Multistage Strike-Slip Fault in the Narrowest Portion of the Qinling Orogen, Central China: Deformation Mechanism and Tectonic Significance

Strike-slip faults play an important role in continental deformation and have been observed widely in nature and analog experiments [1-6]. Strike-slip faults have been discussed in plate-like rigid-body motion models [1] and viscous flow models [3, 7]. Different end-member processes result in different strike-slip fault deformation kinematics, especially for the Altyn Tagh, Haiyuan, West Qinling, East Kunlun, Xianshuihe, and Ailao Shan-Red River faults (Figure 1(a)) [1, 5, 8-12]. In the northeastern Tibetan Plateau, the kinematics of these strike-slip faults have been summarized [5] in the lateral extrusion model [1, 13], transfer fault model [14, 15], rigid bookshelf fault model [16], and nonrigid bookshelf fault model [5]. The interaction between strike-slip fault systems and associated structures commonly involves different deformation mechanisms and continental dynamics [5, 9, 11, 12, 17, 18].

The E–W-trending Qinling Orogen defines a transitional zone between the Tibetan Plateau and the eastern lower-elevation continent (Figure 1), and it features well-developed strike-slip faults, such as the West Qinling, East Kunlun, and North Huicheng Basin faults [17-19] (Figure 1(b)), which facilitate the outgrowth of the Tibetan Plateau [20-25] and lateral extrusion of the Qinling Orogen [26]. The East Kunlun fault may have originally connected the Mianlue suture and marked the south boundary between the West Qinling Orogen and Songpan-Ganzi (Figure 1). The West Qinling fault facilitated the Tibetan Plateau material transportation through left-lateral slipping. The North Huicheng Basin strike-slip fault system developed in the narrowest portion of the Qinling Orogen and acted as the structural boundary between the West and East Qinling Orogens [27-29] (Figures 1 and 2). Many studies have been performed, such as structural analysis [22, 30-36], geochronological dating [22, 36-39], magmatic emplacement [28], and sedimentary basin [40, 41]. Along this strike-slip fault system, prominent salient and recessed structures significantly affect crustal thickening, magmatic emplacement, and basin evolution (Figure 2). However, the deformation mechanisms of the strike-slip fault system and its tectonic significance are unclear, which impedes our comprehensive understanding of continental deformation and related tectonic events. The shortening style of the main strike-slip fault is a key factor in the spatial-temporal deformational kinematics (Figures 2 and 3). If shortening occurs in one direction (Figure 3(b)), the dip direction of the curved thrust faults should be consistent; otherwise, it is the opposite if shortening occurs in two directions (Figure 3(a)).

In this study, we carried out a combination of fieldwork and physical analog modeling (with particle image velocimetry [PIV] mapping) on the North Huicheng Basin strike-slip fault system, which outlines the deformation mechanism of this strike-slip fault system and associated salient and recessed structures. Multiproxy geochronologic data were used to establish the main thermal history of the hanging wall. Integrated research suggests that this multistage strike-slip faulting plays a significant role in the main tectonic events.

Previous studies have identified the main collisional events in the Qinling orogen, characterized by the Proterozoic Kuanping [42], Paleozoic Shangdan, and Triassic Mianlue sutures (Figure 1(b)) [31, 33, 42-44]. The main framework of the Qinling Orogen consists of the southern North China Block, North Qinling Belt, South Qinling Belt, and northern South China Block (Figure 1(b)) [31, 33]. Laterally, the Qinling Orogen was divided into the East and West Qinling Orogens with diverse topography and geology (Figure 1(b)). The East Qinling and West Qinling Orogens are connected with a narrow portion, which features a sharp decrease in width to 60–80 km and underwent greater crustal shortening [28, 36]. The structural boundary of the West and East Qinling Orogens is the NE-striking North Huicheng Basin fault system (Figure 1(b)) [27-29, 35], consisting of a series of substrike-slip faults (Figure 2).

