How Did the Late Paleozoic to Early Mesozoic Tectonism Constrain the Carboniferous Stratigraphic Evolution in the Eastern Qaidam Basin, NW China?

: The eastern Qaidam Basin (EQB), along with its surrounding orogenic belts, witnessed complicated tectonic movements in the period from the late Paleozoic to the early Mesozoic. As strategic succeeding strata, the Carboniferous strata (CST) in the EQB have gradually become a research hotspot in recent years. However, the question of how tectonism controlled the tempo-spatial evolution of the CST has yet to be studied. To resolve these issues, we collated statistics related to unconformities, seismic interpretation, and basin modeling in this study. The results show that the structure of the CST was mostly controlled by NNE-striking faults, namely the Zongjia and Ainan Fault, in the period from the Carboniferous to the Triassic time. During the Carboniferous time, the sedimentation of the CST was controlled by medium-high angle potential normal faults. The CST experienced two stages of tectonic subsidence and subsequent burial: the highest average subsidence and burial rate of 45 m/Ma and 12 m/Ma occurred at 340~285 Ma, decreasing to 15 m/Ma and 7.5 m/Ma between 305 Ma and 250 Ma. However, the maximum burial (~5500 m) took place at ~250 Ma. From the end of the late Permian to the late Triassic (254~195 Ma), the overall exhumation rate of the CST has averaged 38.71 m/Ma, and 75 m/Ma in the southern margin of the Huobuxun Depression. The CST near the piedmont margins of the EQB suffered essential denudation at 254~195 Ma, resulting in small amounts of the residual CST. In these areas, the CST were deformed with a steepening dip during this time and were characterized by the combinations of syncline-anticlinal asymmetric folds with the high-angle interlimb. These findings indicated that the tempo-spatial evolution of the CST was possibly influenced by the sedimentary and tectonic transition, and was a combined response to Paleo-Tethys Ocean subduction, and arc-continental collisions since the late Paleozoic to early Mesozoic periods.


Introduction
Qaidam Basin is one of the three major oil-bearing basins in western China.At present, 34 oil and gas fields have been discovered, mainly in the Gasi depression, the Kunbei depression, Dongping (along with its periphery in the western part of the basin) [1][2][3][4], the Paleogene-Neogene oil-producing formation, the Jurassic source rock primary reservoir, the Paleogene-Neogene reservoir (developed in the northern margin of the basin) [4][5][6], and the quaternary self-generated and self-stored biological gas reservoir (developed in the Sanhu area in the middle of this basin) [7].To find the strategic succeeding strata, the Paleozoic Carboniferous strata (CST) have gradually become a research hotspot in recent years [8].More than 10 Wells in the eastern Qaidam Basin (EQB) have encountered the CST and presented a good oil and gas showing, indicating that the CST have resource potential [8][9][10][11].Furthermore, in the EQB, the petroleum geological conditions of the CST were favorable, and well-arranged elements for reservoir storage were deployed there, making it a conducive secondary structural unit for the accumulation and trapping of oil and gas [10,11].Until now, there has been a relatively unified understanding of aspects such as the source rock types, the hydrocarbon quality assessment, and the hydrocarbon generation of the CST [10,11].However, the mechanisms of hydrocarbon migration and accumulation in the Ounan depression remain to be studied [8], and the deciphering of the spatial-temporal evolution of the CST, for example, will provide a critical basis for petroleum exploration in the EQB.
The Qaidam Basin is a remarkable intermountain basin situated in the northeastern part of the Qinghai-Tibet Plateau, which is encircled by tetragonal mountains (Figure 1a) [12][13][14].Its formation and evolution have been inextricably linked to the evolution of the Tethys Ocean and the collision of multiple continental blocks since the late Paleozoic to Mesozoic time [14][15][16].During this time, numerous tectonic movements occurred in the EQB, which is proved by several low-temperature thermochronological ages and unconformities underlying the Mesozoic strata [17,18].Some researchers believe that the tectonic movements during this period caused the denudation of the CST, contributing a 'high in south, low in north' paleo-topographic pattern in the late Triassic in the EQB [19].In the basin area, Li et al. [8] conducted borehole samples of CST in the northern area of the EQB, and divided four phases of tectonism that could control the sedimentation and post-diagenesis of the CST.In the Olongbuluke area, Peng [20] and Dai et al. [21] analyzed the tectonic regime from the early Permian to the late Triassic, suggesting that the N-S trending compressional stress field caused the deformation of the typical outcrop structures and structural style of the CST in the EQB.These studies show that the surficial arrangement of fractures and morphology of folds have been already studied in the northern area of the EQB.However, significant deformation uncertainties remain in the whole basin due to, in particular, the remarkable paucity of subsurface imprints during this stage [22,23].To be specific, this relates to how the boundary faults reactivated in the EQB and influenced the sedimentation and erosion of the CST.Furthermore, the process of how the CST evolved from the Carboniferous to the Triassic has been poorly constrained.In recent years, several researchers have conducted the thermal modeling of well QY2, arguing that the CST have been buried in a stable status with an average temperature of 150 • C [10], and they have also observed the CST in the most piedmont areas may experience intense denudation due to the late Paleozoic orogenesis [17].In such scenarios, the burial-thermal history of the CST in the whole EQB remained to be studied.
To address the energy exploration and geological issues outlined above, and with the aim of reconstructing the tempo-spatial evolution of the CST, we conducted a comprehensive statistical analysisof unconformities of outcrops and wells in the EQB.Furthermore, we interpreted four seismic profiles in the Huobuxun depression to decipher the deformation characteristics of the CST.And we utilized the method of basin modeling to detect the subsidence/burial and uplift/exhumation history of the CST from the late Paleozoic to the early Mesozoic.Finally, the structural evolution and dynamic mechanism of the EQB were unraveled.[22]), wells/boreholes, structural units, and the position of outcrops and seismic lines in the eastern Qaidam Basin and its adjacent orogenic belt (NE-trending seismic profile AA', as shown in [23]).

