Search

The South China region has undergone a complex tectonic history since the Mesozoic era, characterized by significant tectonic regime transformations. However, the mechanism behind these transformations is still subject to debate. This study aims to elucidate the specific manifestations and stress mechanisms driving the tectonic transformations and their geological background. Through field geological surveys, effective stress inversions and analysis of lithofacies palaeogeography in Fujian Province, we have identified six distinct tectonic episodes that occurred during the Mesozoic: (a) late Middle Triassic to early Late Triassic: intense NNE–SSW compression generated an unconformity between the An'ren Formation and Dakeng Formation; (b) middle Late Triassic to early Middle Jurassic: NNE–SSW extension exerted control over the deposition of the Dakeng Formation, Wenbinshan Formation and Zhangping Formation; (c) late Middle Jurassic to early Late Jurassic: NW–SE compression resulted in an unconformity between the Zhangping Formation and Changlin Formation; (d) late Jurassic to early Cretaceous: NW–SE extension influenced the deposition of the Changlin Formation, Nanyuan Formation and Xiaoxi Formation; (e) late Early Cretaceous to Late Cretaceous: a strike–slip stress field caused an unconformity between the Xiaoxi Formation and Shaxian Formation; (f) post‐Late Cretaceous: NE–SW extension occurred. During the late Middle Jurassic to early Late Jurassic period, there was a notable change in the distribution of lithofacies palaeogeography. The coexistence of NE and E‐W‐trending patterns shifted to a predominant NE‐trending pattern, which was controlled by the palaeostress field transitioning from NNE–SSW extension to NW–SE compression. These changes indicate a tectonic regime transformation in Southeast China during this period. Considering the sedimentary, magmatic and tectonic activities, the tectonic evolution of Southeast China was predominantly influenced by the Palaeo‐Tethys tectonic domain from the Early Triassic to early Middle Jurassic and by the Palaeo‐Pacific tectonic domain from the late Middle Jurassic to the Cretaceous. [ABSTRACT FROM AUTHOR]

The Guang‐Tou‐Shan (GTS) granites, located in the Xing'an Block (XB), record Late Palaeozoic magmatism during the stage of convergence between micro‐blocks in the eastern Central Asian Orogenic Belt (CAOB). We performed a detailed geochronological and geochemical study of GTS granites intending to obtain better constraints on the subduction processes of the Palaeo‐Asian Ocean (PAO) in eastern CAOB. LA–ICP–MS U–Pb dating of zircons from GTS granitic rocks in western XB yielded emplacement ages of 332 ± 2 Ma and 301 ± 2 Ma, respectively, indicating a two‐stage emplacement process. Geochemical analyses show that the Early Carboniferous monzogranite (332 ± 2 Ma) have low values of the A/CNK and A/NK, are enriched in LILE (Rb, Sr, Ba) and LREE (LREE/HREE = 5.6–10.8), and depleted in HFSE (Nd, Ta, Zr, Hf) and P, Ti, which belong to I‐type granites. The Late Carboniferous granodiorite (301 ± 2 Ma) also displays LILE, LREE enrichment, and HFSE depletion with negative Nb–Ta anomalies, and contains relatively high Zr + Nb + Ce + Y contents (495–524 ppm) and high zircon crystallization temperature, indicating the Late Carboniferous granodiorite shows geochemical characteristics similar to those of typical A‐type granites. The two‐stage granitic rocks have εHf(t) values of +7.7 to +11.6 and yield a two‐stage depleted mantle Hf model age of 0.66–0.88 Ga, suggesting that the GTS granitic rocks were derived from partial melting of juvenile arc‐related mafic to intermediate rocks. The results, combined with regional magmatic information, indicate that the eastern CAOB experienced a subduction setting in the Early Carboniferous and extension setting in the Late Carboniferous. This change was most likely caused by the subduction slab break‐off of PAO from the Late Carboniferous. [ABSTRACT FROM AUTHOR]

