Late Permian to Triassic intraplate orogeny of the southern Tianshan and adjacent regions, NW China

Geoscience Frontiers 5 (2014) 83e93

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Research paper

Late Permian to Triassic intraplate orogeny of the southern Tianshan and adjacent regions, NW China Wei Ju a, b, Guiting Hou a, b, * a b

School of Earth and Space Science, Peking University, Beijing 100871, China The Key Laboratory of Orogenic Belts and Crustal Evolution, Ministry of Education, Beijing 100871, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 20 April 2012 Received in revised form 19 February 2013 Accepted 5 March 2013 Available online 22 March 2013

The South Tianshan Orogen and adjacent regions of Central Asia are located in the southwestern part of the Central Asian Orogenic Belt. The formation of South Tianshan Orogen was a diachronous, scissors-like process, which took place during the Palaeozoic, and its western segment was accepted as a site of the final collision between the Tarim Craton and the North Asian continent, which occurred in the late Palaeozoic. However, the post-collisional tectonic evolution of the South Tianshan Orogen and adjacent regions remains debatable. Based on previous studies and recent geochronogical data, we suggest that the final collision between the Tarim Craton and the North Asian continent occurred during the late Carboniferous. Therefore, the Permian was a period of intracontinental environment in the southern Tianshan and adjacent regions. We propose that an earlier, small-scale intraplate orogenic stage occurred in late Permian to Triassic time, which was the first intraplate process in the South Tianshan Orogen and adjacent regions. The later largescale and well-known Neogene to Quaternary intraplate orogeny was induced by the collision between the India subcontinent and the Eurasian plate. The paper presents a new evolutionary model for the South Tianshan Orogen and adjacent regions, which includes seven stages: (I) late Ordovicianeearly Silurian opening of the South Tianshan Ocean; (II) middle Silurianemiddle Devonian subduction of the South Tianshan Ocean beneath an active margin of the North Asian continent; (III) late Devonianelate Carboniferous closure of the South Tianshan Ocean and collision between the Kazakhstan-Yili and Tarim continental blocks; (IV) early Permian post-collisional magmatism and rifting; (V) late PermianeTriassic the first intraplate orogeny; (VI) JurassicePalaeogene tectonic stagnation and (VII) NeoceneeQuaternary intraplate orogeny. Ó 2013, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. All rights reserved.

Keywords: Central Asian Orogenic Belt Northern Xinjiang South Tianshan Ocean Tectonics Tarim-North Asia collision

1. Introduction The Central Asian Orogenic Belt (CAOB; Jahn et al., 2000; Kröner et al., 2007; Windley et al., 2007) or the Altaids (Sengor et al., 1993; Sengor and Natal’in, 1996) is situated among the European Craton

* Corresponding author. School of Earth and Space Science, Peking University, Beijing 100871, China. Tel.: þ86 10 62756422; fax: þ86 10 62758610. E-mail address: [email protected] (G. Hou). Peer-review under responsibility of China University of Geosciences (Beijing)

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to the west, the Siberian Craton to the east and the Tarim Craton and the North China Craton to the south (Fig. 1). It is the largest Paleozoic accretionary orogen in the world and its complicated accretion-collision processes are generally thought to be related to the closure of Paleo-Asian Ocean, which finally resulted in the largest tectonic assembly of continental, active margin and oceanic terranes (Zonenshain et al., 1990; Mossakovsky et al., 1993; Khain et al., 2002, 2003; Kovalenko et al., 2004; Safonova, 2009; Safonova et al., 2011). The CAOB is the world largest region of Phanerozoic continental growth (Coleman, 1989; Jahn et al., 2000, 2004; Xiao et al., 2009; Han et al., 2011). The formation of South Tianshan Orogen was a diachronous, scissors-like process during the Palaeozoic and its western segment was accepted as the site of the final collision zone between the Tarim Craton and the North Asian continent (Chen et al., 1999; Dong et al., 2011; Han et al., 2011). The age of that collisional event

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Figure 1. Simplified tectonic map of Central Asian Orogenic Belt (modified after Sengor et al., 1993; Jahn et al., 2000).

