Bohai Basin Research Articles

Study on crustal thickness and the prediction of prolific depressions: the Bohai Basin as an example

Study on crustal thickness and the prediction of prolific depressions: the Bohai Basin as an example

Analysis on Vertical Variation Characteristics of Eogene Formation Water in Jizhong Depression

Jizhong Depression is located in the middle and west of Bohai Basin. During the Tertiary period, the depression was in a relatively stable state, and the shape of the depression did not change much, but the internal ups and downs of the depression were still continuous. In the Tertiary period, it experienced three development stages: early division, middle communication and late contraction, and three different sedimentary facies appeared accordingly. At the same time, it also caused three shifts of sedimentary centers. In the medium-term communication stage, two large water invasion periods occurred, and in some areas, the sedimentary water medium changed from salty to light twice. Oil and gas in the depression are associated with formation water, and the formation history of formation water is closely related to the formation history of oil and gas reservoirs. They are both in a hydrodynamic system and exchange material components for a long time. The lower tertiary is the main oil-bearing interval in central Hebei, but it is affected by the late tectonic movement and its structural characteristics are complex. Therefore, the hydrochemistry and vertical distribution characteristics of tertiary strata are studied. By effectively studying the distribution characteristics of formation water, it is concluded that the salinity of formation water in the basin tends to increase from edge to center and from shallow to deep, mainly in alternate blocking zones and alternate stagnation zones. The water deposition environment is relatively stable and the general regularity is obvious, which is an effective means to study the migration and distribution of oil and gas.

Study on the Evaluation Method of the Water Saturation Logging of a Low-Resistance Oil Reservoir in the Guantao Formation, Bohai Basin

As an unconventional and hidden special oil reservoir, a low-resistance oil reservoir has huge exploration potential. The formation of low-resistance oil reservoirs in the study area is affected by multiple factors, and the electrical characteristics of low-resistance oil reservoirs are not obvious, which makes it difficult to accurately evaluate the oil- and gas-bearing nature of low-resistance reservoirs by traditional saturation evaluation methods. The water saturation evaluation has become the focus and difficulty of the logging interpretation of low-resistance oil reservoirs in the target blocks. Low-resistance oil reservoirs have various reservoir characteristics and genesis mechanisms. Based on this, this paper firstly analyzes the genesis mechanisms of low-resistance oil reservoirs by combining geological, core NMR, X-diffraction, and well logging data. Secondly, on the basis of clarifying the genesis mechanism and the main controlling factors of the low-resistance oil formation, the additional conductivity correction of clay is carried out by combining it with the petroelectricity experiment, and the petroelectricity parameters are determined. Finally, the saturation of the low-resistance oil formation in the study area is quantitatively evaluated using the Simandoux model. The results show that the high immobile water saturation caused by the complex pore structure and the additional electrical conductivity of clay are the main controlling causes of the low-resistance reservoirs in the study area. Compared with the saturation evaluation effect of the W-S model, the evaluation accuracy of the Simandoux model corrected for the additional electrical conductivity of clay is better than that of the W-S model, which is 0.88, the relative error of its computation is 7.2%, and the model constructed can provide quantitative bases for the dredging of the low-resistance reservoirs in the Bohai Basin.

Sedimentary Organic Facies Division and Hydrocarbon-Generation Potential of Source Rocks in Coal-Bearing Strata-A Case Study of the Upper Paleozoic in Huanghua Depression, Bohai Basin, China.

