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.

Tectonic characteristics and numerical simulation analysis of an arcuate structural belt：A case study of the middle and southern segments of the Red River fault

WANG Chenxu, LI Xi

Objective The southeast Yunnan Arcuate Structural Belt, with the central and southern segments of the Red River fault as its main body, serves as the southwestern boundary of the Sichuan-Yunnan block and the forefront of its south-southeastward movement. However, there is still controversy as to whether its current motion is primarily characterized by transpression or transtension. This debate is strongly correlated with the complex stress-strain patterns surrounding the southeastern margin of the Qinghai-Tibet Plateau. Clarification of the kinematic characteristics and genesis of the southeast Yunnan Arcuate Structural Belt will help to understand the regional tectonic evolution. Methods This study utilized the interpretation of remote sensing images and geological field surveys to identify evidence for late Quaternary tectonic activity in the central and southern segments of the Red River fault. A three-dimensional (3D) geological model tailored to the actual characteristics of the region was established. It considers the influence of lower crustal flow and includes finite-difference numerical simulations with different velocity boundary conditions set at 26.5°N. Results The study reveals: (1) Numerous geomorphic features and fault profiles in the central and southern segments of the Red River fault indicate that most of the faults along the line are predominantly high-angle, northwest-striking with a complex fault rock development. The presence of structural wedges and infill of overlying material has been observed in several typical outcrops. Additionally, there are significant undulations in the lower part of the overlying strata on both sides of the fault. (2) The numerical simulations show that the influence of the far-field stress on the horizontal and vertical deformation of the southeastern margin of the Qinghai-Tibet Plateau is quite different on both sides of 26.5°N, and the deformation is further enhanced by the presence of lower crustal flow. (3) Numerical simulations of velocity fields, maximum shear strain rates, and maximum principal stresses demonstrate that surface motion trajectories, velocity distributions, stress conditions, and deformation accumulation differ among different models for the arcuate structural belt area; the presence of lower crustal flow promotes deformation accumulation and makes the magnitude of the velocity field closer to that of the current GPS horizontal velocity field. Conclusion (1) The numerous geological profiles along the central and southern segments of the Red River fault reveal predominantly normal-strike-slip movement, indicating that the region is currently dominated by the effects of transtension; (2) Current tectonic deformation and landscape evolution in the southeastern margin of the Qinghai-Tibet Plateau are mainly controlled by two different force sources: one is the southeastward movement of material, and the other is the arc-parallel extension and slab rollback beneath the Sunda-Java subduction zone. The presence of lower crustal flow influencess the scale of tectonic deformation in the southeastern margin of the Qinghai-Tibet Plateau. In the southeast Yunnan Arcuate Structural Belt, arc-parallel extension and slab retreat play a more significant controlling role; (3) The initial curved shape of the southeast Yunnan Arcuate Structural Belt is mainly attributed to the influence of material migration towards the southeast and the Xiaojiang sinistral strike-slip fault. Under the constraints of pre-existing structural fabrics and the control of arc-parallel extension and slab retreat, it has led to continuous deformation. Significance The research contributes to the understanding of the current activity of the southeast Yunnan Arcuate Structural Belt and the causes of this activity. It provides a quantitative analysis reference and theoretical basis for the study of the tectonic evolution of the southeastern margin of the Qinghai-Tibet Plateau.

Tectonic transition and extension at the eastern and western ends of the Altyn Tagh fault: insights from triple junctions

