Detrital-zircon geochronology of the Jurassic coal-bearing strata in the western Ordos Basin, North China: Evidences for multi-cycle sedimentation

Geoscience Frontiers xxx (2017) 1e19

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Detrital-zircon geochronology of the Jurassic coal-bearing strata in the western Ordos Basin, North China: Evidences for multi-cycle sedimentation Pei Guo a, Chiyang Liu a, *, Jianqiang Wang a, Yu Deng a, Guangzhou Mao b, Wenqing Wang a a

State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi’an, Shaanxi 710069, China Shandong Provincial Key Laboratory of Depositional Mineralization & Sedimentary Minerals, College of Earth Science and Engineering, Shandong University of Science and Technology, Qingdao, Shandong 266590, China b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 18 June 2017 Received in revised form 25 September 2017 Accepted 3 November 2017 Available online xxx Handling Editor: S. Glorie

The western Ordos Basin (WOB), situated in a tectonic transition zone in the North China Craton, acts as an excellent example for studying the Mesozoic intraplate sedimentation and deformation in Asia. In this study, U-Pb ages for 1203 detrital zircons of 14 sandstone samples collected from 11 sections are presented to unravel the sediment source locations and paleogeographic environments of the EarlyeMiddle Jurassic coal-bearing Yan’an Formation in the WOB. Data show that there are five prominent age groups in the detrital zircons of the Yan’an Formation, peaking at ca. 282 Ma, 426 Ma, 924 Ma, 1847 Ma, and 2468 Ma. Samples from the northern, middle, and southern parts of the WOB contain these five age categories in various proportions. In the northern region, the Yan’an Formation exclusively contains Early Permian detrital zircons with a single age group peaking at 282 Ma, matching well with the crystallizing ages of the widespread Early Permian granites in the Yinshan Belt to the north and the Alxa Block to the northwest. While in the southern region, the Yan’an Formation mainly contains three groups of detrital zircons, with age peaks at 213 Ma, 426 Ma, and 924 Ma. These zircon ages resemble those of the igneous rocks in the Qilian-Qinling Orogenic Belt to the south-southwest. Samples in the middle region, characterized by a mixture age spectrum with peaks at 282 Ma, 426 Ma, 924 Ma, 1847 Ma and 2468 Ma, are previously thought to have mixed derivations from surrounding ranges. However, by referring to the detrital-zircon age compositions of the pre-Jurassic sedimentary successions and combining with paleontological and petrographic analysis, we firstly propose that the sediments of the Yan’an Formation in the middle region were partly recycled from the Triassic and Paleozoic sedimentary strata in the WOB. The occurrence of recycled sedimentation suggests that the Late TriassiceEarly Jurassic intraplate compressional deformation was very intense in the WOB, especially for regions in front of the Qilian Orogenic Belt. Ó 2017, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/).

Keywords: Western Ordos Basin Yan’an Formation U-Pb geochronology Recycled zircon Intraplate deformation

1. Introduction The western Ordos Basin (WOB) is situated in the western part of the North China Craton (NCC), which is a tectonic transition zone between the Ordos Basin, Alxa Block and Qilian Orogenic Belt. It has been widely recognized as an excellent example for studying the Mesozoic intraplate sedimentation and deformation in Asia (Liu,

* Corresponding author. E-mail addresses: [email protected], [email protected] (C. Liu). Peer-review under responsibility of China University of Geosciences (Beijing).