is still a matter of discussion. Some researchers think that the collision occurred in the late Devonian (Xiao et al., 1992; Che et al., 1994; Xia et al., 2004) or late Devonianeearly Carboniferous (Allen et al., 1992; Carroll et al., 1995; Chen et al., 1999; Charvet et al., 2007, 2011; Wang et al., 2007a, 2010, 2011; Lin et al., 2009). The others constrain the time as the earlyemiddle Carboniferous (Coleman, 1989; Windley et al., 1990; Gao et al., 1998; Bykadorov et al., 2003; Yang and Zhou, 2009) or late Carboniferous (Bakirov and Kakitaev, 2000; Burtman, 2006, 2008; Gao et al., 2009; Hegner et al., 2010; Su et al., 2010) or middle Carboniferouseearly Permian (Biske, 1995; Bykadorov et al., 2003; Biske and Seltmann, 2010). The younger timing of the collision was attributed to the latest Carboniferouseearly Permian (Zonenshain et al., 1990; Shi et al., 1994; Chen et al., 1999; Bazhenov et al., 2003; Xiao et al., 2004; Klemd et al., 2005; Gao et al., 2006; Solomovich, 2007; Pickering et al., 2008) or late Permianeearly Triassic (Li et al., 2002, 2005, 2010) or Triassic (Zhang et al., 2005, 2007b) or latest Permianemiddle Triassic (Xiao et al., 2008, 2009). The evidence for the late PermianeTriassic age of that major collisional event comes from two late Permian radiolarian specimens from the Baleigong ophiolitic mélange (Li et al., 2002, 2005, 2010) and from Triassic zircon U-Pb ages obtained from high to ultrahigh pressure metamorphic rocks of the western Tienshan (Zhang et al., 2005, 2007b). However, in spite of different tectonic models, it is widely accepted that the Central Asian Orogenic Belt grew in southward direction, away from Siberia and southern Mongolia (Zonenshain et al., 1990; Sengor and Natal’in, 1996; Xiao et al., 2008, 2009; Safonova et al., 2011; Kröner et al., 2013). It is well known that the Paleozoic and Mesozoic tectonic evolution of Central Asia was dominated by accretion of continental blocks, back/fore/island arc terranes and fragments of oceanic lithosphere (Dewey et al., 1988; Coleman, 1989; Chen et al., 1999; Windley et al., 2007; Xiao et al., 2009; Safonova et al., 2011 and

references therein). However, the post-collisional tectonic evolution of the South Tianshan Orogen and adjacent regions remains debatable. We suggest an earlier intraplate orogenic process, which could have been active during the Mesozoic time. Based on previous studies and recent geological data, the present study focuses on the tectonic evolution of the South Tianshan Orogen (NW China) and adjacent regions after the final collision, and presents a new evolutionary model for the study area to contribute to our understanding of the tectonic evolution and magmatism in the western CAOB. 2. Geology outline The Central Asian Orogenic Belt (CAOB) is located between the Siberian Craton and East-European Craton to the north, and the Tarim Craton and North China Craton to the south (Fig. 1). It encompasses an immense area from the Urals in the west, through Kazakhstan, NW China, Mongolia, and NE China to the Okhotsk Sea in the Russian Far East (Zonenshain et al., 1990; Sengor et al., 1993; Badarch et al., 2002; Xiao et al., 2004, 2008; Kröner et al., 2007, 2013; Windley et al., 2007; Rojas-Agramonte et al., 2011; Safonova et al., 2011). The CAOB is the largest and most complex accretionary collage that has been a site of considerable Phanerozoic crustal growth (Sengor et al., 1993; Jahn et al., 2000; Xiao et al., 2009). Despite its importance, the understanding of the CAOB history is limited because of insufficient detailed studies throughout that vast area. As a result, the Paleozoic tectonic evolution of the orogenic collage remains debatable (Xiao et al., 2009; Han et al., 2010; Wang et al., 2011). The Northern Xinjiang is located in the southwestern part of the CAOB (Fig. 1), including the Chinese Altai, Junggar and Tianshan tectonic domains from north to south, which is a key area, connecting the Kazakhstan orogenic belt to the west and the