The widespread migmatites in the northwestern part of the Sulu Orogen, China, indicate regional anatexis that is of great significance when discussing the tectonic evolution of this continental orogenic belt. Cathodoluminescence (CL) images, U–Pb ages, and in situ trace element compositions of zircons from four pegmatite veins within these migmatites provide clear evidence for the nature of the post-collisional evolution of the Sulu Orogen. The inherited zircon cores reveal that the protoliths of the migmatites were middle Neoproterozoic magmatic rocks (810–620 Ma) of the South China Block. The protoliths underwent two partial melting events. The mantle domains of the inherited zircons record a Late Triassic (222.0–204.0 Ma) partial melting event that occurred during the exhumation and retrograde metamorphism, after ultrahigh-pressure (UHP) metamorphism. Subsequent newly grown zircons record a Middle–Late Jurassic to Early Cretaceous (164.1–125.5 Ma) anatexis event, indicating that the late Mesozoic anatexis started before ca. 164.1 Ma, reached a peak at ca. 152.1 Ma, and ceased at ca. 125.5 Ma. Combined with previous results of studies on the Sulu orogen, the late Mesozoic anatexis suggested that the thickened crust of the Sulu Orogen had started to become unstable before 164.1 Ma. The duration of ~164.1–137 Ma corresponds to a period of transition in the tectonic regime of the Sulu Orogen, enabling the early high-temperature ductile deformation. After ca. 137 Ma, the tectonic regime was fully transformed into extension and the Sulu Orogen underwent rapid thinning and collapse, thus leading to the late medium–low temperature ductile deformation (137–121 Ma) and laying the foundations for the large-scale magmatic emplacement during the late Early Cretaceous (127–115 Ma). These two partial melting events together promoted the rapid exhumation of the Sulu UHP rocks. [ABSTRACT FROM AUTHOR]

This study investigates the stress-strain behavior, failure deformation, and change of gas permeability in shale under compressive stress induced failure. The study is based on triaxial compression of shale from Lower Silurian Longmaxi Formation, South China at different angle of bedding planes. It was found that shale behaves as a brittle materials and exhibits brittle and semi-brittle response to peak strength load at different angles of pre-exist bedding during stress compression. The gas permeability increased after failure deformation for all shale samples. However, samples under vertical and horizontal bedding angles exhibited brittle deformation and significant increase in permeability. While the samples with bedding angles of 30° and 60° showed semi-brittle deformation, and the permeability increased slightly after failure. These results indicate that the pre-existing structure influences the mechanical behavior of shale, which consequently affects its gas permeability. The results of this study are of great significance in shale gas exploration and development in areas of South China which has complex tectonic and pre-existing foliation structures. • Permeability increased after artificial structural deformation for Longmaxi shale. • Shale exhibit brittle and semi-brittle deformation with different stress orientation. • Permeability increased significantly after brittle deformation with compressed parallel and perpendicular. • Permeability increased slightly after semi-brittle deformation with compressed oblique. [ABSTRACT FROM AUTHOR]

The Mesozoic northern Yangtze foreland basin system was formed by continental collision between the North China and South China plates along the Mianlue suture. Synorogenic stratigraphic sequences of Late Triassic to Early-Middle Jurassic age were developed in the northern Yangtze foreland basin. The upper Middle Jurassic Shaximiao Formation consists mainly of thick-bedded terrestrial successions that serve as the main body of the basin-filling sequences, suggesting intense tectonism in the peripheral orogeny of the foreland basin. Laser-ICP-MS U-Pb analysis of 254 detrital zircon grains from sandstone samples and several published Lower-Middle Jurassic samples, detrital compositions, petrofacies, and paleocurrent reconstructions in the northern Yangtze foreland basin indicate that discrete source areas included the Qinling-Dabieshan ranges and the Mianlue suture zone to the north, and the South China plate to the south. The stratigraphic succession and sediment provenance of the foreland basin imply that the early Mianlue oceanic basin, magmatic arc, and nonmarine molasse foreland basin during the period of deposition were modified or buried by the subsequent continent-continent collision between the North China-Qinling-Dabieshan plate and the Yangtze plate during the Jurassic, which followed the oblique amalgamation between these plates during the Middle-Late Triassic. [ABSTRACT FROM AUTHOR]

