The northern margin of Qaidam Basin (NQB) is a complicated deformation belt formed under the collision background of Indian and Eurasia plates^[1,2]. The belt has the most complete Cenozoic sequences in the northern Tibetan Plateau^[3,4,5], making it an ideal place to study the structural deformation in response to the far-field effect of the India - Eurasia collision during the Cenozoic. So far, two contrasting viewpoints, the northward stepwise rise model^[6,7] and the simultaneous uplift model^[8], have been proposed to explain the rise of the Tibetan Plateau, and the key evidences for the latter come mainly from the sedimentary and structural records in the NQB^{[2, 9]}. Limited by study methods and resolution of seismic data, the understandings of the Cenozoic sedimentary history and deformation pattern in the NQB remain controversial. For instance, in terms of stratigraphic chronology, controversies persist on whether the lowermost part of the Cenozoic strata, the Lulehe Formation, belongs to the Early Eocene or Late Oligocene^[7,10-11] and whether it had a north source from the South Qilian mountain or a south one from the Qiman Tagh - East Kunlun mountain^[7,12-13]; in terms of structural deformation, there are multiple models to explain the structural deformation like thrusting^[14,15,16], dextral strike-slip faulting^[1,17-18], sinistral oblique thrust faulting^[19,20] and structural wedge^[2], etc. These uncertainties have restricted to some extent our understanding on the deformation mechanism of the northern Tibetan Plateau. The NQB is also an important hydrocarbon accumulation region in the Qaidam Basin, but without major breakthrough made yet^{[21,22,23,24,25]}, therefore, it is of great significance for the subsequent oil and gas exploration to study the structural deformation in the NQB. In this work, taking the Yuqia - Jiulongshan region where the Meso - Cenozoic strata well expose as the study object, we investigated the features and mechanisms of structural deformation in this region in detail based mainly on the newly-collected seismic data together with field geological survey, drilling data and apatite fission track analysis.

The Yuqia - Jiulongshan region is located in the easternmost part of the Saishitengshan, with the Luliangshan to the southeast and the Sainan Depression to the southwest. It is a complicated deformation zone within the Saishiteng - Aimunike structural belt in the NQB^[1], and consists of a series of NW-striking folds including the Mahaigaxiu, Yuqia, North Yuqia and Jiulongshan anticlines (Fig. 1). In this area, complete Meso - Cenozoic strata crop out, with a thickness of about 4500 m (Figs. 1 and 2). The Middle - Lower Jurassic Dameigou Formation (J_1-2) is comprised of black to greenish grey sandstone and shale intercalated with conglomerate and coal beds, and is the major source rock layer in the NQB. The Upper Jurassic Hongshuigou Formation (J₃) is made up of red mudstone, sandstone and conglomerate. The Lower Cretaceous Quanyagou Formation (K₁) consists mainly of red conglomerate and sandstone intercalated with mudstone, whereas the Upper Cretaceous is completely absent across the entire Qaidam Basin. The Cenozoic is a set of red sandstone and conglomerate interbedded with mudstone about 4000m thick (Fig. 2). From bottom to top, it consists of the Lulehe (E₁₊₂l, 44.2 - 52.0 Ma), Lower Ganchaigou (E₃, 34.2 - 44.2 Ma), Upper Ganchaigou (N₁, 19.5 - 34.2 Ma), Lower Youshashan (N₂¹, 12.9 - 19.5 Ma), Upper Youshashan (N₂², 8.1 - 12.9 Ma), Shizigou (N₂³, 2.5 - 8.1 Ma) and Qigequan (Q_1-2q, < 2.5 Ma) formations. Whilst there remains disputes over the absolute ages of the Cenozoic strata (especially the N₂¹ and underlying ones) in the NQB^{[7, 10-11]}, this has little impact on the results of this study. Previous studies on the Yuqia - Jiulongshan region were concentrated mainly on the sedimentary sequences, environment and the hydrocarbon potential of the J_1-2^[26,27], but rarely covered the Cenozoic deformation leading to the present structural configuration in this area. In this work, we consulted the magnetostratigraphic results from the nearby Dahonggou region by Ji et al.^[11] and the Huaitoutala region in the eastern NQB by Fang et al.^[28,29] for the age assignments of the Cenozoic strata (Fig. 2).