Sedimentary organic facies cover the formation, evolution, and spatial distribution characteristics of organic matter, and they are effective tools for oil and gas resource evaluation and basin prospect prediction. According to the basic organic rock composition of the sedimentary organic facies, combined with the sedimentary facies and organic matter geochemical characteristics of Carboniferous-Permian strata, the characteristics of organic facies and hydrocarbon-generation potential of Upper Paleozoic source rocks in Huanghua Depression are being discussed. The results show that source rocks of Taiyuan and Shanxi Formations in the study area were oil-prone, and the oil-generation potential of mudstone is greater than that of carbonaceous mudstone and coal. The organic facies in the study area can be divided into six types: (1) terrestrial forest organic facies; (2) shallow swamp forest organic facies; (3) deep swamp forest organic facies; (4) deep swamp reed organic facies; (5) flowing water swamp organic facies; and (6) open water organic facies. The Taiyuan Formation is mainly composed of flowing water swamp, deep swamp forest, and shallow swamp forest with a strong hydrocarbon-generation capacity, while the Shanxi Formation chiefly includes organic facies of the deep swamp forest and shallow swamp forest. The deep swamp reed sedimentary organic facies had the highest hydrocarbon-generation potential, while the terrestrial forest sedimentary organic facies had the worst hydrocarbon-generation potential. Coal had a certain oil-generating capacity but was weaker than that of mudstone. Compared with mudstone, coal had a stronger gas-generating capacity.

Buried channels under the northern Bohai Sea: Evidence for a great Bohai Paleolake during the late MIS3

Buried channels are widespread on continental shelves and potentially record Quaternary glacial–interglacial environmental changes. The Bohai Sea, an epicontinental basin, is extraordinarily sensitive to such environmental changes, but the evolution of buried channels there remains poorly understood. Here, combined seismic and sedimentological data were used to investigate the evolution of buried channels in western Liaodong Bay, northern Bohai Sea. We identified over six sets of channels, up to 15 m thick and up to 4 km wide, at different horizons within a terrestrial bed formed during the late Marine Oxygen Isotope Stage (MIS) 3 between 35 and 30 kyr BP. Most of these channels are interpreted as incised valley systems of fluvial origin, with complex combinations of fill facies. They were truncated on their tops by transgressive ravinements, which formed during base-level rise. Our findings suggest that these incised valley systems formed and evolved over multiple cycles of incision/filling controlled by the alternation of subaerially exposed and drowned environments during the late MIS3, and the contemporaneous shoreline advanced and retreated repeatedly within our study area. Given that sea level during this period was ∼10–45 m lower than the buried channels in our study area, the evolution of the multiple channels appears to be dominated by lake level fluctuations rather than global eustasy. We hypothesize that during late MIS3, the Bohai Basin transformed into a great paleolake (>39000 km2) as a result of falling sea levels, separating it from the Pacific Ocean, which is also supported by the microfossils in a core collected from the more basinward location. Our stratigraphic evidence also suggests that lake level fluctuations were intensive and of different amplitudes (∼5 to >15 m), which is consistent with climate proxy records. These fluctuations in the paleolake level were presumably a response to and therefore could provide implications for reconstructing regional climate dynamics.

Resequencing of Rosa rugosa accessions revealed the history of population dynamics, breed origin, and domestication pathways

BackgroundRosa rugosa is a shrub that originated in China and has economic and ecological value. However, during the development of R. rugosa, the genetic background was chaotic, and the genetic structure among different wild populations was unclear, as well as wild and cultivated accessions. Here, we report whole-genome resequencing of wild and cultivated R. rugosa accessions.ResultsA total of 19,041,284 SNPs were identified in 188 R. rugosa accessions and 3 R. chinensis accessions by resequencing. Population genetic analysis revealed that cultivated and wild groups were separated very early. All R. rugosa accessions were divided into 8 categories based on genetic structure: (1) Weihai, Yantai, and Liaoning category, (2) Jilin category, and (3) Hammonasset category (above three are wild); (4) traditional varieties, (5) hybrids between R. rugosa and R. chinensis, (6) Zizhi Rose, (7) Kushui Rose, (8) hybrids between R. rugosa and R. multiflora. We found that the heterozygosity and genetic diversity of wild accessions were generally lower than those of cultivated individuals. The genes that were selected during cultivation were identified, and it was found that these genes were mainly related to environmental adaptation and growth.ConclusionsThe Jilin population was the oldest population and later migrated to Liaoning and then migrated to Yantai and Weihai by sea regression in the Bohai Basin. The Hammonasset naturalized population probably originated from the Jilin population and then experienced separate differentiation. The long-term asexual reproduction pattern of R. rugosa decreased genetic diversity in the wild population. During R. rugosa cultivation, the ancestors of the Jilin population were involved in breeding traditional varieties, after which almost no wild individuals were engaged in breeding. However, in recent decades, cross breeding of R. rugosa started the utilization of wild germplasms. In comparison, some other species play important roles in variety formation. Few genes related to economic traits were selected, suggesting no directional domestication in the R. rugosa cultivation process.