There is an ongoing debate regarding the paleogeographic evolution of the Qaidam Block (37.3 °N/96.4 °E) in northwest China during the late Paleozoic. To provide a reliable constraint on the paleogeographic reconstruction and to determine the relationship with the adjacent area, we performed a paleomagnetic investigation of Early and Late Carboniferous sediments from the Qaidam Block. Stepwise thermal demagnetization successfully isolated high unblocking-temperature characteristic directions from the samples. The tilt-corrected mean direction of the Early Carboniferous sediments is Ds / Is = 298.6°/− 46.4° with α 95 = 4.7° and N = 15 sites, corresponding to a paleopole at − 3.2 °N, 147.5 °E with A 95 = 4.8°. The Late Carboniferous paleomagnetic direction is Ds / Is = 320.1°/− 46.1° with α 95 = 5.3° and N = 8 sites, corresponding to a paleopole at − 15.1 °N, 132.2 °E with A 95 = 5.4°. A positive fold test for the Early Carboniferous paleomagnetic directions, and consistency with the Kiaman reversed superchron for the Late Carboniferous paleomagnetic directions, indicates the primary origin of the characteristic remanence of the studied section. The new paleomagnetic results suggest that the Qaidam Block was located at about 27 °N and did not undergo significant N-S movement during the Carboniferous. However, the Qaidam Block experienced a counter-clockwise rotation of about 21° during this period. Comparisons of the paleomagnetic results from the major blocks of Central Asia suggest that the Qaidam Block may have experienced a relatively rapid northward movement and collided with the Tarim Block after the Late Permian. Based on the paleomagnetic poles of the Qaidam Block and adjacent blocks, we present a tentative paleogeographic reconstruction for central Asia during the Early and Late Carboniferous. [ABSTRACT FROM AUTHOR]

Marine shales in southern China experiences multiple complex tectonic movements, which highly affects shale pore characteristics and pore evolution. The effects of tectonic deformation on nanopore structure and petrophysical properties of organic shales remain unclear. Three ductile deformed shale samples were collected and analyzed through an integrated experiment procedure for a comparative research against undeformed shale samples. Field emission-Scanning electron microscopy (FE-SEM), low pressure N 2 and CO 2 adsorption and mercury injection porosimetry (MIP) analyses were performed to characterize pore structure of the shales. The results reveal that the mineral compositions of these shale samples are controlled by sedimentary environment and source and exhibits no obvious relationship with tectonic deformation. With increasing deformation degree, the micropore proportion gradually decreases (from 17% to 6%), and the macropore proportion increases from 16% to more than 20%. Micropore volumes and proportion in the fold core samples are the smallest (4.2 × 10−3 cm3/g and 6.3%) and account for only one-third of the undeformed shale values, while the meso- and macropore values are larger-1.2 times those of the undeformed shale values. The evolution characteristics and deformation modes of organic matter (OM), interparticle (interP) and intraparticle (intraP) pores and microfractures under ductile deformation were examined. All pore types become interconnected during deformation and form microfractures ranging from a few nanometers to tens of microns, which is the main reason for the enhanced shale connectivity caused by deformation. Under the same porosity conditions, the deformed shale permeability is much higher than the undeformed shale permeability, and stronger deformation corresponds to higher permeability. The fold core permeability is generally higher than the limb permeability and exceeds 30 times. The different ductile deformation positions were divided into five levels, and the corresponding reservoir quality was evaluated. This study is of great significance to better understand the pore structure evolution process for shale exploration and development under tectonic deformation. • Pore structure deformation develops widely during tectonic deformation. • Ductile deformation significantly affects the micro- and macropores of shale. • The fold core permeability is generally much higher than the limb permeability. • The pore interconnected during deformation promotes the development of microfractures and increases the permeability. [ABSTRACT FROM AUTHOR]