South China Plate Research Articles

Wave velocities and anisotropy of rocks: Implication for origin of low velocity zone of the Qinling Orogenic Belt, China

Structure and composition of Earth is of fundamental importance in exploring the dynamic evolution of the crust and mantle. The Qinling Orogenic Belt (QOB) is located between the North China plate and the South China Plate, and is one of the main orogenic belts in China. To explore the composition and origin of anisotropy and the low wave velocity zone of the QOB, ten rock samples (gneiss and schist) were collected from the five sites of the QOB and the P- and S-wave velocities of these samples were measured under 0.6 to 2.0 GPa and 100 to 550 °C. The wave velocities increase with increasing pressure and decreasing temperature. The VP and VS of the schist and gneiss match the velocity of the middle and lower crust of the QOB, indicating that schist and gneiss are important component of the QOB. All the schist and gneiss samples exhibit obvious seismic anisotropy with 1.64%–17.42% for VS and 2.93%–14.78% for VP under conditions of crust and upper mantle. The CPO/LPO and layering distribution of mica in rock samples are the main reasons for this anisotropy. The VS structures below the five sampled sites from seismic ambient noise tomography were built to explore the effect of schist and gneiss on the composition and structure of the QOB. The results indicate that orientation-arranged gneiss and schist driven by the tectonic stresses might be a new origin of the character of VP/VS, seismic anisotropy, and the low velocity zone in the QOB.

Zircon U-Pb geochronology, major, trace elemental, and Sr-Nd-Pb isotopic geochemistry: constraints on the age and magmatic origin of Mesozoic mafic dykes in the Guangxi Zhuang Autonomous Region, South China

ABSTRACT As a direct product of lithospheric extension, the mafic dyke group have become an ideal object for studying the formation, evolution and dynamics of both the crust and continental orogenic belts. Given the scientific significance of mafic dykes and their utility in addressing outstanding questions concerning the origin and development mafic dyke swarms Mesozoic age with-in the South China Plate, in this study, 54 representative mafic dykes within the Guangxi Zhuang Autonomous Region, South China, were examined. Research methods include laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U-Pb age-dating, whole rock geochemistry, and the Sr-Nd-Pb isotope geochemistry of the studied mafic dykes. The results indicate that these rocks formed between 139 Ma and 113 Ma and have typical doleritic and lamellar textures. They fall into the alkaline and shoshonitic series, are characterized by enrichment of light rare earth elements, some large ion lithophile elements (Rb, Ba, and Sr), Th, U, and Pb, and have depleted Nb, Ta, Hf, and Ti. Moreover, the mafic dykes studied have high initial87Sr/86Sr ratios (0.7055–0.7057), negative εNd(t) values (from −15.2 to −11.4), and relatively constant initial Pb isotopic ratios (that are Enriched Mantle-1 (EM I)-like; 206Pb/204Pb = 16.765–16.816, 207Pb/204Pb = 15.429–15.476, and208Pb/204Pb = 36.882–37.144). The above results indicate that the group of dolerites and lamprophyres studied herein were likely derived from magma generated via low-degree partial melting (1.0%–10%) of an EM1-like garnet-lherzolite mantle source. The parental magmas to these fractionated olivine-, clinopyroxene-, plagioclase, and Ti-bearing phases, with negligible crustal contamination during ascent and dyke emplacement. On the basis of existing research and evidence from this study, we propose a reasonable model for the origin of the mafic dykes: the ancient Pacific Plate subducting below the South China plate led to extension of the lithosphere, an upsurge of the asthenosphere, which triggered partial melting of the lower mantle lithosphere in the Early Cretaceous, and the formation of the parent magma to the Mesozoic mafic dykes in the Guangxi Zhuang Autonomous Region study area. The involvement of slab-derived fluids from the subduction of the ancient Pacific Plate was responsible for metasomatism of the mantle source to these mafic magmas.

