The Meso-Cenozoic tectonic activities of the Central Asian Orogenic Belt (CAOB) played an important role in controlling the present-day topography of Central Asia. The Altai orogenic belt is a key component in the southern CAOB; so far, there is still a lack of sufficient constraints on the time and mechanism of its tectonic reactivation since the Mesozoic. In this contribution, we present new zircon and apatite (U-Th)/He and apatite fission track thermochronological data from granitoid samples in the Habahe area, western Altai orogenic belt. Therein zircon (U-Th)/He ages range from ~230 to ~238 Ma, apatite fission track central ages are ~140–157 Ma, and apatite (U-Th)/He ages vary from ~134 to ~149 Ma. Based on the associated thermal history modeling results, the Habahe area underwent a moderate cooling during the Late Triassic to Middle Jurassic (~230–170 Ma) with a cooling rate of ~0.8–1.1℃/Ma and a subsequent moderate to slightly rapid cooling stage during the Middle Jurassic to Early Cretaceous (170–130 Ma) with a cooling rate of ~1.5–2.3℃/Ma. We propose that this prolonged cooling stage occurred under a long-lasting contractional tectonism in the western Altai throughout the early Mesozoic, which was produced by multiplate convergence in East Asia during this period, mainly including the consumption of the Mongol-Okhotsk Ocean in the northeast and the Meso-Tethys Ocean in the south. The region experienced rather limited Late Cretaceous-Cenozoic cooling and exhumation due to insufficient reactivation and weak surficial erosion.
The Central Asian Orogenic Belt (CAOB), also named the Altaids, incorporates the mountainous zone between the Siberian craton to the north and the North China-Tarim cratons to the south (Figure 1(a)). The CAOB was formed by complicated subduction-accretion processes involving a number of continental terranes, island arcs, seamounts, and accretionary wedges, recorded a long history of accretionary orogenesis from the Neoproterozoic to Paleozoic [1-3]. Affected by the far-field effects of Meso-Cenozoic continental collision, parts of the CAOB formed a typical intracontinental basin-mountain system and are ideal places to study the deformation occurring in the continental interior [4-8].
In the past decades, a number of thermochronological studies have been conducted on several key areas of the CAOB, mainly including Siberia Altai-Sayan [9-11], Junggar [12-14], and Tianshan [14-21]. These studies attributed the Mesozoic rock cooling to be brought by far-field effects of distant major tectonic events such as the Qiangtang-Lhasa terrane and Kohistan arc collisions/accretions to the southern edge of the Eurasian continent or the Mongolian-Okhotsk orogeny [11, 14, 20, 22, 23]. These events triggered faults reactivation and intensive intraplate deformation, and the growth of this orogenic belt in its southern part formed the structural architecture that the present-day mountain ranges are built upon.
The Altai orogenic belt, located between the Siberian craton and the Kazakhstan-Junggar plate, is an important component of the CAOB (Figure 1(b)). Altai orogenic belt is geographically divided into the Chinese, Siberia, and Mongolia Altai by national borders. Available thermochronological data indicate that the Mongolia segment of Altai mainly experienced the Early Jurassic and Cretaceous cooling [24, 25], while Siberia Altai experienced three cooling periods, that is, Late Triassic-Early Jurassic, Late Jurassic-Early Cretaceous, and the late Cenozoic . There are also different views on the dynamic mechanisms of these cooling episodes [11, 26, 27].
The Altai region in China is located between the Siberia craton and the Junggar terrane, serving as a key area to verify the process and mechanism of Meso-Cenozoic deformation in Central Asia. Previous thermochronological studies on the thermo-tectonic evolution and structural architecture are rather limited. Yuan et al.  reported a series of apatite fission track data from the Kanas area of the Altai belt (Figure 1(c)); however, a relatively single analysis method cannot provide a comprehensive and systematic recovery of the thermal evolution history of the region. As to another dataset that was more recently provided by Pullen et al.  (Figure 1(c)), it largely contains apatite (U-Th)/He (AHe) ages, and the constraints were put more on the very shallow crust (<1–2 km). Consequently, the cooling process of the western Altai basement is still not very clear based on a relatively limited dataset, calling for further investigations.
