地质力学学报

Formation and evolution of the Yuncheng Salt Lake and sources of the saline ions

KOU Linlin, LI Zhenhong, XIA Caixiang, WEI Lijie, DONG Xiaopeng, WANG Sen, CUI Jiawei, HUANG Ting, NAN Debin

Objective Yuncheng Salt Lake, located within the core area of the Fenwei Graben System in the south-central part of the North China Craton, is the earliest known salt lake in China and worldwide. The formation and evolution of the Yuncheng Salt Lake are closely related to the Cenozoic tectonic domains of the coastal Pacific in eastern China and the Qinghai-Tibet Plateau in western China. However, there are many debates regarding the spatiotemporal processes and key timing associated with these two domains in the Yuncheng Salt Lake. Additionally, the salt lake is rich in sodium, magnesium, chloride, and sulfate ions; however the sources of these saline ions remain unclear. Methods This study investigated the major changes in the Cenozoic tectonic geomorphology of the Yuncheng Basin and its surrounding areas, focusing on the evolution of the Cenozoic river-lake system to establish the formation and evolution stages of the Yuncheng Salt Lake. This study combined the tectonic background and geochemical element analysis of typical strata in the adjacent Zhongtiao Mountains to identify the main sources of saline ions in the Yuncheng Salt Lake. Results At the boundary between the Paleogene and Neogene, there was a widespread angular unconformity across the central and eastern parts of the North China Craton. This unconformity affected the Weihe Basin in the Fenwei Graben System to the west, and not the northeastern edge of the Qinghai-Tibet Plateau, which connects to the western North China Craton. This indicates that the tectonic forces during this period originated mainly from the coastal Pacific tectonic domain of the eastern North China Craton. In the mid-Miocene, a widespread unconformity along the northeastern edge of the Qinghai-Tibet Plateau connect to the western North China Craton. The northeastward uplift and expansion of the Qinghai-Tibet Plateau affected caused the Ordos Basin to rotate counterclockwise, initiating the formation of the Shanxi Graben System within the Fenwei Graben System. The Cenozoic evolution of the Yuncheng Salt Lake is the result of the interaction between the two major tectonic domains of eastern and western China. Before the mid-Miocene, the coastal Pacific tectonic domain predominantly controlled the region. Subsequently, the long-distance effects of northeastward uplift and expansion of the Qinghai-Tibet Plateau began to influence the Yuncheng Salt Lake. The Jixian System Longjiayuan Formation in the Zhongtiao Mountain area is composed of marine carbonate deposits that is the primary source of magnesium ions for the salt lake, with an average magnesium oxide content of 20.92%. The Paleogene Pinglu Group, an arid lake basin deposit rich in gypsum layers, provids sodium, chloride, and sulfate ions to the salt lake, with an average sodium oxide content of up to 2.6%. Conclusion This study suggested that approximatedly 700000 years ago, during the transition between the early and middle Pleistocene, the Yellow River flowed eastward into the sea through the Sanmenxia Gorge, leading to the disappearance of the Sanmen paleolake and the initial formation of the Yuncheng Salt Lake. Approximately 70000 years ago, during the mid-late Pleistocene, the Fen River changed its course and left the Yuncheng Basin, transforming the salt lake from an open to a closed system, ultimately forming the Yuncheng Salt Lake. Subsequently, under the continuous influence of the fault at the northern edge of the Zhongtiao Mountains, a large sedimentary depression formed, accumulating a large amount of saline minerals. Natural salt lake is formed over long periods of precipitation and evaporation. Currently, the saline ions of the Yuncheng Salt Lake mainly originate from adjacent sedimentary strata in the Zhongtiao Mountains and deep mineralization layers. The major fault controlling the salt lake provides the upward migration and injection of saline substances from deep mineralization layers into the salt lake. The marine high-magnesium dolomite of the Longjiayuan Formation of the Mesoproterozoic Jixian System in the Zhongtiao Mountains is a major source of magnesium ions for the salt lake. Significance The research findings provide fundamental geological evidence for the implementation of an ecological protection strategy to restore the lake by reducing salt levels in Yuncheng Salt Lake.