1998; Liu and Yang, 2000; Darby and Ritts, 2002, 2007; Ritts et al., 2004, 2009; Faure et al., 2012; Zhang et al., 2013). Numerous work has been done to study the mechanism for the formation of the famous Late Jurassic western Ordos fold-thrust belt (e.g., Liu, 1998; Zhang et al., 2000; Darby and Ritts, 2002; Huang et al., 2015; Guo et al., 2017) or for the occurrence of the Triassic complicated sedimentation in the WOB (e.g., Liu and Yang, 2000; Ritts et al., 2004). However, much less efforts have been made to explore the Late Triassic compressional deformation event and the subsequent EarlyeMiddle Jurassic sedimentation in the WOB, because the overwhelming Late Jurassic and Cenozoic deformation events in the WOB were so strong that they had

https://doi.org/10.1016/j.gsf.2017.11.003 1674-9871/Ó 2017, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NCND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Please cite this article in press as: Guo, P., et al., Detrital-zircon geochronology of the Jurassic coal-bearing strata in the western Ordos Basin, North China: Evidences for multi-cycle sedimentation, Geoscience Frontiers (2017), https://doi.org/10.1016/j.gsf.2017.11.003

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obscured and overprinted most tectonic and sedimentary traces of previous tectonic events and depositions (Tang et al., 1992). The general absence of tectonic traces and intact sedimentary strata are common in such tectonic transition zones, making it difficult to reconstruct the regional tectonic and sedimentary evolution. In this study, we use the U-Pb ages of detrital zircons in the coalbearing Yan’an Formation to reconstruct the EarlyeMiddle Jurassic provenance and paleogeography of the WOB, which can provide quantitative estimates of sediment derivations despite of the discontinuity of strata. Here, we firstly recognized the occurrence of multi-cycling zircons in the sediments of the Yan’an Formation, which provides a new perspective to study the Late TriassiceEarly Jurassic tectonic event in the WOB. Detrital zircons can be reworked through multiple sedimentary cycles because of their remarkable durability (Fedo et al., 2003; Thomas et al., 2004; Link et al., 2005; Thomas and Becker, 2007; Dickinson and Gehrels, 2009; Dickinson et al., 2009; Pereira et al., 2016), and their occurrence can largely complicate the provenance analysis as multi-recycled sedimentation can be masked, leading to an incorrect interpretation of primary sources (Gehrels et al., 2011; Thomas, 2011). However, their occurrence can also inversely indicate the previous uplifting events of sedimentary beds, which generally cannot be easily perceived by traditional sedimentary methods, especially in complicated tectonic zone, like the WOB.

(Zhang, 2010). The thickness of the Yan’an Formation in this part is generally 200e300 m, which can be correlated to that in the interior of the Ordos Basin. While in the middle region, the thickness of the Yan’an Formation varies considerably, ranging from 200 m to more than 700 m (Fig. 2). Even in the same section, such as in the Wangwa area, formation thickness can change markedly over only 1500-m distance (Guo, 2015), indicating undulating topography. The Yan’an Formation in this part comprises much higher proportion of coarse-grained sandstones (Fig. 2) and much lower proportion of coals. In the southern region, the Yan’an Formation is also 200e300 m thick, but it comprises the thickest coal seams and much finer sediments compared to the sediments in other parts of the WOB (Fig. 2). Two sections, in the Xinjing and Jingyuan areas, contain large amounts of alluvial or braided fluvial conglomerates (Fig. 2), which may correspond to marginal sedimentation. 3. Potential sediment sources Potential sediment sources include mountain ranges surrounding the WOB (Fig. 1B). Pre-Jurassic buried sedimentary and metamorphic units could also have the potential to provide sediments during early basin filling. 3.1. Yinshan Belt

