Yu Wang

The formation of tectonic–magmatic–sedimentary processes during the Permian in the Beishan region represents a highly debated research topic along the southern margin of the Central Asian Orogenic Belt and even globally: does it mark the final subduction and amalgamation of the Paleo-Asian Ocean, or does it instead represent rifting superimposed upon an earlier orogen? New field observations combined with geochemical analyses reveal that the Liuyuan area is dominated by Early Permian basalts, associated with a rifting sedimentary sequence. During the Mid–Late Permian, gabbro–rhyolite associations were emplaced, accompanied by minor lacustrine sedimentation. The late stage was characterized by minor granitic intrusions or dikes with adakitic affinities, culminating in the emplacement of lamprophyre dikes. The basalts and gabbros in the Liuyuan area display mantle-derived geochemical signatures, with compositions intermediate between MORB and OIB. The exposed Permian basalt–rhyolite bimodal magmatic suite represents a genetically integrated rift-related rock series. Geochemical data from the Ordovician granites and schists within the belt reveal adakitic characteristics, implying that the Permian granitic rocks largely represent remelting products of these early granitic and schistose protoliths. Collectively, the lithological characteristics and magmatic associations clearly demonstrate that the tectonic setting during the Early Permian corresponded to a post-collisional extensional environment superimposed upon the early Paleozoic orogenic belt (Caledonian Huitongshan ophiolite–arc accretionary orogen), which subsequently underwent tectonic inversion to form the present-day orogenic structure. This paper proposes a theoretical model wherein the bimodal magmatic suite was generated by the upwelling of enriched asthenospheric mantle material, providing the driving mechanism for rifting. It formed within a post-collisional extensional environment developed over a complex pre-existing orogenic belt and was subsequently inverted, forming the current tectonic belt—a typical intracontinental Pyrenees-type orogeny.

Concurrently, loess deposition, oceanic water systems, and the arid high-plateau environment have undergone significant transformations across different regions, particularly in western and eastern China, which are divided by two north-south-trending gravity gradient belts. What type of tectonic processes have primarily contributed to these fundamental environmental and climatic changes in the China continent over the past 25 million years, or even across the entire Asian continent? During this period, the subduction of the West Pacific Plate facilitated the development of the trough-arc island-basin system in eastern Asia, while the Indo-Asian collision led to the uplift of the Tibetan Plateau and the formation of various tectonic belts, mountain ranges, and rift basins between 25 and 20 Ma. The formation of east-west-, north-south-, and northeast-trending mountain belts and basins, whether in western or eastern China, corresponds to tectonic transformations and geodynamic events along the continental margin that were associated with the Indo-Asian collision and the westward subduction of the Pacific Plate. The emergence of the Asian monsoon during this time interval could be interpreted as a consequence of these tectonic transformations, rather than the direct cause of loess deposition or the complete uplift of the Tibetan Plateau. Furthermore, even in the past 10,000 years, seismic activity and volcanic eruptions have continued to correspond to tectonic processes along the continental margin. Regional and local environmental changes have been directly constrained by tectonic activity in these respective areas.

The theory of plate tectonics has developed over the past 60 years, and geologists have long regarded it as the dominant model for global tectonic processes. However, it still fails to fully explain continental tectonics and deformation. What, then, constitutes global tectonics? The relationship between plate tectonics and intracontinental tectonics, intracontinental orogeny classified according to different tectonic settings and evolutionary characteristics across the globe, are fundamentally absent. Furthermore, the far-field and near-field tectonic stresses, examining their modes of transmission and their roles in tectonic processes, are still unknown.Moreover, with regard to tectonic evolution, the role of the advection mantle in continental tectonics, alongside its influence on the global tectonic framework should be considered, but not merely mantle convection. By mantle advection, along with horizontal and vertical flow patterns, such as mantle upwelling and downwelling responding to the rifting and orogeny, plays a significant role. This includes shear zones and mantle extrusion beneath continents, rotated mantle flow and energy transmission around the continental lithospheric root, could provide new perspectives on tectonic processes. We suggest that various mantle flow patterns, leading to the clear proposition that global tectonics can be defined as the combination of transcontinental tectonics, plate tectonics, and intracontinental tectonics.

Since last 1960’s, the plate tectonics has been played a main role for the study on the Earth’s evolution and global tectonics, but mostly focused on the global divergence and convergence, and focused on the continental margin—subduction and collision. However, the plate tectonics cannot resolve all of the tectonic evolution and reconstruction of the global evolution, like non-rigid blocks and continental lithospheric deformation; and mountain building within the continent; large scales deformation and tectonics in the continental interiors and so on. Thus, “Intracontinental Tectonics and Orogeny” has been studied. Globally, there are lots of tectonics or deformation types have been found and the intracontinental mountain building and the orogeneses have been classified, like types of the Alice Spring in Australia, the Tianshan in Asia and the Pyrenees in Europe. They are with different orogenic frameworks including deformation, magmatism, sedimentation and metamorphism. Also, they have formed in different tectonic backgrounds, such as on the reworking orogenic belt, intracontinental rift-basin deformation, and multiple-stage orogeny between the continental blocks, and linkages to plate tectonics and non-plate tectonics in mechanism and dynamics.

