests that the three subduction zones were simultaneously the Early Caboniferous to Late Permian. active. Therefore, the evolution of the Mianlue suture is closely 4.2.1.1.3. Early to Middle Triassic subduction without new related to these other western sutures and blocks. oceanic crust generation. Li (1998) calculated that the According to the above geochronological data, the collision termination of ocean spreading was about Early Triassic along Qinling–Dabie between the South and North China based on the ocean's width and spreading rates identified cratons is not documented to be diachronous or to be using geochemical methods. Moreover, deep-sea cherts and characterized by a proposed younging and westward migration radiolaria characteristic of marine crust are not observed in the 中国科技论文在线 http://www.paper.edu.cn

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Triassic strata (Feng et al., 1996). This is to say, no oceanic crust the juxtaposition of slices of various composition and nature, is known to have formed during the Triassic. In the range from leading to out-of-sequence strata. This kind of intense bedding- Wudu County to Mianxian County, the metamorphic ages of parallel transposition resulted in the same deformational Sm–Nd and 40Ar/39Ar of 242±21 Ma and 220–230 Ma are sequence and style of the slices and the matrix except for obtained from the subduction-related slices with HP/LT mineral those induced by local stress fields. This style of deformation is assemblage aligned along S1 (Li et al., 1996; Jiang et al., 2000). reminiscent of subduction accretion complexes elsewhere in the Therefore, the entire subduction began in the Early Triassic. world (e.g. Kusky et al., 1997a,b, 2003). Every thrust slice within the main suture belt preserves 4.2.1.2. Pre-collisional subduction–accretion related deforma- “bedding-parallel” intrafolial folds and ductile shear zones, tion and its sequence. Previous research mainly focused produced in the toe of the accretionary wedge at the frontal on the collisional deformation and did not pay much attention to margin of the subduction zone during low angle subduction of the D1 structural association. In many slices and blocks within oceanic crust (Li, 1998; Li et al., 2001b). The easily-deformed the Mianlue suture, there are many intrafolial folds of bedding. volcanic rock layers preserve some small-scale weakly-deformed The bedding-parallel schistosity is locally composed of lenticles, while the strongly deformed layers comprise the phengite, stiplnomelane and other minerals formed under low- present-day schistose “matrix” in the suture. Subduction related temperature and high-pressure metamorphic conditions consis- uplift of the overriding plate resulted in the gravitational tent with metamorphism above a subducting slab, reflecting that instability of the carbonates, cherts, sandstones and mudstones they are products of subduction. These bedding-parallel folds over the low-angle south-dipping plane, which slumped into the are restricted to the meta-cherts and marble strata. However, foredeep and trench. Southward propagation of the decollement they are rarely observed in the easily-deformed volcanic layers between the basement and the cover formed the consistent because of complete replacement in the penetrative, S1-parallel southward vergence of asymmetric bedding-parallel folds, ductile compositional bands. These bedding-parallel folds in the meta- sliding, and stacking. The abundant geochronological data on the cherts and marble are asymmetric, tight and isoclinal. Shear subduction implies that the subduction-related deformation sense indicators show south vergent, north-dipping, low-angle occurred in the Early Triassic (Li et al., 1996; Jiang et al., 2000). and nearly bedding-parallel thrust movement. These bedding- parallel folds resulted from a piggyback sequence of accretion 4.2.2. Geometry and kinematics of syn-collisional deformation of late Paleozoic rocks above a basal decollement. During the Subduction-related structures are overprinted by three same deformation stage, the materials in the suture experienced generations of structures related to the collisional (D2–D3) and strong tectonic mixing. Furthermore, oblique subduction post-collisional (D4) stages of tectono-thermal development and produced boudinage of the competent layers along strike and orogenesis: (1) thermal heating and prograde metamorphism (Li

