Multispectral satellite remote sensing images are very useful to interpret the structural features and sedimentary units in the Pamir–Tian Shan convergence zone with an arid and semi-arid climate condition (Fu et al., 2010). Moreover, the duplication and the absence of strata and weathering make it difficult to identify the spatial distribution of stratigraphic units in the field (Heermance et al., 2007). In this study, multiple sources of satellite remote sensing data were used, which include the ASTER Visible and Near Infrared (VNIR, 15-m ground resolution) and short-wave infrared data (30-m ground resolution) and Sentinel-2 VNIR data (10-m ground resolution). For multispectral remote sensing data, an appropriate band selection of remote sensing data plays a key role in image enhancement, which is useful for geological interpretation and lithological extraction (Figure 2A). The ASTER false-color composite image (R: band ratio 2/1, G: band 3, and B: band ratio 5/8, Figure 2A) was able to distinguish different lithological units in this region. We also imposed ASTER multispectral image on the DEM data from ALOS World 3D-30 m (AW3D30) to generate 3D perspective images of a typical area (Figure 4A), which is beneficial for the geomorphologic feature analysis of KFTB.

Cosmogenic nuclide burial dating, first proposed by Lal and Arnold (1985), relies on various decay constants for ¹⁰Be and ²⁶Al (Granger and Muzikar, 2001). The assumption behind the method is that quartz is exposed to the cosmic rays at the Earth’s surface for a period of time, acquiring certain amounts of ¹⁰Be and ²⁶Al, respectively. When quartz is buried at a sufficient depth, the production of cosmogenic nuclides ceases, and decay becomes the dominant process affecting the concentrations of ¹⁰Be and ²⁶Al (Granger and Muzikar, 2001; Granger et al., 2013). Cosmogenic nuclide burial data were widely used in interpreting exposed and the burial history of sediments or terrace surfaces (Balco and Shuster, 2009; Jungers and Heimsath, 2016) and dating the age of Late Cenozoic conglomerates (Kong et al., 2011; Thompson-Jobe et al., 2018). Compared with other means, cosmogenic nuclide dating is a direct way to measure samples without relying on existing symbolic age (like fossils and volcanic ash), which provided the numerical age of the samples instead of the correlated age or relative age (McCalpin and Nelson, 1996). ²⁶Al/¹⁰Be burial dating is an advantage for dating Pliocene–Pleistocene clastic sediments that are challenging to be dated by other methods (Balco and Shuster, 2009; Thompson-Jobe et al., 2018). So far, we have selected ²⁶Al/¹⁰Be burial dating method to constrain the geological age of Pliocene–Pleistocene conglomerates, which are exposed along the southern flank of ATA. Four samples were collected from the boundary between Atushi Formation and Xiyu Conglomerate (Table 1; Figure 5B). The sample preparations were processed at the cosmogenic nuclide laboratory of the Institute of Geology and Geophysics, Chinese Academy of Sciences in Beijing. The oxides were mixed with niobium and silver metal powders, and both ¹⁰Be and ²⁶Al concentrations were measured by AMS at PRIME Lab at Purdue University. The concentrations of ¹⁰Be/⁹Be were normalized to the NIST standard SRM4325. Half-lives of 1.39 Ma (Korschinek et al., 2010) and 0.71 Ma and high-latitude, sea-level production rates of 4.6 and 31.1 atoms/g/year were used for ¹⁰Be and ²⁶Al in the age calculation, respectively.

Based on the geological and geomorphological interpretation of satellite images and field investigations, our work depicted the geomorphologic expression and stratigraphic units of ATA in detail (Figure 1).

ATA is a growing fold with two asymmetric limbs (steep and narrow in the northern limb and gentle in the southern limb) and a tight hinge zone, with an axial surface dip to the south (Figure 2C). The surface expression of ATA extends to the Old Atushi town in the west and is interacted with MTA in the east, exceeding about 60 km along the axis of the fold (Figure 1C). A water gap formed in the middle part of ATA by the erosion of Boguzi River, and it provides an excellent geological section to observe and document the structural deformation and sedimentary features of ATA (Figure 2). Notably, our results indicated that a tight anticline developed at the east bank of Boguzi River along the southern flank of ATA (Figure 2).