True polar wander (TPW) or equatorial dipole (ED) field hypothesis have the potential to decipher the Ediacaran paleomagnetic enigma, based on the fast apparent polar wander (APW) rate revealed by the orthogonal directional groups. However, determining the final solution is challenging, as the conservatively estimated APW rates for both hypotheses are consistent given current chronological constraints. A precise APW rate is thus urgently needed. Fortunately, two distinct poles and a transition zone were identified from the astronomical calibrated Member 3 of the Doushantuo Formation in the Zhengjiatang section, South China. The APW rate was precisely constrained to be 243 ± 17°/Myr, an extremely fast rate which excludes an origin of fast plate motion or TPW. Based on the current global Ediacaran to Early Cambrian paleomagnetic data, the ED field at ca. 596–570 Ma is suggested to be responsible for the rapid directional change.

The Helanshan Mountain tectonic belt (HTB) is an intraplate deformation belt along the northwestern border of the Ordos Block in the North China Craton. When and why this intracontinental tectonic belt formed, its subsequent uplift and erosion, and the relationships between ranges and adjacent basins remain unclear. To better assess the connections between the temporal and structural activity in HTB, apatite fission-track (AFT) and zircon fission-track (ZFT) analyses were conducted in this study. The lack of adequate FT data from the HTB is a source of contention and dispute. This paper collected samples for AFT and ZFT techniques from the central and southern HTB, trying to improve the research. The ZFT and AFT ages could be divided into the following 7 groups: 279 Ma, 222–213 Ma, 193–169 Ma, 151–147 Ma, 130–109 Ma, 92–77 Ma, and 65–50 Ma. The inverse modeling results of AFT indicate 4 fast cooling episodes of 170–120 Ma, 120–95 Ma, 66–60 Ma, and ~10–8 Ma to the present. Combining the results of FT analysis with radial plot and inverse modeling of AFT, the following eight age groups are believed to reveal the distinct tectonic activities in HTB: the first age group of 279 Ma mainly represented the back-arc extension of the southern HTB; the age group of 222–213 Ma was bounded with NNE-SSE trending contraction between the South China block and North China Craton; the event of 193–169 Ma responded to the post-orogenic collapse followed after the second event; the 151–147 Ma group was interpreted as the eastward extrusion induced by the subduction between Qiangtang and Lhasa blocks; the Early Cretaceous (130–109 Ma) group was not only affected by the rollback of the Pacific Plate, but also denoted the collapse of the thickened lithosphere formed in the Late Jurassic; the Late Cretaceous (92–77 Ma) group was attributed to long-distance impact from the subduction of the Pacific Plate beneath the Eurasian Plate; the event during 65–50 Ma was a correspondence to far-field effect of the onset collision between the Eurasian and Indian Plates; and from 10–8 Ma to the present, the progressive collision of the Indian and Eurasian Plates have a significant impact on the HTB and the northeastern Tibetan Plateau.

The initiation of a continental rift or the birth of an oceanic basin, and their long-term evolution, are challenges in the understanding of the evolution of regional and global tectonics. Rifts develop within the interior or margins of plates, essentially as extensional structures, and they have been studied widely since the discovery of plate tectonics. The eastern segment of the Paleo-Tethys traversed central Asia, its structural features, timing, geological setting, and geodynamic mechanisms are poorly understood. The Songpan-Garze Terrane (SGT) on the northeastern Tibetan Plateau has been regarded as either a remnant of the Paleo-Tethys Ocean or a continental rift basin. Different interpretations of the SGT lead to various tectonic reconstructions between Gondwana and Laurasia. Here, sedimentology, geochemistry, and geochronology have been applied to the southern and northern parts of the SGT to reconstruct the initiation of a Permian rift system. In this area, for the first time, field investigation reveals that the SGT is characterized by a continuous succession of Permian−Lower Triassic terrestrial-neritic deposits and ocean island basalt (OIB)-type basalts, and that it developed as an intracontinental rift basin in the Yangtze Block. Zircons from gabbros with OIB-type geochemical signatures intruded in the southern SGT yield U-Pb ages of 263−254 Ma, constraining the timing of the initial rifting. Proterozoic felsic rocks collected from the southern SGT yield U-Pb ages of 761−865 Ma, and they are petrologically and geochemically comparable to Neoproterozoic basement rocks along the western margin of the Yangtze Block. The Middle−Late Permian volcano-sedimentary sequences of the SGT and their geochemistry and geochronology are comparable to rift sequences in the Panxi region of the western Yangtze Block. Thus, it has been proposed that the SGT originated as an intracontinental rift within the Yangtze Block during the Middle Permian−Early Triassic, and that it continued to deepen as a rift basin through the Middle−Late Triassic.

