Abstract

In response to collision and convergence between India and Asia during the Cenozoic, convergence took place between the Pamir and South Tian Shan. Here we present new detrital zircon U-Pb ages coupled with conglomerate clast counting and sedimentary data from the late Cenozoic Wuheshalu section in the convergence zone, to shed light on the convergence process of the Pamir and South Tian Shan. Large Triassic zircon U-Pb age populations in all seven samples suggest that Triassic igneous rocks from the North Pamir were the major source area for the late Cenozoic Wuheshalu section. In the Miocene, large populations of the North Pamir component supports rapid exhumation in the North Pamir and suggest that topography already existed there since the early Miocene. Exhumation of the South Tian Shan was relatively less important in the Miocene and its detritus could only reach a limited area in the foreland area. Gradually increasing sediment loading and convergence of the Pamir and South Tian Shan caused rapid subsidence in the convergence area. Since ca. 6–5.3 Ma, the combination of a major North Pamir component and a minor South Tian Shan component at the Wuheshalu section is consistent with active deformation of the South Tian Shan and the North Pamir. During deposition of the upper Atushi Formation, a larger proportion of North Pamir–derived sediments was deposited in the Wuheshalu section, maybe because faulting and northward propagation of the North Pamir caused northward displacement of the depocenter to north of the Wuheshalu section.

1. INTRODUCTION

In the Cenozoic, collision and subsequent convergence of the Asian and Indian plates (e.g., Molnar and Tapponnier, 1975; Rowley, 1996; DeCelles et al., 2014) caused extensive intracontinental deformation in Central Asia. After westward retreat of the Para-Tethys in the Paleogene (e.g., Tang et al., 1989; Bosboom et al., 2011, 2014), northward indentation, crustal thickening and exhumation took place in the Pamir Plateau, forming an arcuate syntax (e.g., Burtman and Molnar, 1993; Sobel et al., 2013; Rutte et al., 2017) (Fig. 1A), and leading to convergence between the Pamir and South Tian Shan (Fig. 1A). This process resulted in arc-shaped thrust belts of the Pamir superimposed with imbricated thrust belts of the South Tian Shan (Qian et al., 2011) and widespread deposition of up to 10-km-thick late Cenozoic terrestrial strata in the Pamir–South Tian Shan convergence area and the western Tarim Basin (e.g., Sobel and Dumitru, 1997; Wang et al., 1992) (Fig. 2). These sediments preserve important information for understanding the tectonic process of convergence between the Pamir and the South Tian Shan and its sedimentary and climatic influence to western Tarim Basin or even broader regions (e.g., Chen et al., 2010; Bosboom et al., 2011, 2014; Sun et al., 2013, 2015).

In the study area, several detrital zircon provenance studies have been carried out in the foreland areas of the South Tian Shan and Pamir (Bershaw et al., 2012; Yang et al., 2014, 2017; Sun et al., 2016; Clift et al., 2017; Liu, 2017; Liu et al., 2017). In the South Tian Shan foreland, detrital zircons from the late Cenozoic succession in the Ulugqat section (Fig. 1) have four main age groups: 240–320 Ma, 400–540 Ma, 550–1600 Ma, and 1640–2800 Ma (Yang et al., 2014). They are interpreted to be mainly derived from the South Tian Shan with a minor component from recycling of the northern margin of the Tarim Basin due to Cenozoic uplift of the South Tian Shan (Yang et al., 2014). To the southeast, in the Kangsu section (Fig. 1), detrital zircons from widespread Cretaceous and Jurassic sediments have main age peaks of 276 Ma, 445 Ma, and 819 Ma (Fig. 3), indicating that a well-developed drainage in the Middle Jurassic and Early Cretaceous delivered sediment with a provenance from the South Tian Shan and a recycled component from the Tarim Basin (Yang et al., 2017). Detrital zircons from the modern Rivers, which drains the Pamir–South Tian Shan convergence area, have prominent population peaks at 190–360 Ma and 360–510 Ma (Clift et al., 2017; Liu et al., 2017) (Fig. 3). In the foreland of the northeast Pamir, detrital zircons sampled from Cenozoic successions and the modern river at the Bieertuokuoyi section (Liu, 2017; Liu et al., 2017) (Fig. 1) and the Oytag section (Bershaw et al., 2012; Sun et al., 2016; Liu, 2017) (Fig. 1) have similar age distributions, revealing five main age groups: 30–60 Ma, 75–110 Ma, 200–220 Ma, 280–310 Ma, and 400–470 Ma. Supported by N-directed and/or NE-directed paleocurrent direction, the Eocene, the early Cenozoic–Late Cretaceous, and the early Mesozoic–Paleozoic zircons are interpreted to be sourced from the Central Pamir, the South Pamir and the North Pamir, respectively (Bershaw et al., 2012; Sun et al., 2016).

However, the lack of convergence-related volcanic units and poor exposure of late Cenozoic sediments in the Pamir–South Tian Shan convergence area makes the detailed source-to-sink relation of the Pamir–South Tian Shan convergence area poorly established, hindering our understanding of intracontinental deformation in the convergence area. Previous studies of detrital zircon chronology of the late Cenozoic sediments, as noted above, were conducted in the foreland region of either the northeast Pamir or the South Tian Shan. These localities are tectonically relevant to either the Pamir (e.g., Bershaw et al., 2012; Blayney et al., 2016; Liu, 2017) or the South Tian Shan (Yang et al., 2014) and therefore return results that indicate potential sources from the relevant domains. Establishing the convergence process between the Pamir and South Tian Shan requires examining a section in the center of the convergence area. In this study, the well-exposed late Cenozoic Wuheshalu succession in the Pamir–South Tian Shan convergence area is studied using detrital zircon geochronology, conglomerate clasts counts, and paleocurrent measurements in order to shed new light on changes in the provenance of the late Cenozoic sediments in the convergence zone, and through this, to establish the convergence process between the Pamir and the South Tian Shan in the late Cenozoic.

2. GEOLOGIC BACKGROUND

2.1. Pamir

The Pamir Plateau in the western part of the India-Asia collision belt is an arcuate structural belt, consisting of several accreted E-W–trending terranes resembling the Tibet Plateau terranes with their collision ages generally younging southward (e.g., Yin and Harrison, 2000; Schwab et al., 2004; Angiolini et al., 2013). The Main Pamir Thrust (MPT) and the Pamir Frontal Thrust (PFT) form its northern boundary and separate it from the Alai Basin and the South Tian Shan to the north (e.g., Sobel et al., 2013) (Figs. 1A and 2). The Pamir is separated by the Kashi-Yecheng Transfer System from the western Tarim Basin on the east side (Cowgill, 2010) and the Darvaz fault from the Tajik Basin in the west side (e.g., Strecker et al., 1995) (Fig. 1A).