YI Kexin, JOLIVET Marc, GUO Zhaojie

Objective The Altyn Tagh fault (ATF) is the largest left-lateral strike-slip fault on the northern margin of the Qinghai-Tibet Plateau, extending for about 1600 km. It accommodates a considerable portion of the India-Eurasia convergence and is widely regarded as a key tectonic boundary influencing the plateau’s uplift and outward growth. However, its mode of propagation remains debated. Resolving this debate requires clarifying how the ATF evolved into its present configuration and how it connects with adjacent structures such as the Qilian orogenic belt and the Qimantagh-Eastern Kunlun fault. In this study, we use the concept of triple junctions to investigate key transition zones at the eastern and western ends of the ATF—namely, the Subei and Tula triple junctions—to shed light on the fault’s Cenozoic segmented rupture and bidirectional extension. Methods Triple junction analysis, a fundamental method in plate tectonics, is utilized to assess fault properties and fault stability from a kinematic perspective. Additionally, GPS data and seismic source mechanism solutions are analyzed to characterize the current kinematic behaviors and movement directions of the faults. Results (1) Transition between ATF and Qilian orogenic belt: Subei triple junction. The central segment of the ATF was the earliest to become active during the Cenozoic, generating a compressional horsetail splay on its eastern termination. The Danghe Nanshan fault and Yemahe-Daxueshan fault emerged as part of this horsetail splay. As left-lateral strike-slip motion on the ATF accelerated in the Miocene, large offsets developed between the Tarim, Qaidam, and Qilian blocks, giving rise to a triple junction near Subei. Initially, this triple junction was unstable, and the Qilian block experienced extensional strain relative to the Tarim block, indicating a local stretching environment. To achieve stability, the ATF progressively “straightened” eastward, ultimately supplanting the Yemahe-Daxueshan fault. Its western segment was reoriented to run parallel to the ATF, while the Danghe Nanshan fault remained as the key boundary on the Qilian side. Consequently, a stable triple junction formed at the intersection of the ATF’s central and eastern segments with the Danghe Nanshan fault. At the present leading edge of the ATF’s eastward propagation, the Hongliuxia fault displays a similar evolutionary trajectory, suggesting that the ATF continues to extend by reconfiguring secondary faults. (2) Transition between ATF and Qimantagh-Eastern Kunlun fault: Tula triple junction. The ATF’s central segment spawned a tensional horsetail splay at its western termination, involving the Tula and Baiganhu faults. When large-scale activity on the Eastern Kunlun fault commenced in the Miocene, the Qaidam block began moving relative to the Eastern Kunlun block, producing an unstable triple junction in the Tula region. To achieve a stable configuration, the ATF propagated westward along a more linear path, gradually diminishing activity on the Baiganhu fault. As a result, the stable triple junction—involving the Tarim, Qaidam, and Eastern Kunlun blocks—ultimately localized where the ATF meets the Qimantagh-Eastern Kunlun fault. This westward “straightening” and the concurrent reduction in subsidiary fault activity have fashioned the current tectonic framework at the western end of the ATF. Conclusion (1) The ATF has undergone a segmented rupture–bidirectional extension process throughout the Cenozoic. (2) The Miocene activation of the Yemahe-Daxueshan, the Qimantagh-Eastern Kunlun and other fault systems led to the formation of two triple junctions at Subei and Tula, respectively. These junctions were initially unstable, prompting secondary faults to “shortcut” and realign and leading the ATF to straighten and extend farther east and west. [Significance] This study refines our understanding of how the Altyn Tagh fault expanded along the northern margin of the Qinghai-Tibet Plateau. By applying triple junction concepts to continental blocks, we illustrate how block interactions have governed the ATF’s segmentation and through-going evolution. The proposed segmented rupture–bidirectional extension framework reconciles geological observations of Cenozoic deformation along the ATF. It also underscores the importance of analyzing triple junctions in understanding large-scale tectonic reorganization.

