The Tianshan Mountains, which formed through the prolonged and complex subduction-accretion of the paleo-Asian ocean, experienced multiple widespread erosions and reactivations during the Mesozoic-Cenozoic period. Thus, the Tianshan has become a crucial area to explore the process and mechanism of intracontinental orogeny. The Harlik Mountain located in the easternmost part of Tianshan has special significance in revealing crustal deformation and far-field stress propagation. In this study, we use the apatite (U-Th)/He method to investigate the near-surface exhumation process of Harlik Mountain, which is the result of tectonic events and erosion. The apatite (U-Th)/He data and thermal history modeling reveal three enhanced exhumation events in the Harlik Mountain, i.e., the early Cretaceous, the late Cretaceous-Paleocene, and the middle Eocene. These events are consistent with the exhumation patterns observed throughout the entire Tianshan. Notably, the exhumation rate and amplitude in the early Cenozoic period were significantly higher in the Harlik Mountain compared with the Mesozoic period. Our findings reveal that the exhumation of the Harlik Mountain and the entire Tianshan displayed pronounced spatial-temporal variations throughout the Mesozoic-Cenozoic period. These variations can be attributed to the combined effect of uneven northward compression of the southern Asian margin, the heterogeneous rheological properties of the lithosphere, and the complex structure of the Tianshan.
The Central Asian orogenic belt (CAOB) is bound to the north by the Siberian craton and to the south by the North China-Tarim craton (Figure 1a). After experiencing subduction of the paleo-Asian ocean and multiple stages of collision and accretion between different terranes during the Neoproterozoic to the Permian period, the CAOB has become the largest intracontinental orogeny on Earth [1, 2]. Since the Mesozoic era, the Tianshan has undergone large-scale erosion and multiple stages of intracontinental deformation, which has shaped the complex present topography and the pattern of differential evolution between the east and west Tianshan. However, there is still a lack of consensus regarding the processes and mechanisms of the intracontinental orogeny in the Tianshan, as well as the rules governing the far-field stress propagation within the tectonic plates.
A growing body of evidence supports the view that the modern Tianshan mainly formed during the Mesozoic era due to intracontinental orogeny [3-8]. Additionally, it has experienced multiple phases of crustal deformation during the late Eocene-early Oligocene [5, 9, 10], late Oligocene-early Miocene [11-16], and late Miocene eras [17, 18]. The intense intracontinental orogeny has led to the rapid exhumation of geological bodies. Specifically, the rapid exhumation during the middle Triassic-late Triassic/early Jurassic is believed to be a response to the closure of the Paleo-Asian Ocean and the collision of the Qiangtang terrane with Kunlun terrane or the closure of the Mongol-Okhotsk Ocean [19-21]. The rapid exhumation during the late Jurassic-early Cretaceous is thought to be associated with the collision between the Qiangtang and Lhasa terranes , while the rapid exhumation during the Late Cretaceous-Paleogene is attributed to the collision of the Kohistan-Dras island arc with the Karakoram and the ongoing convergence between the Indian and Eurasian plate . However, these events occurring at the plate margins do not strictly correspond to or exhibit systematic delays in relation to the rapid exhumation events observed in the Tianshan. The tectonic processes at the plate margins, such as subduction, collision, and accretion, are characterized by prolonged durations and complex mechanisms, and the rules and effects of stress propagation are still not well understood. Furthermore, the complete exhumation process of certain regions has not been fully elucidated, which also impedes the understanding of intracontinental orogeny in Tianshan.
