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Long-term and multiple stage exhumation of the Ordos Basin, western North China Craton: Insights from seismic reflection, borehole and geochronological data
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The study of large-scale and long-term sedimentary hiatus and exhumation in vast intraplate basins is of great significance for unravelling their tectonic development, morphodynamics and relationships with petroleum occurrence. The Ordos Basin is an intra-cratonic depression in the western part of the North China Craton that gradually subsides from the Proterozoic to the Mesozoic and contains several post-1.8 Ga unconformities, some of which are related to a series of Cenozoic tectonic events contributing to current plateau setting. While previously published thermochronological data have identified Mesozoic–Cenozoic multiphase cooling events, we report in this paper new apatite fission-track, zircon (Usingle bondTh)/He, and zircon Usingle bondPb geochronology on Paleoproterozoic borehole samples from the oldest sedimentary rocks in the Ordos Basin. Taken together, our data reveal cooling events at ca. 2.44 Ga, 1.97 Ga, 1.84 Ga, 630–570 Ma, 570–277 Ma, 75–15 Ma and ~ 15 Ma. Seismic reflection profiles and associated well cross-section interpretations are used to identify the basin's major unconformities and decipher the stratigraphic pattern. The six recognized unconformities exist between (1) the Precambrian and Cambrian, (2) the Cambrian and Ordovician, (3) the Lower Paleozoic and Upper Paleozoic, (4) the Middle-Upper Triassic and Jurassic, (5) the Jurassic and Lower Cretaceous and (6) the Cretaceous and Cenozoic. The new data allow a re-assessment of the Ordos Basin's amount and timing of erosion and burial. Integrating the existing evidence of exhumation, we interpret the Neoproterozoic cooling and exhumation events as a response to the Neoproterozoic Glaciations during the breakup of Rodina, while Phanerozoic wide subsidence of the Ordos Basin is interrupted by several differential exhumation and cooling events under the multiplate interactions in East Asia. Particularly, late Miocene (ca. 15–8 Ma) exhumation, synchronous topography inversion and deposition of Red Clay involve feedback relationships between tectonics and strengthening of Asian winter monsoon. Our results provide novel insights into of long-term exhumation and related unconformities concerning East Asia orogeny and climate events and impose important constraints on petroleum exploration.
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... Figure 4b-i). The age of these sections is mainly determined by paleomagnetic dating and Hipparion fossils [46,93], with thicknesses ranging from 20 to 120 m, exhibiting an unconformable contact with the underlying Mesozoic and Paleozoic strata [94]. ...
... Thirdly, the Ordos Plateau, situated in the western part of the North China Craton, has an average elevation of 1000 m (Figure 1b,c). Detrital materials eroded from the Taihang, Qinling, and Qilian Shan were transported into the Ordos Basin during the Mesozoic and early Cenozoic periods [34, 94,104]. The Ordos Basin amassed extensive fluvial and lacustrine deposits over time and was subsequently uplifted into a plateau during the late Cenozoic period due to the expansion of the northeastern margin of the Tibetan Plateau [94,121]. ...
... Detrital materials eroded from the Taihang, Qinling, and Qilian Shan were transported into the Ordos Basin during the Mesozoic and early Cenozoic periods [34, 94,104]. The Ordos Basin amassed extensive fluvial and lacustrine deposits over time and was subsequently uplifted into a plateau during the late Cenozoic period due to the expansion of the northeastern margin of the Tibetan Plateau [94,121]. The flat geomorphic feature of the plateau facilitated the significant accumulation of aeolian sediments on its surface [2,94]. ...
Eolian sediments are extensively distributed across the Earth's surface, and their formation is intricately linked to climate change, tectonic activity, and topographic features. Consequently, the investigation of eolian sediments bears great geological significance. The northwest region of China is renowned for hosting the most extensive and thickest Late Miocene-Pliocene red clay deposits globally. Nonetheless, scholars have yet to reach a consensus regarding the precise formation processes of these red clays. The identification of the source region of the red clays is crucial for comprehending their formation mechanism. The correlation of zircon U-Pb age spectra is a frequently utilized method for determining the provenance of eolian sediments. In this study, we compared the previously published zircon U-Pb ages (n = 12,918) of the Late Miocene-Pliocene red clays in the Ordos Plateau with those from the potential provenance regions (n = 24,280). The analysis, supported by the tectonic and climatic background of the region, revealed that the Late Miocene-Pliocene red clay in the Ordos Plateau originates predominantly from the Yellow and Wei rivers, with a minor contribution from the weathering of bedrock in the western North China Craton. The transport of these detrital materials by the East Asian winter monsoon is impeded by the presence of the Qinling and Taihang Shan, resulting in their deposition on the flat surface of the Ordos Plateau. This development of red clay is consistent with the proximal accumulation model, illustrating how the hydrosphere, atmosphere, and litho-sphere interacted to shape the red clay deposits during the Late Miocene and Pliocene periods in the Ordos Plateau.
... During the Cambrian-Ordovician and Late Carboniferous-Permian, the area where the Ordos Basin was located as a part of the large North China Basin widely underwent deposition . During the Cambrian-Ordovician period, carbonate rocks dominated by shallow marine sediments developed in the Ordos Basin, and gypsum-salt layers were sandwiched within the Middle Ordovician Majiagou Formation (Fig. 2;Liu et al., 2006;Xu et al., 2012;Ren et al., 2021a;He et al., 2022;Peng et al., 2023). During the Late Carboniferous-Permian, the Ordos Basin underwent short-lived marine-continental transitional facies sedimentation that formed during the early stage and subsequently transformed into large-scale continental sedimentation Peng et al., 2023). ...
... During the Cambrian-Ordovician period, carbonate rocks dominated by shallow marine sediments developed in the Ordos Basin, and gypsum-salt layers were sandwiched within the Middle Ordovician Majiagou Formation (Fig. 2;Liu et al., 2006;Xu et al., 2012;Ren et al., 2021a;He et al., 2022;Peng et al., 2023). During the Late Carboniferous-Permian, the Ordos Basin underwent short-lived marine-continental transitional facies sedimentation that formed during the early stage and subsequently transformed into large-scale continental sedimentation Peng et al., 2023). The Carboniferous-Lower Permian Formations are essential coal-bearing formations and are the main gas source rocks in the basin. ...
... During the Middle to Late Permian, the basin developed an inland lake-delta sedimentary system, and the large distribution of alluvial fans, braided rivers, anastomosed streams, deltaic plain channels and deltaic front sandbodies jointly constituted the most important reservoir rocks in the basin (Fig. 2;He et al., 2003;Liu et al., 2006Liu et al., , 2021Zhang et al., 2021). As a result of the Caledonian tectonic movement, the main body of the basin lacks Late Ordovician-Early Carboniferous strata, and widespread ancient-weathered crust developed (Fig. 2;Liu et al., 2006;Xu et al., 2012;Peng et al., 2023). ...
Abnormal pressure conditions and spatial variations have direct and powerful influences on the aggregation, dissipation, accumulation and development of hydrocarbons under subsurface conditions, and they are extremely important aspects that must be focused on for safe coal mining and oil and natural gas development. The Ordos Basin, located in the western part of the North China Craton (NCC), contains significant reserves of oil, gas, coal and other mineral resources and is a typical low-pressure basin. This paper conducts an overall analysis and comprehensive comparison of the spatial and temporal variations in the pressure coefficients of oil and gas areas in the Ordos Basin. The analysis is based on a dataset comprising 589 sets of pressure coefficient-depth data from hydrocarbon layers that are uniformly classified in the basin's main oil and gas fields. Notably, high-pressure features develop in the basin only in the gas fields located in the centres of areas with high gas generation intensity, in individual sections of lithological seals and in the gas layers beneath the gypsum salts of the Ordovician Majiagou Formation. The Mesozoic oil layers and Paleozoic gas layers throughout the basin, which are buried at depths up to 4500 m and within a stratigraphic era spanning 3 × 10 8 years, generally dominate low pressures, and these abnormal pressure features are not noticeably affected by spatial or temporal variations. Additionally, the ranges of the pressure coefficients and the distributions of the peak values of the oil and gas layers in the upper-oil and lower-gas areas (UOLGAs) generally resemble each other. The pressure coefficients for gas reservoirs only slightly exceed those for oil reservoirs. These unique features are rare in many large-and medium-sized petroleum bearing-basins worldwide. Excluding the high abnormal pressure data of the subsalt gas layer in the basin, based on the magnitudes, proportions, and distributions of the pressure coefficients of the gas layers, as well as the geological background, it is possible to classify the 10 gas fields in the basin into five distinct categories: (1) weak low-pressure dominated; (2) weak low-pressure dominated, mixed with high-pressure; (3) weak low-pressure dominated, supplemented with normal pressure; (4) normal and weak low pressures coex-isting; and (5) abnormally high pressure dominated. Regarding the spatial distribution, the pressure coefficients of the gas layers exhibit a macroscopic pattern of gradual increase from the northern to the southern regions. Furthermore, the unique characteristics of the generally low pressures in the basin, in the coalfield (mine) gas pressure coefficient, in the gas content, and in the gas explosion frequency correspond to the performance and can be corroborated by each other. This study provides a scientific basis and theoretical foundation for a profound understanding of the accumulation (mining) effects of evolution-modification and the unique features of hydrocarbons and coal seams (mines and fields) in the Ordos Basin, which can help to promote and guide the further exploration and development of energy minerals in different regions of the basin; furthermore, a new type of pressure system has been added to global energy basin pressure systems.
... The Ordos Block has recorded relatively continuous sedimentation from the Proterozoic to the Cretaceous. Its basement is comprised of schists and igneous rocks, associated with multiple tectonic events from the Mesoproterozoic to the Neoproterozoic (Figure 1; Lin et al., 2011;Peng et al., 2023). The overlying Cambrian-Ordovician strata are dominated by continental to deep marine sediments that formed on a passive margin (Lin et al., 2011;Ritts et al., 2009). ...
... The overlying Cambrian-Ordovician strata are dominated by continental to deep marine sediments that formed on a passive margin (Lin et al., 2011;Ritts et al., 2009). The Ordos region was uplifted and eroded during the end of the Ordovician, and the large-scale absence of the Silurian to Lower-Middle Carboniferous sediments in this region represents the response to the Caledonian orogeny (Peng et al., 2023;Zhao et al., 2016). Subsequent deposition of lacustrine, fluvial and marine sediments took place during the Carboniferous-Triassic cratonic stage (Ritts et al., 2009). ...
The Cenozoic topographic growth of the Tibetan Plateau is a pulsed, polyphase process that still requires more constraints. The Cenozoic sedimentary record of the Ningnan Basin, a continental basin located adjacent to the northeastern margin of the Tibetan Plateau, is a key archive for recording the surface evolution of the Tibetan Plateau. This work reports new provenance data (apatite fission‐track, apatite U–Pb dating, and trace element analysis on the same individual grains) from the Oligocene–Pliocene sedimentary sequence that filled the Ningnan Basin. The data set shows variations in provenance patterns through the Miocene which are related to the tectonic evolution of the northeastern margin of the Tibetan Plateau. In contrast to a primary provenance from the Western Ordos Block (WOB) during the Oligocene, the Miocene sediments were mostly derived from the recycling of Mesozoic successions that occur along the northwestern Haiyuan Fault, documenting it was active in the last ca. 15 Myr. These sediments, in turn, were derived from different orogenic blocks but mainly from different segments of the Qilian Mountains. We show that the Late Miocene–Pliocene sediments were primarily derived from transpressional uplift along the Haiyuan Fault, which affected regions such as the Liupan Mountains. Progressive northeastward migration of tectonic stress since the Middle Miocene has induced extensive regional deformation in the northeastern Tibetan Plateau, particularly along the Haiyuan Fault. The provenance record of the neighbouring Cenozoic basins is a key archive for deciphering this tectonic evolution.
... Given that most sample data do not include maturity data such as Ro and T max , the sample statistical results are divided according to the current burial depth of the source rocks, which is divided into 0-1 km, 1-2 km, and 2-3 km. The main basis for this division is that the overall tectonic activity of the basin is relatively complete; that is, the strata in the southern and northern parts of the basin have undergone similar subsidence and uplift processes since the Triassic period 80,81 . Based on this, it is believed that the current burial depth of the Yanchang Formation shale can roughly serve as a benchmark for relative maturity. ...