Organic matter accumulation in response to tectonism: New data from the Upper Permian Dalong Formation black shales in the Western Hubei Basin, South China and its implications for the end-Permian mass extinction

Redox conditions and primary productivity are proposed to be the major controls of organic matter accumulation in sediments. Studies of the impacts of tectonic processes on organic matter accumulation are still limited. We took the Late Permian-Early Triassic Western Hubei Basin, located in the north of South China, as an example. In black shales in the Dalong Formation, bottom-water redox conditions and surface-water productivity are constrained as shown by the analysis of total organic carbon (TOC) as well as geochemical (enrichment of Mo, U, Ni, Cd and Zn) and mineralogical characteristics. The highest productivity is present in the lower Dalong Formation, and the most intensively anoxic and restricted conditions are found in the middle Dalong Formation. The correlation of the sedimentary successions on the two sides of the basin's boundary fault indicates a subsidence of the basement during the deposition of the lower-middle Dalong Formation, followed by an uplift during the deposition of the upper Dalong Formation. When looked at in conjunction with the tectonic background of the basin, the new data in the present study support that: (1) the collision of the South and North China plates controlled the paleoceanographic and hydrographic evolution of the Western Hubei Basin, as well as organic matter accumulation in sediments; (2) upwelling-induced high productivity and good preservation in an anoxic environment jointly contributed to organic matter enrichment in the lower Dalong Formation, while good preservation in an anoxic environment was the major cause of organic matter enrichment in the middle-upper Dalong Formation. This study indicates that tectonism can act as the ultimate drive mechanism for organic matter accumulation in black shales. Our data also suggest that plate tectonism in related to the assembly of East Asian blocks with the main body of Pangea can also be treated as one of the ultimate drive mechanisms of the end-Permian mass extinction as it created the restricted, anoxic Paleo-Tethys Ocean, which greatly disturbed- global seawater circulation.

Metamorphic P-T-t path of Triassic eclogite from the Rongcheng region, eastern China: implications for the tectonic evolution of Sulu orogenic belt

ABSTRACT Interpretation and inversion of P-T (Pressure-Temperature) paths of high-pressure (HP) and ultrahigh-pressure (UHP) metamorphic rocks are critical to understanding the subduction and exhumation histories of an orogen. The Sulu orogen constitutes the eastern part of the Dabie-Sulu orogen, in which, HP metamorphic rocks are relatively poorly as studied compared to the well-documented UHP metamorphic rocks. This study presents the P-T paths of HP eclogites constructed by petrography, mineral chemistry, and pseudosection modelling by using the GeoPS software within the MnNCFMASKHTO (Mn-CaO-FeO-MgO-Al2O3-SiO2-K2O-H2O-TiO2-O2) system. Sample 19LR70–1 mainly consists of omphacite, garnet, quartz, minor rutile, ilmenite, and amphibole. Sample 19LR71–1 is mainly composed of omphacite, garnet, kyanite, and accessory minerals of rutile and amphibole. These two eclogite samples recorded clockwise P-T paths with peak P-T conditions of P ≥ 21.3 kbar, T ≥ 755°C and P ≥ 23.0 kbar, T ≥ 871°C, respectively. Well-correlated U-Pb ages and trace element data show that the protoliths of eclogites were 655–699 Ma and 676–754 Ma, the metamorphic ages were 212–232 Ma, and the time of partial melting were 221–225 Ma. Combined with regional geological data, we conjectured that before 240 Ma, the South China plate was subducted to the North China craton; subsequently, at 240–220 Ma, HP eclogite facies metamorphism occurred at a depth of 60–70 km. This was followed by the exhumation of these HP metamorphic rocks, coupled with concurrent partial melting, around 220–200 Ma.

New data on Silurian (Llandovery) sinacanthids from Wuhan, South China and their biostratigraphic and paleobiogeographic implications

Sinacanthids are a group of Silurian spine-based mysterious taxa, probably representing a lineage of stem chondrichthyans. New sinacanthid fossils attributed to Sinacanthus wuchangensis, S. triangulatus, Sinacanthus sp., Neosinacanthus planispinatus, Tarimacanthus bachuensis, Eosinacanthus shanmenensis, and fin spine morphology 1, 2 and 3 are identified from the upper member of the Qingshui Formation and the Fentou Formation (Telychian, Llandovery) of Wuhan, South China. These sinacanthid fin spines are similar to those from the Rongxi Formation in northwestern Hunan and the Ymogantau Formation in the northwestern Tarim. This study provides further evidence for the presence of diverse sinacanthid faunas from the South China plate. As the part of the Zhangjiajie Vertebrate Fauna of China, they have great significance in the paleogeographic division of vertebrates and the regional stratigraphic division and correlation. The Silurian sinacanthids from China were mainly grouped into three assemblages, i.e., the Wentang/Tataertag assemblage (early Telychian), the Fentou/Ymogantau assemblage (middle Telychian), and the Maoshan assemblage (middle to late Telychian). In addition, the sinacanthids from the South China plate, Tarim plate, and Qiongzhong block belong to the Zhangjiajie Vertebrate Fauna and there was certain extent of migration of sinacanthids among these areas during the Telychian, Silurian.

