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.

Decoupling of Sr-Nd Isotopic Composition Induced by Potassic Alteration in the Shapinggou Porphyry Mo Deposit of the Qinling–Dabie Orogenic Belt, China

In our previous study on petrogenesis of quartz syenite and granite porphyry, the host rocks of the Late Mesozoic Shapinggou Mo deposit in the Qinling–Dabie orogenic belt, we found that the initial Sr isotopic composition of the host rocks is strongly affected by the degree of K-alteration. Here, we provide further isotopic evidence of the host rocks and their minerals to investigate the geochemical behaviour of trace elements and isotopes during the alteration and to explain the phenomenon of decoupling of Sr–Nd isotopic composition. The quartz syenite and granite porphyry are altered by K-alteration in varying degrees and have high K2O and Rb contents and low Na2O, CaO, Sr, and Ba contents. Rock samples of both quartz syenite and granite porphyry have variable Rb/Sr ratios and initial 87Sr/86Sr values (even < 0.70) but contain quite homogeneous εNd(t) values (−12.8 to −14.8). Minerals from the rocks of moderate to intense K-alteration have very low initial 87Sr/86Sr values (even < −17), while those from the weakly altered rocks have 87Sr/86Sr(t) values of 0.7044 to 0.7084. The same phenomenon of the decoupling in Sr–Nd isotopic composition can be observed from several Mo deposits within the eastern Qinling–Dabie orogenic belt. This fact suggests similar hydrothermal features and a comparable origin for both the magmatic rocks and hydrothermal fluids in this belt. A comparison between porphyry Mo and porphyry Cu deposits shows that elements and the Rb–Sr isotope system have different behaviours during the K-alteration, implying distinct material sources and igneous rocks for porphyry Mo and porphyry Cu deposits, respectively.

Seismic evidence for the subduction of North China Block to Yangtze Block beneath Tongbai-Dabie Orogenic belt

&lt;p&gt;The Tongbai-Dabie Orogenic belt formed in the Middle-to-Late Triassic through a collision between the Yangtze Block (YB) and North China Block (NCB) and is a key component of the Central Orogen of China, which is famous on the most extensive high and ultrahigh pressure (HP/UHP) metamorphic zone in the world and marks the irregular suture between the YB and NCB. It is an ideal place to study the ancient orogenic processes between collided continents. In this study, we used a large number of P-wave arrival times recorded by portable and permanent seismic stations to reveal the structure of the crust and upper mantle beneath the Tongbai-Dabie orogenic belt and its adjacent region. Our images show the south-dipping high-velocity anomalies beneath the Tongbai-Dabie orogenic belt and the east-dipping high-velocity anomalies beneath the Tanlu Fault, which represent the southeastward subducted NCB in Mesozoic. While a huge high-velocity anomaly beneath the Wudang Moutin region extending down to 250 km is possible the ancient lithosphere of the Yangtze Craton remnant since the Paleoproterozoic. The northward subducted YB is only limited in the Eastern Dabie terrane and Yangtze foreland. Break-off retained Paleo-Tethyan oceanic slab are revealed at depths from the upper mantle 250 to 400 km. The structure of the crust and upper mantle suggests that the southeastward subduction of NCB resulted in the collision of NCB with YB.&lt;/p&gt;

Lithospheric structure of Hubei Province, central China, from Rayleigh wave tomography: insight into the spatial contact relationship between the Yangtze Plate and the eastern Qinling-Dabie orogenic belt

SUMMARY Knowledge about the spatial contact relationship between the Yangtze Plate and the eastern Qinling-Dabie orogenic belt can not only provide a scientific basis for the exploration of mineral resources, disaster prevention and earthquake prediction in the region, but also play an important role in reconstructing the geological process of the central orogenic belt. Hence, high-resolution lithospheric velocity model is essential to address these geological problems. In this study, using waveform data from 48 stations in Hubei Province and adjacent regions, central China, we invert for a 3-D S-wave velocity structure model of the crust and upper mantle from Rayleigh wave tomography. Our model reveals the complex subduction pattern of the Yangtze Plate to the north and the thrust-nappe tectonics of the Qinling-Dabie orogenic belt along the Mianlue suture with different scales and different deformation strengths. In addition, in the central part of Hubei Province, the local Yangtze slab has been broken into several pieces, among which the upwelling low-velocity anomalies appear. Moreover, the southern margin of the Dabie orogenic belt has undergone thrusting-nappe movement, and a series of associated structures are formed in the northern margin of the middle Yangtze platform. The contact zone between the two blocks in this area is composed of a series of thrust faults with dextrorotation slip component. Finally, based on the 3-D S-wave velocity image of Hubei Province and its vertical cross-section profiles along three different directions, three dynamic models are proposed to explain the spatial contact relationship between the Yangtze Plate and the eastern Qinling-Dabie orogenic belt in different regions.