Regional lithospheric deformation beneath the East Qinling-Dabie orogenic belt based on ambient noise tomography

Summary The Qinling–Dabie orogenic belt, which contain the arc-shaped Dabbashan orocline and is the world's largest belt of HP/UHP metamorphic rocks, formed by a long-term complex amalgamation process between the North China Block and the Yangtze Block. To understand the collision processes and tectonic evolution, we constructed a three-dimensional (3D) S-wave velocity model from the surface to a depth of ∼120 km in the eastern Qinling-Dabie orogenic belt and its adjacent region by inverting 5–70 s phase velocity dispersion data of Rayleigh waves extracted from ambient noise data. Our 3D model reveals low velocities in the middle–lower crust and high velocities in the upper mantle beneath the orogenic belt, suggesting the delamination of the lower crust. Our results support a two-stage exhumation model for the HP/UHP rocks in the study area. First-stage exhumation was caused by the slab breaking away from the subducted Yangtze Block during the Early–Middle Triassic. Partial melting of the lithospheric mantle caused by slab breakoff–related asthenospheric upwelling weakened the lithospheric mantle beneath the orogenic belt, and continued convergence of the two continental blocks led to further thickening of the lower crust. Such processes promoted lower-crust delamination, which triggered the second-stage exhumation of the HP/UHP rocks. In the Dabbashan orocline, two deep-rooted high-velocity domes, that is, Hannan–Micang and Shennong–Huangling domes, acted as a pair of indenters during the formation stage. High-velocity lower crust was observed beneath the Dabbashan orocline. In addition, our 3D model reveals that high-velocity lithospheric mantle extends from the Sichuan Basin to the Dabbashan orocline, with a subhorizontal distribution, providing strong support for the high-velocity lower crust. We also observed the destruction of lithospheric mantle beneath the Yangtze Block; the destruction area is bounded by the North–South Gravity Lineament, suggesting that the destruction mechanism of the Yangtze Block may be similar to the North China Block.

Provenance of Carboniferous strata (the Meishan Group) on the northern margin of the Dabie orogenic belt and implications of oblique convergence between the North and South China blocks

ABSTRACT The newly defined Carboniferous Meishan Group, along the northern margin of the Dabie orogenic belt, provides unique opportunities to document the poorly understood Paleozoic tectonic evolution of the Dabie orogenic belt and the Paleozoic convergence between the North and South China blocks. We apply sandstone petrology, geochemistry, and U-Pb detrital-zircon geochronology to constrain the provenance of the Carboniferous Meishan Group and to document its potential tectonic significance. We conclude that the Meishan Group received most sediment directly from early Paleozoic continental island arc rocks that are currently missing in the Dabie orogenic belt, with minor contributions from middle Neoproterozoic magmatic rocks of the South China Block and recycling of Archean to Proterozoic basement rocks of both the North and South China blocks. Compilation and comparison of detrital zircons and geochemistry data of the Silurian–Devonian and Carboniferous units suggests that all of them share similar source areas, but that individual contributions from each source were different. These results support the hypothesis that the Dabie orogenic belt developed a similar Paleozoic accretionary system, and shares a similar tectonic history, with the Qinling orogenic belt. These provenance patterns can be explained by a model of oblique convergence between the North and South China blocks during the Paleozoic. The South China Block was obliquely subducted beneath the North China Block with its opening to the east, forming an eastward-widening sedimentary basin. As a result, the eastern part of the basin received more sediment from the northern passive margin of the South China Block, while the western part of the basin received more material from the southern active margin of the North China Block.

New Ediacara-type fossils and late Ediacaran stratigraphy from the northern Qaidam Basin (China): Paleogeographic implications

Ediacara-type macrofossils characterize the late Ediacaran Period and are pivotal in understanding the early evolution of animals on the eve of the Cambrian explosion and useful in late Ediacaran biostratigraphy. They have been discovered on almost all major paleocontinents, except the North China and Tarim blocks, as well as on a series of northwestwest–oriented cratonic fragments between the two blocks, including the Olongbuluke terrane of the Qaidam block, where the terminal Ediacaran successions developed. We report a newly discovered terminal Ediacaran biotic assemblage, the Quanjishan assemblage, containing Ediacara-type fossils from the Zhoujieshan Formation of the Quanji Group in the Olongbuluke terrane, Qaidam Basin, northwestern China. The Quanjishan assemblage is dominated by the non-biomineralized tubular taxon Shaanxilithes, which has the potential to be a terminal Ediacaran index fossil, and by the iconic frondose rangeomorph Charnia, which represents the only unambiguous Ediacara-type fossil discovered in northwestern China. The co-occurrence of Charnia and Shaanxilithes from the Quanjishan assemblage likely constrains the depositional age of the Zhoujieshan Formation to be terminal Ediacaran (ca. 550–539 Ma) and the immediately underlying Hongtiegou diamictites to be late Ediacaran, probably representing post-Gaskiers glacial deposition. The occurrence of post-Gaskiers Ediacaran glaciation and similarities between the late Ediacaran–early Paleozoic lithostratigraphic and biostratigraphic sequences in the Olongbuluke terrane of the Qaidam block and the North China block suggest that these two blocks may have been located close to each other during this time period, and situated in the middle to high latitudes instead of the equatorial region.

Supplemental Material: High-precision U-Pb age constraints on the Permian floral turnovers, paleoclimate change, and tectonics of the North China block

Stratigraphy of the study sections, previous geochronology, U-Pb analytical procedures, age results, and Bayesian age modeling.<br>