The East Kunlun Orogenic Belt (E-KOB) stands out as one of the most prominent basin-mountain geomorphic features in the northern interior of the Tibetan Plateau. It records a series of accretion-collision events from the Mesozoic to the Cenozoic. In particular, with the uplifting of the Tibetan Plateau, the E-KOB experienced intracontinental deformation and exhumation in the Cenozoic. Clarifying the exhumation history of the E-KOB is crucial to define the growth time and mechanism of the Tibetan Plateau. In this study, we apply detrital zircon fission-track (ZFT) and apatite fission-track (AFT) analyses on modern river sands in order to constrain the regional exhumation history of the eastern E-KOB. Four peak ages have been identified and interpreted as results of rapid exhumation correlated with intracontinental deformation. Two older peak ages at 144.7–141.0 and 114.6–82.1 Ma are in good accordance with the collision time of the north-south Lhasa-Qiangtang Block along the Bangong-Nujiang suture zone and the subsequent progressive deformation stage toward the north. Peak age at 60.9–45.3 Ma is coeval with the initial timing of the India-Asia collision. The youngest peak age at 25.1–18.3 Ma matches well with the extensive outward and upward growth of the Tibetan Plateau during the Oligocene to Miocene time. The Cretaceous and early Cenozoic rapid exhumations suggest that the E-KOB has been involved in the intracontinental deformation induced by collisions of the Lhasa-Qiangtang and India-Asia from the south. It implies that the northern Tibetan Plateau likely has been elevated or was a structural high before the Eocene. In addition, some of our detrital samples show a younger ZFT peak age than the AFT peak age. We attributed this data bias to the contribution of hydrodynamic sorting and/or lithological difference. The combination of ZFT and AFT dating has advantages in eliminating interfering age signals in detrital thermochronology.

The rising of the Tibetan Plateau is one of the focal issues in continental tectonics. A fundamental question is to determine when and how the Tibetan Plateau has developed through geological time. However, it is widely admitted that the uplift of the Tibetan Plateau is mainly a consequence of the India-Asia collision since the Eocene [1-6]. A growing body of evidence shows that significant crustal thicken and high topography have existed prior to the India-Asia collision [7-11], for example, the Lhasa-Qiangtang collision in the early Cretaceous may have elevated the interior of the Tibetan Plateau [7, 12] and induced some shortening deformation toward the north [13, 14].

The East Kunlun Orogenic Belt (E-KOB), located in the northern interior of the Tibetan Plateau (Figure 1), is a long-term subduction-accretionary orogen between the Qaidam and Qiangtang Blocks caused by the closure of the Paleo-Tethyan Ocean in the early Mesozoic [15]. It, therefore, was believed to be easily reactivated by multistages of intracontinental deformation induced by the amalgamation of various southern continental fragments that jointed Eurasia from the late Mesozoic to Cenozoic, such as the Lhasa-Qiangtang and the India-Asia collisions [13, 14, 16-20]. The reactivation of preexisting old structures in the E-KOB resulted in mountain building and basin filling which are prior to or coeval with the Tibetan Plateau formation. Thus, the E-KOB provides an ideal setting to investigate the intracontinental tectonic activity and detect the growth time and mechanism of the Tibetan Plateau, especially since it archives some information about the early stages of the India-Asia collision-related deformation.

Detrital thermochronology (e.g., detrital fission-track dating) is an effective method for revealing the tectonic uplift and exhumation processes of orogenic belts [21-23]. The detrital samples are taken from modern river sands, which contain full-scale thermal history information of the bedrock from summits to valleys [24-26]. When thicker supracrustal rock uplifts through the partial anneal zone within a short period, the relatively rapid exhumation rate results in peak ages in the grain-age distribution of samples [27, 28]. Analyzing minerals like zircon and apatite in modern river sand samples allows us to construct a grain-age distribution and recognize peak ages for each catchment sample [29, 30]. The peak age and percentage of the grain-age distribution obtained from statistical analysis can serve as indicators for the timing of rapid exhumation events in the source area.