We conducted detailed field investigation of the structural deformation of the Mahaigaxiu, Yuqia, North Yuqia and Jiulongshan anticlines, and accordingly drew measured structural section as shown in Fig. 3.

The Mahaigaxiu anticline in the southernmost part of the study region is separated from the Luliangshan to the south by a fault (Fig. 4a) and gradually disappears toward the northwest (Fig. 1). It is a long and narrow tight anticline, where the Middle - Lower Jurassic Dameigou Formation, the Upper Jurassic Hongshuigou Formation and the Lower Cretaceous Quanyagou Formation expose at the core, Lulehe and Lower Ganchaigou formations expose at the north flank; and complete Cenozoic strata crop out in the south flank. The strata in the south flank are nearly vertical or even overturned, while the strata in the north flank are gentler (Figs. 1, 3 and 4a).

Different from the tight Mahaigaxiu anticline, the Yuqia anticline to the north is a broad anticline with low topographic relief (Fig. 1). A large NE-dipping reverse fault has been identified in the Middle - Lower Jurassic carbonaceous shale near the central axis of the Yuqia anticline where a number of smaller-scale folds and faults appear (Fig. 4b). The strata at the core deform intensely but gradually become stable toward both flanks, where the youngest formation involved in the deformation is Cenozoic Lulehe Formation. There is a secondary anticline and an associated syncline in the southern flank. Further south, a NE-dipping reverse fault separates the the Yuqia anticline from the Mahaigaxiu anticline, making the Jurassic strata juxtapose against the Late Quaternary deposits (Figs. 1, 3 and 4c).

The North Yuqia anticline is separated from the Yuqia anticline to the south by a blind NE-dipping fault (Fig. 1). At the core of this anticline, the carbonaceous shale of the Middle - Lower Jurassic Dameigou Formation crops out. Whilst this formation has folded and faulted severely in some local areas (Figs. 3, 4d and 4e), it is overall gentle in occurrence. In the north flank, the strata are vertical or even overturn, and a nearly vertical fault separates the Dameigou Formation and the Upper Jurassic Hongshuigou Formation (Figs. 1 and 4d). Field observations reveal that the youngest stratum involved in the anticline is the Lower Cretaceous Quanyagou Formation, that is covered by Quaternary deposits upward (Fig. 3).

The Jiulongshan anticline located in the northmost part of the study region is a tight anticline too, similar to the Mahaigaxiu anticline (Fig. 1). At its core, the Cenozoic Lulehe and Lower Ganchaigou formations expose, towards the two flanks, these formations are unconformably overlain by slightly-deformed Quaternary deposits, indicating that the anticline finalized in shape possibly in the Late Quaternary. This anticline is generally asymmetric with a steeper (>60°) northern flank and a gentler (-40°) southern flank (Fig. 3).

Plenty of high-resolution seismic data has been collected along with the oil exploration in the Yuqia - Jiulongshan region over the past decade. We interpreted two NNE-trending seismic profiles crossing the major folds and faults based on surface outcrops and drilling data to reveal the subsurface deformation (Figs. 5 and 6).

Profile B-B° (Fig. 5) crosses the Sainan Depression, Mahaigaxiu anticline, Yuqia anticline, North Yuqia anticline and Jiulongshan anticline in turn from south to north and reaches northward to the front of the South Qilianshan (Fig. 1). The Well Long 2 and Long 4 drilled recently are near this profile. The following features can be seen from this profile. (1) The Mahaigaxiu - Yuqia - Jiulongshan area is overall a box- shaped anticline under the control of the Sainan fault to the south and the Yubei and the Longbei faults to the north. This box-shaped anticline features two steep flanks and a broad core, quite different from the Sainan and the Longbei sags to the south and north, respectively, which are both dominated by weak deformation on flanks and intense subsidence. The basement relief between the box-shaped anticline and the nearby sags can reach 2 - 5 km. (2) The Mahaigaxiu anticline is a secondary fold controlled by the Sainan fault and its secondary back-thrust fault in the southern flank of the box- shaped anticline; whereas the Jiulongshan anticline is mainly influenced by the Longbei fault and its back thrust fault. The Yuqia and the North Yuqia anticlines are secondary folds within the broad core of the box-shaped anticline. Except the Mahaigaxiu anticline, all the other four anticlines are at the western plunging end of the profile with small magnitudes of deformation. (3) The faults on the profile are largely steep and basement-involved, forming a positive flower structure.