In the east Tianshan (Figure 1(b) and 1(c)), low-temperature thermochronology has revealed multiple episodes of deformation and exhumation events since the Cretaceous, with the most significant exhumation occurring during the late Cenozoic [22-26]. However, recent studies based on apatite fission-track analysis suggested that the substantial exhumation in the area occurred solely prior to the India-Eurasia collision [6, 7, 27, 28], and the exhumation during the Cenozoic era is not evident. The apatite (U-Th)/He dating system has a lower closure temperature (~70°C ) and is more sensitive to the thermal evolution of the near-surface low-temperature stages. Compared with the apatite fission-track dating (closure temperature ~110°C ), the (U-Th)/He system can record more recent and updated information. Therefore, it has become an important method for investigating the growth process and patterns of orogenic belts [31, 32]. This study included the Harlik Mountain as the research area, which is located far from the plate boundary at the easternmost end of the Tianshan orogenic belt, and employed the apatite (U-Th)/He dating to reveal its exhumation process since the Cretaceous era. Based on previous research, this study provides new chronological evidence for whether the eastern Tianshan experienced significant exhumation during the Cenozoic era, which also contributes to unraveling the patterns of stress propagation and far-field effects generated by the ongoing plate convergence along the southern margin of Asia. Moreover, it offers new constraints for further understanding the intracontinental orogeny within the Tianshan.
2. Geological Background
As the longest mountain range in the CAOB, the Tianshan extends for more than 2500 km from east to west, spanning Uzbekistan, Kyrgyzstan, Kazakhstan, and Northwestern China. Overall, they display a convergent shape toward the east and diverge toward the west. Within China, the Tianshan orogenic belt can be roughly divided into the west Tianshan and the east Tianshan, with the boundary located at ~88°E. The Tarim basin is situated to the south, while the Junggar basin is located to the north of the Tianshan. In the east Tianshan, the prominent mountains include the Bogda Mountain, Harlik Mountain, and the Moqinwula Mountain, which have high elevation. The region also consists of relatively low elevation and low relief areas such as the Jueluotage and Kuluketage low mountain hills, as well as the Gobi desert. The Turpan-Hami basin and Barkol basin are situated within the mountain ranges (Figure 1(b)).
During the late Carboniferous to early Permian period, following the closure of the paleo-Asian Ocean and the cessation of collision-accretion orogeny, the microcontinental blocks in the east Tianshan underwent tectonic amalgamation and entered a phase of intracontinental orogeny [1, 33, 34]. From the late Permian to early Triassic period, extensive block rotation and reorganization caused significant ductile deformation along major shear zones within the east Tianshan . Following the widespread erosion [36, 37], the east Tianshan experienced multiple episodes of intense rejuvenation processes [5, 14, 19, 38].
The Harlik Mountain is located at the easternmost edge of the Tianshan, stretching ~250 km from west to east. It is bordered by E-W trending faults on the north and south sides adjacent to the Barkol and the Turpan-Hami basin. Furthermore, the Harlik Mountain is connected to the Moqinwula mountain in the east . The Devonian-Carboniferous subduction-collision-related granites and the late Carboniferous-early Permian post-collision-related granites are widespread distributed within the Harlik Mountain . In addition, the exposed sedimentary rocks mainly include the Ordovician, Devonian, and Carboniferous (mostly Carboniferous in the central-western and Devonian in the eastern part), with only minor amounts of Permian strata. Triassic and Cretaceous strata are absent, with limited Jurassic and Paleogene-Neogene continental clastic sediments exposed at the mountain margins (Figure 1(c)). To the south of the Harlik Mountain, the Turpan-Hami basin mainly consists of early-middle Jurassic, late Cretaceous, Oligocene, and Neogene-Quaternary strata, primarily controlled by the northern margin fault of the basin . The Barkol Basin situated to the north of the Harlik Mountain mainly contains early Jurassic, Oligocene-middle Miocene, and Neogene-Quaternary strata, with the absence of middle-late Jurassic and Cretaceous strata .
3. Sampling and Methods
A total of eleven samples from different parts of the Harlik Mountain, aligned in the west-east direction, were obtained. Each of these late Paleozoic granite rock samples weighed ~10 kg (figures 1(c) and 2). Two samples were collected from the western region of the Harlik Mountain, south of the Barkol Lake (HL1405/06); four from the Koumenzi area in the central part (DTS13105/13106, HL1401/03); and five from the southeastern region (HL1407/08/12/13/14) with a horizontal distance of ~20 km between the five samples and significant differences in elevation (~1189–1915 m). Detailed information regarding their longitude-latitude coordinates and lithologies is provided in Table 1.
