Northward Growth of the West Kunlun Mountains: Insight From the Age–Elevation Relationship of New Apatite Fission Track Data

The Cenozoic collision between India and Asia promoted the widespread uplift of the Tibetan Plateau, with significant deformation documented in the Pamir Plateau and West Kunlun Mountains. Low-temperature thermochronology and basin provenance analysis have revealed three episodes of rapid deformation and uplift in the Pamir–West Kunlun Mountains during the Cenozoic. However, there is very little low-temperature thermochronology age–elevation relationship (AER) data on fast exhumation events in this area—especially in the West Kunlun Mountains— leading to uncertainty surrounding how these events propagated within and around the mountain range. In this study, we produced an elevation profile across granite located south of Kudi, Xijiang Province, China, to reveal its exhumation history. Apatite fission track AER data show that a rapid exhumation event occurred at ∼26 Ma in the southern West Kunlun Mountains. When combined with published data, we interpret that the initial uplift events related to the India–Asia collision began in the central Pamir, southern West Kunlun, and northern West Kunlun regions during the Late Eocene, Oligocene, and Middle Miocene periods, respectively. Therefore, the Cenozoic northward growth process occurred from south to north around West Kunlun.


INTRODUCTION
The Cenozoic collision between India and Asia formed the Tibetan Plateau (TP, Figure 1A), a series of intracontinental orogenic belts (Tapponnier et al., 2001;Royden et al., 2008), and induced regional climatic change (Raymo and Ruddiman, 1992). The onset ages of the India-Asia collision in the western Himalaya syntaxis (WHS), central Tibet and eastern Himalayan syntaxis (EHS) remain debated (Leech et al., 2005;Hu et al., 2016). The WHS is dominated by the Pamir Plateau, which is comprised of the northern Pamir, central Pamir, and southern Pamir ( Figure 1B) domains. The northern Pamir has Asian affinity, whereas the central and southern Pamir regions have Cimmerian Gondwanan affinity (Burtman and Molnar, 1993;Li et al., 2020). The West Kunlun Mountains (WK), situated in the southeastern Pamir, are divided into the southern West Kunlun (SWK) and northern West Kunlun (NWK) regions. These three parts of the Pamir extend to the southeast and correspond to the SWK, Songpan-Ganzi Terrane, and Qiangtang Terrane ( Figure 1B; Robinson et al., 2004;Cowgill, 2010). Together, these domains are known as the Pamir-WK. The Cenozoic and pre-Cenozoic geological evolution of the Pamir-WK has been a topic of significant scientific focus over the past 20 years (Robinson et al., 2004;Cowgill, 2010;Li et al., 2020;Cai et al., 2021).
Low-temperature thermochronology is widely used to constrain the cooling histories of plateaus and mountain ranges (Reiners et al., 2005;Guenthner et al., 2013). Moreover, age-elevation relationship (AER) data are extremely valuable for defining the denudation and relief history in a locality, especially when an AER has a transform point (Braun, 2002;Valla et al., 2010). Previous low-temperature thermochronological studies in the study region have shown that the Pamir-WK experienced three rapid exhumation events during the Cenozoic at ∼50-40, ∼25-16, and between ∼10 Ma and the present day (Robinson et al., 2004;Robinson et al., 2007;Amidon and Hynek, 2010;Sobel et al., 2011;Wang et al., 2011;Carrapa et al., 2014;Li et al., 2019). Other studies have shown that a provenance change or increase in sediment flux occurred at ∼40-30, ∼26, and ∼15 Ma (Jiang and Li, 2014;Tang et al., 2015;Blayney et al., 2016;Sun et al., 2016;Wang et al., 2019;Zhang et al., 2019;Sun et al., 2020;Sakuma et al., 2021;Wang et al., 2021). The Paleogene paleotopography in the WK may therefore represent an ancient land surface . Despite this work, it is uncertain how these tectonic events propagated within and around the mountain range. This uncertainty is exacerbated by the paucity of low-temperature thermochronology AER data that constrain the uplift and exhumation rates in this area, especially in the WK. In this study, we collected samples of granite profiles situated south of Kudi, Xijiang Province, China, and used the AER of the apatite fission track (AFT) data to reveal the exhumation history of the SWK. These data record a transformation point within the AER, which means an abrupt tectonic transition in the SWK. We then integrate these results with those of previous studies to interpret the Cenozoic growth history of the WK.