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Mongolia-Chinese Inner Mongolia orogenic belt to the east (Xiao et al., 2009; Han et al., 2011). Its almost complete geological records and excellent exposure of the ophiolites, magmatic arcs, and accretionary wedges make it an ideal place to study its tectonic evolution (Windley et al., 1990; Jahn et al., 2000; Xiao et al., 2004, 2008). Northern Xinjiang can be divided into several tectonic belts: Chinese Altay, East Junggar, West Junggar, and Tianshan. The Tianshan Range is an important Paleozoic collisional orogenic belt, which is of key importance to understand the tectonic evolution of Central Asia. It extends in the east-west direction over a distance of more than 2500 km from Uzbekistan in the west, across Tajikistan, Kyrgyzstan, and southern Kazakhstan in the center, to Xinjiang in western China in the east (Chen et al., 1999; Xiao et al., 2009; Han et al., 2011). In China, the Tianshan Range is generally divided into eastern and western segments along longitude 88 E. The West Tianshan is usually divided into four main tectonic units: the North Tianshan, Yili Block, Central and South Tianshan from north to south. The Yili Block is the eastern part of the Kazakhstan microcontinent and is mainly composed of late Precambrian to Paleozoic rocks. The South Tianshan is the eastern part of a single late Paleozoic orogen between the Tarim Craton to the south and the Kazakhstan-Yili terrane to the north (Allen et al., 1992; Ma et al., 1997; Gao et al., 2009; Han et al., 2011; Fig. 2). It is generally accepted that the South Tianshan Orogen resulted from the closure of the South Tianshan Ocean (also called the

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Turkestan Ocean) followed by the collision between the Tarim Craton and the North Asian continent. The closure of the South Tianshan Ocean was possibly a diachronous, scissors-like process, which started in the east and finished in the west (Chen et al., 1999; Dong et al., 2011). The closure began in the Hongliuhe area of the East Tianshan in the early Devonian. The Hongliuhe ophiolite is crosscut by an undeformed granite dyke with a zircon U-Pb age of 405  5 Ma (Zhang and Guo, 2008). The collision in the Kumux area occurred during the late Devonian to early Carboniferous (Charvet et al., 2007; Dong et al., 2011). The final collision between the Tarim Craton and North Asian continent was in the late Palaeozoic, but the exact timing of this collision is still unclear (Gao et al., 2009; de Jong et al., 2009; Han et al., 2010, 2011; Hegner et al., 2010). The Tarim Craton is located in the southernmost part of the Northern Xinjiang and is bounded by the southern Tianshan melange or the Kokshaal-Kumishi accretionary complex in the north (Biske and Seltmann, 2010). Tarim is one of the largest Precambrian continental blocks in China, underlain by Neoarchean to Paleoproterozoic metamorphic basement (U-Pb zircon ages of 2830 to 1900 Ma; Guo et al., 2003; Deng et al., 2008; Lu et al., 2008; Xiao et al., 2008; Rojas-Agramonte et al., 2011). Those NeoarcheanePaleoproterozoic granitic gneisses, supercrustal rocks, and granitoid plutons are covered by Mesoproterozoic sedimentary and volcanic deposits. All these units were affected by the latest Mesoproterozoic to middle Neoproterozoic tectonothermal events (Lu et al., 2008), which are typically considered to be related to the amalgamation and breakup of the Rodinia supercontinent (Guo

Figure 2. Tectonic sketch map of the Tianshan and northern Tarim (modified from BGMRXUAR, 1993, 1:1,500,000; Shi et al., 1994). Abbreviations: NTOMAC, Northern Tianshan ophiolite melange and accretionary complex; CTVA, Central Tianshan volcanic arc; YT, Yili terrane; STOMAC, Southern Tianshan ophiolite melange and accretionary complex; NTMA, Northern Tarim magmatic arc; NTDB, Northern Tarim deformation belt.