The ATA has deformed Miocene Pakabulake Formation to Xiyu Conglomerate stratigraphic units (Figure 2). As shown in the ASTER false-color composite image (Figure 2A), the color patterns of Pakabulake Formation, Atushi Formation, and Xiyu Conglomerate are displayed as brownish-red, yellowish-green, and purplish-gray, respectively. The field investigations show that the Pakabulake Formation, exposed in the core of the anticline, is consisting of brownish-red argillite, sandy argillite, siltstone, and gypsum (Figures 2B,C). The Atushi Formation is mainly composed of thick pale grayish-yellow sandstone, interbedded with thin brownish-red mudstone and containing a dark gray conglomerate at its top layer. The Xiyu Conglomerate is composed of a massive gray conglomerate containing grayish-yellow thin-bedded sandstone, siltstone layers, or lens (Figure 3E). The contact between Atushi Formation and Xiyu Conglomerate is displayed as a low-angular unconformity (Figure 3D).

The ATF is a south-dipping thrust fault extending along the north flank of ATA, which is affected by a northward thrusting deformation (Figure 1B; Yang et al., 2009; Fu et al., 2010). The fault scarps are developed on Late Quaternary proluvial and alluvial deposits of the Boguzi River along the ATF, showing that ATA is currently still active (Figures 2B,C).

Four fine-grained sandstone and siltstone samples were collected from the upper layer of Atushi Formation to the lower layer of the Xiyu Conglomerate at the southern flank of ATA (XY-01 to XY-04, Figures 5B,C). The sedimentary features of the upper layer of Atushi Formation and Xiyu Conglomerate are quite different. The upper layer of Atushi Formation is characterized by grayish yellow sandstone interbedded with gray conglomerate, which is consisting of small-sized pebbles (1–3 cm) with fair sorting and subrounded psephicity. The Xiyu Formation is composed of dark gray, gray massive conglomerate, containing grayish-yellow thin-bedded sandstone, and siltstone layers or lens. The Xiyu Conglomerate is consisting of large- to medium-sized pebbles (5–10 cm) with bad sorting and subangular psephicity (Figures 5D,E). XY-01 and XY-02 were collected from the grayish-yellow sandstone and siltstone layers. XY-03 was sampled from the sandstone layer interbedded with conglomerate, where a low-angular unconformity has developed. These three samples were collected from the upper layer of Atushi Formation. XY-04 was sampled from the lenticular sandstone layer of Xiyu Conglomerate, near the boundary between the Atushi and Xiyu formations. To minimize production during incision and exhumation after the initial deposition, we collected samples from the sites that were well shielded by at least 30 m of rock. Sample information and ¹⁰Be and ²⁶Al concentrations together with data on burial ages are shown in Table 1.

The ²⁶Al/¹⁰Be ratios of XY-01, XY-02, XY-03, and XY-04, are 2.64 ± 0.38, 2.58 ± 0.27, 2.84 ± 0.26, and 3.02 ± 0.30, respectively. Although the ²⁶Al/¹⁰Be ratio has a statistical difference from the model ²⁶Al/¹⁰Be ratio of 6.76 ± 0.88 at the Earth’s surface, the ²⁶Al/¹⁰Be values of the four buried samples are consistent with each other. For buried samples that are within the range of muon production, the ¹⁰Be and ²⁶Al concentrations will be functions of the initial concentrations at the time of burial, the burial time, and the burial depth as a function of time. Using the ²⁶Al/¹⁰Be ratios, the burial ages range from 1.94 to 1.67 Ma. Our burial ages are almost equal to 1.7 Ma as obtained from the Boguzi River stratigraphic section (Sobel et al., 2006), but these are different from ∼1.9 Ma for Xiyu Conglomerate as obtained by Chen et al. (2002) using magnetostratigraphy.

Previous studies constrained the initial timing of deformation by dating the geologic age of the growth strata but rarely paid attention to the relationship between the growth mechanism of the fold and the sedimentary features of the growth strata (Sobel et al., 2006; Sun et al., 2009; Sun and Zhang, 2009). From the perspective of structural analysis, the growth mechanism of the thrust-related fold is mainly divided into two categories: hinge migration and hinge rotation (Shaw and Suppe, 1994; John and David, 1997; Hubert-Ferrari et al., 2007; Li et al., 2015; Li et al., 2018). Among the folds deformed by hinge migration, the geological age of the growth strata cannot simply represent the initial deformation timing of the fold. In this case, the hinge widening and dip angle might change over the hinge area in the pre-growth strata (Suppe et al., 1992; Salvini and Storti, 2002). On the other hand, the growth of the hinge rotation fold is through limb rotation, with the constant limb length and variable limb dip angle. The measurement of the onset age of the growth strata enabled us to constrain the initial deforming age of the limb rotation fold (Poblet et al., 1997).

Previous studies have already suggested that ATA belongs to the fold of the limb rotation growth mechanism based on the analyses of the geometry and dip panel (Chen et al., 2002), interpretation of seismic profile (Qu et al., 2001; Gao et al., 2013), and the measurement of the fold scarps (Li et al., 2018). According to our field investigation and mapping, we confirmed that the growth mechanism of ATA belongs to limb rotation (Figure 4C). The initial deposition of the growth strata could represent the beginning of the tectonic deformation of ATA.