Geological Settings
EQB is now bounded by the Eastern Kunlun Shan, Qilian Shan-Zongwulong Shan tectonic belt, and Ela Mountain, and is controlled by numerous major faults, such as the Ounan, Ainan, and Zongjia faults.It has formed a typical tectonic pattern of three depressions and two uplifts from the northern to southern EQB throughout geological time, namely the Delingha Depression, Olongbuluk Uplift, Ounan Depression, Emnic Uplift, and Huobuxun Depression [9][10][11] (Figure 1b,c).
Recent studies have documented massive deposits that are well developed in the surrounding orogenic belts of the EQB, which represented long-term sedimentation from the  [22]), wells/boreholes, structural units, and the position of outcrops and seismic lines in the eastern Qaidam Basin and its adjacent orogenic belt (NE-trending seismic profile AA', as shown in [23]).

Geological Settings
EQB is now bounded by the Eastern Kunlun Shan, Qilian Shan-Zongwulong Shan tectonic belt, and Ela Mountain, and is controlled by numerous major faults, such as the Ounan, Ainan, and Zongjia faults.It has formed a typical tectonic pattern of three depressions and two uplifts from the northern to southern EQB throughout geological time, namely the Delingha Depression, Olongbuluk Uplift, Ounan Depression, Emnic Uplift, and Huobuxun Depression [9][10][11] (Figure 1b,c).
Recent studies have documented massive deposits that are well developed in the surrounding orogenic belts of the EQB, which represented long-term sedimentation from the late Paleozoic to early Mesozoic, such as shallow marine shelf facies of clastic and carbonate sedimentary rocks in the Eastern Kunlun Shan (Figure 2), and multiple bathymetric-shallow sequences of clastic rocks in ZWS [23][24][25][26].In the EQB, the CST are constituted by the lower Carboniferous Chuanshangou Formation (Fm.) (C1ch), Chengqianggou Fm. (C1c), Huaitoutala Fm. (C1h), the upper Carboniferous Keluke Fm. (C2k) and the lower part of Zhabusagaxiu Fm. (P1zh), extending from from bottom to top (Figure 3).The lower Carboniferous is characterized by open platforms and paralic facies of bioclastic limestone with thin layers of mudstone and argillaceous limestone [17,27], whereby the limestones of the C1h Fm. are deemed as hydrocarbon source rocks, of which TOC and S1 + S2 were estimated at 0.69% and 0.10% on average, respectively [28].The upper Carboniferous strata developed frequent interbedded sequences of clastic to carbonate rocks and coal-bearing strata [17,29].Of these strata, C2k is mainly dominated by source rocks of mudstones and coals with higher organic abundances of 6.0-10.0%and >60% in TOC [19,28].Permian is mainly composed of sandstone, shale, and limestone with coal [9,29].In the Zongwulong tectonic belt, the upper Carboniferous Zongwulong Fm. is composed of clastic and carbonate rocks that developed in a stable tectonic environment in the late Paleozoic [20,29].The formation of Carboniferous in the EQB was closely related to tectonic settings, sea level, and global climate changes [29,30].However, some Carboniferous strata have undergone denudation due to the inversion of tectonism, which was caused by the thickening of the lower crust [19,21].
late Paleozoic to early Mesozoic, such as shallow marine shelf facies of clastic and carbonate sedimentary rocks in the Eastern Kunlun Shan (Figure 2), and multiple bathymetric-shallow sequences of clastic rocks in ZWS [23][24][25][26].In the EQB, the CST are constituted by the lower Carboniferous Chuanshangou Formation (Fm.) (C1ch), Chengqianggou Fm. (C1c), Huaitoutala Fm. (C1h), the upper Carboniferous Keluke Fm. (C2k) and the lower part of Zhabusagaxiu Fm. (P1zh), extending from from bottom to top (Figure 3).The lower Carboniferous is characterized by open platforms and paralic facies of bioclastic limestone with thin layers of mudstone and argillaceous limestone [17,27], whereby the limestones of the C1h Fm. are deemed as hydrocarbon source rocks, of which TOC and S1 + S2 were estimated at 0.69% and 0.10% on average, respectively [28].The upper Carboniferous strata developed frequent interbedded sequences of clastic to carbonate rocks and coalbearing strata [17,29].Of these strata, C2k is mainly dominated by source rocks of mudstones and coals with higher organic abundances of 6.0-10.0%and >60% in TOC [19,28].Permian is mainly composed of sandstone, shale, and limestone with coal [9,29].In the Zongwulong tectonic belt, the upper Carboniferous Zongwulong Fm. is composed of clastic and carbonate rocks that developed in a stable tectonic environment in the late Paleozoic [20,29].The formation of Carboniferous in the EQB was closely related to tectonic settings, sea level, and global climate changes [29,30].However, some Carboniferous strata have undergone denudation due to the inversion of tectonism, which was caused by the thickening of the lower crust [19,21].