Thermal Evolution, Hydrocarbon Generation, and Heat Accumulation of a High Geothermal Coalfield: A Case Study of Pingdingshan Coalfield, China.

As an important energy base in central China, the Pingdingshan coalfield has abundant coal and geothermal resources. The cooperative exploration of coal and geothermal resources is significant for the comprehensive utilization of energy resources. This work collected coal-bearing samples from the Pingdingshan coalfield to investigate the tectono-thermal evolution of a high geothermal coalfield, especially the present geothermal field and hydrocarbon generation model. The geochemical results show that the Shanxi and Taiyuan source rocks have average R o values of 0.88 and 0.97%, respectively, with an average Rock-Eval T max value of 442 °C. Hydrocarbon generation of source rocks started at ∼205 Ma, with the highest rates at ∼170 Ma, reaching the maximum transformation ratio of 40-50% in the middle of the Early Cretaceous. The age and length of apatite fission tracks (AFTs) indicate that coal-bearing strata underwent significant post-depositional annealing after the Late Permian and suggest an abnormal thermal event that occurred in the Late Mesozoic. Meso-Cenozoic thermal event was mainly caused by the plutonic metamorphism of the Early Jurassic and magmatic thermal metamorphism of the Early Cretaceous, achieving a maximum paleotemperature of ∼140 °C. The magmatic thermal event resulted from the intensive post-orogenic extension of the Qinling-Dabie Orogenic Belt caused by the tectonic transition of the North and South China Plates. The present-day high geotemperature of Pingdingshan Coalfield is dominated by the horst structure caused by the regional extension of the basin-mountain system. The Cambrian limestone with a high thermal conductivity underlying coal measure collects deep heat, forming a heat accumulation center of this horst structure with a heat flow of 74 mW/m2 and a maximum temperature of ∼50 °C nowadays.

Continent‐Continent Collision Between the South and North China Plates Revealed by Seismic Refraction and Reflection at the Southern Segment of the Tanlu Fault Zone

AbstractThe Tanlu Fault Zone (TFZ) is considered as a suture of the intra‐continental collision between the South and North China plates, and it has attracted considerable geological, geochemistry, and geophysical investigations. However, its deep structures still lack clear delineation and the related tectonic processes are still debated without a clear consensus. To better characterize the TFZ as a plate boundary in east China, a 2‐D land seismic survey extending 87 km was conducted across its southern segment to investigate the underlying complex structures. Acoustic full‐waveform inversion (FWI) is applied to the early arrivals of the land seismic data to reconstruct the high‐resolution P‐wave velocity model of the upper crust (above ∼3.5 km). Structures of the crust and the uppermost mantle are further constrained through deep seismic profiling (DSP). The integrated geological interpretation from the FWI and DSP results suggests massive magma upwelling activities, characterized by the reflection‐free zone revealed by DSP and the high‐velocity anomaly recovered by FWI. The TFZ in the study area is found to be a deeply seated fault system with a positive flower structure at its shallow part, which suggests it has undergone a transpression regime. Also, the DSP result provides evidence for the subduction of the South China plate under the North China plate. Finally, a three‐stage model including the compression, transpression, and extension stages is proposed based on the new seismic results to better constrain the tectonic evolution of the southern segment of the TFZ.