The basement of the Qinling Orogen consists of Proterozoic amphibolite, quartzite, and quartz schist (Figures 2(b) and 4(a)) [42]. Silurian outcrops are present in South Qinling and predominantly consist of deep-marine siliciclastic rocks and turbidites [45]. Devonian marine sandstones and Carboniferous continental marginal deposits are also present (Figures 2(b), 4(a) and 5(d)) [46]. Permian to early Triassic siliciclastic and carbonate rocks were deposited in the Huicheng Basin (Figures 2(b), 4(a) and 5(f)). Middle Jurassic Longjiagou Formation lacustrine sandstone, shale, and interlayered coal occur occasionally in the northern Huicheng Basin (Figure 5(e)) [40]. Lower Cretaceous terrestrial conglomerate, sandstone, and other clastic rocks of the Donghe Group unconformably cover the older strata (Figure 5(a)–5(c)) [46]. Neogene–Holocene rocks were deposited in the Huicheng Basin [46] (Figure 2(b)).

Systematic field transects across the main region of the main structural trace were constructed based on a digital elevation model (DEM) image (Figure 2(a)). With field observations, the fault-slip data are analyzed according to the kinematic approach of Marrett and Allmendinger [47] to assess incremental shortening or extension directions. P-B-T axes method is used to calculate the fault plane solutions and to visualize the structural data.

One hanging wall plutonic sample was collected for zircon U‒Pb dating, biotite ⁴⁰Ar/³⁹Ar dating, and apatite fission-track (AFT) analyses. Different closure temperature results can be obtained from the same sample (zircon U‒Pb >900°C [48]; biotite ⁴⁰Ar/³⁹Ar ~300 ± 50°C [49], ; and AFT 60°C–110°C [50]. Under this sampling strategy, the thermal history of the hanging wall can be approximately determined. The detailed geochronological analytical processes are listed in online supplementary material 1.

Based on a compilation of published work [33, 36, 40], DEM images (Figure 2(a)), and field observations, we carried out a new interpretation of the structure of the North Huicheng Basin strike-slip fault system and neighboring region (Figure 2(b) and 2(c)). To the west, the Devonian to Carboniferous rocks form a highly deformed WNW–ESE-trending fold-and-thrust belt, forming salient structures. To the east, the NW–SE-trending fold-and-thrust belt is shortened into fault-related folds as recessed structures (Figure 2(c)). Seven sets of paleostress tensor data from different strata along the North Huicheng Basin fault (Table 1, Figure 2(b)) have been collected, which implies multiple complex tectonic events. Data sets 1 and 3 suggest principal stress orientations σ₁ of 229° and 215°; data set 2 suggests that the principal stress orientation σ₂ is at 325°; data sets 4, 5-old, and 6 suggest principal stress orientations σ₁ at 162°, 335°, and 348°; and data set 5-young suggests a σ₂ orientation of 308° (Table 1, Figure 2(b)).

Zircon U–Pb dating yields a concordant age of 213 ± 0.7 Ma (Figure 6(a)), which is obtained from 18 valid zircons after discordant correction. All the zircon Th/U ratios are larger than 0.1 (Table 2). For biotite ⁴⁰Ar/³⁹Ar dating, we exclude several initial and final stages of the sample because the Ar loss might be related to slight alteration and crystal structure damage. Thus, the ⁴⁰Ar/³⁹Ar ages yield a well-defined plateau age of 203.7 ± 1.8 Ma during the 980℃–1400℃ phases (Figure 6(b); Table 3). For AFT dating, the granite sample yields a pooled age of 56 ± 4 Ma for P (χ²) > 5%, with a mean track length of 11.3 ± 2.4 μm (Figure 6(c); Table 4). The detailed AFT age, Dpar, and length data are listed in online supplementary Tables S1 and S2. In addition, combined with published works, we established the main thermal history of the hanging wall of the North Huicheng Basin fault system (Figure 6(d)). Combined with the paleostress tensor information, the thermal history is useful to discuss the detailed spatial–temporal deformational kinematic process.