The source of uranium is an important research topic related to the exploration of sandstone-type uranium deposits, and potential uranium sources in deep basins are often overlooked. Black organic-rich shale is a common uranium-bearing rock in deep sedimentary basins. However, relatively few studies have investigated the migration of uranium during hydrocarbon generation in and release from uranium-rich shale. In this study, the uranium-rich shale in the Chang 7 member of the Yanchang Formation of the Upper Triassic in the Ordos Basin was selected to investigate the migration of uranium and other trace elements during the thermal maturation of uranium-rich shale via a semiopen pyrolysis simulation system. The gas and liquid products as well as the solid residue were thoroughly analysed by means of multiple instruments. The results showed that uranium significantly migrated before hydrocarbon generation (Ro < 0.61%), with a leaching rate between 12.1% and 18.8%. The leaching rate of uranium during the hydrocarbon generation stage (0.63% < Ro < 1.35%) was relatively low, ranging from 0 to 7.2%. Cu, Pb, Zn, Mo, and other trace elements also migrated considerably during the early stage of thermal evolution, with leaching rates ranging from 2.9 ~ 11.6%. The yield of low-molecular-weight organic acids (LOAs) was the highest in the early stage of thermal maturity, and the LOA yield exhibited a good correlation with the leaching rates of Cu, Pb, Zn, Co, Mo, etc. The generation of LOAs from source rocks was conducive to the leaching and migration of trace elements. Moreover, according to a statistical analysis of published geochemical data, the total organic carbon (TOC) content, uranium content, and U/TOC ratio in shale decreased significantly with increasing burial depth, indicating that uranium migrated significantly upon kerogen hydrocarbon generation during thermal evolution. Therefore, uranium-rich shale is an important deep uranium source in sedimentary basins.
... Geologic time scales of stratigraphic deposition and erosion were defined by combining the stratigraphic chart of Ordos Basin and region-specific geologic evolution features. 51,60 Erosional thicknesses in critical geological periods were calculated using the mudstone acoustic time-difference method in comparison to previous results. 61,62 Detailed procedures for determining the boundary conditions of the geologic models could be found in previous research. ...
The genesis, occurrence, and accumulation of coalbed methane (CBM) are critical to the methane exploration and development. Combining the geological and geochemical data from CBM exploration wells and basin modeling, the genesis and accumulation characteristics of CBM in different regions and depths at the eastern margin of Ordos Basin were elucidated. Regional-scale gas content is controlled by depth and coal rank without turning depth occurs, but blocks perform variably. Three CBM genetic types, that is, secondary microbial, thermogenic, and mixed genesis, were identified based on data of gas components and isotopic composition. Under the influences of tectonic and hydrogeological conditions, secondary microbial gas is widely distributed in shallow coal seams (depth <1350 m) showing mixed gas characteristics, while secondary thermogenic gas dominates in deep coal seams. The extensive variations of carbon isotope of methane raised from desorption–diffusion induced by tectonic uplift, dissolution in flowing groundwater, secondary microbial gas generation, and rapid gas generation enhanced by magmatic thermal. Three CBM accumulation modes have been concluded depending on the depth and coal rank. The Baode mode is characterized by a high-ratio secondary microbial gas replenishment, typically occurring at lower ranks and shallow depths. The Linxing mode is distinguished by magmatic thermal influences, and the Daji mode features massive secondary thermogenetic gas associated with higher ranks and deeper depths. Every mode has the potential to form high-gas-content zones.
... The study area is characterized by the development of carbonate-gypsum salt assemblages, which are present in thick sedimentary deposits and have a broad distribution [30,31]. The Majiagou Formation consists of three types of subfacies: subtidal, intertidal, and supratidal. ...
The marine carbonates in the Ordovician Majiagou Formation in the Ordos Basin have significant exploration potential. Research has focused on their thermal history and hydrocarbon accumulation stages, as these are essential for guiding the exploration and development of hydrocarbons. In this paper, we study the thermal evolution history of the carbonate reservoirs of the Ordovician Majiagou Formation in the east-central Ordos Basin. Furthermore, petrographic and homogenization temperature studies of fluid inclusions were carried out to further reveal the hydrocarbon accumulation stages. The results demonstrate that the degree of thermal evolution of the Ordovician carbonate reservoirs is predominantly influenced by the deep thermal structure, exhibiting a trend of high to low values from south to north in the central region of the basin. The Fuxian area is located in the center of the Early Cretaceous thermal anomalies, with the maturity degree of the organic matter ranging from 1 to 3.2%, with a maximum value of 3.2%. The present geothermal gradient of the Ordovician Formation exhibits the characteristics of east–high and west–low, with an average of 28.6 °C/km. The average paleo-geotemperature gradient is 54.2 °C/km, the paleoheat flux is 55 mW/m2, and the maximum paleo-geotemperature reaches up to 270 °C. The thermal history recovery indicates that the Ordovician in the central part of the basin underwent three thermal evolution stages: (i) a slow warming stage before the Late Permian; (ii) a rapid warming stage from the end of the Late Permian to the end of the Early Cretaceous; (iii) a cooling stage after the Early Cretaceous, with the hydrocarbon production of hydrocarbon source rocks weakening. In the central part of the basin, the carbonate rock strata of the Majiagou Formation mainly developed asphalt inclusions, natural gas inclusions, and aqueous inclusions. The fluid inclusions can be classified into two stages. The early-stage fluid inclusions are mainly present in dissolution holes. The homogenization temperature is 110–130 °C; this coincides with the hydrocarbon charging period of 210–165 Ma, which corresponds to the end of the Triassic to the end of the Middle Jurassic. The late-stage fluid inclusions are in the dolomite vein or late calcite that filled the gypsum-model pores. The homogenization temperature is 160–170 °C; this coincides with the hydrocarbon charging period of 123–97 Ma, which corresponds to the late Early Cretaceous. Both hydrocarbon charging periods are in the rapid stratigraphic warming stage.
... 28 Since the Late Palaeozoic, the Ordos Basin has experienced several stages of differential exhumation, and four recognized unconformities exist between (1) the Middle-Upper Triassic and Jurassic, (2) the Middle and Upper Jurassic, (3) the Jurassic and Lower Cretaceous, and (4) the Cretaceous and Cenozoic. 29 Although the thicknesses of the eroded strata have been estimated for four erosion periods, the simultaneous erosion of different strata caused by the Late Cretaceous exhumation event in the southeastern Ordos Basin has not been considered. 30 Several studies have developed methods to quantify the exhumation history of sedimentary basins. ...
The Ordos Basin is characterized by abundant natural gas resources, and the marine-continental transitional shale gas of the Permian Shanxi Formation has great exploration and development potential. However, few systematic studies have focused on the burial history, thermal maturity, and hydrocarbon generation of the shale, which limits the understanding of shale gas enrichment and resource evaluation. To reveal the shale gas resource potential, we focused on the Shanxi Formation shale in the southeastern Ordos Basin. Net erosion was estimated, and then one-dimensional (1D) and three-dimensional (3D) geological models were constructed using PetroMod to simulate the burial-thermal history and hydrocarbons generated in the Shanxi Formation shale, and finally, the gas generation intensity was evaluated. The results show that four periods of uplift and erosion events have occurred in the study area since the Mesozoic, of which the erosion in the Late Cretaceous was the most severe. The burial center gradually shifted from east to northwest in the study area, and the basin reached the maximum burial depth in the Late Cretaceous and then gradually changed to a monoclinal tilted east to west after uplift and erosion. The Shanxi Formation shale reached the hydrocarbon generation threshold at 233 Ma (Ro = 0.5%), reached the oil generation peak at 200 Ma (Ro = 1.0%), and entered the high maturity stage rapidly (Ro = 1.3%). Currently, the average maturity is approximately 2.48%, which is in the overmature stage. The center of shale maturity was in the southern part of the study area before the Late Jurassic and shifted northeast in the late Early Cretaceous. Cumulative gas generated to date is 44.0 × 10¹² m³, and the center of gas generation was in the middle-eastern region of the study area before the Early-Middle Jurassic and shifted northwest in the Early Cretaceous. This study provides a theoretical basis and guidance for the exploration and development of marine-continental transitional shale in the Ordos Basin.
A precisely constrained and high‐resolution geochronology will enhance our understanding of sedimentary evolution, tectonic influences and climatic variations, all of which contribute to hydrocarbon exploration. According to sedimentary analysis, the North China Craton is believed to have undergone a transition in its sedimentary environment from a marine to a terrestrial lacustrine–fluvial system during the Late Palaeozoic. However, the geochronology and driving forces associated with transgression and regression remain understudied. In this study, we employed a cyclostratigraphic method to analyse the gamma‐ray logging data from a Late Palaeozoic sequence in the Y69 well in the eastern Ordos Basin. We established an astronomical time‐scale for this sequence, spanning approximately 10.83 million years, from ~299.94 ± 0.32 Ma to ~289.11 ± 0.32 Ma, in order to provide a geochronological framework for the evolution of the Ordos Basin. Subsequently, we reconstructed paleolake‐level variations by applying sedimentary noise modelling to the tuned gamma‐ray series and conducted periodicity analysis to identify astronomical signals. Our results indicate that water‐level fluctuations in the eastern Ordos Basin were modulated by ~1.2‐Myr obliquity and ~ 2.4‐Myr eccentricity cycles, suggesting long‐term astronomical forcing on hydrological circulation. The in‐phase correlation between the reconstructed water level and global sea level at ~1.2‐Myr intervals suggests that sedimentation was controlled by oceanic systems through transgression/regression events before ~294 Ma. Conversely, the anti‐phase correlation between the reconstructed water level and global sea level at the same interval indicates that sedimentation was influenced by terrestrial systems through aquifer depletion and recharge after ~294 Ma. This shift in the correlation between water levels and global sea levels reflects the transition from an oceanic to a terrestrial sedimentary environment. These findings provide a high‐resolution geochronological framework for further investigations and offer new insights into hydrological circulation, improving our understanding of the driving mechanisms behind sedimentary evolution in the North China Craton during the Late Palaeozoic Ice Age.
Middle Triassic paleogeography is essential for the paleoclimate change and ecosystem recovery after the end-Permian mass extinction, but is highly debated in the Paleo-Tethys Ocean and North China Craton, eastern Pangaea. Here we report a detailed provenance fingerprinting dataset from Middle Triassic sediments across the North China Craton, Qinling and Songpan-Ganzi and identify the paleo-uplift in the northern North China Craton as a primary sediment source. We propose that a Middle Triassic transcontinental drainage system flowed from the northern North China Craton through the central Qinling into the eastern Paleo-Tethys Ocean along the gradually descending landform, challenging the suggestion that the North China Craton was a great endorheic basin. The finding of this study will provide a better understanding of Middle Triassic physiography and paleogeography of the North China Craton. Such heterogeneous paleotopography and transcontinental drainage played a critical role in the marine-terrestrial ecosystems of the eastern Pangaea.
The evolution and reforming history of the Cenozoic basins in the northeastern Tibetan Plateau (NETP) are significant for understanding the northeastward growth of the Tibetan Plateau. The sediments in these basins archived the spatial‐temporal patterns of the surface uplift and deformation processes in response to remote effects of growth of the NETP. Here, we present an integrated provenance analysis of the Ningnan basin in the NETP to provide direct insights into the basin evolution and further constraints of the Tibetan Plateau's northeastward growth. In this study, the Cenozoic strata (Sikouzi, Qingshuiying, Zhang'enpu and Ganhegou formations) in the Ningnan basin yield complex detrital zircon age distributions featured by predominant age populations at 200–360 Ma and 360–540 Ma with minor Precambrian ages. Detrital zircon U‐Pb ages indicate that the southwestern Ordos Block, the western Qinling orogenic belt, and the eastern Qilian orogenic belt are primary source areas. Additionally, adjacent local paleo‐uplifts (e.g., Xihua, Nanhua, Xiangshan, Liupan Mts.) also provided minor volumes of sediment. In addition, conglomerates and paleocurrent directions of the Sikouzi–Ganhegou formations exhibit an apparent change since the late Miocene, revealing an obvious provenance shift. The quantitatively determined mixing proportions of the surrounding source areas also reveals that the early source‐to‐sink system was replaced during the deposition of the Ganhegou Formation. Combined with previous studies, we suggest that the formation of the Ningnan basin was controlled by normal faults in extension setting and experienced continued sediment accumulation during the Qingshuiying–Zhang'enpu period; since the late Miocene, the significant provenance shift reflects the obvious northeastward growth of the NETP and led to the extinction of the Ningnan basin.
The Yanchang Formation of the Triassic in the Ordos Basin comprises various stratigraphic intervals. The Chang 8 reservoir represents a significant oil-producing section of the Yanchang Formation, and its hydrocarbon accumulation mechanism is complex. In this study, we analyzed the diagenetic evolution and reservoir-forming stages of the Chang 8 member of the Yanchang Formation in the Late Triassic in the Fuxian area, the southern Ordos Basin, via thin-section casting, scanning electron microscopy (SEM), X-ray diffraction, and fluid inclusion petrology and homogenization temperature analyses. The relationship between the petrogenesis and hydrocarbon charging history was analyzed, which provided guidance for identifying and predicting the hydrocarbon reservoir distribution. The results show that the main diagenesis types of the Chang 8 reservoir are compaction, cementation, dissolution, and metasomatism. The comprehensive analysis of the reservoir mineral types, diagenesis, diagenetic sequence, and thermal evolution degree of organic matter shows that the Chang 8 reservoir of the Yanchang Formation is in the A stage of the middle diagenesis stage. Under the overpressure of hydrocarbon generation, oil and gas migrated into the Chang 8 reservoir along fractures and connected pores. The earlier-stage hydrocarbon charging occurred after compaction and later than the early clay film formation and early calcite precipitation, and it also occurred earlier than or simultaneously with the quartz overgrowth. The later hydrocarbon charging occurred after the significant quartz overgrowth and late calcite pore filling. Depending on the homogenization temperature and salinity, the fluid inclusions can be divided into two types: low-temperature, low-salt (90–105 °C, 1.4%–11.2%) fluid inclusions and high-temperature, high-salt (115–120 °C, 2.2%–12.5%) fluid inclusions. According to the analysis of the evolution of the burial history, hydrocarbon charging in the Chang 8 reservoir of the Yanchang Formation in the Fuxian area occurred in two consecutive periods: 133~126 Ma and 122~119 Ma, demonstrating one-scene, two-stage reservoir formation, characterized by simultaneous reservoir densification and hydrocarbon charging. In this research, we precisely ascertained the regional diagenetic characteristics and patterns and periods of hydrocarbon charging, thereby furnishing crucial evidence that deepens the comprehension of sedimentary basin evolution.