In this study, new detrital zircon fission-track (ZFT) and apatite fission-track (AFT) dating for modern river sands are performed and integrated with published bedrock data to calculate cooling rates and constrain the timing of rapid exhumation in the eastern range of E-KOB. Four major exhumation phases are recognized with varying cooling rates. We argue that those rapid exhumations are due to the intracontinental deformation induced by the collision between the Lhasa-Qiangtang and the India-Asia, respectively. Furthermore, we analyze the data bias that shows younger ZFT peak ages than AFT in samples which may be ignored by using a single method of detrital dating.

2.1. Regional Geology

Stretching east-west more than 1200 km between the Qiangtang and Qaidam Blocks, the E-KOB constitutes the geographic and geological unit of the northern Tibetan Plateau (Figure 1). Its evolution comprises subduction-accretion-collision events in the Paleozoic and Mesozoic, recording the closure of the Paleo-Tethyan Ocean [15]. The E-KOB can be simply divided into the North Qimantagh Belt (NQB), the Central East Kunlun Belt (CKB), the Southeast Kunlun Belt (SKB), and the Songpan-Ganzi Terrane by those boundary faults or ophiolitic mélange zones, such as the North Qimantagh Fault, Central Kunlun Fault, and Kunlun Fault from north to south [15]. The NQB is traditionally subdivided into the southern Qaidam Block, and it mainly comprises the Paleoproterozoic high-grade metamorphic basement rocks and overlying discontinuous cover succession of Paleozoic to Mesozoic volcano-sedimentary rocks [31]. The CKB comprises the Precambrian metamorphic basement rocks and voluminous Paleozoic and Triassic granitoids, which cover rocks consisting of the Upper Paleozoic clastic-carbonate and volcano-sedimentary rocks and Triassic lavas interbedded with clastic rocks [15]. The SKB is composed of a few Precambrian metamorphic fragments, Paleozoic-Triassic plutons, and Paleozoic-Triassic sedimentary and volcanic associations [32]. The Songpan-Ganzi Terrane has been overthrust by the mélanges and the South Kunlun Belt. It is broadly covered by 5–15 km thick Triassic turbidites [33]. These different complexes/tectonic slices above are highly deformed and juxtaposed against one another by faults, and the Triassic strata unconformably cover the pre-Mesozoic rocks. Jurassic to Cretaceous strata are absent in the E-KOB, and early Cretaceous ductile deformation is reported along preexist boundary faults [16]. The Paleozoic-Mesozoic structures were reactivated during the Cenozoic. Active faults, like the Kunlun Fault with sinistral strike-slip kinematics, were traced from the E-KOB in the west to the Qinling Orogen in the east [17, 34]. Cenozoic pull-apart basins/intermont basins line these faults [35, 36]. The Qaidam Basin in the north front of the E-KOB started to deposit since the early Cenozoic as well [18, 37, 38].

2.2. River/Drainage System

The eastern E-KOB contains peak elevations of ~5000 m and surface elevations of ~3000 m (Figure 2). Greater than 17,000 km2 drainage area of the Xiangride river covers a substantial portion of the eastern E-KOB and comprises three major secondary tributaries: the Tuosuo river, the Hongshui river, and the Kakete river (Figure 2). These rivers flow through different tectonic units and deeply incise the escarpment of the E-KOB with a depth of >2 km. The Tuosuo river originates from the SKB and drains through the Donggi Cona lake (~260 km2) flowing out into the Hongshui river along the Kunlun Fault from east to west. The Hongshui river flows along the Kunlun Fault from west to east in the SKB and drains into the Xiangride river from south to north in the CKB. The Kakete river flows through the Central Kunlun Fault in the CKB and drains into the Xiangride river from southeast to northwest. The outcropping rocks in the drainage area have great differences (Figure 3). Precambrian metamorphic rocks and late Paleozoic to Triassic granitoids are mainly exposed in the downstream. While Permian accretionary mélange and Triassic neritic facies rock with few metamorphic basement rocks and Paleozoic-Triassic plutons are exposed in the upstream.

