Our study region is situated in the middle of the northwestern margin of the Qaidam Basin, and in the junction area between the NW-NNW-striking structures in the basin interior and the E-W-striking structures adjacent to the ATF (Figure 1). Therefore, this is an ideal place to evaluate the development and interplay of these nearly orthogonal fault systems.

Three major anticlines have developed in this region (Figure 2A). The first is the NNW-striking South Dongping anticline that is characterized by a series of NW-striking, right-stepping, en echelon minor folds at surface, indicating a certain degree of NNW-striking dextral shear (Figure 2B). Quaternary QGQ Fm is outcropped in the core of the anticline. The second is the E-W-striking Dongping anticline to the north. Late Eocene–Oligocene SG Fm is well exposed in the core of this anticline, and deformed by a left-stepping, en echelon array of ESE-striking secondary anticlines, as well as some minor NE-striking left-lateral and NNW-striking right-lateral faults (Wu et al., 2019) (Figure 2C). These secondary structures together suggest a component of E-W-striking sinistral shear along the Dongping anticline (Figure 2C). The third is the NW-striking Hongsanhan anticline in the northwest of the study region. This anticline is a superimposed buckle fold (Du et al., 2019b), and highly asymmetric with a wide southwestern limb and a very narrow northeastern limb (Figure 2A). LLH to XY Fms are well exposed in the anticline. Faults controlling the formation of these anticlines are nearly invisible in the field, because of covering of late Quaternary alluvial deposits and saline mudstone deposited in playas.

A large E-W-striking fault, the Niubei fault, is existent in the north of our study region, and linked with the ATF to the west (Figures 1, 2A). It was a syn-depositional sinistral transpressional fault in ∼53.5–17 Ma, followed by intense reverse faulting during 17–15 Ma and tectonic quiescence since ∼15 Ma (Wu et al., 2019). This pattern was regarded as a response to the tectonic reorganization of the Altyn Tagh fault system in the middle Miocene (Wu et al., 2019).

Fault F5e and F6e are N-dipping basement-involved thrust faults lying in the northernmost of our seismic survey, and connect with the larger E-W-striking Niubei fault to the north through NW-striking F5w and F6w (Figure 3C). They cut and folded the pre-Cenozoic basement and LLH–XY Fms, resulting in the southward tilting of the basement and the development of a local angular unconformity between the SY Fm and underlying strata (Figure 5A). This unconformity gradually changes to conformable contact toward the basin interior. Strata above the unconformity are only slightly deformed. Growth strata consisting of LXG to SZG Fms are observed in the hanging wall of F6e, with the best occurring in the XY Fm.

The N-dipping F8 is a steep basement-involved fault to the southwest of F6e. It cuts the basement upward into the UXG Fm, resulting in the occurrence of growth strata, though unapparent, in the UXG Fm that is overlain conformably by the non-growth SG Fm (Figure 4C). Together with the northern end of F1n, F8 controls the shape of the Dongping anticline, with a back-thrust developing in the hanging wall and soling in the mudstone in the LLH Fm. According to the consistency between deformed UXG-XY Fms, which is unconformably overlain by horizontal QGQ Fm atop F8, and the overlying syn-tectonical SY-SZG Fms to the south (Figure 4C), we interpret that F8 should be also active during the deposition periods of SY and SZG Fms. F8 extends against the F1 toward east (Figure 3C). Fault F3 has similar geometry to F8, and is located on the other side of F1. It slightly deformed the LLH–UXG Fms, with the appearance of growth strata in the UXG Fm (Figure 5A).

Fault F9 is another basement-involved fault in the footwall of F1 and located to the south of F3 (Figure 5A). It dips to the south, and cuts the basement upward into the UXG Fm, with the development of growth strata in the UXG Fm overlain conformably by non-growth SG Fm.