Statistics of Unconformities
Unconformity is an effective tool that can be used to constrain the cursory time of strata deformation/exhumation and tectonism throughout geologic time.Effective methods that can be used to detect unconformities include field reconnaissance, wells, and seismic interpretation [32].Angular unconformities commonly developed in the margin of the EQB and its peripheral orogenic belts, while parallel unconformities mostly developed inside the basin, especially in the Huobuxun Depression [23].These unconformities allowed us to detect how the CST interacted with tectonism [33].In drawing on previous studies of typical outcrops, wells, boreholes, and seismic profiles [8][9][10][11]19,23], we collected and characterized unconformity in the EQB for the basis of reconstructing the tempo-spatial evolution of the CST.

Seismic Interpretation
To further specify how CST were only controlled/deformed by tectonism that occurred before the Cenozoic time, we selected four seismic profiles (BB', CC', DD', EE') while considering the almost zero dip angle of overlying strata.We drew on the well-logging combination, horizon calibration, and seismic phase identification to carry out comprehensive seismic interpretations by using Landmark software.Previous studies gave us the information we needed to define the age of seismic reflection and faults of each seismic profile [20,26,27,29,[34][35][36].

Basin Modeling
With the advancement of basin modeling techniques, the analysis of basin tectonic subsidence (uplift) and burial thermal characteristics has become a widely utilized approach in the comprehensive study of superimposed sedimentary basins.This approach has been extensively investigated and reported by the literature [8,10,11,[37][38][39][40][41][42][43].In this study, we utilized BasinMod ® 2009 software to simulate the tectonic subsidence and burial thermal history of 10 wells in the EQB (well location shown in Figure 1).The constraining conditions for modelling are as follows: (1) the present terrestrial heat flow in the Qaidam Basin (53.1 mW/m 2 ) and the average annual earth surface temperature (18 • C) [10,11]; (2) the exhumation time around ~264-249 Ma and ~225-180 Ma, constrained by regional unconformities and low-temperature thermochronological data [10,11,19].All the modeling results mentioned in this study have been tested by the measured values (Ro) obtained from wells and boreholes.1 show, in relation tosome areas in the northern margin of this basin, that the Dachaidan Depression, Maihai-Dahonggou Depression, and Hongshan Depression barely developed the CST.Unconformities mainly developed between the mid-upper Jurassic strata and crystalline basement in these areas, which means that the tectonism, followed by fierce denudation, occurred before ~174 Ma.Additionally, we can observe the unconformities between the Cenozoic strata and Precambrian basement, which were revealed by Well GQ1.This indicates that another tectonism occurred in the period from the Mesozoic time to the Cenozoic time.In the southern margin of this basin, eight wells and the Alagertai (ALGET) area showed that the CST were well developed in the Huobuxun Depression, while unconformities between the Quaternary strata and Precambrian basement were well developed in the piedmont areas.This indicates that the denudation was not much fiercer than in northern areas, while the CST were entirely denuded in the most piedmont areas.The unconformity between upper Triassic and upper Carboniferous strata in the ALGET area suggests a tectonism occurred between the Permian and early Triassic time in the EQB.For depressions in the middle of the EQB, most unconformities developed between the upper Carboniferous and middle Jurassic strata.The Dongdagou (DDG) area developed the angle unconformities between the upper Permian and middle Jurassic strata [44].These findings indicate that the overlying strata of CST have been exhumated and eroded in the middle of this basin, which allows us to constrain the tectonism time in the EQB from the early-middle Permian to the mid-late Triassic.In addition, the para-conformity in the Baishugou (BSG) area contacted the thick deep gray limestone of the C1h Fm. and continent-marine facies of the C2k Fm. [45], indicating that a moderate tectonic event occurred during the sedimentation of the CST.Overall, the unconformities in the EQB constrain two phases of tectonism that occurred between (1) the early-middle Permian and mid-late Triassic; and (2) the: late Triassic and middle Jurassic.

