Paleocene initial indentation and early growth of the Pamir as recorded in the western Tarim Basin

Tectonophysics 772 (2019) 228207

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Paleocene initial indentation and early growth of the Pamir as recorded in the western Tarim Basin

T

Shijie Zhanga, Xiumian Hua,*, Eduardo Garzantib a b

State Key Laboratory of Mineral Deposit Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210023, China Department of Earth and Environmental Sciences, Università di Milano-Bicocca, 20126 Milano, Italy

ARTICLE INFO

ABSTRACT

Keywords: Uplift and indentation of Pamir Western Tarim Basin Provenance change Paleogeographic evolution

It is presently unclear as to when and how the Pamir were uplifted to their current elevation, because a Cenozoic sedimentary record is lacking on the Pamir Plateau, and the Pamir domes only record metamorphism associated with crustal shortening since ∼37 Ma. Studies of the tectonic and geomorphological evolution of the Pamir in the early stages of the India–Asia continental collision are also lacking. The western Tarim Basin at the northeastern margin of the Pamir provides an ideal opportunity to investigate their formation and evolution. Four Cretaceous–Paleogene stratigraphic sections were examined in this study. From these, we infer that the West Kunlun Mountains were topographic highs during the Mesozoic that divided the adjacent Central Pamir–Tianshuihai terrane from the Tarim Basin. This paleo-topographic configuration lasted until a marine transgression occurred in the Aksai Chin and Central Pamir areas in the latest Cretaceous. Provenance analysis indicates that sediments in the western Tarim Basin were derived solely from the West Kunlun Mountains until the Paleocene. The appearance of Pamir-derived detrital zircons in Paleocene and Eocene strata of the Tarim Basin suggests that the Pamir was experiencing uplift and reached an elevation similar to, or even higher, than the West Kunlun Mountains. At that time, the sandstone composition documents a provenance change from a stable continental block to an active thrust belt of the proto-West Kunlun Mountains, indicating that the western Tarim Basin evolved from an intraplate to a foreland basin setting. During the Paleocene, alluvial fan conglomerates were deposited along the northeastern margin of the Pamir, and a thick gypsum unit began to be deposited in the Tarim Basin. All these lines of evidence indicate that the north-verging thrusting, foreland basin subsidence, uplift, and northward indentation of the Pamir began in the Paleocene (62–58 Ma).

1. Introduction The arcuate Pamir indentor is one of the most remarkable geomorphic features on Earth, and is located in the western part of the Tibetan Plateau at elevations between 4000–7700 m. However, it is unclear when and where uplift occurred in the Pamir Plateau. Although the high elevation of the Pamir is generally considered to be a consequence of the India–Asia continental collision that began at ca. 60 Ma, early Cenozoic tectonic shortening has rarely been documented (Molnar and Tapponnier, 1975; Tapponnier and Molnar, 1976; Hu et al., 2015). Studies of Pamir gneiss domes have documented prograde metamorphism and crustal thickening since 37–35 Ma, but the tectonic and geomorphological evolution of the Pamir prior to ∼37 Ma remains unclear (Smit et al., 2014; Stearns et al., 2015; Rutte et al., 2017a). Climate change in Central Asia is closely related to uplift and northward indentation of the Pamir, with consequent westward retreat of the

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proto-Paratethys Sea, reduced evaporation, blocking of water vapor from westerly winds, and aridification in Central Asia (Ramstein et al., 1997; Carrapa et al., 2014, 2015). Although the temporal and spatial evolution of Pamir uplift has important geological and climatic significance, the pre-Oligocene tectonic history of the region is still poorly understood. The impact of the continental collision between India and Asia on Pamir tectonics and climate change is controversial. This is mainly because: (1) large-scale crustal shortening and surface uplift led to extensive erosion of pre-, syn-, and post-collisional sediments (Schmidt et al., 2011; Sobel et al., 2013; Rutte et al., 2017b; Chapman et al., 2018a); and (2) the Pamir gneiss domes experienced high-grade and subsequent retrograde metamorphism, which overprinted geological information on the initial stage of crustal thickening during the early Cenozoic (Schmidt et al., 2011; Stearns et al., 2015; Rutte et al., 2017a). There are three hypotheses regarding the timing of initial uplift and

Corresponding author. E-mail address: [email protected] (X. Hu).

https://doi.org/10.1016/j.tecto.2019.228207 Received 29 April 2019; Received in revised form 7 September 2019; Accepted 16 September 2019 Available online 09 November 2019 0040-1951/ © 2019 Elsevier B.V. All rights reserved.

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Fig. 1. (A) Geological map of the Pamir and West Kunlun orogens (after Pan et al. (2004); Schwab et al. (2004), and Chapman et al. (2018b)). Sources of U–Pb zircon ages and εHf(t) values: 1–Chapman et al. (2018b); 2–Stearns et al. (2015); 3–Schwab et al. (2004); 4–Jiang et al. (2012); 5–Jiang et al. (2013); 6–Jiang et al. (2008); 7–Wei (2018); 8–Zhang et al. (2005); 9–Gao et al. (2013); 10–Zhang et al. (2006); 11–Yu et al. (2011); 12–Aminov et al. (2017); 13–Robinson et al. (2004); 14–Robinson et al. (2007); 15–Wang et al. (2013); 16–Liu et al. (2014); 17–Xiao et al. (2005); 18–Zhang et al. (2016); 19–Liu et al. (2015); 20–Cui et al. (2007); 21–Cui et al. (2006); 22–Hu et al. (2016). Upper Cretaceous shallow marine carbonates exposed discontinuously from Central Pamir to the Tianshuihai terrane (modified from Pan et al. (2004)) are shown in blue. (B) Simplified tectonic map of the Pamir–Tibetan region (modified from Robinson et al. (2004)). (C) Stratigraphy and petrography of Paleogene conglomerates deposited in the foreland of northeastern Pamir (Maerkansuhe, Akqiy, Oytag, Gaizihe, and Yigeziya sections modified from Chen et al. (2018); Qimugen section from this study; Aertashi section modified from Tang et al. (1989)). Four panels at lower left are detailed geological maps around the locations of the four measured sections: (D) Akqiy; (E) Bora Tokay; (F) Tuoyunduke; and (G) Qimugen. The location of sample 15YJ02 collected near the Tuoyunduke section is shown in (F). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).