2.3. Bedrock Thermochronology Data from the Eastern E-KOB

Published bedrock thermochronology data from the eastern E-KOB are summarized in Figure 2. Samples from previous studies are mainly collected from the late Paleozoic to Triassic granitoids and the Precambrian basement outcropped along the river valley. Hornblende and mica 40Ar/39Ar dating for those basements and granitoids close to the Central Kunlun Fault indicate a rapid cooling event resulting from late Triassic emplacement and metamorphism [39]. 40Ar/39Ar, Rb/Sr, and ZFT studies have documented a late Jurassic-early Cretaceous rapid exhumation probably related to ductile deformation along the fault zone [16, 40-43]. AFT and (U-Th)/He dating are used to constrain the timing of Cenozoic reactivation and exhumation of the E-KOB [44-46]. Most of the age results trace the involvement of the E-KOB into the northward growth of the Tibetan Plateau at ca. 30–15 Ma [19, 47-50], while a small amount of data predates the uplift of the E-KOB to the Eocene [46, 51, 52].

We collected four sand samples from modern Xiangride river sediments of the E-KOB: one sample (HSX46) was collected from a site near the entry point of the Xiangride river into the Qaidam Basin; the remaining three samples were obtained from the main tributaries, including the Tuosuo river (HSX04), Hongshui river (HSX41), and Kakete river (HSX42; Figure 2). We aimed to collect ~5 kg of medium- to coarse-grained sand from several locations within ~30 m of each other along the modern river bank [53]. Detrital samples not only reflect the geochronological age of the source rock but also allow for efficient statistical sampling over large areas. Discrete grain-age components are concentrated and known as peak ages by statistical. Peaks of the grain-age distributions are truly representative mirror ages of bedrock in the source regions, which is a more adequate time-window to investigate orogenic processes [29, 30]. Sample locations are shown in Figure 2.

Our attention is focused on ZFT and AFT dating, reflecting the last steps of the exhumation process from 3 to 8 km depth to the surface. Zircon and apatite minerals are concentrated by standard mineral separation procedures, including electromagnetic and heavy liquid techniques, followed by hand-picking for further refinement. We prepare four zircon mounts for each sample. The polished mounts are etched in a KOH:NaOH eutectic solution (in a molar ratio: 1:1) at 227℃–230℃ for either 12, 16, 20, or 24 hours to reveal fossil tracks [54]. The apatite grains are mounted in epoxy resin, then ground, polished, and finally etched by 5.5 N HNO3 for 20 seconds at 21℃ [55]. Zircon and apatite mounts are then covered with a uranium-free muscovite external detector and irradiated at the nuclear reactor facility of Oregon State University in Corvallis, Oregon, USA. Neutron fluence is monitored by using the RM541 (ZFT) and IRMM-540R (AFT) uranium glass. The neutron irradiation produces induced fission tracks, which can be revealed in the external detectors through etching with 40% HF (Hydrofluoric Acid) at room temperature for 30 minutes. Based on the age standards of Fish Canyon zircon and Durango apatite, the fission-track age is determined using the zeta approach with values of 109.0 ± 4.3 (1σ) and 297.1 ± 15.7 (1σ) [56]. Tracks are counted using an automated stage and Zeiss AxioImager microscope at 625× magnification in transmitted light at the thermochronology laboratory at the Northwest University, Xi’an, China.

Here, we report new 822 detrital ZFT ages and 553 detrital AFT ages from four modern river sand samples. Analyzed samples appear as a mixture of different age components and show a large span in ZFT and AFT grain ages (Figure 4). Kernel density estimation is used to find statistically significant and appropriately measured age populations. Based on the mixture algorithm techniques of Galbraith and Green [57], fission-track grain-age distributions were decomposed into discrete main grain-age components as peak ages. Kernel density estimation and peak ages with errors (1σ) of each sample are calculated from the pooled population ages using Density Plotter [58]. In Figure 4, we exhibit the ZFT and AFT data from modern river sands in the E-KOB and display the radial plots, probability density, kernel-density estimate curves, and histograms for each age population. We provide peak-fitting results as peak ages (P1–P4, Table 1) for each age population with a 1σ error depicting the width of the population, expressed as a percentage of the population size relative to the total amount of grains measured. The χ2 test is observed in all samples to be <0.1, confirming that the detrital grains originate from different source rocks.