Silurian phyllocarid crustaceans (Phyllocarida, Archaeostraca) from South China

In the Silurian, the suborder Caryocaridina of the phyllocarids was replaced by the suborder Ceratiocaridina as the dominant group. The latter did not achieve a global distribution until the late Silurian. In the early Silurian, ceratiocarids were a little-diversified group and palaeogeographically restricted mainly to Laurussia. Although previous studies have mentioned phyllocarid fossils from the Silurian of the South China plate, these have never been systematically described. This study describes three genera and four species (including one new genus, two new species, and two undetermined species), i.e. Cugocaris future gen. et sp. nov., Gonatocaris wuhanensis sp. nov., Gonatocaris sp. and Warneticaris sp., based on 38 phyllocarid specimens from the Fentou Formation in Wuhan. Cugocaris future gen. et sp. nov. is distinguished by an elliptic carapace ornamented by sinuous and anastomosing longitudinal striae, a styliform telson, and slender furcal rami. Gonatocaris wuhanensis sp. nov. is characterized by anastomosing longitudinal striae on the carapace, greatest carapace width in the centre of the carapace, and three different types of ornamentations on the surface of the abdominal segments. A phylogenetic analysis of 41 Palaeozoic phyllocarids reveals that Gonatocaris, Rolfecaris and Cugocaris gen. nov. form a monophyletic group. Thus, a new family, Gonatocarididae fam. nov., is proposed. This family is characterized by a carapace lacking median dorsal plate, rostral plate, a well-developed anterior carapace horn, and ornamented with prominent raised, wavy, anastomosing longitudinal striae. A palaeoecological analysis suggests that G. wuhanensis was a fairly active swimmer, while C. future was necto-benthic with relatively weak swimming abilities. This study not only extends the known morphological range of phyllocarids, but also has implications for the functional morphology, taxonomy, and evolution of archaeostracans as a whole. http://zoobank.org/urn:lsid:zoobank.org:pub:4B6B60B6-19F1-4A58-AC65-3B1AEE5D0939

Linking South China Plate to Arabian margin of Gondwana: Significance for Cambrian global plate reconstruction

Large-scale plate convergence processes led to the assembly of the Gondwana supercontinent during the Ediacaran–Cambrian. Herein, we present the results from petrological and geochemical studies on fine-grained sedimentary rocks belonging to the Lower Cambrian strata from the western part of South China Plate (SCP) in the Yangtze terrane, to reveal the location of SCP in the Gondwana supercontinent. Our results show that these rocks are of high compositional and textural maturity and belong to the same tectono-sedimentary cycle. The rare earth element (REE) distribution curve standardized by Post-Archean Australian Shale is similar to that for typical post-Archean shale. Characteristics of major and trace elements indicate that these fine-grained sedimentary rocks might mainly be derived from moderately felsic magmatic provenance with only a minor supply from mafic–ultramafic sources. Trace element ternary discriminant plots reveal the geotectonic setting as a continental island arc, although some passive margin sediments might have also been involved; R-type cluster analysis based on trace elements and REEs also shows that the tectonic setting of the sample provenance area is most likely to be that of continental island arcs and active continental margins. Combined with the published detrital zircon geochronological data of SCP and global tectonic-magmatic activity during the Cambrian, we propose that the Lower Cambrian clastic sediments in SCP were mainly supplied by the Cadomian island arc along the northern margin of Gondwana during the Ediacaran–Early Cambrian. Based on the above analysis, we believe that SCP might have been located closer to Arabian Plate than to Australian Plate.

High-resolution crustal and upper mantle shear-wave velocity structure beneath the central-southern Tanlu fault: Implications for its initiation and evolution

The Tanlu fault is a well-known lithosphere-cutting, strike-slip fault in eastern China and it has played an important role in regional plate tectonics since the middle Mesozoic. However, the initiation mechanisms and tectonic evolution of the Tanlu fault remain controversial, in part due to the absence of constraints from deep structures. In this study, we inverted 5–150 s surface-wave dispersion data collected from 253 permanent stations and 111 temporary stations in the vicinity of the central-southern Tanlu fault and obtained both isotropic and azimuthally anisotropic shear-wave velocity (Vs) models in the crust and upper mantle simultaneously. The new Vs models reveal unprecedented multiple patterns of anomalies. There is, for example, an arc-shaped pattern of fast directions found in the South China plate (SCP), providing direct evidence of the sudden termination of the Tanlu fault at its southern end; orogen-parallel fast directions and high-velocity anomalies were detected beneath the Dabie and Sulu orogens, which are believed to be the residue of the delaminated lithosphere. Based on the evidence collected, we propose a four-stage tectonic model for the Tanlu fault. The tectonic model suggests that the area has been subjected to multiple geologic processes since the Early Mesozoic, including oceanic-continental collision, continental-continental collision, break-off of oceanic slabs, lithospheric delamination, and convective erosion resulting from corner flow. As such, the proposed tectonic model reconciles the newly discovered Vs anomalies as well as various previous findings, providing profound insight into regional deformations and the evolution of the Tanlu fault and its adjacent areas.