The Pamir could be divided into the North, Central, and South Pamir with different basement rocks separated by Paleozoic-Mesozoic sutures (e.g., Burtman and Molnar, 1993; Schwab et al., 2004; Robinson et al., 2012). The North Pamir consists of the Permian-Triassic Karakul-Mazar belt and the Kunlun Terrane (e.g., Schwab et al., 2004; Robinson et al., 2004, 2007). It is bounded by the Torbashi thrust (Robinson et al., 2012) and the south-dipping late Triassic–Early Jurassic Tanymas suture (e.g., Burtman and Molnar, 1993) to the south (Fig. 1A). The North Pamir has mainly Carboniferous-Triassic-Jurassic ages related to the Kunlun arc of the Karakul-Mazar terrane and Songpan-Ganzi terrane (e.g., Schwab et al., 2004; Weislogel, 2008) (Fig. 3). This is consistent with modern river detrital zircon results from the North Pamir which have a Triassic main age peak (e.g., Carrapa et al., 2014; Blayney et al., 2016; Rittner et al., 2016). The minor amount of early Paleozoic ages (ca. 400–500 Ma) (Carrapa et al., 2014; Liu et al., 2017) in the North Pamir are associated with sources from the northern and southern Kunlun magmatic belts (Schwab et al., 2004) (Fig. 3).

The Central Pamir, consisting of Paleozoic, Triassic-Jurassic (meta)sedimentary rocks and several Cenozoic gneiss domes, is separated by the Cretaceous Rushan-Pshart zone from the South Pamir (e.g., Yin and Harrison, 2000; Robinson et al., 2007; Schmidt et al., 2011) (Figs. 1A and 3). Magmatic rocks in the Vanj complex of the Central Pamir yield ages of 41–36 Ma (Chapman et al., 2018). Detrital zircons from modern rivers draining the Central Pamir reveal main age peaks of ca. 32 Ma (He et al., 2018) and ca. 40 Ma (Lukens et al., 2012) (Fig. 3).

The South Pamir is bounded by the Rushan-Pshart suture to the north and the mid-Triassic Wakhan-Tirich suture zone (Angiolini et al., 2015) from the Karakoram terrane to the south (Zanchi and Gaetani, 2011) (Fig. 1A). Widespread Cretaceous and Cenozoic igneous rocks (Fraser et al., 2001; Schaltegger et al., 2002; Stübner et al., 2013) (Fig. 3), Precambrian-Paleozoic metamorphic rocks, and Triassic and Jurassic sedimentary rocks (Vlasov et al., 1991; Schwab et al., 2004) constitute the South Pamir. Exhumation and metamorphism of gneiss domes occurred in the South Pamir during the Cenozoic (e.g., Schwab et al., 2004; Stübner et al., 2013; Stearns et al., 2015). Detrital zircons from modern rivers draining the South Pamir have a main age peak of ca. 102 Ma, with minor late Cenozoic ages (Lukens et al., 2012; Blayney et al., 2016; Chapman et al., 2018) (Fig. 3).

To the south of the Pamir are the Karakoram terrane and the Kohistan-Ladakh arc. The Karakoram terrane comprises metamorphic rocks exhumed from the lower crust, and Cretaceous and Cenozoic intrusive, and sedimentary rocks (Fraser et al., 2001). The Kohistan-Ladakh arc, mostly consisting of Late Cretaceous plutons, was accreted onto the southern Asian margins along the north-dipping Shyok zone in the Late Cretaceous (e.g., Schwab et al., 2004; Heuberger et al., 2007; Khan et al., 2009) (Fig. 1A).

2.2. South Tian Shan

The E-W– to ENE-WSW–trending Tian Shan is an ∼2500 km intracontinental range stretching across central Asia (Fig. 1A). It formed through complex accretions of island arcs and amalgamations of continental lithospheric blocks during the late Paleozoic (e.g., Windley et al., 1990; Xiao et al., 1992; Alekseev et al., 2009). During the Cenozoic, the South Tian Shan was reactivated by far-field effect of the India-Asia convergence (e.g., Molnar and Tapponnier, 1975; Yin et al., 1998; Jolivet et al., 2010), creating ∼10-km-thick late Cenozoic basin fills and basinward propagation of thrust belts in the foreland regions (e.g., Sobel and Dumitru, 1997; Wang et al., 2002; Heermance et al., 2007).

Available U-Pb zircon ages from basement rocks in the South Tian Shan area west of Kepingtage were compiled to identify the potential provenance age peaks in this study (see references in Fig. 3). These results reveal two remarkable age groups: 250–364 Ma (sub-peak at 286 Ma) and 374–500 Ma (sub-peak at 400 Ma) (Fig. 3). The 250–364 Ma age group corresponds to the collision between the Central Tian Shan and the Tarim block during the late Paleozoic (e.g., Gou et al., 2012; Huang et al., 2012). The 374–500 Ma age group could be related to magmatism during the Ordovician-Devonian subduction of the South Tian Shan oceanic lithosphere (e.g., Gao et al., 2009; Yang and Zhou, 2009). Late Cretaceous–Paleogene zircons, from basalts in the Tuoyun basin, possibly relate to a small plume rooted at a shallow level in the asthenosphere (Sobel and Arnaud, 1999; Liang et al., 2007).

2.3. Western Tarim Basin

The Tarim Basin is a large inland basin with a stable basement that developed in the Archean and Neoproterozoic (e.g., Lu et al., 2008; Cheng et al., 2017). The western Tarim Basin is now bounded by the South Tian Shan to the north and the northeast Pamir to the southwest (Fig. 1A). From the Late Cretaceous to the Paleogene, five major marine incursions through the Alai Valley left widespread shallow marine deposits in the western Tarim Basin (e.g., Tang et al., 1989; Burtman, 2000; Bosboom et al., 2011). During the Neogene, up to 10-km-thick sediments of terrestrial facies derived from the adjacent mountain belts were deposited in the basin (Wang et al., 1992), providing a good opportunity to study the tectonic evolution and provenance changes related to convergence of the Pamir and the South Tian Shan.

2.4. Late Cenozoic Tectonics in the Convergence Area

In the Pamir–South Tian Shan convergence area, the complex tectonic context and discrete Cenozoic sedimentary successions are caused by pluses of basinward migration of late Cenozoic fault belts.

The northeast Pamir is bounded by the MPT, PFT, and Kashi-Yecheng Transfer System (Fig. 1). The MPT places the North Pamir over the Mesozoic and Cenozoic strata (BGMRXUAR, 1993; Sobel et al., 2013) (Figs. 1 and 2). It was active in the early-middle Miocene (Sobel and Dumitru, 1997; Bershaw et al., 2012) and became inactive since the Pliocene (Sobel et al., 2013). The PFT is the north boundary of present Pamir, located tens of kilometers north of the MPT (Fig. 1). It became active in the western Kashi Basin in the Pliocene (e.g., Fu et al., 2010; Thompson et al., 2015). In the Alai Valley, the PFT became active in the middle Miocene and remains active (Coutand et al., 2002). The PFT accommodates most of the crustal shortening since late Miocene to Pliocene and places Cretaceous and Cenozoic strata over younger basin fills (e.g., Coutand et al., 2002; Li et al., 2012; Thompson Jobe et al., 2018) (Fig. 1). In the eastern Pamir, the dextral Kashi-Yecheng Transfer System divides the Pamir and the southwestern Tarim Basin (Fig. 1). It has been active in the late Miocene (5–6 Ma) or earliest Pliocene (e.g., Cowgill, 2010; Sobel et al., 2011; Cao et al., 2013).

