2023 Vol. 29, No. 5

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2023, 29(5): .
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Contents
Contents
2023, 29(5): 1-2.
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Tectonic System & Structural Geology
Fission track ages and Mesozoic tectonic uplift in the Niushoushan-Luoshan area on the western edge of the Ordos Basin
TIAN Zhaoyang, CHEN Hong, LIU Xinshe, GONG Wangbin, ZHAO Weibo, KANG Rui
2023, 29(5): 599-617. doi: 10.12090/j.issn.1006-6616.2023030
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The western margin of the Ordos Basin and its adjacent regions have undergone a complex tectonic evolution from the Mesozoic to the Cenozoic era. However, the question of tectonic uplift since the Cenozoic era remains a topic of contention, and the regional thermal evolution history necessitates precise chronological evidence. Situated close to the thrust belt within the western margin of the Ordos Basin, the Niushoushan-Luoshan area holds pivotal significance in unraveling the Mesozoic tectonic events within the basin's confines. Through a meticulous exploration employing apatite fission track (AFT) analysis and thermal history simulation, this study delineates the Mesozoic uplift sequence and its temporal confines in the Niushoushan-Luoshan area. The results reveal that the Mesozoic uplift within this region predominantly occurred during the Middle Jurassic period (170 Ma) to the end of the Early Cretaceous (110 Ma). Furthermore, we observe a slightly earlier onset of uplift in the Luoshan area (170 Ma) compared to the Niushoushan area (160 Ma). This uplift is primarily attributed to the north-eastward extrusion of the Qilian orogenic belt. Combining our findings with existing research, we propose that the Cenozoic uplift in the western margin of the Ordos Basin and its adjacent areas started during the Late Triassic, comprising two distinct phases: the first phase unfolding from the Late Triassic (220 Ma) to the end of the Early Jurassic (185 Ma), and the second phase occurring from the Middle Jurassic (175 Ma) to the end of the Early Cretaceous (110 Ma); the uplift in the Niushoushan-Luoshan area is part of the second phase of uplift along the western margin of the Ordos Basin. The two Cenozoic tectonic uplift phases along the western margin of the Ordos Basin display characteristics of north-to-south and southwest-to-northeast propagation, respectively. It is inferred to be associated with the Late Triassic collision between the North China and South China blocks, as well as the movement of the Lhasa Block converging toward the northeast during the Middle to Late Jurassic.

Energy Resources Geology
Accumulation conditions and exploration potential of deep natural gas in the Qaidam Basin
LI Jian, TIAN Jixian, WANG Bo, ZHU Jun, ZHOU Fei, LI Senming, SHAO Zeyu, LI Haipeng, QIAO Bohan
2023, 29(5): 618-630. doi: 10.12090/j.issn.1006-6616.2022084
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This paper examines the enrichment conditions of deep natural gas reservoirs in the Qaidam Basin and delineates the exploration potential utilizing seismic, geological, geochemical, well-logging, and drilling data. The findings indicate the presence of two high-quality gas source formations, namely the Jurassic and Paleogene formations, along the northern margin and the western part of the basin, respectively. The formations both exhibit advanced evolution and robust gas-generating capacity. The deep layers along the northern margin consist of bedrocks and Paleogene clastic reservoirs, while the western deep layers feature Paleogene lacustrine carbonate reservoirs. The reservoirs west of Qaidam Basin are widely distributed on the plane and vertically form multiple reservoir cap combinations. The primary pores, dissolution pores, fractures, and other pore types developed in the reservoirs are considered as the storage space for deep gas accumulation. The continuous active deep faults serve as high-quality channels for deep gas sources; furthermore, the formation of deep structures is well-matched with natural gas generation. The deep hydrocarbon source rocks in the western Qaidam Basin are characterized by early and continuous hydrocarbon generation. Early-generated liquid hydrocarbons undergo high-temperature cracking into gas during later burial, resulting in a robust gas-generating capacity and significant potential for deep resources. The widely developed salt rocks, argillaceous rock, and abnormally high-pressure layers in the deep Qaidam Basin contribute to preserving deep natural gas. In conclusion, it is believed that deep gas reservoirs in the Qaidam Basin are enriched in the traps around hydrocarbon-generating sags with developed faults. Key favorable areas for deep-seated natural gas exploration include the basement rocks of the ancient piedmont uplift in the northern margin of Qaidam, the Paleogene clastic rocks in the central structural belt, and the carbonate rocks along the Yingxiongling structural belt in the western part of the basin.