A Sidneyia-Like Euarthropod from the Guanshan Biota (Cambrian Series 2, Stage 4), Eastern Yunnan, Southwest China

Sidneyia inexpectans was first excavated in Laurentia, and since then Sidneyia and Sidneyia-like euarthropods have been reported from different continents. Here we describe a new Sidneyia-like euarthropod from the Guanshan biota and, its preserved dorsal exoskeleton and appendages resemble those of S. inexpectans. The discovery in the Guanshan biota provides a new data point of Sidneyia-like euarthropod in Cambrian Stage 4 and a second occurrence on the South China plate.

The Proto- and Palaeo-Tethys tectonic evolution in Southeastern Tibetan Plateau: Constraints from detrital zircon dating of metasedimentary rocks from the Diancang Shan complex

ABSTRACT Metamorphic rocks from the Diancang Shan (DCS) complex along the Ailao Shan-Red River belt between the South China plate and Simao-Indochina block are studied in this contribution to constrain the Proto- to Palaeo-Tethys tectonic evolution of the eastern Tethys domain. LA-ICP-MS zircon U-Pb geochronological dating is conducted in this study on the metasedimentary rocks (garnet-biotite-plagioclase gneisses, biotite-quartz schists and phyllonites) from different stratigraphical horizons (Proterozoic and Triassic) of the DCS complex. The analyses yielded three major age peaks at ca. 248 Ma, ca. 441 Ma, ca. 1000–900 Ma and one subordinate age peak at ca. 828 Ma. The youngest ages for the samples help to constrain the maximum ages of deposition of the complex at Early Carboniferous and Late Triassic, respectively. The existence of the diagnostic age peak at ca. 440 Ma and the euhedral-subhedral morphology of the detrital zircons in the Palaeozoic and Mesozoic strata may indicate that there were early Palaeozoic magmatic activities along the eastern margin of the Simao-Indochina block. At the meantime, the age peak at ca. 440 Ma was never reported in the early Palaeozoic strata from the western margin of the South China plate, which supports the existence of an early Palaeozoic rift zone, i.e., the Proto-Tethys Jinsha Jiang-Ailao Shan-Song Ma rift zone, being consistent with geochemical discriminations. Furthermore, we conclude, from the significant difference between detrital zircon age data of Silurian to Devonian sedimentary rocks from the Simao-Indochina block and South China plate, the opening of the Palaeo-Tethys ocean since Early Carboniferous, the absence of evidence for the Palaeozoic orogeny, and the successive sedimentation from the Cambrian to Triassic along the western margin of the South China plate, that the Proto-Tethys rifting and subsequent Palaeo-Tethys ocean evolution along the Jinsha Jiang-Ailao Shan-Song Ma zone formed actually a complete ‘Wilson Cycle’ from early Palaeozoic to the Late Triassic.

Mesozoic–Cenozoic tectonic controls on the formation of large‐scale metamorphic rock buried‐hill reservoirs in Bozhong Sag, eastern China