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et al., 2005; Deng et al., 2008; Lu et al., 2008; Xu et al., 2009). In the northwestern Tarim Craton, late Precambrian to Devonian shallow marine to non-marine limestone and sandstone are unconformably overlain by Carboniferous and Permian deposits (Carroll et al., 1995). The late Paleozoic magmatism at the northern margin of the Tarim Craton was thought as reflection of arc or subduction (Chen et al., 1999). Thus, the northern margin of the Tarim Craton includes Permian continental deposits, bimodal dykes (Yang et al., 2007) and several granitoid plutons (Fig. 3). 3. Closure of the South Tianshan Ocean The period from the Sinian to the Cambrian was a time of continental breakup setting. The widespread occurrence of the late Sinian tillites at the northern margin of the Tarim Craton, north and south of the Yili terrane and Kazakhstan block (Wang et al., 1990; Mikolaichuk et al., 1997) indicates that both the Tarim Craton and the North Asian continent were parts of the Rodinia supercontinent in the late Neoproterozoic time (Xu et al., 2005). The South Tianshan Ocean probably opened at ca. 428e450 Ma (Hao and Liu, 1993; Long et al., 2006; Wang et al., 2007d). Therefore, the North Asian continent and the Tarim Craton were separated by the South Tianshan Ocean during the late Ordovician to the early Silurian (Chen et al., 1999; Bazhenov et al., 2003; Gao et al., 2009; Qian et al., 2009). The northward subduction of the South Tianshan Ocean likely started in the middle Silurian or early Devonian time (Allen et al., 1992; Che et al., 1994; Chen et al., 1999; Gao and Klemd, 2003; Gao et al., 2006), and continued until the late Devonian or early Carboniferous (Fig. 4; Chen et al., 1999). The subduction led to the formation of a middle Silurian to early Carboniferous magmatic arc along the southern margin of the Kazakhstan-Yili terrane (Gao et al., 2009; Han et al., 2011). In the southern Tianshan, the ophiolitic mélanges mainly occur in the Kokshaal accretionary complex in Kyrgyzstan (Biske and Seltmann, 2010; Safonova and Santosh, 2013) and in the

Figure 4. Evolution of the South Tianshan Orogen and adjacent regions during the late Ordovician to early Permian.

Baleigong, Heiyingshan, Kulehu and Kumux orogenic belts in China from west to east. The Baleigong, Heiyingshan and Kulehu orogenic belt look like thrust sheets over the passive margin of the northern Tarim Craton (Han et al., 2011). The diabase-gabbro yielded a zircon

Figure 3. Distribution of Paleozoic plutons in the South Tianshan Orogen and adjacent regions as well as the Tarim Large Igneous Province of continental flood basalts, northwest China. The late Carboniferous and Permian age data are shown.

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age of 450  2 Ma (LA-ICP-MS, Wang et al., 2007c) in the Baleigong ophiolitic mélange. A cumulate gabbro of the Heiyingshan ophiolitic mélange yielded a zircon U-Pb age of 392  5 Ma (LA-ICP-MS, Wang et al., 2011). Two gabbro samples from the Kulehu ophiolitic mélange yielded zircon ages of 425  8 Ma (SHRIMP, Long et al., 2006) and 418  3 Ma (LA-ICP-MS, Ma et al., 2007) respectively. A granodiorite pluton with arc affinity of the Kumux ophiolitic mélange intruded the Silurian strata and yielded a zircon age of 424  1 Ma (TIMS, Zhang et al., 2007a). All these facts are indicative of a diachronous, scissors-like closure of the South Tianshan Ocean, which started first in the east and continued and finished in the west (Chen et al., 1999; Dong et al., 2011). This suggests that the western segment of the South Tianshan Orogen in the contiguous regions of China and Kyrgyzstan is a key site for constraining the time of the final collision. The earliest closure is recorded in the Hongliuhe area of the East Tianshan and probably took place in the early Devonian. There, the Hongliuhe ophiolite is crosscut by an undeformed granite dyke with a U-Pb zircon age of 405  5 Ma (Zhang and Guo, 2008). The collision recorded in the Kumux area occurred during the late Devonian to early Carboniferous (Charvet et al., 2007; Dong et al., 2011). In addition, the lack of marine Devonian sediments in the Kuruktag region suggests that the eastern part of the Tarim Craton collided with the North Asian continent in the Devonian time (Chen et al., 1999). Evidence for the final collision comes from the Sm-Nd isochron age of 343  44 Ma for the eclogites, 40Ar-39Ar ages of 345e340 Ma for glaucophane, and the 40Ar-39Ar and Rb-Sr white mica ages of 335e310 Ma of the Tianshan HP-LT metamorphic belt in NW China. Consequently, the final collision between the Tarim Craton and the North Asian continent probably occurred during the late Carboniferous (Figs. 4 and 5; Gao and Klemd, 2003; Klemd et al., 2005; Gao et al., 2006, 2009, 2011; Han et al., 2011). There are numerous Permian granitoid plutons in the western segment of the South Tianshan Orogen and adjacent tectonic units (Fig. 3). Those plutons intrude Paleozoic sedimentary strata and late Paleozoic thrust sheets (Gao et al., 1998; Solomovish and Trifonov, 2002), implying that they temporally post-dated the collision between the Tarim Craton and the North Asian continent. Consequently, the Permian plutons are usually interpreted as products of post-collisional magmatism (Solomovish and Trifonov, 2002; Konopelko et al., 2007, 2009; Solomovich, 2007; Han et al., 2011). Important, the Permian plutons also occur in adjacent areas around the South Tianshan Orogen (Fig. 3). If we accept the idea about the Triassic age of the South Tianshan Orogen, the widely accepted northward subduction model may suggest that the Permian plutons were generated by subduction-related magmatism and confined only to the southern margin of the Kazakhstan-Yili terrane (Windley et al., 2007; Burtman, 2008; Gao et al., 2009; Dong et al., 2011; Han et al., 2011). Therefore, the Permian plutons in the western segment of the South Tianshan Orogen emplaced after the collision, and the oldest late Carboniferous plutons with zircon ages of ca. 300 Ma (Fig. 3) mark the upper time boundary of the collision. On the other hand, the stitching plutons with the oldest ages of w300 Ma and the 299 Ma low-pressure high-temperature metamorphic assemblages in the western segment of the South Tianshan Orogen and adjacent regions definitely constrain the pre-Permian timing of the collision between the Tarim Craton and the North Asian continent. More evidence for the pre-Permian collision comes from paleomagnetic data indicating the amalgamation of the North Asian continent, Siberian and Tarim continental blocks at about 300 Ma (Bazhenov et al., 2003). Therefore, based on the geochronological data discussed above, we suggest that the collision between the Tarim Craton and the North Asian continent, and the final amalgamation of the southwestern Altaids had terminated in the late Carboniferous (Fig. 4;