Secondary porosity generation in sandstones constrained by the fault movement, fluid flow, and mudstone–sandstone interactions, offshore Bohai Basin, China

Secondary porosity generation in sandstones constrained by the fault movement, fluid flow, and mudstone–sandstone interactions, offshore Bohai Basin, China

Prediction of Sedimentary Microfacies Distribution by Coupling Stochastic Modeling Method in Oil and Gas Energy Resource Exploitation

In view of the problem that a single modeling method cannot predict the distribution of microfacies, a new idea of coupling modeling method to comprehensively predict the distribution of sedimentary microfacies was proposed, breaking the tradition that different sedimentary microfacies used the same modeling method in the past. Because different sedimentary microfacies have different distribution characteristics and geometric shapes, it is more accurate to select different simulation methods for prediction. In this paper, the coupling modeling method was to establish the distribution of sedimentary microfacies with simple geometry through the point indicating process simulation, and then predict the microfacies with complex spatial distribution through the sequential indicator simulation method. Taking the DC block of Bohai basin as an example, a high-precision reservoir sedimentary microfacies model was established by the above coupling modeling method, and the model verification results showed that the sedimentary microfacies model had a high consistency with the underground. The coupling microfacies modeling method had higher accuracy and reliability than the traditional modeling method, which provided a new idea for the prediction of sedimentary microfacies.

Quantitative characterization of volcanic expansion fractures based on thermodynamic coupling analysis

In recent years, volcanic reservoirs in the Bohai Sea have become the focus of oil and gas exploration and development. Volcanic activity during the Cenozoic provided favorable conditions for the development of hydrocarbon traps. These volcanics also result in difficulties during the drilling and development of oil and gas fields. Furthermore, the high-temperature magma induced thermal expansion in the surrounding rocks, which changed the local stress field and affected the fracture distribution characteristics of the surrounding strata. The complexity of the fracture distribution makes predicting lost circulation difficult and causes substantial economic losses. Therefore, it is necessary to characterize the volcanic perimeter expansion-type fractures effectively. The distribution of volcanic rocks is identified through logging and seismic data, with perimeter fracture development characteristics being analyzed. The temperature conditions of the volcanic activity were determined through core mineral analysis and thin section observation. Based on the laws of energy conservation and geomechanics, the principle of thermal coupling was applied to quantitatively predict the thermal expansion fractures caused by volcanic activity using a finite element numerical simulation method. To determine the extent of volcanic influence on lost circulation, and the development characteristics of fractures under the influence of these rocks were analyzed. The results provide an important reference for drilling and volcanic reservoir prediction in the Bohai Basin and guide for elsewhere.

Molecular Characterization of Naphthenic Acids and Naphthenates in the Oil Sludge: A Case Study in Bohai Basin, China

Oil sludge deposition is a common phenomenon during oil transportation, storage, and processing. Detailed molecular characterization of sludges is a prerequisite for understanding the mechanisms of emulsification, naphthenate, and asphaltene deposition. In this study, naphthenic acids and their salts in the oil sludge from an offshore oil platform were characterized by negative ion electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry and high-temperature chromatography (HTGC). The results show that the oil sludge is a stable emulsion consisting mainly of water and crude oil. An insoluble fraction, which accounted for roughly 3 wt % of the sludge, was found containing a large amount Ca and Fe. The organic matter in solid was extracted by toluene/formate acid extraction and was identified as large-molecular binary, ternary, and quarternary naphthenic acids, including the C80 tetraacids (so-called “ARN”). HTGC results showed that the ARN accounted for more than half of the polybasic acids. It was considered that the Ca2+ in the formation water and large molecular polyacids in the crude oil is the key factor for the sludge formation; small molecular naphthenic acids and other polar components in the crude oil stabilize the emulsion of the sludge.