3.1. Apatite (U-Th)/He Analysis
The analysis of apatite (U-Th)/He was conducted at the (U-Th)/He laboratory in the Institute of Geology and Geophysics, Chinese Academy of Sciences. The rocks were first crushed, and the apatite mineral was separated using heavy liquid and magnetic separation techniques. Subsequently, a single mineral separation was performed under the microscope. Five apatite grains with high crystal quality, absence of cracks or inclusions, uniform transparency, and a diameter larger than 60 µm were selected from each sample. The geometric characteristics of the selected grains, including length, width, height, and cone length, were measured. The grains were encapsulated in platinum capsules. The encapsulated grains were heated using a 970 nm Nd-YAG laser, and the concentration of He was determined using an Alphachron MK2 noble gas mass spectrometer. The concentrations of U and Th were determined using a Thermo Fisher X2 series ICP-MS instrument. The alpha correction factor was calculated using the geometric parameters of the particles measured under a microscope. The detailed experimental procedure was described in References 9, 41. After obtaining the (U-Th)/He age of single apatite grains, we employed the IsoplotR software  to identify and exclude outliers based on the modified Generalized Chauvenet Criterion. The software was also used to calculate the weighted mean age, utilizing a random effects model that considered both analytical uncertainty and an overdispersion term. A comprehensive explanation of the calculation methods was described in Reference 42.
3.2. Thermal History Modeling
This study utilized the QTQt software to model the thermal history, as recorded in the AHe data of the samples. Based on Bayesian theorem, QTQt uses the Markov chain Monte Carlo method to simulate and calculate, takes the possible time-temperature paths within a specific range of the sample as prior information, and then inverts the optimal thermal evolution model . During the simulation process, QTQt considers the effects of particle radius, effective U concentration, cooling rate, etc. Given that the Harlik Mountain has developed a large amount of Carboniferous-Permian granites, and regional-scale ductile shear deformation occurred during the Triassic period, we took the 40Ar/39Ar ages of biotite and K-feldspar and their corresponding closure temperatures as the constrained point (220 ± 30 Ma, 350℃ ± 50℃ ), and the current surface temperature was set as 15℃ ± 5℃ . In particular, HL1406 did not show good simulation results because the birth and death values differed significantly in the case of a single constrain point of 220 ± 30 Ma and 350℃ ± 50℃. Therefore, the AFT age of 100.6 ± 7.2 Ma and the corresponding closure temperature of 90℃ ± 30℃  from the same geological unit were used as another constrain point for HL1406 to obtain a good simulation result. For samples with no significant correlation between single-grain age and Rs or eU, the thermal kinetic parameters of  were used during the simulation process, whereas for samples with a linear correlation between single-grain age and eU, the thermal kinetic parameters of Radiation Damage Accumulation and Annealing Model  were used.
During each simulation, 200,000 iterations were performed to search the possible cooling curves. Based on this, the maximum likelihood and weighted average thermal evolution curves were obtained. The simulation range was generally set as AHe age ± AHe age. When the acceptance rate was between 0.2 and 0.6, the birth and death values were low enough, with a birth/death ratio of ~1, for the simulation results to be considered acceptable.
4.1. Apatite (U-Th)/He Ages
The results of the apatite (U-Th)/He analysis are presented in Table 2 and Figure 3. It was observed that there was a certain degree of dispersion in the single-grain ages within the same sample. For HL1405 (1873 m), obtained from the west Harlik Mountain, four single-grain (U-Th)/He ages were obtained, and the weighted mean age was determined to be 71.38 ± 4.34 Ma, except for one outlier with an age of 44.56 ± 2.37 Ma. The five single-grain ages of HL1406 (1969 m) could be divided into two groups: 20.63 ± 1.23 Ma (three grains) and 45.15 ± 3.29 Ma (two grains). The weighted mean age of all five grains was calculated to be 28.23 ± 9.53 Ma. No significant correlation was observed between the single-grain ages and the Rs or eU values for HL1405 and HL1406 (Figure 3).