Our results also show that the deformed late Cenozoic strata of Miocene Pakabulake Formation, Pliocene Atushi Formation, and Xiyu Conglomerate developed from the core to the southern limb of ATA (Figure 2C). Particularly, we carefully document the subtle change of bed dipping angles, bed width, and sedimentary features around the boundary of the growth and the pre-growth strata. Based on these criteria, we identified the sign layer that can represent the initial deposition of the growth strata (Figure 5). The sign layer consists of alluvial conglomerate interbedding with sandstone at the top layer of Atushi Formation. The low-angular unconformity starting from the dip angles of the sign layer are abruptly varying from 56° to 45° (Figures 2C, 5B). Meanwhile, the width of the strata under the sign layer is constant (W₄, W₅, and W₆ in Figure 5C), but upward of the sign layer, the width of the strata is distinctly becoming thinner toward the crest, indicating that they accumulated during deformation (W₁, W₂, and W₃ in Figure 5C).

The geological age of Pliocene–Pleistocene deposits in the KFTB is still controversial in previous studies (Chen et al., 2002; Sobel et al., 2006; Heermance et al., 2008). By using magnetostratigraphy dating data, Chen et al. (2002) suggested that the boundary ages between the Atushi Formation and Xiyu Conglomerate on the east bank of the Boguzi River and Ganhangou sections are ∼1.9 and ∼1.0 Ma, respectively. Consequently, they firstly proposed that the initial deformation time of ATA might start at ∼1.4 and ∼1.2 Ma in the above-mentioned two sections. However, the magnetostratigraphic sampling section located on the east bank of Boguzi River, where a secondary-scale anticline developed, led to the duplication of the Pliocene–Pleistocene strata (Figures 2, 4A). The duplication of sedimentary stratigraphic units might bring confusion to the magnetochronological age data which relies on the continuous deposition of sedimentary strata. Moreover, the fine sandstone and argillite layers suitable for paleomagnetic study are very limited in the boundary between Atushi Formation and Xiyu Conglomerate. Thus, the timing of Xiyu Conglomerate age from a magnetochronological study remains uncertain, and accurate age dating of the growth strata and detailed research of structural deformation are urgently needed to constrain the Pliocene–Pleistocene tectonic deformation of ATA.

In this study, the ²⁶Al/¹⁰Be burial age of the growth strata indicates that the initial deformation time of ATA can be traced to 1.79 ± 0.16 Ma, which is almost equal to 1.7 Ma (Sobel et al., 2006) and is different from ∼1.4 Ma (Chen et al., 2002). Meanwhile, we proposed that the initial deposit time of Xiyu Conglomerate is about 1.67 ± 0.18 Ma, which is different from ∼1.9 Ma in the eastern bank of Boguzi River section and ∼1.0 Ma at the Ganhangou section. This phenomenon may be caused by the diachronous deposition of Xiyu Conglomerate (Chen et al., 2000; Chen et al., 2007a; Chen et al., 2007b; Heermance et al., 2007; Qiao et al., 2016). The initial tectonic deformation age of ATA is similar to the middle Kalayuergun anticline (2.6–1.7 Ma) located at the western end of Baicheng-Kuqa foreland fold-and-thrust belt (Lv et al., 2019).

The previous studies have constrained the initial deformation time of west Atushi anticline (ca.3.8 Ma; Thompson-Jobe et al., 2018) and the Ganhangou section of Atushi anticline (1.2 Ma, Chen et al., 2002), respectively. Combined with the initial deformation age of the west bank of the Boguzi River section of Atushi anticline (1.79 Ma in this study), it can be concluded that the ages of the growth strata eastward along the Atushi anticline were younger. Therefore, the fold might have propagated eastward during the Pliocene to Quaternary, and the rate of lateral propagation is 18.6–24.3 km/Ma.

Figure 6 shows that the initial deformation ages of thrust–fold belts are becoming younger from MZF to the north flank of KA in front of southwestern Tian Shan (Chen et al., 2002; Sobel et al., 2006; Chen et al., 2007a; Heermance et al., 2008; Jia et al., 2015), which indicated the southward multi-stage propagation processes in the KFTB. Meanwhile, recent studies also proposed that the initial deformation ages of the thrust–fold belts in front of northeastern Pamir are also becoming younger, which suggests a northward propagation process in northeastern Pamir (Figure 6; Chen et al., 2005, Chen et al., 2007a; Thompson-Jobe et al., 2018). The boundary of two opposite-verging thrust–fold belts along the PTCZ is likely distributed along KA (Figure 6; Fu et al., 2010).