The S-dipping (60–70°) fault F10 is located to the southwest of F8 and linked with F2s and F2n to the east (Figure 3C). It cuts through the basement rocks up to the SZG Fm, resulting in thinning of LXG and SY-SZG Fms in the hanging wall (Figure 5C). To the east, the S-dipping basement-involved F2n cuts and folds the LLH–SZG Fm in the hanging wall, forming a secondary anticline of the larger South Dongping anticline (Figure 3C). Obvious F2n-related growth strata are observed in the SY and SZG Fms that are unconformably overlain by the weakly deformed QGQ Fm (Figure 5C). Whilst minimal thickness change has been detected in the XY Fm, we still attribute it as growth strata comparing with the underlying SG Fm that thickens toward F2n. F2n lies against the N-S-striking F1 to the east. Fault F4 with similar characteristic of F2n develops on the other side of F1. It cuts and folds the LLH–SZG Fms that are unconformably overlain by the nearly horizontal QGQ Fm, with well-developed growth strata in the SY and SZG Fms but inapparent growth strata in the XY Fm (Figure 5A).

Fault F1s (also termed Pingdong Fault) strikes approximately NNW in map view (Figure 3C). It dips ∼50–70° toward west, cuts the basement rocks upwards into the SY Fm, forming the South Dongping anticline in the hanging wall (Figure 4A). This fault is one of the major faults in western Qaidam Basin, because it has accumulated more than one thousand meters of basement uplift, and controls the distribution of the Mesozoic strata. Drilling data reveal that Mesozoic (mainly Middle Jurassic) strata are only preserved in the footwall of F1s, but missing in the hanging wall, indicating that it was likely a preexisting reverse fault prior to the Cenozoic. Growth strata related to F1s are clearly observed in the SY and SZG Fm that are unconformably overlain by less deformed Qigequan (QGQ) Fm (Figure 4A). Some NE-dipping back-thrusts develop in the hanging wall of F1s, and sole into the LXG and LLH Fms. They are en echelon right-stepping distributed in the map view (Figure 3C), indicating a component of dextral shear of F1s. Fault F1n is the northward continuation of F1s to the north of F8, and disappears further north. It does not have as much displacement as F1s, and together with fault F8, controls the shape of Dongping anticline. Growth strata are not observed in the LLH–XY Fms in the hanging wall, and the overlying SY and SZG Fms, which are overlain by horizontal QGQ Fm, are missing on top of F1n (Figure 4C).

The approximately N-S-striking fault F2s is located to the south of F2n and F10 (Figure 3C). It dips steeply to the east, and extends from the basement upward into the SY Fm (Figure 4A). F2s forms the western boundary fault of the box-shaped South Dongping anticline, which is characterized by a nearly wide and horizontal core but narrow and steep limbs. Some thin-skinned, SW-dipping back-thrusts, which are in en echelon right-stepping arrangement, develop in its hanging wall, indicating a certain component of dextral shear on F2s. Growth strata related to F2s occur in the SY -QGQ Fms.

The NW-striking F5w and F6w dip toward the NE, and cut upward from the basement to the SG Fm that is well exposed at surface or unconformable overlain by the QGQ Fm (Figures 4C, 5B). Pre-Cenozoic basement in the hanging walls tilts toward north, different from that of the conterminous F5e and F6e to the east (Figures 3C, 5B). Growth strata are not well developed in the remaining LLH–SG Fms in the hanging walls of F5w and F6w (Figure 4C).

The NW-striking, NE-dipping F7 is a steep basement-involved fault to the southwest of F6w, elevating the basement in the hanging wall to be hundreds of meters higher than that in the footwall (Figures 4B, 5B,C). A SW-dipping back-thrust develops on top of F7, constituting a flower structure indicative of strike-slip shear. LLH–SY Fms have been intensely deformed in the hanging wall of this back-thrust, forming the Hongsanhan anticline at surface. Growth strata are not observed in the LLH–XY Fms in the southern limb of the Hongsanhan anticline (Figures 4B, 5B). F7 likely connects the basement-involved E-W-striking fault F8 to the east.