In the central Harlik Mountain, there was no significant correlation between the single-grain ages and the Rs or eU of the four grains from DTS1305 (2104 m). Except for one minimum age of 27.12 ± 1.53 Ma, the other three grains yielded a weighted mean age of 37.25 ± 3.18 Ma. For DTS1306 (2272 m), four single-grain ages were obtained, which showed greater scatter. The mean age of the two older grains was 58.24 ± 4.28 Ma, and a significant positive correlation between (U-Th)/He age and eU was observed (four grains), indicating the influence of radiation damage on age dispersion. Among the four single-grain ages of HL1401 (2751 m), except the outlier age of 165.5 ± 9.5 Ma, the weighted mean age of the other three grains yielded a mean age of 56.12 ± 7.53 Ma, which also exhibited a linear relationship between single-grain ages and eU. Whereas the five single-grain ages obtained from HL1403 (2751 m) could be divided into two groups: 65.2 ± 4.88 Ma (two grains) and 46.81 ± 2.84 Ma (three grains). The weighted mean age of all five grains was 53.45 ± 7.66 Ma. Among these five grains, the three younger grains had higher eU content, while all five grains showed a positive correlation with Rs.
The (U-Th)/He ages of the samples collected from the southeast region of Harlik Mountain displayed good consistency within the margin of error and primarily concentrated in the early Cretaceous period (~125–110 Ma). The weighted average (U-Th)/He age of the eighteen individual grains was determined to be 118.8 ± 4.4 Ma. Among these grains, the mean age of the four grains from HL1407 (1189 m) was 120.86 ± 7.80 Ma, and their individual ages showed a positive correlation with Rs but no relation to eU. The mean age of the four grains from HL1408 (1347 m) was 115.58 ± 8.04 Ma with good consistency among the single grain ages. Except for the largest grain with an age of 241.07 ± 13.07 Ma, the three other grains from HL1412 (1659 m) yielded a mean age of 125.62 ± 7.89 Ma. Meanwhile, the weighted mean age of the four grains from HL1413 (1835 m) was 109.6 ± 13 Ma. In the case of HL1414 (1915 m), except for one grain with an age of 137.88 ± 7.30 Ma, the three other grains yielded a mean age of 110.51 ± 6.61 Ma. Notably, the individual ages of these grains showed a negative correlation with Rs and eU.
4.2. Modeling Results
The simulation results exhibited significantly different thermal histories of the west, central, and southeast Harlik Mountain. However, the cooling histories of different parts of Harlik Mountain since the late Mesozoic era were all characterized by moderate to slow rates and indicated the influence of tectonics in controlling exposure levels of rocks. In the west Harlik Mountain, HL1405 exhibited a moderate cooling rate from the late Mesozoic to the early Cenozoic era, with a cooling rate of ~2℃/Myr, which indicated that this region had exhumed to the near-surface position at approximately 60 Ma. HL1406, another sample from the west Harlik Mountain, remained in the partial retention zone of apatite (U-Th)/He system for a prolonged duration, followed by an accelerated cooling phase since the middle Eocene period, with an average cooling rate of ~1.44℃/Myr since ~45 Ma. In the central Harlik Mountain, the cooling histories of different locations did not exhibit any regular variation in the N-S direction. After the relatively rapid cooling (~ 2.5℃–2℃/Myr) from the late Cretaceous to the early Eocene period, different tectonic units of the central Harlik Mountain gradually exhumed to the near-surface position during the Eocene era. As to the southeast Harlik Mountain, five samples collected from different locations yielded identical AHe ages within the range of error. Although their inversion simulation results displayed some variations, they all indicated a relatively rapid cooling (~2℃/Myr) during the early Cretaceous and a slower cooling (<0.5℃/Myr) since the late Cretaceous period.
5.1. Differential Exhumation of the Harlik Mountain since the Cretaceous Era
The cooling history recorded by thermochronological data is the thermal response of tectonic or erosional exhumation. This study focused on the apatite (U-Th)/He data with relatively low closure temperatures. Thus, the simulation results provided good constraints on low temperature (below ~90°C) near-surface processes rather than limited constraints on the cooling paths during the high-middle temperature ranges. The magmatism in the east Tianshan ceased in the late Permian period, and the paleo-geothermal gradient had been continuously decreasing since the Triassic era. According to the maturity of organic matter in the foreland sedimentary strata, the paleo-geothermal gradients decreased from ~35°C/km during the Cretaceous to ~24.2°C/km during the Paleogene-Neogene period . Combining with the simulation results (Figure 4), the apatite (U-Th)/He data obtained from different parts of the Harlik Mountain mainly experienced three relatively rapid exhumations, i.e., the early Cretaceous, late Cretaceous–early Eocene, and late Eocene era.