northward indentation of the Pamir: late Oligocene–early Miocene (Sobel and Dumitru, 1997; Bershaw et al., 2012; Jiang and Li, 2014; Blayney et al., 2016; Cheng et al., 2017), middle–late Eocene (Yin et al., 2002; Carrapa et al., 2015; Sun et al., 2016; Kaya et al., 2019), or Paleocene (Chen et al., 2018; Zhang et al., 2018). The late Oligocene–early Miocene age is supported by structural (Jiang and Li, 2014; Cheng et al., 2017), sedimentological (Zheng et al., 2015; Blayney et al., 2019), provenance (Bershaw et al., 2012; Blayney et al., 2016), and thermochronological evidence (Sobel and Dumitru, 1997; Cao et al., 2015; Cheng et al., 2017). Although faulting of the basin margin, increased tectonic subsidence, and a greater supply of coarse detrital sediments in adjacent foreland basins occurred during the mid-Cenozoic, initial uplift and northward indentation of the Pamir may have begun earlier, possibly during the middle to late Eocene in association with major regression of the Tarim–Tajik Sea or during the Paleocene in response to initial continental collision between India and Asia. The western Tarim Basin, adjacent to the northeastern margin of the Pamir, preserves a continuous sedimentary succession from the Jurassic onwards (Compiling Group for Xinjiang Regional Stratigraphic Chart, 1981; Zhou, 2001; Jia, 2004), thus providing an ideal opportunity to investigate these unresolved scientific issues. We examined the Cretaceous–Paleogene sedimentary record of the

western Tarim Basin to investigate the possibility of early Cenozoic uplift of the Pamir and northwestern margin of the Tibetan Plateau (Figs. 1 and 2). Field observations, paleocurrent directions, conglomerate and sandstone petrography, and detrital zircon U–Pb geochronological and Hf isotopic data from four stratigraphic sections show that zircons from the Pamir reached the Tarim Basin as early as the Paleocene. Provenance analysis and documentation of facies changes in the Tarim Basin, Central Pamir, and Aksai Chin, combined with existing thermochronological data, indicate that initial uplift and indentation of the Pamir and associated thrust deformation along the northern margin of the Pamir–West Kunlun began during the Paleocene, much earlier than previously thought. 2. Geological setting The Pamir indentor is located between the Hindu Kush and Karakorum Ranges in the south, the Tajik Basin in the northwest, the Tian Shan in the north, and the Tarim Basin in the northeast (Fig. 1). The Pamir is subdivided into northern, central, and southern terranes by suture zones, which were formed during closure of oceanic seaways within the Tethys realm (Burtman and Molnar, 1993; Angiolini et al., 2013; Cao et al., 2015; Robinson, 2015; Chapman et al., 2018b). The 2

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Fig. 2. Stratigraphic columns of the four sections (Akqiy, Bora Tokay, Tuoyunduke, and Qimugen) measured along the northeastern margin of the Pamir, showing sample locations, conglomerate compositions, paleocurrent data, and facies interpretations. Colours in the logs correspond with the rock colours observed in the field.

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Tashkorgan and Karakorum faults separate the Pamir from the Tibetan Plateau in the east (Robinson et al., 2007; Cowgill, 2010). Before indentation, the Pamir terranes are thought to have had a general E–W trend and been the western continuation of accreted terranes in Tibet (Burtman and Molnar, 1993; Cowgill, 2010; Sun et al., 2016). Collision between the Qiangtang terrane and Eurasia occurred in the Late Triassic–Early Jurassic (Xiao et al., 2002; Ding et al., 2013; Cao et al., 2015). Magmatic activity was not recorded in the West Kunlun orogen after this event. However, south of the West Kunlun orogen, multiple stages of magmatism occurred during the late Mesozoic–Cenozoic in the Pamir and Tibetan domains (Pan et al., 2004; Schwab et al., 2004; Cao et al., 2015; Chapman et al., 2018b). As such, the magmatic, tectonic, and erosional history of the Pamir can be investigated with detrital zircon U–Pb ages and εHf(t) data. The main terrane and igneous units are described as follows.

Pamir terrane consists mainly of the relatively undeformed Karakul–Mazar granitoid belt and an accretionary complex of Carboniferous–Triassic age, both of which were formed during northward subduction in the Paleotethys Ocean and final docking of the Central Pamir block to the Kunlun block (Burtman and Molnar, 1993; Xiao et al., 2002; Schwab et al., 2004; Ding et al., 2013). The Karakul–Mazar granitoids are mainly Late Triassic in age (∼241–216 Ma) (Schwab et al., 2004), coeval with similar rocks exposed along the Karakax Fault in the northern Tianshuihai terrane (Fig. 1) (Pan et al., 2004; Schwab et al., 2004; Wei, 2018). The granitoids intruded into Carboniferous to Permian metasedimentary and metavolcanic rocks and Triassic deepsea turbidites (Burtman and Molnar, 1993; Xiao et al., 2002; Schwab et al., 2004; Ding et al., 2013). In modern rivers draining North Pamir, U–Pb ages of detrital zircons cluster between 240 and 190 Ma, with εHf (t) values of –4 to +6 (Figs. 3 and 4A) (Carrapa et al., 2014; Blayney et al., 2016).

2.1. Tarim Basin

2.4. Central Pamir terrane

The Tarim Basin has a rhomboid shape, and is bounded by the Tian Shan orogen to the north, the West Kunlun orogen to the south, and the Altyn–Tagh orogen to the southeast. Most of this basin is occupied by the Taklimakan Desert, with a few scattered outcrops along its margins. The major episodes of igneous and metamorphic activity in the Tarim block occurred during the Neoarchean (2800–2500 Ma), Paleoproterozoic (Orosirian; 2000–1800 Ma), Neoproterozoic (1000–600 Ma), and early Permian (290–270 Ma) (Zhang et al., 2013). These events are inferred to represent the formation and reworking of the Tarim continental basement (Zhang et al., 2013; Wang et al., 2014), assembly and breakup of the Columbia and Rodinia supercontinents (Wang et al., 2014), and eruption of continental flood basalts (Zhang et al., 2010), respectively. These events correspond with age spectra of detrital zircons found in both Mesozoic–Cenozoic strata and modern river sands in the western Tarim Basin, which cluster at 2800–2500, 2000–1800, and 1000–600 Ma (Fig. 3).

The Central Pamir terrane is a continental block correlated to the Band-e Bayan block in Afghanistan to the west (Girardeau et al., 1989; Montenat, 2009), and is also correlated to the southern part of the Tianshuihai terrane to the east (Schwab et al., 2004). Central Pamir is separated from South Pamir by the Rushan–Pshart suture zone. It consists of Paleozoic to Late Cretaceous metasedimentary rocks, gneiss domes with various protoliths, and Late Cretaceous (80–70 Ma), Eocene (42–36 Ma), and Miocene (20–10 Ma) igneous rocks (Schwab et al., 2004; Rutte et al., 2017b; Chapman et al., 2018b). εHf(t) values of these igneous rocks range from −5 to +5 (Chapman et al., 2018b). The sporadic Late Cretaceous igneous rocks, exposed mainly in the southern part of the Central Pamir terrane, are interpreted as resulting from extension associated with roll-back of the Neotethyan oceanic slab (Chapman et al., 2018b). The Eocene magmatism has been interpreted as originating from the delamination of Pamir–Tibetan lithospheric mantle and possibly lower crust (Chen et al., 2013; Chapman et al., 2018b). Eocene igneous rocks with similar geochemical characteristics can be traced along strike in the southern Tianshuihai terrane and northern Qiangtang terrane (Chung et al., 2005; Chapman et al., 2018b). Miocene magmatism was associated with the transition from crustal thickening to extensional exhumation, and is typically attributed to subduction followed by collision and slab break-off of the deep Indian and Asian lithospheres beneath the Pamir (Ducea et al., 2003; Jiang et al., 2012; Chapman et al., 2018b). U–Pb ages of detrital zircons from modern river sands are characterized by a peak at 43–33 Ma, and a minor age cluster at 78–61 Ma, with εHf(t) values of –6 to +4 (Figs. 3 and 4A), reflecting rapid erosion of Cretaceous–Paleogene igneous rocks (Lukens et al., 2012; Carrapa et al., 2014).

