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To test the idea that the voluminous upper Middle to Upper Triassic turbidite strata in the Songpan-Ganzi complex of central China archive a detrital record of Dabie ultra-high-pressure (UHP) terrane unroofing, we report 2080 single detrital U-Pb zircon ages by sensitive high-resolution ion microprobe-reverse geometry (SHRIMP-RG) and laser ablation-inductively coupled plasma--mass spectrometry (LA-ICP-MS) analysis from 29 eastern Songpan-Ganzi complex sandstone samples. Low ( doi: 10.1130/B26606.1

The East Gobi basin of Mongolia is a poorly described Late Jurassic-Early Cretaceous extensional province that holds great importance for reconstructions of Mesozoic tectonics and paleogeography of eastern Asia. Extension is especially well recorded in the structure and stratigraphy of the Unegt and Zuunbayan subbasins southwest of Saynshand, Mongolia, where outcrop and subsurface relationships permit recognition of prerift, synrift, and postrift Mesozoic stratigraphic megasequences. Within the synrift megasequence, three sequences developed in response to climatic and rift-related structural controls on sedimentation. Where best exposed along the northern margin of the Unegt subbasin, each of the synrift sequences is bounded by unconformities and generally fines upward from basal alluvial and fluvial conglomerate to fluvial and lacustrine sandstone and mudstone. Resedimented ashes and basalt flows punctuate the synrift megasequence. Rifting began in the Unegt subbasin prior to 155 Ma with coarse alluvial filling of local fault depressions. Subsidence generally outstripped sediment supply, and fresh to saline lacustrine environments, expanding southward with time, dominated the Unegt-Zuunbayan landscape for much of latest Jurassic-Early Cretaceous time. Episodic faulting and volcanism characterized the basin system for the balance of the Early Cretaceous. A brief period of compressional and/or transpressional basin inversion occurred at the end of the Early Cretaceous, prior to deposition of a widespread Upper Cretaceous overlap sequence. The driver(s) of Late Jurassic-Early Cretaceous extension remain uncertain because southeast Mongolia occupied an intraplate position by the beginning of the Cretaceous. Extension in the East Gobi basin was coeval with collapse and extension of early Mesozoic contractional orogenic belts along the northern and southern borders of Mongolia and probably was a linked phenomenon. Strike-slip faulting associated with collisions on the southern Asian and Mongol-Okhotsk margins likely also played a role in late Mesozoic deformation of the East Gobi region, perhaps partitioning the Gobi from apparently coeval large-magnitude contractional deformation in the Yinshan-Yanshan orogenic belt south of the study area in Inner Mongolia. Keywords: Asia, Mesozoic, Mongolia, rifting, sedimentary basins, tectonics.

The Yagan-Onch Hayrhan core complex is an important geological reference point for the study of the tectonic evolution of Asia. Tectonic movement in this area indicate a subhorizontal south-southeast directed extension, which became largely prominent during the early Cretaceous period. High-strain extensions, which were recently recognized in certain parts of eastern Asia, were believed to have occurred in southern Mongolia.

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 noah 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 felsic volcanic 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.

Provenance interpretations from sandstone detrital modes are widely used as criteria for inferring tectonic settings of ancient sedimentary basins. Although global schemes of sandstone petrography generally work quite well, they fail to adequately identify the correct tectonic setting of sands of modern basins of Xinjiang Uygur Autonomous Region, northwest China, because of the complicated tectonic history of sediment-source terranes. Modern sands of the Junggar and Turpan basins are markedly litharenitic because of derivation from upper Paleozoic arc sequences that form the bedrock of the northern Tian Shan and Bogda Shan. In contrast, sands of the Tarim basin are compositionally much more varied, ranging from quartzofeldspathic to litharenitic. Tarim sands reflect derivation from plutonic and thrusted sedimentary and metamorphic terranes exposed in surrounding mountain ranges. Extrabasinal carbonate sedimentary-lithic fragments, intentionally omitted from provenance analyses by some workers, are important provenance indicators in Tarim samples. Interpreted in terms of schemes for global sandstone composition, Junggar and Turpan basin sands suggest an arc-related setting; Tarim sands suggest settings with erosional access to arc terranes, basement (for example, rifts), and sedimentary-metamorphic sequences (for example, foreland basins). Modern basins of Xinjiang, however, do not easily fit any of these tectonic settings, and composition of modern sand reflects relict source terranes rather than current tectonic setting. In fact, Xinjiang basins are difficult to fit into most basin classifications. Modern Xinjiang basins are deformed at their margins by active thrust belts, and to a lesser extent, by strike-slip faults associated with the Cenozoic Himalayan collision, and were built upon Mesozoic retroarc-foreland basins. We term the modern basins of Xinjiang 'collisional successor basins' in recognition of their tectonic setting and history. The highly varied composition of Xinjiang basin sands may be typical of collisional successor basins, particularly where the tectonically accreted substrate of the basins consists of numerous and diverse small terranes.

A coherent blueschist terrane near Aksu, northwestern China, is unconformably ] overlain by unmetamorphosed sedimentary rocks of Sinian age (ca. 600-800 Ma). The pre-Sinian metamorphic rocks, termed the Aksu Group, have maintained stratal continuity and were derived from shale, sandstone, basaltic volcanic rocks, and minor pelagic sediments. They have undergone multistage deformation and transitional blueschist-greenschist facies metamorphism, and are represented by strongly foliated chlorite-stilpnomelane-graphite schist, stilpnomelane-phengite psammitic schist, greenschist, and minor quartzite, metachert, and metamorphosed ironstone. Metamorphic minerals present in blueschists include crossitic amphibole, epidote, chlorite, albite, quartz, and actinolite. Mineral parageneses and composition of sodic amphibole suggest blueschist facies recrystallization at about 4 to 6 kbar and about 300-400 [degrees] C. Post-tectonic alkali diabase dikes cut the Aksu Group. The basal conglomerate of the overlying Sinian to Eocambrian sedimentary succession contains clasts of both the metamorphic and dike rocks, clearly demonstrating that conditions required for blueschist facies metamorphism were attained at least 800 m.y. ago.

Recently completed sedimentologic and petrologic studies of Oligocene and Miocene strata in the Temblor Range (San Joaquin basin) and Santa Cruz Mountains (La Honda basin) permit detailed reconstructions of paleogeography, as well as new estimates of displacement along the San Andres fault. During the Oligocene and Miocene, the San Juaoquin and La Honda basins were contiguous. The southwestern margin of the San Joaquin-La Honda basin was tectonically unstable and paleogeographically complex, with several small deep-sea fans fed by sediment derived from nearby subaerial uplifts of Franciscan and granitic basement. One of these deep-sea fans, represented by the Temblor Formation of the southern Temblor Range and the Vaqueros Sandstone of the central Santa Cruz Mountains, apparently has been displaced about 315 to 320 km by post-early Miocene (post-Zemorrian, about 23 Ma) right-lateral slip along the San Andreas fault. This new estimate of displacement places an additional constraint on interpretations of San Andres slip history, but ambiguities remain. The time of initiation of San Andres displacement is poorly known, and available data do not permit discrimination between a history of relatively continuous slip versus a history of episodic slip.