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VÌ> Late Paleozoic tectonic evolution of the northern West Chinese Tianshan Belt ÊÊÊÊÊ ÊÊÊÊÊ Downloaded by [University of Tasmania] at 15:31 16 June 2015 Bo Wang a,b,*, Michel Faure b, Dominique Cluzel b, Liangshu Shu a, Jacques Charvet b, Sebastien Meffre c, Qian Ma a a Department b of Earth Sciences, Nanjing University, Nanjing, 210093, China ISTO UMR 6113, University of Orléans, F45067, Orléans, Cedex 2, France c School of Earth Sciences, University of Tasmania, Hobart, Australia Received: 13/02/06, accepted 03/05/06 Abstract The northern West Chinese Tianshan is divided into three subunits: Carboniferous turbidite, ophiolitic mélange and Yili magmatic arc. Stratigraphical and petrological studies suggest that the turbidite and ophiolitic mélange form a subduction complex. The ophiolitic mélange that forms the North Tianshan suture was a result of intra-oceanic tectonism and subsequent redeposition and deformation during the subduction of the North Tianshan oceanic basin. The Yili arc-type granitoids are constained by single zircon U-Pb radiochronology between 361 and 309 Ma. The first-hand kinematic results on the deformed turbidite suggest that this suture zone was reworked by a Permian ductile dextral strike-slip fault. An evolutionary model of the study area allows three events to be distinguished: 1) Late Devonian to Carboniferous subduction of the oceanic basin below the Yili Block producing Yili magmatic rocks and subduction complex, 2) Late Carboniferous complete closure of this basin, 3) Permian right-lateral strike-slip faulting generating pull-apart basins and alkaline magmatism. A prominent reactivation during the Indo-Eurasia collision provoked the northward thrusting of the Paleozoic units upon the Cenozoic sediments of the Junggar Basin, consequently, hiding the bulk of this Late Paleozoic suture. © 2006 Lavoisier SAS. All rights reserved Keywords: Paleozoic subduction; strike-slip shearing; mélange; zircon U-Pb dating; Tianshan 1. Introduction The Tianshan range, extending E-W over 3000 km from NW China to Kazakhstan and Kyrgyzstan, separates the Tarim Basin to the South from the Junggar Basin to the North. It is a key region for understanding the Late Paleozoic geodynamic evolution of Central Asia. The Paleozoic Tianshan orogenic belt is considered to result from the accretion and/or collision of continental blocks, magmatic arcs and subduction com* Corresponding author. Tel: +33 (0)2 38 49 46 60 - Fax: 33 (0) 2 38 41 73 09 E-mail address: bo.wang@univ-orleans.fr © 2006 Lavoisier SAS. All rights reserved plexes [1-6]. The Tianshan orogenic belt can be subdivided in several ways. Geographically, the West Chinese Tianshan (WTS) develops from Urumqi to the Chinese border (Fig. 1). Topographically, it consists of two E-W elongated ranges surrounding the Yining basin, which is also called “Yili Block”. From a tectonic point of view, the Chinese Tianshan orogen is generally divided into North, Central and South domains. The Yili Block, located between the North and Central Tianshan domains, played an important role on the Downloaded by [University of Tasmania] at 15:31 16 June 2015 238 Bo Wang et al. / Geodinamica Acta 19/3-4 (2006) 237-247 Fig. 1: Structural map of northern West Chinese Tianshan belt (modified from XBGMR [11]). Insert A shows the location of the study area in Central Asia, inset B defines the North Tianshan and the Yili Block. 1, Main Tianshan Shear Zone (MTSZ); 2, the Northern Central Tianshan Fault after Gao et al. [18]; 3, North Tianshan Fault; 4, Aibi Lake Fault; 5, Sailimu- Jinghe Fault; 6, Houxia Fault. Paleozoic evolution of WTS, but its tectonic feature and the relationship with the North Tianshan are still poorly constrained. This study aims at clarifying the Late Paleozoic tectonic evolution of the northern WTS Belt, and we focused on the ophiolitic mélange, turbidite, the Yili magmatic arc as well as shear zone crossing the highway from Dushanzi to Nalati (Fig. 1 and 2). The tectonic significance of ophiolitic rocks is discussed and a geodynamic evolutionary model of the North Chinese Tianshan is proposed. 