2.2. West Kunlun terrane The West Kunlun terrane is located along the northwestern margin of the Tibetan Plateau and is bounded by the Trans-Alai–Pamir thrust belt and the Kashgar–Yecheng transfer system to the north, and the Karakax Fault to the south (Cowgill, 2010) (Fig. 1). The West Kunlun terrane consists mainly of Precambrian metamorphic basement, Paleozoic to lower Mesozoic carbonate and clastic strata, and two major arc-related magmatic and volcanic–sedimentary units of Paleozoic age (Xiao et al., 2002; Cowgill et al., 2003; Pan et al., 2004; Schwab et al., 2004). However, recent studies interpreted the metamorphic rocks as the remnants of accretionary wedges formed in the Neoproterozoic–Cambrian, or even in the Triassic, as a result of subduction of Prototethys or Paleotethys oceanic lithosphere (Yang, 2013). Two igneous rock units formed in association with this subduction (Fig. 1), which are an older suite ranging in age from ∼513 to 384 Ma (Cowgill et al., 2003; Schwab et al., 2004; Ye et al., 2008; Liu et al., 2014) and a younger suite with ages of ∼338 to 216 Ma (Cowgill et al., 2003; Schwab et al., 2004; Jiang et al., 2013; Liu et al., 2015). Detrital zircons from modern river sands sourced from the West Kunlun orogen show distinctive 500–400 and 300–220 Ma age peaks with εHf(t) values ranging widely from –15 to +12 (Figs. 3 and 4A) (Carrapa et al., 2014; Blayney et al., 2016; Rittner et al., 2016). Magmatism of Jurassic to Cenozoic age is lacking in this terrane.

2.5. South Pamir terrane and Karakorum ranges The South Pamir terrane, located south of the Rushan–Pshart suture, includes Precambrian and Paleozoic metamorphic rocks, Carboniferous to Paleogene strata, and Triassic, Cretaceous, and Paleogene granites and diorites (Burtman and Molnar, 1993; Schwab et al., 2004; Chapman et al., 2018a). Extensive volcanic rocks, coeval with emplacement of the South Pamir batholith and Qiangtang magmatism, range in age from 130 to 95 Ma, with εHf(t) values from −15 to 0 (Chapman et al., 2018b). This volcanism is interpreted to document an active continental arc above the low-angle northward subduction of Neotethyan oceanic lithosphere, followed by collision along the Shyok suture and accretion of the Kohistan island arc to the Karakorum (Gaetani et al., 1993; Aminov et al., 2017; Chapman et al., 2018b). U–Pb ages of detrital zircons carried by modern rivers draining the South Pamir terrane cluster between 120 and 90 Ma, with a distinctive peak at ∼105 Ma, and εHf(t) values of −15 to +7 (Figs. 3 and 4A) (Lukens et al., 2012). The Karakorum Range comprises an Ordovician to Cretaceous

2.3. North Pamir terrane The boundary between North Pamir and West Kunlun, although not clearly defined, is generally accepted to be the Ghez Fault, and the southern boundary is represented by the Tanymas suture zone (Robinson et al., 2004; Schwab et al., 2004; Cowgill, 2010). The North 4

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Fig. 3. Normalized probability density plots (PDP) for detrital zircon U–Pb ages from modern river sands and bedrock in potential source areas. References: ①–Rittner et al. (2016); ②–Carrapa et al. (2014); ③–Blayney et al. (2016); ④–Lukens et al. (2012); ⑤–Lukens et al. (2012); Carrapa et al. (2014); Blayney et al. (2016); ⑥–Schaltegger et al. (2002); Jagoutz et al. (2009); Ravikant et al. (2009); Bosch et al. (2011); Bouilhol et al. (2011, 2013); ⑦–Yang et al. (2014); ⑧–Carrapa et al. (2015).

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Fig. 4. Zircon U–Pb age versus εHf(t) plots. (A) Data for bedrock and sands of modern rivers draining potential source areas (after Ravikant et al. (2009); Bouilhol et al. (2013), and Carrapa et al. (2014)). (B) Data for detrital zircons from Cretaceous–Eocene sandstones analysed in this study.

The Kashi Group includes (from bottom to top) the Aertashi, Qimugen, Kalataer, Wulagen, and Bashibulake formations (Compiling Group for Xinjiang Regional Stratigraphic Chart, 1981; Tang et al., 1992; Zhou, 2001). A thick, white gypsum and anhydrite unit is overlain by three grey–green limestone units alternating with three purple–red (gypsiferous) argillite and argillaceous sandstone units, documenting three cycles of shallow marine to sabkha and coastal plain sedimentation (Tang et al., 1992; Zhang et al., 2018). Abundant foraminifera, bivalves, coccoliths, and dinoflagellates, combined with magnetostratigraphic data, suggest a Selandian to Priabonian depositional age (Tang et al., 1992; Zhang et al., 2018; Kaya et al., 2019). Facies analysis shows that the western Tarim Basin experienced five transgressive–regressive cycles from the mid-Cretaceous (Cenomanian) to the late Eocene (Priabonian) (Zhang et al., 2018). Middle and inner ramp facies characterize transgressions, whereas coastal plain, lacustrine, and fluvial facies characterize regressions (Tang et al., 1992; Zhang et al., 2018). Initial transgression and transition from coastal plain to carbonate tidal flat environments is documented by purple–red or orange–red argillite interbedded with siltstone passing upwards to massive limestones with laminoid fenestrae or miliolids. The third transgression is distinct, and marked by a transition from orange–red argillite to thickly bedded gypsum reaching thicknesses of tens of meters to > 200 m in the western Tarim Basin, as indicated by outcrops exposed along the basin margin and boreholes within the basin (Zhang et al., 2018). While the gypsum was being accumulated, thick conglomerates were being deposited along the northeastern margin of the Pamir (Fig. 1C) (Chen et al., 2018; Zhang et al., 2018). This conglomerate unit belongs the Paleogene Kashi Group, found only along the northeastern margin of the Pamir, which decreases progressively in thickness from west to east. The sudden influx of clast-supported conglomerates with cobbles and boulders interbedded with pebbly sandstone, sandstone, and some silty argillite, may document a major tectonic event in this area.

sedimentary succession that grades northwards into black slates (Gaetani et al., 1990), a central area with mid-Cretaceous to Paleogene quartz diorites and granodiorites (Debon et al., 1987), and amphibolitefacies metasediments and related migmatitic domes in the south (Rolland et al., 2001). 2.6. Kohistan and Ladakh arcs The Kohistan and Ladakh arcs, located between the Shyok suture to the north and the Indus suture to the south, were formed by northward subduction of Neotethyan oceanic lithosphere during the Cretaceous (Bouilhol et al., 2013). Magmatic activity lasted from 112 to 40 Ma, and igneous zircons are characterized by εHf(t) values that cluster between +2 to +15 and range from −7 to +15 (Figs. 3 and 4A) (Ravikant et al., 2009; Bouilhol et al., 2013). 2.7. Stratigraphy and sedimentology of the western Tarim Basin The Tarim Basin is a long-lived sedimentary basin that records multiple tectonic events (Jia, 2004). Its western part contains a > 10km-thick sedimentary succession deposited since the Jurassic, which includes the Cretaceous–Paleogene Yingjisha and Kashi groups that are the main focus of this study. The Yingjisha Group comprises (from bottom to top) the Kukebai, Wuyitake, Yigeziya, and Tuyiluoke formations (Compiling Group for Xinjiang Regional Stratigraphic Chart, 1981; Tang et al., 1992; Zhou, 2001). Two limestone units alternating with two orange–red (gypsiferous) argillite units record two sedimentary cycles that mark the transition from shallow marine to sabkha and coastal plain environments (Tang et al., 1992; Zhang et al., 2018). Abundant foraminifera, bivalves, dinoflagellates, and brachiopods in the marine strata indicate a Cenomanian to Danian depositional age (Zhang et al., 2018; Kaya et al., 2019). 6