Sedimentary paleo-environment and organic matter enrichment in the Lucaogou Formation of the Jimsar Sag
LIU Jin, WANG Jian, TAN Jingqiang, CUI Haisu, LIU Miao, ZHENG Yu, SONG Xueqi, YUAN Qiang
2023, 29(5): 631-647. doi: 10.12090/j.issn.1006-6616.2022127
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The Lucaogou Formation in the Jimsar Sag is the main target layer of shale oil exploration and development in continental basins. However, its paleo-environmental information and organic matter enrichment mechanism still need to be determined. In order to investigate the paleo-environment of the Permian Lucaogou Formation, based on core and thin section observation, we carried out major and trace element analyses on 26 source rocks from Well J10025 by using X-ray fluorescence spectroscopy(XRF) and inductively coupled plasma mass spectrometry(ICP-MS). This analysis, combined with GC-MS biomarker data and typical sedimentary structures, reflects the paleo-environmental characteristics of the Luchaogou Formation. Geochemical indicators such as C-value, Sr/Ba, V/Cr, Pr/Ph, P, and Co show that the Lucaogou Formation was generally developed in a saline lake environment under an arid to semi-arid climate, with relatively deep water during deposition and hypoxic to anoxic conditions. The upper member was deposited in a semi-arid, brackish lake with weak reducibility, deep water body, and high productivity. In contrast, the lower member was deposited in an extremely dry, highly saline, and strongly reductive shallow lake with low productivity. The paleo-environment of the upper member fluctuated wildly, while that of the lower member was relatively stable. The correlation analyses between paleo-environmental indicators and TOC show that the preservation conditions and dilution rate of organic matter have limited influence on organic matter enrichment of the Lucaogou Formation. At the same time, the primary productivity is the main controlling factor.

Geo-hazards & Engineering Geology
Study on the deformation mechanism and large deformation control method of a strongly weathered carbonaceous slate tunnel in western China
WANG Zhijiao, XIE Di, FAN Jinyan, MAO Yuting, TAO Zhigang
2023, 29(5): 648-661. doi: 10.12090/j.issn.1006-6616.2023020
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In response to the significant soft rock deformation challenges encountered during the construction of the Minxian Tunnel along the Lanzhou-Haikou Expressway (G75), this study conducted a comprehensive analysis of soft rock types and the underlying mechanical mechanisms governing the deformation of the surrounding rock. It presented tailored mechanical transformation strategies to address diverse mechanical mechanisms. Also, it introduced the application of anchor cable with high pre-tightening force, constant resistance and large deformation, a proven solution widely employed in mining and rock engineering. Furthermore, the research proposed a high-prestress and active and passive combined support technique, encompassing pre-reinforced retaining structure, optimally arranged active retaining structure with long and short NPR anchor cables, steel arches, and permanent retaining structure of shotcrete. By implementing numerical simulations and on-site monitoring, the results demonstrated a remarkable reduction in the maximum deformation of the surrounding rock in the test section to only 73 mm, and the pre-tightening forces applied to the anchor cable with constant resistance and large deformation ranged from 280 to 300 kN, underscoring the effectiveness of the optimized retaining technique in controlling surrounding rock deformation. This research highlights the pivotal role of retaining structure with constant resistance and yielding support, which significantly improves deformation control.