Profile C-C° (Fig. 6) is very close to the measured section A-A° (Fig. 1) in the field. It cuts through the Yuqia anticline, North Yuqia anticline, Jiulongshan anticline and Longbei sag from south to north. As the Yuqia anticline is located in the southern end of the profile, no seismic reflection signals were collected from there, the geological interpretation there was made based mainly on field investigation data. Well Long 7 drilled recently is in the middle of this profile. Though shorter than profile B-B°, the profile C-C° shows quite similar structural features with profile B-B°: (1) the area is an overall box-shaped anticlinorium under the control of the Sainan, Yubei and Longbei faults; (2) clearly-imaged asymmetric Yuqia and North Yuqia anticlines are largely influenced by NE-dipping reverse faults, which is consistent with field observations (Fig. 3), and (3) the steep basement-involved faults make up an overall positive flower structure.

In order to quantify the intensity of the Cenozoic deformation in the study area, we conducted palinspastic reconstruction for the profile B-B° by using the base of the Lulehe Formation as the reference and following the area conservation (Fig. 5). As profile C-C° does not cross the entire region, it wasn’t reconstructed. As described above, faults in the study area comprise a typical positive flower structure, implying these faults have some strike-slip components. In this scenario, the palinspastic reconstruction along 2-D profile is unable to fully quantify the true deformation intensity, yet it can still provide an estimation of the crustal shortening amount. The results of the palinspastic reconstruction show that the present and reconstructed lengths of the profile B-B° are 31.5 km and 36.8 km (Fig. 7), respectively. Correspondingly, the amount and ratio of crustal shortening are 5.3 km and 14.4%, respectively. This ratio is consistent with the reported Cenozoic deformation intensity in the adjacent areas^[30], indicating spatially stable NE-SW crustal shortening in the NQB. The reconstructed section also reveals that the Mesozoic strata, especially the Quanyagou Formation, vary in thickness obviously on two sides of the Sainan and the Yubei faults, which likely resulted from the pre-Cenozoic (probably Cretaceous) activities and/or Cenozoic strike-slip movement of these two faults.

In order to better delineate the sedimentary and structural evolution of the study area in the Early Cenozoic, we collected a sample (YQ01) from the yellowish-green sandstone layer at the top of the Dameigou Formation in the core of the North Yuqia anticline (Fig. 2), and made detrital apatite fission track (AFT) analysis of this sample. As the AFT single-grain age and track length are sensitive to temperature, they can document the thermal history of the analyzed sample below 110 °C, and be used to estimate the burial history of the sample. Herein we used this method to quantitatively estimate the maximum depositional thickness of the Cenozoic strata at the core of North Yuqia anticline where the entire Cenozoic has been completely eroded away. We compared the estimated thickness with those of the adjacent regions to get a picture of the overall Cenozoic evolution history in the study area. The sample processing followed the procedure proposed by Tian et al.^[31], and the AFT analysis was conducted in the London Geochronology Center. The results of the single-grain ages and fission track lengths are listed in Table 1 and plotted in Fig. 8a and 8b, respectively.

It can be seen from the experimental results that the ages of most grains have an error of 15 - 25%, few (only two) larger than 40%. These ages are dispersedly distributed between 140 and 303 Ma. More than 70% single-grain ages are no later than the depositional age of the sample (late Middle Jurassic), yet the rest are of Late Jurassic age and younger, implying that the sample has experienced partial annealing after deposition. The measured fission track lengths range from 9.6 μm to 15.5 μm and display bimodal distribution, with a majority peaking at 13 - 14 μm, a minority shorter than 10 μm and the minimum value of 9.62μm. All these indicate that the sample had been buried and reheated to a temperature equivalent to the AFT partial annealing zone (75 - 110 °C) after the deposition. The fission tracks have partially annealed due to the reheating, resulting in the occurrence of single-grain ages younger than the depositional age of the sample and track lengths in bimodal distribution. Since the single-grain ages are mainly calculated according to the distribution of the tracks, the partial annealing may also bring about error to the single-grain ages.