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Fig. 5. Stratigraphic correlation of Upper Cretaceous shallow marine limestones from the Tajik Basin across the Central Pamir and Aksai Chin to the Tarim Basin, showing the regression of epicontinental seas from Central Pamir–Aksai Chin at the end-Cretaceous.

than the mid-Paleocene (Deng, 1998). The sandstones overlying these limestones in the Akbaital–Rangkul imbricate thrusts, which were dated as Campanian to Maastrichtian in the Central Pamir (Burtman and Molnar, 1993; Rutte et al., 2017b), yield detrital zircons with a youngest U–Pb age of ∼75 Ma (n = 2) (Rutte et al., 2017b). Farther to the west in the Band-e Bayan block, pithonellas and Globotruncana spp. collected from bedded limestones indicate a Senonian age (Girardeau et al., 1989; Montenat, 2009). Thus, a shallow sea extending from the Band-e Bayan block and Central Pamir to the eastern Tianshuihai terrane existed along the southern margin of the West Kunlun orogen until the Maastrichtian (Fig. 5). The Central Pamir and Aksai Chin regions remained as low-lying areas along the southern margin of the paleoKunlun highlands throughout the late Mesozoic. The Upper Cretaceous limestones are unconformably overlain in the Aksai Chin (Wen et al., 2000) by red siltstones and sandstones with conglomerate and breccia layers in the Wuzibieli mountain pass region (Compiling Group for Xinjiang Regional Stratigraphic Chart, 1981; Wen et al., 2000), and by a conglomerate of possible Paleogene age in the Akbaital–Rangkul imbricate thrust region and Kalaktash–Kozyndy klippen of the Central Pamir (Rutte et al., 2017b). These continental clastic deposits, which formed in alluvial fan and flood plain environments of a piedmont, document a major regression and large-scale tectonic uplift beginning in the latest Cretaceous or Paleocene.

3. Previous studies of the Central Pamir–Aksai Chin During the Permian–Triassic, a magmatic arc formed along the southern part of the West Kunlun terrane due to northward subduction of Paleotethyan oceanic lithosphere (Xiao et al., 2002; Ding et al., 2013). Following collision with the southern Qiangtang terrane in the Late Triassic–Early Jurassic, shortening and surface uplift induced the formation of topographic relief in the West Kunlun Mountains (Ding et al., 2013; Cao et al., 2015; Robinson, 2015). To the south of the mountains, widespread deposition of shallow marine carbonates with diverse faunas (bivalves, gastropods, corals, and sponges) characterized the Central Pamir, South Pamir, and Aksai Chin during the Jurassic (Fig. 5) (Wen et al., 2000; Rutte et al., 2017b). In the Akbaital–Rangkul thrust sheets and Kalaktash–Kozyndy klippen of the Central Pamir, Upper Jurassic limestones are overlain by Lower Cretaceous strata containing interbedded sandstone and conglomerate (Rutte et al., 2017b). The region between the Wuzibieli mountain pass and Aksai Chin was not the site of sedimentation at that time (Wen et al., 2000). A narrow belt of Upper Cretaceous shallow marine limestones is exposed from the Band-e Bayan block in Afghanistan in the west, to the Bartang thrust sheet, the Akbaital–Rangkul imbricate thrust region, the Wuzibieli mountain pass of the Central Pamir, and the Aksai Chin in the east, where it is known as the Tielongtan Group (Fig. 1) (Compiling Group for Xinjiang Regional Stratigraphic Chart, 1981; Burtman and Molnar, 1993; Wen et al., 2000; Pan et al., 2004; Rutte et al., 2017b). The Tielongtan Group contains rudists and planktonic foraminifers of Turonian to Campanian age, and locally abundant orbitolinids suggest initial deposition during the Cenomanian (Wen et al., 2000; Luo, 2010). The Tielongtan Group is cross-cut and thermally affected by volcanic rocks dated at 60 Ma, which constrains the depositional age to be older

4. Methods We measured and made detailed sedimentological observations of four Cretaceous–Paleogene stratigraphic sections widely spaced along the northeastern margin of the Pamir in the western Tarim Basin (Xinjiang Uygur Autonomous Region, China). From west to east, these 7

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are the Akqiy, Bora Tokay, Tuoyunduke, and Qimugen sections (Fig. 2), which range in thickness from 100 m to > 1000 m. We collected 542 samples at a sampling interval of 1–2 m. Microfacies analysis was carried out on carbonate samples, and provenance analysis was performed on locally intercalated sandstones and mixed carbonate–siliciclastic layers. Twenty-seven samples were selected for petrographic, geochronological, and isotopic studies. Stratigraphic ages and correlations amongst these four sections, as well as an analysis of the sedimentary facies and paleowater-depth, were presented by Zhang et al. (2018). 4.1. Sedimentary petrography and paleocurrent directions Conglomerate provenance was investigated in the field by counting at least 80 clasts at each of two sites in the Akqiy section using a 10 cm grid. Paleocurrent data were obtained from clast imbrication. Data were corrected to the horizontal with Stereonet 9.5 software. The average trough axis orientation of 15–20 observations at each site was determined on a stereographic plot. Sandstones were classified according to the nomenclature of Garzanti (2019), although carbonate grains were not considered because of uncertainties involved in the distinction of their extrabasinal versus intrabasinal origin in some samples. More than 400 points were counted on each of 21 less-altered sandstone samples following the Gazzi–Dickinson method, in which crystals or clasts larger than 62.5 μm contained within rock fragments are reclassified as single grains (Ingersoll et al., 1984). Results are shown in Supplementary Material S1.

Fig. 6. Compiled Cretaceous–Paleogene paleocurrent statistics for the western Tarim Basin.

4.2. Detrital zircon U–Pb geochronology and Hf isotopes

limestone in the Kezilesu Group; 64% sandstone and 36% limestone in the Aertashi Formation) (Fig. 2). Thin-section observations indicate that limestone clasts in both units contain faunas typical of Paleozoic carbonate platforms, and are significantly different from faunas found in the Cretaceous–Paleogene marine carbonates of the Tarim Basin. Clast imbrication indicates paleocurrent directions toward the NNW in mid-Cretaceous strata, and toward the N and NNE in Paleocene strata (Figs. 2 and 6).

