Late Triassic tectonic stress field of the southwestern Ordos Basin and its tectonic implications: Insights from finite-element numerical simulations

Crustal deformation is the result of tectonic stress fields during geologic history; therefore, clarifying the characteristics of the paleotectonic stress fields can reveal the genetic mechanisms of relevant geologic records, such as the formation of basins and the initiation and growth of fractures within the crust (McKinnon and Barra, 1998; Tuckwell et al., 2003; Ding et al., 2012; Jiu et al., 2013). Generally, a paleotectonic stress field can be reconstructed based on the results of field mapping, geophysical exploration, and geochemical analysis (Darby and Ritts, 2002, 2007; Bobrov et al., 2022; Li et al., 2022b). However, structural deformations observed in the field are usually the result of tectonic superimposition and may represent the strongest or latest paleotectonic stress field, not linked to a specific geologic time period. In particular, in prototype basin analysis, which is generally concerned with the primary conditions and distribution of the sedimentary facies and syndepositional deformation (Liu and Yang, 2000), structural superimposition usually leads to varied inferences about the prototype basin nature and paleotectonic stress field. Moreover, geophysical and geochemical analyses can be employed only when high-quality data are available. Notably, finite-element numerical simulation is another widely applied technical method for studying the paleotectonic stress field in the research of tectonics, earthquakes, and petroleum, using different software packages, including ANSYS, FEM, ALGOR, ABAQUS, PATRAN, and NASTRAN (Tapponnier and Molnar, 1976; England and Houseman, 1986; Jiu et al., 2013; Zhao et al., 2016; Guo et al., 2016, 2019; Fang et al., 2017; Yin et al., 2018; Hu et al., 2019; Ju et al., 2019; Liu and Ma, 2019; Li et al., 2022a; Wu et al., 2022). This method can reconstruct the paleotectonic stress field without limits in time and space and predict the orientation, strength, and distribution of the stress and strain. The boundary conditions are mainly set based on plate-scale tectonic dynamics (Wu et al., 1997; Zhang, 2011). Furthermore, an important criterion for testing the credibility of a numerical model is whether the simulation results can be validated by contemporaneous structural deformations, which are usually considered as “tangible fossils” of the paleotectonic stress field (e.g., Wu, 1984; Zhang et al., 1995, 2006). Therefore, if the study area with multistage tectonic superimposition is largely covered by thick Cenozoic sediments, and/or the high-quality geophysical and geo-chemical data cannot be obtained, then finite-element numerical simulation is a better choice to reconstruct the paleotectonic stress field and further reveal the mechanism of crustal deformation.

The Ordos Basin is a Mesozoic inland fluvial and lacustrine superimposed basin, largely covered by Cenozoic sediments (Liu et al., 2008; Ma et al., 2019). It is located in the interior of the Ordos block, which makes up the western part of the North China craton (Zhao et al., 2005), and it connects the Alxa block through the western active belt of the Ordos block to the west, the Central Asian orogenic belt through the northern active belt of the North China craton to the north, the eastern part of the North China craton through the Taihangshan tectonic belt to the east, and the Qilian and Qinling orogens to the south (Fig. 1A). The Late Triassic tectono-sedimentary evolution processes in the southwestern Ordos Basin included early to middle Late Triassic (ca. 231–218 Ma) deposition and late to terminal Late Triassic (ca. 218–200 Ma) deposition, uplift, and erosion (Fig. 2; Liu et al., 1996; Song et al., 2009, 2010). However, the nature of the prototype basin of the southwestern Ordos Basin during the Late Triassic remains controversial. Generally, the southwestern Ordos Basin is believed to have been a foreland basin developed under a NE-SW compressive stress field throughout the Late Triassic (Tang et al., 1992; Liu et al., 1996; Xu et al., 2006; Zhang et al., 2006; Li et al., 2012). Alternatively, it may have also been an extensional graben basin associated with regional strike-slip faulting during the early Late Triassic that transformed into a compressional basin during the late Late Triassic (Song et al., 2009, 2010; Yin et al., 2019; Li et al., 2020). Furthermore, the associated tectonic dynamics were probably dominated by either the northward drift of the Qiangtang terrane (Liu et al., 1996; Xu et al., 2006; Zhang et al., 2006; Song et al., 2009, 2010) or the scissor-like collision from east to west between the North China craton and Yangtze block (Song et al., 2009, 2010).

