Late Paleozoic tectonic amalgamation of northwestern China: Sedimentary record of the northern Tarim, northwestern Turpan, and southern Junggar Basins

摘要

Sedimentary rocks contained in basins adjacent to the Tian Shan provide a long and complex record of the late Paleozoic continental amalgamation of northwestern China, complementing that provided by rocks preserved within the range. This record, which comprises dramatic changes in sedimentary facies, sediment dispersal patterns, sandstone provenance, and basin subsidence rates, broadly supports previous interpretations of a two-part evolution of the Tian Shan: Late Devonian to Early Carboniferous collision of the Tarim continental block with the Central Tian Shan, followed by collision of this combined block with island arcs in the north Tian Shan and Bogda Shan in Late Carboniferous–Early Permian times. The first collision resulted in widespread angular unconformities within the Tarim basin. Continued convergence following the collision created a long-lived flexural foredeep along the northern margin of the Tarim block, which received at least 2000 m of Lower Carboniferous through Lower Permian fluvial and marine sediment derived from the interior of Tarim. Subsequent Early Permian continental extension of the northern Tarim basin resulted in the deposition of interbedded nonmarine siliciclastic sedimentary rocks and mafic to felsicvolcanic rocks. Sandstone within this interval was derived from the paleo–Tian Shan, and is composed predominantly of lithic volcanic grains similar to the rhyolite. In contrast to the Tarim basin, calc-alkaline volcanic rocks and volcanogenic sedimentary rocks dominated Carboniferous and Permian sedimentation in the northern Turpan and northwestern Junggar basins. Volcanic arcs remained active in the North Tian Shan and Bogda Shan through the early Late Carboniferous, depositing a kilometers-thick interval of deep marine sediment-gravity flows in the northwestern Junggar basin. Major arc magmatism ceased in the Late Carboniferous in response to closure of the oceanic basin between the combined Tarim/Central Tian Shan block and the North Tian Shan/Bogda Shan arcs. Upper Carboniferous through Lower Permian rocks in the northwestern Junggar basin compose the sedimentary fill of a bathymetric basin of oceanic depth (on the northern side of the volcanic arcs), culminating in a 1000-m-thick marine regressive sequence. Middle to Upper Permian sandstones were derived from the uplifted paleo–Tian Shan and bear the distinctive provenance imprint of granitic rocks presently exposed within the range. Late Permian subsidence of the Junggar basin accommodated >5 km of nonmarine sediments; however, the cause of this subsidence and its relationship to regional tectonic events remain controversial.