The relative rapid exhumation during the early Cretaceous period occurred in the southeast Harlik Mountain (Figures 2(c) and 5), and the five samples taken from different elevations within the residual titled planation surface, which as a separate block, yielded consistent AHe ages within the error range (Figure 3(c)). This exhumation had an average exhumation rate of ~0.05–0.07 km/Myr and an exhumation amplitude of ~1.7–2 km. It had been gradually exhumed to the near-surface position in the beginning of the late Cretaceous era and entered a stage of long-term slow exhumation. This further confirmed that the planation surface was formed in the late Cretaceous-early Eocene period . However, the specific time of the planation surface tilting is still undetermined.
From the Late Cretaceous to the Neogene era, the paleo-geothermal gradient significantly decreased from ~35°C/km to ~24.2°C/km , indicating that the cooling during this period was the result of both exhumation and paleo-geothermal variation. During the late Cretaceous-early Cenozoic, the relative enhanced exhumation occurred both in the west and central Harlik Mountain. In the central Harlik Mountain, the average exhumation rate during the late Cretaceous (~80–66 Ma) was ~0.03–0.06 km/Myr, consistent with the exhumation rate obtained from the age-elevation method (~0.04 km/Myr) [6, 7]. During the Paleocene-Eocene, the average exhumation rate sharply increased to ~0.10–0.12 km/Myr. In the west Harlik Mountain, the average cooling rate was ~1.4°C/Myr during ~90–55 Ma, while the average exhumation rate increased from ~0.04 km/Myr during the late Cretaceous to ~0.06 km/Myr during the Paleocene-Eocene era, with an exhumation amplitude of ~1.5 km. Since the late Eocene, the relative rapid exhumation events only occurred in the west Harlik Mountain, with an average exhumation rate of ~0.08 km/Myr and an exhumation amplitude of ~2.9 km since ~35 Ma. Notably, the activity of blind or small faults within HL1405 and HL1406, along with differential surface erosion, is likely to be a contributing factor to their disparate cooling and exhumation process (Figure 4). The rapid exhumation during the early and the late Cretaceous in the Harlik Mountain had also been recorded by the detrital apatite from the Cenozoic sediments in the Turpan-Hami basin .
Overall, the rapid exhumation events from the early Cretaceous to the Eocene in the Harlik Mountain displayed a general trend from east to west over time. However, there were differences in the exhumation recorded for different samples even in the same area of the Harlik Mountain (Figure 4), reflecting the significant influence of complex fault activities on the differential exhumation, which was also confirmed by the asymmetric drainage systems and the late Cretaceous fault thrust at the southern margin of the planation surface .
Within the east Tianshan, significantly different exhumation was observed among different mountains, which directly reflected upon the varied exhumation level of the sedimentary rocks. For example, the Bogda Mountain was mainly composed of Carboniferous volcanic and carbonate rocks, with Permian-Neogene deposits distributed on both sides of the range [3, 46], while the Harlik Mountain located further east exposed a large amount of Ordovician-Silurian strata (Figures 1(c) and 2), indicating that the Harlik Mountain experienced more intense exhumation. In addition, both AFT age differences between the interior and exterior of Moqinwula Mountain and the gradual decrease in AFT ages from east to west indicated the differential exhumation of the crust caused by tectonics .