In front of the South Tian Shan, late Cenozoic thrust belts developed along the foreland and are cut by the NW-SE–trending, dextral Talas-Fergana strike-slip fault, which was activated at ca. 25–16 Ma (Yang et al., 2014; Bande et al., 2017) (Fig. 1). Activation of the South Tian Shan foreland thrust belt west of the Talas-Fergana fault is poorly constrained. The conglomerate deposits became prevalent along the foreland since the Pliocene (Yang et al., 2014, 2015; Chen et al., 2015). At the foreland area east of the Talas-Fergana fault, uplift of the hinterland structure began at ca. 25–20 Ma (Sobel et al., 2006; Heermance et al., 2008), followed by rapid deformation in the Kashi Thrust Belt since ca. 19 Ma, a decrease in the shortening rate between ca. 13.5–4.0 Ma, and a final acceleration of propagation at ca. 4.0 Ma (Heermance et al., 2007). This faulting caused the growth of anticlines generally parallel with the basin boundary and basinward propagation of conglomerate deposits (e.g., Chen et al., 2007; Heermance et al., 2007, 2008; Thompson Jobe et al., 2018).

In the Pamir–South Tian Shan convergence area west of the Talas-Fergana fault, numerous W-E–trending, N- or S-directed faults developed between the PFT and the South Tian Shan foreland thrust belt (Fig. 1). These faults, which extend ∼5 km to tens of kilometers and penetrate to shallow depth, cut strata from Mesozoic to Quaternary (Figs. 1 and 2) and are likely to be activated by convergence of the Pamir and the South Tian Shan during the Pliocene and Quaternary. The south-dipping Wuheshalu fault, in front of the PFT, thrusts Pliocene and Miocene strata over Miocene strata (Figs. 1 and 2). Growth strata are not observed in the Atushi Formation in the hanging wall of the Wuheshalu fault, although these might have been eroded during Pliocene thrusting. Alternatively, growth strata may not have formed, in which case, the Wuheshalu fault should have become active during the Quaternary.

3. SAMPLING AND ANALYTICAL METHODS

3.1. Sampling

Seven sandstone samples of at least 2 kg each were collected from the Miocene Anjuan Formation to the Pliocene Atushi Formation in the late Cenozoic succession of the Wuheshalu section. The Xiyu Formation was not sampled, due to difficulties in accessing the outcrops (Fig. 4F).

3.2. LA-ICP-MS Zircon U-Pb Dating

Ages of detrital zircons are believed to be unaffected by erosion, transportation, and deposition processes; therefore, they have been widely used in determining the potential provenance of sedimentary rocks (e.g., Cawood and Nemchin, 2000; Liu et al., 2013). In this study, zircons from seven sandstone samples collected from the Wuheshalu section have been U-Pb dated using laser ablation–inductively coupled plasma–mass spectroscopy (LA-ICP-MS).

More than 300 zircon grains were selected from each sample (except for the sample WHSL-2 with 235 grains) using conventional heavy liquid and magnetic techniques, and handpicking under a microscope. Cathodoluminescence (CL) images were obtained to study zircon internal morphology and to select potential sites for isotopic analysis that avoided fractures and inclusions (Boggs and Krinsley, 2006; Mange and Wright, 2007) (Fig. S1 in the GSA Data Repository Item1).

After CL imaging, LA-ICP-MS U-Pb dating was performed at the Key Laboratory of Orogenic Belts and Crustal Evolution, Peking University. Zircons with obvious metamorphic features on the CL images were excluded during dating. The laser beam had a diameter of 32 μm and a depth of 20–40 μm. The common lead was corrected following the method described by Andersen (2002). After that, Glitter 4.4 (Griffin et al., 2008) was used for data reduction. Grains which are >10% disconcordant (disagreement between the 206Pb/238U and 207Pb/235U ages) or Th/U < 0.1 (Corfu et al., 2003; Hoskin and Schaltegger, 2003) were not included in the following discussions because of their low accuracy. 207Pb/206Pb ages were chosen for older (>1000 Ma) zircons, and 206Pb/238U ages were chosen for younger (<1000 Ma) ones (Compston et al., 1992; Griffin et al., 2004; Gehrels, 2014) because of the counting statistics and reliability of the ages. After removing grains with discordance >10%, 94–99 valid ages are obtained from each sample. The final concordia ages and diagrams were obtained using Isoplot 4.15 (Ludwig, 2008) and DensityPlotter 7.0 (Vermeesch, 2012) (Fig. S2).

3.3. Conglomerate Clast Counts

In the Wuheshalu section, four conglomerate beds were selected for conglomerate clast counting in 1 m2 areas for each sample (Fig. S3). Grain sizes have been measured for both long (D) and short axes, and grain roundness and clast lithology have been counted for up to ∼200 clasts in each site. Site Gc-4 has too many large clasts; thus, only 146 clasts are counted in the 1 m2 area. Following the division proposed by McLane (1995), gravels are subdivided into granule (D ≤ 64 mm), pebble (64 <D ≤ 256 mm) and cobble (D > 256 mm) and roundness is grouped into well-rounded, rounded, sub-rounded, sub-angular, angular, and very-angular. According to the outcrop, lithologies of clasts are grouped into sandstone, pebbly sandstone, granite, limestone, shell limestone, gypsum, quartz, and magmatite (excluding granite).

3.4. Paleocurrent Measurements

Paleocurrent directions were determined from unidirectional tabular and wedge-shaped cross bedding in the late Cenozoic succession of the Wuheshalu section. All the orientation data were measured by a magnetic compass with their local magnetic declination anomaly corrected. For each site, stratum orientation and 8–10 cross stratifications are measured and corrected for tilted bedding (e.g., Lin et al., 2010; Wang et al., 2017).

4. STRATIGRAPHY AND AGE OF THE WUHESHALU SECTION

4.1. Stratigraphy of the Wuheshalu Section

In the Pamir–South Tian Shan convergence area and the western Tarim Basin, previous works divided the late Cenozoic strata into the Wuqia Group (the Keziluoyi, Anjuan, and Pakabulake formations), the Atushi Formation, the Xiyu Formation, and overlying strata (e.g., BGMRXUAR, 1993; Chen et al., 2002; Jia et al., 2004). A summary of published description of regional late Cenozoic strata is presented in Table 1.

The Wuheshalu section is located ∼40 km west of Wuqia, between the PFT and the Wuheshalu fault in the convergence area between the Pamir and the South Tian Shan (Figs. 1, 2, and 4A). This ∼15-km-long section exposes late Cenozoic successions along the Kezilesu River southwest of the Wuheshalu Village, forming the south limb of an E-W–trending anticline (Fig. 1B and 1C). This section has not been described previously.