Deformation Characteristics of the CST
Seismic profiles BB' (Figure 3a,b) and CC' (Figure 3c,d) are NE-SE trending, extending from the Zongjia fault to the southern flank of the Xietie Shan and Ainan fault.Unconformities also developed between the Carboniferous and the overlying Mesozoic and Cenozoic strata.The characteristics of seismic reflection in the CST are generally subparallel-chaotic configurations with poor continuity, along with jumbled-, chained-and worm-like structures (Figure 3b,f).Owing to medium-high acoustic impedance, the CST are generally characterized by a substantial seismic amplitude response.Of these two seismic profiles, some medium-high angle faults play a major role in controlling the CST, especially in the southern front of Emnic Uplift and the northern flank of the Eastern Kunlun Shan, where the CST have been deformed.When the stratigraphic characteristics from both footwall and hanging wall are compared, obvious differences in deformation and thickness around each specific fault are apparent in the seismic profiles, which we define as inversion structures [46].In the central area of the EQB (F14-18, F22, F25, and F28), the dip angle of all faults varies from 60-80 • , and the thickness of CST in these faults' hanging walls is slightly greater than in the footwalls (Figure 3a,e).F15 and F16 are both inversion structures, and the hanging wall of F15 deposited thicker CST than the footwalls, due to the extensional setting in the early Carboniferous.F16 cut through the lower Carboniferous strata, and the upper Carboniferous strata were deformed into a combination of syncline-anticlinal asymmetric folds, of which interlimb angles are 150-170 • .These structures indicate a moderate tectonic transition after the consolidation of overlying strata.The same characteristics are also shown in F23 and F24, along with potential secondary faults (Figure 3c).However, in contrast, at the northern margin of the Huobuxun Depression, which is close to the piedmont zone, the CST were strongly compressed by the Ainan fault and its peripheral faults.For example, F28 and F29 cut through all CST and dislocated them into a combination of closed-open shaped folds with a dip angle varying from 30-40 • (Figure 4g).Additionally both the lower Carboniferous strata (with high but asymmetric interlimb angles of 40-60 • (Figure 3d)), and the upper and lower Carboniferous strata (with symmetric single synclines (Figure 3g)) indicate an intense deformation after the sedimentation of the CST.
Seismic profiles DD' (Figure 4a,b) and EE' (Figure 4c,d) are NE-SE trending and are located in the eastern Huobuxun Depression, extending from the Zongjia Fault and Ainan Fault, and crossing through the elongated Emnic Uplift.The seismic reflection of the CST is similar to that of seismic profiles BB' and CC'.In these two profiles, normal faults with medium-high angles and folds related to extensional tectonic settings are shown in the eastern area of the Depression.These structures controlled the filling of sediments in the hanging walls in the late Carboniferous time, and the strata were then compressed and deformed in the later time.For example, 60-90 • normal faults and nearly 80 • secondary minor normal faults were parallel to the main faults, such as F32, F37, and F44 (Figure 4a,b,g).These faults imply a crustal stretching in the early Carboniferous that accumulated the maximum sedimentation of the CST.On seismic profile DD', rollover anticlines are welldeveloped in the CST [46], whose basement and detachment are identified by deep-domain broadband seismic profiles [47].On the basis of the occurrence of the internal strata of Carboniferous (the black dotted line indicating the different dip angles of the upper and lower strata), we speculate that the sediments on the hanging walls of formerly-existing faults slid down or even collapsed along the fault plane in the early Carboniferous, and also conclude that a tectonic inversion occurred after sediment filling during the late Carboniferous to early Permian time.Similar to the observation of profiles BB' and CC', on both sides of the margin in profiles DD' and EE', the angle of faults is seen to be steep.These faults reactivated later in the Mesozoic to Cenozoic time, directly controlling the CST, leading to uplift and intensified denudation (e.g., F40 in Figure 4).In contrast, in the internal and central area of the Huobuxun Depression, some faults (e.g., F36 in Figure 4) only cut through the CST, causing it to be weakly deformed.Collectively, in the EQB, the CST were mostly controlled by medium-high angle faults.In the internal area of depression, the deformation trail of symmetric folds with low-angle interlimb remains in place.Faults were capable of cutting through the whole CST and causing them to be weakly deformed, but their influence on the residual thickness of the CST was negligible.In contrast, near the piedmont zone, combinations, both of syncline-anticlinal asymmetric folds with the high-angle interlimb, and deformed strata with steepening dip, are welldeveloped, suggesting a fierce tectonism in the EQB as a result of intensified uplift and denudation from the late Carboniferous period onwards.