The exhumation age signals from the E-KOB are revealed by our ZFT and AFT dating of modern river detrital samples (Figure 3). The Tuosuo river catchment (Sample HSX04) dominantly covers Permian-Triassic shallow marine strata and a part of Paleozoic-Triassic granitoids in the SKB. There exhibits three ZFT peak ages at 26.2 ± 8.7 (0.8%, P1), 60.9 ± 8.9 (10.4%, P2), and 102.6 ± 5.9 Ma (88.8%, P3); and three AFT peak ages at 34.6 ± 4.5 (4.3%, P1), 75.1 ± 5.8 (36.8%, P2), and 123.4 ± 7.8 Ma (58.8%, P3). The catchment of Hongshui river (Sample HSX41) covers Permian accretionary mélange and Triassic neritic facies rock in the SKB, as well as a portion granitoids near the CKB. ZFT dating yields three peak ages at 20.8 ± 4.2 (0.6%, P1), 52.4 ± 5.7 (10.2%, P2), and 103.2 ± 5.1 Ma (87.9%, P3); while AFT peak ages are at 25.4 ± 2.7 (8.3%, P1), 61.5 ± 4.9 (35.3%, P2), and 108.9 ± 7.2 Ma (56.4%, P3). It is unusual that peak ages of ZFT from these two samples are younger than that of AFT, which is abnormal according to the closure temperature theory (as explained in the data analysis section). The catchment of Kakete river (Sample HSX42) covers granitoids and metamorphic basement rocks near the CKB, and four peak ages of ZFT and AFT are observed. Two younger ZFT peak ages are 25.1 ± 6.1 (1.0%, P1) and 56.5 ± 7.3 Ma (8.6%, P2); two older ZFT peak ages are 112.0 ± 18.0 (64.0%, P3) and 143.0 ± 24.0 Ma (26.0%, P4). Similarly, AFT dating yields two younger peak ages of 18.3 ± 3.0 (2.0%, P1) and 45.3 ± 6.5 Ma (7.9%, P2) and two older populations with peak ages of 89.8 ± 7.2 (54.9%, P3) and 141.0 ± 12.0 Ma (36.0%, P4). Sample HSX46 is collected in downstream of the Xiangride river, which brings together all the tributaries. Three dominant age populations are observed including ZFT peak ages at 60.9 ± 5.5 (19.7%, P2), 114.6 ± 7.5 (44.9%, P3), and 144.7 ± 6.7 Ma (35.3%, P4) and AFT peak ages at 55.6 ± 8.7 (16.0%, P2), 82.1 ± 7.3 (49.0%, P3), and 143.0 ± 11 Ma (35.0%, P4). There is no peak signal P1, probably because some age signals localized in the fault zones are likely to be smoothed out when all signals are observed together. Taken together, the majority of signals (>80%) are concentrated in the early Cretaceous, while only a small proportion of signals are distributed among the Paleocene–Eocene (10%–20%) and Oligocene–Miocene (<10%). These four peak ages imply major cooling events affecting the E-KOB during the late Mesozoic to Cenozoic.

5.1. Detrital Data Bias Analysis

Detrital ZFT peak ages of samples HXS04 and HSX41 are younger than that of AFT ages (Table 1). This data bias can potentially be introduced into detrital datasets by natural or artificial processes, such as hydraulic sorting [59], mineral fertility [60], intrinsic zircon character [61], and artificial laboratory treatment and analysis [62]. Even though intrinsic sources of zircon bias, such as α-damage and U concentration, may influence the closure temperature system of ZFT [63], it is unlikely to make a closure temperature as low as AFT. At the same time, the artificial bias is almost eliminated by using the zeta approach [64]. The geological constraints are crucial to the detrital bias in the natural environment [65, 66]. Thus, we attribute our data bias to natural factors including hydraulic sorting and mineral fertility impact as below.