The Ordos Basin is among China's largest petroliferous basins, with its southwestern part being a key exploration area. Prior studies indicated that the basin's internal structure was relatively simple, featuring minor developmental faults and primarily stratigraphic–lithologic reservoirs. However, recent research has identified numerous strike‐slip faults in the basin, and their relationship to oil and gas accumulations remains unclear. This study, using integrated interpretations of field outcrops, imaging logging and 3D seismic reflection data, clarifies the characteristics, morphology and formation mechanisms of multi‐period faults in the southwestern Ordos basin. Additionally, the study investigates the relationship between these faults and oil and gas accumulations. Results show that Mesozoic fractures in the southwestern basin are primarily NE‐ and NW‐trending. Seismic profiles reveal that these faults exhibit complex geometries, including upright structures in the Middle to Upper Triassic and floral structures in the Cretaceous. Coherence slices show that Lower Jurassic faults have linear structures NE‐ and NW‐trending, while Cretaceous faults exhibit parallel linear structures ENE‐trending. The study of fault displacement and morphology suggests two evolutionary patterns for Mesozoic faults in the basin: layered development and basement‐activated faulting. The widespread ENE‐ and NW‐trending faults represent a specific mode of tectonic stress transfer in stable cratonic areas with minimal basement fault influence. Conversely, some ENE‐trending faults are significantly influenced by basement activation during various geological periods, penetrating deeply into strata and exhibiting distinct segmentation on a planar scale. This differential fault development results in an uneven distribution of Jurassic oil and gas reservoirs. Significant accumulations of Jurassic oil and gas are found in the ENE‐trending tension–torsional strike‐slip sections, whereas many NW‐trending faults may negatively impact oil and gas reservoirs. This study elucidates the characteristics of Mesozoic faults in the southwestern Ordos Basin, offering valuable guidance for oil and gas exploration and development in the region.
In recent years, tight sandstone oil and gas have been an important area for unconventional oil and gas exploration and development in China. It is of great significance to clarify the pore-throat structure characteristics of tight sandstone reservoirs to guide production practices. This study takes the tight sandstone of the sixth member of the Yanchang Formation in the Huaqing area, Ordos Basin, as an example, based on experimental methods such as high-pressure mercury intrusion, cast thin sections and scanning electron microscopy. At the same time, the pore-throat structure of tight sandstone reservoirs is divided into three types using the tube-bundle and spherical fractal models. The corresponding pore and throat radius distribution, pore-throat combination mode and influencing factors of various pore-throats are studied. The results show that the fractal dimension of type I pore-throats is the smallest, and the distribution of their pore-throat radii is the most uniform. They are dominated by intercrystalline pores and dissolution pores with tube-bundle throats and small pores with small throats. Type II pore-throats have the largest fractal dimension and the worst pore-throat uniformity. They are dominated by residual primary intergranular pores with necked throats and large pores with small throats. The type III pore-throat fractal dimension is in the middle, mainly composed of residual dissolved intergranular pores with pore-reduced throats, sheet-like and curved sheet-like throats, and large pores with large throats. The influence of different pore-throat combinations on the reservoir is reflected in the different characteristics of mercury injection parameters. The main influencing factors for the differences in the fractal dimensions of different pore-throats are diagenesis, rock composition and pore-throat combination type. Diagenesis and rock composition, in turn, affect the type and development degree of pore-throats, as well as the combination of pore-throats. The purpose of this study was to clarify the internal connection modes of different homogeneous pore-throats and their influencing factors, enrich the theoretical basis for the study of tight sandstone reservoirs and provide theoretical guidance for their exploration and development.
Global mean sea level is a key component within the fields of climate and oceanographic modelling in the Anthropocene. Hence, an improved understanding of eustatic sea level in deep time aids in our understanding of Earth’s paleoclimate and may help predict future climatological and sea level changes. However, long-term eustatic sea level reconstructions are hampered because of ambiguity in stratigraphic interpretations of the rock record and limitations in plate tectonic modelling. Hence the amplitude and timescales of Phanerozoic eustasy remains poorly constrained. A novel, independent method from stratigraphic or plate modelling methods, based on estimating the effect of plate tectonics (i.e., mid-ocean ridge spreading) from the ⁸⁷Sr/⁸⁶Sr record led to a long-term eustatic sea level curve, but did not include glacio-eustatic drivers. Here, we incorporate changes in sea level resulting from variations in seawater volume from continental glaciations at time steps of 1 Myr. Based on a recent compilation of global average paleotemperature derived from δ¹⁸O data, paleo-Köppen zones and paleogeographic reconstructions, we estimate ice distribution on land and continental shelf margins. Ice thickness is calibrated with a recent paleoclimate model for the late Cenozoic icehouse, yielding an average ∼1.4km thickness for land ice, ultimately providing global ice volume estimates. Eustatic sea level variations associated with long-term glaciations (> 1 Myr) reach up to ∼90m, similar to, and is at times dominant in amplitude over plate tectonic-derived eustasy. We superimpose the long-term sea level effects of land ice on the plate tectonically driven sea level record. This results in a Tectono-Glacio-Eustatic (TGE) curvefor which we describe the main long-term (>50 Myr) and residual trends in detail.
In order to determine the provenance and variation characteristics of sandstone-type uranium deposits located in the southwest Ordos Basin, U−Pb geochronology and Hf isotope analyses were conducted on detrital zircons from the Late Mesozoic strata of the SD01 well in the Zhenyuan area. The detrital zircon U−Pb ages of four samples exhibited four main peaks at 250–330, 420–500, 1 720–2 000, and 2 340–2 580 Ma, with a small number of zircons dated at 770–1 100 Ma. The detrital zircon age spectrum and further restriction provided by the in-situ Hf isotopic data suggest that the provenance of each stratum was mainly derived from the crystalline basement rock series (Khondalites, intermediate-acidic intrusive rocks, and metamorphic rocks) of the Alxa Block to the northwest and the Yinshan Block to the north, with minor amounts of Caledonian magmatic rocks and Jingning Period rocks from the western part of the northern Qilian orogenic belt to the west and the northern Qinling orogenic belt to the south. The provenance of the sandbody has not changed significantly and is of the Middle Jurassic-Early Cretaceous. The clear variations in the zircon ages of the samples from the Zhiluo and Anding formations were likely influenced by climate change during the Middle-Late Jurassic. The Triassic zircon age (<250 Ma) first appeared in Early Cretaceous strata, suggesting that tectonic activity was relatively strong in the northern Qinling orogenic belt during the Late Jurassic and produced extensive outcrops of Indo-Chinese granite, which were a source of basin sediments.
Lithosphere of cratons and orogens generally reacts differently to tectonic events. Although these differences are mostly clear during the orogenic phases, understanding how they respond to tectonic reactivation is still challenging. Here, we report the first detailed apatite fission‑track (AFT) study pinpointing the gradual transition between cratonic and orogenic lithosphere, using the case study of the São Francisco craton (SFC) and the adjacent Araçuaí‑West Congo Orogen (AWCO), eastern Brazil. The collision that built the AWCO partially affected the inherited rift structures of the Paramirim Aulacogen, embedded in the São Francisco‑Congo paleocontinent. Our data reveal a differential Phanerozoic exhumation between closely interspaced areas affected and not affected by the AWCO deformation. Samples from the SFC present slow and protracted basement cooling during the Phanerozoic, while samples from the orogen display rapid exhumation since the Eocene. An intermediate ~ N–S zone of c.40 km shows lower magnitude basement cooling during the Cenozoic, possibly because the propagation of AWCO deformation decreases towards the craton interior. Within the orogen, the Rio Pardo salient is the main reactive structure and probably results from the deformation of a master fault, inherited from its precursor rift. Here, we show how the magnitude of Phanerozoic denudation may be deeply associated with previous events of lithosphere weakening.
Here, we present seven new zircon (U-Th)/He (ZHe) ages and three new zircon fission track (ZFT) ages analyzed from an age-elevation profile (Machu Picchu, Peru). ZFT data present ages older than those obtained with other thermochronological data, whereas the ZHe data interestingly present ages similar to those obtained with apatite (U-Th)/He (AHe). It has been proposed that He retention in zircon is linked to the damage dose, with an evolution of the closure temperature from low values associated with a low α-dose (<10^16 α/g), subsequently increasing before decreasing again at a very high α-dose (>10^18 α/g). Studies have focused on He diffusion behavior at high α-dose, but little is known at low doses. We propose that the ZHe closure temperature at α-dose ranging from 6 × 10^15 to 4 × 10^16 α/g is in the range of ~60–80 °C. This value is lower than that proposed in the current damage model ZRDAAM and demonstrates that the ZHe and AHe methods could have similar closure temperatures at low α-dose (i.e., similar ages). These new data strengthen our previous geological conclusions and even highlight a cooling rate approximately twice as important as that deduced from AHe and apatite fission track data alone at Machu Picchu.
The Qilian Shan, which is located along the northeastern margin of the Tibetan Plateau, plays a key role in understanding the dynamics of the outward and upward growth of the plateau. However, when and how tectonic deformation evolved into the geographic pattern which is currently observed in the Qilian Shan are still ambiguous. Here, apatite fission track (AFT) thermochronology and sedimentology were conducted to interpret the low-temperature tectonic deformation/exhumation events in well-dated Late Miocene synorogenic sediment sequences in the Xining Basin, which is adjacent to the southern flank of the Qilian Shan. These new low-temperature thermochronological results suggest that the Qilian Shan experienced four stages of tectonic exhumation during the late Mesozoic–Cenozoic. The Late Cretaceous exhumation events in the Qilian Shan were caused by the diachronous Mesozoic convergence of the Asian Plate and Lhasa Block. In the early Cenozoic (ca. 68–48 Ma), the Qilian Shan quasi-synchronously responded to the Indian–Asian plate collision. Subsequently, the mountain range experienced a two-phase deformation during the Eocene–Early Miocene due to the distal effects of ongoing India–Asia plate convergence. At ca. 8 ± 1 Ma, the Qilian Shan underwent dramatic geomorphological deformation, which marked a change in subsidence along the northeastern margin of the Tibetan Plateau at that time. Our findings suggest that the paleogeographic pattern in the northeastern Tibetan Plateau was affected by the pervasive suture zones in the entire Qilian Shan, in which the pre-Cenozoic and Indian–Asian plate motions reactivated the transpressional faults which strongly modulated the multiperiodic tectonic deformation in northern Tibet during the Cenozoic. These observations provide new evidence for understanding the dynamic mechanisms of the uplift and expansion of the Tibetan Plateau.
The Ordovician system of the Ordos Basin is rich in oil and gas resources, but the tectonic evolution of Ordovician strata and controls on natural gas generation and accumulation are poorly understood. In this study, we integrate field observations, seismic and drill core interpretation, and geochemical data analysis [total organic carbon (TOC) and vitrinite reflectance (R o)] to address this issue. Results show a clear depositional axis, with thick (>2500 m) Ordovician sediments in the southern half of the basin, and thinner strata in the central and northern parts. R o values range from 2.04% to 3.45%, and 55 of 73 samples have TOC > 0.23 wt%. Our novel findings made as below: (i) Ordovician strata have high contents of organic matter and that thermal evolution reached the dry gas stage, favorable for natural gas generation. (ii) Ordovician strata have recorded multiple tectonic events and experienced erosion that resulted in the formation of a karst type residuum in the central and northeastern parts of the basin. (iii) An Early Cretaceous tectono-thermal event controlled the generation and accumulation of natural gas, and this was the peak period of gas generation of Ordovician strata. (iv) Natural gas accumulation and adjustment occurred during the Cretaceous and Cenozoic, respectively. The Ordos Basin has experienced strong deformation and denudation since the Late Cretaceous. Collectively, the deformation was more intense in the south and weaker in the north, preventing hydrocarbon preservation in the southern part of the basin. These findings provide new insights to guide oil and gas exploration of the Ordos Basin.