Low-temperature thermochronological techniques, including zircon and apatite fission track and (U-Th)/He dating, measure the timing and rates at which rocks approach the surface and cool as a result of exhumation [30, 31]. These methods have been widely applied to quantify the thermotectonic history and constrain landscape evolution of many orogenic belts in the past few decades [12, 15, 19]. In this contribution, we conducted zircon and apatite (U-Th)/He and apatite fission track thermochronology on four representative granitoids collected from the westernmost segment of the Chinese Altai orogenic belt. Using well-constrained inverse thermal history modeling results, we first present the Mesozoic reactivation events occurring in the Habahe area, western Chinese Altai. Together with the published thermochronological and structural data, we then discuss the intracontinental deformation and its related geodynamics in the Altai orogenic belt and the adjacent areas since the Mesozoic.
2. Geological Background
The Altai orogenic belt underwent a long-term process of accretional and collisional orogenesis from the Neoproterozoic to Paleozoic. It consists of a series of fault-bounded tectonostratigraphic units, based on magmatic activity, metamorphism, stratigraphy, and deformation pattern [32-35]. Several large-scale ~E-W trending faults (e.g., Irtysh, Tesbahan, Kurti faults, etc.) developed across the Altai orogenic belt and have experienced multiple-stage deformation in the late Paleozoic [36-38] and have played important roles in controlling regional magmatism and mineralization [4, 39]. Paleozoic to early Mesozoic magmatic activities have been frequent in the study area, the ages of the intrusive rocks from the middle Altai to the Iytrsh tectonic belt gradually become younger from north to south , displaying various crystallization ages of the early Paleozoic (~470–360 Ma), late Paleozoic (~355–270 Ma), and early Mesozoic (~245–190 Ma) [34, 40-43], Zircon U-Pb dating yielded an age of 313 ± 5 Ma for the studied Tuokesalei intrusion . The Habahe area is located in the western Chinese Altai (Figures 1(c) and 2), Paleozoic metasediments are well exposed in the region, including the (early Paleozoic) Habahe Group, (Early Devonian) Kangbutiebao Formation, and (Middle Devonian) Altay Formation.
During the early Paleozoic, the Kazakhstan-Junggar plate subducted beneath the southern Siberian margin, leading to voluminous continental magmatic activity along this orogenic belt [4, 32, 45]. In the end of the late Paleozoic, the convergence between the Siberia craton and Kazakhstan plate resulted in the development of large-scale strike-slip faults in and around the belt, and their deformation was accompanied by intrusion of massive synkinematic plutons and dykes [35, 46-48]. Following the Permian closure of the Paleo-Asian Ocean that represents the final accretion and amalgamation of the western CAOB, ongoing tectonic events at the plate margins of the Eurasian continent continued to affect the continental interior [22, 49]. Stress that propagated from the plate margins drove reactivation and intracontinental deformation in Central Asia throughout the Mesozoic and Cenozoic; it shaped the topography of the Tianshan and Altai orogenic belts [14, 15, 20-22]. In order to decipher the relationship between plate margin collisions and the intracontinental tectonic response, many studies have focused on understanding the exhumation and uplift history of these mountain ranges, particularly the Tianshan orogenic belt [6, 8, 27, 50, 51]. The distribution of thermochronological data in Central Asia with respect to major Paleozoic structures suggests that Meso-Cenozoic deformation in Central Asia was often controlled by intensive reactivation of the structural architecture inherited from the earlier tectonic evolution of the region (e.g., Paleozoic sutures and shear zones). Typical cases include the Talas-Fergana fault [52, 53], the Atbashi-Inylchek suture zone , the Main Tianshan shear zone , and the Kangguer-Huangshan shear zone .