2. Geological setting and stratigraphy The WOB is situated in a tectonic transition zone on the western margin of the Ordos Basin, bounded by the Qinling Orogenic Belt (QinOB) to the south, the Qilian Orogenic Belt (QiOB) to the southwest, the Alxa Block (AB) to the northwest, and the Yinshan Belt (YS) to the north (Fig. 1A and B). In the Meso-Neoproterozoic, there existed an aulacogen (i.e., the Helan Aulacogen) in the WOB, as the failed branch of the paleo-Qilian Sea (Sun and Liu, 1983; Lin et al., 1995). In the subsequent Paleozoic, regional extension and marine transgression occurred twice in the WOB, where thick marine carbonates and siliciclastic rocks were deposited (Lin and Yang, 1991; Lin et al., 1995; Guo et al., 2015). By the end of the Triassic, the WOB was entirely in an intraplate setting (Darby and Ritts, 2002; Ritts et al., 2009), but continued to subside and deform in response to the far-field stresses from the northward assembly of the Tibetan blocks to the southern margin of Eurasian Plate, the closure of the MongolOkhost Ocean, the continuous compression between the NCC and Yangtze Plate, and the subduction of the Paleo-Pacific (e.g., Enkin et al., 1992; Zorin, 1999; Darby et al., 2001; Darby and Ritts, 2002; Ritts et al., 2009; Faure et al., 2012). As a weak zone of strain concentrations (Darby and Ritts, 2002; Ritts et al., 2009), the WOB contained the most complete and thickest sedimentary successions during the Phanerozoic but also underwent the strongest deformation during the Meso-Cenozoic (Tang et al., 1992; Guo et al., 2015). The subsidence of the NCC during the EarlyeMiddle Jurassic has been interpreted as a result of stress relaxation after the Triassic intense compression (Jin et al., 1999; Zhao et al., 2000). During the early deposition of the Yan’an Formation, several residual highlands remained in the Ordos Basin, separated by a large EW-trending river (i.e., the Gan-Shan Channel) and its four main branches in the WOB (Zhao et al., 1999; Zhao and Chen, 2006). The uneven topography in the WOB resulted in the various thickness and petrography of the Yan’an Formation in different sedimentary areas (Fig. 2). In the northern WOB, the Yan’an Formation mainly consists of fine- to middle-grained sandstones and mudstones, intercalated with thin-bedded coals (Fig. 2). Conglomerates can be observed in the basal part of the Yan’an Formation in the Helanshan section

The Yinshan Belt (YS) is located to the north of the WOB and is a typical intraplate orogeny within the northern NCC (Davis et al., 1998). Main early Precambrian geological events and key tectonic events in the YS include: an old continental nuclei and main crustal growth took place at 2.7e2.9 Ga; micro-blocks amalgamated to form a coherent craton at 2.5 Ga; Paleoproterozoic mobile belts came into being at 2.3e1.95 Ga; and lower crust uplifted as a whole and a mafic dyke swarm occurred at 1.8 Ga, associated with continental rifting and intrusion of orogenic magmatic associations (Zhai, 2010, 2011). During the Phanerozoic, a large Late PaleozoiceEarly Mesozoic granite belt occurred in the northern NCC as the result of the continued orogenesis of Central Asia Orogenic Belt to the north (Zhang et al., 2007a, 2009; Wan et al., 2009). The intrusive time of these granites has been dated, ranging of 325e235 Ma (e.g., Luo et al., 2007, 2010; Wan et al., 2009). Plentiful granites now expose in the YS (Fig. 3) (Luo et al., 2007, 2010). 3.2. Alxa Block The Alxa Block (AB) is triangular in shape and situated to the northwest of the WOB (Fig. 1A and B). It was not a uniform early Precambrian craton like the NCC, but a Precambrian composite ribbon continent which originally detached from a main cratonic accretionary and collisional margin and underwent a multi-stage tectono-thermal evolution during global plate margin reorganization (Song et al., 2017) and multi-stage magmatism and ductile deformation in the Phanerozoic (Yang et al., 1988; Zhou and Yu, 1989). In the early Mesozoic, influenced by the collision of the NCC and the Yangtze Plate, the AB was uplifted and strongly deformed (Yang et al., 1988; Darby and Ritts, 2002; Darby and Gehrels, 2006; Zhang et al., 2013, 2015; Yuan et al., 2015). Until in the middle Middle Jurassic, this place started to subside and receive sediments (the Zhiluo Formation) (Zhang, 2010), indicating that during the EarlyeMiddle Jurassic the AB was in a relatively high relief and could represent a potential source for the WOB. Principal magmatism in the AB can be divided into four stages: Early Paleoproterozoic (2400e2200 Ma); MiddleeLate Paleoproterozoic (2000e1810 Ma); Neoproterozoic (1000e800 Ma); and Phanerozoic (525e175 Ma) (Zhou and Yu, 1989; Zhang et al., 2012).