In general, the field observations indicate that the structures can be divided into three domains: one has a salient shape (domain I) in the western region, the second has a recessed shape (domain II) in the eastern region, and the third (domain III) is bounded by the North Huicheng Basin strike-slip fault system. This regionalized structural domain is consistent with the individual plate-like movement from the Global Positioning System (GPS) velocities and crustal uniform strain rate fields [51]. The plate-like movement of the West and East Qinling Orogens induced multiple left-lateral strike-slip motions in domain III and controlled the associated structures in domains I and II. This interpretation can well explain the coeval development of the North Huicheng Basin strike-slip fault system and alongside salient and recessed structures. In particular, we focus on how strike-slipping controls salient and recessed structures in one or two shortening directions, which is the key controversial issue in this study. The southwestward thrust fault (Figure 5(g)) in the east side recessed fold-and-thrust belt indicates a consistent shortening direction with that of the salient structures (Figure 3(b)). This evidence is consistent with published cross-sections (Figure 4(b), cross-section I-I’; Figure 5(h), cross-section II-II’) [36]. In this way, the one-direction shortening model (Figure 3(b)) is most favored based on field observations. Furthermore, the paleostress tensor data indicate that it underwent multiple left-lateral strike-slip faulting and complex tectonic events in this region (Figure 2(b)).

Sandbox modeling has been widely used to investigate continental deformation [52-59], such as strike-slip faults and associated structures [60-62]. Inspired by the published sandbox models, the analog model here is set up as one direction shortening, with one fixed lateral vertical glass wall and one free boundary (Figure 7). Plates A and B are used to illustrate the West and East Qinling deformation domains, respectively. Plate A is pushed through a mobile backstop to shorten the model (Figure 7(a)). With a transfer force through the plate border, plate B was passively pushed forward, while plate A was moving (Figure 7(a)). The experimental velocity can be illustrated by the following equation:

where V_b^e is the efficient velocity parallel to the shortening direction in plate B, μ is the correlation coefficient, V_a is the efficient velocity parallel to the shortening direction on plate A, V is the shortening velocity with respect to the mobile backstop (Figure 7), and α is the plate border angle. Since gravity is equal for both the prototype and model and the densities have the same order of magnitude, the concept of dynamic similarity implies that the ratio of stresses is nearly equal to the ratio of lengths [63, 64]. The used sand has an angle of internal friction of ca. 42°, a density of ca. 1560 kg/m⁻³, and negligible cohesion. The ratio was chosen to be 1.5 × 10⁻⁶ (Table 5).

The analog experiments were performed in the Tectonophysics Experiment Platform of the Laboratory of Deep Earth Science and Exploration Technology, Ministry of Natural Resources, China. During the shortening, we collect images of the experiment at 5 seconds intervals with a Canon 80D camera. Then, we recorded and calculated the displacement and rotation fields via PIV (Figure 8 and -continued). In map view, the displacement was usually accommodated by thrust sheets, which developed in a forward direction, and the distance was evenly distributed (figures 8 and 9). The frontier displacement field obtained from particle image velocimetry in the plate A domain was parallel to the shortening direction (Figure 8). However, the particles displayed counterclockwise motion in the transition zone and perpendicular motion to the shortening direction on plate B (Figure 8 and -continued). In a cross-sectional view, the sand layers were thickened around a frontal fault ramp. When a thrust sheet migrated forward, the shortening was accommodated by a new thrust at the front of the mobile sand layer (Figure 9(c) and 9(d)). The thrust sheets in plate A (cross-section a-a’) occurred more frequently, and the thrust sheet distance was larger than that in plate B (cross-section b–b’; Figure 9(e) and 9(f)). The highest point of each thrust sheet grew gradually under continuous shortening (Figure 9(e) and 9(f)). The topographic slope was ~32° in the cross-section a–a’ forelimb, while it was ~30° in the cross-section b–b’ forelimb (Figure 9(c) and 9(d)).