Tectonic Subsidence and Burial Thermal Evolution
Basin modeling results show that, in the EQB, the CST experienced rapid tectonic subsidence with an average rate of 45 m/Ma from the early-late Carboniferous to the end of the late Carboniferous (360~303 Ma) (Figure 5).Notably, Well ZK3-2 exhibited the highest subsidence rate of the CST (approximately 65 m/Ma, with a maximum subsidence of about 2250 m), while Well QDC1 presented the lowest subsidence rate, of around 33 m/Ma.Over time, the burial capacity of the CST in EQB continually increased from the onset of Carboniferous, culminating in a peak of 4600 m (observed in borehole ZK3-2), and an average burial rate of approximately 12 m/Ma.Well GQ1 showed the smallest buried volume of the CST at about 1350 m, along with relatively high rates of burial.The onset time of subsidence and burial of the CST for Well HC1 and Well GQ1 was approximately 316 Ma, compared to the earlier times observed in the Delingha and Ounan depressions.During the late Carboniferous to late Permian period (305~250 Ma), the subsidence rate of the CST in the EQB gradually decreased, resulting in an average of approximately 15 m/Ma.The burial cumulant of the CST increased continuously, reaching a maximum of 5500 m.The wells, including Well QDD1, QDD2, CY2, and HC1, etc., recorded a maximum subsidence and burial cumulant of the CST between ~265 Ma and 250 Ma.However, as shown in Borehole ZK3-2 and Well HSC1, the CST did not reach the maximum burial depth.At ~290 Ma, some regions in the EQB witnessed a rapid decline in CST burial rates, and the most significant decrease fell from 50 m/Ma to 7.5 m/Ma (Figure 5).Overall, during this period, the subsidence and burial rates of the CST in the EQB slowed down.It is noteworthy that the CST reached its maximum burial depth in the major depressions.
From the end of the late Permian to late Triassic (254~195 Ma), a strong tectonic inversion response began to take place in the EQB at approximately 226 Ma, while the CST showed a weak subsidence during 254~226 Ma.Since ~226 Ma, the overall exhumation rate of the CST has averaged 38.71 m/Ma.The CST close to the orogenic belt in the EQB experienced a higher denudation rate, such as 75 m/Ma in the southern margin of Huobuxun Depression (well HC1) and 42 m/Ma in the northern margin of Delingha Depression (well DC1) (Figure 5).The piedmont zone also exhibited a significant amount of thinning.Specifically, in the southern margin of the Huobuxun Depression and the northern margin of the Delingha Depression, a small residual amount of the CST remained.At ~206 Ma, only 321 meters of the CST remained in the well HC1 (Figure 5).Conversely, in the SHG area, the thickness of the CST moderately decreased, and the denudation rate shown in Well CY2 was about 15 m/Ma.Owing to significant accumulation in the early phase, the thicker CST remained in the SHG area and other Ounan Depression areas after tectonic uplift and denudation.The thickness of the CST in Well CY2 and Well QDD1 at ~200 Ma were 3229.8m and 2703.64 m, respectively (Figure 5).During the late Carboniferous to late Permian period (305~250 Ma), the subsidence rate of the CST in the EQB gradually decreased, resulting in an average of approximately 15 m/Ma.The burial cumulant of the CST increased continuously, reaching a maximum of 5500 m.The wells, including Well QDD1, QDD2, CY2, and HC1, etc., recorded a maximum subsidence and burial cumulant of the CST between ~265 Ma and 250 Ma.However, as shown in Borehole ZK3-2 and Well HSC1, the CST did not reach the maximum burial depth.At ~290 Ma, some regions in the EQB witnessed a rapid decline in CST burial rates, and the most significant decrease fell from 50 m/Ma to 7.5 m/Ma (Figure 5).Overall, during this period, the subsidence and burial rates of the CST in the EQB slowed down.It is noteworthy that the CST reached its maximum burial depth in the major depressions.