Fig. 12. Simplified map showing the relation between the Mianlue suture and the strike–slip faults (①②from He et al. (1997);③, ④ from Li (1998); ⑤ from Zhong and Zhang (1997); ⑥, ⑦ from Li (1998); and this study). Except for stereodiagram, others are plan view maps; stress field map is in the upper-right corner. 中国科技论文在线 http://www.paper.edu.cn

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et al., 2003), leading to the development of a ductile basal shear boundary effect continued until the later intracontinental zone; (2) out-of-sequence thrusting at thermal peak conditions adjustment (Yin and Nie, 1993; Li et al., 2001a,b,c,d). 3) The involving the incorporation of basement slices into the thrust east–west striking foreland formed during the south-directed belt; and (3) post-thermal peak out-of-sequence thrusting in the brittle–ductile thrusting (Dong et al., 2005). Then, 4) the dextral Mianlue Suture. Post-kinematic emplacement of the Guang- and/or sinistral strike–slip faults were superimposed locally on toushan granite provides a minimum age of 200 Ma for stage 2 the suture, locally re-orienting fold axes (B2) from shallow to (Zhang et al., 1996). The crystalline basement blocks north of steep plunging near the strike–slip faults. The sinistral strike– the Mianlue suture were extruded laterally (eastward or slip faults formed after the D2 folds. westward) out of the belt during the D2–D3 episodes. The D2 and D3 episodes folded the thrust belt and its footwall basement 4.2.3. Intracontinental adjustment of post-collisional deforma- in a thick-skinned fashion about east–northeast (D2) and west– tion (post-Late Triassic) northwest (D3) axes. During the collisional stage (D2–D3), all The suture was intruded by the 200 Ma Guangtoushan strata except for parts of the accretionary prism (Li et al., 1999) granite (Fig. 2), truncating all fabrics in the suture. This shows underwent intense deformation. These strata are the middle that the ocean had closed by the Late Triassic leading to the Triasssic and pre-Triassic. During the early collisional stage, D2 collision of the South China craton with the Qinling–Dabie deformation was dominated by the formation of folds with microblock and deposition of nonmarine sedimentary forma- vertical axial surfaces and brittle–ductile thrusting. The D3 tions (Liu and Zhang, 1999; Yong et al., 2003; Meng et al., deformation is dominated by buckling and brittle thrusting. The 2005). Therefore, all younger deformations are related to irregular nature of the plate boundary is thought to have led to the intracontinental processes such as continued convergence and formation of various structural styles during collision along crustal thickening, A-type subduction, post-orogenic extension- the suture. In total, the two folding episodes are responsible for the al collapse, strike–slip adjustments of intracontinental blocks, isolation of the thrust belt in a klippe, for the regional west–east and syn- to post-collisional foreland basin formation and related plunge and for the second-stage extrusion of the HP metamorphic deformation. rocks from the middle crust to the present erosional surface. In the south Dabashan area (Fig. 1), post-Triassic peak For the entire Mianlue suture of the Qinling orogenic belt, orogenesis led to the filling and deformation of the syn- the collisional event is diachronous from earlier in the eastern collisional foreland basin, and subsequent Late Jurassic to Early segments (such as Late Permian to Early Triassic of the Dabie– Creataceous filling of the basin, driven by oblique intraconti- Sulu segment (Yin and Nie, 1993), and corrlelatives in Korea nental A-type subduction. Late Jurassic to Early Cretaceous (Oh and Kusky, in press), to later in the western segments (such post-collisional deformation deformed the basin again, resulting as Late Triassic of the A'nyemaqen in the west Qinling orogenic in the formation of a large-scale arcute thrust-fold belt in the belt (Xu et al., 1996)). In terms of the Gaochuan–Kangxian Dabashan segment of the suture. This process also resulted in segment, the strong Late Triassic collision produced some the main east–northeast trending structural line of the Paleozoic collision-type granites of 209 Ma–219 Ma (Li et al., 1996; Sun strata over the north Hannan complex west of the Dabashan et al., 2002b). Meanwhile, the upper Triassic strata that flank the arcute thrust-fold belt. Meanwhile, structural analysis of folds suture are