The Great Unconformity is a widely distributed surface separating Precambrian rocks from overlying Phanerozoic sedimentary sequences. The causes and implications of this feature, and whether it represents a singular global event, are much debated. Here, we present new apatite (U‐Th)/He (AHe) thermochronologic data from the central Canadian Shield that constrain when the Precambrian basement last cooled to near‐surface temperatures, likely via exhumation, before deposition of overlying early Paleozoic sedimentary sequences that mark the Great Unconformity. AHe data from 11 samples (n = 57) across a broad region define a similar date‐eU pattern, implying a common thermal history. Higher eU (>25 ppm) apatite form distinct flat profiles of reproducible dates at ∼510 ± 49 Ma (mean and 1σ standard deviation), while lower eU (<25 ppm) apatite define a positive date‐eU trend with younger dates. The data patterns, geologic context, and thermal history modeling point toward >3 km of erosion across the entire ∼450,000 km² study area between 650 and 440 Ma, followed by modest reheating during later burial. Plume activity associated with intracratonic basin formation or continental rifting/breakup may have caused this erosion event. The post‐650 Ma timing of the last major sub‐Great Unconformity exhumation phase in this region implies a late Great Unconformity that is younger than inferred elsewhere in North America. This suggests that this feature is likely the result of multiple temporally distinct erosion events with differing footprints and mechanisms.
The Shanxi Rift System (SRS) is a prominent intracontinental rift in eastern Eurasia. However, its tectonic origin remains enigmatic, as the timing of rift initiation and its subsequent evolution is not well constrained. To evaluate the cooling history of rift flanks, we present joint apatite fission track (n = 15) and apatite (U‐Th‐Sm)/He (n = 62) thermochronological study across the Huo Shan and the Zhongtiao Shan in the central and southern SRS, respectively. Inverse modeling of the thermochronological data yields two episodes of enhanced exhumation during the Cenozoic. Both ranges record rapid cooling circa 50‐35 Ma, coeval with a phase of widespread rifting across entire North China. Data from the Zhongtiao Shan record renewed cooling from ∼8 Ma to the present, following a protracted near‐isothermal condition. Considering the SRS in the context of plate reconstructions, we propose that the Eocene rift initiation is triggered by the subduction of the trench‐parallel Izanagi‐Pacific mid‐ocean ridge followed by subduction of the Pacific plate. Tectonic quiescence along the Shanxi rift during Oligocene and Miocene time reflects slow thermal subsidence as the Pacific subduction regime was established. In late Miocene, faults associated with the SRS were reactivated in dextral transtension linked to fault systems that extend outward from the northeastern Tibetan Plateau. This kinematic reorganization implies a fundamental change in force balance throughout North China. Overall, our results reflect the changing influence of tectonic regimes along the eastern Eurasian plate boundary and intracontinental deformation associated with the India‐Eurasia collision.
The detrital ZUPb data obtained from the Lower Cretaceous stratum display features of proximal sediments and indicate that there existed at least three phases (i.e. 224 Ma, 247 Ma and 348 Ma) of tectonomagmatic activity caused by the closure of the Paleo-Asian Ocean. The detrital AFT and part of the ZUPb dating results reveal three cooling events, including ca. 106 Ma, ca. 143–149 Ma and ca. 174–178 Ma. This new discovery of a three-period cooling event is in agreement with the different stages of the Yanshanian Movement (Dong, et al., 2008). Firstly, ca. 174–178 Ma cooling is indicative of a post-magmatic cooling and orogenic period of the southern Central Asian Orogenic Belt, approximately matching the timing of episode A of the Yanshanian Movement. The ca. 143–149 Ma cooling may record Late Jurassic regional-scale thrust, folding and exhumation of the Yingen-Ejinaqi Basin in response to episode B of the Yanshanian Movement. The region converts from Jurassic compression to post-orogenic extension during the Cretaceous. Finally, the ca. 106 Ma rapid cooling reflects the contemporaneous rifting and volcanic activity of the region. These new results reveal that the eastern Yingen-Ejinaqi Basin experienced three-phase tectonomagmatic activity during 348 Ma–224 Ma and three-stage cooling events during the Yanshanian Movement between 178 Ma-106 Ma. Based on the tectonic background, the six period (i.e., ca. 106 Ma, 143–149 Ma, 174–178 Ma, 224 Ma, 247 Ma and 348 Ma) tectonic reformation events here detected are probably related to plate convergence and intracontinental orogenesis in East Asia (Dong et al., 2008).
This study presents a comprehensive low‐temperature thermochronometric data set from the Shanxi Rift, Taihangshan, and eastern Ordos block in North China, including new apatite fission track and apatite (U‐Th‐Sm)/He data and published apatite and zircon fission track and (U‐Th‐Sm)/He data. We use these data and new thermal history inversion models to reveal that the Shanxi Rift and Taihangshan experienced an increase in cooling rates between ca. 110–70 Ma and ca. 50–30 Ma. A preceding ca. 160–135 Ma cooling event is generally restricted to the western rift margin in the Lüliangshan and Hengshan. In contrast, the ca. 50–30 Ma cooling event was widespread and occurred coevally with the opening of the Bohai Basin and slip across the NNE‐striking Eastern Taihangshan fault. In the southern rift zone, however, exhumation beginning ca. 50 Ma was likely associated with fault block uplift across the ESE–striking Qinling and Huashan faults, which accompanied the extensional opening of the Weihe Graben. Coeval fault slip along the NNE–striking Eastern Taihangshan faults and ESE–striking Qinling and Huashan faults was associated with NW‐SE extension in North China related to oblique subduction of the Pacific plate under Eastern Asia and slow convergence rates. The Shanxi Rift is commonly attributed to Late Miocene and younger extension, but our new thermochronologic data do not precisely record the onset of rifting. However, our inversion models do suggest ≤∼50°C of Neogene–Quaternary cooling, consistent with ≤∼2 km of footwall uplift across most range‐bounding faults.
Cratons are ancient regions of relatively stable continental fragments considered to have attained long‐term tectonic and geomorphic stability. Low‐temperature thermochronology data, however, suggest that some cratons have experienced discrete Phanerozoic heating and cooling episodes. We report apatite fission track, and apatite and zircon (U‐Th)/He low‐temperature thermochronology data from the Archean Pilbara craton and adjacent Paleoproterozoic basement, NW Australia. Inverse thermal history simulations of this spatially extensive data set reveal that the region has experienced ~50–70°C cooling, which is interpreted as a response to the unroofing of erodible strata overlying basement. The timing of cooling onset is variable, mainly ~420–350 Ma in the southern and central Pilbara‐eastern Hamersley Basin and ~350–300 Ma in the northern Pilbara, while the westernmost Pilbara‐central Hamersley Basin does not record a significant Paleozoic cooling event. These differences are attributed to variations in sedimentary thickness and proximity to adjacent rift basins, which lack Archean age zircons in their Paleozoic strata. The onset of Paleozoic cooling coincides with the timing of the episodic intraplate late Ordovician‐Carboniferous Alice Springs Orogeny. This orogeny is thought to have resulted from far‐field plate margin stresses, which in turn caused the opening of the adjacent Canning Basin, to the north and east of the craton. We propose that basin development triggered a change of base level, resulting in denudation and the crustal cooling event reported here. Our results provide further evidence for the transmission of far‐field forces to cratons over hundreds of kilometers and support the view that cratons have experienced geomorphic changes during the Phanerozoic.
The multiphase intensive intra-continental deformation of the Eurasian continent in the Cenozoic caused by Indo–Asian and Pacific–Asian collisions has been studied extensively. However, its Cenozoic intra-continental deformation process and dynamics remains poorly constrained. The western North China Plate of the Eurasian continent is characteristic of the Cenozoic faulted basin system around Ordos Block, and is a critical region to determine this deformation process. Here, new structural data and fault kinematic analysis, coupled with new geochronological results, delineate a two-stage Cenozoic tectonic evolution in the region, providing new structural evidence to decipher the intra-continental deformation due to Indo–Asian and Pacific–Asian collisions. The first stage is characterized by the formation of faulted basins in the northwestern and southeastern margins of the Ordos Block, dominated by a pure-shear mechanism. This stage further comprises the NW–SE extension spanning the Eocene to Late Miocene (ca. 10.5 Ma), and the subsequent basin inversion triggered by the NW–SE compression during the Late Miocene (ca. 10.5-9.5 Ma). Its tectonism is associated mainly with the far-field effect of the northwestward subduction of the Pacific Plate. The second stage is characterized by the development of the Shanxi and Hetao Basins in the eastern and northern parts of the Ordos Block, respectively, and an intensive mountain-building process in the western part since the Late Miocene (ca. 9.5 Ma), which is connected with a simple-shear mechanism. This stage is furthermore divided into three alternating episodes of shortening and extension events. These resulted predominantly from far-field responses to the northeastward growth of the Tibetan Plateau and partially from the Pacific subduction.
This study delineates reservoir and non-reservoir zones in the Lower Cretaceous Dariyan Formation as units that are characteristic of the stratigraphic section representative of portions of the Persian Gulf offshore area. Reservoir rock types are categorized by textural and diagenetic properties. Static flow zones were delineated by porosity and permeability measurements of cored intervals. Electrofacies were prepared from clusters of petrophysical data to define reservoir zones for areas lacking wells with cored intervals. The attributes of these reservoirs are integrated into a sequence stratigraphic framework. This research indicates that rock types and reservoir zones of the Dariyan Formation differ in the studied fields located to the west and to the east in the Persian Gulf. These differences are interpreted to have resulted from a differing tectono-stratigraphic framework that controlled depositional facies and subsequent diagenesis. For example, reservoirs associated with the lower and upper carbonate units of the Dariyan Formation have different lithofacies and diagenetic modifications that resulted from deposition at two intrashelf basins at areas to the northwest and to the southeast in the Persian Gulf, and subsequent exposure to meteoric water flows during subaerial exposure.
The Gangdese magmatic arc, southern Tibet, was products of the Mesozoic and Cenozoic magmatism during the long-lasting subduction of the Neo-Tethyan oceanic lithosphere and the subsequent India-Asia collision, respectively, and therefore, it is a natural laboratory for studying accretionary and collisional orogenesis, as well as growth and reworking of the continental crust. Based on a synthesis of available results of magmatism, metamorphism and mineralization in this region, the formation and evolution history of the Gangdese arc is divided into five stages, namely, the early subduction of the Neo-Tethyan lithosphere, the subduction of the Neo-Tethyan mid-oceanic ridge, the late subduction of remnant Neo-Tethyan lithosphere, the collision of India and Asia, and the postcollisional stage. The first stage, lasting from Late Triassic to Middle Cretaceous, is characterized by the normal subduction of the Neo-Tethyan oceanic lithosphere and the formation of subduction-related arc magmatic rocks, and during this stage, the long-term mantle-derived magmatism resulted in the significantly growth of juvenile crust throughout the Gangdese arc, together with the generation of a giant porphyry Cu deposit in the western segment of the Gangdese arc. The second stage, happened in Late Cretaceous, was related to the subduction of the active Neo-Tethyan mid-oceanic ridge. In this stage, the upwell of the asthenosphere along the slab window resulted in extensive partial melting of upwelling asthenosphere, subduction slab and hinging-wall mantle wedge, which in turn, resulted in formation of diversity magmatic rocks and high-temperature metamorphic rocks. At the same time, underplating and accretion of the voluminous mantle-derived magma induced in the significantly growth and thickening of the Gangdese arc crust, and high-temperature and high-pressure metamorphism and partial melting of the thickened lower crust. The third stage, the latest Late Cretaceous, is characterized by the subduction of remnant Neo-Tethyan oceanic lithosphere and the normal arc magmatism. The fourth stage is represented by Paleocene to Middle Eocene magmatic flare-up, which was induced by the roll-back and breakoff of the subducted Neo-Tethyan slab during the Indo-Asian collision. This stage is characterized by the significantly thickening and partial melting of juvenile and old crusts, and extensive mixing of the mantle- and crust-derived magmas. The generated I-type granites inherited the chemical compositions of arc-type magmatic rocks, and also have the geochemical features of adakites. A series of large and giant Pb-Zn deposits related to the old crust-derived granites formed in the northern part of the Gangdese arc. The latest postcollisional stage is characterized by the continuous thickening and formation of the thickened lower crust-derived adakitic rocks during the Late Oligocene to Middle Miocene and many large and giant adakitic rock-related porphyry Cu-Au-Mo deposits formed in the eastern Gangdese arc in this stage. The multistage magmatic, metamorphic and mineralization processes provide excellent constraints for the Gangdese tectonic evolution from the Neo-Tethyan ocean subduction to India-Asia continental collision.