3. Sampling and Methodology
In this study, four samples were collected from the Tuokesalei intrusion and diorite veins, ~40 km northwest of the Habahe county for apatite fission track, apatite, and zircon (U-Th)/He dating analyses (Figure 2). The Tuokesalei intrusion consists of four small, rounded bodies emplaced in the South Altai (Figure 1(c)), all of them intruded the metamorphosed terrigenous clastic strata of the Altay Formation. They are mainly composed of gray, medium-grained, tonalite (Figures 3(a) and 3(b)), the mineral compositions include plagioclase (50%), amphibole (20%), quartz (15%), and biotite (5%) (Figures 3(c)–3(h)). Plagioclase phenocrysts (0.2, 5 mm) are subhendral, polysynthetically twinned, and there are different degrees of sericitization. Quartz phenocrysts (0.2, 2 mm) are xenomorphic. Muscovite phenocrysts (0.2, 1 mm) are enhedral. Biotite crystals (0.7, 1.5 mm) are also subhedral and commonly contain inclusions of zircon and apatite (Figures 3(c)–3(h)). Mafic microgranular enclaves are common but randomly distributed in the intrusion. It is mainly composed of plagioclase, biotite, hornblende, and other minerals, and a small amount of quartz veins can be observed in the rock mass. Sample descriptions including their locations and lithological data can be found in Table 1.
3.2. Experimental Process
3.2.1. Apatite Fission Track Dating
Apatite fission track dating was performed in the Department of Earth Sciences at the University of Arizona, USA, using the external detector method. First, apatite crystals were handpicked under a binocular microscope, then embedded in epoxy resin, grinded, and polished. After that grains were etched by 5.5 M nitric acid for ~20 seconds at ~21°C to reveal spontaneous fission tracks. Thermal neutron irradiation was carried out at Oregon State University, USA, where neutron injection was monitored using uranium glasses IRMM 540R. Induced tracks in muscovite detector were revealed by etching with 48% HF for 20 minutes after irradiation. Finally, spontaneous and induced fission track densities were counted and calculated using an Olympus BX61 microscope at 1250× magnification with a Kinetek automated grading system. Confined fission track lengths and Dpar values were measured and calibrated using the FTStage software. Central ages were calculated (with 1σ error) using the zeta calibration method IUGS recommended by Hurford and Green . Based on calibrating multiple Durango and Fish Canyon Tuff apatite standards, zeta calibration factor of 351.9 ± 3.8 a·cm2 was obtained [57, 58].
3.2.2. Zircon and Apatite (U-Th)/He Dating
Apatite and zircon (U-Th)/He analyses were performed at the National Institute of Natural Hazards, Ministry of Emergency Management of China. Conventional rock crushing and mineral separation procedures were used to separate apatite and zircon grains from the rocks. Apatite grains with euhedral morphology showing no visible inclusions were selected under a microscope, and only grains that are >70 μm in both length and width were considered to be suitable for (U-Th)/He dating. Grain dimensions were measured from digital photographs for an α ejection correction, the selected particles were then loaded into niobium (zircon) and platinum (apatite) capsules. After that He extraction and measurement of zircon (U-Th)/He (ZHe) and AHe were conducted by a PrismaPLus QME 220 quadrupole mass spectrometer. Determination of U and Th was conducted on a coupled Agilent 7500 quadrupole Inductively coupled plasma-mass spectrometry (ICP-MS). The α ejection correction was applied to each crystal for calculating and correcting the (U-Th)/He ages. Four single-grain aliquots were analyzed for both ZHe and AHe dating for chosen samples. More detailed analytical processes can be found in Shen et al. .
3.3. Thermal History Modeling
Cooling histories of the three western Altai granitoids (ALT 1302, 1303, and 1305) were further derived by using the HeFTy program . The main input data include the apatite fission track spontaneous and induced track densities, length frequent distributions, and kinetic parameters Dpar. Monte Carlo simulation was used in our modeling operation. The starting of the model was set as 210–230℃ for temperature at 260–280 Ma. We use the multikinetic fission track annealing model of Ketcham et al. , and the initial track length was set as 16.3 µm . Available AHe and ZHe data were integrated using the radiation damage accumulation and annealing (RDAAM; ), and the helium diffusion (ZRDAAM; ) models. Since all samples come from outcrops, we constrained the present-day surface temperature at 15 ± 10℃.