Lake sorting is inevitable in our sample of the Tuosuo river (HSX04) because detrital samples are taken from the outfall of the Donggi Cona lake. When the Tuosuo river flows through the Donggi Cona lake, coarse- to medium-grained minerals from a distant source are deposited at the bottom of the lake, and only some fine-grain minerals flow out of it. There are more apatite and relatively fewer zircons due to their different densities (zircon 4.65 and apatite 3.20 g/cm3 [59]). Hence, the majority of distal age signals may be lost in ZFT but partially retained in the AFT. Another reason for this bias is the fertility difference between zircon and apatite caused by lithological differences. As shown in Figure 3, abundant Triassic limestone and Permian accretionary mélange are exposed in the eastern and southern catchments of the Tuosuo river. These rocks contain apatite but negligible zircon minerals, resulting in the preservation of cooling signals in the apatite. Meanwhile, Precambrian metamorphic rocks and late Paleozoic–Triassic granitoids are distributed in the northern drainage, which contributes the vast majority of zircon and apatite in the modern river sediments despite the area limited. The single-grain zircon we collected is likely to originate from granitoids and metamorphic rocks in the northern Donggi Cona lake; however, these single-grain zircons likely provide a localized record of cooling signals. Taken together, the combined effects of lake hydraulic sorting and mineral fertility make older detrital ZFT ages no record or lack, which explains why ZFT peak ages show enormous bias in the Tuosuo river.

Even if there is no large-scale lake sorting, the detrital bias is still observed in the sample of Hongshui river (HSX41) caused by lithological variation. In general, the units mainly consisting of metamorphic and plutonic rocks have higher zircon and apatite fertility than those largely consisting of sedimentary rocks [65, 66]. As lithology shown in Figure 3, the Permian accretionary mélange and Triassic neritic facies rocks are extensively developed in upstream, which contain few apatite and no zircon minerals. On the contrary, the exposed source rocks in downstream are mostly the Precambrian metamorphic rocks and late Paleozoic to Triassic granitoids that contain rich fertility in zircon and apatite. Obviously, zircon and apatite fertility values are systematically higher in the downstream than the upstream. For sediment transport in the drainage of Hongshui river (HSX41), detrital minerals are carried from lower fertility (SKB) to higher fertility (CKB). Based on the age-grain distribution of fission-track analyses, the peak ages will result in a deviation toward higher fertility [66], probably causing the younger ZFT peak ages than AFT. However, the peak ages of 108.9–103.2 Ma from the Hongshui river are consistent within the range of the permitted errors.

Taken together, new detrital datasets demonstrate that hydraulic sorting and mineral fertility are significant sources of detrital FT bias. Based on the aforementioned factors, neither HSX04 nor HSX41 can represent the exhumation history of the E-KOB. Thus, we define the rapid exhumation stages and quantify the cooling rate mainly based on peak ages from samples HSX42 and HSX46.

5.2. Rapid Exhumation and Cooling Rates

Rocks are brought to the earth’s surface by exhumation through the partial annealing zone of the ZFT and AFT [67], successively crossing their closure temperature T at a cooling age t (AFT: 110℃ ± 10℃ [68]; ZFT: 240℃ ± 20℃ [69]). Four detrital FT peak ages for modern river sands represent four rapid cooling phases in the E-KOB: 144.7–141.0, 114.6–82.1, 60.9–45.3, and 25.1–18.3 Ma, respectively. Here, we calculate four phases of the exhumation/cooling rates by using the peak ages and closure temperature of the ZFT and AFT and collect the previous bedrock thermochronology data from river drainages to plot the T–t paths and AFT age-distribution histogram (Figure 5).