When comparing structures, the experimental results and prototype have a high fitness, which can be used to explore how the strike-slip fault controls salient and recessed structures in nature. In the main strike-slip zone (domain II/II’), evidence of a simple shear component is visible, and the axial plane of anticlines changes to a nearly E–W trend (Figure 10), which is confirmed by the particle counterclockwise rotation via PIV mapping in the sandbox experiment (Figure 8 and -continued). In addition, the structures display multiple left-lateral strike-slip motions, which occurred both in the experiment and in the prototype (Figure 10). With the accommodative effect of strike-slip faulting, transtensional faulting can result in some pull-apart basins (Figure 10).

Domain I (Figure 10(a)) is controlled directly by the motion of plate A, which is very similar to the shape of the salient fold and thrust belts distributed in domain I’ (Figure 10(b)). Both in the experiment and prototype, the thrust faults are distributed at approximately equidistant distances, and some coeval fault-related folds also developed (Figure 10). The passive markers (circles with a diameter of 3 cm, Figure 8) in this domain are compressed parallel to the shortening direction and show that the deformation in domain I is pure shear. The salient thrusts and folds gradually terminate along the North Huicheng Basin strike-slip fault system, and their eastern segments display left-lateral strike-slip motion (Figure 10). In domains III and III’, the thrusts and folds are recessed and curve opposite to the shortening direction. Some of the thrust faults and folds should connect at the beginning and are truncated during the shortening process, which is evidenced by the experimental results (Figure 10).

According to the experimental results, it can be inferred that the highest point of each thrust sheet and topographic slope should grow during shortening. Because of the complex weathering and erosion, we can no longer obtain related information in nature. However, in general, the fact that individual plate-like movement induces strike-slipping and controls salient and recessed structures is confirmed by the experimental results (Figure 10).

The evidence from geology, geophysics, and geochronology indicates that the Qinling Orogen evolved into intracontinental tectonic processes since the closure of the Paleotethys Ocean [65]. In the Early Triassic, a foreland basin formed in the Huicheng region, which controlled the tectonic setting and resulted in neighboring structures [36, 41]. In the Middle Triassic, the subduction of the Paleotethys oceanic crust ended the foreland basin in the Huicheng region [41]. In the Late Triassic, widespread syn- to postcollisional plutons were emplaced at 200–230 Ma [31, 38, 66], which is consistent with our zircon U–Pb ages (213 ± 0.7 Ma; Figures 6(a) and 11(b)). Previous analog experiments have confirmed that magma can rise along fault-ramp structures [67-75]. The weak middle crustal zone [36, 76-78] can be activated as a thrust fault ramp [79], such as the ramp in Figure 4(b). This fault-ramp structure might have allowed magma to be emplaced ca. 900°C at 213 Ma (zircon U–Pb [48]; Figures 6(d) and 12(a)). Then, the hanging wall pluton was transported upward to the 300°C ± 50°C isotherm at 203 Ma (biotite ⁴⁰Ar–³⁹Ar dating [49]).

In the Late Triassic to Jurassic, due to continuous compression from the North and South China Blocks, the Qinling Orogen was mainly characterized by strike-slip faulting and pull-apart basin formation along the major boundary faults [32, 43, 65]. In the North Huicheng strike-slip fault system, the Miaoping fault underwent south-directed thrusting as well as left-lateral strike-slip faulting following the late Triassic collision [28, 80]. Along the Niyang fault, the left-lateral strike-slip faulting transitioned to transtensional faulting and controlled the local development of the half-graben basin during the Middle Jurassic (Figures 11(b) and 12(b)). These strike-slip fault-controlled pull-apart basins are well represented both in our sandbox experiment (Figure 10(a), domain II) and in field observations (Figure 11(a), paleostress sets 2, 9, 10, 11, 12, and 13). Therefore, the Jurassic basin was likely induced by the (1) continuous development of left-lateral strike-slip faulting along the North Huicheng Basin fault system [28], not (2) regional postcollisional collapse [40].