4.1. AFT Dating Results
Apatite fission track dating was performed on samples ALT1302, 1303, and 1305, and detailed results are shown in Table 2. For each sample, the central age is calculated based on 20 grains. The P(χ²) values for each sample were all higher than 0.97, indicating a relatively concentrated age of single particles. Their central ages were determined to be 140.1 ± 7.5, 148.8 ± 6.7, and 156.8 ± 9.8 Ma, respectively (Table 2). Mean track lengths (MTLs) of the three samples vary between ~13.6 and ~14.5 μm (Table 2; Figure 4); they are relatively long confined track lengths in consideration of an intracontinental region [65, 66], suggestive of less intensive thermal annealing. The average etches diameter (Dpar) values of the samples range from ~1.91 to ~2.12 μm, which were larger than the Dpar values of the Durango apatite standards (~1.75 μm), indicating relatively stronger resistance to fission track annealing .
4.2. (U-Th)/He Dating Results
Samples ALT1305 and 21ALT01 were also dated by ZHe and AHe methods, and results are shown in Table 3. The two samples yielded quite consistent ZHe single-grain ages, four repeats from sample ALT1305 give a weighted mean ZHe age of 238.0 ± 3.9 Ma and that of 230.1 ± 3.9 Ma was calculated for 21ALT01. While four grains of sample ALT1305 produced a mean AHe age of 148.8 ± 4.2 Ma, and sample 21ALT01 displays mean AHe age of 133.6 ± 2.5 Ma. Comparison between (U-Th)/He and fission track ages within single sample shows that dating methods with higher isotope closure temperature generally display older corresponding ages. And all of them are much younger than the rock formation age (313 ± 5 Ma) , indicating the reliability of the data. It is noted that the obtained (U-Th)/He dataset shows low degree of single-grain age dispersion. The age-eU and grain size plots show no clear positive correlation (Figure 5), indicating that radiation damage did not significantly affect helium diffusion  and the crystal size did not affect (U-Th)/He ages.
4.3. Thermal History Modeling Results
All three samples (ALT1302, 1303, and 1305) in this study yielded good thermal modeling results (GOF (good of fit)>0.7). Inverse thermal history models are detailed in Figure 6 and compiled in Figure 7. It is observed that all the three rocks exhibit similar cooling paths (i.e., the weighted average paths); they entered the ZHe PRZ in the latest Permian and cooled out of the AHe PRZ in the Early Cretaceous, followed by a prolonged slow cooling since the Late Cretaceous (Figures 6 and 7). There three samples all display a more complex three-stage cooling history, and a turning point of cooling rate (to a faster one) in the Middle Jurassic can be observed (Figure 6). The modeling shows a relatively slow cooling between ~230 and 170 Ma with a cooling rate of ~0.8–1.1℃/Ma and a subsequent accelerated cooling stage between 170 and 130 Ma with a cooling rate of 1.5–2.3℃/Ma. For a more accurate description in our further discussion, we define rates of <~0.5℃/Ma, ~0.5–2℃/Ma, and >~2℃/Ma as slow, moderate, and rapid cooling regarding an intracontinental setting, respectively (based on empirical values) [14, 20].
5.1. Long-Lasting Mesozoic Accelerated Cooling
Inverse thermal history modeling results in this study show that the three samples underwent similar cooling processes (i.e., a moderate cooling between ~230 and 170 Ma and a moderate to slightly rapid cooling stage between 170 and 130 Ma). In the adjacent areas, different Early Jurassic (~195–185 Ma) and Cretaceous AFT ages were obtained from the Mongolia Altai [24, 25, 67, 68]. Siberia Altai-Sayan also experienced a rapid cooling phase that occurred in the Late Triassic-Early Jurassic period, as revealed by titanite fission track method [11, 27]. And apatite fission track thermochronology revealed a subsequent Late Jurassic-Cretaceous (~150–72 Ma) cooling pulse [7, 22, 69]. All these results obtained from the Altai orogenic belt and adjacent regions indicate that this belt experienced accelerated cooling during the Mesozoic, but the exhumation process differs spatiotemporally.