InSAR deformation observation and regional severe earthquake hazard analysis of the 2016 and 2022 strong earthquake activities in Menyuan, Qinghai

CHENG Yan, JIANG Yanan, HOU Zhongjian, ZENG Rui, LUO Huiyuan

Objective Over the past 30 years, Menyuan Country, Qinghai, has experienced three strong earthquakes, demonstrating active seismic activity. The focal mechanisms of these earthquakes showed remarkable differences and the epicenters were all located near the Lenglongling fault. Methods In order to quantitatively analyze the activity characteristics of the Menyuan seismic sequence using InSAR technology to obtain the coseismic deformation field of the Menyuan earthquake of 2016 and 2022 and establish an appropriate fault model, the fine slip distributions of the two earthquakes were obtained through steepest decent method (SDM). Furthermore, the static Coulomb stress changes on the faults in the region and its surroundings caused by the Menyuan M_w 6.6 earthquake in 2022 were evaluated. Results The coseismic deformation field in 2016 exhibited a single elliptical uplift center, with predominantly thrust motion. In contrast, the spatial distribution of the deformation field in 2022 was more complex, showing a Y-shaped distribution, and there was a slight variation in the rupture direction from west to east, with primarily horizontal deformation. Conclusion The two earthquakes also differed in their slip patterns and shallow and deep structural styles. In 2016, the seismic activity was weak, with a slip zone existing at depths between 8 km and 12 km, where the maximum slip was only 0.23 m. The fault plane had a low inclination angle, which is a characteristic of deep-seated slip. By contrast, the 2022 earthquake was a typical shallow-focus earthquake featuring three distinct slip areas. The primary rupture occurred in the Lenglongling section, concentrated in the shallow part from 1 km to 7 km, with a maximum displacement of 3.22 m. A significant slip also occurred along the western extension of the Lenglongling fault, reaching a maximum slip of 2.59 m. The Tuolaishan section experienced rupturing mainly between 3 km and 8 km, with the maximum slip recorded as 2.1 m. Significance Combined with the analysis of the Menyuan earthquake activity in 1986, it is inferred that the Menyuan earthquake sequence is dominated by the activity of the Lenglongling fault, which is continuously adapting to new structures and stress adjustments in its northeast extension and shows compressive-shortening activity trend. The 2022 Menyuan Mw 6.6 earthquake had a significant impact range, and earthquake hazards need to be continuously monitored and studied further, especially in areas where static Coulomb stress changes exceed the hazard threshold.

Spatial structure characteristics and formation mechanism of the ancient Deda landslide elucidated using the microtremor survey method in Sichuan Province, China

QIU Zhendong, GUO Changbao, YANG Zhihua, WU Ruian, YAN Yiqiu, ZHANG Yiying, JIN Feng, CHEN Wenkai

Objective The topography and geological structure of the eastern Qinghai-Tibet Plateau are highly complex. High development density and intricate spatial structures of large ancient landslides in alpine canyon areas, such as those in the Jinsha River, contribute to this complexity. In addition, hazards resulting from creep deformation and the resurgence of these ancient landslides are severe. The ancient Deda landslide, situated in Deda Township, Batang County, Sichuan Province, is a significant ancient landslide influenced by the Chalong-Ranbu fault activity. The spatial structural characteristics of this ancient landslide are complex, with localized resurgence deformation at the landslide front. Methods Various methods were included in the current study, such as remote sensing interpretation, on-site investigations, the microtremor survey method (MSM), and engineering geological drilling to elucidate further the spatial structural characteristics of the ancient Deda landslide. Results The research revealed that the ancient Deda landslide can be divided into three sections in the plan view: the Deda I landslide (I), the Deda II landslide (II), and the rear wall of the ancient Deda landslide (Ⅲ). Using MSM combined with drilling verification, a scheme for classifying the shear wave velocities of shallow and deep sliding zones was constructed, with a relative error in sliding zone depth identification ranging 2.6% ~ 4.8%. This paper showed that the Deda I landslide features two sliding zones, with a burial depth of 18.7 ~ 20.1 m for the shallow sliding zone (S1-1) and 36.2~49.9 m for the deep sliding zone (S1-2). The volume of the Deda I landslide is approximately 8.7×10⁶~ 12.0×10⁶ m³. The Deda II landslide has one sliding zone (S2) with a 25.2 ~ 38.6 m burial depth and a landslide volume of approximately 6.3×10⁶~ 9.6×10⁶ m³. Conclusion A comprehensive analysis suggested that the formation of the ancient Deda landslide was the result of various factors, including fault structures, rainfall infiltration, and river erosion. The complex landslide structure and its genesis were identified as the primary controlling factors for landslides in a state of creep deformation. [Significance] The research methods and insights presented in this study can serve as a reference for the spatial identification and risk prevention of large ancient landslides on the eastern Qinghai-Tibet Plateau.