Please cite this article in press as: Guo, P., et al., Detrital-zircon geochronology of the Jurassic coal-bearing strata in the western Ordos Basin, North China: Evidences for multi-cycle sedimentation, Geoscience Frontiers (2017), https://doi.org/10.1016/j.gsf.2017.11.003

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Figure 1. (A) Tectonic location of the Ordos Basin (modified after Liu et al., 2005). (B) Location of the WOB in relation to the Ordos Basin, Yinshan Belt, Alxa Block, and Qilian-Qinling Orogenic Belt. This map shows potential source rocks for the detrital zircons of the Yan’an Formation in the WOB, adapted on the basis of the 1/2,500,000 geological map of China (China Geological Survey, 2004). (C) A detailed geological map of the WOB and sample locations (modified after Zhang et al., 2011). The locations and numbers of the sections in Fig. 2 are indicated.

Please cite this article in press as: Guo, P., et al., Detrital-zircon geochronology of the Jurassic coal-bearing strata in the western Ordos Basin, North China: Evidences for multi-cycle sedimentation, Geoscience Frontiers (2017), https://doi.org/10.1016/j.gsf.2017.11.003

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Figure 2. Regional stratigraphic correlation of the Yan’an Formation in the WOB. The thickness, lithological associations, and proportion of coal seams vary greatly between different sections. The location of the sections is shown in Fig. 1C. Two sections at Jingyuan and Xinjing areas are highlighted in the inset box as the Yan’an Formation in these two sections is much thinner and mainly consists of coarser grains, possibly indicating marginal sedimentation. Drill data from sections 3, 5, 6, 7, and 9 were obtained from the Ningxia Geology and Mineral Survey Institute, outcrop data for section 1 was obtained from Wang (2013); outcrop data for sections 2, 4, 8 and 10 were collected from Yang et al. (2011).

Neoproterozoic metamorphic rocks are the most important portion for the basements of the AB and distribute mainly in its western part (Geng and Zhou, 2010), with two age peaks at w992e978 Ma and w826e807 Ma (Song et al., 2017); while the middle-upper Paleoproterozoic rocks mainly expose in the eastern part (Geng et al., 2010), having a higher potential to be the sources for the sediments of the WOB. Permian magmatism mainly resulted from the accretionary orogenic events in the Central Asian Orogenic Belt; the ages of these rocks are concentrated in the range of 289e252 Ma (Fig. 3) (Dan et al., 2014; Zhang et al., 2015), and there are significant outcrops in the Bayanwula Shan and Langshan, in the easternmost of the AB (Fig. 3) (e.g., Geng and Zhou, 2012; Dan et al., 2014, 2015; Zhang et al., 2015; Wang et al., 2015, 2016). 3.3. Qilian Orogenic Belt The NW-trending Qilian Orogenic Belt (QiOB) is located to the southwest of the WOB. During the Precambrian, the Qilian area was situated in a continental rift, which recorded an ocean basin evolutionary cycle from closure to continental collision in the early Paleozoic to Devonian (e.g., Xu et al., 2010a,b; Song et al., 2013b). In the Carboniferous, this area underwent marine transgression from

the west, which was then connected to the North China Sea to the east in the Late Carboniferous (Yan et al., 2008; Yan and Yuan, 2011; Guo et al., 2015; Zhang et al., 2016a). During the late Triassic, responding to the collision between the Qiangtang Block and Kunlun Block in West China, a regional deformation event occurred in the QiOB (Zhao and Jin, 2011), evidenced by the widespread ejective folds in the southern and middle QiOB (Tong, 2016) and the zircon fission-track ages (Jolivet et al., 2001). Although several intermountain basins developed locally during the EarlyeMiddle Jurassic, the QiOB still remained as a highland compared to the WOB; thus, it was another potential source area (Jin et al., 1999; Zhao and Jin, 2011). The most prominent tectono-thermal event in the QiOB was the Early Paleozoic orogenic event (Fig. 3). Coincident with this event, large volumes of OrdovicianeDevonian granites intruded into the QiOB (Fig. 3). The age range of these granites is 534e400 Ma, peaking at 438 Ma (Yang et al., 2006; Pei et al., 2007). The second significant magmatism occurred in the Neoproterozoic, evidenced by large volumes of Neoproterozoic magmatic and metamorphic rocks in its eastern regions (Fig. 3). The ages of these crystalline rocks can be subdivided into three stages: 902e981 Ma, 839e862 Ma, and 738e799 Ma (Pei et al., 2012; Gao et al., 2013b).