2. Structure of the northern WTS The WTS consists of several units bounded by strike-slip faults (Fig. 1). The north side of the Central-South Tianshan Belt is bordered by the Main Tianshan Shear Zone (MTSZ, fault 1 in Fig. 1) [4, 5, 7], which separates the Central-South Tianshan Belt from the “Bogda Arc” [4]. To the West it merges with fault 2 separating the Yili Block to the Northwest from the Central-South Tianshan Belt to the south. To the North, the North Tianshan Fault (NTF, fault 3 in Fig. 1) [8], which is also named Junggar Fault [9] or Borohoro Fault [10], divides longitudinally the northern range of WTS along the Borohoro Range into the Yili Block and the North Tianshan Domain. Aibi Lake Fault (fault 4 in Fig. 1) is the northwest extension of the NTF. Sailimu-Jinghe Fault (fault 5 in Fig. 1) separates the Yili Block from the “Bole Block”. Although a detailed discussion of the Bole Block is beyond the scope of this paper, it is worth noting that it strongly differs from the Yili Block and therefore is a very peculiar domain in the tectonic framework of WTS. Lastly, the “Houxia Fault” (fault 6 in Fig. 1) separates Carboniferous arc-related rocks that are exposed around the Turfan Basin from the North Tianshan terrigenous rocks (Fig. 1). In the following sections, we present northern WTS located to the east of the Sailimu-Jinghe Fault, and up to Urumqi. This area can be subdivided into three lithotectonic units: 1) Carboniferous turbidite, 2) ophiolitic mélange and 3) Yili magmatic arc (I, II and III, respectively in Fig. 2). The units I and II constitute one single subduction complex, i.e. the North Tianshan subduction complex, but the contrasted lithology allows the distinction of two different units. 2.1. Carboniferous turbidite The northern slope of the Borohoro Range consists of a turbiditic formation developing WNW-ESE for 300 km long and about 20 km wide. On the basis of plant fossils, these terrigenous rocks are assigned to the Bayingou Formation of Late Carboniferous age [11]. This unit is estimated to be between 5,000 and 10,000 m thick although it is difficult to establish because of possible tectonic duplication. Sandstone beds present variable thickness ranging from a few centimeters to 1 m (Fig. 3a, b). Typical Bouma sequences can be observed. Some deep-water ichnofossils (Chondrites sp. and Helminthoidda Labyrinthica) were found in sandstone [11] indicating deep sea fan deposition. Sandstone grains and conglomerates pebbles consist of terrigenous, volcanic, plutonic and siliceous clasts with only minor carbonate Downloaded by [University of Tasmania] at 15:31 16 June 2015 Bo Wang et al. / Geodinamica Acta 19/3-4 (2006) 237-247 Fig. 2: Crustal scale cross section from Dushanzi to Nalati showing the polyphase deformation: Carboniferous D1 thrusting, Permian D2 shearing, and Cenozoic D3 thrusting. The dextral strike-slip fault partly reworks the Carboniferous suture (thickness of strata are after Liu and Li [24]). S, D, C, P, T, J, K, E, N and Q represent Silurian, Devonian, Carboniferous, Permian, Triassic, Jurassic, Cretaceous, Paleogene, Neogene and Quaternary, respectively. clasts. Petrographic study indicates that the plutonic clasts are dominantly composed of granodiorite, diorite and gabbro. The volcanic clasts have calc-alkaline geochemical features [12]. Although detailed mapping is not available, load casts, graded bedding and cross laminations allow us to recognize both normal and upside down sequences that infer isoclinal folding or thrust stacking. Up-to-the-North thrust faults are also observed. Therefore, the turbidite series was likely involved in north verging recumbent fold and thrust sheets. 