Chuanbo Shen

Late-stage foreland growth of China’s largest orogens (Qinling, Tibet): Evidence from the Hannan-Micang crystalline massifs and the northern Sichuan Basin, central China

This paper addresses the timing of final foreland growth of China’s largest orogens: the Mesozoic Qin Mountains (Qinling) and the Cenozoic Tibetan Plateau. In particular, we ask when the front of the Qinling orogen fold-thrust belt was emplaced, and when the northern Sichuan Basin was affected by the eastward growth of the Tibetan Plateau. We employ zircon and apatite fission-track and (U-Th)/He dating in the Proterozoic crystalline rocks of the Hannan-Micang massifs and the sedimentary rocks of the northern Sichuan Basin. The Hannan-Micang rocks remained in the zircon fission-track partial annealing zone (240 ± 30 °C) throughout the Paleozoic–Middle Triassic (481–246 Ma). From the late Middle Jurassic (ca. 165 Ma) to the early Late Cretaceous (ca. 95 Ma), enhanced cooling and exhumation, with rates of 1.2–2.5 °C/m.y. and 0.04–0.10 mm/yr, respectively, record propagation of the Qinling orogen into its leading foreland; the timing of foreland growth is supported by sedimentologic evidence, i.e., regional variation in sediment thickness and depocenter migration. Negligible cooling and exhumation since the Late Cretaceous (ca. 95 Ma) likely mark the end of the foreland fold-thrust belt formation and the subsequent persistence of a low-relief landscape that occupied extensive parts of central China; cooling and exhumation rates of 0.38–0.70 °C/m.y. and <0.02 mm/yr characterize this tectonic stagnation period. Accelerated cooling (4–5 °C/m.y.) since the Late Miocene (13–8 Ma), derived from apatite fission-track temperature-time path models, signifies involvement of the Hannan-Micang massifs and the northern Sichuan Basin into the eastward-growing Tibetan Plateau. Their inclusion into the plateau growth initiated faulting and stripped off 1.4–2.0 km of rock from the Hannan-Micang massifs and northern Sichuan Basin.

Sichuan Basin and beyond: Eastward foreland growth of the Tibetan Plateau from an integration of Late Cretaceous‐Cenozoic fission track and (U‐Th)/He ages of the eastern Tibetan Plateau, Qinling, and Daba Shan

Combining 121 new fission-track and (U-Th)/He ages with published thermochronologic data, we investigate the Late Cretaceous–Cenozoic exhumation/cooling history of the eastern Tibetan Plateau, Qinling, Daba Shan, and Sichuan Basin of east-central China. The Qinling orogen shows terminal southwestward foreland growth in the northern Daba Shan thrust belt at 100–90 Ma and in the southern Daba Shan fold belt at 85–70 Ma. The eastern margin of Tibet Plateau experienced major exhumation phases at 70–40 Ma (exhumation rate 0.05–0.08 mm/yr), 25–15 Ma (≤1 mm/yr in the Pengguan Massif; ~0.2 mm/yr in the imbricated western Sichuan Basin), and since ~11–10 Ma along the Longmen Shan (~0.80 mm/yr) and the interior of the eastern Tibet Plateau (Dadu-River gorge, Min Shan; ~0.50 mm/yr). The Sichuan Basin records two basin-wide denudation phases, likely a result of the reorganization of the upper Yangtze-River drainage system. The first phase commenced at ~45 Ma and probably ended before the Miocene; >1 km of rocks were eroded from the central and eastern Sichuan Basin. The second phase commenced at ~12 Ma and denudated the central Sichuan Basin, Longmen Shan, and southern Daba Shan; more than 2 km of rocks were eroded after the lower Yangtze River had cut through the Three Gorges and captured the Sichuan-Basin drainage. In contrast to the East Qinling, which was weakly effected by Late Cenozoic exhumation, the West Qinling and Daba Shan have experienced rapid exhumation/cooling since ~15–13 Ma, a result of growth of the Tibetan Plateau beyond the Sichuan Basin.

Low-Temperature Thermochronology Constraints on the Mesozoic-Cenozoic Exhumation of the Huangling Massif in the Middle Yangtze Block, Central China

The Huangling (黄陵) massif is an important area to understand the tectonic evolution of the northern Middle Yangtze Block. Integrating previously published thermochronology data with new zircon and apatite fission track, and apatite (U-Th)/He thermochronometry, the Meso-Cenozoic exhumation history of the Huangling massif has been quantitatively studied. Based on the data and the time-temperature thermal history modelling results, the exhumation process of the Huangling massif can be divided into four stages: the slow cooling stage during 200-150 Ma; the rapid cooling event at ca. 150-80 Ma; a period of relative thermal stability during ca. 80-40 Ma, and an increase in cooling thereafter. Two rapid cooling/exhumation indicate two tectonic events in the northern Middle Yangtze Block. The rapid exhumation between ca. 150-80 Ma is likely related to a wide range Cretaceous intra-continental reactive and deformation in the eastern China. The accelerated cooling after ca. 40-30 Ma may result from a far-field effect of the India-Asia collision.