Structural Evolution of EQB
From the end of the late Permian to late Triassic (254~195 Ma), a strong tectonic inversion response began to take place in the EQB at approximately 226 Ma, while the CST showed a weak subsidence during 254~226 Ma.Since ~226 Ma, the overall exhumation rate of the CST has averaged 38.71 m/Ma.The CST close to the orogenic belt in the EQB experienced a higher denudation rate, such as 75 m/Ma in the southern margin of Huobuxun Depression (well HC1) and 42 m/Ma in the northern margin of Delingha Depression (well DC1) (Figure 5).The piedmont zone also exhibited a significant amount of thinning.Specifically, in the southern margin of the Huobuxun Depression and the northern margin of the Delingha Depression, a small residual amount of the CST remained.At ~206 Ma, only 321 meters of the CST remained in the well HC1 (Figure 5).Conversely, in the SHG area, the thickness of the CST moderately decreased, and the denudation rate shown in Well CY2 was about 15 m/Ma.Owing to significant accumulation in the early phase, the thicker CST remained in the SHG area and other Ounan Depression areas after tectonic uplift and denudation.The thickness of the CST in Well CY2 and Well QDD1 at ~200 Ma were 3229.8m and 2703.64 m, respectively (Figure 5).

Structural Evolution of EQB
During the late Paleozoic sedimentation period, the whole EQB underwent widespread sedimentation [9][10][11][23][24][25][26].The sediment deposition was relatively stable, and normal faults have been welldeveloped [17,27].During the late Carboniferous to late Triassic time, large-scale tectonism occurred, resulting in the formation of a series of NW-trending uplifts, which were the embryonic forms of present-day mountains [10].In the Emic Uplift and Olongbuluk Uplift, all these tectonic units were controlled by previous potential normal faults.Then, the NNE-striking faults were reactivated and reversed, resulting in the CST in the hanging wall of the Zongjia and Ainan Faults being almost completely eroded.A series of NW-and NNW-trending fold structures controlled by faults were formed inside the depressions (Figure 6).Owing to the fault reactivity, the CST in the hanging wall was exhumated and suffered varying degrees of erosion, while the residual thickness of CST in the footwall was relatively thicker than in the hanging wall, at about 800 m to 2000 m.As a result, inversion structures with reversed faults extending to a maximum depth of 4 km developed in the residual depressions (Figure 6).All of these features were potentially influenced by the intensifying diastrophism during this period [10,19].

A Response to Early Tectonism: In the Carboniferous Time
Some scholars believe that in the Qaidam Basin sediments were formed in the passive continental margin setting after the Carboniferous to Permian time, up until the closure of the Zongwulong Trough at the end of the Late Permian [19,20].In recent years, with the discovery of detrital zircon ages and the obtaining of magmatic rocks dating results in both the Eastern Kunlun orogenic belt and northern Qaidam [17], an increasing number

A Response to Early Tectonism: In the Carboniferous Time
Some scholars believe that in the Qaidam Basin sediments were formed in the passive continental margin setting after the Carboniferous to Permian time, up until the closure of the Zongwulong Trough at the end of the Late Permian [19,20].In recent years, with the discovery of detrital zircon ages and the obtaining of magmatic rocks dating results in both the Eastern Kunlun orogenic belt and northern Qaidam [17], an increasing number of researchers believe that the Qaidam Basin developed in a back-arc setting since the Carboniferous time.At this time, a large range of plutons invaded the basin and its periphery due to the continuous subduction of the South Kunlun Ocean [17].Specifically, a sequence of volcanic arcs developed in the southern area, which was potentially attributable to Kunlun oceanic crust subduction, accompanied by arc magmatism and trench clogging [17].
Since the early Carboniferous, the margin of the PaleoTethys Ocean in the Eastern Kunlun orogenic belt began expanding continuously [48][49][50].It has been shown that, during this time, a large and long strip-shaped area of continental shelf deposits developed in the western Qinling terrane [50], which mainly consisted of carbonate and clastic rocks [17].In the area extending from the south to north of Kunlun to Qadiam terrane, 347~308 Ma ophiolite discovered in South Kunlun [31,47] indicated that, from the early late Carboniferous to the end of the late Carboniferous, this belt was the mid-ocean ridge of the Kunlun Ocean.In the northern Eastern Kunlun orogenic belt, flysch formations, OIB, pillow basalts, and seamounts were widely developed [25,31,48], and laminar carbonate sedimentary rocks with oryctocoenose were found in the seamount of Buqingshan [24].In the south of EQB, Lalingzaohuo, and other areas, shallow sea-slope sedimentary facies were found to have developed [20,27].The sea level in the Eastern Kunlun orogenic belt to Qaidam terrane therefore gradually became shallower after the early Carboniferous; during this time, the EQB also experienced back-arc subsidence.
In response to the development of the tectonic and sedimentary environment in the period from the early Carboniferous to late Carboniferous, magmatism in the Kunlun area was also active [49,51].In the south of EQB, there was a shoal and sea-land cross-zone, which is attested to by the siltstones of the C1h Fm. in the southern margin of Huobuxun Depression [27], there was widespread limestone in themuds and Siphonophyllia of the lower Carboniferous strata in the Dulan, Xiangride, and Guanjiaoya areas [48].At the end of the Carboniferous, the sedimentary environment transformed into a shallow sea high-energy paleoenvironment, as a result of changes of deep-sea sediments and lithofacies with Giganioproductus that occurred in the SHG area [48].Additionally, as the intracontinental extensional tectonism took place at this time, normal faults were welldeveloped in the Huobuxun Depression (Figures 3 and 4), and the rates of deposition became rapid.The CST were therefore mainly controlled by the early extensional tectonic settings.The medium-high angle faults and rapid rates of deposition can be considered to be an active response that is consistent with continental rift margin theory [52].