The Qilian Shan, at the northeastern frontier of the Tibetan Plateau, is a key area for studying the expansion mechanism of the Tibetan Plateau. Although previous thermochronology and paleomagnetic studies indicate Neogene northward expansion of the northern Qilian Shan, there is a distinct temporal gap in knowledge relative to the tectonic history of the southern Qilian Shan. This has hindered a complete understanding of the Cenozoic deformation pattern of the entire Qilian Shan. To study the growth history of the southern Qilian Shan, apatite fission track (AFT) data have been acquired from Zongwulong Shan and the Huaitoutala section. AFT thermal history modeling from the former shows a rapid cooling episode occurred at ~18–11 Ma, which is interpreted as marking the onset of intensive exhumation in the southern Qilian Shan. Within the Huaitoutala section, detrital grain up‐section shows progressively decreasing peak AFT ages followed by an age increase from midsection, implying that a sediment‐recycling event occurred at approximately 7 ± 2 Ma. Together with a shift in paleocurrent directions, this change marks the onset of Late Miocene deformation of the northern Qaidam Basin. Combined with previous studies on the deformation time of the Qilian Shan, our findings suggest that both the northern and southern Qilian Shan region grew outward synchronously in opposite directions during the Neogene. This resulted in the formation of a flower structure, which had an important impact on the deformation pattern of north Tibet. The synchronous outward expansion may have been triggered by the removal of mantle beneath north Tibet.
The Great Unconformity, a profound gap in Earth’s stratigraphic record often evident below the base of the Cambrian system, has remained among the most enigmatic field observations in Earth science for over a century. While long associated directly or indirectly with the occurrence of the earliest complex animal fossils, a conclusive explanation for the formation and global extent of the Great Unconformity has remained elusive. Here we show that the Great Unconformity is associated with a set of large global oxygen and hafnium isotope excursions in magmatic zircon that suggest a late Neoproterozoic crustal erosion and sediment subduction event of unprecedented scale. These excursions, the Great Unconformity, preservational irregularities in the terrestrial bolide impact record, and the first-order pattern of Phanerozoic sedimentation can together be explained by spatially heterogeneous Neoproterozoic glacial erosion totaling a global average of 3–5 vertical kilometers, along with the subsequent thermal and isostatic consequences of this erosion for global continental freeboard.
The Cambrian Period is the first period of the Phanerozoic Eon and witnessed the explosive appearance of the metazoans, representing the beginning of the modern earth-life system characterized by animals in contrary to the Precambrian earth-life system dominated by microbial life. However, understanding Cambrian earth-life system evolution is hampered by regional and global stratigraphic correlations due to an incomplete chronostratigraphy and consequent absence of a highresolution timescale. Here we briefly review the historical narrative of the present international chronostratigraphic framework of the Cambrian System and summarize recent advances and problems of the undefined Cambrian stage GSSPs, in particular we challenge the global correlation of the GSSP for the Cambrian base, in addition to Cambrian chemostratigraphy and geochronology. Based on the recent advances of the international Cambrian chronostratigraphy, revisions to the Cambrian chronostratigraphy of China, which are largely based on the stratigraphic record of South China, are suggested, and the Xiaotanian Stage is newly proposed for the Cambrian Stage 2 of China. We further summarize the integrative stratigraphy of South China, North China and Tarim platforms respectively with an emphasis on the facies variations of the Precambrian-Cambrian boundary successions and problems for identification of the Cambrian base in the different facies and areas of China. Moreover, we discuss stratigraphic complications that are introduced by poorly fossiliferous dolomite successions in the upper Cambrian System which are widespread in South China, North China and Tarim platforms.
Cretaceous strata are widely distributed across China and record a variety of depositional settings. The sedimentary facies consist primarily of terrestrial, marine and interbedded marine-terrestrial deposits, of which marine and interbedded facies are relatively limited. Based a thorough review of the subdivisions and correlations of Cretaceous strata in China, we provide an up-to-date integrated chronostratigraphy and geochronologic framework of the Cretaceous system and its deposits in China. Cretaceous marine and interbedded marine-terrestrial sediments occur in southern Tibet, Karakorum, the western Tarim Basin, eastern Heilongjiang and Taiwan. Among these, the Himalayan area has the most complete marine deposits, the foraminiferal and ammonite biozonation of which can be correlated directly to the international standard biozones. Terrestrial deposits in central and western China consist predominantly of red, lacustrine-fluvial, clastic deposits, whereas eastern China, a volcanically active zone, contains clastic rocks in association with intermediate to acidic igneous rocks and features the most complete stratigraphic successions in northern Hebei, western Liaoning and the Songliao Basin. Here, we synthesise multiple stratigraphic concepts and charts from southern Tibet, northern Hebei to western Liaoning and the Songliao Basin to produce a comprehensive chronostratigraphic chart. Marine and terrestrial deposits are integrated, and this aids in the establishment of a comprehensive Cretaceous chronostratigraphy and temporal framework of China. Further research into the Cretaceous of China will likely focus on terrestrial deposits and mutual authentication techniques (e.g., biostratigraphy, chronostratigraphy, magnetostratigraphy and cyclostratigraphy). This study provides a more reliable temporal framework both for studying Cretaceous geological events and exploring mineral resources in China.
The Carboniferous period lasted about 60 Myr, from ~358.9 Ma to ~298.9 Ma. According to the International Commission on Stratigraphy, the Carboniferous System is subdivided into two subsystems, i.e., Mississippian and Pennsylvanian, including 6 series and 7 stages. The Global Stratotype Sections and Points (GSSPs) of three stages have been ratified, the Tournaisian, Visean, and Bashkirian stages. The GSSPs of the remaining four stages (i.e., the Serpukhovian, Moscovian, Kasimovian, and Gzhelian) have not been ratified so far. This paper outlines Carboniferous stratigraphic subdivision and correlation on the basis of detailed biostratigraphy mainly from South China, and summarizes the Carboniferous chronostratigraphic framework of China. High-resolution biostratigraphic study reveals 37 conodont zones, 24 foraminiferal (including fusulinid) zones, 13 ammonoid zones, 10 brachiopod zones, and 10 rugose coral zones in the Carboniferous of China. The biostratigraphic framework based on these biozones warrants the precise correlation of regional stratigraphy of China (including 2 subsystems, 4 series, and 8 stages) to that of the other regions globally. Meanwhile, the Carboniferous chemo-, sequence-, cyclo-, and event-stratigraphy of China have been intensively studied and can also be correlated worldwide. Future studies on the Carboniferous in China should focus on (1) the correlation between shallow- and deep-water facies and between marine and continental facies, (2) high-resolution astronomical cyclostratigraphy, and (3) paleoenvironment and paleoclimate analysis based on geochemical proxies such as strontium and oxygen isotopes, as well as stomatal indices of fossil plants.
Provenance of the silt on the Chinese Loess Plateau (CLP) is essential for the paleo-environmental interpretation of the thick eolian deposits, but is highly debated. The controversy mainly comes from the multi-interpretation of similar geochemical signatures of potential source areas (PSAs). This work applies the (²³⁴U/²³⁸U) activity ratio as a new source tracer, because it can distinguish particles with different transporting and recycling histories regardless of petrological origin. The (²³⁴U/²³⁸U) activity ratio of a fine particle decreases progressively after its production because of the increasing fraction of the ²³⁴U precursor ejected out of the particle surface, due to the recoiling effect associated with the α decay of 238U. Distinct spatial patterns of (²³⁴U/²³⁸U) can be found for the sediments in PSAs and the loess on the CLP. When combined with other constraints, the new findings indicate that the CLP loess can be best explained by the mixing of three end-member dust sources on the northwestern transportation trajectory, namely (1) the Gobi Desert, (2) the Ordos Desert, and (3) the Qilian Mountains. A contribution from the Yellow River is also possible but may not be significant. The identified source partition implies that the eolian silt is mainly produced by processes in the 'High Asia' mountains and, partly, by erosion of the exposed clastic rocks. This new constraint on the production and transportation of eolian dust has great implications for the proxy interpretation of loess related to atmospheric circulation, dust accumulation rate, and chemical indexes.
A series of global major geological and biological events occurred during the Permian Period. Establishing a highresolution stratigraphic and temporal framework is essential to understand their cause-effect relationship. The official International timescale of the Permian System consists of three series (i.e., Cisuralian, Guadalupian and Lopingian in ascending order) and nine stages. In China, the Permian System is composed of three series (Chuanshanian, Yansingian and Lopingian) and eight stages, of which the subdivisions and definitions of the Chuanshanian and Yangsingian series are very different from the Cisuralian and Guadalupian series. The Permian Period spanned ∼47 Myr. Its base is defined by the First Appearance Datum (FAD) of the conodont Streptognathodus isolatus at Aidaralash, Kazakhstan with an interpolated absolute age 298.9±0.15 Ma at Usolka, southern Urals, Russia. Its top equals the base of the Triassic System and is defined by the FAD of the conodont Hindeodus parvus at Meishan D section, southeast China with an interpolated absolute age 251.902±0.024 Ma. Thirty-five conodont, 23 fusulinid, 17 radiolarian and 20 ammonoid zones are established for the Permian in China, of which the Guadalupian and Lopingian conodont zones have been served as the standard for international correlation. The Permian δ¹³Ccarb trend indicates that it is characterized by a rapid negative shift of 3–5‰ at the end of the Changhsingian, which can be used for global correlation of the end-Permian mass extinction interval, but δ¹³Ccarb records from all other intervals may have more or less suffered subsequent diagenetic alteration or represented regional or local signatures only. Permian δ¹⁸O{ainpatite} studies suggest that an icehouse stage dominated the time interval from the late Carboniferous to Kungurian (late Cisuralian). However, paleoclimate began to ameriolate during the late Kungurian and gradually shifted into a greenhouse-dominated stage during the Guadalupian. The Changhsingian was a relatively cool stage, followed by a globally-recognizable rapid temperature rise of 8–10°C at the very end of the Changhsingian. The ⁸⁷Sr/⁸⁶Sr ratio trend shows that their values at the beginning of the Permian were between 0.70800, then gradually decreased to the late Capitanian minimum 0.70680–0.70690, followed by a persistent increase until the end of the Permian with the value 0.70708. Magenetostratigraphy suggests two distinct stages separated by the Illawarra Reversal in the middle Wordian, of which the lower is the reverse polarity Kiaman Superchron and the upper is the mixed-polarity Illawarra Superchron. The end-Guadalupian (or pre-Lopingian) biological crisis occurred during the late Capitanian, when faunal changeovers of different fossil groups had different paces, but generally experienced a relatively long time from the Jinogondolella altudensis Zone until the earliest Wuchiapingian. The end-Permian mass extinction was a catastrophic event that is best constrained at the Meishan section, which occurred at 251.941±0.037 Ma and persisted no more than 61±48 kyr. After the major pulse at Bed 25, the extinction patterns are displayed differently in different sections. The global end-Guadalupian regression is manifested between the conodont Jinogondolella xuanhanensis and Clarkina dukouensis zones and the end-Changhsingian transgression began in the Hindeodus changxingensis-Clarkina zhejiangensis Zone. The Permian Period is also characterized by strong faunal provincialism in general, which resulted in difficulties in inter-continental and inter-regional correlation of both marine and terrestrial systems. © 2018, Science China Press and Springer-Verlag GmbH Germany, part of Springer Nature.
The surface uplift of the Tibetan Plateau (TP) is among the most important geological events in Earth's history, but the growth of the eastern TP margin that yielded the thickened crust and abrupt topography remains controversial. In this paper, six sample apatite fission-track (AFT) analyses of three boreholes in the Xiji Basin are applied to constrain the Mesozoic-Cenozoic uplift and exhumation history of the eastern Qilian Shan, northeastern TP. Most of the AFT ages range from 134 ± 8 Ma to 117 ± 6 Ma, except for the shallowest sample, with a younger age of 65.9 ± 3.2 Ma. Thermal history modeling indicates that the eastern Qilian Shan experienced a three-phase differential cooling history: (1) widespread rapid cooling during the Middle Jurassic-Early Cretaceous (ca. 174-120 Ma, Stage 1), (2) regional cooling during the Late Cretaceous-Paleocene (ca. 80-60 Ma, Stage 2) and (3) widespread cooling during the Eocene-early Miocene (ca. 40-20 Ma, Stage 3). We conclude that the Middle Jurassic-Early Cretaceous was the main stage of the growth and thickening of the northeastern TP, which was related to the upper-crustal horizontal shortening of the eastern Qilian Shan. Thermal history modeling of the youngest sample and seismic profile analysis imply significant reactivation of the Liupanshan fault zone during ca. 80-60 Ma. This locally intense uplift and deformation on the eastern margin of the Qilian Shan during the Late Cretaceous was likely induced by the closure of the Neo-Tethys and the continued shortening of the Lhasa-Qiangtang block. Late Eocene-Early Miocene (ca. 40-20 Ma) cooling of the eastern margin of the Qilian Shan records the coeval crustal extension and exhumation of the northeastern margin of the TP as the far-field effect of subduction of the western Pacific plate. Our AFT data detect no intense late Cenozoic reactivation information for the Xiji region, which indicates that the ca. 15-7 Ma rapid uplift triggered by fault reactivation was only located in the Haiyuan-Liupanshan fault zone, along the eastern margin of the Qilian Shan, northeastern TP. This multi-phase exhumation history is not beneficial for regional hydrocarbon exploration.