Intracontinental deformation in Central Asia was thought to be interrelated with plate-margin processes. Throughout the Meso-Cenozoic, the Eurasian landmass suffered multiphase of compression as a result of the convergence and collision between (micro-) continents and/or island arcs at the southern plate margin. The strains and stresses generated by convergence and collision are transmitted northward to the interior of Eurasia along the weak zone of the lithosphere, these correspond to older sutures and structures inherited from earlier collision and accretion events. In this way, it led to the reactivation and establishment of the intracontinental orogenic belts [70, 71]. Available (low-temperature) thermochronological records of the Altai and Tianshan orogenic belts and surrounding areas documented several enhanced cooling events during the Meso-Cenozoic, which resulted in increased regional denudation, older faults reactivation, and topographic generation [11, 22, 72]. Moreover, processes like slab break-off or postorogenic collapse may also have an impact on the deformation of interior Eurasia. For example, Triassic cooling within the Kyrgyz Tianshan is thought to reflect the Qiangtang-Eurasia collision and/or rifting/subsidence in the West Siberian basin, and the mid-Cretaceous cooling event can be interpreted as an isostatic response to the slab-break-off model [8, 27].
Mesozoic cooling in the Siberia and Mongolia Altai is generally considered to result from the far-field effects generated by coeval collisional events occurring along the Eurasian active margin from the south [9, 73]. One possible triggering factor for the Triassic basement cooling is the accretion/collision of the Qiangtang block with the Kunlun terrane in the southern margin of the Eurasian continent or, alternatively, the distal effects related with the Mongol-Okhotsk orogeny (~160–110 Ma) [74, 75]. During this period, the inner cooling rate of Altai-Sayan in Siberia accelerated, and large-scale fault activation occurred. The associated cooling may reflect a dynamic topographic response to either increased loading on the crust from the growing Mongol-Okhotsk orogen or the slab break-off model.
According to our new inverse modeling results, moderate cooling of the rock mass in the study area (westernmost Chinese Altai) commenced at ~240–230 Ma, generally consistent with the onset of the Qiangtang and Kunlun-Qaidam collision. However, a time-lag usually exists between the onset of distant collision and the time of enhanced basement cooling in the continental interior under a compressional regime due to the fact that several million years’ erosion is usually needed (particularly in places without efficient external drainage system like Central Asia) for significant unroofing and exhumation after mountain uplift . In this regard, the Early-Middle Triassic cooling in the western Chinese Altai is more likely related with late Paleozoic to earliest Mesozoic tectonic movements in the southwestern CAOB.
Late Paleozoic postorogenic tectonic evolution of the southwestern CAOB was characterized by eastward extrusion of orogenic segments along large-scale strike-slip faults [76, 77]. In the Altai orogenic belt, the sinistral Irtysh shear zone was active during the Permian (~283–252 Ma, by both mica 40Ar/39Ar and zircon U-Pb dating) with transpressional deformation as a result of the collision between the Junggar terrane and Chinese Altai orogenic belt [78, 79]. In addition, recent structural studies of the region strengthen the interpretation that sinistral transpression with a large thrust component resulted in widespread exhumation in the Chinese Altai orogenic belt and along the Irtysh shear zone during the early-middle Permian [43, 80, 81]. Considering that the Habahe area is very close to the western segment of the impressive Irtysh shear zone (Figure 1(b)), it is reasonable to propose that the transpressional movement along this fault during the Permian to earliest Mesozoic in a degree affected the deformation and exhumation of the Habahe rock mass.
The Habahe area experienced an accelerated cooling stage in the Middle Jurassic and Early Cretaceous (Figure 7). Whereas a number of scholars related this period of exhumation to the far-field effects of the coeval Mongol-Okhotsk orogeny to the northeast of the Tianshan, Junggar, and Altai [8, 15, 82]. It is suggested that the closure of the Mongol-Okhotsk Ocean in its western part took place during the Triassic. The consumption of the eastern Mongol-Okhotsk Ocean and related Mongol-Okhotsk orogeny may have occurred later in the Jurassic and Early Cretaceous, based on the evidence of significant decrease of magmatism in the late Mesozoic . But the scissors-like closure of the Mongol-Okhotsk Ocean was more likely to have impact on the regions like the Transbaikalia, North Mongolia, and interior North China craton in the east [14, 84, 85].