Compressional Geodynamic Process: From the Late Carboniferous to Triassic
Two phases of tectonic events were recorded in the basin in the periods from the late Carboniferous to early Permian and from the late Permian (~298 Ma) to early Triassic (~258.9-225Ma) [10,11,19].These events were closely related to the subduction and reduction of the Zongwulong Ocean, the subduction and gradual closure of the remnant Paleo-Tethys Ocean, and the collision and merger of the Kunlun-Qaidam arc and Songpan-Ganzi terrane [53,54].Meanwhile, in this period, the subsidence and burial rates of the CST decreased, and the CST were then exhumated and eroded in the EQB.The structure of the CST was primarily controlled by mid-high angle faults.The spatial-temporal evolution of the CST was controlled by such compressional tectonic settings.However, the dynamic mechanism behind this spatial-temporal evolution has yet to be studied.
Sun et al. [17] and Wu et al. [55] believe that volcanic island arc developed in the Eastern Kunlun orogenic belt during this period.The Zongwulong rift basin in the northern margin of EQB still existed and was filled by detritus from the Qilian Shan orogenic belt [17,25].In the northern EQB, the original low-energy coastal intertidal environment was transforming into a high-energy coastal counterpart during this time [26,29].This can be proven by the stratigraphic characteristics, as the thickness of limestone, combined with thePseudoschwagerina of an upper sequence of the upper Carboniferous strata transferred into the lagoon or barrier island facies, coal mudstone, and lenticular deltaic sandstone in the C 2 zh Fm. [29].In the early Permian, the lower Permian strata presented subangular mineral grains (such as quartz and feldspar) in the SHG area, which suggests that a significant hydrodynamic force may control the sedimentation process [29], and that a coastal environment has become more volatile.Therefore, in the period from the late Carboniferous to early Permian, the decreasing rate of CST's subsidence and burial could be attributed to the transition of the tectonic environment.Meanwhile, it was possibly a response to eustatic sea-level changes and the influx of sediments from distal sources.
From the early Triassic to the late Triassic, the remnant Paleo-Tethys Ocean gradually subducted, and the Kunlun-Qaidam arc was still merging with the Songpan-Ganzi terrane [53,54].A large number of I-and S-type granitoid (255 Ma) developed in the Eastern Kunlun orogenic belt.And the early Triassic granite magmatics developed in the Zongwulongshan tectonic belt and the northern Olongbuluk micro-block [50,55].Furthermore, at the ZFT peak age of 243 Ma, and the age of sericite 40 Ar/ 39 Ar (~258-236 Ma) in the Hala Lake area [56], pluton and thermochronological ages indicate that, in the margin of EQB, magma was not only emplaced, but intrusive rocks were also subject to a cooling process.This confirms the tectonic environment of the margin of EQB was active in the early Triassic.In the basin, the ZFT ages in the SHG area (~249.5 Ma) and ZHe ages in the Dulan area (~258.9Ma) [19,40] imply that tectonism may have occurred later than in the orogenic belts.Liu et al. [19] argued that, in the early Triassic, the tectonism amplitude of the northern margin of the EQB was relatively low and there were still several potential transporting channels for sediments.The sedimentary effect of 'weak subsidence and a small amount of burial' was presented from ~250 Ma to 230 Ma.Since the middle Triassic, continuous intensification of collision orogenesis, intense forebulge flexural elevation, and orogen unroofing took place in the broad Kunlun-Qaidam arc, and was synchronous with the generation of a foreland basin in the South Kunlun Belt [25].In such scenarios, the lower crust of the Kunlun-Qaidam arc thickened [57], and considerable flisch formations deposited in the Songpan-Ganzi terrane absorbed large amounts of shortening of this kind [58].In response to this intense orogenesis, the Qilian Shan terrane collided with the Olongbuluke terrane, and fierce exhumation occurred in the northern margin of the Qaidam Basin.This can be proven by referring to the 300~400 • C ductile shear deformation observed in the Zongwulong Group and the ZHe ages (~238-225 Ma) in the EQB [19,20].Under the buffering effect of the dual collision orogenic belts, the compressional effect inside the basin was weaker than in the orogenic belts [19].This can be verified by referring to the inversion structures with brittle deformation of the CST, which mainly developed in the Huobuxun Depression.Moreover, Liu et al. [19] sugggested that, during the middle Triassic to the middle of the late Triassic, the Luliangshan and Emnic Mountains continued to thrust onto the Huobuxun Depression.Overall, from the early Permian to the late Triassic, the intensity of tectonic inversion in EQB increased gradually, and the CST were rapidly exhumated and eroded.Small amounts of residual CST in the margin of EQB, and the brittle and deformed counterpart inside the basin can be attributed to the final response to Paleo-Tethys Ocean subduction, and intense arc-continental collisions since the late Paleozoic and early Mesozoic.