Please cite this article in press as: Guo, P., et al., Detrital-zircon geochronology of the Jurassic coal-bearing strata in the western Ordos Basin, North China: Evidences for multi-cycle sedimentation, Geoscience Frontiers (2017), https://doi.org/10.1016/j.gsf.2017.11.003

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Figure 3. Distributions of crystalline rocks in surrounding tectonic units, showing primary potential sources for detrital zircons in the WOB. The age ranges of the magmatic rocks in the Qinling were extracted from Luo (2013) and Wang et al. (2009b), those for the AB are from Gao et al. (2013a), those for the Langshan are from Dan et al. (2015), and those for the Yinshan are from Luo et al. (2010). The age distribution curves on the right for the Yinshan are modified from Chen et al. (2012), while those for the AB are after Zhang et al. (2012) and Dan et al. (2014), those for the north QinOB are after Zhang et al. (2016a), and the north QiOB are after Yang et al. (2015) and the references therein.

3.4. Qinling Orogenic Belt The Qinling Orogenic Belt (QinOB), located to the south of the Ordos Basin and once as a part of the Yangtze Plate in the Paleoproterozoic, represents a composite orogenic belt. It underwent multi-stage accretion and collision between different continental blocks (Dong and Santosh, 2016), which resulted in multi-stage magmatism in the Neoproterozoic, Paleozoic, and Mesozoic (Lu et al., 2004). By the early Neoproterozoic, granitic metamorphic rocks, mainly in the age range of 960e840 Ma, were associated with the collision of North and South Qinling blocks. In the Paleozoic, in contrast, this collision reverted to subduction and was accompanied by the intrusion of granites, which can be divided into three age ranges, 505e470 Ma, 455e422 Ma, and 415e400 Ma (Wang et al., 2009a,b). In the Triassic, a collisional orogeny formed between the South Qinling Block and the Yangtze Plate along the

Mianlue suture, with widespread Triassic granites intruding into the QinOB (Qin and Lai, 2008; Qin et al., 2009) (Fig. 3). 3.5. Early Paleozoic strata In recent years, an increasing number of U-Pb geochronological data of detrital zircons from the pre-Jurassic sedimentary strata in the WOB and adjacent tectonic units have been published, which will contribute to the valid interpretation of multi-cycle sedimentation (Fig. 4). On the basis of sample locations and their age compositions, the Early Paleozoic samples are divided into three categories: detrital zircons derived from the NCC, comprising one sample (Fig. 4A) (Darby and Gehrels, 2006); detrital zircons from the AB, comprising five samples (Fig. 4B); and detrital zircons from the Qi-QinOB (Qilian-Qinling Orogenic Belt), comprising five samples (Fig. 4C).

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Please cite this article in press as: Guo, P., et al., Detrital-zircon geochronology of the Jurassic coal-bearing strata in the western Ordos Basin, North China: Evidences for multi-cycle sedimentation, Geoscience Frontiers (2017), https://doi.org/10.1016/j.gsf.2017.11.003

Figure 4. Diagram showing published age distributions of detrital zircons from pre-Jurassic strata in the WOB and adjacent units. Red curves represent Lower Paleozoic strata, purple curves represent Upper Paleozoic strata, and yellow curves represent Triassic strata. Citations are given for each curve while N denotes the number of detrital zircons. Division of the Lower Paleozoic samples: A represents samples influenced by the North China; B represents samples influenced by the Alxa Block; C represents samples influenced by the Qilian-Qinling Orogenic Belt.