Dynamics of wave generation and dissipation processes during cold wave events in the Bohai Sea

Wave dynamics were investigated by applying the wind-wave model Simulating Waves Nearshore to the semi-enclosed Bohai Sea during cold wave events. Wind quality was examined by comparing three wind data sources with buoy observations, and then numerical experiments were conducted to investigate the impacts of model physics settings on simulated waves. After model calibration, the wave dynamics were examined from the aspects of response to various wind conditions (speed, fetch, duration), wave dominant component (wind wave or swell), as well as the dissipation processes during a pair of cold wave events (e.g., northwesterly in 2014 and northeasterly in 2015) with the consideration of spatial differences. The maximum significant wave heights during two cold wave events are similar (e.g., 3.5 m in the central Bohai Basin), which attributes to the impacts of limited fetch difference and insufficient high wind duration. Wind wave is the dominant component, which is reflected in the wave spectra (e.g., wave age less than 1.2). Spatial regions of wave dissipations controlled by whitecapping, bottom friction, and depth-induced breaking during cold wave events are quantified. Whitecapping dominates the wave energy dissipation in deep-water areas, while the bottom friction dissipation controls the major dissipation process as waves propagate towards the coast (e.g., at 5–10 m water depths). Depth-induced dissipation only occurs over a narrow strip along the southern coastal area but is significantly beyond the others. Depth-induced dissipation decreases suddenly when reaches a flat bottom, and it decreases with the increase of the width of the bottom slope.

Integrated water-quality management indicators from river to sea: A case study of the Bohai Sea, China

Considering the interrelatedness of river and bay ecosystems, river and bay water quality management is shifting to integrated management across coastlines. Here, an integrated management indicator for the coordinated and efficient nitrogen abatement of the Bohai Sea and its basin was proposed. The terrigenous total nitrogen (TN) allocated load was optimized under the dual water quality constraints for both river and bay using a simulation–optimization method. The contributing jurisdictions were identified by their TN overload rates, and their responsibility apportionment rate for specific nitrogen-polluted segment was quantified. Integrated TN reduction scheme resulted in a 29 % greater reduction in bay and river nitrogen pollution than the equal proportion reduction approach. In 18 % of the watersheds in the Bohai basin, the water quality standards of the river were more restrictive than the standards of the bay. Integrated management scheme has higher coordination of river and sea management objectives.

Paleoenvironmental modes and organic matter enrichment mechanisms of lacustrine shale in the Paleogene Shahejie Formation, Qikou Sag, Bohai Bay Basin