Whilst all presented faults in our study region are reverse in section view, strike-slip shear is expected to have occur on these faults based on following observations. First, they are in the vicinity of the left-reverse Altyn Tagh fault system. Second, they are all high-angle basement-involved faults and form many flower or half-flower structures. Third, present-day GPS velocity in the western Qaidam Basin is generally NE-directed (Wang et al., 2017; Wang and Shen, 2020; Figure 1), oblique to their strikes. And fourth, strata in the hanging walls of F1s and F2s, which are cut by some secondary faults, are not balanced across faults in 2-D profiles (Figure 4A). The E-W-striking Dongping anticline, which is controlled by fault F8, consists of a set of en echelon distributed, ESE-striking secondary anticlines, as well as some minor NE-striking left-lateral and NNW-striking right-lateral faults (Figure 2C), suggesting a component of sinistral shear on F8. This is consistent with the kinematics of the larger E-W-striking Niubei fault to the north, which has been testified as a left-reverse fault (Wu et al., 2019). The NNW-striking South Dongping anticline bounded by F1s and F2s to the west and east, respectively, however, features a series of NW-striking, right-stepping, en echelon minor folds at surface (Figure 2B) and secondary faults at depth (Figure 3C), indicating dextral shear on F1s and F2s.

Based on the above descriptions of fault-related growth strata, we interpreted activity time of these faults and thus delineated the spatial and temporal patterns of fault development in our study region. The results are listed in Table 2 and Figure 6, and reveal different deformation patterns of the two orthogonal fault systems. The E-W-striking faults were generally initiated during the deposition periods of the LXG–XY Fms (43.8–15.3 Ma) in an out-of-sequence order, but most of them ceased their activity prior to the deposition of SY Fm (∼15.3 Ma) with limited weak reactivations. Activity of the NW- to NNW-striking faults, however, generally commenced in the deposition period of SY Fm (∼15.3–8.1 Ma), much later than that of the E-W-striking faults, and continue to the present. We further calculated the intensity of fault activity over time by measuring the fault throw of each period. The result (Figure 7) reveals that the E-W-striking faults were intensely active during the deposition periods of UXG (41.5–35.5 Ma) and XY (23–15.3 Ma) Fms, but became weakly active, or even inactive in the deposition period of SY–QGQ Fm (<15.3 Ma), when intense activity on the NNW-striking faults happened.

The above analyses and interpretations reveal two orthogonal basement-involved reverse fault systems that contrast in geometries, kinematics, and temporal development in the NW margin of the Qaidam Basin. The E-W-striking faults are generally N-dipping, oblique to the ATF, and left-reverse. They result in southward tilting of the basement and form a local unconformity between the SY Fm and underlying strata (Figure 5A). They were intensively active prior to the deposition of SY Fm (∼15.3 Ma), but in a state of tectonic quiescence with limited weak reactivtions henceforth (Figure 7). This deformation pattern, which has been reported by Wu et al. (2019), is only observed in the NW margin of the Qaidam Basin and has never been found elsewhere. It is thus reasonable to attribute the development of these E-W-striking faults to the tectonic movement of the Altyn Tagh fault system that bounds the Qaidam Basin to the northwest. The NNW- to NW-striking faults are approximately perpendicular to the Altyn Tagh fault system, and form under right-laterally transpressional setting during the deposition periods of SY Fm (Figure 7), much later than the initiation of the E-W-striking faults. These deformation characteristics are similar to the NW- to NNW-striking faults that dominate the first-order tectonic framework in the interior of the modern Qaidam Basin (Mao et al., 2016).

Taken the deformation features of these two fault systems together, our findings reveal a change of structural deformation from E-W-striking sinistrally transpressional faulting to NNW- to NW-striking right-laterally transpressional faulting in the middle Miocene in the NW margin of the Qaidam Basin. This change was likely attributed to the middle Miocene reorganization of the Altyn Tagh fault system, as previously proposed by Wu et al. (2019). Specifically, the present-day active central segment of the ATF may not form during the deposition periods of LLH to XY Fms. The North Altyn Fault and the Jinyanshan Fault to the north defined the northwestern boundary of the Qaidam Basin, constituting a large restraining bend on the eastern termination of the Altyn Tagh fault system at the time (Wu et al., 2019; Figure 8A). The E-W-striking faults formed in this left-reverse transpressional restraining bend. The central segment of the ATF started to form in the late deposition period of the XY Fm (∼17 Ma), cutting through the restraining bend (Wu et al., 2019). The strike-slip efficiency of the ATF was very low in its initial stage due to lack of smooth fault traces. In this scenario, deformation along the ATF was dominated by vertical uplift rather than horizontal displacement, as attested by physical analog (Hatem et al., 2017), resulting in intense reverse faulting on the E-W-striking faults that formed the local angular unconformity below the SY Fm. The present ATF was finally established at ca. 15.3 Ma when it completely cut through the bend and the E-W-striking faults in the NW Qaidam Basin (Wu et al., 2019). Left-slip strain was then largely localized on the ATF, resulting in an abrupt decrease in the intensity of tectonic activity of the E-W-striking faults. Fast slip on the ATF was absorbed by NE-SW-directed crustal shortening over the entire northern Tibetan Plateau (Zhang et al., 2007; Zheng et al., 2013), which triggered the widely distributed NNW- to NW-striking reverse faulting in the Qaidam Basin. As the NNW- to NW-striking faults are oblique to the regional maximum principle stress (σ₁) and lie against the sinistral ATF with a large angle, a component of dextral shear is manifested.