2.2. Ophiolitic mélange Ophiolitic rocks are present within the turbiditic formation. According to the available geological maps [11, 13], these rocks crop out discontinuously for about 250 km long and 5~15 km wide (Fig. 1). In the regional geology, they are referred to as the Shadawang Formation [11]. Several areas are already well acknowledged to investigate the ophiolites [14-19]. In the Bayingou section, 30 km to the south of Dushanzi (Fig. 1), the dominant rocks are serpentinized peridotite (Fig. 3c), gabbro, diabase, basalt, chert, plagiogranite and rare limestone. Black or red scaly mudstone and light yellow-green greywacke often surround the other rock types. Mafic greywacke that might be easily confused with gabbro corresponds actually to gabbroic sandstone. In the field, the ophiolitic rocks crop out in two ways: either as continuous sequences of massive basalt, pillow lava and overlying red chert (Fig. 3d) that develop for a few tens 239 meters, or as centimetre to kilometre size isolated bodies of mafic-ultramafic or sedimentary rocks included in a schistose mudstone matrix. In the latter occurrence, the blocks exhibit without any regular organization but distribute rather randomly in the matrix. Sandstone phacoids are included in the scaly mudstone (Fig. 3e). Mafic and ultramafic blocks occur as olistoliths within the turbidite. Pebbly mudstone bearing angular blocks of gabbro, basalt, sandstone, chert and limestone are described in Gurt and Motoshalagou sections [8, 17], northwest of Bayingou. On the basis of geochemistry, three types of mafic rocks are distinguished: N-MORB, OIB and IAT [14, 15, 17], indicating the genesis of oceanic basin. These lithological and geochemical features are compliant with the interpretation of the rocks as an ophiolitic suite. However, the absence of coherent ophiolitic bodies larger than one kilometre or so, the widespread blocky habitus of the rocks and the importance of sedimentary facies opposite to the magmatic ones suggest that this whole suite represent a mélange unit formed during the closure of an oceanic basin. The cherts associated with the mafic rocks yield Late Devonian to Early Carboniferous radiolarians and conodonts [15, 17], and one plagiogranite block yields a zircon U-Pb SHRIMP age of 325±7 Ma [20]. Both suggest a Late Paleozoic age for ophiolite formation. The tectonic significance of the ophiolitic mélange and its geodynamic setting will be discussed in the forthcoming sections. 2.3. Yili magmatic arc The Yili Block contains voluminous volcanic rocks of Carboniferous age [11, 21] (Fig. 1). The volcanic rocks consist of basaltic andesite, andesite, rhyodacite, dacitic andesite, tuff and volcano-sedimentary rocks. They are closely associated with limestone and shallow water clastic deposits. The evolution of sedimentary facies of the Yili Block infers a progressive change from a gently subsiding platform during Bo Wang et al. / Geodinamica Acta 19/3-4 (2006) 237-247 Downloaded by [University of Tasmania] at 15:31 16 June 2015 240 Fig. 3: Field photographies of the North Chinese Tianshan subduction complex. (a, b) Carboniferous turbidite dipping at high angle to the south; (c) decameter-size serpentinite block in Bayingou ophiolitic mélange; (d) pillow lavas and overlying red chert and pelite in mélange; (e) green sandstone (S), Late Devonian-Early Carboniferous red chert (C), tholeiitic basalt (B) included in red pelitic matrix of the mélange; (f) Slate at the south of the mélange, the subvertical cleavage contains a sub-horizontal mineral-stretching lineation. the Early Carboniferous towards a Late Carboniferous filling up environment [22]. Geochemical analyses of the volcanic rocks show that they belong to calc-alkaline series and formed in a continental active margin setting [22, 23]. Arc-type granodiorite, diorite and tonalite are widespread within the Yili Block (Fig. 4a). The granodiorite is mainly composed of plagioclase, hornblende and minor 241 Bo Wang et al. / Geodinamica Acta 19/3-4 (2006) 237-247 Table 1: Zircon U-Pb data of granitoids from the Yili Block Plots Ratios 207Pb/235U 1σ 206Pb/238U Ages 1σ 207Pb/206Pb 1σ 206Pb/238U 1σ 207Pb/206Pb 1σ 207Pb/235U 1σ Disc.