nonmarine sediments characteristic of intracontinen- from the Zhenba area of the south Hannan block indicates that tal evolution. An east–west striking, foreland basin also formed the earlier structures exhibited a near east–west striking fold at the same time (Liu and Zhang, 1999). The position of pattern consistent with the results of deformation analysis for successive basin margins can be related to a thrust structure to peak syn-collisional structures. This suggests that the present- the north. This localized deformation must have terminated day north–northwest striking arcute fold belt is controlled by before 200 Ma because the Guangtoushan pluton emplaced into the indentation of the competent Hannan complex with the the suture is undeformed. Qinling microblock during oblique intracontinental subduction, A synthesis of the structural profiles described above reveals leading to the rotation of the strata in this area. Until this event, that the collision-related deformation in the suture can be fold axes in lower Paleozoic strata in the Fangxian range east of summarized as follows: 1) the early collision-related deforma- the Dabashan arcute belt (Fig. 1), remained at a small angle to tion is related to N–S directed contraction including regional the major thrust fault of the Dabashan arcute belt. At this stage, folding forming the E–W trending, horizontal fold axes. The D2 the Dabashan arcuate belt was cut by a conjugate set of NW- fold axial planes form a crenulation cleavage S2, locally striking dextral and NE-striking sinistral strike–slip faults exhibiting growth high-pressure minerals such as phengite, (Fig. 12). Metamorphic (Li et al., 2003) and structural stiplnomelane and others. 2) the later collision-related deforma- observations suggest that rocks of the middle level under the tion is dominated by south-directed brittle–ductile thrusting. Foping dome extruded eastward about 20 km due to the offset The irregular subducted plate boundary led to the variation of dip of the Wudang dome at the same time or soon after the angles and the curved shape of thrust planes and fold symmetry. indentation of the Hannan complex. For example, we infer that the subducted plate boundary had In general, the locations where promontories on one margin gentle dips from the town of Guozhen to west of Wenxian. caused indented locations on the opposing margin exhibit However, it was steep east of Youshui village (Fig. 2). In more higher-grade metamorphism than the embayment or non- easterly regions, it was also gentle from the Dabasha area to the indented areas. For instance, the Mianlue suture from Changba easterly Chengkou–Fangxian area (Fig. 1). We consider that this in Lueyang County to Youshui village in the east (Fig. 2) has 中国科技论文在线 http://www.paper.edu.cn

138 S. Li et al. / Gondwana Research 12 (2007) 121–143 higher grade metamorphism than nearby regions, suggesting thrusting mainly occurred in the Dabashan arcute belt range, that the lower crust was uplifted to shallower levels (Li, 1998; perhaps being a composite structure formed after the later stage Li et al., 2003). In the segment of Kangxian County to Lueyang of the oblique indentation, and the strike–slip faulting is County, the suture is offset by a series of NW-striking brittle secondary. However, strike–slip faulting is characteristic of the strike–slip faults. Based on S–C fabrics, en echelon quartz Qinling microblock. This indentation also produced the last veins (Fig. 12) and striae, it is deduced that the earlier strike– uplift of the Foping and Madao domes (Fig. 12). slip faulting was mainly sinistral, and later ones became locally dextral. Regional paleo-stress field analysis shows that the later 4.2.3.1. Strike–slip faulting, indentation, extrusion and oblique dextral strike–slip faulting may be related to the NE-directed intracontinental subduction (J1–K1). Closure of the Minalue oblique shortening, the regional NW-directed extension and the Ocean between Kangxian County and Gaochuan Town led to local stress fields are associated with block extrusion (Zeng peak deformation and metamorphic conditions in the South et al., 2001; Wang et al., 2001; Zhang et al., 1996). Meanwhile, China Craton and the Qinling Microblock beginning in Late

Fig. 13. A model showing deformation dynamics in the Mianlue suture zone (see text for details). 中国科技论文在线 http://www.paper.edu.cn

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Fig. 13 (continued).