The Paleogene Shahejie Formation (Shahejie Fm) of Bohai Bay Basin has ignited unprecedented interest among scholars owing to another breakthrough achieved in shale exploration besides Kongdian Fm. However, this breakthrough, particularly high output submember—the third Member of the Paleogene Shahejie Formation (Mbr 3) lacks elaborate study on paleoenvironment and its covariations with organic matter (OM) concentration for shales, which impedes the further acquaintances of OM distribution and shale oil–gas possibility. The present study adopts advanced approaches to gain insights into the deposition process of shales in the Mbr 3, Qikou Sag, Bohai Basin. Petrological characteristics, organic geochemistry, and an abundant element database offer a solid foundation for performing master controlling factor analysis and establishing deposition modes for OM accumulation and preservation. Our results indicate that the entire Mbr 3 has potential. Shales in the Mbr 3 of the Shahejie Fm were deposited under a temperate environment with weak weathering and sediment influx surrounded by suboxic freshwater. However, the organic-enriched shale in the upper Mbr 3 is a more desirable exploration layer. Disparate paleoproductivity support and clastic detritus sediment afforded a diminished consequence of OM accumulation between the upper Mbr 3 (L1–L2 in this study) and middle Mbr 3 (L3 in this study). Our investigation suggests that the OM concentration in the shales was dominated primarily via paleoproductivity and secondarily via redox reactions. Clastic sediment influx sets the primarily passive effect, while effect of salinity was minimal. Two sedimentary modes were established to interpret the variability between L1–L2 and L3 during the two depositional stages. The lake expansion period of L3 resulted in shales being deposited mainly on a gentle slope with a shallow water column containing insufficient OM susceptible to redox condition compared to the optimal state in the semideep lake in L1–L2 stage. The results provide an integrated reconstruction of paleosedimentary environment for the target layer and offer a solid basis for further exploration through coupling relationship between sedimentary environment and OM enrichment in the study area.

Ultimate expellable potentials of source rocks from selected super basins: What does “world class” look like?

An important step in the evaluation of a play or prospect is to consider the potential supply of petroleum charge, which is ultimately constrained by masses and volumes supplied by the source bed. The two factors limiting the mass of petroleum expelled from the organic matter in the source bed are (1) its initial expulsion potential and (2) the cumulative fraction of potential that has been expelled up to its maximum state of maturation. To evaluate the initial expulsion potential, we introduce a workflow to estimate the ultimate expellable potential (UEP), divided into an oil potential (UEO) and a gas potential (UEG), which represent the cumulative masses of oil and gas that can be expelled upon complete maturation of the source rock. For use in resource estimation, these masses can be converted to surface volumes of oil and gas per unit area (million stock tank barrels per square kilometer and billion standard cubic feet per square kilometer or million barrels of oil equivalent per square kilometer, respectively). The UEP (UEO, UEG) can be mapped across the depositional extent of the source bed, just as a reservoir depositional system is mapped. These potentials per unit area constitute the first level of quantitative resource estimation in the evaluation of a play or prospect. We show examples of UEP (UEO, UEG) mapping based on available public data. Three of the example source rocks are aquatic organofacies that have charged major conventional petroleum systems: the Callovian to Oxfordian marine organofacies of the Arabian Basin, Saudi Arabia; the Volgian marine organofacies Bazhenov Formation of the West Siberian Basin, Russia; and the Eocene–Oligocene lacustrine freshwater organofacies Shahejie Formation of the Bohai Basin, China. We also include an unconventional system: the uppermost Devonian–lowermost Mississippian marine organofacies Bakken Formation of North Dakota, United States. The UEPs of the studied source rocks in the Arabian Basin and West Siberian Basin, with tens of millions of barrels of oil equivalent per kilometer over large areas, define world class in marine source rocks since these basins are ranked number one and number two in the world by oil endowment. Until more data are available on other lacustrine basins, we offer the UEP of the studied Bohai Basin source rock as an example. In contrast, the UEP of the Bakken Formation source rocks (combined Upper and Lower Members) is relatively modest despite its world class unconventional oil endowment. The Bakken's effectiveness, despite its relatively low UEP, reflects the negligible migration saturation losses involved in charging the Middle reservoir Member. This illustrates that the commonly touted term world class can be rather meaningless. It needs to be considered in context given the task in hand: the greater the (vertical) migration losses incurred in charging reservoirs, the higher the UEP needed to create the charge needed to overcome them.