5.2. Multi-Plate Convergence and Its Far-Field Effect on the East Tianshan
After the collision and accretion orogeny of the Tianshan, multiple stages of complex crustal deformation occurred during the Mesozoic-Cenozoic intracontinental orogeny. The extensive development of the early-middle Jurassic coal seams in the Bayinbuluke and Turpan-Hami basins within the Tianshan, as well as the Kuche, Tarim, and Junggar basins on either side, indicated that the ancient Tianshan had been eroded to a low relief landscape with low elevation mountains [19, 47]. The low relief and low elevation topography of the east Tianshan continued till the early-middle Jurassic period and only provided limited detrital materials to the basins on either side of the mountains [40, 48]. Since the late Jurassic, significant changes occurred in the sedimentary sources of the Turpan-Hami basin. At the same time, Bogda Mountain started to uplift, along with the intense thrust-folding of Moqinwula Mountain , which blocked the southward migration of materials from the Junggar basin. Furthermore, the Turpan-Hami basin received significant sediments from the Harlik Mountain until the late Cretaceous period [39, 48-50]. Therefore, the rapid exhumation in the southeast Harlik Mountain during the early Cretaceous was most likely a response to the initial uplift since the widespread planation.
The rapid exhumation during the early Cretaceous, late Cretaceous-Paleogene, and middle Eocene in the Harlik Mountain can be widely compared with the entire Tianshan. In the east Tianshan, the Bogda Mountain [23, 26, 51], the Moqinwula Mountain , and the Yaman Su-Dananhu area that is located in the south of the Turpan-Hami basin [6, 7] underwent significant exhumation during the early Cretaceous. The reactivation of the Kanggur-Huangshan ductile shear zone under compressional or transform compressional conditions during the early Cretaceous also led to widespread rapid exhumation . The absence of early Cretaceous sedimentations in the Turpan-Hami basin and Barkol basin further indicates that the east Tianshan experienced regional uplift and erosion during this period . During the late Cretaceous-Paleogene, the strong folding and thrusting activities of the Bogda Mountain and Moqinwula Mountain rapidly exhumed the deep-seated rocks and caused significant changes in the regional topography [22, 52]. However, areas south of the Turpan-Hami basin, such as the Jueluotage, have experienced slow exhumation since the late Cretaceous, which may be an important reason for the notable difference in topography of the Tianshan between the north and south of the Turpan-Hami basin. Except for the Harlik Mountain, the rapid Eocene exhumation processes were only reported in the Bogda Mountain in east Tianshan [22, 23, 25]. Furthermore, the absence of Cretaceous sediments in the Tianshan, along with an acceleration in sedimentation rates and an increase of coarse clastic sedimentation along the northern margin of the Tarim basin and the southern margin of the Junggar basin during the late Cretaceous-Paleogene , also suggest that the widespread rapid exhumation happened during the early Cretaceous and the late Cretaceous-Paleogene period, in both the west Tianshan within China and the Tianshan outside China [5, 8, 13, 14, 21, 53-55]. However, compared with the east Tianshan, the west Tianshan within China and the Tianshan outside China exhibit younger thermochronological ages and have undergone more intense and widespread exhumation since the Eocene . This is consistent with the observed higher strain, elevation, and relief landscape in the west Tianshan compared with the east Tianshan . It is worth noting that while rapid exhumation events such as those in the early Cretaceous, late Cretaceous-Paleogene, and early Eocene era occurred, there were still many areas in the Tianshan undergoing slow exhumation [5, 9, 21, 55, 58]. Thus, the Mesozoic-Cenozoic crustal deformation and exhumation of the Tianshan exhibited pulsating patterns and significant spatial-temporal variations .
Currently, the dynamics behind the rapid exhumation in the early Cretaceous and the late Cretaceous-Paleogene in the Tianshan are mostly attributed to significant tectonic events at the distant plate boundaries. The closure of the Mongol-Okhotsk ocean, the low-angle subduction of the Bangong-Nujiang ocean beneath the Asian continental lithosphere, the collision of the Qiangtang and Lhasa terranes, as well as the India-Eurasia collision [6, 7, 14, 59], were considered to be an important cause of crustal deformation and exhumation in the Tianshan. Recent studies have revealed that due to the retreat subduction of the oceanic basin and the rotation of the Siberian plate, the Mongol-Okhotsk Ocean started eastward scissor-like closure from west to east since the late Triassic (~220 Ma) and continued until the complete closure of the ocean basin in the late Jurassic (~160–150 Ma) era [60, 61]. The closure of the Mongol-Okhotsk ocean resulted in the collision of the Siberian and the Amuria-North China plates, as well as the formation of the Mongol-Okhotsk Orocline. However, there is no record of exhumation events during the early Cretaceous in regions such as the Hangay plateau and Siberia , and the initial uplift of Gobi-Altay, which is closer to the Mongol-Okhotsk Ocean, occurred in the late Miocene-early Pliocene period . Therefore, the closure of the Mongol-Okhotsk Ocean and the subsequent orogeny are more likely to influence Mongolia and North China [63, 64] rather than the Tianshan, which is located further to the west.