GEOLOGICAL SETTING
The Pamir-WK is broadly salient and has been displaced northward over the Tarim-Tajik basins along the Main Pamir thrust system and Darvaz fault ( Figure 1B). The Pamir-WK can be divided into four tectonic terranes: the NWK, the northern Pamir-SWK, the central Pamir-Songpan-Ganzi, and the Southern Pamir-Karakorum-Hidu Kush-Qiangtang, which are separated by the Kudi suture, Tanymas-Karakax suture, and Rushan-Pshart-Jinsha suture, respectively ( Figure 1B). The former two terranes have affinity to Asia, whereas the latter two terranes previously split away from the Gondwanan (Burtman and Molnar, 1993;Li et al., 2020). The southwestern Tien Shan was situated north of the Pamir-WK prior to the shortening of strata during the Cenozoic, with estimates of shortening between both domains ranging between ∼50 and 100 km (Chen et al., 2018;Li et al., 2020) and ∼300 km (Burtman and Molnar, 1993).
The WK is bordered by the Tarim Basin to the north, the Pamir Plateau to the northwest, and the Songpan-Ganzi terrane to the south. It is a mountainous region ∼700 km long, ∼100-130 km wide, and contains peaks up to ∼7,600 m high. The Tarim Basin has an elevation of <1,500 m, while the frontal orogenic fold belt between the Tarim Basin and the Tiklik fault has an elevation of 1,500-2,500 m. To the south of the Tiklik fault, the WK itself has an elevation of 3,000-6,000 m. The WK can be divided into the northern and southern parts by the Kudi suture ( Figure 1B), which formed due to the closure of the Proto-Tethys Ocean (Matte et al., 1996). The WK initially formed during the Paleozoic-Mesozoic and experienced a complex strike-slip to compressive evolution (Yin and Harrison, 2000;Arnaud et al., 2003;Laborde et al., 2019). The WK reached its current elevation due to reactivation of pre-existing faults during the Cenozoic India-Asia collision (Matte et al., 1996;Yin and Harrison, 2000;Jiang et al., 2013;Laborde et al., 2019).
To the north of the WK, the Tarim Basin contains extensive Mesozoic-Cenozoic deposits and has an average thickness of ∼1,200 m. Today, the Tarim Basin is an endorheic basin surrounded by mountain ranges: the Pamir-WK to the southwest, the Altyn Tagh Mountains to the southeast, and the Tien Shan to the north. These ranges previously provided abundant sediment that infilled the Tarim Basin, with the Pamir-WK having been the main sedimentary provenance for the SW Tarim Basin during the Mesozoic-Cenozoic, especially during the Cenozoic (Jiang and Li, 2014;. The Mesozoic strata in the SW Tarim Basin include Jurassic (Shalitashi, Kansu, Yangye and Kuzigongsu Formations) and Cretaceous (Kezilesu and Yengisar Groups) sediments (Sobel, 1999), and the Cenozoic strata include the Kashi Group (Aertashi, Qimugen, Kalatar, Ulagen and Bashibulake Formations), Wuqia Group (Keziluoyi, Anjuan and Pakabulake Formations), Artux Formation and Xiyu Formation (Jia et al., 2004;Liu et al., 2017a).