The studied section presents the late Cenozoic sedimentary succession of the Miocene Anjuan and Pakabulake, the Pliocene Atushi, and the Plio-Pleistocene Xiyu formations. The lower part of the Anjuan Formation is interrupted by two thrust faults near the core of the anticline (Fig. 4B). The formations have been previously mapped (BGMRXUAR, 1993), and this study relies on their formation designations and mapping.

In the Wuheshalu section, the Miocene Anjuan Formation consists of ∼547 m of brownish red sandstone-mudstone complexes. Upsection, the thickness of fine sandstone beds increases from ∼2 cm to ∼40 cm, and the bedforms change from laminated into lens-shaped beds. The thickness of brownish red mudstone beds varies from ∼0.5 m to ∼5 m. Sedimentary structures in this formation include ripple and cross-bedding. Paleocurrent directions are N-directed in the upper Anjuan Formation (Fig. 5).

The Miocene Pakabulake Formation consists of ∼3531-m-thick sandstone-mudstone-siltstone packages and conformably overlies the Anjuan Formation (Fig. 4A). The lower part (∼433 m thick) begins with well-developed fine-grained sandstone sheets (Fig. 4A) and changes gradually into medium-grained thick sandstone channels. Mud cracks, cross-bedding, ripples, and burrows are found in the sandstone channels in the lower part of the Pakabulake Formation. The middle and upper Pakabulake Formation comprises mudstone-pelitic sandstone-medium-grained sandstone packages. Mudstone and pelitic sandstone beds are relatively more abundant in the middle part (1795 m thick). The thickness and proportion of sandstone increase significantly in the upper part (1303 m thick) of this formation (Fig. 4D). Paleocurrent directions are N-directed at the lower Pakabulake Formation and E-directed at the top of this formation.

The Pliocene Atushi Formation (662 m thick) conformably overlies the Pakabulake Formation with a transition from sandstone-mudstone packages to massive conglomerate layers (Fig. 4E). This formation consists of two upward-coarsening mudstone-(pebbly/coarse-grained) sandstone-conglomerate (sheet/channel) sequences. Trough cross-bedding is developed in the sandstone and pebbly sandstone beds. Conglomerate beds, sometimes lenticular with basal erosive surface, with thickness reaching ∼4 m are sandy matrix–supported in the lower and middle parts and change to clast-supported at the top of the Atushi Formation. The imbricated gravels have grain sizes ranging from 7 mm to 500 mm and are mostly composed of sub-rounded and sub-angular, brownish red/yellowish green sandstone and limestone. Paleocurrent directions are E-directed at the bottom and shift to N-directed at the upper part of the Atushi Formation.

In the study area, the Xiyu outcrops were inaccessible; therefore, we were unable to describe them to the same level of detail as the other units. The Xiyu Formation is a massive, thick-bedded, dark gray conglomerate with sparsely interbedded sandstone lenses that overlies the Atushi Formation with an angular unconformity and is marked by a distinct transition from light gray to dark gray (Fig. 4F).

4.2. Age Correlation

In this study, the age framework of the Wuheshalu section is based on correlation with nearby late Cenozoic magnetostratigraphically dated sections. Recently, several magneto-stratigraphic works had been carried out in the study area (Chen et al., 2002, 2015; Heermance et al., 2007; Tang et al., 2015; Thompson et al., 2015; Yang et al., 2015; Qiao et al., 2016, 2017; Liu et al., 2017; Thompson Jobe et al, 2018). However, due to the lack of fossils and synchronous volcanic deposits, variable thickness and lithofacies, and poor preservation, the late Cenozoic chronostratigraphic framework throughout the Pamir–South Tian Shan convergence area and the western Kashi Basin still remains in dispute. In the Kashi Thrust Belt, previous magnetostratigraphic studies (e.g., Chen et al., 2002; Heermance et al., 2007; Qiao et al., 2016) documented the late Cenozoic growth of the Kashi Thrust Belt. The late Cenozoic stratigraphy of the Kashi foreland differs from those in the Wuheshalu section (Heermance et al., 2007). The work of Qiao et al. (2017) at the Sankeshu section, ∼40 km west of the Wuheshalu section, covers the upper Pakabulake Formation and the Pliocene Atushi Formation, with basal age of the Atushi Formation at ca. 7 Ma and basal age of the Xiyu Formation at ca. 2.6 Ma or younger. This study lacks description of the late Cenozoic sedimentary. Several missing chrons around the boundary of the Pakabulake Formation and the Atushi Formation makes the basal age of the Atushi Formation not so convincing (Qiao et al., 2017) (Fig. 5). The magnetostratigraphy of Tang et al. (2015) at the Baxbulake section, ∼12 km north of the Wuheshalu section, covers only the Oligocene Bashibulake Formation and the early Miocene Keziluoyi Formation (Fig. 5). The magnetostratigraphy at the Tierekesazi section (or the Mine section), ∼20 km northwest of the Wuheshalu section, covers the Miocene-Quaternary (Chen et al., 2015; Yang et al., 2015) (Fig. 5). These two studies have different subdivisions for the Miocene strata: the Keziluoyi Formation of Yang et al. (2015) includes the Keziluoyi and Anjuan formations of Chen et al. (2015), and the Anjuan Formation of Yang et al. (2015) is equal with lower part of the Pakabulake Formation of Chen et al. (2015) (Fig. 5). The correlations of the magnetozones in these two studies are similar and are supported by apatite fission track age (Yang et al., 2014) and pollen samples (Chen et al., 2015) (Fig. 5). Because the sedimentary characteristics of the Tierekesazi section resemble the Wuheshalu section, we choose to correlate the Wuheshalu section with the Tierekesazi section in order to give age estimations for the sampling horizons.

In the Tierekesazi section, the pattern of the magnetozones from ca. 20 Ma to ca. 14 Ma resembles the reversal pattern of the GPTS 2012 (Gradstein et al., 2012). This correlation is supported by the pollen from the middle Pakabulake Formation (Chen et al., 2015) and apatite fission track age of the Yang et al. (2015) (Fig. 5). This part could be used to correlate with the strata from the Anjuan Formation to the Pakabulake Formation in the Wuheshalu section. In the Wuheshalu section, the Anjuan Formation is characterized by mudstone-sandstone packages with the sandstone/mudstone ratio generally increasing upwardly and sandstone changing from thin layers to channels (Chen et al., 2015; Yang et al., 2015) (Fig. 5). These sedimentary characteristics can also be found in the Tierekesazi section (ca. 19.6–17.4 Ma, Yang et al. [2015]; ca. 20.4–17.1 Ma, Chen et al. [2015]). Therefore, we suggest the Anjuan Formation in the Wuheshalu section has an age interval of <20.4–ca. 17.1 Ma, as the lower part is interrupted by faults. Sample WHSL-1 was collected from the first sandstone channel of the Wuheshalu section; this is loosely correlated with the first sandstone channels at the Tierekesazi section, given an age estimation of ca. 19 Ma (Fig. 5). Sample WHSL-2 in the sandstone sheets overlying the Anjuan Formation of the Wuheshalu section is given an age estimation of ca. 17 Ma (Fig. 5). Upsection, sample WHSL-3 at the first thick sandstone channel can correlate with the first thick sandstone channels with thickness of ∼5-m-thick and extending more than 100 m laterally in the Tierekesazi section (Chen et al., 2015); this is given an age estimation of ca. 16 Ma (Fig. 5). In the Tierekesazi section, conglomerate–pebble sandstone–coarse-grained sandstone packages that first appeared at ca. 14.4 Ma (Yang et al., 2015) or 13.9 Ma (Chen et al., 2015) (Fig. 5) are not observed throughout the Pakabulake Formation of the Wuheshalu section. This difference might be caused by limited distribution of these coarse sediments sourced from the South Tian Shan (Chen et al., 2015; Yang et al., 2015) and may also be consistent with the N-directed paleocurrent directions in the Wuheshalu section. Upsection, the correlation of magnetozones from ca. 13 Ma to ca. 6 Ma in the Tierekesazi section is not as convincing (Chen et al., 2015). Thus, the upper Pakabulake Formation of the Tierekesazi section is not correlated or used for age control at the Wuheshalu section.