As the Cenozoic is in unconformable contact with the underlying Mesozoic strata in the NQB^[32], the reheating of the sample might occur in the Late Jurassic - Early Cretaceous and/or the Cenozoic. Previous study shows that the residual thickness of the Upper Jurassic - Lower Cretaceous is 1500 m at maximum, and the NQB in the Late Cretaceous was dominated by intense deformation and erosion with fairly weak deposition^[33]. These suggest that the Mesozoic strata where the sample was collected is likely less than 1500 m thick, which is far smaller than that of the Cenozoic strata nearby (Fig. 2). We thus inferred that the reheating of the sample mainly took place in the Cenozoic era. Since the old single-grain ages and large track lengths dominate in the measured results (Table 1 and Fig. 8), we think that the sample only experienced slightly partial annealing and likely reached the middle - upper part of the AFT partial annealing zone (75 - 90 °C). Previous studies show that in the NQB, the paleo-geothermal gradient from the Paleogene to Early Neogene was 26 °C/km ^[34] and the average surface temperature was around 15 °C. Thus the maximum burial depth of the collected sample was calculated at around 2900 m since the Cenozoic.

In addition, we also conducted thermal history modeling of the sample by using the HeFTy program^[35]. Three time - temperature constraints were applied in the modeling. (1) The analyzed apatite grains were firstly exposed to the AFT partial annealing zone (~110 °C) prior to 180 Ma when they were still in the provenance rock. (2) The sample deposited at 160 - 180 Ma and was near the surface with a temperature of 20 ± 10 °C at that time. (3) The sample reached the maximum burial depth (corresponding to the temperature of < 90 °C) during the deposition period of the Lower Ganchaigou to Shizigou formations (2.5 - 40.0 Ma). The modeling results (Fig. 8c) indicate that the sample was reheated to merely 40 - 60 °C in the late Cenozoic. Given a paleo-geothermal gradient of 26 °C/km^[34], this indicates that the maximum burial depth of the sample after deposition is 1000 - 1700 m. It should be noted that, since the collected sample is clastic rock, the apatite grains in it might come from different provenances and have different thermal histories before deposition, the modeling results have large uncertainty. But they still demonstrates that the sample was buried at a maximum depth of <2900 m since Cenozoic. Since the sample is collected in the upper part of the Middle Jurassic and that Mesozoic strata above the sample is about 300 m thick (Fig. 2), we infer that the original thickness of the Cenozoic strata in the North Yuqia area was <2600 m.

Though it is generally believed that the Cenozoic deformation in the NQB is dominated by reverse faulting and folding^{[2, 36]}, the kinematic process associated with the deformation re-mains controversial, and multiple models like thrusting^[14], dextral slip faulting^{[1, 18]}, sinistral slip thrusting^[19,20] and structural wedge ^[2], etc were proposed to explain the process. Both field survey and seismic profiles (Figs. 3, 5 and 6) show that the Yuqia - Jiulongshan region is an overall box-shaped anticline controlled by a number of high-angle reverse faults, such as the Sainan, Yubei and Longbei faults, etc., which is characterized by overall basement uplift but has no long-distance low-angle thrusting or complex structural wedge. The high- angle reverse faults in the study area comprise a typical positive flower structure, suggesting certain strike-slip components. Although we haven’t find any steps or scratches to measure directly the kinematics of these faults in the field, geological mapping and interpretation of remote sensing images reveal many dextral strike-slip faults and associated large-scale drag-folds. For instance, a clear drag-fold affected by dextral strike-slip on a basement-involved fault has been found to the north of the West Yuqia anticline; and there is a north-dipping fault in the core of the North Yuqia anticline which slips dextrally and cut through the Jurassic, and there are some dextral slip faults at the two flanks of the Mahaigaxiu anticline (Fig. 1). These evidences indicate that the Cenozoic deformation of the Yuqia - Jiulongshan region is typically dominated by dextral slip thrusting, which is consistent with regional stress field. The NQB is nearly perpendicular to and truncated by the 1600 km long sinistral slip Altyn Tagh fault to the northwest, and therefore should have opposite strike-slip component to that of the Altyn Tagh fault according to the Riedel shear model. Previous observations of the distribution and orientation of the surface folds and faults in other parts of the NQB also confirmed the existence of dextral strike-slip component^[18]. The dextral faulting in the study area likely represents the main pattern of the structural deformation in the Late Cenozoic in the NQB; whereas the reported sinistral strike-slip is likely local and secondary deformation that was influenced by this dextrally transpressional stress field.