SAMPLING AND METHODS
Geological mapping and sampling were performed over several years in the WK in collaboration with the China Geological Survey. Survey routes were situated near the G219 highway, which runs from Xinjiang to Tibet, and granite samples were collected from southern Kudi. From north to south, this area consists of the NWK, SWK, and Songpan-Ganzi ( Figure 2). The NWK includes Carboniferous and Mesoproterozoic rocks of the Changcheng System and Silurian and Ordovician granites. The SWK contains Mesoproterozoic rocks of the Changcheng System, alongside granite of various ages, and other deformed rocks. The Songpan-Ganzi terrane only shows exposed Silurian rocks. A topographic profile from the Songpan-Ganzi to the NWK shows that these three terranes attain maximum elevations exceeding 4,500, 3,000, and 2,700 m ( Figure 3) Triassic (T γδβJ ) intrusion between its highest (4,814 m) and lowest (3,540 m) points (Figures 2, 3 and Table 1).
All granite samples were crushed and pulverized, and constituent minerals were concentrated by using standard magnetic and density separation techniques. Individual apatite grains were handpicked from the concentrates and used for fission track dating via the external detector method, following the procedures documented in our previous publication (Liu et al., 2017b). Initially, apatite grains were mounted and polished to expose the centers of as many grains as possible and were then immersed in 5.5 N HNO 3 for 20 s at 21°C to reveal natural fission tracks. Fission track sample mounts, age standards (Fish Canyon and Durango) and IRMM540R dosimeter samples were irradiated together in a thermalized reactor located at Oregon State University, United States, using a thermal neutron fluency of 1.0 × 10 16 n cm −2 . U-free muscovite external detectors were etched in 40% HF for 40 min at 20°C to reveal their induced fission tracks. Fission tracks were counted on a Zeiss microscope at the Chinese Academy of Geological Sciences, China, using an Autoscan system (produced in FIGURE 2 | quartz diorite; SξγβS, Silurian medium-grained biotite moyite; SηγβS, Silurian medium-grained biotite monzogranite; TηcJ, Triassic medium-grained monzogranite; TγδβJ, Triassic medium-grained, porphyritic biotite granodiorite; TηγβS, Triassic medium-grained, porphyritic biotite monzogranite; TηδoJ, Triassic medium-to fine-grained quartz monzodiorite; Tδo-ηγS, Triassic mixed magmatic granite; P αμ+ξ , altered andesitic porphyrite and dacite; P ss , sandstone intercalated with sericitic and silty slate; C mb , bioclastic dolomitic limestone and marble; S 1 W sl , silty slate and phyllite; TS a , segment A in sandstone of the Triassic Sailiyakedaban Group; and TS b , segment A in conglomerate of the Triassic Sailiyakedaban Group. Songpan-Ganzi Terrane: S 1 W b , Formation B of the Silurian Wenquan Group, which contains gray, medium-bedded, and fine-grained arkosic sandstone intercalated with silty, Didymites-bearing slate; and S 1 W c , Formation C of the Silurian Wenquan Group, which contains thickly bedded, moderate-to fine-grained quartz sandstone and fine-grained arkosic sandstone intercalated with some silt and slate.   The zeta (ζ) value is 272.78 ± 15.99. Abbreviations are as follows: N, number of individual grains dated; Rho-S, spontaneous track density (×10 5 tracks cm −2 ); Ns, number of spontaneous tracks counted; Rho-I, induced track density in external detector (muscovite) (×10 5 tracks cm −2 ); Ni, number of induced tracks counted; Rho-D, induced track density in external detector adjacent to dosimeter glass (×10 5 tracks cm −2 ); Nd, number of tracks used to determine Rho-D; P(χ 2 ) (%), Chi-square probability (Galbraith, 1984); Mean Dpar, arithmetic mean diameter of fission-track etch figures parallel to the crystallographic c-axis.
Frontiers in Earth Science | www.frontiersin.org December 2021 | Volume 9 | Article 784812 Australia) in manual mode, set to a magnification of ×1,000. The zeta (ζ) value of 272.78 ± 15.99 was obtained using Durango and Fish Canyon apatite standards (Hurford and Green, 1983;Naeser and Cebula, 1985). More than 20 grains were chosen from each sample. All ages were determined to be within an error of 1σ using the computer code "Trackkey" (Dunkl, 2002).