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Gao et al., 2009, 2011; Hegner et al., 2010; Han et al., 2011). During the collision, the accretionary complex on the southern margin of the North Asian continent and its hosted ophiolitic mélange were thrust over the passive margin of the northern Tarim Craton. 4. Major stages of intraplate orogeny In post-Permian time, after the closure of the South Tianshan Ocean and final collision of the Tarim Craton and the North Asian continent, the southern Tianshan and adjacent regions experienced post-collisional orogeny (Shu et al., 2004; Gao et al., 2009; Han et al., 2011). The Permian clockwise rotation of the Tarim Craton in respect to the Yili block probably resulted in large-scale strikeslip movements. The Permian strike-slip faulting was in agreement well with the timing of numerous transcurrent faults in the Tianshan Orogen and adjacent regions (Shu et al., 1999; LaurentCharvet et al., 2003; de Jong et al., 2009). It may be related to the break off of the subducted oceanic slab or as a result of the delamination of a thickened lithospheric root, which was accompanied by upwelling of the asthenosphere. The oldest stitching plutons within the collisional zone and the lowpressure high-temperature metamorphism belt began to exist, followed by extensive Permian molasse sedimentation. The postcollisional stage resulted in granitoid magmatism and intracontinental volcanism manifested throughout the whole South Tianshan Orogen and adjacent regions (Figs. 4 and 6; Shu et al., 2004; Simonov et al., 2008; Han et al., 2011). Occurrence of molasse is a critical issue for constraining the timing of collisional orogeny. In the southern Tianshan, the oldest molasse deposited in the early Permian and was overlain by the late Permian to Triassic continental deposits (Shu et al., 2004, 2007; Han et al., 2011). The Triassic units in the western segment of the South Tianshan Orogen consist of polymictic conglomerate, sandstone, siltstone, shale and minor coal beds with abundant plant fossils. There are no marine or volcanic rocks. All this suggests an intracontinental setting during the Triassic period (BGMRXUAR, 1993; Han et al., 2011). A picrite-basalt-rhyolite suite, which erupted between 291 and 275 Ma in the northern Tarim uplift region, together with extensive Permian magmatism from elsewhere in the Tarim Basin, erupted over an area >300,000 km2, and is characterized by rapid eruption (Tian et al., 2010). The geochemical characteristics of quartz syenite porphyry within associated dykes suggest that these dykes were formed in a within-plate rifting setting at 277 Ma or slightly later (Yang et al., 2007). Yang et al. (2007) proposed that the felsic magmas which were derived from the mantle intruded into the middle-upper crust and were emplaced as dykes. These dykes are indicative of large-scale extension, which took place in the southern Tianshan, especially in the Tarim Basin, during the Permian. There have been obtained many age estimations for the deformation in the Tianshan Mountains and for the erosion of its relief, which suggest that the reactivation of the mountain range and the growth of Tianshan topography began during the early Miocene at the oldest. However, recent data of fission-track analysis suggest that the uplifting of the Tianshan started well before the onset of the Tertiary reactivation (Chen et al., 2011). The apatite fission-track analysis and the (U-Th)/He dating of apatite and zircon indicate that the inherited Paleozoic structures were once reactivated in the late Paleozoic to early Mesozoic time (Jolivet et al., 2010). Evidence for the intracontinental deformation in Tianshan and adjacent regions during the late Permian to Triassic comes from the analysis of sedimentary strata (e.g. Hendrix et al., 1992), seismic cross sections (Figs. 7 and 8), results of previous studies (e.g. Jolivet et al., 2010) and other geological data. This intracontinental deformation could have resulted from the collision between the