Tectonic evolution significantly influenced the sandstone-type uranium mineralization in the continental basins of northern China. To better understand the relationship between the tectono-thermal evolution and uranium-ore-forming process, we selected the southwestern Ordos Basin as the research subject. Detrital apatite and zircon fission track analyses of samples from the Early Cretaceous succession were conducted in this work. The results revealed five stages of tectono-thermal events since the Late Mesozoic: (i) the Late Jurassic–Early Cretaceous (165∼140 Ma), (ii) the late Early Cretaceous (130∼100 Ma), (iii) the Late Cretaceous (100∼60 Ma), (iv) the Late Paleogene–Early Neogene (55∼20 Ma), and (v) the Late Miocene–present (<10 Ma). These activities are recognized as the results of the multiplate convergence of the Siberian, paleo-Pacific, and Indian plates since the Mesozoic. Furthermore, the Late Jurassic–Early Cretaceous uplifting of the western Ordos Basin supplied the abundant sedimentation materials and uranium materials for the Early Cretaceous succession in the basin, which acted as excellent ore-bearing layers. The regional extension in the Early Cretaceous has led to an increasing geothermal gradient, which caused the maturity of hydrocarbon source rocks and escape of oil and gas. The following tectonic revisions supplied the pathways and favorable locations for oil and gas accumulation and uranium mineralization. The latest activity has driven the reactivation and re-enrichment of the uranium mineralization since the Late Miocene.
Bleaching of red beds, a type of hydrocarbon-induced alteration, is generally attributed to redox reactions between ferric iron minerals and hydrocarbon-bearing solutions. Herein, we report sandstone bleaching occurs interbedded with the coal- and dark mudstone-bearing strata at shallow depths below two unconformity surfaces separating sandstone formations of Triassic-Jurassic age in the Ordos Basin, China. Field observations, petrography, and geochemistry suggest that uplift events controlled the formation of red beds via supergene alteration and bleaching via hydrocarbon circulation. The color of sandstones below the unconformities grade from red, yellow, and white colors at shallow depths (few meters to tens of meters) to dark yellow, gray-green and gray colors at deeper depths. Organic matter (carbonaceous plant debris) and pyrite in the unaltered sandstone gave rise to the gray color. The red/yellow sandstones are characterized by the presence of extensive iron oxide/hydroxide grain coatings, exhibit intense dissolution and extensive kaolinization of detrital feldspar and biotite and lithics and are mainly composed of detrital quartz. The white, bleached sandstone presents similar petrographic characteristics as the unbleached sandstone except for a lack of iron oxide/hydroxide cements. δ18OVSMOW (9.8‰ to 15.8‰) and δDVSMOW (−103‰ to −119‰) values of kaolinite, and chemical indices of alteration of sandstones indicate a weathering origin for the kaolinite and the dissolution of labile minerals in the red and yellow sandstones. The original color of the bleached sandstone was gray during very early diagenesis and shifted to red/yellow due to the oxidation of pyrite and ferromagnesian silicate minerals (e.g., biotite) into hematite or goethite cements by the meteoric water circulation during regional uplift following the deposition of each formation. Supergene alteration associated with unconformities also created significant secondary porosity, and allowed later hydrocarbons to flow along the unconformities. The lithological properties of the weathered rocks below unconformities are highly heterogeneous both vertically and laterally and have a significant influence on fluid flow. This study provides direct evidence for hydrocarbon migration along unconformities and improves understanding of fluid-rock interaction in subsurface reservoirs.
The Asian interior contains the Central Asian Orogenic Belt (CAOB, the longest accretionary orogen on Earth), the largest mid-latitude desert, and the most widespread late Cenozoic airborne dust deposits (Red Clay) in the Chinese Loess Plateau. This Quaternary loess deposit is underlain by aeolian Red Clay, which has a basal age of ~7 Ma throughout most of the Loess Plateau (except for several sites with latest Oligocene age in its western margin). By integrating the most recent knowledge of the reactivation and uplift history of the CAOB and the northeastern (NE) Tibetan Plateau with the magnetostratigraphic ages of the aeolian Red Clay, we provide evidence for coeval timing between the reactivation of the CAOB-NE Tibetan Plateau and the accumulation of the Red Clay, both of which began at ∼7 Ma. We suggest that the reactivation of the CAOB and the NE Tibetan Plateau played an important role in the deposition of the Red Clay in the latest Miocene-Pliocene through its control of the production of vast amounts of silt-sized debris created by frost weathering and of the transportation by the enhanced strength of the northwesterly wind systems. Detrital zircon ages demonstrate that the source materials of the late Cenozoic Red Clay were mainly derived from a mixed provenance of enlarged alluvial fans coherently linked to the late Cenozoic uplift of the CAOB and the NE Tibetan Plateau. These relations provide the basis for understanding the connections between the deep Earth (lithosphere) and surface processes and their impact on the ecosystem in Asia.
The thermochronological history of orogens along plate margins provides unique constraints on regional tectonic evolution. Given existing thermochronological data that identify late Mesozoic–Cenozoic multiphase cooling events, we perform new apatite fission-track and (U-Th)/He low-temperature thermochronology on Wula Shan and Dahong Shan rocks to reveal the first stages of cooling along the southern Yin Shan orogenic belt, northern margin of the North China Block (NCB). Our new thermal history modeling suggests that cooling of the southern Yin Shan began in the Late Triassic (ca. 230 Ma), with rapid cooling at ca. 210–178 Ma and ca. 10~ Ma in the Wula Shan and ca. 200–160 Ma in the southern Dahong Shan. Late Triassic-Early Jurassic cooling stages are related to uplift and exhumation of the northern NCB, which given the tectonic setting of the late Indosinian event, are mainly interpreted as signatures of Mongol–Okhotsk Ocean subduction. Subsequently, enhanced plate convergence of East Asia with closure of the Mongol–Okhotsk Ocean led to intense thrust-nappe and intracontinental orogeny during the early Yanshanian. Importantly, our new finding of ca. 10 Ma rapid cooling of the Wula Shan at a rate of ~3.5 °C/Ma corresponds to coeval uplift and synrift basin sedimentation in the Ordos Block periphery, western NCB, which could mark the response to craton-scale rotational deformation. The completion of this low-temperature thermochronological database allows us to reappraise the Mesozoic exhumation and ca.10 Ma reactivation history and its implications for the southern Yin Shan.
In order to clarify the hydrocarbon accumulation significance and exploration prospect of the unconformity caused by the Huaiyuan movement in the Ordos Basin, this paper studies the spatial distribution and structural plane characteristics of this unconformity and its relationships with hydrocarbon accumulation by observing field outcrops and cores and analyzing logging data, based on the previous research results and the interpretation results of 2D and new 3D seismic data. And the following research results are obtained. First, the unconformity was mainly formed in the Floian Age of Early Ordovician and widely occurs at the bottom of the Jiawang Formation and the top of the Sanshanzi Formation and the related tectonism lasts 30 Myr. Second, basal conglomerate and sandstone less than 1 m in thickness are developed above the unconformity at the edge of the basin, while thin mudstone, argillaceous dolomite (limestone) and marl are developed above the unconformity in the central part of the basin. Third, the unconformity structurally consists of three layers, including a basal conglomerate layer, a paleosoil layer and a fully–semi weathered carbonate layer from top to bottom, among which, the last one is 20–90 m in thickness with developed dissolution fractures and pores to form a quality reservoir. Fourth, the unconformity results in the development of a series of large valleys landforms, which incise the Lower Ordovician–Upper Cambrian. Fifth, the unconformity can act as a good channel for hydrocarbon migration, and it connects with the unconformity caused by the Caledonian movement within the paleo-uplift of the Ordos Basin, which is favorable for the the hydrocarbon of different sources in the west side of the basin eastwards to migrate, accumulate and form a gas reservoir. In conclusion, the deep Lower Paleozoic related to the Huaiyuan unconformity is expected to be an important natural gas exploration field in the Ordos Basin.
Thermal regime and thermal history are of significant important in basin dynamics and hydrocarbon generation of source rocks. The sedimentary basins in three typical cratons of China, North China Craton (NCC), Yangtze Craton (YC) and Tarim Craton (TC), underwent completely different tectonic background and dynamic mechanism, which resulted in the differentials of their thermal regimes. In this paper, the thermal regimes of the Bohai Bay Basin in the eastern NCC, Tarim Basin in TC, and Sichuan basin in the Upper YC were studied. There has some lower thermal background in the area of western China than eastern side of the NCC. The mean heat flow value of Tarim basin is 42.5 mW/m², 53.8 mW/m² in the Sichuan basin and 66.7 mW/m² in the eastern NCC. And the temperature at 10000 m ultra-depth in the Tarim Basin also shows lower values than that of Sichuan and Bohai Bay basins. The thermal history, modeled from thermal indicators of vitrinite reflectance (Ro), equivalent vitrinite reflectance data (Requ), fission track and (UTh)/He data of apatite and zircon, evolved differently because of the different tectonic history in these basins. The Tarim basin underwent a cooling history with the heat flow of 55–65 mW/m² in the Cambrian decreasing to the present-day of 40–50 mW/m². The Meso-Cenozoic thermal history of the Bohai Bay basin experienced four evolutionary stages with two heat flow peaks in the late Early Cretaceous and in the Middle to Late Paleogene, which correspond to the destruction peaks of the eastern NCC and had heat flow values of 82–86 mW/m² and 81–88 mW/m², respectively. The Sichuan basin was stable with heat flow value of less than 65 mW/m² from the Late Sinian to Late Paleozoic, and the heat flow increased rapidly to the peak value (75–100 mW/m²) at the end of Early Permian by thermal effect of the Emeishan mantle plume. From Mesozoic to present, the heat flow decreased to the present value (60–70 mW/m²). In addition, the comparation of thermal characteristics of typical cratons around the world, including heat flow, thermal history, lithospheric thermal structure and thermal lithospheric thickness, also have been caried out. The differential thermal histories of Tarim and Sichuan basins resulted in the different thermal evolution of Cambrian source rocks. This study may provide some new geothermal evidence for hydrocarbon generation and phase distribution of oil and gas in the deep and/or ultra-deep basin.
Differential tectonothermal evolutionary history of the southern part of the Western Ordos Basin, the southern part of the Liupanshan Mountains, and the northwestern part of the Weibei Uplift was revealed since Late Jurassic by integrating low-temperature thermochronology of apatite fission-track, apatite (U-Th-Sm)/He and zircon (U-Th)/He along with vitrinite reflectance. Inverse modeling results suggest four cooling stages among these three regions since Late Jurassic. Uplift-cooling in the Southwestern Ordos Basin commenced simultaneously with that in the South Liupanshan Mountains in Late Jurassic at ∼164-160 Ma. During Late Cretaceous, the entire southwestern region uplifted and cooled since ∼118 Ma in the Southwestern Ordos Basin and in the Northwest Weibei Uplift, and since ∼105 Ma in the South Liupanshan Mountains. During Early Cenozoic, the Southwestern Ordos Basin and the Northwest Weibei Uplift continued uplift-cooling, while parts of the South Liupanshan Mountains subsided. Since Miocene, a rapid uplift-cooling event of the South Liupanshan Mountains commenced at ∼24 Ma, and at ∼8 Ma for the Southwestern Ordos Basin and the Northwest Weibei Uplift. In general, the differential evolutionary process of the southwestern North China Plate began in Late Jurassic, continued in Cretaceous and reaching a cooling peak during Miocene with heterogeneous start-time and cooling rate of different sub-units, and mainly controlled by the regional uplift-cooling event of the Qinling-Qilian Orogen in Late Jurassic - Early Cretaceous and by the Qinghai-Tibet Plateau event since Paleocene.
The North China Craton (NCC) experienced important tectonic events during the Meso-Cenozoic as implied by many geological studies. Combined with existing low-temperature thermochronological data, new apatite fission track (AFT) and zircon (UTh)/He data from the Lüliangshan and Taihangshan mountains in this study reveal that the central NCC experienced tectonothermal events during the Meso-Cenozoic, i.e., ca. 150 Ma, 110–80 Ma, 50–40 Ma and 30–20 Ma. The Late Jurassic event caused the formation of the Lüliangshan basement-involved anticline. The main folding and thrusting in the central NCC developed in the Mesozoic instead of the Cenozoic and resulted from the low-angle subduction of the Paleo-Pacific ocean plate beneath the Eurasian Plate during the Late Jurassic; and the Late Cretaceous tectonic event stopped the Early Cretaceous extension across the eastern Asia, thrusting and sinistral strike-slip faulting occurred across the central NCC, which was possibly caused by a collision along the southeastern boundary of eastern Asia. The early Cenozoic event in central NCC was coeval with an event occurring throughout eastern Asia and may have been a regional response to changes in the movement direction and velocity of the Pacific Plate, and this event may have resulted in the formation of rift basins in the eastern NCC. The whole central NCC experienced uplift during this period in isostatic readjustment to this extension.