Triassic. The collision affected the entire Qinling microblock segment of the suture basically represents the primary location due to its soft basement. Major Chevron type folds are well of closure of the Mianlue ocean. However, in the subsequent developed, especially in the Foping area, and overprinted by oblique intracontinental subduction, the Yangpingguan sinsitral duplex thrusting. The steep collisional boundary geometry and strike–slip fault west of the Hannan complex perhaps shows the flexural rebound of the overriding plate of both the north Ordos thrust fault exhumed from deep levels. Additionally, the ESE block and the Hannan complex resulted in the disappearance of dipping, Huayingshan dextral strike–slip fault, probably shows the foreland basin on the northern margin of the Hannan gently ENE dipping thrust fault relationships (Zhang et al., complex. In the Carnian (Late Triassic), the Dabashan foreland 1996). This resulted in the near E–W compression acting on the basin was a nonmarine foreland molasse basin in an strata of the Gaochuan slice, the Donghe slice and the Wudang intracontinental collision stage. Within the microblock, the dome (Figs. 1 and 2) to form the third generation of NNW or pre-existing faults are typically preserved as brittle structures. NNE striking gentle folds that were superimposed on the second Most are WNW dextral strike–slip faults, whereas others generation of WNW striking folds. include ENE-and NNE-striking sinistral strike–slip faults. Both fault sets accommodated the movement between different sub- 4.2.3.2. Foreland thrust tectonics in the north margin of the blocks of the microblock. Pull-apart basins along these fault South China craton (T3–K1). The foreland deformation is belts were filled by lower Jurassic to lower Cretaceous closely related to collisional processes rather than the oceanic sedimentary and (volcanic) rocks. The stress-field analysis subduction process (Dong et al., 2005). A Mesozoic foreland- (Fig. 12) indicates NE-ENE directed oblique A-type subduction basin complex formed along the northern South China craton of the South China craton under the microblock showing that it during A-type subduction of the South China plate under the was in an oblique intracontinental subduction stage, consistent Qinling–Dabie orogenic belt along the Mianlue suture. As the with the formation of major strike–slip faults. South China craton moved northwestwards and was obliquely At shallow structural levels, the ophiolitic blocks crop out subducted under the Qinling–Dabie (Middle–Late Triassic), a along the bulged border of the competent Hannan complex and flysch-filled foredeep developed in the Diebu–Songpan in the foreland basin. However, the foreland basin sequence is western part of the northern South China craton (Zhang et al., preserved poorly in this area. The geophysical profile reveals 1996, 2004a,b). During the Late Triassic, a nonmarine molasse that the present-day geometry of the suture shows some steeper basin first formed in the eastern part of the northern South China collisional faults resulting in the poorly developed lower angle craton in response to initial collision there (Liu et al., 2005; thrusts from the late stages of orogenesis. The indentation of the Meng et al., 2005). This clastic wedge prograded over the Hannan complex into the Qinling microblock led to a lowering former marine basin and was accompanied by a change from of the angle of thrusting in the suture, the development of high-sinuosity river systems flowing into basinal lakes, to strike–slip faults on the flank of the Hannan complex, and higher gradient braidplains. Complete oceanic closure along the obduction of ophiolites and detachment of the earlier thrusted Mianlue suture during the Middle Jurassic produced a more Xixiang slice to the root belt of the suture. Therefore, this extensive east–west molasse basin with rivers, deltas and lakes 中国科技论文在线 http://www.paper.edu.cn

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(Liu et al., 2005). The Dabashan foreland fold-thrust belt in the belt continuously migrated eastward to the frontal thrust zone, at northern margin of the South China craton, closely related to the the same time, this molassic clastic wedge prograded eastward Mianlue suture, is a result of normal and oblique subduction of and the earlier Jurassic molasse was folded. This wedge was the South China craton beneath the Qinling–Dabie microblock cannibalized forming new Cretaceous–Tertiary molasse depos- in the Indosinian and Yanshanian periods. its, resulting in the migration of the foredeep from northwest to The southern Dabashan thin-skinned foreland fold-thrust southeast. Similar foredeep migration