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- Final Amalgamation Processes of the Southern Altaids: Insights from the
Abstract: AbstractThe Permian–Triassic tectonic setting is still controversial in the southern Altaids. The Beishan orogen is an ideal region to address the final tectonic of the Altaids. These systematic mapping, geochemistry, and geochronology studies on the Houhongquan ophiolitic mélange in the south Beishan are conducted to address this issue. New mapping reveals that the Houhongquan ophiolitic mélange consists of blocks of gabbro, basalt, chert, granite, and strongly deformed and cleaved sandstone in the southern Beishan. The studies reveal that the mafic fragments are relics of normal-mid-ocean ridge (N-MOR) and suprasubduction zone (SSZ) types of oceanic lithosphere. The four sandstone matrix samples yield the maximum depositional ages of 222±5 Ma, 233.8±2.3 Ma, 263.4±2.5 Ma, and 263.5±2.8 Ma, respectively, indicating that the youngest sandstones were tectonic emplaced in the Houhongquan ophiolitic mélange after ca. 222 Ma. The sandstone matrices display two types of age spectra. Early Permian sandstones have a single Devonian to Early Permian peak age patterns, indicating the existence of an independent Permian intraoceanic arc. In contrast, Late Triassic sandstones have multiple peaks with some Precambrian zircons, suggesting that they were sourced from a continental arc. Accordingly, we consider that the Houhongquan ophiolitic mélange tectonic was emplaced in the intraoceanic island arc during the Middle Permian and docked to a continental margin arc during the Late Triassic. Thus, we argue that the terminal amalgamation timing of the southern Altaids was probably during ca. 222-217 Ma.
PubDate: Wed, 22 Feb 2023 00:00:00 GMT
- Corrigendum to “A Simulation of Water Conservation Policy Impacts on
Long-Range Climate Change: A CGE Analysis”
Abstract: In the article titled “A Simulation of Water Conservation Policy Impacts on Long-Range Climate Change: A CGE Analysis” [1], an affiliation of author Juan Gao was omitted in error. This affiliation has been added to the affiliation list above as number 3, and the author affiliations have been corrected.
PubDate: Mon, 20 Feb 2023 00:00:00 GMT
- Characteristics of In Situ Desorption Gas and their Relations to Shale
Components: A Case Study of the Wufeng-Longmaxi Shales in Eastern Sichuan
Basin, China
Abstract: AbstractIn situ desorption gas measurement can be used to evaluate shale gas potential, sweet spot prediction, and production strategy optimization. However, gas contents and carbon isotope compositions of in situ desorption gas and the relationship to reservoir properties and shale compositions are not systematically studied from the actual production situation. In this study, 63 core shales of Wufeng-Longmaxi formation from the YY1 well in the eastern Sichuan Basin were subjected to TOC (total organic carbon), solid bitumen reflectance (Rb), maceral fractions of kerogen analysis, and X-ray diffraction (XRD) analysis to obtain shale compositions, and 10 selected samples were conducted on low-pressure N2/CO2 (N2/CO2GA), mercury injection capillary pressure (MICP), and field emission scanning electron microscopy (FE-SEM) tests to acquire reservoir properties. Meanwhile, 60 samples were also subjected to in situ desorption tests to obtain shale gas content, and 5 selected samples were used to investigate variations in shale gas compositions and carbon isotopes during the desorption process. Results indicated that transient rates of shale gas during desorption process are significantly controlled by desorption time and temperature. In terms of in situ desorption process, total gas is divided into desorbed gas and lost gas. Desorbed gas is mainly comprised of CH4, N2, CO2, and C2H6, with desorption priorities of N2 > CH4 > CO2 ≈ C2H6, which are consistent with their adsorption capacities. The δ13CH4 values tend to become heavier during desorption process, varying from -37.7‰ to -16.5‰, with a maximum increase amplitude of 18.8‰, whereas the change of δ13C2H6 value, from -38.2‰ to -34.8‰, is minor. Desorbed gas shows carbon isotope reversals, due to that preferential desorption of 12C-CH4 during desorption process results in δ13C value less negative in CH4. The tested desorbed gas, lost gas, and total gas ranged 0.088 to 1.63 cm3/g, 0.15 to 3.64 cm3/g, and 0.23 to 5.20 cm3/g, respectively. Shale gas content, i.e., desorbed gas and lost gas, is controlled primarily by TOC content and organic matter (OM)-hosted nanometer-size pores. Clay mineral content is negatively correlated with shale gas content, due to that, clay mineral pores are more easily compacted during burial and occupied by water molecules. Compared with shale gas reservoirs in North America, the studied shale reservoir has high brittle mineral content and permeability, which is considered to have great potential of shale gas resource and to be the next commercial development zone in south China.