In this study, we constructed two paleotectonic stress field simulation models for the southwestern Ordos Basin during the early to middle and late to terminal periods of the Late Triassic by applying the finite-element numerical method based on regional tectonic dynamics. The simulation results were validated using contemporaneous structural deformations and regional tectonothermal records. Accordingly, we suggest a tectonic switch in the nature of the prototype basin of the southwestern Ordos Basin during the Late Triassic period.

Tectonically, the geodynamics of the North China craton during the Triassic may have involved the closure of the Paleo-Asian Ocean to the north, closure of the Paleo-Tethys Ocean to the south, and subduction of the Paleo-Pacific Ocean to the east (Stampfli and Borel, 2002; Zhu et al., 2012; Xiao et al., 2018; Zhao et al., 2018a, 2018b; Wang et al., 2022a; Wei et al., 2022; Wu et al., 2019b). In particular, the Paleo-Asian Ocean had already closed before the Middle Triassic to form the Central Asian orogenic belt (Fig. 1B; Xiao et al., 2018; Wang et al., 2022b, 2022c), but the northern active belt of the North China craton was still active and characterized by intracontinental deformation during the Triassic–Cretaceous (Wang et al., 2017, 2021; Zhang, et al., 2020), indicating massive energy consumption by the postorogenic dynamics of the Central Asian orogenic belt. Furthermore, the Ordos block was already welded together with the Alxa block and the eastern part of the North China craton before the Carboniferous (Fig. 1A; Zhao et al., 2005, 2012; Yuan and Yang, 2015; Zhang et al., 2016; Song et al., 2017; Wang et al., 2019). Although the Paleo-Pacific Ocean was most likely subducting beneath the eastern margin of the North China craton during the Late Triassic (Guan et al., 2022; Wei et al., 2022; Zhu et al., 2022), the intracontinental deformation in the Taihangshan tectonic belt (Fig. 1A) developed as early as the Middle–Late Jurassic (Wang et al., 2016; Zhang, et al., 2020, 2021), suggesting that the far-field effects of this subduction were not the dominant tectonic dynamics of the Ordos Block during the Late Triassic. Notably, the southwestern Ordos Basin is located at the junction between the Ordos block and the Qinling-Qilian-Kunlun orogenic belt (Fig. 1A). The subduction and subsequent collisional events of the Paleo-Tethys Ocean during the Triassic could have acted directly on the southwestern Ordos Basin (Stampfli and Borel, 2002; Zhao et al., 2018a, 2018b), and thus may have been the dominant tectonic dynamics for this region during the Late Triassic. Moreover, the Eastern Qinling Ocean, a branch of the Paleo-Tethys Ocean between the North China craton and the Yangtze block, may have closed from east to west, and the Ordos block may have rotated counterclockwise during the early to middle Late Triassic (Fig. 1B; Wu and Zhu, 1990; Ma and Yang, 1993). The Songpan-Ganzi and Qiangtang terranes drifted to the north during the Late Triassic and may have accreted to the North China craton by the end of Late Triassic (Fig. 1B; Yin and Harrison, 2000; Wu et al., 2019a).

The most extensively developed Triassic sedimentary sequence in the Ordos Basin is the Yanchang Formation, which is also where major oil and gas reservoirs were discovered in previous petroleum exploration. This formation mainly comprises inland fluvial, delta, and lacustrine facies clastic rocks and can be further subdivided into 10 lithologic units (Fig. 2; Deng et al., 2018; Fu et al., 2020). It contacts the underlying Middle Triassic Zhifang Formation and overlying Jurassic Yan’an Formation by parallel and angular unconformities, respectively (Song et al., 2010). The Yanchang Formation has long been considered to be part of the classic Upper Triassic section (e.g., Li et al., 2009; Song et al., 2010; Yin et al., 2019). However, recent zircon U-Pb dating of the tuff layer samples interbedded within the unit 7 layer yielded ages of ca. 241–222 Ma, indicating that the boundary between the Middle and Upper Triassic most likely developed within the Yanchang Formation (Yang and Deng, 2013; Deng et al., 2018, and references therein). Thus, the Yanchang Formation in the Ordos Basin can be divided into Middle (units 10–7; T₂y^10–7) and Late (units 6–1; T₃y^6–1; Fig. 2) Triassic sections. The contemporaneous sedimentary sequence in the southwestern Ordos Basin is named the Kongtongshan Formation (T_2–3k; Fig. 2), and it is mainly composed of purple to brown conglomerates with sandstone and mudstone interlayers (Song et al., 2009, 2010; Yang et al., 2014; Shi et al., 2015). Additionally, the Yaoshan Formation (T₃–J₂ys), which was deposited during the terminal Late Triassic to Middle Jurassic, is composed of lacustrine to marsh facies deposits with coal interlayers (Fig. 2), the outcrops of which are mainly observed along the western margin of the Ordos Basin (Feng et al., 2009; Song et al., 2013).

