Late Paleozoic Tectonic Amalgamation of Northwestern China: Sedimentary Record of the Northern Tarim, Northwestern Turpan, and Southern Junggar Basins
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Late Paleozoic tectonic amalgamation of northwestern China: Sedimentary record of the northern Tarim, northwestern Turpan, and southern Junggar Basins A. R. Carroll Exxon Production Research Company, P.O. Box 2189, Houston, Texas 77252-2189 S. A. Graham M. S. Hendrix* Department of Geological and Environmental Sciences, Stanford University, D. Ying Stanford, California 94305-2115 D. Zhou } ABSTRACT dominantly of lithic volcanic grains similar Ren et al., 1987; Coleman, 1989; Wang et to the rhyolite. al., 1990). The origin and history of these Sedimentary rocks contained in basins In contrast to the Tarim basin, calc-alka- elements, however, and the nature of the adjacent to the Tian Shan provide a long line volcanic rocks and volcanogenic sedi- growth of the Asian continent during this and complex record of the late Paleozoic mentary rocks dominated Carboniferous time remain controversial. In contrast to the continental amalgamation of northwestern and Permian sedimentation in the northern better-understood collisional histories which China, complementing that provided by Turpan and northwestern Junggar basins. mark the closure of the Tethyan oceanic rocks preserved within the range. This Volcanic arcs remained active in the North realm (e.g., the Alpine and Himalayan col- record, which comprises dramatic changes Tian Shan and Bogda Shan through the lisions), the central Asian ‘‘Altaid’’ orogenic in sedimentary facies, sediment dispersal early Late Carboniferous, depositing a ki- belts (cf. S¸engo¨r et al., 1993) generally lack patterns, sandstone provenance, and basin lometers-thick interval of deep marine sed- intact ophiolites or other means of clearly subsidence rates, broadly supports previous iment-gravity flows in the northwestern defining sutures between previously distinct interpretations of a two-part evolution of Junggar basin. Major arc magmatism tectonicblocks.Reactivationofintraplateor- the Tian Shan: Late Devonian to Early Car- ceased in the Late Carboniferous in re- ogenic belts during subsequent interplate in- boniferous collision of the Tarim continen- sponse to closure of the oceanic basin be- teractions (cf. Hendrix et al., 1992) has fur- tal block with the Central Tian Shan, fol- tween the combined Tarim/Central Tian ther obscured these boundaries, making lowed by collision of this combined block Shan block and the North Tian Shan/Bogda interpretation of original Paleozoic struc- with island arcs in the north Tian Shan and Shan arcs. Upper Carboniferous through tures nearly impossible. One approach to Bogda Shan in Late Carboniferous–Early Lower Permian rocks in the northwestern this problem has been to identify magmatic Permian times. The first collision resulted Junggar basin compose the sedimentary fill fronts associated with ancient magmatic arcs in widespread angular unconformities of a bathymetric basin of oceanic depth (on and their associated accretionary com- within the Tarim basin. Continued conver- the northern side of the volcanic arcs), cul- plexes, which might then be used as struc- gence following the collision created a long- minating in a 1000-m-thick marine regres- tural markers to reconstruct plate interac- lived flexural foredeep along the north- sive sequence. Middle to Upper Permian tions prior to final amalgamation (S¸engo¨r et ern margin of the Tarim block, which sandstones were derived from the uplifted al., 1993). This approach, although promis- received at least 2000 m of Lower Carbon- paleo–Tian Shan and bear the distinctive ing, is hampered by the difficulty in distin- iferous through Lower Permian fluvial and provenance imprint of granitic rocks pres- guishing between different subduction-ac- marine sediment derived from the interior ently exposed within the range. Late Per- cretion complexes formed at approximately of Tarim. Subsequent Early Permian conti- mian subsidence of the Junggar basin the same time, and by the structural dislo- nental extension of the northern Tarim ba- accommodated >5 km of nonmarine sedi- cation of these complexes along later strike- sin resulted in the deposition of interbedded ments; however, the cause of this subsidence slip faults. Reconstructions derived from the nonmarine siliciclastic sedimentary rocks and its relationship to regional tectonic present positions of ancient subduction and mafic to felsic volcanic rocks. Sand- events remain controversial. zones are therefore nonunique. stone within this interval was derived from An alternative (and complementary) the paleo–Tian Shan, and is composed pre- INTRODUCTION source of data lies in the sedimentary basin sequences preserved in areas within and ad- Northwestern China and adjacent areas jacent to the Altaid orogenic belts. These of central Asia comprise a collage of dispar- deposits possess the singular advantage that they contain a vertically stacked and rela- *Present address: Department of Geology, ate tectonic elements that amalgamated University of Montana, Missoula, Montana during the late Paleozoic (e.g., Burrett, tively undeformed record of erosion (due to 59812. 