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The first category contains two prominent Paleoproterozoic age groups, peaking at 1952 Ma and 2709 Ma (Fig. 4A). The second category comprises distinctive late Mesoproterozoiceearly Neoproterozoic zircons, in the age range of 912e1173 Ma, with minor Paleozoic zircons (Fig. 4B). The third category contains a prominent Late Ordovician group in the age range of 445e482 Ma. One sample from the Tianshui section contains an additional age peak at 984 Ma, indicating that Neoproterozoic sources might distribute inhomogeneously in the Qi-QinOB (Fig. 4C). 3.6. Late Paleozoic and Triassic strata The late Paleozoic and Triassic sediments in the WOB contain zircon grains with broadly similar age populations (Zhang, 2007b;

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Song et al., 2010; Zhang et al., 2016a). There are three major age groups in these pre-Jurassic sediments, including the Early Paleoproterozoic cluster peaking at ca. 2500 Ma, the late Paleoproterozoic cluster peaking at 1850 Ma, and the Phanerozoic cluster with several subordinate peaks (Fig. 4D). Zircon grains within these age compositions show similar derivations from northerly sources in the YS, which seems paradoxical for the samples from the Kongtongshan and Qianyang sections as they are geographically much closer to the Qi-QinOB (Fig. 4D). One Late Triassic sample in the northern margin of the QiOB encompasses age populations similar to the Early Paleozoic samples in this region but distinct to coeval samples in the WOB (Zhang et al., 2014) (Fig. 4C).

Figure 5. (A) Triangular QFL plot for the frame work of the Yan’an Formation sandstones in the middle WOB (Dickinson et al., 1983). (B) Petrographic images of the Yan’an Formation sandstones from the WOB, showing the occurrence of magmatic (yellow arrow) and metamorphic (blue arrow) lithic grains in the Jingyuan section, siltstone lithic grains (green arrows) and carbonate lithic grains (red arrows) in the Mahuanggou and Tanshan sections.

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4. Sampling and analytical methods Seven sandstone samples were mainly collected from the outcrops of the Yan’an Formation in the southern-central part of the WOB. Sample location and lithology are briefly described in the supplementary table. The sandstone sample in the Xinjing section was fine-grained and collected from a small fluvial sandstone body, with argillaceous components in it; while the samples in the Mahuanggou, Shiganggou, Jingyuan, and Tanshan sections were mainly middle- and coarse-grained and collected from large channel sandstone bodies, where large-scale inclined beddings or trough cross-beddings can be found. Two samples were collected from two outcrops in the Ankou section; one was coarse-grained (Ankou-1) collected from a coarse meandering river sandstone body and the other was fine-grained (Ankou-2) from sandstone lenses in floodplain mudstones. Procedures of sample pretreatment include: (1) zircon crystals were randomly selected to be fixed on epoxy resin for later dating, irrespective of their color, size (large enough for dating), or crystal form; (2) zircon crystals were polished to expose their cores washed with 3% HNO3 to remove the contamination on crystal surface; and (3) the cathode luminescence (CL) images were taken using a Mono CL3þ manufactured by Gatan, USA. Approximately 80e100 analyses were conducted on each sample using LA-MC-ICP-MS in the State Key Laboratory of Continental Dynamics at Northwest University, China. The ICP-MS used for dating is an Agilent 7500 produced by Agilent Technologies, USA, while the laser-ablation system is a GeoLas200M produced by Lambda Physik, Germany. The acquisition routine comprised 15-s blank measurements (laser off), 15-s peak acquisition (laser on), with a delay of ca. 30 s between analyses to purge the previous sample and allow a return to background signal intensities. The laser beam diameter and pit depth were 32 mm and 15 mm,