In the catchment of Kakete river (Sample 42, Figure 5(a)), almost unaffected by potential bias, the analysis yields four phases of exhumation/cooling by T–t paths. The first rapid cooling phase exhibits a cooling rate of ~65.0℃/Myr and an exhumation rate of 2.16 mm/yr (assuming a geothermal gradient of 30℃ ± 5℃/km) from 143.0 to 141.0 Ma. Ages between ~112.0 and 89.8 Ma indicate the relatively decreased cooling rate of ~5.9℃/Myr (exhumation rate of ~0.19 mm/yr). These ages (~97 Ma) are reported by previous AFT in the low elevation. The third stage shows an exhumation rate of ~0.38 mm/yr with cooling ~11.6℃/Myr in Paleocene (56.5-45.3 Ma), and the youngest cooling phase is observed with cooling ~19.1℃/Myr (exhumation ~0.63 mm/yr) from late Oligocene to early Miocene (25.1-18.3 Ma).

Sample 46 from the Xiangride river exhibits consistent peak ages between ZFT and AFT. It might be affected by detrital bias from the upstream. The rapid cooling stages are calculated and treated as indicator only (Figure 5(b)). The fastest cooling phase is limited at 144.7–143.0 Ma with a cooling rate of ~76.5℃/Myr (exhumation ~2.55 mm/yr). The second rapid cooling episode is discovered with a cooling rate at ~4.0℃/Myr and exhumation of ~0.13 mm/yr between 114.6 and 82.1 Ma. The Cenozoic rapid exhumation phase is recorded with exhumation rate of ~0.81 mm/yr and a cooling rate of ~24.5℃/Myr between 60.9 and 55.6 Ma. There is no peak signal P1, probably due to the exposure of partially reset ages localized in a small area. Age signals are likely to be smoothed out when all signals are observed together.

As described above, four peak ages in the age populations represent correlated rapid exhumation stages of the E-KOB. They include two older stages of cooling at 144.7–141.0 (cooling rate of ~76.5℃/Myr–65.0℃/Myr) and 114.7–82.1 Ma (cooling ~5.9℃/Myr–4.0℃/Myr) and two younger stages of Cenozoic cooling which are at 60.9–45.3 Ma (cooling rate of ~24.5℃/Myr–11.6℃/Myr) and at 25.1–18.3 Ma (cooling ~19.1℃/Myr).

6.1. Late Mesozoic Exhumation Prior to the India-Asia Collision

In the northern Tibetan Plateau including the E-KOB, the vast majority of thermochronology studies focused on dating their Cenozoic tectonics in order to test the growth time and mechanism of the Tibetan Plateau [44-47]. The Jurassic to Cretaceous was considered as a tectonic stagnation period, and related thermal activities were sporadically reported and rarely discussed, though an increasing number of thermochronology data indicate the existence of the late Mesozoic deformation [14, 41]. Our detrital FT datasets captured two earlier rapid exhumation stages at 144.7–141.0 and 114.6–82.1 Ma (Table 1). We propose that these rapid exhumations represent two phases of intracontinental deformation in the E-KOB induced by the collision and progressive convergence between the Lhasa and Qiangtang Block along the Bangong-Nujiang suture zone during the late Jurassic-Cretaceous [7, 70-72].

The first rapid exhumation occurred at 144.7–141.0 Ma with an extremely fast cooling rate up to ~76.5℃/Myr–65.0℃/Myr. Previous thermochronological and geological research supports the late Jurassic to early Cretaceous fast exhumation in the E-KOB and northern Tibet. For example, 40Ar-39Ar and Rb-Sr dating on the metamorphism basement and Triassic granitoids recorded fast cooling from ca. 140 to 120 Ma in the central E-KOB [16, 40, 41]. Based on ZFT dating, Chen et al. [73, 74] and Yuan et al. [43] reported a rapid cooling event at ~147–133 Ma, representing a mineralizing tectonic event in the Harizha-Halongxiuma mining area of the eastern E-KOB. This early Cretaceous rapid cooling is also widely recorded in the Qimantagh area, the Altyn Tagh Orogen, and Qilian Orogen in the northern Tibetan Plateau [14, 75, 76] (Figure 6). The ~140 Ma fast exhumation stage was synchronous with the initial collision between the Lhasa Block and Qiangtang Block along the Bangong-Nujiang suture zone in the late Jurassic-early Cretaceous [7, 70-72]. Thus, we propose that this rapid exhumation in the E-KOB was a result of intracontinental deformation triggered by the far-field effects of the initial collision between the Lhasa and Qiangtang Blocks. Although, the related early Cretaceous sediments are rarely found in the Songpan-Ganzi Terrane and southern Qaidam Basin on both sides of the E-KOB, Vincent and Allen [13] reported that the foreland basin north far to the northern Qilian Orogen accumulated abundant of early Cretaceous sediments. We infer that the E-KOB and Qaidam Basin have uplifted as a highland or structural high in geomorphology during the late Jurassic-early Cretaceous.