We constructed two paleotectonic stress field models using the finite-element software ANSYS 10. For simplicity, the two models were configured as two-dimensional models, considering only the plane stress and strain, and the vertical stress was ignored. To reduce the boundary effects, the modeling ranges were set at 90°E–126°E and 30°N–44°N, including the Central Asian orogenic belt, North China craton, Tarim craton, Qinling-Qilian-Kunlun orogenic belt, Yangtze block, and Qiangtang–Songpan-Ganzi terranes (Fig. 3). Furthermore, because the northern active belt of the North China craton could largely have accommodated the postorogenic dynamics of the Central Asian orogenic belt and the farfield effects of Paleo-Pacific Ocean subduction did not reach the Taihangshan tectonic belt during the Late Triassic (Wang et al., 2016, 2017, 2021; Zhang, et al., 2020, 2021), the scissor collision between the North China craton and Yangtze block and the northward drift of the Qiangtang–Songpan-Ganzi terranes were considered to be the dominant tectonic dynamic mechanisms in the two models. Additionally, only the boundary faults were adopted in this model, and the involved tectonic units were grouped into three types: paleo-orogenic belt, active belt, and craton or block. As the northern and southern boundaries of the Ordos block were active during the Late Triassic and the western boundary may have been reactivated during compression, they were set as active belts and defined by six-node triangular elements (Fig. 3). The eastern boundary of the Ordos block, the Taihangshan tectonic belt (Fig. 1A), did not reactivate until the Jurassic. Therefore, it was not considered as an active boundary in the simulations. The other two types of tectonic units were divided into eight-node quadrangle elements (Fig. 3) but with different material property parameters (Table 1).

During the early to middle Late Triassic, the tectonic stress field of the southwestern Ordos Basin was most likely dominated by the scissor collision between the North China craton and the Yangtze block. Accordingly, an external force was applied at the southeastern boundary that gradually decreased from east to west in the first paleotectonic stress field model (Fig. 3A). The western boundary was set as a fixed boundary, whereas the other three boundaries were free. From the late to terminal Late Triassic, the maximum compressive force operating on the Ordos block was probably caused by the northward compression of the Songpan-Ganzi and Qiangtang terranes. Correspondingly, an external force was applied to the southwestern boundary that gradually decreased from west to east in the second paleotectonic stress field model, wherein the eastern boundary was constrained, and the other three boundaries were free (Fig. 3B).

The simulations of the paleotectonic stress fields of the southwestern Ordos Basin during the Late Triassic complied with the elastic theory. Notably, the positive and negative rules of stress in elastic theory are contrary to those in rigid structural theory. Namely, the direction of the external force is opposite to that of the stress. Therefore, in the simulation models, the compressive forces were defined by the trajectories of the minimum principal stress (σ₃, green vectors in Fig. 4), and the maximum principal stress trajectories (σ₁, blue vectors in Fig. 4) represented the extensional forces. Accordingly, the contour maps of the maximum principal stress (σ₁) reflect the variations in the tensile stress, wherein the positive and negative stress values indicate extensional and compressional deformation, respectively (Figs. 5A and 5B). Moreover, the contour maps of the strain reflect variations in the deformation strength (Figs. 5C–5F). Additionally, the distribution patterns of stress and strain are only qualitative simulations of regional deformation, and their values are not necessarily consistent with the actual paleotectonic stress field.

The simulation results of the paleotectonic stress field of the southwestern Ordos Basin during the early to middle Late Triassic suggest that the axial orientations of σ₁ and σ₃ were NE-SW and NW-SE, respectively, and the absolute values of σ₁ were generally higher than those of σ₃ (Fig. 4A). As the preexisting faults at the western margin of the Ordos block strike nearly N-S, the angles between the fault strikes and the axial orientations of σ₁ are in the range of ~45°–60°, whereas those between the fault strikes and the axial orientations of σ₃ are relatively small (~30°–45°). Such stress conditions would lead to the activation of sinistral strike-slip faulting along the western margin of the Ordos block. Furthermore, the stress values of the southwestern Ordos Basin on the contour map of the maximum principal stress (σ₁) are almost positive and higher than those of surrounding regions (Fig. 5A), indicating the development of extensional deformation during the early to middle Late Triassic. Correspondingly, the normal strain values of the southwestern marginal areas of the Ordos Basin are also positive and higher than those of the adjacent areas (Fig. 5C), and the shear strains are concentrated in the western active belts of the southwestern Ordos Basin (Fig. 5E). In addition, the tensile stress and normal strain values of the northwestern Ordos Basin are both nearly zero or negative (Figs. 5A and 5C), but the shear strain values of the northern part of the western active belt of the Ordos block are much higher than those of the Alxa block and the interior of the Ordos Basin (Fig. 5E), indicating that strike-slip faulting was predominant during the early to middle Late Triassic. Thus, the strike-slip faulting and extensional deformation became weaker and stronger, respectively, from north to south along the western margin of the Ordos Basin. These results suggest that the southwestern Ordos Basin was most likely an extensional basin associated with strike-slip faulting along the western margin of the Ordos block during the early to middle Late Triassic.

