The Permian marks an important, yet poorly understood, tectonic transition in the Tian Shan region of northwestern China between Devonian-Carboniferous continental amalgamation and recurrent Mesozoic-Cenozoic intracontinental orogenic reactivation. The Turpan-Hami basin accommodated up to 3000 m of sediment and is ideally positioned to provide constraints on this transition. New stratigraphic data and mapping indicate that extension dominated Early Permian tectonics in the region, whereas flexural, foreland subsidence controlled Late Permian basin evolution. Lower Permian strata in the northwestern Turpan-Hami basin consist of coarsegrained debris-flow and alluvial-fan deposits interbedded with mafic to intermediate volcanic sills and flows. In contrast, Lower Permian rocks in the north-central and northeastern Turpan-Hami basin unconformably overlie a Late Carboniferous volcanic arc sequence. These Lower Permian strata include possible shallow-marine carbonate rocks and thick volcanic and volcaniclastic rocks that are in turn overlain by littoral-to profundal-lacustrine facies. Above a regional Lower Permian/Upper Permian unconformity, regional sedimentation patterns record the development of a more integrated sedimentary basin. The Upper Permian is entirely nonmarine and can be correlated east-west along the depositional strike of the basin. The lower Upper Permian consists of a broad belt of

We employ petrographic and advanced geochemical techniques to better document the evolution of the Turpan-Hami basin based on the unique geologic histories of the arc terranes that served as potential sources of Turpan-Hami deposits. First, a provenance study of Permian through Cretaceous sandstone of the Turpan-Hami basin reveals temporal and spatial changes in dominant source terranes that provided detritus to the basin. points to the initial uplift and unroofing of the largely andesitic Bogda Shan to the north, which first shed its sedimentary cover as it emerged to become the partition between the Turpan-Hami and southern Junggar basins.Second, geochronological, trace-element, and Sm-Nd isotopic variations among granitoids in the late Paleozoic Tian Shan orogenic belt provide a further test of Mesozoic uplift of the Bogda Shan. On the basis of previous models of crustal compositions throughout the South, Central, and North Tian Shan, Bogda Shan, and East and West Junggar terranes, we infer that isotopically enriched granitic cobbles (average ⑀Nd i ‫؍‬ Ϫ0.50, n ‫؍‬ 6) contained in Lower Triassic deposits in the north-central Turpan-Hami basin were derived from the continental crustal Central Tian Shan terrane, south of Turpan-Hami, and not from the more oceanic North Tian Shan, Bogda Shan, and East and West Junggar terranes, north of the Turpan-Hami basin. We therefore infer that the ancestral Bogda Shan had not been uplifted by the Early Triassic, and that prior to this time, a unified Junggar-Turpan-Hami basin existed during Late Permian deposition of extensive lacustrine deposits.

The Turpan-Hami basin is a major physiographic and geologic feature of northwest China, yet considerable uncertainty exists as to the timing of its inception, its late Paleozoic and Mesozoic tectonic history, and the relationship of its petroleum systems to those of the nearby Junggar basin. To address these issues, we examined the late Paleozoic and Mesozoic sedimentary record in the Turpan-Hami basin through a series of outcrop and subsurface studies. Mesozoic sedimentary facies, regional unconformities, sediment dispersal patterns, and sediment compositions within the Turpan-Hami and southern Junggar basins suggest that these basins were initially separated between Early Triassic and Early Jurassic time.Prior to separation, Upper Permian profundal lacustrine and fan-delta facies and Triassic coarse-grained braided-fluvial-alluvial facies were deposited across a contiguous Junggar-Turpan-Hami basin. Permian through Triassic facies were derived mainly from the Tian Shan to the south, as indicated by northward-directed paleocurrent directions. This is consistent with the sedimentary provenance of Triassic sandstone (mean Qm 29 F 29 Lt 42 , Qp 23 Lvm 49 Lsm 28 , and Qm 51 P 25 K 24 ) and conglomerate ( & 32% granitic clasts) in the northern Turpan-Hami basin. We interpret a relative increase in quartz and feldspar concentration and a relative decrease in volcanic lithic grains in the northern Turpan-Hami basin to reflect unroofing in the Tian Shan and exposure of late Paleozoic granitoid rocks. In addition, two basinwide unconformities of Late Permian-Early Triassic and Early Triassic-middle-Late Triassic age attest to deformation within the Turpan-Hami basin and associated continued uplift and erosion of the Tian Shan.

Uranium-lead (U-Pb) geochronology studies commonly employ the law of detrital zircon: A sedimentary rock cannot be older than its youngest zircon. This premise permits maximum depositional ages (MDAs) to be applied in chronostratigraphy, but geochronologic dates are complicated by uncertainty. We conducted laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) and chemical abrasion–thermal ionization mass spectrometry (CA-TIMS) of detrital zircon in forearc strata of southern Alaska (USA) to assess the accuracy of several MDA approaches. Six samples from Middle–Upper Jurassic units are generally replete with youthful zircon and underwent three rounds of analysis: (1) LA-ICP-MS of ∼115 grains, with one date per zircon; (2) LA-ICP-MS of the ∼15 youngest grains identified in round 1, acquiring two additional dates per zircon; and (3) CA-TIMS of the ∼5 youngest grains identified by LA-ICP-MS. The youngest single-grain LA-ICP-MS dates are all younger than—and rarely overlap at 2σ uncertainty with—the CA-TIMS MDAs. The youngest kernel density estimation modes are typically several million years older than the CA-TIMS MDAs. Weighted means of round 1 dates that define the youngest statistical populations yield the best coincidence with CA-TIMS MDAs. CA-TIMS dating of the youngest zircon identified by LA-ICP-MS is indispensable for critical MDA applications, eliminating laser-induced matrix effects, mitigating and evaluating Pb loss, and resolving complexities of interpreting lower-precision, normally distributed LA-ICP-MS dates. Finally, numerous CA-TIMS MDAs in this study are younger than Bathonian(?)–Callovian and Oxfordian faunal correlations suggest, highlighting the need for additional radioisotopic constraints—including CA-TIMS MDAs—for the Middle–Late Jurassic geologic time scale.

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