In the Wuheshalu section, the transition from the Pakabulake Formation to the Atushi Formation is marked by a massive conglomerate overlying a sandstone-mudstone package (Fig. 4E), and these can also be observed at the Ulugqat and Tierekesazi sections (Wang et al., 2014; Chen et al., 2015). Magnetostratigraphic studies in the Tiereksazi and Sankeshu sections reveal ages of ca. 5.2 Ma and ca. 7 Ma for the bottom of the Atushi Formation. Nevertheless, these two sections lack independent age constraints (Chen et al., 2015; Qiao et al., 2016) (Fig. 5), making these results not as convincing. In the Kashi Thrust Belt, magnetostratigraphic study of Heermance et al. (2007) and Qiao et al. (2016) revealed basal ages of ca. 5.3 Ma and ca. 6 Ma for the Atushi Formation. These results are supported by Pliocene fossils such as Hyacypris manasensi, Ilyocypris errabundis, Candona (Candona) neglecta, Candona (Pseudocandona) subequalis, and Eucypris notabilis, etc., found in the Atushi Formation near the Atushi City (XJUARRSCG, 1981; Jia et al., 2004). The Atushi Formation is highly time transgressive in the south Tarim Basin. With the discovery of a ca. 11 Ma volcanic ash layer in the Xiyu Formation (Zheng et al., 2015), the underlying Atushi Formation has basal ages varying from ca. 23 Ma to ca. 4.6 Ma in the south Tarim Basin (Zheng et al., 2000, 2010, 2015; Sun, 2006; Sun et al., 2008). These sections are hundreds of kilometers away from the study area, and the volcanic ash layers have not been found in the Xiyu Formation that developed in the foreland area of South Tian Shan. In general, we estimate the basal age of the Atushi Formation in the Wuheshalu section to be ca. 6–5.3 Ma (Table 2). Due to the difference in the sampling horizons, sample WHSL-5 at the top of the Pakabulake Formation is estimated to be the lower limit age of ca. 6 Ma, and sample WHSL-6 at the bottom of the Atushi Formation is estimated to be the upper limit age of ca. 5.3 Ma. Sample WHSL-4 was collected from the middle Pakabulake Formation. Its age, loosely constrained by the formation time interval divided by the thickness of the distribution of the Pakabulake Formation, is estimated to be ca. 9 Ma (Table 2).

In the Wuheshalu section, the Xiyu Formation is made up of massive, thick-bedded, dark gray, and clast-supported conglomerates that unconformably overlie the light gray Atushi Formation (Fig. 4F). Researches about the Xiyu Conglomerate in the Kashi Thrust Belt suggest that it is highly time-transgressive (e.g., Chen et al., 2002; Heermance et al., 2007; Thompson Jobe et al., 2018), with basal ages varying from ca. 15.5 ± 0.5 to 0.7 ± 0.1 Ma, generally younging from hinterland to the basin (Heermance et al., 2007) and overlying Cretaceous-Pliocene strata (Chen et al., 2002; Heermance et al., 2007; Qiao et al., 2016). This sequence reflects progressive shortening, deformation and basinward propagation of the thrust belts (e.g., Chen et al., 2002; Heermance et al., 2007; Qiao et al., 2016). In the Wupoer Piggyback Basin, the Xiyu conglomerate has basal ages that vary from ca. 6 Ma to ca. 1.9 Ma (Li et al., 2013; Thompson et al., 2015; Thompson Jobe et al., 2018). At the Ulugqat section, electron spin resonance dating constrained the basal age of Xiyu Formation to ca. 1 Ma (Wang et al., 2014). Due to high diachroneity of the Xiyu Conglomerate and lack of age control in the Wuheshalu section, the basal age of the Xiyu Formation in the Wuheshalu section remains unsolved.

5. RESULTS

5.1. U-Pb Geochronology of Detrital Zircons

Zircon grains from all 7 samples are mostly sub-rounded to sub-angular with long axes ranging from 40 to 120 µm (Fig. S1). Well-developed oscillatory zoning could be observed on the CL images (Fig. S1) and all of the Th/U ratios are >0.1, indicating a magmatic origin for these zircons (Belousova et al., 2002).

Zircon U-Pb ages from all 7 samples from the late Cenozoic Wuheshalu section range from 13 ± 0.3 Ma to 3377 ± 32 Ma. Age distributions of 7 samples are characterized by 5 main age groups: 190–250 Ma, 280–350 Ma, 390–480 Ma, 550–650 Ma, and 850–1020 Ma, and minor components of 13–180 Ma and 1020–3400 Ma (Fig. 6). Age group 190–250 Ma has the largest population in all 7 samples (Fig. 6). Moving upsection, the proportions of zircons in age groups 280–350 Ma, 390–480 Ma, and 550–650 Ma increase at the base of the Pakabulake Formation (ca.17 Ma, WHSL-2), decrease in the lower Pakabulake Formation (ca. 9 Ma, WHSL-3), increase again at the top of the Pakabulake Formation (ca. 6 Ma, WHSL-5), and decrease again in the upper Atushi Formation (WHSL-7). Rare Late Cretaceous–late Cenozoic zircons appear at the bottom of the Pakabulake Formation (WHSL-2) and from the middle Pakabulake Formation to the Atushi Formation (WHSL-4, WHSL-5, WHSL-6, and WHSL-7).

5.2. Conglomerate Clast Counts

Site Gc-1 (GPS: 39.6374°N, 74.6535°E) is located at the bottom of the Atushi Formation. Gravels (7 ≤ D ≤ 74 mm) in this site consist of mostly sub-rounded and sub-angular granules with only one pebble. Sandstone (46%) and limestone (33%) clasts are the major components of the gravel lithologies in this site, with minor components of magmatite (except for granite) (12%) and quartz (8%) clasts (Figs. 7 and S3).