Spatiotemporal evolution of interseismic coupling and stress accumulation near an asperity on a vertical strike-slip fault: Insights from three-dimensional viscoelastic numerical simulation

LI Yebo, HUANG Luyuan, YAO Rui, TIAN Yiwei, YANG Shuxin

Objective Understanding the kinematic state and stress accumulation near fault protuberances is crucial for accurate assessment of earthquake hazards. Interseismic coupling (ISC) is a widely used method for characterizing the kinematic behavior of faults. Despite its importance, the correlation between the spatial distribution of ISC and the positioning of fault asperities, areas of increased frictional resistance, has not been extensively studied. Furthermore, the influence of the rheological properties of Earth materials on the temporal and spatial evolutions of slip deficits and shear stress accumulation in close proximity to these asperities remains poorly understood. Methods We developed a set of three-dimensional (3D) elastic and viscoelastic finite element models to investigate the effects of fault asperities on interseismic deformation and stress accumulation. These models incorporate vertical strike-slip faults and use sophisticated contact algorithms to simulate the mechanical locking associated with asperities. Our innovative approach, referred to as the “binary fault-locking approach”, simplifies fault behavior into a binary system, categorizing states as either “locked” or “unlocked”. The present study analyzes the spatial and temporal variations in the ISC and shear stress accumulation rates around a single asperity, providing novel insights into the mechanics of fault systems. In addition, we validate the efficacy of the “binary fault-locking approach” by applying it to the Xianshuihe fault, thereby reinforcing the relevance of our findings to real-world fault behavior. Through this study, we aim to enhance our understanding of fault mechanics and improve earthquake hazard assessments, which ultimately contributes to more effective risk-mitigation strategies. Results Because of the mechanical locking of the asperity, a fault-sliding surface within a certain distance from the asperity cannot slide freely, resulting in a slip deficit in an area centered around the asperity. Consequently, the degree of fault-locking displays a ring-shaped attenuation pattern centered on this asperity. Under purely elastic conditions, the ISC and shear stress accumulation rates near the vicinity of the asperity remained constant over time. Conversely, under viscoelastic conditions, the contours of the ISC and shear stress accumulation in the areas surrounding the asperity expanded with time under loading, and the effects of temporal changes in the locking degree became more pronounced. In scenarios where the viscosity differs on either side of the fault, the interseismic deformation and stress accumulation rate of the fault are primarily controlled by the rheological properties of the material on the side with a lower relaxation time, owing to the different relaxation times on either side of the fault. Conclusion (1) Because of the continuity of the medium, although the region adjacent to an asperity is not fully locked, its slip velocity is still lower than the movement velocity of block, resulting in a spatial pattern of decreasing ISC outward from the fault asperity. (2) Viscoelastic effects regulated the deformation near a fault asperity, leading to an increase in the spatial extent of the ISC over time. (3) The ISC can serve as an approximate indicator of the shear stress accumulation rate. Irrespective of viscoelastic effects, a value of approximately 0.5 can be used as the threshold for moderate to strong locking, and shear stress accumulation is insignificant below this value. (4) Considering the spatially nonuniform fault coupling along the Luhuo-Kangding segment of the Xianshuihe fault, the simulated surface velocities closely matched the GPS observations, thus confirming the reliability of the method. Significance This study establishes an important connection between ISC and shear stress accumulation rate, providing valuable insights for identifying potential seismic hazards. Overall, this study emphasizes the intricate interactions between fault dynamics and geological structures, and highlights the significance of detailed modeling for understanding earthquake mechanisms. By addressing the gaps in knowledge regarding the influence of protuberances on fault behavior, this research contributes valuable information to the field of seismic hazard estimation, thereby enhancing our ability to effectively mitigate earthquake risks.

Exploration and application of in-situ stress estimation method based on core disking phenomenon of boreholes

YAN Shaokun, WANG Chenghu, GAO Guiyun, LIU Jikun, DAI Xiangqian

Objective Rock core disking is a typical phenomenon that occurs in environments with high in-situ stress. The geometric characteristics and section shape of rock disking are related to the state of in-situ stress, and the site where this phenomenon occurs may be unsuitable for measuring in-situ stress. To obtain more comprehensive in-situ stress data from a wider range of sources, according to this phenomenon, in-situ stress estimation is conducted based on the internal relationship between the in-situ measurement data and the original in-situ stress, and the obtained result can supplement the in-situ stress measurement data. Methods According to the relevant hypotheses and theories worldwide, the change in core section stress and core internal energy during the core disking phenomenon was analyzed. The physical and geometric characteristics of the disked rock cores were measured, combined with the stress state of the original rock, and an in-situ stress estimation formula based on core disking characteristics was constructed. The results were compared with those of the other formulas. Results The geometric characteristics of 73 representative rock disks in the 30–120 m depth section of the Dalianshan borehole in Dandong, Liaoning Province, where the phenomenon of rock core disking occurs, are measured, and the physical properties of the core are tested. The value of the in-situ stress in this section is estimated using the established in-situ stress estimation formula to supplement and perfect the measured hydraulic fracturing data and better reveal the law of in-situ stress variation. Conclusion This section is estimated according to other in-situ stress estimation formulas based on the phenomenon of core disking proposed by scholars worldwide; the calculated results either deviate from reality or are dispersed in the distribution. Compared with these formulas, the in-situ stress data estimated by this formula are more in line with reality and meet the stress conditions for the generation of core disking. Significance The results show that the results obtained using this method can complement and perfect the in-situ stress data of the core disking depth section.