Palinspastic Reconstruction and Geological Evolution of Jurassic Basins in Mongolia and Neighboring China

Journal of Palaeogeography 2013, 2(3): 306-317 DOI: 10.3724/SP.J.1261.2013.00033 Tectonopalaeogeography and palaeotectonics Palinspastic reconstruction and geological evolution of Jurassic basins in Mongolia and neighboring China Wu Genyao* Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China Abstract The important event in Jurassic tectonics in Mongolia was the subduction and closure of the Mongolia-Okhotsk ocean; correspondingly, basin evolution can be divided into two main stages, related to the orogeny and collapse of the orogenic belt, respectively. The developing of Early-Middle Jurassic basins to the north of the ocean resulted from back-arc extension. The fossil sutures, from the China-SE Asia sub-continent to the south of the ocean, were rejuvenated by subduction-related orogeny; in addition, the Yanshanian intra-continental movement occurred. Three Early-Middle Jurassic molasse basins were developed by movement in Inner Mongolia, all of which stretched westwards (or northwards) into Mongolia; therefore, the molasse basins in eastern and southern Mongolia had the same geometric and kinematic features as the basins in the Inner Mongolia. Owing to the collapse of the Mongolia- Okhotsk orogenic belt, a group of rift basins developed during the Late Jurassic. In eastern Mongolia, the NE orientated extensional basins were controlled by the neogenic NE-structure. The contemporary basins in southern Mongolia and the neighboring areas in China were constrained by remobilization (inherited activation) of the latitudinal or ENE-directional basement structures. Three stages can be recognized in the evolution of the Early-Middle Jurassic basins after reversal; the basins also experienced four episodes of reformation. Key words back-arc extension, molasse basin, collapse rift, basin evolution, basement structure, neogenic structure, Jurassic, Mongolia and neighboring China 1 Introduction* tinuously from the latest Late Paleozoic to the Middle Ju- rassic based on the nonmarine strata of the Upper Perm- The Jurassic system is widely distributed in Mongolia ian-Lower Jurassic (?) and the unconformity between the and its environs; especially, the Lower-Middle Jurassic Lower-Middle Jurassic and the Upper Jurassic-Lower series contains coal-bearing strata of great economic va- Cretaceous in the Noyon Uul region of the South Gobi lue. Up to now, the Jurassic primary sedimentary basins in (Ömnö Govi) province. Afterwards, Hendrix et al. (2001) Mongolia have not been reconstructed; in addition, ideas and Lamb et al. (2008) suggested that crustal contraction regarding the tectonic settings for these basins are contro- took place in North China, Northwest China and southern versial. For example, there are two completely different Mongolia during the Triassic-the Jurassic with a com- opinions about Early-Middle Jurassic basins bordering pressive stress field, which resulted in the closure of the China and Mongolia. Hendrix et al. (1996) claimed that Paleo-Asian ocean during the latest Permian, and the succ- intra-continental compressive deformation occurred con- eeding collision between the Mongolia and China conti- nents, which persisted from the Triassic to the Jurassic. * Corresponding author. Email: [email protected]. Graham et al. (2001) pointed out that the Khamarkhoovor Received: 2013-01-31 Accepted: 2013-04-05 Formation of the Lower-Middle Jurassic in the East Gobi Wu Genyao: Palinspastic reconstruction and geological evolution of Vol. 2 No. 3 Jurassic basins in Mongolia and neighboring China 307 (Dorno Govi) Basin is a molasse formation. Meanwhile, the Mongolia-Okhotsk ocean (Zorin et al., 1993), and to there exists an opposite opinion, namely, a tensile or trans- the east was the Panthalassa (or Paleo-Pacific) ocean (Wu tensile tectonic setting for the Jurassic basins. Kimura and Yano, 2007) respectively. et al. (1990) suggested that the Mesozoic basins in East During the Early Mesozoic in Mongolia and adjacent ar- Asia were controlled by the tenso-shear fractures related eas of Russia, the key structure was the Mongolia-Okhotsk to the collision-extrusion structures. Ritts et al. (2001) ocean, which stretched eastwards and linked up with the demonstrated that the Lower Jurassic strata in the western Panthalassa. The ocean subducted northwards and closed Daqing Shan of the Yinshan Mountains, Inner Mongolia, in the latest Early Jurassic-early Middle Jurassic (Zorin, are deposited in a rift basin. In addition, the Yagan-Onch 1999). According to this theory, the evolution of Jurassic Hayrhan metamorphic core complex bordering China and basins in Mongolia can be divided into two main periods, Mongolia supports further evidence for extension (Webb related to the orogeny during the Early-Middle Jurassic et al., 1999). Wang et al. (2002) proposed that the exten- and to the collapse of the orogenic belt during the Late Ju- sion structure controlling the metamorphic core complex rassic respectively. started, at least, from the latest Early Jurassic and conti- The tectonic setting