The second stage of the exhumation event took place in 114.6–82.1 Ma with a relatively decreased cooling rate of ~5.9℃/Myr–4.0℃/Myr. Combined with a peak age at 108.9–102.6 Ma along the Kunlun Fault, this phase of cooling is probably related to localized fault activity in the early Cretaceous. Published thermochronology data reported this cooling event mainly localized along the fault zones [16, 40, 41, 43-46]. For example, Arnaud et al. [40] and Liu et al. [41] recorded significant thermal activity from ca. 120 to 100 Ma (center at ca. 100 Ma) along the Xidatan Fault-west segment of the Kunlun Fault, using 40Ar-39Ar and Rb-Sr dating. They related the cooling to regional ductile deformation and rapid exhumation. The ongoing convergence between the Lhasa and Qiangtang Blocks could have invoked the preexist old structures of the E-KOB [7, 12-14, 40].

The late Mesozoic rapid exhumation in the E-KOB traces the intracontinental deformation in the northern interior of the Tibetan Plateau. Evidence indicates that large area of southern Asia experienced crustal shortening and thickening before the India-Asia collision (Figure 6), including the Lhasa and Qiangtang Blocks in central Tibet [7, 12, 77, 78], the northern margin of the Tibetan Plateau [13, 14, 79, 80], and the Longmen Shan region along the eastern margin [81, 82]. Accordingly, these substantial crustal shortening probably have given rise to the crustal thickening and surface uplifting in Tibet prior to the India-Asia collision.

The Cretaceous shortening deformation in the E-KOB is still not well documented so far, possibly due in large part to the absence of Jurassic-Cretaceous strata and the strong Cenozoic tectonic activity overprinting the Cretaceous deformation [18]. More detailed work on structural analysis needs to be performed in the future. However, our detrital FT dating establishes a first-order correlation of rapid rock exhumation to the intracontinental shortening in the E-KOB during the Cretaceous.

6.2. Two Stages of Cenozoic Exhumation and Rise of the Tibetan Plateau

Two younger age signals at 60.9–45.3 and 25.1–18.3 Ma are recorded by detrital FT datasets (Table 1), which are closely related to the uplift of the Tibetan Plateau during the Cenozoic. This is consistent with previous studies which suggested that the India-Asia collision was divided into two periods (~50 and ~25 Ma) [1-6].

The age signal of 60.9–45.3 Ma showed the initial response collision stage of the E-KOB in the early Cenozoic (cooling rate ~24.5℃/Myr–11.6℃/Myr). Previous similar signals are reported rarely in the E-KOB but mostly in the Qilian Orogen and northern Qaidam Basin [20, 37]. However, an increasing number of thermochronology evidence reveals the same rapid exhumation of E-KOB in this period [46, 51, 52]. The 40Ar/39Ar fault gouge, AFT, and (U-Th)/He dating suggest that north-south oriented crustal shortening in the E-KOB initiated between 65 and 50 Ma and ceased by 43 Ma [46]. Wang et al. [51, 52] indicate that far-field impacts of the initial India-Asia collision resulted in the formation of the highest relief in the E-KOB at ~40–35 Ma. What is more, a growing body of evidence suggests that large-scale tectonic exhumation occurred throughout the northern and eastern Tibetan Plateau within 10 Myr of the initial India-Asia collision (Figure 6), such as the Altyn Tagh Orogen [14], Qilian Orogen [75, 76], west Qinling Orogen [44], and Longmen Shan region [5, 83, 84]. In addition, the oldest Cenozoic strata (Luluhe formation) and the balanced cross-section of the Qaidam Basin record initial sedimentation and continuous compression since the beginning of Cenozoic (65-44.2 Ma) [85, 86]. Therefore, we suppose that the rapid exhumation of the E-KOB was caused by the initial India-Asia collision during the Paleocene–Eocene, and it established the preliminary basin-mountain geomorphic feature of the northern Tibetan Plateau.