Tectonothermal evolution of the eastern Tibetan Plateau Foreland: Fission-track thermochronology of the southern Dabashan fold-thrust belt

Apatite fission-track dating and thermal-history modeling were carried out on samples from the Dabashan (大巴山), a fold-thrust belt, northeast of the Sichuan (四川) Basin and east of the Tibetan Plateau. A first cooling event in the Late Cretaceous is followed by a prolonged period of thermal stability with exhumation rates of ≤0.025 mm/a, as determined from age vs. elevation relationships. The preservation of age vs. elevations relationships and the lack of distinct age changes across tectonic structures indicate that the Dabashan fold-thrust belt formed prior to the Late Cretaceous, consistent with the current view of Triassic-Early Cretaceous shortening. Relatively short mean track lengths (∼12 μm) indicate that the samples remained in the partial annealing zone for a prolonged time. The knick points in the best-fitting temperature-time models suggest that the onset of late-stage accelerated cooling commenced at ≤11 Ma. Related exhumation rates are 0.3–0.2 mm/a assuming geothermal gradients of 20 and 30 °C/km. We speculate that this late-stage event results from eastward growth of the Tibetan Plateau and overstepping of the Sichuan Basin, it is likely responsible for the youthful morphology of the Dabashan.

Thermochronology quantifying exhumation history of the Wudang Complex in the South Qinling Orogenic Belt, central ChinaC. SHEN AND OTHERSExhumation of the Wudang Complex

The Wudang Complex located in the central part of South Qinling, has been inferred to be a segment of the Yangtze Craton involved in the orogen. In this study, the cooling/exhumation history of the Wudang Complex is revealed through combined published geochronology data and new apatite fission-track results. Three rapid exhumation episodes related to relevant geodynamic events have been identified. Previous 40 Ar– 39 Ar and (U–Th)/He data indicate that the most significant exhumation, induced by the collision between the North and South China Blocks, occurred from c. 237 to 220 Ma after long-term subsidence and sedimentation of the passive continental margin. The second exhumation event, related to the long-distance effect of the Pacific subduction, occurred during the period from c. 126 to 90 Ma. Following the late Cretaceous – Eocene peneplanation stage, the final late Cenozoic exhumation since c. 15 Ma may be attributed to the combined effect of the eastward growth of the Tibetan Plateau uplift and the Asian monsoon.

Neoproterozoic–Cambrian petroleum system evolution of the Micang Shan uplift, northern Sichuan Basin, China: Insights from pyrobitumen rhenium–osmium geochronology and apatite fission-track analysis

The Neoproterozoic strata of the Sichuan Basin are a key target for oil and gas. To evaluate the hydrocarbon evolution and its relationship with tectonic events in the Micang Shan uplift, northernmost Sichuan Basin, we apply solid bitumen geochemistry (bitumen reflectance and fluorescence) and Re-Os geochronology. The geochemistry of the solid bitumen indicates that it is highly mature pyrobitumen that formed contemporaneously with dry-gas generation during oil thermal cracking. The pyrobitumen is enriched in both Re (∼106–191 ppb) and Os (∼3030–5670 parts per trillion). The Re-Os isotope data imply an Early Jurassic date for pyrobitumen formation, which coincides well with age estimates from fluid-inclusion data and basin modeling. The Re-Os date for pyrobitumen formation coupled with previously presented apatite fission-track (AFT) analysis show that exhumation of the Neoproterozoic strata occurred during the Cretaceous in the Micang Shan uplift. This extensive uplift led to the erosion of any potential gas reservoirs and surface exposure of bitumen-bearing Neoproterozoic strata. In contrast, the more southern and central portions of the Sichuan Basin have experienced less severe exhumation and, as a result, Neoproterozoic-sourced gas systems are present. This study shows that, through the combined application of Re-Os and AFT methodologies, the timing of gas generation and subsequent erosion of any potential gas reservoirs in the Micang Shan uplift, northern Sichuan Basin, can be quantified. Moreover, the Re-Os and AFT data illustrate the potential to constrain the timing of gas generation in petroleum systems worldwide.

Nature and Evolution of the Ore-Forming Fluids from Nanmushu Carbonate-Hosted Zn-Pb Deposit in the Mayuan District, Shaanxi Province, Southwest China

The Nanmushu carbonate-hosted Zn-Pb deposit is located in the Mayuan district of Shaanxi Province, a newly discovered metallogenic district next to the Sichuan Basin, in the northern margin of the Yangtze Block, which is the largest and the only one that is currently mined in this district. The δ34S values of sulfides are characterized by positive values with a peak around