(%) Downloaded by [University of Tasmania] at 15:31 16 June 2015 XJ581 MA23C2 0.1044 0.0108 0.0167 0.0003 0.0480 0.0054 107 2.1 99 264 101 10 2.1 MA23C5 0.1541 0.0098 0.0245 0.0003 0.0449 0.0029 156 1.9 -61 159 145 9 1.9 MA23C11 0.1794 0.0094 0.0247 0.0004 0.0520 0.0029 157 2.4 284 126 168 8 2.4 MA23C7 0.3593 0.0103 0.0482 0.0004 0.0531 0.0016 303 2.4 332 69 312 8 2.4 MA23C8 0.3708 0.0182 0.0482 0.0009 0.0533 0.0027 303 5.4 342 115 320 14 5.4 MA23C12 0.3861 0.0109 0.0489 0.0005 0.0565 0.0016 306 3.4 471 61 332 8 3.4 MA23C9 0.3592 0.0123 0.0490 0.0004 0.0526 0.0019 309 2.6 311 83 312 9 2.6 MA23C3 0.3647 0.0085 0.0494 0.0003 0.0523 0.0013 311 2.1 300 57 316 6 2.1 MA23C6 0.3685 0.0101 0.0502 0.0004 0.0528 0.0014 316 2.4 320 62 319 8 2.4 MA23C10 0.3646 0.0095 0.0510 0.0004 0.0515 0.0014 321 2.4 263 61 316 7 2.4 MA23C4 0.3707 0.0153 0.0515 0.0005 0.0514 0.0021 324 3.2 261 93 320 11 3.2 MA23C1 0.3907 0.0150 0.0532 0.0006 0.0520 0.0021 334 3.5 285 93 335 11 3.5 MA23M11 0.3494 0.0216 0.0483 0.0008 0.0537 0.0033 304 5.3 357 138 304 16 5.3 MA23M9 0.3560 0.0214 0.0486 0.0007 0.0540 0.0033 306 4.7 373 138 309 16 4.7 MA23M7 0.3673 0.0261 0.0488 0.0008 0.0558 0.0040 306 5.4 445 157 318 20 5.4 MA23M8 0.3342 0.0239 0.0487 0.0010 0.0530 0.0039 306 6.6 329 168 293 18 6.6 MA23M12 0.3612 0.0289 0.0490 0.0010 0.0551 0.0044 307 6.2 417 179 313 22 6.2 MA23M3 0.3588 0.0226 0.0492 0.0009 0.0565 0.0035 308 5.5 473 136 311 17 5.5 MA23M4 0.3832 0.0247 0.0495 0.0008 0.0578 0.0038 309 5.0 521 144 329 18 5.0 MA23M10 0.3306 0.0245 0.0491 0.0008 0.0513 0.0039 310 5.4 256 174 290 19 5.4 MA23M6 0.3662 0.0256 0.0495 0.0008 0.0560 0.0040 310 5.3 453 160 317 19 5.3 MA23M2 0.3953 0.0227 0.0500 0.0009 0.0594 0.0034 312 5.7 583 123 338 17 5.7 MA23M5 0.3250 0.0246 0.0504 0.0010 0.0502 0.0040 318 6.2 202 187 286 19 6.2 MA23M1 0.3705 0.0280 0.0511 0.0010 0.0574 0.0045 319 6.6 508 173 320 21 6.6 XJ583 Disc. (%) denotes percentage of discordance quartz as well as biotite (Fig. 4b). Single zircon grains of one granodiorite and one microdiorite samples collected along the highway from Dushanzi to Nalati (Fig. 1 and 2) are dated using a Hewlett Packard HP 4500 ICP-MS fitted with a Nd-YAG Laser operating at 213 nm at the University of Tasmania (Australia). The U-Pb isotopic results are presented in Table 1 and in Fig. 5. The granodiorite XJ583 yields an age of 309±3Ma (MSWD=0.63) with an isolated peak in the histogram. The microdiorite XJ581 presents a main peak age of 315±3Ma (MSWD=4.6) with 4 out of 120 analyses scattering from the main peak in histogram, their relatively high U/Pb ratio is interpreted as due to a Pb loss. Geochronology of the calc-alkaline granites from other places in the Yili Block (Work in progress) indicates that the oldest age for arc-related magmatism is 361 Ma. This plutonic event that lasted for about 50 millions years was contemporaneous with congenetic volcanic activity, i.e. basalt and dacitic andesite from the same area yielding SHRIMP U-Pb zircon ages ranging from 354 Ma to 313 Ma [23]. The tectonic setting and age of the Yili magmatic rocks are consistent with those of the Middle Devonian to Carboniferous Bogda calc-alkaline volcanic rocks [4, 24], and sharply contrast with those of the Permian intra-plate granites, alkaline basalts or continental tholeiites and associated felsic rocks that crop out in Nileke and Baiyanggou areas (Fig. 1) [11, 25]. 3. Polyphase deformation in the northern WTS Three principal phases of deformation, called D1 to D3, are recognized in the northern WTS Belt. They are presented here in the retro-tectonic order (from younger to older) in order to remove the effects of younger deformations on the older ones. Downloaded by [University of Tasmania] at 15:31 16 June 2015 242 Bo Wang et al. / Geodinamica Acta 19/3-4 (2006) 237-247 3.1. Cenozoic intracontinental thrusting (D3) Fig. 4: (a) Field photography of arc-type granitoids in the Yili Block; (b) Micro-photography of a granodiorite showing the plagioclase (Pl), hornblende (Hbl), quartz (Qtz) and biotite (Bi). The northern piedmont of the Tianshan Belt, at the contact with the Junggar Basin, consists of a thick succession (nearly 10 km) of Triassic-Neogene terrigenous deposits formed by fluvial erosion of the range (Fig. 2) [26]. Kilometre scale north verging folds and high to intermediate angle brittle thrusts accommodate a N-S shortening of the south margin of the Junggar Basin. According to detailed structural, geomorphological and magneto-stratigraphic results, the Paleozoic rocks (either turbidites or volcanic rocks) are thrusted over the Mesozoic to Neogene continental sediments with a throw of several tens kilometres [27-30]. As a consequence, prominent tectonic features, such as the Late Paleozoic suture between the Yili and Junggar blocks, have been concealed and cannot be recognized in the field (Fig. 2). 