U–Pb ages of detrital zircons were obtained on ten sandstone samples. Zircon grains were separated by crushing, elutriation, magnetic separation, and then handpicked, mounted in epoxy resin, and polished. U–Pb dating was conducted by laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) at the State Key Laboratory of Mineral Deposits Research, Nanjing University, Nanjing, China. Laser sampling was performed using a New Wave UP193 laser ablation system with a spot size of 32 μm and a repetition rate of 10 Hz. An Agilent 7500a ICP–MS instrument was used to acquire ion signal intensities. GLITTER 4.4 was used for calculating results and relevant isotopic ratios (Jackson et al., 2004). Results are shown in Supplementary Material S2. In situ zircon Hf isotope analyses were performed on four samples following U–Pb dating at the State Key Laboratory of Mineral Deposits Research, Nanjing University, using a Neptune (Plus) multiple-collector ICP–MS attached to a New Wave ArF 193 nm laser ablation system. Spot analyses were ablated with a laser beam diameter of 35 μm, 8 Hz repetition rate, and energy of 15.5 J/cm2. For each spot, 200 analyses were obtained, and the mean 176Lu/177Hf, 176Yb/177Hf, and 176Hf/177Hf ratios were calculated. The decay constant for 176Lu of 1.867 × 10–11 yr–1 proposed by Söderlund et al. (2004) was adopted. Results are shown in Supplementary Material S3.

5.2. Sandstone petrography Cretaceous sandstones are mainly feldspatho-quartzose, whereas Cenozoic sandstones are mostly feldspatho-litho-quartzose and lithofeldspatho-quartzose with a marked increase in lithic fragments (Figs. 7 and 8). Cretaceous sandstones are dominated by volcanic lithics, whereas Cenozoic sandstones contain both volcanic and low-grade metamorphic lithics (classification after Garzanti and Vezzoli, 2003). Therefore, detrital modes document a distinct provenance change between the uppermost Cretaceous and Paleocene strata. 5.3. Detrital zircon U–Pb ages and Hf isotopes U–Pb ages and Hf isotopic signatures of detrital zircons in strata of the western Tarim Basin document a major provenance change during the early Paleocene (Figs. 4B and 9 ). Upper Jurassic and Cretaceous sandstones (samples 15YJ02, 15YJ06, 16AK02, 15TY20, 15TY79, and 16AK99) are characterized by distinctive 500–400 and 300–240 Ma age peaks. A smaller 240–200 Ma age peak is invariably present in all of the younger samples (samples 16BE30, 16BE36, 15QM79, and 15QM159). Paleocene–Eocene (63–41 Ma) and sporadic Cretaceous ages (120–90 Ma) also occur, whereas the 500–400 and 300–240 Ma age peaks become less pronounced from the bottom to top of the Eocene section. Detrital zircons with Paleozoic ages yielded εHf(t) values ranging widely from –15 to +12, clustered between –9 and +4 for Cambrian–Devonian zircons (500–400 Ma) and –3 and +9 for

5. Results 5.1. Conglomerate petrography and paleocurrent directions In the Akqiy section, conglomerate clasts were counted both at the bottom of the section in the Lower Cretaceous Kezilesu Group, and in the Paleocene Aertashi Formation at the top of the section. Conglomerates with clasts of 2–15 cm in diameter and interbedded with moderately to well sorted pebbly sandstone and sandstone were deposited in distal alluvial fan or braided river environments. All clasts in both units consist of sedimentary rocks (54% sandstone and 46%

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Fig. 7. Petrographic differences between the Upper Cretaceous quartz-rich (Kukebai Formation) and upper Eocene lithic-rich (Bashibulake Formation) sandstones. Q = quartz; K = K-feldspar; P = plagioclase; L = lithic fragments (Lv = volcanic, Ls = terrigenous clastic, Lc = carbonate, and Lm = metamorphic).

Permian–Triassic zircons (300–240 Ma) (Fig. 4B). The rare Triassic zircons (240–200 Ma) yielded εHf(t) values between –6 and +5, and the only mid-Cretaceous (97 Ma) grain has an εHf(t) value of –0.3. Paleocene–Eocene (63–41 Ma) zircon εHf(t) values vary between −5 and +5. Zircons from a tuff bed in the lower part of the Qimugen Formation (sample 15QM184; weighted-mean age = 57.6 Ma) yielded εHf(t) values from −2.0 to +0.5 (Zhang et al., 2018). Sun et al. (2016) reported εHf(t) values between –9 and +5 for Cretaceous detrital zircons and between –7 and +2 for Paleogene detrital zircons from the Oytag section, which is consistent with the values obtained in this study.

6. Discussion 6.1. Paleocene provenance change In Jurassic to Paleogene sandstones, detrital zircons show predominant Cambrian–Devonian (500–400 Ma) and late Carboniferous–Triassic ages (300–240 Ma), with εHf(t) values ranging from –9 to +4 and –3 to +9, respectively (Figs. 4B and 9). These signatures are consistent with those of zircons in widely exposed granitoid batholiths and arc-related volcanic rocks, as well as in modern river sands, from the West Kunlun orogen (Figs. 1 and 3) (Blayney et al.,

Fig. 8. Sandstone petrography. (A) QFL plot. (B) Lithic plot. Q = quartz; F = feldspars (K = K-feldspar and P = plagioclase); L = lithic fragments (Lv = volcanic, Ls = sedimentary, and Lm = metamorphic).

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Fig. 9. Normalized probability density plots (PDP) for detrital zircon U–Pb ages from sandstones in the four studied sections as compared with the data shown in Fig. 3.

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2016; Rittner et al., 2016; Sun et al., 2016). The 240–200 Ma detrital zircons with εHf(t) values between –6 and +5, which are rare in Cretaceous sandstones but common in Paleogene sandstones, are similar to zircons from granitoids exposed in the Karakul–Mazar and northernmost Tianshuihai regions and river sand derived from these areas, indicating provenance from either North Pamir or the area along the Karakax Fault in the northern Tianshuihai terrane (Carrapa et al., 2014; Blayney et al., 2016). Detrital zircons with ages between 122 and 97 Ma and εHf(t) values from –7 to +2 are typical of batholiths in the South Pamir and Qiangtang terranes, and river sand derived from these terranes (Lukens et al., 2012; Carrapa et al., 2014; Chapman et al., 2018b). Igneous rocks of the Central Pamir and Rushan–Pshart suture zone, and modern river sand derived from these areas yield U–Pb zircon ages that cluster between 78–61 and 43–33 Ma, with εHf(t) values of –5 to +5 (Lukens et al., 2012; Carrapa et al., 2014; Chapman et al., 2018b). However, detrital zircons dated at 61–43 Ma have not been reported from the Pamir (Figs. 1 and 3). These may have been derived from igneous rocks that have either been completely eroded or are still not identified. A provenance from the Central Pamir–Aksai Chin region is likely based on the following lines of evidence.