The collisional process of the peri-Gondwana Qiangtang block to the Eurasian margin more or less came to an end during the Middle to Late Jurassic, and it is observed that there is a turning point of cooling rate (to a faster one) for three samples, ALT1305 in the Middle Jurassic (~180–170 Ma, Figure 7). Recently, He et al. [14, 20] suggested that the Late Jurassic to Early Cretaceous cooling pulses observed in the Tianshan orogenic belt and Junggar terrane could be related to the contemporary subduction of the Meso-Tethys oceanic plate [86, 87]. The closure of the Paleo-Tethys Ocean caused the southern Eurasian margin to jump southward, initiating the subduction of the Meso-Tethys Ocean [72, 88]. Subduction-related magmatism in the northern Lhasa terrane and migration of continental arc magmatism into the South Pamir terrane are evidence to reflect northward directed, low-angle to flat-slab subduction of the Tethyan oceanic lithosphere [89, 90]. A low-angle plate subduction beneath the Eurasian continental lithospheric mantle could have generated considerable crustal shortening within the continental interior . Evidence of fault activity was also found in the area, such as Malkakuri fault, which is a secondary fault of Irtysh fault and a regional deep fault. During the Jurassic to Cretaceous, it experienced several tectonic movements, forming a N-W trending and intense extrusion structure. The locally occurs an inverse “S” shape, and a large scale of thrust faults was developed, accompanied by dextral strike-slip. Along both sides of the fault, there are extrusion fracture or mylonitized zones with a width of hundreds of kilometers. Therefore, the subduction of the Meso-Tethys oceanic plate may have triggered intracontinental deformation of the Jurassic-Early Cretaceous Altai orogenic belt.
In addition to extensive Early Cretaceous exhumation, parts of the Tianshan are also suggested to have experienced continued deformation in response to the Lhasa collision-accretion event to the southern Eurasian margin, potentially triggering further exhumation in the belt [14, 91, 92]. Moreover, previous studies have also identified Late Cretaceous Kohistan-Dras collision as an instigator of exhumation in the central and eastern segments of the Tianshan, resulting in continued deformation in some localized regions [8, 14, 15, 20, 27]. Anyway, mid- to Late Cretaceous accelerated basement cooling is not traced by our data in the western Chinese Altai, this on the other hand demonstrates that the Meso-Cenozoic intracontinental deformation in Central Asia is quite inhomogeneous, and reliable thermochronological data from various key areas are of great importance for regional comparison.
The three samples from the Habahe area experienced three cooling stages during the Mesozoic and exhibited a cooling rate transition at ~180–170 Ma; although the cooling rate attains to rapid one (1.5, 2.3℃/Ma; in intracontinental tectonic setting), their comparable cooling paths are still quite linear (Figure 7). Consequently, it is suggested that the long-lasting (Triassic to earliest Cretaceous) exhumation in the western Altai, as recorded by the Habahe granitoids, was likely to result from long-existing intracontinental deformation since the latest Paleozoic under “episodic” contractional environments, which were produced by various intermittent major tectonic events. It is noteworthy that the adjacent West Junggar experienced a long Mesozoic cooling [12, 93, 94], hence, may have a relatively consistent tectonic history with the Junggar area.