in other foreland basins belts south of the Chengkou–Fangxian fault, presently has a has been equated with rates of plate convergence (e.g., Bradley southwestward curvature, and can be divided into a thrust belt in and Kusky, 1986). the south, merging north into a thrust-fold belt and frontal fold On the other hand, the Jurassic, Cretaceous and Tertiary belt (He et al., 1997; Dong et al., 2005). The sedimentary depocenters also migrated southwestward along the Longmeng- successions of the Mesozoic foreland basins suggest that there shan fold-thrust belt (Liu et al., 1990; Yong et al., 2003), reflecting were three phases of depositional history associated with ocean that the thrust belt has not only compressive components but also closure and development: the Late Triassic phase coeval with a sinistral strike–slip component. These transpressive character- syn-orogenic collision, the Early–Middle Jurassic phase istics are also shown in en echelon folds and depocenter equated with intense post-Late Triassic buckling of the basin migration. All these reflect that the Jurassic to Early Cretaceous, basement, and in which the middle Jurassic Mianxian Group oblique, intracontinental indentation of the Hannan complex into underwent the intense compressive deformation, and the Late the Qinling microblock resulted in the extrusion of the Bikou Jurassic and Early Cretaceous phase in which the Lower block and the Donghe slice of which the north border is the Shiyan Cretaceous deposits were thrust over the Upper Cretaceous in fault and the south border is the Bashan arc-shaped fault (Li, 1998; the Sichuan basin (Liu et al., 2005). Dong et al., 2005). The Longmen Shan and Shiyan faults formed The kinematics of the southern Dabashan thrust-fold belt as sinistral faults. However, the Zhuangyuanbei and Bashan faults indicates that in the Late Triassic, the stair-shaped thrusting west of the Guangtoushan pluton initiated as dextral faults. It is south of the Chengkou–Fangxian fault at first initiated in the noted that the Bikou block is also indented northward, but less deep crust. In both the Huijunba–Wuliba area at the southern than the Hannan complex, showing the relative and southwest- Shaanxi province and the Heping–Wuxi area at the Bashan ward escape effects (Fig. 12). The middle Jurassic in the Mianlue curvature, the structural lines show the WNW or near E–W suture north of Mianxian County underwent steep northward strikes, consistent with those in the suture. This resulted from thrusting in the Late Jurassic. the N–S compression during the head-on collision of the South China craton to the Qinling microblock in the syn-collisional 5. Collision leading to multiple-stage large-scale extrusion: deformational episode. However, the Longmen Shan thrust belt discussion is probably related to the eastward extrusion and resistance of the Songpan–Gangze block including Bikou block. From the The Qinling–Dabie orogenic belt is a zone of long-lived Jurassic to Early Cretaceous, the southern Dabashan thrust belt shortening with multistage extrusion events that formed during propagated forward from deep to upper levels, and from the Mesozoic times in central China. Based on all the characteristics north root to the south thrust front. The deep-to-shallow in the Kangxian to Gaochuan and neighboring segments of the thrusting perhaps is related to the synchronous, horizontally suture, especially the kinematics, sedimentary rock associa- eastward 20 km extrusion of the Foping midcrust due to the tions, and igneous rock distributions, we propose a three-stage offset of the Wudang dome (Li, 1998). By the Late Jurassic, the structural-tectonic model (Li, 1998; Li et al., 2002, 2003). deformation propagated to the present-day frontal location The first stage involves opening and spreading of an ocean (Dong et al., 2005). At the same time, there are a few reverse basin from the Early Devonian to Late Permian (Feng et al., thrust and out-of-sequence faults developed in the intense 1996; Meng et al., 1996; Zhang et al., 1996; Lai et al., 1997, deformational domain that accommodated the displacement 1999, 2003, 2004; Fig. 13A). Many lines of evidence suggest between the different blocks (He et al., 1997). Both the fault that the Bikou block may have been located south of the strikes and the fold axes in these blocks are dragged into a NNW “Gonghe” graben, comprising a triple RRR junction together direction and the faults are dextral strike–slip faults (Fig. 12) with the Mianlue ocean (Zhang et al., 2004a,b). This area may (He et al., 1997). These result from the irregular shape of the be the initial RRR location of opening of the Mianlue Ocean. passive margin, oblique subduction of