erosion of Central Pamir and southern Tianshuihai, between the endCretaceous and earliest Cenozoic. Since then, the Central Pamir–southern Tianshuihai and South Pamir have supplied sediment to the Tarim Basin. The provenance change documented by the detrital zircons reflects erosion of the West Kunlun orogen and successive contributions from drainage areas, including first the North Pamir, next the Central Pamir (or southern Tianshuihai), and finally the South Pamir or Qiangtang terranes. The notable up-section decrease in detrital zircons dated at 500–400 and 300–240 Ma derived from West Kunlun indicates progressive dilution while the source areas expanded to include the Central and South Pamir. 6.2. Initial growth of the Pamir during the Paleocene Various independent lines of evidence outlined below suggest that initial tectonic growth and indentation of the Pamir occurred in the Paleocene, and was associated with thrusting and formation of a foreland basin along the northern margin of the Pamir–West Kunlun. 6.2.1. Facies changes In the western Tarim Basin, a prominent change in sedimentary facies occurred in the Paleocene (Fig. 2). During the third transgression dated as mid-Paleocene, the thick gypsum deposits (> 200 m) of the Aertashi Formation, which lack limestone and terrigenous interbeds, were rapidly deposited in the western Tarim Basin. In contrast to the thin-bedded gypsum layers interbedded with tidal carbonates deposited at the margins of the sedimentary basin due to sea-level change, the thick basin-filling evaporites resulted from tectonic isolation of the basin from ocean waters (Kendall, 1996; Warren, 2016). The Tarim Basin at this time was a restricted bay receiving only periodic injection or seepage of seawater brines from an adjacent seaway. A viable explanation for such a major environmental change is the geomorphological evolution associated with northward indentation of the Pamir, inducing accelerated localized subsidence and formation of a topographic barrier hampering the free exchange of seawater with the proto-Paratethys in the west. Deposition of the conglomerate unit exposed in northernmost Pamir (Tang et al., 1992; Chen et al., 2018) contemporaneous with the accumulation of thick gypsum can be ascribed to either tectonic (Zhang et al., 2018) or climatic change (Kaya et al., 2019). Tectonic control is suggested by the following lines of evidence.

(1) 61–43 Ma zircons are common in Paleogene strata of the Tarim and Tajik basins (Fig. 3), indicating a source area common to both (Carrapa et al., 2015; Sun et al., 2016). This source area cannot be the West Kunlun, North Pamir, or Tian Shan because they all lack igneous rocks of this age. (2) Magmatic activity in the Kohistan and Ladakh arcs continued from 112 to 40 Ma (Bouilhol et al., 2013), but zircon εHf(t) values are mostly positive (+2 to +15), in contrast to the εHf(t) values of −5 to +5 for the 61–43 Ma detrital zircons (Fig. 4). (3) Volcanic rocks with ages between 61 and 43 Ma are common in the eastern Tianshuihai and Qiangtang terranes, which are along-strike correlations of Central Pamir (Deng, 1998; Chung et al., 2005; Chapman et al., 2018b). As such, Paleogene magmatism may have also occurred in Central Pamir. (4) Zircon εHf(t) values of igneous rocks dated at 43–33 and 78–61 Ma in Central Pamir are consistent with those of detrital zircons dated at 61–43 Ma from the Tarim Basin (Fig. 4) (Carrapa et al., 2014; Chapman et al., 2018b), which suggests a similar magmatic source. (5) Magmatic intrusions dated at ∼80–70 and ∼42–36 Ma occur sporadically in the northwestern Central Pamir and Rushan–Pshart suture zone (Fig. 1A), where Cenozoic strata are mostly lacking, indicating widespread Cenozoic erosion (Rutte et al., 2017b; Chapman et al., 2018b). (6) Detrital zircons dated at 61–43 Ma are common in Paleogene strata of the Tarim and Tajik basins, decrease notably in Neogene strata, and are missing in river sand from Central Pamir (Lukens et al., 2012; Cao et al., 2014; Carrapa et al., 2014, 2015; Sun et al., 2016), which is consistent with complete erosion of source rocks.

(1) The deposition of conglomerates and thick gypsum in the Tarim Basin, the appearance of detrital zircons derived from the Central Pamir, and a progressive increase in subsidence rates all occurred simultaneously during the Paleocene (Fig. 10). Uplift and indentation of the Pamir, consequent reactivation of thrust faults along the basin margin, and formation of a foreland basin represent a plausible scenario for such a major paleogeographic change. (2) The conglomerate unit is limited to the northeastern margin of the Pamir (74–76 °E) and does not continue to the east (Fig. 1), suggesting that it was controlled by uplift and indentation of the Pamir rather than by regional climatic conditions. (3) The stratigraphic thickness of the Paleogene Kashi Group reaches a maximum at the northern edge of the Pamir, whereas it gradually decreases from west to east (Tang et al., 1989). This is more likely to be related to the uplift and indentation of the Pamir and initial formation of the foreland basin to the west of the Tarim Basin. Tectonic subsidence was significant in the foreland, but decreases with increasing eastward distance from the Pamir. (4) Limestone clasts in the conglomerate contain fossils typical of Paleozoic carbonate platforms, implying provenance from Paleozoic limestone exposed to the south of the Akqiy section along the northern margin of the West Kunlun (Pan et al., 2004). All clasts were derived from sedimentary rocks, which is consistent with shallow erosion levels during the initial stages of tectonism.

Detrital zircons document a major provenance change in the Paleocene in the studied sections (Fig. 9). The 500–400 and 300–240 Ma age peaks characterizing Upper Jurassic–Cretaceous sandstones indicate sediment provenance mainly from the West Kunlun Mountains. Sporadic zircon grains dated at 240–200 Ma indicate a source area not farther south than the North Pamir. Zircons dated as 63–41 and 122–97 Ma were found in upper Paleocene to lower Eocene and overlying midEocene strata. Their source areas are Central Pamir–southern Tianshuihai and South Pamir, respectively. The Central Pamir and southern Tianshuihai regions were occupied by shallow seas during the Late Cretaceous (Fig. 5) (Compiling Group for Xinjiang Regional Stratigraphic Chart, 1981; Burtman and Molnar, 1993; Wen et al., 2000; Luo, 2010; Rutte et al., 2017b). These seas would have trapped detrital minerals transported to the Tarim Basin from the south. However, detrital zircons with ages of 63–41 and 122–97 Ma began to appear in the Tarim Basin, reflecting the uplift and 11

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Fig. 10. Generalized stratigraphic column for Cretaceous to Paleogene strata in the southwestern Tarim Basin (modified after Zhang et al. (2018)). The paleowaterdepth curve and tectonic–subsidence rates are from Zhang et al. (2018), and zircon U–Pb ages and εHf(t) values are from Chapman et al. (2018b). Weighted-mean εHf (t) values of Mesozoic to Cenozoic igneous rocks are plotted as boxes with 2σ uncertainty error bars; single detrital zircon εHf(t) analyses are plotted as blue dots. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).

than, the West Kunlun Mountains (Fig. 11C). The change in topographic profile, declining from east to west in the Late Cretaceous and from south to north in the Paleocene, suggests a tectonic driving mechanism, which we ascribe to uplift of the Central Pamir. Clast imbrication in the Akqiy section indicates paleocurrent directions toward the NNW in mid-Cretaceous strata and toward the NNE in Paleocene strata (Fig. 6). In the Oytag section, ∼63 km east of the Akqiy section, measured paleocurrent directions are to the N and NNW in Cretaceous strata, but toward the NE in Cenozoic strata (Sobel, 1999; Bershaw et al., 2012; Sun et al., 2016). Farther to the east, in the Qimugen and Aertashi sections, paleocurrent indicators are scarce in Cretaceous strata (Sobel, 1999; Zheng et al., 2010; Cao et al., 2014) and dominantly to the NE in Cenozoic strata. As such, significant changes in paleocurrents between the