5.2. Cenozoic Slow Exhumation Process
Following the Mesozoic, no thermochronological evidence (both AFT and AHe) for Cenozoic cooling was found in the areas studied in this contribution. The low elevation (<1500 m) of the western Chinese Altai orogenic belt, relative to other ranges in Central Asia, makes this result unsurprising. As shown by the thermal history modeling results, all the three granitoids cooled out of the AHe temperature-sensitive window (~75–50℃; ) in the Cretaceous, with less than 30℃ cooling occurring during the Cenozoic. This means that the exhumation of the region in the Cenozoic is generally less than 1 km. Although evidence for late Cenozoic activity along adjacent structures such as the Junggar fault, Dalabute fault, and the bounding faults of the Altai orogenic belt attests to their recent reactivation [96, 97], modern deformation in generating the uplift and orogenic erosion failed to result in deep exhumation. It is noted that a series of recent studies have argued for less intensive (generally <1.5–1 km) basement exhumation in some high relief and high elevation domains in Central Asia [14, 21, 98, 99], which were previously considered to display late Cenozoic AFT apparent ages [100, 101]. On the other hands, in the past few years, a growing number of thermochronological studies have revealed that the majority of Central Asia (pre-Mesozoic) basement rocks show Mesozoic (low-temperature) thermochronological signals, and Cenozoic rapid and massive exhumation only occurred at very localized regions [14, 20-102, 103,103]. Our results, hence, confirm that the western Chinese Altai is also dominated by Mesozoic accelerated rock cooling showing Cretaceous apatite fission track and (U-Th)/He ages, comparable with those in the central and eastern Chinese Tianshan. And the impact of the India-Asia collision was not identified.
Jepson et al.  recently explored the relationship between climate and erosion in Eurasia and found that there is a certain relationship between available thermochronological ages and associated historical precipitation in the Altai orogenic belt, with relatively low precipitation since the Late Cretaceous. This may also account for limited surface erosion of the region during the Cenozoic. In addition, based on sedimentary records in the Junggar basin, during Late Triassic-Early Jurassic, humidity is thought to have reached a maximum in the Junggar basin as evidenced by the retrogradation of deltaic and fluvial facies and the expansion of deep-water facies [104, 105]. Since the end of Cretaceous, the Junggar basin has been affected by the global climate cooling and the uplift of the Tibetan plateau, and the degree of dryness has been continuously enhanced . This arid climate limited the denudation of Junggar and its adjacent areas to a certain extent in the late Mesozoic and Cenozoic . Therefore, the weak Cenozoic mountain uplift and less strong surficial erosion led to limited Cenozoic basement denudation in the Habahe area of the western Chinese Altai.
This study presents new zircon and apatite (U-Th)/He and apatite fission track results for four Paleozoic tonalite samples from the Habahe area. This multimethod approach allows us to put further constraints on the Meso-Cenozoic thermo-tectonic evolution of the (western) Altai orogenic belt in the southern CAOB. The following conclusions can be derived:
All the low-temperature thermochronological signals (ZHe, AFT, and AHe) acquired in the western Chinese Altai are Mesozoic in age, comparable with the published results in the adjacent areas.
Associated inverse thermal history models reveal that the Habahe area experienced a moderate cooling stage (~0.8–1.1℃/Ma) between ~230 and 170 Ma, and a subsequent moderate to slightly rapid cooling stage (1.5, 2.3℃/Ma) between ~170and 130 Ma, followed by a cooling stagnation until the present.
Mesozoic cooling in the study area may not be related with a single triggering factor. Instead, major tectonic events including the subduction and closure of the Mongol-Okhotsk Ocean in the northeast and the Meso-Tethys Ocean in south resulted in long-term compression in the western Altai orogenic belt during the early Mesozoic. While Cenozoic deformation is rather weak.
We would like to thank the three anonymous reviewers whose comments helped to improve the quality of the manuscript, and associate editor Dr. Feng Cheng for his editorial handling. This study was financially supported by the China Geological Survey (Grant no. DD20230213), the National Natural Science Foundation of China (Nos. 42127801, 41830216, and 41873060), the National Key Research and Development Project (No. 2019YFA0708601), and the Outlay Research Fund of Institute of Geology, Chinese Academy of Geological Sciences (Grant no. J2207). This is a contribution to IGCP 662.
Conflicts of Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
The sampling information and compiled apatite fission track, and zircon and apatite (U-Th)/He data used to support the findings in this study are included within the paper, but detailed counting data and measurements will be available from the corresponding author upon reasonable request.