As the two orthogonal fault systems spatially overlap, we observe two aspects of interplay between them.

First, the relatively older E-W-striking faults were offset by younger NNW-striking faults. Specifically, F8 and F3 are two E-W-striking, N-dipping basement-involved faults in the western and eastern sides of the NNW-striking F1, respectively. They both extend from the basement upward into the UXG Fm, resulting in slight thinning of the UXG Fm in its hanging wall. We interpret that these two faults were likely active and linked to each during the deposition period of UXG Fm, but became tectonically inactive in the subsequent deposition periods of SG and XY Fms. Right-laterally transpressional fault F1 was initiated since the deposition period of SY Fm (∼15.3 Ma), offsetting F8 and F3, and resulting in reactivation of F8 in its hanging wall. Similarly, F2n and F4 are two E-W-striking, S-dipping basement-involved faults in the western and eastern sides of the F1, respectively. Analysis of associated growth strata indicates that they both became active since the deposition of XY Fm (∼23 Ma). Similar geometries, kinematics, activity time and spatial coincidence imply that they were possibly linked to each other prior to the deposition of SY Fm (∼15.3 Ma), but were offset right-laterally by F1 since then. If our above correlations are correct, ∼2 km right-lateral offset is predicted on F1 since ∼15.3 Ma.

Second, the younger NNW-striking faults curved to link with the preexisting E-W-striking faults. This is well documented by the N-S-striking F2s, which curves toward the east to link with the E-W-striking F2n to the northeast. Similar situation also occurred on NW-striking F5w and F6w, which curve on their southeastern ends to connect with F5e and F6e, respectively. This common pattern indicates that preexisting faults may result in lateral heterogeneity of rocks involved in the deformation, and thus change the local stress field to generate complex fault geometries.

Our findings bear meaningful implications for the development of FTB in a transpressional setting. Typical contractional-dominated FTBs are usually characterized by long-distance, thin-skinned over-thrusting and expands from hinterland to inland with increasing width over time, following the critical wedge taper theory (Dahlen et al., 1984; Suppe, 2007), such as those in the nearby SW and N margins of the Tarim Basin (Li et al., 2012; Cheng et al., 2017). However, in a transpressional-dominated FTB, faults are usually high-angle to facilitate vertical uplift and lateral offset rather than horizontal shortening, and the width of fault zone may abruptly decrease in response to strain localization on the neighboring boundary transpressional fault (Frost et al., 2009; Zhao et al., 2016; Wu et al., 2019), such as the tectonic quiescence of the E-W-striking faults since ∼15.3 Ma. Moreover, faults with contrasting geometries, kinematics, and activity time, like the E-W-striking and NNW- to NW-striking faults in this study, could develop in different time and space due possible to highly-heterogeneous stress field in a transpressional-dominated FTB. These different faults may interplay with each other to generate very complex fault systems. In brief summary, transpressional-dominated FTBs evolve in a more complicated way than the contractional-dominated ones, with highly spatio-temporal heterogeneity in fault development. More site-based case studies should be conducted to obtain the underlying primary principles, possibly in combined use of surface geology, subsurface geophysical data as well as physical analog and/or numerical modeling.