of Early-Middle Jurassic basins in nued to the Late Cretaceous. northern Mongolia, specifically, to the north of the Mongo- In the light of tectonopalaeogeography (Feng, 2003, lia-Okhotsk suture zone, was different when compared to 2009; Feng et al., 2012) and orogen-palaeogeography the basins in central-southern Mongolia which was situ- (Wu, 2005, 2007), and based on a study of the regional tec- ated to the south of the suture zone. In total, the Mongolia- tonic framework, this paper will focus on the palinspastic Okhotsk ocean subducted northwards under the accreted reconstruction of the Early-Middle Jurassic and the Late Siberian subcontinent. The northern part of Mongolia was Jurassic primary basins respectively, and will approach the located in a back-arc region and the development of the tectonic setting of the basins and related geological evolu- basin was constrained by back-arc extension. Erdenetsogt tion. et al. (2009) reported that the coal-bearing nonmarine Saikhan Formation of Lower-Middle Jurassic age filled 2 Regional tectonic framework about twenty scattered grabens that might indicate a group of isolated small basins related to the initial subduction of Structurally, Mongolia is attached to the Central Asian the oceanic crust and occurred in the back-arc region of an Orogenic Belt, to the north and the south of the orogen- Andean-type mountain arc. These scattered grabens were ic belt are the Siberian and Sino-Korean subcontinents further developed with the ongoing subduction and super- (cratons) respectively. The orogenic belt grew out of the imposed by a unified large (fault-) depression, named the Paleo-Asian ocean, which was a wide archipelago occurr- Orhon-Selenge Basin in the Figure 1. The southwestern ing in the Neoproterozoic and Paleozoic. Because of multi- and the southeastern boundaries of the basin, being NW- ple and varied directional subductions of the oceanic crust and NE-running respectively, were close to and parallel to during the geo-history, accompanied by multiple arc-arc the Mongolia-Okhotsk suture zone. The sediments in the (continent) collisions (Wu, 1998; Windley et al., 2001), basin of the Saikhan Formation, 670 m thick (Table 1), is both subcontinents were accreted and enlarged succes- characterized by thick conglomerates in the lower mem- sively. By the end of the Permian, the Paleo-Asian ocean ber with a thickness of 250 m. The upper member of the was closed (Heubeck, 2001), followed by the collision of formation consists of sandstone, siltstone and shale with the two subcontinents in the Triassic. interlayers of coal seams. The Sino-Korean subcontinent to the south of the Paleo- To the south of the suture zone existed an amalga- Asian ocean was enlarged northwards by accretion of a se- mated continental mass, namely, the China-SE Asia sub- ries of arc terrains from the Paleozoic to the Triassic. The continent. It is a composite entity, welded and combined by simultaneous closure of the Qinling-Dabie ocean in central a series of cratons and massifs. The fossil sutures and deep China and the Paleotethyan ocean in southwestern China, fractures within the composite continent represent active and the following collision and amalgamation of the Sino- tectonic zones, which will be rejuvenated and will control Korean, the Yangtze and the Shan-Thai subcontinents re- the intra-plate (intra-continental) orogeny in the case of sulted in the creation of the China-SE Asia subcontinent the inter-plate orogeny occurring on the outer side of the (Wu, 1997). The latter was still surrounded by oceans: to composite continent (Wu, 2000, 2002). In Inner Mongolia the southwest was the Neotethyan ocean, to the north was and adjacent areas, there are four fossil sutures; three of 308 JOURNAL OF PALAEOGEOGRAPHY July 2013 them relate to the present study; they are, from South to segment of the suture zone, only one ophiolite, namely, the North, the Caledonian Bayan Qagan-Ondor Sum-Kedan- Adaatsag ophiolite, was defined with a zircon U-Pb age shan suture, the Hercynian Solen Mountain-Erlian-Hegen of 325.4±1.1 Ma (Early Carboniferous; Tomurtogoo et al., Mountain suture, and the Hailar-Huma suture (Wu, 2006). 2005). The Hangay area was uplifted during the Permian; Subduction and related closure of the inter-plate orogeny in addition, the NW-striking segment might be transpres- of the Mongolia-Okhotsk ocean immediately re-activated sional and acted as the transitional zone in compressive- all fossil sutures; at the same time, intra-continental oroge- contracted events since the Permian. It may have become nies occurred, which are called the Yanshanian movement a left-lateral shearing fracture in the Early-Middle Jurassic in China. Each Yanshanian orogenic belt (fold-and-thrust (Tomurtogoo et al., 2005) and constrained transpressional belt) constrained a molasse basin (Fig. 1) that are called fractures in western Mongolia to orientate in a northwest the Yinshan-Yanshan, the Erlian and the Hailar Basins re- direction. The major basin-controlling faults thrust north- spectively (Wu, 2007; Wu et al., 2008a).

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