Crystal habit in igneous rocks provides a window to understand magmatic processes or reveal crystallization environments. Generally, we can obtain the two-dimensional (2D) crystal habits directly from the thin section, which is easy to access. However, the three-dimensional (3D) habit cannot be directly observed in thin sections and needs the stereological conversion from 2D habits. Statistical methods have been developed for stereological conversion, but they cannot identify mixed habits. Our study uses the distributions of the cut-sections of pre-set habits to match the unknown sample and enumerates habit combinations to find the best-match results for mixed habits. The specialized program, HabitEst3D, is developed according to our model in this study. The program is written in Python and is a cross-platform with a user-friendly graphical interface. The input data are the aspect ratio of 2D sections. After setting the parameters, the program finds the best-match estimations fitting the sample, visualizes the results, and saves them in multiple file formats. The program is robust and is not sensitive to outliers to obtain more accurate results. It traverses all possible combinations and needs memory and time but effectively explores the mixed crystal habits in the sample, contributing to investigating magmatic processes in more detail.

The theory of plate tectonics suggests that deformation occurs mainly along plate boundaries; however, compression can result in the formation of orogens and basins within intracontinental settings. During these two tectonic processes, the sedimentation and environmental changes occur in response to marginal and intracontinental deformation. Early Jurassic – Early Cretaceous deformation and basin formation along the Qinling orogenic belt and the northwestern Sichuan Basin in central–SW China are ideal for investigating a reactivated tectonic belt and basin formation. We studied the Lower Jurassic – Lower Cretaceous sedimentary sequences and structures along the northwestern margin of the Sichuan Basin, and obtained detrital zircon U–Pb ages for these rocks. The structures show that deformation migrated SE-wards and S-wards into the Sichuan Basin along the Longmen Shan, Micang Shan and Daba Shan tectonic belts during middle–late Mesozoic time. The Lower Jurassic oligomictic conglomerates have a smaller grain size and thicken towards the south, indicating protracted transport from a northern source. The conglomerates deposited near-source record post-orogenic south-vergent thrusting during the Late Triassic – Early Jurassic epochs. The Lower Cretaceous conglomerates and sandstones have multiple sources, which indicate that they were rapidly deposited near their source, synchronous with thrusting that occurred in response to coeval SE-wards and S-wards thrusting in the Longmen Shan and Daba Shan tectonic belts during the Late Jurassic – Early Cretaceous epochs. Detrital zircon grains from the Lower Jurassic – Lower Cretaceous sedimentary rocks yielded age peaks of 2600–2200, 1850–1600, 850–700, 540–400, 250–180 and 180–140 Ma. A comparison of these ages with those of surrounding exposed rocks indicates that the sediments in the northwestern Sichuan Basin were supplied from the Qinling orogenic belt, the northwestern Yangtze Block, the south margin of the North China Block and the Songpan–Garzê Terrane. The youngest peaks of detrital zircon U–Pb ages at 207 and 159 Ma constrain the two stages of intracontinental shortening and highlight the link between intracontinental deformation and sedimentation.

The NE–NNE-trending Yuan-Ma Basin in central South China, an overprinted basin, is important for understanding the transition in Mesozoic intracontinental deformation in South China from compressional to extensional settings. A detailed sedimentary and structural cross-section across the basin reveals the Upper Triassic – Lower Jurassic black coal-bearing shale, greyish-green sandstone and brick-red claystone, and the Middle Jurassic brick-red sandstone, pebbly sandstone and conglomerate in the eastern segment of the basin. The Lower Cretaceous brick-red coarse sandstone, pebbly sandstone and siltstone occurred in the western and central segments, as well as fault breccia and Lower Cretaceous sandstone at the western margin of the basin. Detrital zircon U–Pb dating by laser ablation inductively coupled plasma mass spectrometry shows that the magmatic and metamorphic zircons yield significant age clusters at 900–700, 500–350 and 300–150 Ma, as well a minor age cluster at 120–100 Ma. Synthesizing the stratigraphic sequences, structures, isotopic dating results and palaeocurrent data, we infer that the Yuan-Ma Basin experienced three evolutionary stages and tectonic settings: (1) during Late Triassic – Early Jurassic time, the Yuan-Ma Basin was related to the diachronous progressive intracontinental deformation as a result of the early Mesozoic Xuefeng intracontinental orogeny in South China; (2) during Middle–Late Jurassic time, the Yuan-Ma Basin was related to intracontinental compression in South China; and (3) during late Early Cretaceous time, the Yuan-Ma Basin was constrained by the intracontinental extension that occurred in eastern China. These three stages, a result of various tectonic regimes, caused the intracontinental deformation that was controlled by the evolution of the continents and their margins.