The Songliao Basin in northeast China in the eastern part of the Central Asia Orogenic Belt is the largest accretionary orogen belt in the world. However, to date, previous studies have not extensively examined the dynamic mechanism of its formation and evolution, especially the pre-Cretaceous. In this study, the Mesozoic–Cenozoic tectonic style, stratigraphic records, sedimentary time, magmatic activity, and tectonic evolution in the Songliao Basin were systematically analysed and summarised. We propose that Songliao Basin experienced multi-stage basin superposition processes: the foreland compression stage, syn-rift and post-rift stage, and structural inversion stage. After Palaeozoic deposition, the basin experienced a strong orogeny with the formation of the Triassic–Middle Jurassic foreland basin. During the syn-rift period, three tectonic movements occurred at the end of the Shahezi Formation (Late Valangnian, ~135 Ma), the end of first member of the Yingcheng Formation (Late Hauterivian, ~129 Ma), and the end of the Yingcheng Formation (Late Barrenian, ~125 Ma) in the Songliao Basin, which ended the evolutional history of the rift. In the post-rift period, three tectonic events occurred at the end of the Nenjiang Formation (Middle Campanian, ~79 Ma), the Mingshui Formation (Late Maastrichtian, ~66 Ma), and the Yi'an Formation (Late Chattian, ~23 Ma), which determined the deformation characteristics of the middle and shallow strata of the basin, such as folding and reverse faults. The rudimentary shape formed at the end of the Nenjiang Formation, was finalised at the end of the Mingshui Formation, and formed at the end of the Yi’an Formation. The largest depth and width influence of the two events was at the end of the Mingshui and Yi’an formations, which played a decisive role in the generation, migration, and accumulation of oil and gas in the middle and shallow strata of Songliao Basin. After a series of asthenospheric mantle upwelling, crustal uplift and extension, and volcanic activity, Songliao Basin was eventually formed. The formation and structural evolution of the Songliao Basin were mainly influenced by the Paleo-Asian Ocean, the Mongol-Okhotsk Ocean, and the Pacific Ocean. The closure of the Paleo-Asian Ocean in the Late Permian-Early Triassic was influenced by the Siberian Plate, which began to contact and collide with the Xingan-Mongolian Block from west to east, causing the Mongol-Okhotsk Ocean to close in a scissor-like structure from west to east in the Early–Middle Jurassic. The subduction and movement of Pacific Rim tectonic systems, such as the Farallon, Izanagi, Pacific, and Philippine plates, led to the formation of Songliao Basin, which has experienced a foreland period via compressive pressure since the Triassic, syn-rift period and post-rift period via extensional stress, and then a structural inversion period.
With continuous improvements in oil and gas exploration technology and the increasing demand for oil and gas resources, the focus of oil and gas exploration has shifted from deep to ultradeep reservoirs. This study targeted the carbonate karst reservoirs of the Ordovician Ma5 Member in the southeastern Ordos Basin. Based on the analysis of cores, thin sections, cathodoluminescence, scanning electron microscopy (SEM), carbon and oxygen isotopes and trace elements, we discuss the development characteristics, types and influencing factors of paleokarst reservoirs. Petrology and geochemistry show three kinds of karstification in the study area: quasi-contemporaneous karst, epigenetic karst and deep burial karst. The quasi-contemporaneous and epigenetic paleokarsts were mainly controlled by freshwater, where the soluble components of the strata were preferentially dissolved. The deep burial karstification was dominated by the reaction product H2S from thermochemical sulfate reduction (TSR) between sulfate minerals and organic matter, during which hydrothermal fluid provided the required activation energy for the reaction. The observations of cores and thin sections show that the paleokarst reservoirs are composed of different kinds of dissolution pores and fractures and cemented by multistage calcite and siliceous cements. Influenced by freshwater leaching and hydrothermal fluid, the three paleokarst processes led to negative anomalies in δ¹⁸O and δ¹³C, while the low δCe, low δEu and rare earth element (REE) partition modes confirm that the intensity of freshwater leaching was stronger than that of hydrothermal fluid. The controlling factors of the paleokarst reservoirs included acidic fluids, paleogeomorphology and paleostructure in each diagenetic stage. According to the structural background and core characteristics, we establish a development model and spatial evolution model of the paleokarst reservoirs in the southeastern Ordos Basin.
Post-depositional modification is commonly found in sedimentary basins, particularly in the eastern Tethys Tectonic Domain, and is related to special geological settings and evolution processes of the region. The Chinese continent is composed of numerous small-scale blocks that underwent intensive tectonic movements. Paleogeography reconstruction for a modified basin is fundamental for objectively understanding the basin dynamics and exploration of hydrocarbon, sandstone-type uranium deposits, and other mineral resources. As a large-scale intra-cratonic depression basin in the western part of the North China Craton (NCC), the Mesozoic Ordos Basin underwent multi-stage modification; as a result, only the western part was preserved. In this study, the paleogeography in Bathonian–Oxfordian ages of the Jurassic was reconstructed via integrated approaches including stratigraphic correlation, sedimentary facies analysis, provenance analysis, and fission track analysis. The original sedimentary boundary of the Ordos Basin during this period is approximately outlined. We determine that the northern and southern borders reach the Cenozoic Hetao Basin and the Cenozoic Weihe Basin, respectively, and that the eastern, northwestern, and southwestern borders lie to the west of Taihang Mountain, at the Langshan–Bayanhot Basin–Kexueshan, and to the west of Liupan Mountain, respectively. The original sedimentary extent of the Ordos Basin was almost twice that of the present-day residual basin. The evolution of this basin in the Mesozoic recorded the evolution and destruction processes of the NCC. The NCC shows that N–S- and W–E-trending compressional setting occurred during the Triassic and the Jurassic, respectively. The interactive activities of adjacent tectonic domains played a critical role in subsidence dynamics for the intra-cratonic Ordos Basin. This study provides further insights for exploration of hydrocarbon and sandstone-type uranium deposits in the Ordos Basin.
Proterozoic sedimentary covers in the Helan-Qianli Mountain area, western margin of the North China Craton (NCC), include the Huangqikou, Wangquankou and Zhengmuguan formations from bottom to top. The Huangqikou Formation, mainly distributed in the Middle Helan Mountain (MHM), North Helan Mountain (NHM) and Qianli Mountain (QM), is composed of conglomerate, quartz sandstone and silty-argillaceous shale that are locally metamorphosed into quartzite and slate. Detrital zircon U-Pb dating and stratigraphic correlation bracket that the Huangqikou Formation deposited during 1.75-1.60 Ga. Petrologic and geochemical features suggest that the provenance was sourced from felsic rocks and pre-existed sedimentary rocks. U-Pb ages of detrital zircons from the Huangqikou Formation in the MHM area yield age ranges at 2.5-2.35 Ga, 2.2-2.0 Ga and 1.95-1.8 Ga, while those in the NHM-QM areas yield age ranges at 2.5-2.4 Ga, 2.0-1.95 Ga and 1.85-1.78 Ga. The remarkable geochronological and isotopic similarity suggests that the early Paleoproterozoic granitic gneiss in the Ordos Block and Zhaochigou complex in the Khondalite Belt are the dominant source rocks for the Huangqikou Formation in the MHM. However, the provenances of the Huangqikou Formation in the NHM-QM are dominated by the granitoids in Guyang area of the Yinshan Block, the S-type granites and paragneisses in the Khondalite Belt and the TTG gneisses, Bayanwulashan complex and Boluositanmiao complex in the Alxa Block, as well as a small amount basement rocks in Alxa Block. The characteristics of geochemistry, provenance and regional geological data indicate that the Huangqikou Formation deposited in a rift basin. Besides, detritus sourced from the Alxa Block in the clastic sedimentary rocks of the Huangqikou Formation imply that the Alxa Block was already a part of the NCC before ca.1.75 Ga. The U-Pb age and Lu-Hf isotopic characteristics of detrital zircons from the Statherian show that rapid generation of juvenile crust in the western Block was at 2.9-2.7 Ga, yet the ca.2.5 Ga magmatic activity strongly reworked pre-existed crust with negligible juvenile crust generation. The magmatic activities quite possibly developed in the Western Block during the global magmatic shutdown (2.45-2.2 Ga).
We present new apatite U-Th-Sm/He (AHe; n = 51), apatite fission track data (AFT; n = 12), and zircon U-Th/He (ZHe; n = 8) data for two elevation transects in the north-central Shanxi Rift, North China. Low-temperature thermochronologic data combined with forward and inverse time-temperature history models reveal a Precambrian to Quaternary thermal history characterized by: (1) cooling to <∼50 °C during the Proterozoic, consistent with the development of a regional unconformity above Neoarchean–Paleoproterozoic cratonic basement rocks; (2) reheating to <∼180 °C due to sediment burial during the Paleozoic to Mesozoic; (3) cooling at a rate >3.5 °C/Ma during the Late Jurassic to earliest Cretaceous Yanshanian orogeny; (4) a possible ca. 120-90 Ma reheating event due to elevated geothermal gradients and/or local sediment burial; (5) Late Cretaceous (ca. 110-65 Ma) cooling contemporaneous with regional extension in eastern Asia and denudation of the paleo-Taihangshan highlands; and finally, (6) post ca. 10 Ma cooling associated with extension in the Shanxi Rift. AFT dates from the deepest exhumed structural positions of the sampled footwall blocks are mostly >65 Ma and AHe dates tend to be highly dispersed within samples. AFT inverse and AHe forward model results indicate that samples were at temperatures of <∼75 °C by ca. 70 Ma. Despite the early Cenozoic and older AFT and AHe dates, metamict zircon grains with high effective uranium (eU >∼750 ppm) yield young ZHe dates of ca. 13-9 Ma, consistent with Late Miocene exhumation. We argue for the onset of latest cooling by ca. 10 Ma based on these ZHe dates; however, the precise timing for the onset of rifting remains uncertain. The results further suggest that Late Miocene–Quaternary extension in the north-central Shanxi Rift is responsible for ≤∼2.5 km of exhumation, such that published Quaternary extension and fault throw rates are significantly (>100%) higher than long-term rates inferred from the thermochronologic data.
This study provides an integrated interpretation of the Mesozoic ‐ Cenozoic tectonothermal evolution of Permian strata in the Qishan area of the southwestern Weibei Uplift, Ordos Basin. Apatite fission‐track and apatite/zircon (U‐Th)/He thermochronometry, bitumen reflectance, thermal conductivity of rocks, paleotemperature recovery and basin modeling were used to recover the Meso‐Cenozoic tectonothermal history of Permian Strata. The Triassic AFT data has a pooled age of ∼180 ± 7 Ma (one age peak) with P(χ2) = 86%. The average corrected apatite (U‐Th)/He age of two Permian sandstones is ∼168±4 Ma and a zircon (U‐Th)/He age from Cambrian strata is ∼231±14 Ma. Bitumen reflectance and maximum paleotemperature of two Ordovician mudstones are 1.81%, 1.57% and ∼210°C, ∼196°C respectively. After undergoing a rapid subsidence and increasing temperature in the Triassic by a thermal abnormal, the Permian experienced four stages of cooling‐uplift history after the time when the maximum paleotemperature reached in the late Jurassic: (1) A cooling stage (∼163 Ma ‐ ∼140 Ma) with temperatures ranging from ∼132 °C to ∼53 °C and cooling rate of ∼3 °C/Ma, erosion thickness of ∼1900 m and an uplift rate of ∼82 m/Ma; (2) A cooling stage (∼140 Ma ‐ ∼52 Ma) with temperatures ranging from ∼53 °C to ∼47 °C and cooling rate less than ∼0.1 °C/Ma, erosion thickness of ∼300 m and an uplift rate of ∼3 m/Ma; (3) (∼52 Ma ‐ ∼8 Ma) with ∼47 °C to ∼43 °C and ∼0.1 °C/Ma, erosion thickness of ∼500 m and an uplift rate of ∼11 m/Ma; (3) (∼8 Ma – present) with ∼43 °C to ∼20 °C and ∼3 °C/Ma, erosion thickness of ∼650 m and an uplift rate of ∼81 m/Ma. Tectonothermal evolution history of the Qishan area in the Triassic was controlled by the interaction of the Qinling Orogeny and the Weibei Uplift, and the exposed Permian strata had the earliest uplift‐cooling time compared to other parts within the Weibei Uplift. The early Eocene at ∼52 Ma and late Miocene at ∼8 Ma, as two significant turning points after which both the rate of uplift and the rate of temperature changed rapidly, were two key time of uplift‐cooling history for the exposed Permian in the Qishan area.