RESULTS
Between 22 and 24 grains were analyzed for AFT in each sample, and the results are listed in Table 1 and shown in Figure 4. For this measurement, the value of Zeta (ζ) (Galbraith, 1990) for the Kudi section. The plots were drawn using Radialplotter (Vermeesch, 2009). The color code displays uranium concentrations (left) or chlorine concentrations (right) for each sample. Frontiers in Earth Science | www.frontiersin.org December 2021 | Volume 9 | Article 784812 Figure 5 shows AER data using the central ages. The ages of samples with the three highest elevations are near ∼26 Ma (KDW52, KDW55 and KDW60), while the ages of the two low-elevation samples are near ∼15-16 Ma (KDW61 and KDW62). A clear transition point in AER data can be seen in Figure 5 at ∼26 Ma.
Based on source-to-sink theory, sedimentary provenance analysis in a basin can effectively decipher the evolutionary history of adjacent ranges (Fedo et al., 2003;Najman, 2006;Kimbrough et al., 2015;Koshnaw et al., 2018;Coutts et al., 2019;Nordsvan et al., 2020;Resentini et al., 2020). Basin analysis has been applied to several mountain fronts in the Pamir-WK region, which has constrained the evolutionary history of its adjacent ranges. The dominance of Cenozoic northward-directed paleocurrents in the SW Tarim Basin indicates that the basin sediments were mainly derived from its southern margin (Sobel, 1999;Bershaw et al., 2012;Cao et al., 2014;Zhang et al., 2019;. Interestingly, an ∼45 Ma peak in detrital zircon U/Pb ages is documented only in the central Pamir and first appears in Eocene strata in the SW Tarim and Tajik basins (Blayney et al., 2016;Sun et al., 2016;Wang et al., 2019;Zhang et al., 2019;Sun et al., 2020;Wang et al., 2021). Previous documents indicated that this magmatic activity represented the Late Eocene rapid uplift in the central Pamir region, based on late Eocene detrital apatite fission track ages and regional tectonic movements Zhang et al., 2019;Wang et al., 2021). Moreover, detrital zircons with an age peak of ∼45 Ma are absent in sedimentary rocks that formed at ∼26 Ma in the Oyitag and after ∼26.5 Ma in the Aertashi sections of the SW Tarim Basin (Figure 6; Blayney et al., 2016;Sun et al., 2016), indicating that the influx of sediments from the central Pamir region was hindered by the growth of the mountains to the northern side of the basin. Based on our new data, we interpret that the SWK experienced rapid uplift at ∼26 Ma, which restricted sediment flux into its northern basins.
Both the low-temperature thermochronology performed herein and previous basin sedimentary provenance analyses confirm that the SWK experienced rapid uplift at ∼26 Ma, which restricted sediments sourced from central Pamir region from being transported into its northern basins. Paleomagnetic data show an abrupt increase in mean magnetic susceptibility at ∼26 Ma in the Baxbulak section of the Alai Valley, although this has previously been interpreted as recording tectonic activity in the southwestern Tien Shan (Tang et al., 2015). Nonetheless, this rapid exhumation event (∼25-16 Ma) is also documented in the northern Pamir region (Amidon and Hynek, 2010), indicating that this event may record regional-scale movement on the northwestern Tibetan Plateau.