On the southern margin of the Asian plate, the Bangong-Nujiang ocean began northward subduction during the early Jurassic. Its progressive closure from east to west since the late Jurassic continued until the early Cretaceous, until it completely disappeared by the beginning of the late Cretaceous. This closure led to a comprehensive collision between the Lhasa and Qiangtang terranes along the Bangong-Nujiang suture zone [56, 65, 66]. During the late Cretaceous, the Kohistan arc experienced intra-arc extension due to the Neo-Tethyan slab roll-back. Subsequently, the Kohistan arc collided with the Karakoram terrane along the Shyok suture zone at ~90 Ma . Toward the end of the late Cretaceous and the early Cenozoic, as the Neo-Tethys ocean closed, the Indian and Eurasian continents collided with each other at ~65–55 Ma [67-69]. However, further research is needed to determine the existence of a diachronous collision . The continuous northward subduction of the Indian plate led to the formation of a huge intra-continental deformation domain within the Asian continent, stretching ~2000 km from north to south and ~3000 km from east to west . These Mesozoic-Cenozoic collision and accretion processes along the southern margin of the Asian continent, which are characterized by prolonged duration and northward propagation of stress, resulting in continuous compression in the CAOB. It is evident that these distant tectonic events do not strictly correspond in time to the rapid exhumation events with strong spatial-temporal variations in the Tianshan. We suggest that this discrepancy is closely related to the complex composition, preexisting structures, and highly heterogeneous rheological properties of the lithosphere in the Tianshan [24, 72]. In general, the continuous northward movement of the terranes along the southern margin of Asia has provided a driving force for the reactivation of the Tianshan. The uneven transmission of stress through clockwise rotation of the Tarim block to the Tianshan [72-74] leads to continuous compression of the Tianshan during the Mesozoic-Cenozoic, which served as the primary deformation domain that absorbed a significant portion of the crustal deformation. Furthermore, the extremely complex preexisting structures, uneven stress compression, as well as the strong lateral and vertical heterogeneous rheological properties of the lithosphere resulted in a strong and varied spatial-temporal exhumation among different tectonic units in the Tianshan.
In this study, we use the apatite (U-Th)/He method and thermal history modeling to investigate the near-surface exhumation process of Harlik Mountain. This work provides new chronological evidence for whether the eastern Tianshan experienced significant exhumation during the Cenozoic era. Based on our investigations, the main conclusions are as follows:
The apatite (U-Th)/He ages obtained from the Harlik Mountain reveal three distinct episodes of rapid exhumation that occurred in the early Cretaceous, late Cretaceous-Paleogene, and middle Eocene eras. These enhanced exhumation events exhibited widespread comparability across the entire Tianshan.
Apatite (U-Th)/He data records more recent and shallower information compared with apatite fission track data. The Harlik Mountain experienced intense exhumation during the early Cenozoic, which can be attributed to the compression caused by the ongoing northward movement of the Indian plate.
The strongly varied spatial-temporal exhumation during the Mesozoic-Cenozoic in the east Tianshan was influenced by both far-field tectonic events along the southern margin of Asia and the rheological properties of the Tianshan lithosphere.
We are profoundly grateful to the editor and two anonymous reviewers; their constructive comments and conscientious suggestions greatly improved this work. We also extend our gratitude to Prof. Wen Chen and Dr. Jingbo Sun for their assistance during fieldwork and apatite (U-Th)/He analysis. This work is supported by the National Natural Science Foundation of China (No.42241161 and 41873063), the Geological survey on surface processes and earth system evolution (No. DD20221644), and the China Postdoctoral Science Foundation (2021M703196).
Conflicts of Interest
The authors declare that they have no conflict of interest.
Data supporting the conclusions of this study are available from the text and the corresponding author upon reasonable request.