Sedimentary environment and organic enrichment mechanisms of lacustrine shale: A case study of the Paleogene Shahejie Formation, Qikou Sag, Bohai Bay Basin

Recent Chinese shale oil (or gas) exploration efforts have focused on the Shahejie Formation of Bohai Basin. However, this work has been handicapped by a lack of state-of-the-art investigations of the targeted shale. Specifically, there is a need for an improved understanding of the role that paleoenvironmental aspects of the basin played in the organic enrichment of these deposits. The present study provides the insights into the paleoenvironmental and depositional history of various of shales of the Es1 Member of Paleogene Shahejie Formation, Qikou Sag, Bohai Basin, and addresses the controlling factors of organic matter (OM) accumulation and preservation in these deposits based on petrographic characteristics, organic geochemistry, and major and trace elements data. Our results suggest that the organic-enriched Es1 oil shale is a desirable exploration target, especially felsic and mixed shale. The felsic shale, mixed shale, dolostone and carbonate shale accumulated under relatively humid climatic conditions but in saline lacustrine environment. Climate, salinity, and redox conditions appear to have varied at least three times during the depositional history of the studied interval. Paleoenvironmental conditions favored stable productivity levels and only a modest supply of clastic detritus sediment resulting in a diminished rate of deposition. Felsic and mixed shale deposits that have the greatest source potential accumulated under more humid atmospheric and oxygen-depleted environmental conditions than dolostone and carbonate shale. The present study suggests infers that organic carbon content of the Es1 oil shale was controlled principally by salinity and secondarily by primary productivity and redox conditions.

Water-rock interactions and origin of formation water in the Bohai Bay Basin: A case study of the Cenozoic Formation in Bonan Sag

The faulted lacustrine Bohai Basin in eastern China contains abundant hydrocarbon resources. In these reservoirs, understanding the sandstone diagenesis and the resulting formation water provides a means to unravel the evolution processes in the basin. In most cases, the lack of isotopic and trace element analysis tests in this type of basin limits the research on the origin and evolution of formation water in this area. We have used multivariate statistical methods to classify the geochemical characteristics of the formation water for the Cenozoic Formation of Bonan Sag in the Bohai Bay Basin. Analysis of correlations among the evolution processes of different ions in different types of formation water provides an understanding of the primary factors influencing the ion content. We also evaluate the water-rock interactions of different types of formation water to evaluate their geologic significance, and we find three types. Type I formation water includes a mixture of river water, lake water, and atmospheric precipitation and exhibits weak water-rock interactions. Type II formation water contains primitive freshwater and brackish lake water that has undergone an evolution process similar to that of type I formation water, but that was followed by evaporation and concentration, the dissolution and precipitation of calcite and iron calcite, and feldspar dissolution. Type III formation water, which is a product of rock reconstruction, originates from saline lake sediment water. After undergoing evolution processes similar to those of types I and II, type III formation water is also affected by dissolution of evaporite, albite, dolomite, and iron dolomite. Thus, type III formation water is the product of water-rock interactions such as precipitation, SO42− reduction, and pyrite precipitation in which the water-rock reaction controls the development mechanism and characteristics of the reservoir space.

Transformation of sediment delivery and dispersal patterns controlled by relay-ramp evolution along the boundary fault of a lacustrine rift: The Eocene Shahejie formation, Dongying Sag, Bohai Bay Basin, NE China