Buried‐hill reservoirs are important targets for petroleum exploration. The number of buried‐hill oil–gas fields with reservoirs hosted in metamorphic rock is low, and metamorphic rock buried‐hill reservoirs deeper than 4,500 m (ultra‐deep) are even rarer. Ultra‐deep metamorphic rock buried‐hill reservoirs are characterized by highly heterogeneous lithologies and structure and have complex formation mechanisms, but such reservoirs have received relatively little research attention compared with reservoirs hosted in other rock types and at shallower depths. In the Bozhong Sag in the Bohai Bay Basin, Archean metamorphic rocks are buried at depths of 4,500–6,000 m, and their anticlinal structures are thus defined as ultra‐deep buried hills. Owing to their burial depth and compacted overburden, tectonism has been a key control on the formation of high‐quality reservoirs by causing deformation such as fractures. In this study, using results of drill‐core analyses, thin‐section imaging, and rock mechanical tests, we investigated the distribution and formation mechanisms of high‐quality reservoirs within the Bozhong 19‐A metamorphic rock buried hill. We also examined the formation mechanism of fractures in the Bozhong 19‐A buried hill with respect to early‐stage initiation by compression and late‐stage reactivation by transpression. Results show that intense compression‐related overthrusting caused by collision between the South China and North China plates during the Indosinian (252–237 Ma) orogeny generated initial fractures in the metamorphic rock buried hill and the formation of numerous NW‐trending nappe structures. Subsequent Yanshanian (163–66 Ma) sinistral transpression not only reactivated pre‐existing fractures, but also resulted in the development of numerous new fractures. In addition, in low‐stress regions such as anticline flanks and the lower parts of structures, compression closed the pre‐existing fractures, leading to lower fracture densities. Results of acoustic emission mechanical testing indicate that Himalayan (66–0 Ma) extensional stress was insufficient to produce large‐scale fracture systems in the Archean metamorphic rock, but was crucial for reactivating pre‐existing early‐stage fractures. Thus, the Himalayan period was a key interval for the formation of effective fractures. Therefore, rocks that underwent large‐scale overthrusting during early‐stage (Indosinian) fracturing and subsequent late‐stage (Yanshanian and Himalayan) reactivation favoured the development of ultra‐deep metamorphic rock buried‐hill reservoirs. This study should provide guidance for petroleum exploration in buried hills in superimposed basins that underwent early‐stage compression, late‐stage transpression, and final extension.

First record of ostracods from the Upper Ordovician red-coloured marine sandstones of the Tierekeawati Formation in Tarim Basin, NW China: implications on palaeoenvironment and palaeobiogeography

Ostracods are described for the first time from the red-coloured marine sandstones of Arisu section (Arisu red beds) of the Upper Ordovician Tierekeawati Formation in Kalpin area of northwestern Tarim Basin, Xinjiang Uygur Autonomous Region (Xinjiang), Northwest China. Twenty-two species belonging to thirteen genera are described and figured. The ostracod fauna suggests a probable Sandbian–Katian age for these beds. The palaeoecological assemblage of ostracod fauna implies the deposition in a nearshore-offshore environment during a regression when the Arisu red beds of the Tierekeawati Formation were laid down in the Tarim Basin. Many cosmopolitan and provincial genera were present in diversified ostracod fauna of the Arisu red beds, suggesting the possible biogeographic relationships among the Tarim, Tibet, and South China plates, as well as Europe and North America continents during the Late Ordovician. Ostracods experienced faunal exchanges between Laurentia and the Tarim Plate during the Late Ordovician Period.

Timing of Triassic tectonic division and postcollisional extension in the eastern part of the Jiaodong Peninsula

The Jiaodong Peninsula is a key region for researching the interaction between the North and South China Plates. Tectonic relationships between collision, exhumation of ultra-high-pressure (UHP) slabs, strike-slip faulting and gold mineralization, are still ambiguous. The eastern part of the Jiaodong Peninsula (Eastern Jiaodong), which includes Triassic intrusions and is less affected by the Tan-Lu Fault Zone, is a key area to examine exhumation dynamics in detail. Systematic field mapping and zircon U–Pb dating of Triassic intrusions establishes that: (1) The UHP wedge in the eastern part of the Jiaodong Peninsula can be divided into the lateral and frontal ramps of a thrust and nappe system. Dating of samples from Donglinghou (244.7 ± 4.2 Ma) and Qingyutan (233.8 ± 8.1 Ma) areas indicates that the collision happened at or before the Middle Triassic. (2) Postcollisional extension intrusions including the Shidao granitoid (216.2 ± 2.4 Ma), and the Chengshantou granitoid (cut by a dolerite dyke dated at 210.5 ± 1.0 Ma), generally strike NE and occurred in a metamorphic core complex below the Upper Jiashan-Xiangshui detachment (U-JSXS). (3) Regional faults in the Jiaodong Peninsula exploited syncollisional foliations of the UHP wedge, which resulted in faults dipping towards the NW and SE. The reactivation of the Lower Jiashan-Xiangshui shear zone (L-JSXS) and its overprinting upon the Tan-Lu fault system may have caused another major episode of exhumation of the syncollisional wedge, and could have been responsible for an extensional environment that favored gold mineralization.