Cenozoic Northward Growth of West Kunlun Mountains
The WK is divided into southern and northern domains, with the former extending to the northern Pamir region ( Figure 1B). Sedimentary provenance analysis indicates that the WK and northern Pamir region had certain paleoelevations prior to the Cenozoic (Cao et al., 2015;Blayney et al., 2016;Li et al., 2020), which supports rapid uplift in the northern Pamir region during the late Paleocene-early Eocene (∼50-40 Ma) (Amidon and Hynek, 2010;Carrapa et al., 2015;Chen et al., 2018). As the sedimentary provenance in the SW Tarim and Tajik basins did not change between the Late Cretaceous and the Early Eocene, the topography of the nearby ranges must also not have changed during this time. The first quasi-synchronous rapid uplift of the central Pamir region occurred in the Late Eocene (40-30 Ma), and provided a new sediment flux into the SW Tarim and Tajik basins (Blayney et al., 2016;Wang et al., 2019;Zhang et al., 2019;Sun et al., 2020;Wang et al., 2021), although this occurred at the earliest time of ∼47 Ma in the Oytag section of the Tarim Basin (Sun et al., 2016). The second regional-scale rapid uplift in the SWK (this study) and northern Pamir region (Amidon and Hynek, 2010) occurred during the Oligocene; this lasted at least ∼9 Ma (from 25 to 16 Ma) in the northern Pamir region, but there are no geochronological data to constrain its duration in the SWK. The >1,000-km-long Karakorum Fault developed during this Oligocene uplift event (Lacassin et al., 2004;Li et al., 2007;Valli et al., 2007;Valli et al., 2008) and subsequently played a vital role in the evolution of the WHS (Cowgill, 2010). Finally, a third episode of rapid uplift began in Frontiers in Earth Science | www.frontiersin.org December 2021 | Volume 9 | Article 784812 FIGURE 6 | U-Pb detrital zircon data shown as normalized kernel density plots for rocks from the Oytag (Sun et al., 2016) and Aertashi (Blayney et al., 2016) sections in the SW Tarim Basin. The shaded bar represents the populations diagnostic of the central Pamir provenance with a detrital zircon U-Pb peak age of ∼45 Ma. The ages in the brackets after the sample ID represent the sedimentary ages. N indicates the number of measured detrital zircon grains.
Frontiers in Earth Science | www.frontiersin.org December 2021 | Volume 9 | Article 784812 the WK and northern Pamir during the Middle Miocene, with this event continuing to the present day and shaping the current landscape (Cao et al., 2013;Thiede et al., 2013;Cao et al., 2015;Blayney et al., 2019).
Our new data combined with published results show that at least three rapid uplift events occurred in the Pamir-WK during the Cenozoic (Figure 7), but how did each of these events influence the tectonic evolution of the WK? The ∼45 Ma peak of detrital zircon U/Pb ages indicates that the central Pamir region experienced the first uplift event, although no equivalent ages are recognized in the WK, despite its northward extension (i.e., the northern Pamir) recording this event. Moreover, if the WK had experienced this uplift at this time, the sediments from the central Pamir region would not have been deposited in the SW Tarim Basin. Therefore, we believe that the first uplift event only took place south of the WK. Our AFT data confirm that the second uplift event occurred in the SWK, which restricted sediments derived from the central Pamir region from entering the SW Tarim Basin. Prior to this study, no Oligocene thermochronological data were reported from the NWK, which implied that this second major uplift event did not affect the NWK. Furthermore, while thermochronological data confirm that the third uplift event occurred in the NWK (Sobel and Dumitru, 1997;Sobel et al., 2011;Chapman et al., 2017) and northern Pamir region (Sobel et al., 2011;Thiede et al., 2013;Figures 1, 7), no previous data have shown that this event affected the SWK. Our data from the Kudi profile indicate that the phase of relatively slow exhumation lasted from ∼26 to ∼15 Ma ( Figure 5). Furthermore, as the SWK currently has higher elevation than the NWK (Figure 7), the southern domain likely experienced a prolonged period of uplift than the northern domain. Therefore, we suggest that the SWK possibly also experienced the third documented uplift event. Based on these data, three Cenozoic uplift events should first occur to the south of the WK, SWK, and NWK during the Eocene, Oligocene, and Mid-Miocene (Figure 7). Therefore, northward growth should take place around the WK, possibly caused by the stepwise growth (Tapponnier et al., 2001) or continuous deformation  of the Tibetan Plateau.
Sedimentary provenance analysis in the SW Tarim Basin also supports the interpreted northward growth of the WK. Detrital zircons with age peaks of ∼45 Ma are absent in sedimentary rocks that formed at ∼26-15 and ∼14-11 Ma in the Aertashi section of the SW Tarim Basin ( Figure 6; Blayney et al., 2016), which indicates that sediments from the central Pamir region could not freely enter its northern basin. The reason for this limited movement is most likely due to being restricted by uplift of the WK. These events are shown in Figure 8 as a tectonic model for the WK. During the Late Cretaceous, the central Pamir-Songpan-Ganzi region had not experienced uplift, whereas the WK had a greater elevation, allowing the paleorivers (e.g., the Pishan River, the Yarkang River and others) to erode headward toward the Songpan-Ganzi terrane. During the Eocene, the Paratethys Ocean transgressed into and retreated from the Tarim Basin, and the central Pamir region experienced initial Cenozoic uplift, which allowed the paleorivers (e.g., the Pishan River and the Yarkang River) to supply new detrital zircon grains with ∼45 Ma peak ages. During the Oligocene, the SWK experienced abrupt uplift, which restricted the sedimentary flux from the central Pamir region

DATA AVAILABILITY STATEMENT
The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author.

AUTHOR CONTRIBUTIONS
This manuscript was designed and written by DL. This manuscript was supported by funds from DL, HL, and JP. The apatite fission track data were measured by YW. Other co-authors attended the field and figure work, including CG, MB, YZ, PW, FL, and SW.

ACKNOWLEDGMENTS
We sincerely thank editors and two reviewers for their constructive comments. We thank GeoEditing and American Journal Experts for the entire paper review and writing improvement. . During the Oligocene, the SWK experienced abrupt uplift, which was higher than the NWK and Central Pamir-Songpan-Ganzai terrane (C). From the Mid-Miocene to present, the NWK and SWK both experienced abrupt uplift (D).