2) [8]. The age of the shearing is not settled yet, on the basis of paleomagnetic data in the West Tianshan Belt in Kyrgyzstan, Bazhenov et al. [31] proposed that a dextral strike-slip event occurred during the Late Permian to Early Jurassic. However, in the study area, Jurassic coal bearing sandstone that covers the volcanic rocks are not deformed by the ductile shearing, that should therefore be older than Jurassic. Moreover, in East Chinese Tianshan, the MTSZ (fault 1 in Fig. 1) is dated at 280-250 Ma from syn-kinematic biotites by Ar-Ar method [4, 7]. Since the NTF and MTSZ appear to be cartographically continuous, a Permian age can be tentatively inferred for the NTF. 3.2. Permian dextral strike-slip faulting (D2) 3.3. Carboniferous north-directed thrusting (D1) As observed along the Dushanzi-Nalati highway and other parallel routes, the southern part of the turbidite unit is lithologically dominated by black mudstone, sandstone, minor chert and volcani-clastic rocks. Although attributed to the Devonian [11], these rocks remain undated since they are mainly clastic. They underwent a ductile deformation characterized by a steeply dipping slaty cleavage (Fig. 3f) with a subhorizontal mineral-stretching lineation. In the field, kinematic criteria are rare, but sigmoidal cleavage, lensoids and asymmetrically sheared clasts suggest a dextral sense of shear, which is confirmed by microscopic observations (Fig. 6). Quartz and feldspar clasts exhibiting asymmetric pressure shadows (Fig. 6a), shear bands (Fig. 6b), sigmoidal biotite (Fig. 5c), sheared andalusite or elongated quartz ribbons with oblique sub-grain fabrics (Fig. 5d) are common microstructures. Therefore, the present boundary between the turbidite, ophiolitic mélange and the Yili Block is a ductile rightlateral strike-slip fault, i.e. the NTF (fault 3 in Fig. 1, Fig. In the turbidite unit, a series of tight isoclinal folds marked by siliceous layers are locally well developed. Due to the intense D2 dextral shearing, it is difficult to state whether these folds were formed during the D2 event or ealier. If the latter is the case, the fold asymmetry indicates a northward vergence. Bedding-parallel shear zones, sometimes marked by chlorite or illite coatings and N-S trending slicken-lines indicate a north-directed shearing. Similar low temperature shear zones can be observed around the ophiolite blocks in the mélange. These thrust faults are difficult to date, indeed, some of them might have been formed during the Cenozoic D3 event. However, on some surfaces, shearing related horizontal slicken-lines overprint steeply dipping striaes. This structural succession allows us to infer that the Carboniferous turbidite and ophiolitic mélange units experienced a top-to-the-north shearing before the D2 right-lateral shearing. Thus a Late Carboniferous age appears likely for this D1 event. Downloaded by [University of Tasmania] at 15:31 16 June 2015 Bo Wang et al. / Geodinamica Acta 19/3-4 (2006) 237-247 243 Fig. 5: Concordia diagrams of ICP-MS U-Pb zircon analytical results and histograms showing the age distribution for the arc-related calc-alkaline granitoids from the Yili Block (see Fig. 1 and 2 for samples localities). According to the mélange classification proposed by Raymond [32] we interpret this ophiolitic mélange as a sheared olistostrome with exotic blocks. 