The formation of the Dabashan orocline, central China: Insights from high-resolution 3D crustal shear-wave velocity structure

The Dabashan orocline is a large thrust belt in central China. Determination of its high-resolution three-dimensional (3D) crustal shear-wave (S-wave) velocity structure can enhance our understanding of the intracontinental evolution of the South China plate and North China plate. In this study, we estimated the Rayleigh wave phase and group velocities at periods of 5–50 s, P-wave receiver functions, and Ps and SsPmp travel times from 63 permanent stations and 29 portable stations; we also constructed a 3D S-wave velocity model and crustal thickness and Vp/Vs ratio maps of the Dabashan orocline and adjacent areas. The crustal thickness and Vp/Vs ratio maps show prominent thick crust (50–55 km) and high Vp/Vs ratios (1.8–1.85), suggesting a mafic lower crust in the Dabashan orocline. The 3D S-wave velocity structure shows northeast-dipping low-velocity anomalies in the upper crust of the Dabashan orocline, and high velocities extended from near the surface to the lower crust in the Hannan-Micang and Shennong-Huangling domes, suggesting deep roots of the two domes. We propose that the two domes acted as rigid indenters and rotated clockwise together with the South China plate, penetrating into the Qinling-Dabie orogenic belt. Weak sediments in the upper crust of the Dabashan orocline were compressed and blocked by the two domes and the rigid Qinling-Dabie orogenic belt, resulting in vertical accretion of the sediments and crust and subsequent formation of the northeast-dipping low-velocity anomalies and the thick crust beneath the Dabashan orocline.

Mercury spikes at the Devonian-Carboniferous boundary in the eastern part of the Rhenohercynian Zone (central Europe) and in the South China Block

Mercury (Hg) chemostratigraphy is an efficient tool for tracing distal volcanism, as suggested in many recent studies. In our research, we focused on Hg and other trace element contents in two Devonian-Carboniferous boundary sections from two different and spatially distant areas. The Lesní lom section (Czech Republic) was located in the south-eastern part of the Laurussia plate and the Duli (Guangxi, China) section in the South China Plate. In both studied sections, the highest Hg, Hg/Fexs or Hg/Al values occurred at the Devonian-Carboniferous boundary. In Lesní lom, organic matter remineralization was connected with origin of pyrite which sequestered a part of the Hg content. The decoupling of Hg and TOC within the Duli section strata and the significant correlation with Al and Fe may suggest that Hg is hosted primarily by clay particles, derived from the landmass into the marine realm. Mercury was also incorporated into pyrites but the poor correlation between Hg and TS (total sulfur) indicates a possible loss of S during oxygenation events. In the Lesní lom section, and partly in the Duli section, Fe and Mn redox cycling may have played an important role in sediment Hg content. Both sections were influenced by widespread hydrothermal and volcanic activity, indicated by positive Eu anomalies, MSI, Fe/Ti, Zr/Al2O3 and Ti/Al2O3 values. It remains to be clarified whether, besides the interplay of local magmatic activity and redox oscillations, there may have been some additional influence of the multi-phase Viluy Large Igneous Province, although its younger phase (364.4 ± 1.7 Ma) slightly precedes the Hg anomalies obtained.

Provenance and tectonic setting of the Lower Cambrian Niutitang formation shales in the Yangtze platform, South China: Implications for depositional setting of shales