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Figure 5. Schematic model illustrating the final collision of the South Tianshan Orogen (modified from Van der Voo et al., 2008; Han et al., 2011).

Songpan-Ganzi terrane and palaeo-Eurasian plate (Dewey et al., 1988; Hendrix et al., 1992), and this late PermianeTriassic deformation was possibly the first intraplate orogenic process in the South Tianshan Orogen and adjacent regions. The uplift of the Tianshan and the subsidence of adjacent sedimentary basins were analogous in many aspects to the modern uplift of the Himalaya range driven by the India-Eurasia collision (Dewey et al., 1988; Hendrix et al., 1992; Liu and Wang, 1995). The first intracontinental deformation event reactivated the pre-existed structural patterns in the Tianshan and adjacent territories and additionally created new deformational structures. A number of reverse faults created during that intracontinental event caused the thrusting of the Tianshan orogen overlaid on the North

Tarim regions, which created a Triassic flexural basin in the North Tarim regions, namely the Kuche basin (Fig. 6). In addition, the composition of sandstone may also reflect the deformation process. The composition of Mesozoic sandstones in western China basins is variable suggesting a change of the tectonic regime in the Tianshan and adjacent regions during that time. Hendrix (2000) showed that the temporal variations in the Triassic sandstone composition of the Tianshan are consistent with the late PermianeTriassic tectonic and/or deformation event. That event resulted in intensive mountain growth and its related increased erosion and deposition of compositionally immature sandstone in adjacent basins (Hendrix et al., 1992; Hendrix, 2000; Shu et al., 2004).

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thickness of the NeoceneeQuaternary sedimentary sequence (Hendrix et al., 1992; Shu et al., 2003). The India-Eurasia collision initiated significant large-scale intracontinental deformation in the Tianshan and its neighboring areas, such as the southward thrusting of the South Tianshan over the Tarim Basin and the northward thrusting of the North Tianshan over the Junggar Basin. The thrusting, in turn, caused a strong coupling between the basins and the ranges, and eventually resulted in the most distinctive modern landscapes in Central Asia (Windley et al., 1990; Yin et al., 1998; Allen et al., 1999; Shu et al., 2003). 5. A new evolutionary model for the South Tianshan Orogen

Figure 6. Evolution of the South Tianshan Orogen and adjacent regions during the late Permian to Quaternary.

The Tianshan and its neighboring areas were in a tectonic quitescence setting during the JurassicePalaeogene period, accompanied by weak processes of extension though (Figs. 6 and 7; Han et al., 2011). A similar tectonic period has been also recognized in the northern Tibet, southern Mongolia, and Siberia (Mossakovsky et al., 1993; Kovalenko et al., 2004; Yuan et al., 2006; Jolivet et al., 2010; Rojas-Agramonte et al., 2011; Kröner et al., 2013). Many previous researchers have shown that the recent structural deformation and seismicity in the Tianshan are related to the remote effect of the Cenozoic India-Eurasia collision (Molnar and Tapponnier, 1975; Allen et al., 1994; Yang et al., 2001). During the Neocene to Quaternary time, the Tianshan region was reactivated by the processes of compression and thrusting, which resulted in the uplifting and formation of tectonic nappes, respectively. Consequently, the erosion increased and the Tarim, Junggar and Turpan-Hami basins, which are located on the both sides of the Tianshan orogenic belt, experienced another period of subsidence and sedimentation, resulting in the formation of a very large