PubDate: Thu, 16 Feb 2023 00:00:00 GMT
- The Role of Isostasy in the Evolution and Architecture of Fold and Thrust
Belts
Abstract: AbstractWarmer conditions prevalent in the hinterland of orogenic systems facilitate local ductile flow underneath the surface load, making Airy-like local isostasy more prevalent in these domains. In contrast, flexural isostasy better describes the regional response to surface loading of more rigid lithospheres. Here, we explore how the interaction between horizontal tectonic mass transfer and vertical isostatic mass transfer, through either elastic flexure or viscous flow, impacts the overall architecture of fold and thrust belts. We compare numerical models of fold and thrust belts under either an Airy-like ductile isostasy boundary condition or a flexural-like regional isostasy boundary condition. Our experiments suggest that when ductile flow is involved in accommodating isostatic adjustment, subsidence is rather local, larger, and results in narrower, less elevated fold-thrust belts with a complex internal architecture consisting of prominent steeply dipping faults. When isostatic subsidence is controlled by lithospheric flexure, the tilting of the basement on 10 s of km scale facilitates the outward propagation of fold-thrust belts. The internal architecture is simpler and involves prominent basement-parallel décollements. The outcome is wider fold and thrust belts with higher topographies. A change in lithospheric elastic thickness does not significantly affect fold-thrust belt structural styles. Our results are compared to natural examples from the Subandean zone.
PubDate: Sat, 04 Feb 2023 00:00:00 GMT
- Petrogenesis of Mesozoic Magmatic Suites in the Jiaodong Peninsula:
Implications for Crust-Mantle Interactions and Decratonization
Abstract: AbstractThe North China Craton (NCC) has thick lithosphere in the Paleozoic (>200 km) but appears to be decratonized in the Mesozoic. However, the actual processes operated in the lower crust-deep mantle are still unclear. The Mesozoic granitic rocks can provide important clues to the NCC decratonization mechanism. Here, we conducted trace element modeling to check whether partial melting of the Archean lower crust can generate these Mesozoic magmatic suites. Meanwhile, zircon Hf isotope analysis was conducted to reveal crust-mantle interaction processes and further give constraints on the decratonization of the NCC. Zircon Hf isotope data of the Linglong, Guojialing, and Aishan suites, the mafic microgranular enclaves (MMEs) in the Guojialing suite, and mafic dykes display minor differences: the Linglong (160–150 Ma), Guojialing (~130 Ma), and Aishan (118–116 Ma) suites have zircon εHft=–25.4 to –14.5, –15.3 to –10.4, and –23.1 to –11.9, respectively. The Cretaceous mafic dyke (126 Ma) has a highly negative εHft value (–22.8 to –17.7). Meanwhile, the MMEs (in the Guojialing granodiorite, DCW-2A, 129 Ma) have zircon εHft=–13.0 to –8.9. Temperature-pressure conditions calculated using amphibole compositions for both the Guojialing granodiorite and its MMEs are basically identical, implying possible magma mixing. Our modeling results show that certain trace elements (e.g., Tb, Yb, and Y) have to be retained in the source to match the composition of the Linglong suite, which requires substantial garnet residues (high-pressure melting) in the Jurassic. The Early Cretaceous garnet-dominated lower crust is Yb-/Y-enriched but depleted in elements like Sr and La. Therefore, it could not form geochemical features like high Sr/Y and La/Yb ratios akin to the Guojialing suite. Integrating the modeling results and zircon Hf isotope data, we propose that the crust in the eastern NCC had thickened and partially melted by dehydration to produce an eclogitic residue containing a large amount of garnet (>50% by weight) during the Jurassic (Linglong granite), whereas upwelling of hot and hydrous mafic magma from the asthenospheric mantle induced fluxed melting of both the lower crust and lithospheric mantle in the Early Cretaceous, during which the lithospheric mantle and part of the lower crust in the Jiaodong were removed by the convective mantle. About 10 Mys later while the Aishan suite formed, the crust was not thick anymore, and melting occurred under moderate pressure which does not necessarily require abundant garnet as the residue phase.