The simulation results of the paleotectonic stress field of the southwestern Ordos Basin during the late to terminal Late Triassic display a NW-SE axial orientation of σ₁, orthogonal to the NE-SW axial orientation of σ₃ (Fig. 4B). The absolute values of σ₁ are generally lower than those of σ₃, and the angles between the preexisting fault strikes (nearly N-S) at the western margin of the Ordos block and σ₃ vectors are beyond 45° (Fig. 4B). The southwestern Ordos Basin and adjacent areas are all characterized by negative stress values on the contour map of the maximum principal stress (σ₁), suggesting a compressive stress field for the entire region (Fig. 5B). Moreover, both the normal and shear strain values along the southwestern marginal areas of the Ordos Basin are negative and concentrated in the active belts (Figs. 5D and 5F). All these results indicate a compressive stress field, and the southwestern Ordos Basin was most likely a compressional depression basin during the late to terminal Late Triassic.

According to the simulation results, the southwestern Ordos Basin was an extensional basin associated with strike-slip faulting during the early to middle Late Triassic and a compressional depression basin during the late to terminal Late Triassic. However, these results need to be validated by regional geologic correlations to test their credibility.

During the early to middle Late Triassic, the tectonic stress field of the Ordos Basin was chiefly controlled by the scissor collision from east to west between the North China craton and the Yangtze block, and the Ordos block was probably rotating counterclockwise and moving northward relative to the Alxa block (Fig. 1B; Wu and Zhu, 1990; Ma and Yang, 1993). It is noteworthy that both the concentration of shear strain in the western active belt and the low angles (<45°) between the preexisting fault strikes and the axial orientations of compressive stress (σ₃) suggest the possibility of sinistral strike-slip faulting along the western margin of the Ordos block (Figs. 4A and 5E). This speculation can be verified by the NW-SE sinistral strike-slip fault with en echelon distributed secondary faults identified in the three-dimensional (3-D) seismic profile of the middle Yanchang Formation in the southwestern Ordos Basin (He et al., 2021). Furthermore, the positive values of extensional stress (σ₁) and strain in the southwestern Ordos Basin during the early to middle Late Triassic (Figs. 5A and 5C) indicate an extensional tectonic setting with extensional deformation and rapid near-source sedimentation. The Kongtongshan Formation in the southwestern Ordos Basin mainly consists of conglomerate layers, and their sedimentary provenance not only includes orogenic materials from the Qilian and Qinling orogenic belts but also basement and early Paleozoic deposits from the interior of the Ordos block (Yang et al., 2014; Shi et al., 2015). Notably, many normal faults within Late Triassic strata have been identified from different seismic profiles in the southwestern Ordos Basin, and they are sometimes conjugate and usually control the thicknesses of sedimentary layers. However, they do not truncate the overlying Jurassic strata (e.g., fig. 6–8 in Qiu, 2011; fig. 3–14 in Sun, 2018). This implies the nature of syndepositional normal faulting and is consistent with an extensional sedimentary environment. The conjugate normal faults observed in the Kongtongshan Formation in the southwestern Ordos Basin may reflect the field outcrops of syndepositional normal faults from the seismic profiles, and the orientations of the fault surfaces are also in accordance with NE-SW extension (Fig. 6A). In addition, Late Triassic normal faults have also been reported in the Maling oilfield to the north of Qingyang County and the Liupanshan Basin (Fig. 1C; He et al., 2004), and they were also identified from the 3-D seismic profile of the upper Yanchang Formation in this area (He et al., 2021). Moreover, the extensional stress and strain decreased from the southwestern margin to the northeastern interior of the Ordos Basin (Figs. 4A, 5A, and 5C). This is consistent with the distribution features of the early to middle Late Triassic sedimentary sequences, which are deeper and thicker in the southwest but shallower and thinner in the northeast (Fig. 1C). Additionally, the regional extensional tectonic setting is also in accordance with the extension-related magmatism formed during the early to middle depositional period of the Yanchang Formation in this region, including the alkaline granitic and subvolcanic rocks and dolerites within the southwestern Ordos Basin (ca. 241–224 Ma; Li, 2011; Weng et al., 2012; Luo et al., 2022), continental rift basalts in the Ruqigou area, western margin of the Ordos block (ca. 241 Ma; Wang et al., 2005; Yang et al., 2010), and ultramafic and alkaline magmatic associations in the adjacent Qinling orogenic belt (ca. 235–209 Ma; Ding et al., 2015; Gong et al., 2016). Therefore, the southwestern Ordos Basin was probably an extensional basin associated with western strike-slip faulting during the early to middle Late Triassic.