Upsection, site Gc-2 (GPS: 39.6392°N, 74.6467°E) is from the lower part of the Atushi Formation. This conglomerate (7 ≤D ≤ 160 mm) has more pebbles (13%) and sub-angular gravels than site Gc-1. Clast lithologies in this site are also sandstone (43%) and limestone (46%), resembling site Gc-1 (Figs. 7 and S3).

Site Gc-3 (GPS: 39.6519°N, 74.6129°E) is from the upper part of the Atushi Formation. This conglomerate (7 ≤D ≤ 95 mm) consists of 95% granules and 5% pebbles. Most of the gravels are sub-rounded and sub-angular. Lithologies of gravel clasts are mainly sandstone (54%), limestone (26%), and magmatite (excluding granite) (12%) (Figs. 7 and S3).

Site Gc-4 (GPS: 39.6512°N, 74.6050°E) is from the horizon close to the top of the Atushi Formation. Gravels (11 ≤ D≤ 504) are mainly granules but have the most pebbles (25%) and two cobbles. Gravels are mostly sub-angular and sub-rounded. Clasts in this site have the widest range of lithologies, which are mainly sandstone (42%), pebbly sandstone (11%), granite (10%), limestone (14%), quartz (12%), and magmatite (excluding granite) (10%) (Figs. 7 and S3).

6. SEDIMENTARY PROVENANCE ANALYSIS

As summarized in Sections 2.1 and 2.2, the main zircon U-Pb age groups representing Pamir basement rocks are late Cenozoic–Late Cretaceous, 210–247 Ma and 400–500 Ma. The main age groups of the South Tian Shan basement are 250–364 Ma and 400–500 Ma. Thus, the distinctive age groups of late Cenozoic–Late Cretaceous, and 210–247 Ma in the Pamir and 250–364 Ma in the South Tian Shan are useful for differentiating between the two source areas (Fig. 3). However, potential recycling of zircons in the study area hinders recognition of the South Tian Shan provenance in the late Cenozoic. In the Middle Jurassic and Early Cretaceous, the development of a drainage system at the front of the South Tian Shan deposited widespread sediments in the area, both in the present foreland areas of the South Tian Shan and the Pamir (Sobel, 1999; Yang et al., 2017) (Fig. 1). These Jurassic and Cretaceous sediments contain a significant amount of zircons in the age groups of 250–364 Ma and 400–500 Ma derived from the South Tian Shan basement rocks (Fig. 3) (Yang et al., 2017). Therefore, it is possible that recycled South Tian Shan basement rock zircons (e.g., age groups 250–364 Ma and 400–500 Ma) from Jurassic and Cretaceous sediments deposited in the location of the present Pamir foreland (Fig. 1C) were eroded and transported northward in the late Cenozoic. In addition, Precambrian zircons are interpreted being sourced either from the Precambrian basement of the Tarim Basin or recycled from source rocks containing the Precambrian zircons (e.g., Gu, 1996; Zhang, 2000; Zhang et al., 2001; Qu et al., 2003; He et al., 2005; Ding et al., 2008). Thus, the Paleozoic and Precambrian zircons, which are minor components in each sample, are not unequivocal for precise provenance discussion.

All 7 samples contain a large number of angular and euhedral zircons from the 247–210 Ma age group (Fig. 6), consistent with the Triassic igneous rocks in the North Pamir (Schwab et al., 2004) (Fig. 1A). Together with the N-directed paleocurrent directions, this data indicates that the North Pamir was the major source area for the Wuheshalu section from ca. 19 Ma to ca. 3 Ma (Fig. 6).

At ca. 17 Ma (sample WHSL-2, bottom of the Pakabulake Formation) (Fig. 5), the increase in the Paleozoic and Precambrian zircon components (Fig. 6) is synchronous with the transition from laminated sandstone-mudstone complexes to sandstone sheets, likely indicating an increase of stream power and size of the source area of the Wuheshalu section. Uncommon Paleocene zircons in the sample WHSL-2 (Fig. 6) are also reported in the late Cenozoic succession in the Aertashi section of the west Kunlun Mountain belt (Blayney et al., 2016; Yang et al., 2018) and the Oytag section of the northeast Pamir (Sun et al., 2016). These Paleocene zircons have ages concordant with igneous rocks in the Kohistan arc and Karakoram (Schwab et al., 2004; Heuberger et al., 2007), suggesting derivation from these regions. However, in this case the north-flowing rivers would have to cross the Pamir terranes to transport the zircons to the Wuheshalu section; while the sample WHSL-2 contains neither Eocene zircons from the Central Pamir nor Late Cretaceous to Cenozoic zircons from the South Pamir (Figs. 3 and 6). Thus, the Paleocene zircons in the sample WHSL-2 are more likely to reflect magmatic suites in the Pamir that have been totally eroded or remain to be dated (Chapman et al., 2018).

In the sample WHSL-4 (ca. 9 Ma, middle Pakabulake Formation) (Fig. 6), the ca. 36 Ma zircon grains are interpreted to be derived from igneous rocks of the Central Pamir (Lukens et al., 2012; Chapman et al., 2018). The ca. 13 Ma zircon ages might be sourced from the mid-Miocene Dunkeldik volcanic belt (Ducea et al., 2003; Hacker et al., 2005), the potassic Taxkorgan intrusive complex (Ke et al., 2006; Robinson et al., 2007; Jiang et al., 2012), or the gneiss domes of the South Pamir (Stearns et al., 2015; Rutte et al., 2017; Chapman et al., 2018).

At the end of Pakabulake Formation time (ca. 6 Ma) (Fig. 6), the increasing proportion of Paleozoic and Precambrian zircons in sample WHSL-5 and the change in paleocurrent directions from N- to E-directed suggest enhancement of erosion at their related source areas in the convergence area (Fig. 5) and a combination of provenance from the South Tian Shan and the Pamir. The Eocene, Paleocene, and Late Cretaceous zircon grains in this sample indicate sediment derived from the Pamir terranes (Schwab et al., 2004; Lukens et al., 2012; Thompson Jobe et al., 2018).

Since the deposition of the Atushi Formation, conglomerate started to accumulate in the Wuheshalu section. Sample WHSL-6 from the bottom (ca. 5.3 Ma) of the Atushi Formation (Fig. 5) has a similar zircon age distribution characteristics as sample WHSL-5 (Fig. 6). This, together with E-directed paleocurrent directions, indicates a combination of sediments from the Pamir and the South Tian Shan. The Eocene and Paleocene zircon grains indicate sediment sourced from the Pamir terranes (Schwab et al., 2004; Lukens et al., 2012).

In the sample WHSL-7 from the upper Atushi Formation (Fig. 5), the proportion of the 210–247 Ma age group increases. Combined with the N-directed paleocurrent directions, this suggests enhanced sedimentary transported from the North Pamir. Two Paleocene zircon grains probably reflect materials derived from the Pamir terranes.