4. Discussion 4.2. Geodynamic evolution of the northern West Tianshan 4.1. Implication of the ophiolitic rocks The bulk architecture of the northern WTS Belt is due to a poly-orogenic evolution. As recognized by many authors on the basis of geological and geophysical studies [27-30], the Paleozoic rocks are thrusted to the north upon the Junggar Basin. Consequently, the primary Paleozoic structures are partly erased or concealed by the Cenozoic ones. Moreover, the D2 shearing might also hide initial relationships between the tectonic elements that formed the North Tianshan domain. On the basis of high pressure metamorphic rocks with Sm-Nd and Ar-Ar ages around 350-315 Ma, some previous researchers proposed that the Yili magmatic arc resulted from the closure of a “South Tianshan Ocean” situated to the south of the Yili Block [18, 33-35]. However, older K-Ar ages ranging from 482 Ma to 415 Ma on these metamorphic rocks both in NW China and in Kyrgyzstan [31, 36] suggest that the high pressure event might be older than Yili arc magmatism. Moreover, there is no detailed structural analysis nor kinematic evidence supporting a northward subduction of “South Tianshan Ocean” below the Yili Block. Thus, on the basis of the previous studies and our own results, we propose a geodynamic model accounting for the evolution of the North Tianshan domain and the Yili Block (Fig. 7). In The formation of the North Tianshan ophiolitic mélange is still controversial, and it was interpreted either as in situ disrupted ophiolites [15] or as klippes [18]. Since all the typical lithologies are represented in the field, previous researchers regarded these rocks as an ophiolitic nappe that was thrusted upon the turbidite [15-17]. However, a complete continuous ophiolitic sequence is lacking, and the oceanic rocks are always disrupted and mixed with sediments. In addition, the ophiolitic blocks and the surrounding sedimentary rocks often display sheared boundaries. Sedimentary as well as tectonic processes may form a mélange [32]. In most outcrops, mafic and ultramafic rocks are mixed together with gabbroic sandstone, greywacke and pelite. This suggests that the oceanic rocks underwent an intraoceanic tectonic event that was responsible for unroofing of peridotite and gabbro, shearing and subsequent re-deposition on the ocean floor. Finally, the already mixed magmatic and sedimentary rocks are included in the turbidite, as trench fill deposits during subduction. The sheared block-in-matrix structure supports a tectonic process active during accretion. 244 Bo Wang et al. / Geodinamica Acta 19/3-4 (2006) 237-247 Downloaded by [University of Tasmania] at 15:31 16 June 2015 Fig. 7 Simplified geodynamic evolution of the northern West Chinese Tianshan Belt. In Late Devonian-Early Carboniferous, south directed subduction of an oceanic basin below the Yili Block led to the formation of a magmatic arc and an accretionary complex. In Late Carboniferous, the oceanic basin was closed, and the ophiolitic mélange was re-deformed. In Permian, the suture zone was reworked by dextral strike-slip fault. During Cenozoic, the intracontinental reactivation induced the thrusting of the North Tianshan Domain upon the Junggar sedimentary basin, the Paleozoic suture became “cryptic”, i.e. hidden below the Paleozoic rocks. Fig. 6: Microscopic-scale shear criteria in slate along the North Tianshan Fault showing dextral ductile deformation. (a) feldspar clasts with asymmetric quartz and biotite pressure shadows; (b) shear-bands; (c) sigmoidal biotites and asymmetrically sheared feldspar clasts; (d) elongated quartz ribbons with an oblique shape fabric of recrystallized grains. In the field, the subvertical foliation contains a subhorizontal E-W trending mineralstretching lineation (Fig. 3f). Mineral abbreviations: Fds, feldspar; Bio, biotite; Qtz, quartz. our geodynamic model, the Yili magmatic arc is considered as the result of the south-directed subduction of an oceanic lithosphere located to the north of the Yili Block, remnants of this oceanic lithosphere are found in the ophiolitic mélange. Thus we assume that in Late Devonian to Carboniferous times, the subduction of an oceanic basin (i.e. “North Tianshan oceanic” [15]) below the Yili Block produced the magmatic arc and