The boundary fault of the Dongying Sag is divided into isolated and scattered segments and small-scale relay zones, which are important geomorphic features controlling sedimentation in the early stage of rift evolution. Previous studies have shown changes in facies in deposits of thousands of meters in thickness on fault-bounded margins, where deep-water fan systems are vertically stacked with delta deposits in the same way as in many basins around the world. However, changes in sedimentation processes and sedimentary systems and their relationship with the evolution of the main boundary fault remain a matter of debate. We focus on the transition from shallow-water to deep-water systems and their responses to relay-ramp evolution in the Yanjia Subsag of the Eocene Dongying Sag in the Bohai Basin. A relatively simple assemblage of local structures provides an ideal opportunity to investigate the impact of relay-ramp evolution on the early synrift deposits. An integrated dataset of 3D seismic volumes, cores and wireline logs is used to analyze the evolution of the Chennan Fault, to delineate thirteen lithofacies, and to recognize two depositional styles (including fan deltas and nearshore subaqueous fans) and their distribution during different stages of the relay-zone evolution. In the early stage of rifting, the depositional style changes, from the development of axial and transverse systems in the early to middle period to the development of transverse systems in the late period; this is mainly controlled by the evolution of the relay ramp. The change is accompanied by a rapid decrease in the rate of sediment supply, architectural changes in transport conduit from the relay ramp to a canyon, and local geomorphic changes that control transport routes, sediment delivery, and depositional locations. Floods ultimately become dominant transport mechanism with a new system, such that hyperpycnal flows form, leading to another transformation of the sedimentary systems from a fan-delta system to a nearshore subaqueous fan system. On the basis of these transformations in sedimentary systems we are able to propose new recommendations for reservoir exploration in continental rift basins, which include exploration potential, source reservoir cap assemblage, and spatiotemporal evolution of plays.

Regional Exhumation and Tectonic History of the Shanxi Rift and Taihangshan, North China

AbstractThis 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.

The structural evolution of pull‐apart basins in response to changes in plate motion

AbstractPull‐apart basins are structural features linked to the interactions between strike‐slip and extensional tectonics. Their morphology and structural evolution are determined by factors such as extension rate, the basin length/width ratio, and changes in extension direction. In this work, we investigate the effect of a change in the plate motion direction on a pull‐apart basin's structure, using analogue modelling experiments with a two‐layer ductile‐brittle configuration to simulate continental crust rheology. We initially impose orthogonal extension on an interconnected rift and strike‐slip system to drive pull‐apart development. Subsequently, we rotate the relative motion vector, imposing transtensional deformation and continuing with this new relative motion vector to the end of the experiment. To compare with natural examples, we analyse the model using seismic interpretation software, generating 3D fault structure and sedimentary thickness interpretations. Results show that the change in the direction of plate motion produces map‐view sigmoidal oblique slip faults that become normal‐slip when deformation adjusts to the new plate motion vector. Furthermore, sediment distribution is strongly influenced by the relative plate rotation, changing the locus of deposition inside the basin at each model stage. Finally, we compare our observations to seismic reflection images, sedimentary package thicknesses and fault interpretations from the Northern Gulf of California and find good agreement between model and nature. Similar fault arrays occur in the Bohai Basin in northern China, which suggests a rotational component in its evolution. More broadly, such similar structures could indicate a role for oblique extension and fault rotation in any pull‐apart basin.

Magnetostratigraphic and uranium-series dating of fossiliferous cave sediments in Jinyuan Cave, Liaoning Province, northeast China

Chronological sequences of Quaternary terrestrial mammalian faunas can provide important information about the evolutionary history of mammals, regional biostratigraphy and paleoenvironmental changes. Here we report the results of studies of a thick, nearly-continuous sedimentary sequence from Jinyuan Cave, in Liaoning Province, northeast China; the sequences contains four mammalian faunas ranging in age from the Late Pliocene to the late Middle Pleistocene. Detailed paleomagnetic and U-series age determinations date the upper unit of the sedimentary sequence to ~2.2–0.3 Ma. The results suggest that cave development and infilling were closely associated with the tectonic evolution of the Bohai Basin, including the subsidence of the basin and the uplift of the surrounding mountains, such as Luotuo Hill. Together with the large mammalian faunas from both south and north China, we suggest that these mammalian faunas were smaller in size and lower in species diversity during the Middle Pleistocene Transition (MPT) (1.2–0.7 Ma) than in the earlier and later Pleistocene. This indicates that major climatic variations during the MPT in north China may have reduced their taxonomic abundance and diversity. The extremely warm and wet interglacial climates that followed the MPT in monsoonal East Asia provided an ecological niche for the evolution and migration of these mammalian faunas, resulting in an increase in the numbers and diversity of mammals in East Asia.