1974; Zonenshain and Gorodnitsky, 1977; orogenic uplift) and sedimentation (due to GSA Bulletin; May 1995; v. 107; no. 5; p. 571–594; 18 figures; 1 table. 571 CARROLL ET AL. Figure 1. Location map depicting Carboniferous-Permian outcrops and sedimentary basins of northwestern China (modified from Chen et al., 1985). Boxes indicate the approximate locations of Figures 4, 8, and 10. tectonic subsidence) and therefore avoid employed. As a result, numerous miscon- the evolution of the adjacent orogenic belts. many of the difficulties inherent in unravel- ceptions concerning the geology of the sed- This study is based on fieldwork during the ing complex structural overprints. They of- imentary basins of northwest China have be- summers of 1987, 1988, 1991, and 1992 by ten provide a more continuous and precise come embedded in the literature. As an workers from Stanford University, the Chi- record than that available from deformed example, the Junggar basin has been de- nese Academy of Geological Sciences, and belts. To the extent that such deposits scribed repeatedly as a Late Permian ‘‘back- the Xinjiang Bureau of Geology and Min- record distinctive patterns of regional sub- arc basin’’ (Hsu¨, 1988, 1989) despite the fact eral Resources. Although reconnaissance in sidence or sedimentary provenance, they that its known Upper Permian and younger nature, the data presented here provide a may also aid estimates of the history and sedimentary fill is exclusively nonmarine basis for evaluating alternative hypotheses magnitude of offset on hypothesized strike- (Carroll et al., 1990; Allen et al., 1991; Car- for the evolution of northwestern China and slip faults. Key to the utility of these deposits roll et al., 1992). provide a starting point for more compre- is a detailed understanding of the sedimen- This study focuses on areas adjacent to hensive future studies. tary facies, paleo-sediment dispersal direc- the Tian Shan (shan is Chinese for moun- tions, sandstone provenance, and basin geo- tains) in Xinjiang Uygur Autonomous Re- PALEOZOIC TECTONICS OF history they encompass. Unfortunately, very gion, northwestern China, and provides new NORTHWESTERN CHINA few well-documented, firsthand geologic field data on Carboniferous and Permian field data from northwestern China have outcrop exposures of sedimentary rocks of The Tarim basin (Fig. 1) is underlain by been reported in the western literature. Past the southern Junggar, northwestern Turpan, one of the major cratonal blocks of China, attempts to synthesize such data in a plate and northern Tarim basins (Fig. 1) that bear with Precambrian basement exposed all tectonic framework have relied heavily on directly on the history of late Paleozoic tec- along its periphery (Zhang et al., 1984; Chen (often incomplete) reports from the Chi- tonic amalgamation. We present here a mul- et al., 1985). Paleozoic rocks reach total nese literature, which in many instances tifaceted sedimentary basin analysis, includ- thicknesses of up to 12 km in the northeast- were derived from still earlier studies. Al- ing sedimentary facies, paleocurrent, and ern Tarim basin (Tian et al., 1989), and total though this approach has revealed some of sandstone provenance analyses, and recon- sediment fill may exceed 15 km (Lee, 1985). the principal attributes of the geology of structions of late Paleozoic basin subsid- Published gravity data indicate a subsedi- northwestern China, it has also prevented ence. These data provide a unique record mentary crustal thickness of only 25 km independent evaluation of the primary data not only of the basins themselves, but also of (Lee, 1985), suggesting that Tarim was 572 Geological Society of America Bulletin, May 1995 LATE PALEOZOIC TECTONIC AMALGAMATION, CHINA thinned by continental extension, likely dur- Tian Shan and, along with scattered ultra- in the full outcrop. Photographic documen- ing the Late Proterozoic (‘‘Sinian’’). Subse- mafic rocks and radiolarian cherts, are as- tation of selected outcrop facies are in- quent thermal subsidence resulted in the sociated with what is informally known as cluded both here and in Carroll et al. (1990). deposition of shallow marine carbonates to the ‘‘Junggar ocean’’ (Carroll et al., 1990). In addition to describing sedimentary facies, marine shale and sandstone in the Cambrian The Junggar and Turpan basins (Fig. 1) ap- we measured paleocurrent indicators where and Ordovician in the northwestern Tarim pear to be underlain by a basement of