The four conglomerate clast counting sites are from the bottoms and tops of two upward-coarsening sequences in the Atushi Formation (Fig. 5). The lithologies of the gravels are mainly brownish red/yellowish green sandstone and (shell) limestone, consistent with the lithologies of the Paleogene sediments in the Pamir–South Tian Shan convergence area (e.g., Bosboom et al., 2014). In the study area, the Paleogene sediments are widely exposed in the hanging wall of the PFT and poorly exposed in the hanging wall of the Wuheshalu fault (Fig. 1B). Since the Pliocene, the MPT became less active (Sobel et al., 2013) and the PFT became active (e.g., Fu et al., 2010; Thompson et al., 2015). Together with the observed majority of North Pamir detrital zircons (samples WHSL-6 and WHSL-7) and N-directed paleocurrents at the upper Atushi Formation, we suggest that gravels of the conglomerate in the study area are mainly derived from the hanging wall of the PFT.

7. IMPLICATIONS FOR THE PAMIR–SOUTH TIAN SHAN CONVERGENCE PROCESS DURING THE LATE CENOZOIC

Although the Wuheshalu section in the Pamir–South Tian Shan convergence area is closer to the South Tian Shan (Fig. 1A), Cenozoic and Mesozoic detrital zircons from the Pamir terranes comprise the largest component in the late Cenozoic sediments. It is difficult to determine whether the minor components of Paleozoic and Precambrian zircons are derived from the Pamir or the South Tian Shan, although paleocurrent directions are primarily N-directed. These features could be partly explained by a much higher amount of exhumation in the Pamir than the South Tian Shan in the late Cenozoic (summarized by Sobel et al., 2013), thereby delivering more sediment from the Pamir than the South Tian Shan. This interpretation is supported by the higher sedimentary accumulation rate of the Wuheshalu section (>16.6 cm/k.y. to 34.3 cm/k.y.) (Fig. 5) compared to the Tierekesazi section in the South Tian Shan foreland area (12.3 cm/k.y. to 22.8 cm/k.y.) (Chen et al., 2015).

7.1. Miocene

The Miocene sedimentary succession in the Wuheshalu section consists of generally upward-coarsening sandstone-mudstone complexes. The largest populations of detrital zircons were sourced from Triassic igneous rocks in the North Pamir. The ca. 17.1 Ma to ca. 6–5.3 Ma, ∼3531-m-thick Pakabulake Formation is much thicker than the Tierekesazi section (∼1681 m, Chen et al., 2015), the Baxbulake section (∼2000 m, Qiao et al., 2017) in the South Tian Shan foreland, and the Bieertuokuoyi section (>800 m, Liu, 2017) in the northeast Pamir foreland, suggesting that the Wuheshalu section has a much faster sedimentary accumulation rate and rapid subsidence. This rapid subsidence event is probably caused by crustal flexure due to the increasing sedimentary loading and compression between the Pamir and South Tian Shan. The relatively larger Miocene sediment thickness and higher sedimentary accumulation rate of the Wuheshalu section (>4078 m, 16.6–34.3 cm/k.y.) and the Bieertuokuoyi section (>4118 m, >22.9 cm/k.y.) (Liu, 2017) compared to the South Tian Shan foreland area (2352 m, 12.3–16.3 cm/k.y.) (Chen et al., 2015) implies that that the depocenter of the Pamir–South Tian Shan convergence area was closer to the Pamir at this time. These observations indicate that the North Pamir provided a large amount of sediments since the early Miocene (ca. 19 Ma), implying that there was a northward-flowing drainage system and therefore that the North Pamir had relatively high topography with respect to the Wuheshalu section. The topography of the North Pamir was probably built during multiple rapid deformation events. The middle Eocene deformation in the North Pamir was recorded by accelerated exhumation at the Karakul Lake of the North Pamir (Amidon and Hynek, 2010) and the northeast Pamir (Cao et al., 2013), consistent with the Eocene alluvial conglomerate reported at northern and northeastern foreland of the Pamir (Chen et al., 2018). These results reveal crustal thickening and tectonic uplift of the North Pamir (Amidon and Hynek, 2010) and early motion of the Kashi-Yecheng Transfer System (Cao et al., 2013), which likely built an initial high topography for the North Pamir. Deformation at the Oligocene-Miocene boundary is interpreted from the rapid exhumation widely observed in and around the Pamir (e.g., Sobel and Dumitru, 1997; Amidon and Hynek, 2010; Rutte et al., 2017), reflecting plateau uplifting and thrusting of the MPT related to the India-Asia convergence and break-off of the down-going Indian plate (Negredo et al., 2007). In response to the N-S–directed India-Asia convergence, these events likely created E-W–trending topography in the North Pamir and a N-directed drainage system sourced from high relief along the northern boundary of the North Pamir (Fig. 8).

The small size of the Paleozoic South Tian Shan basement rock component in the Wuheshalu succession, with an unknown proportion derived from recycling of zircons deposited in Mesozoic strata in the Pamir foreland area, indicates that the South Tian Shan contributed little or no sediment to the depocenter of the convergence area in the Miocene. This could be explained by a smaller exhumation amount, slower exhumation rates (e.g., Sobel et al., 2006; De Grave et al., 2012), and relatively lower topography in the South Tian Shan compared to the Pamir during the Miocene. As summarized by Sobel et al. (2013), the South Tian Shan has an average amount of Cenozoic exhumation of ∼3–5 km, less than the ∼7–10 km of the North Pamir (Sobel et al., 2013). Moreover, dextral slip along the Talas-Fergana fault since early Miocene (ca. 25 Ma) (Bande et al., 2015) would lead to northward displacement of the South Tian Shan region to the west of the Talas-Fergana fault. This would reduce the amount of deformation absorbed by the development of the foreland regions of the South Tian Shan west of the Talas-Fergana fault and would cause more shortening in the foreland on the eastern side of the fault compared to the western side (Heermance et al., 2007; Chen et al., 2015). All these factors would lead to relatively less sediment from the South Tian Shan to shed into the convergence area than the North Pamir (Fig. 8).

The appearance of the late Cenozoic–Late Cretaceous zircons in the Wuheshalu section reflects the development of a drainage network connecting the Pamir terranes and the deformation of the Pamir terranes (e.g., Stübner et al., 2013; Stearns et al., 2015; Rutte et al., 2017). As discussed in Section 6, the uncommon Paleocene zircon grains that first appeared at ca. 17 Ma (Fig. 5) likely came from magmatic suites in the Pamir that are totally eroded or yet to be dated (Chapman et al., 2018), indicating that the N-directed drainage connecting the Pamir terranes already existed since that time. In sample WHSL-4 (ca. 9 Ma) (Fig. 6), the ca. 13 Ma and ca. 36 Ma zircon grains are consistent with the age of complexes and gneiss domes in the eastern South and Central Pamir (e.g., Ducea et al., 2003; Robinson et al., 2007; Rutte et al., 2017) and the Vanj complex in the Central Pamir (Chapman et al., 2018), respectively, suggesting that the NE-directed rivers from the Central Pamir and the N-directed river from the southeast Pamir which flowed to the convergence area had developed by this time. These river networks might have been created and modified during Miocene exhumation in the Central and South Pamir (Thiede et al., 2013; Stearns et al., 2015) and/or normal sense shear-zone deformation in the Central Pamir (Rutte et al., 2017). The Late Cretaceous zircon grains that appeared in the latest Miocene (ca. 6–5.3 Ma) (Fig. 6) are consistent with the Late Cretaceous igneous rocks of the South Pamir (e.g., Fraser et al., 2001; Stübner et al., 2013; Thompson Jobe et al., 2018), likely reflect that N-directed rivers flowing from the South Pamir to the North Pamir existed at that time, and might be triggered by exhumation of the South Pamir domes in the late Cenozoic (Stübner et al., 2013; Chapman et al., 2018). Due to the scarcity of these late Cenozoic–Late Cretaceous zircons, their deficiency in the other samples could be caused by either complex variation of relative topography elevation difference in the Pamir due to diverse deformation and loss in the U-Pb zircon dating process.

