The role of O2 in the evolution of early animals, as represented by some members of the Ediacara biota, has been heavily debated because current geochemical evidence paints a conflicting picture regarding global marine O2 levels during key intervals of the rise and fall of the Ediacara biota. Fossil evidence indicates that the diversification the Ediacara biota occurred during or shortly after the Ediacaran Shuram negative C-isotope Excursion (SE), which is often interpreted to reflect ocean oxygenation. However, there is conflicting evidence regarding ocean oxygen levels during the SE and the middle Ediacaran Period. To help resolve this debate, we examined U isotope variations (δ238 U) in three carbonate sections from South China, Siberia, and USA that record the SE. The δ238 U data from all three sections are in excellent agreement and reveal the largest positive shift in δ238 U ever reported in the geologic record (from ~ -0.74‰ to ~ -0.26‰). Quantitative modeling of these data suggests that the global ocean switched from a largely anoxic state (26%-100% of the seafloor overlain by anoxic waters) to near-modern levels of ocean oxygenation during the SE. This episode of ocean oxygenation is broadly coincident with the rise of the Ediacara biota. Following this initial radiation, the Ediacara biota persisted until the terminal Ediacaran period, when recently published U isotope data indicate a return to more widespread ocean anoxia. Taken together, it appears that global marine redox changes drove the rise and fall of the Ediacara biota.
Abstract We combine a ∼485 km‐long seismic reflection profile and a S‐wave speed transect from surface‐wave tomography, to reveal the lithospheric deformation mode of the intracontinental Qinling orogeny, central China. We observe a thick lithosphere keel in the convergence zone between the Yangtze Block and the North China Craton (NCC) and a shallow‐crustal (8–15 km depth) décollement that extends into the lower crust of the Qinling Orogen. Combining with surface structural geology and magmatism, we interpret these seismic findings as kinematically linked features formed by renewed intracontinental convergence between the NCC and the Yangtze Block in the late Mesozoic. We highlight that the ∼40 km lithospheric thickening in the convergence zone was likely balanced by > 130 km thin‐skinned crustal shortening along a crustal‐scale strain‐transfer décollement, and was responsible for the occurrence of late Mesozoic magmatism (∼160–100 Ma) at the southern edge of the NCC.
Abstract: Many equiaxial dome‐like structures developed in the north segment of the Xuefengshan orocline, Central China are obviously inconcordant with the NE‐trending linear structures in this area, which contain important records for understanding the structural framework and evolution of this belt. In this paper, taking one of the typical dome‐like structures in the Xuefengshan orcline (e.g. Moping dome‐like structure) as an example, based on its structural framework interpratatoin, superposed deformation analysis and paleo‐stress fields reconstruction, we propose the Moping dome‐like structure is composed of two populations of different‐striking thrust‐fold structures, ∼E‐trending and NE‐striking structures, indicative of two‐stages shortening, ∼N‐ and NW‐striking, respectively. Together with the geochronological analysis, we suggest the first stage of shortening occurred in Late Triassic to Early Jurassic, due to the Indosinian intercollisional orogeny of the Yangtze Block and the North China Block The second occurred during Late Jurassic–Early Cretaceous owing to Yanshanian intracontinental orogeny, leading to the intensive superposition of the NE‐trending structures onto the ∼E‐trending structures, and the final ocurrence of the Moping dome. Thus, our study indicates the Xuefengshan arc‐shape belt also experienced two‐phase deformation, and resulted from the superposition of NE–SW structures onto ∼E–W structures in Late Jurassic–Early Cretaceous, which could provide new structural evidence for probing the Mesozoic tectonic framework and evolution of the Xuefengshan orocline.
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