During the Late Jurassic to the Early Cretaceous, the Qinling Orogen underwent an intense intracontinental orogeny, which resulted from the southward intracontinental subduction of the North China Block [65]. The lower Cretaceous Donghe Group is in unconformable contact with the underlying strata in the northern Huicheng Basin (Figure 5(a) and 5(c)), indicating this tectonic event [36]. Meanwhile, large-scale extrusion tectonics [80] developed slightly later or within these compressional tectonics [40, 78]. The regional escaped tectonics resulted in some strike-slip and associated early Cretaceous basins in the Huicheng region [40], which is supported by our field observations (Figure 11(a), paleostress data 4, 5, and 7). If this is true, the development of left-lateral strike-slip faulting along the North Huicheng Basin fault could have facilitated the development of the pull-apart basin (Figure 12(b)), which can be confirmed by our analog result (Figure 10(a)).

In the Late Cretaceous to Cenozoic, though there are many controversial disputes about the late Mesozoic and Cenozoic basin age [27, 81, 82] and exhumation process [22, 37, 83, 84] in Huicheng region, two tectonic episodes have been reconstructed in most area of the northeastern Tibetan Plateau based on sedimentary geology [27, 29, 35, 85], paleostress data (Figure 11(a), possible paleostress sets 6, 14, and 17) [22, 29, 35, 40], and aAFT data (Figure 11(c)) [22, 36, 37, 39]. In the first episode, long-lasting tectonic quiescence continued with slow exhumation throughout the Late Cretaceous–Paleogene [22, 37] (Figure 11(c)), and the coeval basin was formed. Moreover, the plutons in the hanging wall of the North Huicheng Basin fault remained in the partial annealing zone (PAZ) for a long time (Figures 6(d) and 11(c)), which might be attributable to near-horizontal thrusting along a fault-ramp structure (Figure 12(c)). In the second episode, the late Miocene−early Pliocene (9, 4 Ma) rapid cooling [22, 37, 39] signifies exhumation from the upper PAZ (90°C and 60°C) to ambient temperatures (Figures 6(d), 11(c) and 12(c)). During these two episodes, the outward growth of the Tibetan Plateau facilitated left-lateral strike-slip and thrusting along the North Huicheng Basin fault system, which predominantly shortened the crust in this region [36].

1. A combination of fieldwork and physical analog modeling outlines the deformation mechanism of the North Huicheng Basin strike-slip fault system and associated structures. The ENE–WSW-trending North Huicheng Basin fault system was induced by the plate-like movement of the West and East Qinling Orogens, which underwent multiple strike-slip faulting and controlled the shape of salient and recessed structures. The scaled physical analog experiment results confirm this hypothesis and reveal the complex spatial–temporal deformational kinematic process of the strike-slip fault system and alongside salient and recessed structures.

2. Combined with published works, multiproxy geochronological dating (zircon U‒Pb age of 213 Ma, biotite ⁴⁰Ar/³⁹Ar age of 203 Ma, and AFT age of 56 Ma) outlines the main thermal history of the hanging wall of this strike-slip system. Based on the above facts, the integrated research suggests that this multistage strike-slip faulting played a significant role in the main tectonic events, that is, the late Triassic magmatic emplacement, the Jurassic/Cretaceous local pull-apart basin, and Cenozoic rapid exhumation driven by Tibetan Plateau growth.

We thank the Prof. Jiyuan Yin and two anonymous reviewers for their critical, careful, and constructive reviews that have helped improve the clarity and modeling of the manuscript. Thanks for the kind help from Qian Wang for sample dating, Dr. He Su, Wei Yu, and Ph.D. candidate Daxing Xu for sandbox experiment modeling. This study is financially supported by the Fundamental Research Funds for the Chinese Academy of Geological Sciences (JKYQN202301), China Geological Survey (DD20230008, DD20221643-5), the National Key Research and Development Program of China (2018YFC0603701), and the Cooperative Project between the Chinese Academy of Geological Sciences and the SinoPec Shengli Oilfield Company (No. P22065).

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

The authors declare that all the readers can access the data supporting the conclusions of the study on the Lithosphere website or editor office.