Conclusions
In this study, we decipher how tectonism played a significant role in the spatial-temporal evolution of the CST.We finally reconstructed the structural evolution of EQB and discussed the dynamic mechanism that operated from the late Paleozoic to the early Mesozoic.
(1) Since the early Carboniferous to the middle Permian, the sedimentation of the CST in the EQB was controlled by medium-high angle normal faults.The CST have experienced two main stages of tectonic subsidence and subsequent burial, with rapid, slower andeventually sluggish rates.This may be related to the intra-continental extensional tectonic settings in the back arc.(2) From the end of the late Permian to the late Triassic, the CST were deformed, with a steepening dip characterized by combinations of asymmetric folds with the high-angle interlimb.The CST rapidly exhumated and finally thinned.The CST near the piedmont margins of EQB suffered essential denudation, resulting in small amounts of remnants being deposited in the hanging wall of the Zongjia and Ainan Fault.This evolution could be attributed to the combined response to Paleo-Tethys Ocean subduction, and fierce arc-continental collisions since the late Paleozoic and early Mesozoic.

Figure 1 .
Figure 1.(a) Geographical location map of the Tibetan Plateau and its surrounding areas; (b) Geomorphological and fault system map of Qaidam Basin and its periphery; (c) Regional geological map (unconformity originated from[22]), wells/boreholes, structural units, and the position of outcrops and seismic lines in the eastern Qaidam Basin and its adjacent orogenic belt (NE-trending seismic profile AA', as shown in[23]).

Figure 1 .
Figure 1.(a) Geographical location map of the Tibetan Plateau and its surrounding areas; (b) Geomorphological and fault system map of Qaidam Basin and its periphery; (c) Regional geological map (unconformity originated from[22]), wells/boreholes, structural units, and the position of outcrops and seismic lines in the eastern Qaidam Basin and its adjacent orogenic belt (NE-trending seismic profile AA', as shown in[23]).

Figure 3 .
Figure 3. Seismic profiles in the western Huobuxun Depression.The locations of these profiles are shown in Figure 1c.(b,f) Uninterpreted profiles.(a,c-e,g,h) Interpreted profiles.

Figure 4 .
Figure 4. Seismic profiles in the western Huobuxun Depression.The locations of these profiles are shown in Figure 1c.(b,f) Uninterpreted profiles.(a,c-e,g) Interpreted profiles.

Geosciences 2024 , 18 Figure 5 .
Figure 5.The tectonic subsidence curve (Left) and burial curve (Right) of Carboniferous from the early Carboniferous to late Triassic periods in EQB.

Figure 5 .
Figure 5.The tectonic subsidence curve (Left) and burial curve (Right) of Carboniferous from the early Carboniferous to late Triassic periods in EQB.

18 Figure 6 .
Figure 6.Tectonic evolution of AA's seismic profile from late Paleozoic to present in EQB.

Figure 6 .
Figure 6.Tectonic evolution of AA's seismic profile from late Paleozoic to present in EQB.

Table 1 .
Statistics of unconformities in the EQB.