Field investigation and seismic profiles (Figs. 3, 5 and 6) show the youngest stratum involved in the deformation is the late Miocene Shizigou Formation in the south flank of the Mahaigaxiu anticline. The Cenozoic strata there are nearly vertical with dipping angles of 70 - 80° (Fig. 3), implying that the strongest deformation must have occurred after the deposition of the Shizigou Formation, i.e., after ca. 2.5 Ma, resulting in intense folding of the whole Cenozoic packages. Moreover, sedimentary formations in the south flank of the Mahaigaxiu anticline have changed upward from the upper part of the Upper Youshashan formation mudstone, sandstone and pebbly sandstone to a large set of conglomerate (Fig. 2), reflecting fast uplift of the South Qilian Shan to the north since the Late Miocene, which is consistent with previous studies^{[29, 37-38]}.

As mentioned above, the Cenozoic strata in the North Yuqia anticline are merely 2600 m thick at maximum, much thinner than those in the south flank of the Mahaigaxiu anticline (about 3886.5 m, Fig. 2). Besides, the Lulehe Formation is only 492 m thick in the Yuqia area as revealed by the Well Long 2 and merely 251 m from measurement in the Jiulongshan area further north, both much thinner than that in the south flank of the Mahaigaxiu anticline (680.5 m, Fig. 2). Therefore, we inferred that the Yuqia - Jiulongshan region was a paleo-uplift that had been slightly active in the Early Cenozoic and experienced intensive structural deformation during and/or after the deposition period of the Shizigou Formation. Previous sedimentary and thermochronological studies also show that the NQB started tectonically active in the Early Cenozoic^[5,36,39-42]. This conclusion provides a new line of evidence that the far-field effect of the Cenozoic India - Eurasia collision had transmitted to the northern Tibetan Plateau in Early Cenozoic, although the magnitude then was much weaker than nowadays.

Based on the above analysis, we come to the conclusion that the Cenozoic structural evolution in Yuqia - Jiulongshan region can be divided into two stages. In the early stage from the Early Cenozoic to the deposition period of the Shizigou Formation (Miocene), this region was a slightly active paleo-uplift possibly under the control of the activity of the Sainan fault (Fig. 9a, 9b). In the subsequent late stage after the deposition period of the Shizigou Formation (Pliocene and Quaternary), the Sainan and Longbei faults were intensively active, resulting in intense folding and uplift of the study area and the development of a broad box-shaped anticline. Local secondary faults remolded this box-shaped anticline, forming the multiple secondary anticlines we see at surface at present (Fig. 9c). This means that, similar to the Mahai - Dahonggou uplift to the south^[43], the Yuqia - Jiulongshan region has been a long active paleo-uplift during the Cenozoic era, which is likely because the Sainan fault was a long-term active fault since the Late Mesozoic^{[39, 44]}. As the Yuqia - Jiulongshan paleo- uplift is near the Sainan sag rich in Middle - Lower Jurassic source rocks to the south, it may be a favorable zone for hydrocarbon accumulation and has high potential of hydrocarbon exploration, like the nearby Mahai - Dahonggou uplift.

We conducted detailed analysis of the Cenozoic deformation features in the Yuqia - Jiulongshan region in the NQB, based on field investigation, seismic data, drilling and logging data, and thermochronological dating, and drew the following conclusions.

The Yuqia - Jiulongshan region is a large box-shaped anticline on the whole, steep at the two flanks and wide and gentle at the core controlled by the Sainan and the Longbei faults to the south and north, respectively. The Mahaigaxiu and the Jiulongshan anticlines are secondary folds affected by secondary faults in the flanks of the box-shaped anticline; whereas the Yuqia and the Northern Yuqia anticlines are secondary folds in the wide and gentle core of the box-shaped anticline.

Faults in the Yuqia - Jiulongshan region are mostly high-angle and basement-involved ones, which comprise into a positive flower structure. These faults have a certain dextral strike-slip component aside from the reverse dip-slip that has resulted in 15.3% of crustal shortening. The dextral reverse faulting might represent the primary deformation feature in the NQB during the Late Cenozoic.

The Yuqia-Jiulongshan region was a slightly active paleo-uplift since Early Cenozoic controlled by the weak activity of the Sainan fault to the south, leading to relatively thin Cenozoic deposits compared with those in the basin interior. The intensive folding deformation there occurred during or after the deposition of Shizigou Formation. Close to the Sainan sag containing abundant Middle-Lower Jurassic source rocks, the Yuqia-Jiulongshan region has high oil and gas exploration potential.