7.2. Post Miocene

Since ca. 6–5.3 Ma, conglomerate started to be deposited at the Wuheshalu section. The detrital zircon ages in the Atushi Formation are mainly sourced from the Triassic igneous rocks in the North Pamir and the gravel lithologies are consistent with the Paleogene sediments at the hanging wall of the PFT. We suggest that the sediments are still mainly sourced from the North Pamir and the hanging wall of the PFT, driven by high topography, accelerated northward deformation, and propagation of the Pamir (e.g., Sobel et al., 2013; Thompson et al., 2015; Blayney et al., 2016) and activation of the PFT in the northeast Pamir at ca. 5 Ma (Fu et al., 2010; Thompson et al., 2015) (Fig. 8). E-directed paleocurrent direction at ca. 6–5.3 Ma (Fig. 5) and an increase of the Paleozoic and Precambrian zircon components (Fig. 5) likely indicate a minor provenance contribution from the South Tian Shan. This is supported by significant basinward propagation of the foreland thrust belts (Heermance et al., 2007; Thompson Jobe et al., 2018) and development of alluvial conglomerate in its foreland area (e.g., Heermance et al., 2007; Wang et al., 2014; Chen et al., 2015) since the Pliocene. The combination of major North Pamir provenance and minor South Tian Shan provenance indicates that the Wuheshalu region was close to the depocenter of the convergence area at ca. 6–5.3 Ma.

An increase in the gravel size of the conglomerate occurred between the lower (long axis ranging from 7 to 74 mm) and the upper (long axis up to 500 mm) Atushi Formation (Fig. 7). A similar pattern was also found in the early Pliocene sediments of the Bieertuokuoyi Piggyback Basin (Thompson et al., 2015). This coarsening likely indicates that the source area became more proximal. More heterogeneous lithological composition of the gravels at the upper Atushi Formation implies enhancement of erosion in their related source areas and/or contribution from new source area. The characteristics of the conglomerate, together with the N-directed paleocurrent direction and the decrease of the Paleozoic and Precambrian zircon components could be explained by northward thrusting of the PFT and/or Wuheshalu fault (Figs. 2 and 8) and north-vergent thrusting of the North Pamir. These events caused northward displacement of depocenter from the location of Wuheshalu section to farther north, and led to a decrease of the sedimentary accumulation rate after ca. 6–5.3 Ma (Fig. 5) and a relatively greater proportion of sediment from the North Pamir in the Wuheshalu section.

8. CONCLUSIONS

Our detrital zircon analysis of samples collected at the late Cenozoic Wuheshalu section provides a comprehensive understanding for the evolution of the Pamir–South Tian Shan convergence area. Seven detrital zircon samples from the Miocene-Pliocene succession of the Wuheshalu section present zircon ages ranging from 13 ± 0.3 Ma to 3377 ± 32 Ma, with five main age groups: 190–250 Ma, 280–350 Ma, 390–480 Ma, 550–650 Ma, and 850–1020 Ma and other minor groups lying at 13–180 Ma and 1020–3400 Ma. The large populations of zircons from the 190–250 Ma age group in all the seven samples suggests that Triassic igneous rocks from the North Pamir were the main source area for the Wuheshalu section since the early Miocene (ca. 19 Ma). Detrital zircon and conglomerate clast counting results from Pliocene sediments support the connection that the North Pamir remained the main source area for the Wuheshalu section and that the South Tian Shan likely provided only a minor amount of detritus to the Pamir–South Tian Shan convergence area in the early Pliocene. During deposition of the upper Atushi Formation, sediments delivered from the North Pamir to the Wuheshalu section had increased.

These data provide new constraints for the convergence process of the Pamir and South Tian Shan. The large North Pamir detrital zircon component present in all of the Miocene samples from the Wuheshalu section implies that topographic relief in the North Pamir existed since at least the early Miocene. The South Tian Shan had a smaller amount of exhumation in the Miocene and its sediments could only reach a limited area in the foreland area. In the Pamir–South Tian Shan convergence area, gradually increasing sediment loading mainly sourced from the Pamir and convergence of the Pamir and South Tian Shan caused rapid subsidence. Since ca. 6–5.3 Ma, a combination of a major North Pamir component and minor South Tian Shan component of sediment deposited at the Wuheshalu section is consistent with northward propagation of the North Pamir and activation of the PFT as well as accelerated uplift and deformation in the South Tian Shan and southward propagation of its foreland thrust belts. The Wuheshalu section was close to the depocenter of the convergence area at this time. During deposition of the upper Atushi Formation, the North Pamir detrital zircon component had increased in the Wuheshalu section, reflecting an increase of sediments derived from the North Pamir, caused by continued faulting and northward propagation of the North Pamir. In turn, this caused northward displacement of the depocenter to the north of the Wuheshalu section, resulting in an increased proportion of sediment that was derived from the North Pamir to be deposited at the Wuheshalu section.

ACKNOWLEDGMENTS

This research is supported by the National Science Foundation of China (grant nos. 41330207, 41720104003, 41702205, 41472181, 41472182, and 41402170), the National SandT Major Project (grant nos. 2017ZX05008-001 and 2017ZX05003-001), the MOST of China (grant no. 2016YFC0600402), the Fundamental Research Funds for the Central Universities (grant no. 2018FZA3008), and the Open Fund of the Key Laboratory of Submarine Geosciences State Oceanic Administration. We thank Dr. Kongyang Zhu in Zhejiang University and Dr. Valby van Schijndel at the University Potsdam for constructive advice on detrital zircon analysis and to Dr. Fang Ma for help with detrital zircon U-Pb dating. We also thank M.S. Shengqiang Chen in Zhejiang University for his assistance during the field work. Special thanks to James B. Chapman and Jessica A. Thompson Jobe for their constructive comments and advice.

1GSA Data Repository Item 2019161, Table A1: Detailed information about detrital zircon U-Pb analytical data, and Figures S1–S3: Representative CL images of zircons, U-Pb concordia diagrams of the analyzed samples, and photographs of the four sites (Gc-1, Gc-2, Gc-3 and Gc-4) for conglomerate clast counting in the Wuheshalu section, is available at http://www.geosociety.org/datarepository/2019, or on request from editing@geosociety.org.
Gold Open Access: This paper is published under the terms of the CC-BY-NC license.