The formation of sandstone-hosted uranium deposits has been linked to upward migration of hydrocarbons in many sedimentary basins. However, hydrocarbon-induced diagenetic alterations associated with uranium mineralization and preservation are rarely discussed. Here, we report the bleaching of red sandstone to green in the Middle Jurassic Zhiluo Formation from the uranium deposits in the northern Ordos Basin, China. The original gray sandstone underwent oxidation and was reddened by the downward flow of meteoric water from the northern margin of the basin during the Late Cretaceous–Cenozoic uplift. This process resulted in the leaching of preore-stage uranium concentration and oxidation of preore pyrite in the organic matter-rich sandstone. Uranyl and sulfate ions were carried downward through the host sandstone by oxygenated groundwater. Meanwhile, hydrocarbons migrated upward and reacted with the downward flowing oxidizing fluids, which induced uranium mineralization at the redox interface between the red and gray sandstone where iron was reduced to Fe ²⁺ and incorporated into pyrite cement. The highly negative ³⁴ S values (−58.0‰ to −33.4‰ V-CDT)of the pyrite indicate that it originated from bacterial sulfate reduction during hydrocarbon biodegradation. Abundant organic acids and CO 2 were generated from hydrocarbon oxidation, resulting in the dissolution of feldspars and the precipitation of kaolinite in the mineralized sandstone. With the formation of the Cenozoic Hetao graben to the north, the gravity-driven flow of meteoric water was limited, resulting in progressive halting of epigenetic uranium mineralization. Then, the upward hydrocarbons dominated the fluid system throughout the northern slope of the basin and bleached the previous red sandstone updip, which set up a strong reducing environment and protected the early formed uranium deposits at the paleo-redox front from oxidation and remobilization. Due to the oxygen and sulfur supply absence, the iron released from hematite during bleaching was immobilized and incorporated into Fe-rich chlorite that imparts the green color, rather than into pyrite or reprecipitation as hematite. This study provides insights for the processes of hydrocarbon-water-rock interactions associated with uranium mineralization and preservation, which can serve as exploration guides for sandstone-hosted uranium deposits.
While a general concensus has recently been reached as to the causal relationship between the subduction of the west Pacific plate and the destruction of the North China Craton, a number of important questions remain to answer, including the initial subduction of west Pacific plate beneath the eastern Asian continent, the position of west Pacific subduction zone during the peak period of decratonization (i.e., Early Cretaceous), the formation age of the big mantle wedge under eastern Asia, and the fate of the subducted Pacific slab. Integration of available data suggests that the subduction of the western Pacific plate was initiated as early as Early Jurrasic and the subduction zone was situated to 2,200 km west of the present-day trench in the Early Creataceous, as a result of eastward migration of the Asian continent over a distance of ca. 900 km since the Early Cretaceous. The retreat of the subducting west Pacific plate started ∼145 Ma ago, corresponding to the initial formation of the big mantle wedge system in the Early Cretaceous. The subduction of the Pacific slab excerted severe influence on the North China Craton most likely through material and energy echange between the big mantle wedge and overlying cratonic lithosphere. The evolution history of the west Pacific plate was reconstructed based on tectonic events. This allows to propose that the causes of phases A and B for the Yanshanian orogeny were respectively related to rapid low-angle subduction and to lowering subduction angle of the west Pacific plate. At ca. 130–120 Ma, the subduction of the west Pacific plate was characterized by increasing subducting angle, slab rollback and rapid trench retreat, leading to the final stagnation of the subducting slab within the mantle transition zone. This process may have significantly affected the physical property and viscosity of the mantle wedge above the stagnant slab, resulting in non-steady mantle flows. The ingression of slab-released melts/fluids would significantly lower the viscosity of the mantle wedge and overlying lithosphere, inducing decratonization. This study yields important bearings on the relationship between the subduction of the west Pacific plate and the evolution of the lithospheric mantle beneath the North China Craton.
Although it remains deeply hidden and largely enigmatic, the basement of the Ordos Basin has played a pivotal role in the evolution of the western part of North China Craton. Diverse gneiss samples collected from boreholes advanced into the deep basement of the Ordos Basin are investigated by integrating petrography, phase equilibria modeling and geochronology. The results reveal the basement gneisses have different protoliths with ages extending from the late Archean to the Paleoproterozoic. Medium to low pressure metamorphism from amphibolite to granulite facies (4.7–7.3 kbar and 740–810 °C) is constrained for the basement gneisses, using phase equilibrium modeling in the MnNCKFMASHTO system. Evidence is provided of metamorphic monazite growing later than zircon in the same basement gneisses, and multi-stage Paleoproterozoic orogenesis is delineated using metamorphic zircon and monazite ages from the basement gneisses. The basement gneisses from most boreholes across the basin are comparable to those in adjoining Paleoproterozoic belts in terms of metamorphic and geochronological characteristics. The results of the study suggest, in combination with previous data, that a large area of the basement was involved in the Paleoproterozoic orogenis. The northern part of the Ordos Basin basement is closely associated with the Khondalite Belt.
The Mesozoic Western Pacific subduction system significantly impacted the North China and South China blocks along the East Asian continental margin and influenced the tectonic, magmatic, metallogenic and geomorphic evolution of the region. However, the dynamics and impact on the zone along the East Asian ocean-continent connection zone remain debated. Here we provide a comprehensive synthesis of the state-of-the-art information from deformation analysis, magmatism, geochronology, tomography and other fields from this region. We evaluate first the pre-Yanshanian (pre-Jurassic) final assembly of blocks and the Late Triassic formation of the unified continental margin in East China. We then focus on the Jurassic and Cretaceous geological processes in the East Asian ocean-continent connection zone. The temporal and spatial evolution of structural propagation, sedimentary depocentre, age zonation and migration of magmatism, as well as the large-scale tectono-morphological inversion in the Earth surface system combined with deep processes, are probed. In the early Yanshannian Period (Early and Middle Jurassic, 200–160 Ma), the destruction of the North China Craton (NCC) was mainly affected by the westward early-stage layered rollback, and stepwise delamination and thinning of its continental lithosphere, resulting in the early Yanshanian westward migration of tectonism and magmatism. Coevally, the combined effect of the closure of the Mongal-Okhotsk Ocean to the north and the subduction of the Bangong-Co- Nujiang Ocean to the south imparted an overall compressional setting in the East Asian Ocean-Continent Connection Zone (EAOCCZ). The centres of asthenospheric upwelling and mantle extrusion at depth continued to migrate eastward, driving the eastward lithosphere thinning with periodic and alternating extension and compression. The South China Block experienced a westward flat subduction during the early Yanshanian Period, resulting in the westward propagation of deformation and magmatism, followed by late two-stage delamination to induce the eastward tectono-magmatism. The difference in tectono-magmatic styles between the North China and South China blocks is a result of the different mechanisms and syles of the deep delamination processes under the superconvergence regime of the East Asian and adjacent plates. Especially delamination under North China generted the northwestward layered and fractured subcontinental lithospheric mantle, whereas under the eastern South China Block, the oceanic lithospheric mantle of the Paleo- Pacific Plate that underwent flat subduction, or continental garnet peridotite mantle. In the middle Yanshanian Period (Late Jurassic to early Early Cretaceous, 160–125 Ma), the EAOCCZ underwent escape tectonics to form some basins related to strike slip faulting. Generally the extensional basins in the tails of the triangular-shaped escape blocks are perpendicular to the extrusion direction. The transtensional or transpressional basins are controlled by the strike slip faults distributed on both sides of the triangular block, and the flexural basins occur in front. In the late Yanshanian Period (late Early Cretaceous-Late Cretaceous, 125–65 Ma), the Paleo-Pacific (Izanagi) Plate subducted NNW-ward beneath the Eurasian continent, and the subduction angles changed gradually following eastward mantle extrusion induced by the closure of the Okhotsk Ocean to the north and Bangong-Nujiang Ocean to the south, as well as the rollback and subduction retreat of the Paleo-Pacific Plate to the east. The EAOCCZ gradually experienced lithospheric collapse and the formation of metamorphic core complexes, as well as obvious landscape reversal. During 70–45 Ma, the Izanagi-Pacific Ridge subducted beneath the EAOCCZ to induce wide uplift resulting in the formation of the Cenozoic dextral transtension-related basins.
Pangea is the youngest supercontinent in Earth's history and its main body formed by assembly of Gondwana and Laurasia about 300–250 Ma ago. As supported by voluminous evidence from reliable geological, paleomagnetic and paleontological data, configurations of major continental blocks in Pangea have been widely accepted. However, controversy has long surrounded the reconstructions of East Asian blocks in Pangea. To determine whether or not the East Asian blocks were assembled to join Pangea before its breakup, we carried out geological and paleomagnetic investigations on East Asian blocks and associated orogenic belts, supported by a NSFC Major Program entitle “Reconstructions of East Asian blocks in Pangea”. Our results indicate that the breakup of Rodinia around 750 Ma ago led to the opening of the Proto-Tethys and Paleo-Asian oceans in East Asia, with the former separating the South China, North China, Alex Qaidam and Tarim blocks from other East Asian blocks at the margins of Australia and India, whereas the Paleo-Asian Ocean existed between the East Asian blocks and Siberia-Eastern Europe. The Proto-Tethys Ocean closed in the early Paleozoic (500–420 Ma), leading to the collision of South China, North China, Alex, Qaidam and Tarim with other East Asian blocks at the northern margin of Gondwana. The subduction of the Paleo-Asian Ocean formed the Central Asian Orogenic Belt, the largest accretionary orogen in Earth's history, and its closure was diachronous, with its western, central and eastern segments closing at 310–280 Ma, 280–265 Ma and 260–245 Ma, respectively, leading the Tarim, Alex and North China blocks to join Eastern Europe-Siberia as part of Pangea. During the early Devonian (420–380 ma), the East Paleo-Tethys Ocean opened with two branches, of which the north branch is called the Mianlue Ocean that separated the Tarim-Qaidam-Central Qilian-Alex and North China blocks in the north from North Qiangtang-Indochina-South China in the south, and the south branch is the stricto sensu East Paleo-Tethys Ocean that separated North Qiangtang-Indochina-South China from the Sibumasu and South Qiangtang-Lhasa blocks at the northern margin of Gondwana. In the Triassic, the East Paleo-Tethys Ocean (stricto sensu) closed along the Longmu Co – Shuanghu – Changning – Menglian – Inthanon belt, leading to the collision of North Qiangtang-Indochina-South China with Sibumasu and South Qiangtang-Lhasa, forming a single southern continent, which then collided with the Tarim-Qaidam-Central Qilian-Alex and North China blocks to form a coherent East Asian continent that had become part of Pangea by 220 Ma, when the Mianlue Ocean closed, leading to the formation of the E-W-trending Central China Orogenic System.
The view that cratons are tectonically and geomorphologically inert continental fragments is at odds with a growing body of evidence partly based on low-temperature thermochronology (LTT) studies. These suggest that large areas of cratons may have undergone discrete episodes of regional-scale Neoproterozoic and/or Phanerozoic heating, and cooling from modestly elevated paleotemperatures. Cooling is often attributed to the km-scale erosion of overlying low-conductivity sediments, rather than to removal of large sections of crystalline basement. Independent evidence for sedimentary burial includes: preservation of outliers, the sedimentary record in intracratonic basins, and sedimentary xenoliths entrained within kimberlites periodically emplaced into cratons. Further, stratigraphic and isotopic data from basinal sediments proximal to some cratons carry a record of the detritus removed, which can be linked temporally to cooling episodes in their inferred cratonic source areas. Differences in denudation rates reported from cratonic basement reconstructed from LTT data (long-term) and cosmogenic isotope and chemical weathering studies (short-term) reflect the strong contrast in erodibility potential between cover sediments since removed and the preserved crystalline rocks. Underlying processes involved in cratonic heating and cooling may include one of, or a complex interplay between: proximity to sediment sources from elevated orogens forming extensive foreland basins, structural deformation transmitted by far-field horizontal stresses from active plate boundaries, and the development of dynamic topography driven by vertical mantle stresses. Dynamic topography may also explain elevation changes observed in some cratons, where no clear deformation is apparent. LTT studies from classic cratons in Fennoscandia, Western Australia, Southern Africa, and Canada are reviewed, with emphasis on different aspects of their more recent evolution.
The Yanchang Formation is extensively developed in the Ordos Basin and its surrounding regions. As one of the best terrestrial Triassic sequences in China and the major oil-gas bearing formations in the Ordos Basin, its age determination and stratigraphic assignment are important in geological survey and oil-gas exploration. It had been attributed to the Late Triassic and regarded as the typical representative of the Upper Triassic in northern China for a long time, although some scholars had already proposed that the lower part of this formation should be of the Middle Triassic age in the mid-late 20th century. In this paper, we suggest that the lower and middle parts of the Yanchang Formation should be of the Ladinian and the bottom possibly belongs to the late Anisian of the Middle Triassic, mainly based on new fossils found in it and high resolution radiometric dating results. The main source rocks, namely the oil shales and mudstones of the Chang-7, are of the Ladinian Age. The upper part of the Yanchang Formation, namely the Chang-6 and the above parts, belongs to the Late Triassic. The uppermost of the Triassic is missed in most parts of the Ordos Basin. The Middle-Upper Triassic Series boundary lies in the Yanchang Formation, equivalent to the boundary between Chang-7 and Chang-6. The Ladinian is an important palaeoenvironmental turning point in the Ordos Basin. Palaeoenvironmental changes in the basin are coincidence with that of the Sichuan Basin and the main tectonic movement of the Qinling Mountains. It indicates that tectonic activities of the Qinling Mountains are related to the big palaeoenvironmental changes in both the Ordos and Sichuan Basins, which are caused by the same structural dynamic system during the Ladinian.