Based on the above review on the geology and tectonics of Tianshan and recent geochronological data, we proposed a new evolutionary model for the South Tianshan Orogen and adjacent regions, which includes seven evolutional stages since the opening of the Tianshan or Turkestan Ocean (Figs. 4 and 6): (I) late Ordovicianeearly Silurian; (II) middle Silurianemiddle Devonian; (III) late Devoniane late Carboniferous; (IV) early Permian; (V) late PermianeTriassic; (VI) JurassicePalaeogene and (VII) NeoceneeQuaternary. (I) During the late Ordovician to the early Silurian, the North Asian continent and the Tarim continental block broke away from their parent supercontinent Rodinia, which resulted in the opening of the South Tianshan (or Turkestan) Ocean. (II) During the middle Silurian to the middle Devonian, the oceanic plate of the Tianshan Ocean subducted in the northward direction, beneath the North Asian continent. (III) The late Devonianelate Carboniferous was a period of the ocean crust accretion and closure of the South Tianshan Ocean, which resulted in the collision between the Tarim Craton and the Kazakhstan-Yili composite continental terrane. (IV) The early Permian post-collisional stage was accompanied by granitoid magmatism, molasse sedimentation, intraplate rifting and its related eruption of voluminous basaltic lavas, which formed the Tarim Large Igneous Province.

Figure 7. Seismic profile interpretation map of Line S93-127.

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Figure 8. Seismic profile interpretation map of Line S90-181.

(V) The late PermianeTriassic was a period of the first intraplate orogeny, which induced orogenic uplifting in the adjacent regions. (VI) The JurassicePalaeogene was a period of tectonic stabilization and calm in the South Tianshan Orogen and adjacent regions, accompanied by weak processes of extension. (VII) The NeoceneeQuaternary second stage of intracontinental orogeny was induced by the remote effect of the ongoing India-Eurasia collision, which started in the beginning of the Cenozoic and reactivated the range-bounding thrust and reverse fault systems, including the strike-slip fault systems of the Tianshan regions.

post-collisional magmatism and rifting; (V) late PermianeTriassic the first intraplate orogeny; (VI) JurassicePalaeogene tectonic stagnation and (VII) NeoceneeQuaternary intraplate orogeny. Acknowledgments We would like to express our gratitude to Prof. M. Santosh, Dr. Inna Safonova and anonymous reviewers who considerably improved this article in many aspects. This work was supported by the National Natural Science Foundation of China (Grant Nos. 40772121, 40314141 and 40172066) and China National Project No. 973 (2009CB219302). The paper is also a contribution to IGCP Project #592 “Continental construction in Central Asia” supported by UNESCO-IUGS.

6. Conclusions References The formation of the South Tianshan Orogen was a diachronous, scissors-like process, which took place during the Palaeozoic. The western segment the South Tianshan Orogen was a site of the final collision between the Tarim Craton and the North Asian continent, which occurred in the late Palaeozoic. Based on previous studies and recent geochronogical data, we suggest that the final collision between the Tarim Craton and North Asian continent occurred during the late Carboniferous. Therefore, the Permian was a period of intracontinental environment in the southern Tianshan and adjacent regions. We propose that an earlier, small-scale intraplate orogenic stage occurred in the late Permian to Triassic time, which was the first intraplate process in the South Tianshan Orogen and adjacent regions. The later large-scale and well-known Neogene to Quaternary intraplate orogeny was induced by the collision between the India subcontinent and the Eurasian plate. The evolution of the South Tianshan Orogen and adjacent regions proceeded in seven stages: (I) late Ordovicianeearly Silurian opening of the South Tianshan Ocean; (II) middle Silurianemiddle Devonian subduction of the South Tianshan Ocean beneath the North Asian continent; (III) late Devonianelate Carboniferous closure of the South Tianshan Ocean and collision between the Kazakhstan-Yili and Tarim continental blocks; (IV) early Permian

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