accretionary complex composed of turbidite and ophiolitic mélange. The closure of the North Tianshan oceanic basin in Late Carboniferous resulted in the northward obduction and redeformation of the ophiolitic mélange. Then a prominent change led to the end of N-S convergence and the beginning of intracontinental transcurrent tectonics. This event was due to either: 1) collision between the continental blocks with Precambrian basements (the Yili Block and the Junggar block, which is alternatively considered as a trapped Paleozoic ocean [37]), or 2) the progressive change from N-S convergence during the earlier stages to NE-SW convergence in the final stages. The intra-continental transcurrent tectonism is represented by the Permian dextral strike-slip faulting. By comparison with eastern Chinese Tianshan, this event is assumed to take place between 280 and 250 Ma [5, 7]. This lateral transcurrent faulting was likely responsible for local crustal thinning and opening of pull-apart basins (e.g. the Turfan Basin in Fig. 1) [38, 39] in which marine deep-water sediments and alkaline pillow lavas accumulated, e.g. in Baiyanggou area [25]. The pull-apart basins are also associated with intra-plate magmatic rocks, such as alkaline granite, basalt, continental tholeiite and felsic volcanic rocks [11, 38]. On the basis of magmatic, sedimentologic and tectonic evidence, the Permian intra-continental large-scale strike-slip tectonics appears to be geodynamically distinct from the Late Devonian to Carboniferous oceanic convergence that built up the West Chinese Tianshan Belt. Therefore, any reconstruction of the Late Paleozoic Central Asia should take into account these lateral displacements. For instance, the Permian transcurrent faulting was geometrically likely to trigger the Bo Wang et al. / Geodinamica Acta 19/3-4 (2006) 237-247 245 Fig. 7: Simplified geodynamic evolution of the northern West Chinese Tianshan Belt. In Late Devonian-Early Carboniferous, south directed subduction of an oceanic basin below the Yili Block led to the formation of a magmatic arc and an accretionary complex. In Late Carboniferous, the oceanic basin was closed, and the ophiolitic mélange was re-deformed. In Permian, the suture zone was reworked by dextral strike-slip fault. During Cenozoic, the intracontinental reactivation induced the thrusting of the North Tianshan Domain upon the Junggar sedimentary basin, the Paleozoic suture became “cryptic”, i.e. hidden below the Paleozoic rocks. Downloaded by [University of Tasmania] at 15:31 16 June 2015 lateral displacement of the Bogda Arc from its original place. Such displacement that was suggested to provoke strike-slip imbrication at the scale of the whole Central Asia Orogen [40] is still need to be documented quantitatively. A Paleomagnetic study that might provide such a constraint on the amount of E-W displacement is presently in progress. 5. Conclusion Poly-orogenic events and multiple tectonic overprints arise some difficulties for reconstructing the geodynamic evolution of the Tianshan Belt. Taking into account the Cenozoic events, our study provides some new evidence to better understand the Paleozoic evolution of the northern WTS Belt. The polyphase evolutionary model proposed in this paper is comparable with the tectonic framework of the adjacent areas, and might be used to interpret the geodynamics of Central Asia Orogen. Regionally, Late Paleozoic subduction, accretion and collision of the Junggar Block play an important role on the building of the Central Asia, and the Permian post-collisional transcurrent event is widely recorded, but remains to be fully understood. Acknowledgements We thank the Bureau of National project 305 (Xinjiang Uygur autonomous Region, China) for field assistance. This is a contribution to the State Key Project for Basic Research of China (2001CB409804). Dr. J. Gao and S. Dominguez are thanked for valuable advices to improve this paper. References [1] Coleman R.G., Continental growth of Northwest China, Tectonics 8 (1989) 621-635. 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