Though the age peak P1 (25.1-18.3 Ma) comprises only a smaller proportion (<10%) with a relatively rapid cooling rate of ~19.1℃/Myr, it is consistent with the localized bedrock thermochronology ages of nearby the fault or low elevation [19, 47]. This timing of rapid exhumation and deformation is keeping with the northward growth and uplift of the Tibetan Plateau since the Oligocene–Miocene. Previous thermochronology research reveals that the rapid cooling event during ~30–20 Ma locally overprinted the Kunlun Fault [16, 40, 41]. The Kunlun Fault was reactivated by thrust and strike-slip movements between 35 and 15 Ma [34, 44, 46] with a strike-slip rate of between 11 and 5 mm/yr [87-89]. Meanwhile, a series of pull-apart basins and pressure rides started forming in the E-KOB [35, 36]. In addition, an increase in sedimentation rate and the Cenozoic tectonic deformation of the Qaidam Basin indicates that the E-KOB suffered a rapid uplift during the 25–16 Ma [86]. The exhumation event during the Oligocene to early Miocene appears to be a widespread phenomenon around the Tibetan Plateau (Figure 6), indicating that large-scale crust shortening deformation of the Tibetan Plateau occurred during this period [46, 47, 90]. These observations, combined with our FT data, imply that the Oligocene to early Miocene is one of the most important periods for northward shortening deformation along the thrust and strike-slip faults in the E-KOB caused by the outward and upward growth of the Tibetan Plateau.

The available detrital ZFT and AFT data on the E-KOB constrain the polyphase exhumation stages at the E-KOB accommodating its tectonic reactivities. The principal results are as follows:

  1. Four peak ages at 144.7–141.0, 114.6–82.1, 60.9–45.3, and 25.1–18.3 Ma are recorded by detrital ZFT and AFT dating on modern river sands, representing four stages of rapid rock exhumation caused by intracontinental deformation in the E-KOB.

  2. The rapid exhumations at 144.7–141.0 and 114.6–82.1 Ma trace the effects of crustal shortening across the northern Tibetan Plateau induced by the collision of the Lhasa and Qiangtang Blocks since the early Cretaceous. The E-KOB may have uplifted as a high land or served as a structure high prior to the India-Asia collision.

  3. The rapid exhumation at 60.9–45.3 Ma dates the early Cenozoic deformation of the E-KOB, reflecting the E-KOB has been involved in the growth of the Tibetan Plateau synchronously with the India-Asia collision onset. The youngest rapid exhumation at 25.1–18.3 Ma indicates the outward and upward growth of the Tibetan Plateau during the Oligocene to Miocene.

  4. Dating of ZFT and AFT together can effectively recognize the data bias caused by lithological difference and/or hydrodynamic sorting in detrital thermochronology analysis.

The sampling information and compiled zircon and apatite fission track 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.

The authors declare that they have no conflict of interest.

Authors acknowledge two anonymous reviewers for their constructive suggestions to prepare the manuscript. This work was supported by the National Natural Science Foundation of China (Grants: 42172257, 42272266, and 41672199), National Key R&D Program of China (Grant: 2022YFE0203800), and MOST Special Fund from the State Key Laboratory of Continental Dynamics, Northwest University.

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