the South China craton At the same time, the Qinling–Dabie microblock moved and the foreland thrusting and propagation at the Bashan northward and the northern Shangdan Ocean shrunk into a segment, synchronously. During Late Jurassic through Early relict sea basin. Cretaceous, the depocenter of the nonmarine molasse basin The second stage involves subduction of the Mianlue migrated continually from east to west because of intraconti- oceanic crust and subsequent collision of the South and North nental deformation associated with clockwise rotation of the China cratons during shrinking of the ocean basin and South China craton relative to the North China craton. In this shortening of associated deposits (Fig. 13B). From Early time interval, the basin was again dominated by fluvial and Permian to Middle Triassic, subduction and subduction-related lake-delta deposition and rivers continued to disperse sediments deformation occurred between the Shangdan suture and the southwards into the basin (Liu et al., 2005). During Jurassic Mianlue subduction zone north of the ocean. South of the ocean, through Early Cretaceous times, the Longmen Shan fold-thrust however, new extension formed a wider ocean in the west than 中国科技论文在线 http://www.paper.edu.cn

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the east. At the same time, the Bikou block moved eastward and played some roles in the exhumation of ultrahigh-pressure the upper and middle Trassic strata in the present-day Songpan– rocks from beneath the Dabie (Maruyama et al., 1994; Hacker Ganze orogen west of the Bikou block formed, indicating the et al., 2000; Xu et al., 2002a,b; Ratschbacher et al., 2003; closure of the Longmen Shan seaway and the westward Faure et al., 2003; Tang et al., 2003; Zhang et al., 2004a,b; migration of the sediments (Yong et al., 2003). Until Late Liu et al., 2004, 2005). Wang et al. (2003) even suggested Triassic, two plates were converging and colliding, which led to that the Dabie HP-UHP metamorphic rocks were originally the formation of a foreland basin in the south. However, no located beneath the Foping dome, then it was extruded foreland basin developed north of the Hannan complex due to eastward to its present-day location. Here our structural data its competence (Yong et al., 2003). Therefore, the Xixiang slice don't support a linkage between the multiple-stage (early was thrust southwestward (with vertical extrusion) as a weakly- southward/vertical and late eastward/lateral), large-scale (both deformed thrust slice from the southwest ramp of the Dabasha about20km)extrusionintheQinlingorogentothe thrust during the earlier thrusting stage. exhumation of UHP-HP metamorphic rock in the Dabie During the third stage, block adjustment of post-collisional orogenic belt. In summary, based on insights from the deformation was under the oblique intracontinental deep- evolution of the Mianlue suture, the D2 and D3 episodes in subduction regime (Dong et al., 2005; Fig. 13C). During Late the Mianlue suture and adjoining areas are not responsible for Triassic to Early Cretaceous, the dynamics of the Pacific and other and associated with the two-stage extrusion of the Dabie plates on the Pacific realm had more and more influence on the UHP-HP terranes from the narrowest part of the Qinling Central China orogen (Hacker et al., 2000; Ratschbacher et al., orogen to the present erosional surface (more than 350 km 2003; Zhang et al., 2004a,b). The blocks in the Qinling–Dabie away from the Foping dome, Wang et al., 2003). orogen and its neighbors accommodated various stress fields in the different tectonic parts. Highly variable structural styles charac- Acknowledgements terize this period. The last deformation in the Longmen Shan orogen and the Dabashan thrusts indicates that the intracontinental This study was funded by a China NSFC grants 40472098, subduction of the northeast part of the South China craton beneath 40002015 and 49732080, the Major State Basic Research the Qinling–Dabie microblock was related to the delamination of Development Program of China (TG1999075505), Geological the subducted Mianlue oceanic slab (Li, 1998). Investigation Project of China Geological Survey (200013000169), Based on the above description of the major faults, sutures and also supported by US NSF grant EAR-02-07886 awarded and the deformation history, the Mianlue suture and its to T. Kusky. We thank Prof. Wang Tao and an anonymous neighboring blocks involved southward thrusting and south- reviewer for their constructive review and suggestions. ward and upward-oblique extrusion of about 20 km above the autochthonous basement during the D1 episode. 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