Quaternary Geology & Environment
Palynological records and paleoclimatic significance during the middle and late Late Pleistocene in the Qingshuihe Basin, Ningxia
WEI Lijie, LI Zhenhong, LI Mingtao, DONG Xiaopeng, CUI Jiawei, KOU Linlin
2023, 29(5): 662-673. doi: 10.12090/j.issn.1006-6616.2023015
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Desertification and drought in inland Asia are closely related to human habitation and social sustainability in western China. The climate in western China has experienced multiple shifts in temperature and aridity since the Late Pleistocene. Investigating this region's Late Pleistocene paleoclimatic changes contributes to predicting future climate trends. This study analyzes the pollen records from the Qingshuihe Basin in southern Ningxia during the middle to late Late Pleistocene, revealing four distinct pollen assemblage zones from bottom to top. These zones reflect paleovegetation and paleoclimate changes during the middle to late Late Pleistocene. The results indicate that during the middle Late Pleistocene (75-50 ka B.P.), the region featured a predominantly Artemisia-dominated steppe landscape with a cool and relatively dry climate. In the late Late Pleistocene (25-15 ka B.P.), the area experienced alternations between desert-steppe or steppe and forest-steppe, transitioning gradually to desert-steppe. During this period, the winter monsoon strengthened, leading to progressively cooler and drier conditions, with the climate shifting from cool and slightly moist to cold and arid. The further study of the palynological records during the middle and late Late Pleistocene in the Qingshuihe Basin of southern Ningxia holds significant implications for a proper understanding of the natural environmental history of the Loess Plateau and informs decision-making for its management.

Active Tectonics & Earthquake
Trigger mechanism and dynamic causes of the Taiwan earthquake sequence on September 17, 2022
HUANG Shaohua, WAN Yongge, FENG Gan, LI Xiao, GUAN Zhaoxuan
2023, 29(5): 674-684. doi: 10.12090/j.issn.1006-6616.2023056
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On September 17th and 18th, 2022, two earthquakes struck Taiwan Province of China in Taitung County and Hualien County, respectively, measuring MS6.5 and MS6.9 in magnitude, followed by multiple aftershocks. Both seismic events were situated along the Longitudinal Valley Fault and exhibited a reverse strike-slip mechanism. The geological setting in this area is intricate, as the Longitudinal Valley Fault zone represents a subduction zone where the late Mesozoic Paleo-Pacific plate converges with the East Asian continental margin, resulting in a predominant thrust-type tectonic stress background. However, historical earthquake data in this region have indicated a prevalence of reverse-faulting earthquakes. To address the causes of these reverse strike-slip fault earthquakes and their relationship with the tectonic stress field in the area, we first reconstructed the tectonic stress field by analyzing the focal mechanisms of past earthquakes within the study area. The resulting stress field was characterized by compressive stress oriented with an azimuth of NWW. Subsequently, this stress field was projected onto fault planes with various strike and dip angles. This analysis revealed that certain fault joints experienced more significant relative shear stress and lower relative normal stress, suggesting that these joints were more prone to dislocation, leading to earthquakes of the reverse fault type, reverse strike-slip type, and strike-slip type. Furthermore, the proximity and timing of the two earthquakes within a two-day span prompted an examination of their potential triggering relationship. Researchers calculated the Coulomb rupture stress changes caused by the MS6.5 earthquake on the rupture plane of the MS6.9 earthquake and its sliding direction. Their analysis indicated an increase of approximately 0.02 MPa in Coulomb rupture stress, suggesting that the Taitung MS6.5 earthquake may have triggered the Hualien MS6.9 earthquake. This study holds significant importance for understanding the seismogenic mechanism of the Longitudinal Valley Fault and gaining insights into the geodynamics of the study region.