Geochemical compositions of the Lower Cambrian Niutitang Formation shales in the southeastern Yangtze Platform margin were investigated for provenance, tectonic setting, and depositional environment. The shale samples are characterized by higher abundances of large ion lithophile elements (Cs, Ba, and Pb), lower abundances of high field strength elements (Cr, Sc, and Co) and transition elements (Th, Zr, Hf, Nb, and Ta) relative to average shale. North American shale composition (NASC) -normalized rare earth element (REE) patterns are observed, with negative Ce anomalies, negative Eu anomalies, and positive Y anomalies. The chemical index of alteration (CIA) varies from 68.67–74.93. Alkali and alkaline element contents and CIA values suggest that the source rocks have undergone moderate weathering. The index of compositional variability (ICV), Zr/Sc and Th/Sc ratios vary from 0.53 to 1.07, 5.31 to 8.18 and 0.52–1.02, respectively. ICV values and relationships between Zr/Sc and Th/Sc ratios indicate negligible sedimentary recycling. The Al2O3/TiO2 (14–26) and TiO2/Zr (56–77) ratios imply that the source rocks of the investigated shales had intermediate igneous compositions. However, Cr/V ratios and a La/Th–Hf discrimination diagram suggest that the intermediate compositional signal of the source rocks was derived from a mixture of 75% mafic and 25% felsic igneous rocks rather than intermediate igneous rocks. The major source was the Jiangnan continental island arc with bimodal igneous rocks, lying to the south of the study area, together with a contribution from granites and gneisses uplifted and eroded in the Yangtze Block. Discrimination of tectonic setting using major and trace elements indicates that the source rocks originated in a transitional setting from active continental to passive margin, consistent with the failed intracontinental rift model for the evolution of the South China plate. The Niutitang Formation shales were deposited in a rift basin setting under conditions of anoxic bottom water in a redox-stratified water column, with organic-rich shales prospective for shale-gas production being found in deep-water downslope and basin environments rather than the shallow-water shelf.

A new Cathaysiorthis (Brachiopoda) fauna from the lower Llandovery of eastern Qinling, China

AbstractFollowing the end Ordovician mass extinction, brachiopod faunas were commonly of low diversity, generally rare in abundance, and restricted to shallow‐water environments in many places. The Cathaysiorthis fauna, first described from South East China, is the most diverse brachiopod fauna in the world succeeding the Hirnantia fauna, and has modified our previous views on the recovery of shelly benthos after the extinctions. Here, we report a newly‐discovered fauna with abundant Cathaysiorthis from the lower Zhangwan Formation (lower–middle Rhuddanian, lower Llandovery) in Qinling, Xichuan County, Henan Province, China. This fauna is slightly younger than that previously documented from South East China, and the evolution of Cathaysiorthis fauna, in terms of its composition, is discussed. The Qinling Cathaysiorthis fauna has a narrower bathymetric range (Benthic Assemblage Zone 3) than that of the fauna from South East China (BA 2–5) and occurred on the Qinling–Dabie Microplate located on the northwest margin of the South China Plate. The migration of the Cathaysiorthis fauna from the South China plate to Qinling–Dabie was achieved by larvae that used ambient ocean currents and possibly drifted from the Jiangnan Region of South East China to Qinling. The Qinling Cathaysiorthis fauna includes 13 species belonging to 12 genera. Two new species, Cathaysiorthis xichuanensis sp. nov. and Aegiria apta sp. nov. are established and described herein, and ten other species are discussed.

Multistage Remobilization of the Southwestern Margin of the South China Plate: Insights From Zircon U‐Pb Geochronology and Hf Isotope of Granitic Rocks From the Yao Shan Complex, Southeastern Tibet Plateau

AbstractThe Yao Shan complex, located at the southwestern margin of the South China plate, to the east of the Sanjiang‐Southeast Asia Tethyan domain, and at the northern extension of the Day Nui Con Voi massif of the Ailao Shan‐Red River shear zone, provides a reasonable example to study the multistage tectonic remobilization of continental plates. In this paper, we present new zircon LA‐ICP‐MS (Laser Ablation Inductively Coupled Plasma Mass Spectrometry) U‐Pb dating results and Hf isotopic data of the granitic plutons and dikes from the Yao Shan complex. Zircon U‐Pb ages of the granitic intrusions mainly concentrate on three periods: 825–780, 82–69, and 31–29 Ma. The zircons are characterized by mainly negative εHf(t) values with TDMC ages of 2637–1645, 1964–1610, and 2077–1400 Ma, respectively. The results show that the Yao Shan complex record evidences for three episodes of extensive partial melting of middle and lower crustal Precambrian crystalline basement rocks. They contributed to the granitic magmatism in relation to tectonic processes and structural deformation from Neoproterozoic, Late Cretaceous to Oligo‐Miocene. Therefore, we argue that the southwestern margin of the South China plate is the weakness for multistage remobilization throughout the tectonic evolution of the plate after its primary cratonization. Remobilization of the plate margin is related to oceanic plate subduction, triggered by rifting in a passive continental margin setting, induced by extension at back‐arc setting, or resulted from continental extrusion.