Mechanics and analog modeling of the Huo-Ma-Tu thrust sheet in the southern Junggar Basin fold and thrust belt

YANG Geng, CHEN Zhuxin, LU Xuesong, ZHANG Yuqing

Objective Understanding the mechanical development of thrust sheets is fundamental, yet challenging, to comprehensively understand the deformation processes of thrust belts. Various models explain the mechanics behind thrust sheet development, yet significant controversies persist. Methods This study takes a comprehensive approach, focusing on the southern Junggar Thrust Belt. We combine a variety of methods including surface geological surveys, seismic reflection profiles, and drilling data analysis with mechanical and physical modeling to thoroughly investigate this issue. Results (1) Based on surface geological surveys, seismic data interpretation, and drilling data, we confirm that thrust faults have developed in the core and southern limb of the Huo-Ma-Tu anticline. These faults extend southward beneath the front of anticlines, forming the extensively distributed Huo-Ma-Tu thrust sheet, which exhibits none to weak internal structural deformation. (2) Drilling data from the Huo-Ma-Tu structural belt clearly show that the frontal thrust faults and detachment faults have developed in layers with abnormally high fluid pressure, indicating that the thrust sheet is a combination of strong deformation sheets and weak detachment faults. Analysis of in-situ formation pressure data suggests that the thrust faults within these overpressure layers can segregate fluid pressure coefficients between the hanging wall and the footwall. (3) Using the geometric deformation characteristics of the Huo-Ma-Tu thrust sheet obtained from seismic profiles and drilling data, a simplified mechanical model is established. This model calculates the mathematical relationship between the horizontal compressive stress-to-gravity ratio at the back of the thrust sheet, the geometric parameters of the thrust sheet, the fault friction coefficient, and the fault dip angle. Separate equations are provided for thrust sheets without fluid overpressure and those with fluid overpressure detachment layers. (4) A physical model of the development characteristics of the Huo-Ma-Tu thrust sheet in the Southern Junggar Thrust Belt confirms that the deformation pattern of such rigid thrust sheets aligns with the structural interpretation from seismic profiles. This supports the validity of the simplified mechanical model in reflecting the actual geological conditions. Conclusions The simplified mechanical model demonstrates that the required horizontal tectonic stress-to-gravity ratio at the back of the thrust sheet decreases significantly with increasing fluid pressure coefficient. The physical modeling results also verify that the deformation pattern of strong deformation sheets/weak detachment fault thrust sheets generally conform to the structural interpretations of the seismic profiles. The experiments reveal that the large displacements at the base of the thrust sheet result from the accumulation of small-scale displacements coinciding across multiple segments. Significance This study proposes simplified rectangular with triangular geometric models of thrust sheets, which can preliminarily explain the kinematics and dynamics of thrust sheets, especially those with fluid overpressure. The derived mathematical relationships accurately describe the geometric, kinematic, and dynamic relationships of thrust sheets and are robustly validated by physical simulation experiments, reinforcing the reliability of our findings.

Avoidance distance and influence range of active faults: A case study of Litang fault

YANG Zhen, ZHONG Ning, ZHANG Xianbing, YU Hao, GUO Changbao, LI Haibing

Objective Active faults can not only trigger strong earthquakes, but also cause engineering breaks, creep deformation, and geological disasters, which seriously threaten the planning, construction, and safe operation of major projects. Methods In this study, based on the statistics of coseismic surface ruptures, the avoidance distances of faults with different properties and the ranges of extremely strong, strong, less strong, and moderately affected areas of active fault zones are established. Remote sensing, a geological survey, dislocation landforms, and shallow seismic exploration were used to analyze, the avoidance distance and affected area of the Litang fault on the Qinghai-Tibet Plateau. Conclusion This study concludes that the hanging wall and footwall avoidance distances of normal faults are not less than 30 m and 15 m, respectively; the hanging wall and footwall avoidance distances of thrust faults are not less than 45 m and 15 m, respectively; the avoidance distance of strike-slip faults is not less than 30 m. The Litang fault is mainly characterized by strike-slip with normal fault components, and its hanging-wall avoidance distance is 30 m and its footwall avoidance distance is 15 m. The extremely strong, strongly, less strong, and moderately affected areas of the Litang fault zones are 154 m, 154-500 m, 500-1000 m, and 1000-3000 m, respectively. [Significance] The research provides basic data for the planning and construction of major projects and the spatial control of the national land.

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