Quantitative research of the impact of Shunyi fault activity on the ground fissures in the Beijing Capital International Airport, China
REN Yazhe, FENG Chengjun, QI Bangshen, GE Weiya, TAN Chengxuan, MENG Jing
2023, 29(5): 685-703. doi: 10.12090/j.issn.1006-6616.2023063
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The Shunyi fault in the Beijing Plain region is a significant late Pleistocene active fault. The Beijing Capital International Airport (BCIA) is situated within the central segment of the Shunyi Fault. Since 2010, ground fissures on the airport's runway have progressively worsened, with a maximum vertical displacement difference of up to 20 cm, severely affecting airport safety. Presently, the impact of Shunyi fault activity on the ground fissures at the BCIA is primarily described qualitatively. This paper, based on research regarding the geometric structure of the Shunyi Fault, Quaternary activity, and investigations of the ground fissures at the Capital Airport, employs the fault dislocation model to quantitatively analyze the influence of the Shunyi Fault's creep-sliding activity as well as the 1996 Shunyi ML 4.5 earthquake on the formation of airport ground fissures. The study also discusses the increasing risk of ground fissure hazards in the event of a potential strong earthquake along the Shunyi fault in the future. The preliminary results suggest that with a vertical activity rate of 0.6 mm/year, 46 years of Shunyi Fault's creep-sliding activity results in differential subsidence of no more than 2.5 cm on both sides of the airport ground fissures, contributing to approximately 20% of the formation and development of these fissures. The impact of the Shunyi ML 4.5 earthquake on the formation of airport ground fissures is minimal. However, if a future M 7.0 earthquake occurs on the Shunyi fault, the estimated maximum land differential subsidence between two sides of the fault could reach 104 cm, increasing the risk of airport ground fissure disasters by a factor of five. Ground differential settlement due to groundwater extraction from the upper and deeper layers of the Shunyi fault's hanging wall contributes about 70% to the formation and development of airport ground fissures, remaining the primary factor. The study's results provide essential scientific references for precise prevention and control of ground fissure disasters at BCIA. Furthermore, to further reveal the causes and mechanisms of delayed geological disasters along the Shunyi fault, such as ground fissures and land differential subsidence, it is recommended to implement dynamic monitoring of cross-fault crustal displacement or deformation at crucial segments.

Discussion on the magnitude or intensity limitation of paleoearthquake events
GAO Yunpeng, LIU Jing, HAN Longfei, SHAO Yanxiu, YAO Wenqian, XU Jing, HU Guiming, WANG Zijun, QU Ziyi, XU Enmin
2023, 29(5): 704-719. doi: 10.12090/j.issn.1006-6616.2023034
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The magnitude is an important parameter that characterizes the size of earthquakes. However, in paleoearthquake studies, it is difficult to precisely determine the rupture parameters closely related to seismic moment, making it challenging to directly calculate the magnitude of events. Researchers often assume that event sequences consist of characteristic earthquakes with similar magnitudes or use empirical relationships based on known magnitudes of historical earthquakes to estimate magnitudes. However, previous studies have shown that the assumption of characteristic earthquakes is overly simplistic, and magnitude estimation based on empirical relationships is limited by various errors. Therefore, there is a pressing need to explore new methods to improve the reliability of magnitude assessments for ancient earthquake events. In recent years, the successful application of three-dimensional combination trenches has demonstrated that these trenches contain rich deformation information about events, confirming the feasibility of assessing event sizes within trenches. Using the example of the Copper Mine trench on the Altyn Tagh fault, this article utilizes the deformation intensity revealed within the trench, including vertical displacement, deformation zone width, and total tensile strain, to estimate the scale of the event sequence. Data analysis results indicate that the deformation intensity parameters have a certain positive correlation with the relative magnitude, and there is also some correlation among these parameters. Therefore, the information on deformation intensity within the trench can be used to assess the relative magnitude of events, and fully exploring the deformation information within trenches can provide valuable insights and references for the reasonable evaluation of the magnitude of paleoearthquake events. This underscores the importance of considering such information in paleoearthquake research.