6.2.2. Paleo-drainage changes The areal distribution of the Upper Cretaceous limestone belt indicates that the Central Pamir–Aksai Chin sea was connected with the proto-Paratethys Sea occupying the Band-e Bayan block and Tajik–Afghan Basin (Figs. 5 and 11A) (Golonka, 2007). Given that the sea retreated from east to west, Central Pamir rivers should have drained toward the Band-e Bayan block in Afghanistan at that time (Fig. 11 B). However, in the Paleocene–Eocene, rivers cut through the West Kunlun orogen and headward into South Pamir, as indicated by the successive appearance of detrital zircons from the Central Pamir–Aksai Chin and South Pamir in sedimentary strata of the Tarim and Tajik basins. This Paleocene reorganization of drainage patterns in the Central Pamir, documented by provenance analysis of detrital zircons, indicates that Central Pamir–Aksai Chin became as high as, or higher

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Fig. 11. Sketch maps showing the inferred paleogeographic and paleo-drainage evolution of the study area. (A) During the Late Cretaceous (∼85 Ma), the West Kunlun orogenic highlands separated two shallow marine basins. Sediments were delivered to the western Tarim Basin from the West Kunlun terrane and North Pamir. (B) In the early–middle Paleocene (∼62 Ma), the Central Pamir–Aksai Chin area was the site of conglomeratic sedimentation and/or the formation of a tectonically induced unconformity following regression. A belt of conglomerates began to be deposited in the piedmont along the northeastern Pamir margin, while a thick gypsum unit filled the Tarim Basin. Rivers feeding the southwestern Tarim Basin were sourced from North Pamir. (C) During the early Eocene, detrital zircons were sourced by rivers from Central and South Pamir, which flowed northward into the southwestern Tarim Basin. At this time, the Pamir was as high as, or even higher than, the West Kunlun Mountains.

partial annealing or retention zones rather than being evidence of cooling during erosional exhumation (Sobel and Dumitru, 1997; Sobel et al., 2013). However, proximal tectonism during the latest Cretaceous–Paleocene in West Kunlun–Pamir represents a viable alternative explanation. As explained by Cao et al. (2013), the ages may indicate that the Kashgar–Yecheng transfer system was active during the early Cenozoic. Moreover, extensive thermochronological data from Pamir and the northwestern Tibetan Plateau indicate active shortening, uplift, and exhumation at this time. In the North Pamir, apatite and zircon (U–Th)/ He ages of Triassic granitoids record a period of accelerated exhumation at ∼50–40 Ma (Amidon and Hynek, 2010). In the Central Pamir, although evidence of the earlier tectonism has largely been overprinted, prograde metamorphic monazite ages as old as ∼50 Ma obtained for lower-crustal xenoliths lacking retrograde reactions in Miocene volcanic rocks indicate that crustal thickening and plateau formation were already occurring during the Paleocene–Eocene (Ducea et al., 2003). In the South Pamir, (U–Th)/He cooling ages of apatite and zircon in bedrock range from latest Cretaceous to early Paleogene, and also record regional exhumation associated with initial crustal thickening (Chapman et al., 2018a). In the northern Karakorum, a marked angular unconformity is overlain by Campanian conglomerates, and no strata younger than the Campanian are preserved or have been deposited (Gaetani et al., 1990, 1993). Although this is the last geological event recorded in the northern Karakorum, monazite as young as 63 Ma in sillimanite-bearing metapelites of the southern Karakorum indicate crustal shortening and thickening during the early Cenozoic (Fraser et al., 2001). In the Qiangtang and Lhasa blocks of the Tibetan Plateau, east of the Pamir, most thermochronological ages (biotite and K-feldspar 40 Ar/39Ar, apatite fission-track, and apatite (U–Th)/He) are concentrated between 75 and 43 Ma, and generally interpreted as plateau growth during the Late Cretaceous to early Cenozoic (Rohrmann et al., 2012).

Cretaceous and Paleocene were documented in most stratigraphic sections along the northeastern margin of the Pamir (Fig. 6). During the Cretaceous, paleocurrent directions toward the N, perpendicular to the West Kunlun Mountains, suggesting that the northern margin of the Pamir was relatively straight and E–W-trending. Subsequently, paleocurrent directions progressively rotated from N to NE, which is consistent with the onset of northward indentation, when the northern edge of the Pamir became gradually curved and the strike of the West Kunlun rotated from E–W to NW–SE (Fig. 11). 6.2.3. Provenance changes Sandstone petrography documents an abrupt provenance change between the latest Cretaceous and Eocene (Figs. 7 and 8). Cretaceous sandstones are mainly feldspatho-quartzose with minor polycrystalline quartz and volcanic lithic fragments, which is a composition generally found in the interior of continental blocks (Dickinson, 1985; Garzanti, 2016). Source rocks may have been located within the West Kunlun orogen, including arc-related granites and associated pyroclastic rocks. However, Paleogene sandstones are mostly litho-feldspatho-quartzose and feldspatho-litho-quartzose, indicating an orogenic provenance and deposition in a foreland basin. Source rocks included Paleozoic sandstones and limestones exposed along the northern margin of the West Kunlun to the south of the studied sections. Low-grade metamorphic and volcanic rock fragments increase up-section, suggesting increasing supply from the thrust belt as it propagated basinward. The absence of schist fragments indicates that the Precambrian metamorphic basement of the West Kunlun (Pan et al., 2004) was not being exhumed during this early orogenic stage, when the erosion level was very shallow. 6.2.4. Thermochronological evidence Fission-track and (U–Th)/He ages of apatite and zircon that cluster between ∼70 and 40 Ma are widespread in both Cenozoic strata of the western Tarim Basin and bedrock in the West Kunlun orogen (Sobel and Dumitru, 1997; Cao et al., 2013; Sobel et al., 2013). However, these ages were interpreted as being partially reset during residence in the

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6.2.5. Alternative hypotheses Several studies have suggested that initial indentation and uplift of the Pamir began in the middle–late Eocene (∼40 Ma) (Yin et al., 2002; Carrapa et al., 2015; Sun et al., 2016; Kaya et al., 2019), based on the following lines of evidence.

Acknowledgments This work benefited from discussions with Anlin Ma, Zhong Han, and Yiwei Xu, and the assistance of Waili Mahemuti in the field. We are grateful to Zhengxiang Li, Alexander Robinson, James B. Chapman, Guillaume Dupont-Nivet, and an anonymous reviewer for their constructive suggestions during various phases of manuscript revision. This study was financially supported by the National Key R&D Plan (Grant No.2017YFC0601405), the National Natural Science Foundation of China (NSFC)International Exchanges Scheme (41511130067), a NSFC–Newton Advanced Fellowship (41761130076), and Program B for Outstanding PhD Candidates of Nanjing University. All data are listed in the references or archived in the Supporting Information.

(1) A significant increase in tectonic subsidence in the Tarim and Tajik basins at ∼40 Ma (Yin et al., 2002; Carrapa et al., 2015; Kaya et al., 2019). (2) Regression of the proto-Paratethys Sea in the Tarim and Tajik basins at ∼40 Ma (Carrapa et al., 2015; Sun et al., 2016; Kaya et al., 2019). (3) The petrographic compositions of mid-Eocene sandstones (∼46 Ma in the Tarim Basin and ∼40 Ma in the Tajik Basin) indicate an orogenic provenance (Yin et al., 2002; Carrapa et al., 2015).