PubDate: Mon, 23 Jan 2023 00:00:00 GMT
- Stress State of the Stable Part of the Pacific Plate Predicted by a
Numerical Model of Global Mantle Flow Coupled with Plate Motion
Abstract: AbstractThe Pacific plate, which is the largest oceanic plate on Earth, has implications for the general understanding of plate dynamics, including the origin of intraplate stress and the driving force for plate motion. However, this is currently limited by the scarcity of geophysical and geological observational data. In this study, an instantaneous global mantle flow calculation was performed to predict the intraplate stress field and stress regimes on the Pacific plate using a geodynamic model based on the density anomaly structure of the mantle converted from a seismic tomography model incorporating subducting plates. The numerical results demonstrate that the southern part of the Pacific plate is dominated by a normal faulting regime. In contrast, the northern part is dominated by a thrust faulting regime, in which the tensional stress axes in the older and stable part of the Pacific plate tend to be oblique to the direction of plate motion. This suggests that the stress state of the Pacific plate is almost neutral (i.e., neither compressional nor tensional) along the direction of plate motion. Furthermore, shallow positive buoyancy-induced asthenospheric flow is essential for reproducing the observed plate motion of the Pacific plate.
PubDate: Thu, 19 Jan 2023 00:00:00 GMT
- A Method to Determine the Chain Pillar Width considering Coal Burst and
Goaf Ignition Dual-Hazard Management
Abstract: AbstractHigh rock stress and ground temperature pose great threats to the routine production of longwall top coal caving (LTCC) panels. In this risky condition, the width of the chain pillar is considered a factor adjustable for controlling coal burst and goaf ignition hazards. However, a contradiction, as suggested by longwall experience, is that narrowing the pillar helps coal burst prevention but negatively leads to higher self-ignition potentials, while widening the pillar restrains goaf ignition but increases the likelihood of coal burst. This paper conducted a case study on a longwall panel from Tangkou Mine, China. The paper first analysed stress, elastic strain energy, and goaf temperature variation with varying pillar widths, by which the coal burst risk index δr and goaf ignition risk index Qs were defined and correlated to pillar width D. Further, a pillar width determination method considering coal burst and goaf ignition dual-hazard management was developed by means of the operating point principle. By this method, a reasonable width range was defined by plotting both correlation curves δr=fD and Qs=gD on a chart, followed by optimal width determination according to the intersection of both curves and further verification via a field trial.
PubDate: Tue, 17 Jan 2023 00:00:00 GMT
- Applicability of the Continuum Mapping Approach for Groundwater Flow
Modeling in 3D Fracture Networks
Abstract: AbstractA methodology for simulating groundwater flow in three-dimensional (3D) stochastic fracture rocks based on a commonly used finite-difference method is presented in this paper. Different realizations of fracture networks are generated by the fracture continuum method (FCM), in which appropriate 3D cuboids are used to describe the geometry of fractures. Then, the effects of different parameter distributions on the fracture networks indicated that the length, orientation, and density of fractures all play significant roles in the connectivity of fractures in this methodology. Greater length and density and wider orientation range of fractures lead to greater connectivity. The proper contrast in hydraulic conductivities between the fractures and matrix is found to be approximately 105 due to the contribution of fluid flow in the matrix which can be ignored. It is shown that the fracture density plays a key role in stabilizing the equivalent hydraulic conductivity (Ke) of the fracture networks. Furthermore, the greater length and closer orientation of fractures to the general flow direction, the larger Ke of the generated fracture networks possess. The findings of this study can help for a better understanding of the mechanism of FCM and the influence of geometry characteristics on the hydraulic conductivity of FCM models.
PubDate: Mon, 16 Jan 2023 00:00:00 GMT
- Research on Water-Conducting Fractured Zone Height under the Condition of
Large Mining Height in Yushen Mining Area, China
Abstract: AbstractAccurately predicting the development height of the water-conducting fracture zone (HW) is imperative for safe mining in coal mines, in addition to the protection of water resources and the environment. At present, there are relatively few fine-scale zoning studies that specifically focus on predicting the HW under high-intensity mining conditions in western China. In view of this, this paper takes the Yushen mining area as an example, studies the relationship between the water-conducting fissure zone and coal seam mining height, coal seam mining depth, hard rock scale factor, and working face slope length, finally proposing a method to determine the development height of the HW based on multiple nonlinear regression models optimized using the entropy weight method (EWM-MNR). To compare the reliability of this model, random forest regression (RFR) and support vector machine regression (SVR) models were constructed for prediction. The findings of this study showed that the results of the EWM-MNR model were in better agreement with the measured values. Finally, the model was used to accurately predict the development height of the hydraulic conductivity fracture zone in the 112201 working face of the Xiaobaodang coal mine. The research results provide a theoretical reference for water damage control and mine ecological protection in the Yushen mine and other similar high-intensity mining areas.
PubDate: Tue, 10 Jan 2023 00:00:00 GMT
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