Fundamental Geology & Regional Geology
Petrogenesis and geological significance of migmatitic gneiss in Mulantou area, Hainan Island
HU Zailong, ZHAO Xiaoming, WANG Yong, WEI Changxin, LYU Zhaoying, LYU Changyan
2023, 29(5): 720-735. doi: 10.12090/j.issn.1006-6616.2023067
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In the coastal area of Pujian to Mulantou on Hainan Island, a set of medium to deep metamorphic rocks (Mulantou complex) has been discovered, composed of migmatite, dolomite, shale, and amphibolite, with migmatite being the predominant lithology. This study selected well-developed and typical migmatitic gneisses as the research focus and conducted systematic zircon U-Pb isotope dating and petrological and geochemical studies. The results indicate that the protolith of the Mulantou migmatitic gneisses was intermediate basic volcanic rock formed around 276 Ma. These rocks exhibit geochemical characteristics of island-arc calc-alkaline basalt, suggesting a tectonic setting related to the subduction of the Paleo-Tethys Ocean. Early anatectic metamorphism occurred around 261 Ma, indicating a tectonic environment related to the collision between the South China Block and the Indochina Block. Later metamorphism took place around 248 Ma, signifying a tectonic environment associated with extension following the collision between the South China Block and the Indochina Block. Therefore, the Mulantou migmatitic gneisses preserve a comprehensive record of the tectonic evolution in Hainan Island from the Early Permian to the Early Triassic. They represent the geological consequences of events such as the closure of the Paleo-Tethys Ocean, the collision between the South China Block and the Indochina Block, and the subsequent extension. The discovery of these rocks provides new insights into the eastern extension of the Song Ma suture zone.

Evolution of metamorphic processes in the Neoarchean mafic granulites of the Qingyuan Terrane in northern Liaoning, North China Craton
CUI Runze, WEI Chunjing
2023, 29(5): 736-756. doi: 10.12090/j.issn.1006-6616.2023049
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Multiple interpretations exist regarding the tectonic evolution model of the Neoarchean North China Craton, requiring a more in-depth study of metamorphic processes. Systematic petrographic observations, mineral chemical analysis, phase equilibrium modeling, and zircon dating were conducted on the basic granulites from Qingyuan in northern Liaoning to elucidate their metamorphic evolution processes and geological significance. The selected samples of mafic granulites were divided into the garnet-bearing domain (19DJ07-GD) and garnet-free domain (19DJ07-NGD), with the garnet-bearing region displaying a banded and inhomogeneous distribution. Both domains exhibit two generations of granulite facies assemblages. In the garnet-bearing domain, the first-generation metamorphic mineral assemblage includes garnet + clinopyroxene + orthopyroxene + hornblende + biotite + plagioclase + quartz. Notably, the first-generation plagioclase (Pl1) exhibits a complex compositional zoning, with anorthite content (xAn) increasing from the core to the mantle and then decreasing towards the rim. Similarly, the titanium component zoning in the first-generation amphibole (Amp1) follows a pattern of increasing from the core to the mantle and then decreasing towards the rim. Based on mineral assemblages and corresponding component zoning, it is inferred that the first-generation granulite facies metamorphic process followed a counterclockwise P-Tpath, involving a pre-peak P-T rise stage and a post-peak P-T drop stage. Phase equilibrium modeling constrains the peak conditions at 0.8~0.9 GPa/900~950 ℃, indicative of high-ultrahigh-temperature (HT-UHT) metamorphism conditions. Zircon dating results yielded a post-peak cooling age of 2498±6.9 Ma (MSWD=0.39). Considering the regional "dome-and-keel" tectonics, the counterclockwise P-T path, and the metamorphic timing of supracrustal rock nearly synchronous with late-stage TTG magmatic pulses, the UHT metamorphism of the supracrustal rocks is believed to be controlled by the unique Archean vertical tectonics/sagduction system. The second-generation metamorphic assemblage is characterized by locally formed coronas or symplectites of garnet + quartz ± clinopyroxene, representing high-pressure (HP) granulite facies metamorphism associated with a Paleoproterozoic orogenic event.