Appendix A. Supplementary data

However, the acceleration of tectonic subsidence at ∼40 Ma in the Tarim and Tajik basins was preceded by a slow increase that started in the Paleocene (Fig. 10) (Zhang et al., 2018). Numerous foreland basins are characterized by initially slow subsidence followed by a sudden increase in subsidence, which is a trend associated with a temporal change in depositional setting from distal to proximal (DeCelles and Giles, 1996). The onset of slow tectonic subsidence during the Paleocene can thus be considered as evidence of foreland basin formation considerably earlier than the middle–late Eocene. It is difficult to establish whether sea-level changes in the protoParatethys, culminating in the final retreat of the sea from the Tarim and Tajik basins, were controlled by tectonics, climate, or eustatic sealevel changes. Changes in paleowater-depth during the middle–late Eocene in the Tarim Basin are consistent with global eustatic fluctuations (Fig. 10), including the ∼40 Ma regression, which might thus be ascribed to a global sea-level fall. However, the paleowater-depth curve reconstructed for the middle Paleocene–early Eocene is markedly different from the global sea-level curve, suggesting a tectonic control (Zhang et al., 2018). The orogenic provenance of mid-Eocene sandstones from the Tarim and Tajik basins (Fig. 8) confirms that tectonism was active at that time, whereas the change in provenance from cratonic to orogenic took place notably earlier, between the end-Cretaceous and Paleocene.

Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.tecto.2019.228207. References Amidon, W.H., Hynek, S.A., 2010. Exhumational history of the north central Pamir. Tectonics 29. Aminov, J., Ding, L., Mamadjonov, Y., Dupont-Nivet, G., Aminov, J., Zhang, L.-Y., Yoqubov, S., Aminov, J., Abdulov, S., 2017. Pamir Plateau formation and crustal thickening before the India-Asia collision inferred from dating and petrology of the 110–92 Ma Southern Pamir volcanic sequence. Gondwana Res. 51, 310–326. Angiolini, L., Zanchi, A., Zanchetta, S., Nicora, A., Vezzoli, G., 2013. The Cimmerian geopuzzle: new data from South Pamir. Terra Nova 25, 352–360. Bershaw, J., Garzione, C.N., Schoenbohm, L., Gehrels, G., Tao, L., 2012. Cenozoic evolution of the Pamir plateau based on stratigraphy, zircon provenance, and stable isotopes of foreland basin sediments at Oytag (Wuyitake) in the Tarim Basin (west China). J. Asian Earth Sci. 44, 136–148. Blayney, T., Dupont‐Nivet, G., Najman, Y., Proust, J.N., Meijer, N., Roperch, P., Sobel, E.R., Millar, I., Guo, Z., 2019. Tectonic evolution of the Pamir recorded in the western Tarim Basin (China): sedimentologic and magnetostratigraphic analyses of the Aertashi section. Tectonics 38, 492–515. Blayney, T., Najman, Y., Dupont‐Nivet, G., Carter, A., Millar, I., Garzanti, E., Sobel, E.R., Rittner, M., Andò, S., Guo, Z., 2016. Indentation of the Pamirs with respect to the northern margin of Tibet: constraints from the Tarim basin sedimentary record. Tectonics 35, 2345–2369. Bosch, D., Garrido, C.J., Bruguier, O., Dhuime, B., Bodinier, J.-L., Padròn-Navarta, J.A., Galland, B., 2011. Building an island-arc crustal section: time constraints from a LAICP-MS zircon study. Earth Planet. Sci. Lett. 309, 268–279. Bouilhol, P., Jagoutz, O., Hanchar, J.M., Dudas, F.O., 2013. Dating the India–Eurasia collision through arc magmatic records. Earth Planet. Sci. Lett. 366, 163–175. Bouilhol, P., Schaltegger, U., Chiaradia, M., Ovtcharova, M., Stracke, A., Burg, J.-P., Dawood, H., 2011. Timing of juvenile arc crust formation and evolution in the Sapat Complex (Kohistan–Pakistan). Chem. Geol. 280, 243–256. Burtman, V.S., Molnar, P.H., 1993. Geological and geophysical evidence for deep subduction of continental crust beneath the Pamir. Spec. Pap. Geol. Soc. Am. 281, 1–76. Cao, K., Wang, G.-C., Bernet, M., van der Beek, P., Zhang, K.-X., 2015. Exhumation history of the West Kunlun Mountains, northwestern Tibet: evidence for a long-lived, rejuvenated orogen. Earth Planet. Sci. Lett. 432, 391–403. Cao, K., Wang, G.C., Beek, P.V.D., Bernet, M., Zhang, K.X., 2013. Cenozoic thermo-tectonic evolution of the northeastern Pamir revealed by zircon and apatite fission-track thermochronology. Tectonophysics 589, 17–32. Cao, K., Xu, Y.D., Wang, G.C., Zhang, K.X., van der Beek, P., Wang, C.W., Jiang, S.S., Bershaw, J., 2014. Neogene source-to-sink relations between the Pamir and Tarim Basin: insights from stratigraphy, detrital zircon geochronology, and whole-rock geochemistry. J. Geol. 122, 433–453. Carrapa, B., DeCelles, P.G., Wang, X., Clementz, M.T., Mancin, N., Stoica, M., Kraatz, B., Meng, J., Abdulov, S., Chen, F., 2015. Tectono-climatic implications of Eocene Paratethys regression in the Tajik basin of central Asia. Earth Planet. Sci. Lett. 424, 168–178. Carrapa, B., Mustapha, F.S., Cosca, M., Gehrels, G., Schoenbohm, L.M., Sobel, E.R., DeCelles, P.G., Russell, J., Goodman, P., 2014. Multisystem dating of modern river detritus from Tajikistan and China: implications for crustal evolution and exhumation of the Pamir. Lithosphere 6, 443–455. Chapman, J.B., Robinson, A.C., Carrapa, B., Villarreal, D., Worthington, J., DeCelles, P.G., Kapp, P., Gadoev, M., Oimahmadov, I., Gehrels, G., 2018a. Cretaceous shortening and exhumation history of the South Pamir terrane. Lithosphere 10 (4), 494–511. Chapman, J.B., Scoggin, S.H., Kapp, P., Carrapa, B., Ducea, M.N., Worthington, J., Oimahmadov, I., Gadoev, M., 2018b. Mesozoic to Cenozoic magmatic history of the

7. Conclusions Based on stratigraphic changes in depositional environments, paleocurrent directions, and sediment provenance in the western Tarim Basin, we draw the following conclusions. (1) The West Kunlun orogen was a topographic high dividing the Central Pamir–Tianshuihai from the Tarim Basin between the Jurassic and Paleocene. Throughout that time, detritus supplied to the western Tarim Basin was derived from the West Kunlun orogen. (2) Initial uplift and indentation of the Pamir, associated with thrusting and formation of a foreland basin along the northern margin of the Pamir–West Kunlun region, began in the Paleocene. This was coeval with the deposition of conglomerates and a thick gypsum unit in the western Tarim Basin. (3) Paleocurrent directions in Cretaceous strata were toward the N, perpendicular to the West Kunlun Mountains, and rotated towards the NE during the Paleocene. (4) Provenance analysis suggests that drainage patterns evolved by headward erosion across the West Kunlun orogen and North Pamir during the Cretaceous, and across the Central Pamir and South Pamir during the early Paleogene.

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