During the late to terminal Late Triassic, the dominant tectonic dynamics for the Ordos Basin changed to northward compression of the Songpan-Ganzi and Qiangtang terranes (Fig. 1B). The tectonic stress field simulated for this period was characterized by NE-SW compressive stress (Fig. 4B) and the concentration of compressive strain in the surrounding active belts (Figs. 5D and 5F). This may have led to tectonic uplift and related compressional deformation in the western and southern marginal areas of the Ordos block. Notably, the deep-lake facies vanished, and the depocenter of the shallow-lake facies sediments migrated northeastward over time in the southwestern Ordos Basin during the late to terminal Late Triassic, wherein the upper part of the Yanchang Formation was completely denuded in the southwestern margin (Fig. 1D; Li et al., 2009). This is consistent with tectonic uplift and denudation processes occurring from southwest to northeast. Furthermore, top- to- the- NE thrust faults can be identified from seismic profiles in the southwestern Ordos Basin (Sun, 2018). These faults cut the Late Triassic Yanchang Formation and the underlying strata but do not truncate the overlying Jurassic sequences (e.g., fig. 3–23 in Sun, 2018), which probably indicates an active period during the late stage of the Late Triassic but before the Early Jurassic. The top- to- the- NE thrust fault observed in the southwestern margin of the Ordos Basin has Mesoproterozoic dolomite (Pt₂) and Yanchang Formation sandstone as the hanging wall and footwall (Fig. 6B), respectively, which constrained the development of the fault to the later part of or after the Late Triassic. In addition, previous field mapping suggested that west- to- east thrusting was the dominant structure in the western Ordos block during the Early Jurassic, with a strike-slip component (Yang, 2018). Furthermore, the depositional thickness of the Yaoshan Formation thins to the east (Feng et al., 2009), in accordance with a compressional depression basin that would have developed during tectonic uplift along the western margin of the Ordos block. Thus, the southwestern Ordos Basin was most likely a compressional depression basin during the late to terminal Late Triassic.

These modeling results and observations provide a new interpretation of the tectonic setting and evolutionary processes of the Ordos Basin in the Late Triassic. A switch may have occurred in the tectonic stress field of the Ordos Basin during the Late Triassic. The dominant external forces probably changed over time from the scissor collision between the North China craton and the Yangtze block to northward compression of the Songpan-Ganzi–Qiangtang terranes. Correspondingly, we speculate that the southwestern Ordos Basin was an extensional basin associated with strike-slip faulting during the early to middle Late Triassic but transformed into a compressional depression basin during the late to terminal Late Triassic.

The southwestern Ordos Basin was dominated by NE-SW extensional stress and strain and characterized by the development of syndepositional normal faults in the early to middle Late Triassic. We hypothesized that it was an extensional basin associated with strike-slip faulting along the western margin of the Ordos block. In the late to terminal Late Triassic, the southwestern Ordos Basin transformed into a compressional depression basin by NE-SW compressive stress and strain, accompanied by regional tectonic uplift and compressional deformation in the western and southern marginal belts of the Ordos block.

We are very grateful to the three anonymous reviewers for their constructive comments. Special thanks are due to Christopher Spencer for handling this manuscript. This work was financially supported by the National Science Key Foundation of China (no. 90814005 and 41102072), the State Key Laboratory of Continental Dynamics, Northwest University in China (no. BJ081334), the Key Laboratory of Tectonic Geology and Oil and Gas Resources of Ministry of Education (no. TPR-2012–20), the China Geological Survey (no. 12120113039900 and DD20190011), and the Science Foundation of Education Department of Shanxi Province, China (no. 12JK0479).