respectively, and one U/Pb detrital zircon and one zircon NIST610 were analyzed between every five samples. For data processing, the age of each 91500 standard zircon was modified in the range of 1060e1065 Ma, while the element concentration of each NIST610 standard zircon was also modified within a certain range. Sample zircons can then be processed on the basis of 238U, 206Pb, and 207Pb ratios; younger zircons (<10 Ga) adopted the ages of 206Pb/238U while older zircons (>10 Ga) adopted the ages of 207Pb/206Pb. Data processing was conducted using the GLITTER program (ver4.0, Macquarie University), while age calculations and plot formation utilized Isoplot (ver2.49) (Ludwig, 1991). More details about analytical methods can be found in Gao et al. (2004) and Yuan et al. (2004), while the common lead correction method followed Andersen (2002). Concordant diagrams were made using ISOPLOT 3.0 for Excel (Ludwig, 2003), while probability density and kernel distributions were created using DensityPlotter 7.3 (Vermeesch, 2012). 5. Results 5.1. Lithology and petrology Conglomerates of the Yan’an Formation are absent in sampled outcrops; sandstones are mostly medium- to coarse-grained, with sub-angular to sub-rounded grains, and variedly sorted. Quartz grains are predominantly found, followed by lithic and feldspar grains, the latter two of which account for less than 25% of the total grains (Fig. 5). Twenty-eight samples of the Yan’an Formation sandstones from seven sections in the middle region are plotted in the triangular QFL compositional diagrams (Dickinson and Suczek, 1979; Dickinson et al., 1983) (Fig. 5A). Most data fall within the recycled orogenic tectonic setting, with a few exceptions in the craton interior setting.

Figure 6. CL images of representative detrital zircons from the Yan’an Formation sandstones in the southern-central WOB. In each case, the white circle represents an ablation diameter spot size of 32 mm. Sample locations are shown in Fig. 8.

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Figure 7. Concordia plots of the Yan’an Formation in the southern-central WOB.

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There are many magmatic and metamorphic lithic contents in the Yan’an Formation of the Jingyuan section, such as aphanite and phyllite rocks (Fig. 5B); while in the Mahuanggou and Tanshan sections in the middle WOB, we can observe a lot of sedimentary debris rocks, such as the siltstone debris (Fig. 5B). Particularly noteworthy is the occurrence of carbonate debris in the sandstones of the Yan’an Formation (Fig. 5B). Their contents show great variations both areally and stratigraphically, accounting for 6%, 8%, 1% and 12% of the total grains in the Tanshan, Tandonggou, Yaoshan and Mahuanggou sections in the middle WOB (Fig. 5B). While in the Mahuanggou section, the contents of the carbonate debris decrease upward, indicating the uncertainty of the carbonate sources. 5.2. U-Pb ages The zircon populations in all sandstone samples comprise predominantly sub-angular to sub-rounded grains, and vary from subspherical to elongate in shape (Fig. 6). No systematic correlation is evident between the shape, color, angularity and U-Pb ages. For example, the samples Ankou-1 and Ankou-2 were collected from two close outcrops in the Ankou section; although different in zircon grain size (Fig. 6), the two samples display similar age spectra (Figs. 7 and 8). The original U-Pb isotopic data of the seven samples in this study are shown in the supplementary data, and their age distribution patterns are presented in concordia plots (Fig. 7) and agebin histograms by Kernel Density Estimate (Fig. 8), respectively. For better understanding the provenance of the Yan’an Formation in the whole WOB, we synthetically presented the age populations of 14 sandstone samples (combined with seven samples from the northern-central part) from 11 sections in the WOB and adjacent tectonic units (Fig. 8). Composite age-distribution curves for the detrital zircons of the Yan’an Formation in the WOB are shown in Fig. 9A. Although 1203 age analyses are presented in Fig. 8, the consistent age content based on the large collection numbers in the northern region may define a dominant age peak at the expense of others. Thus, we chose three samples (Zhuozishan, Helanshan-1, Helanshan-2) from the northern region and a total of 774 data points are presented in the composite age-distribution curve for the whole WOB (Fig. 9A). Results show that there are five major age clusters for the Yan’an Formation (Fig. 9A) in the WOB. Of these, the most prominent peaks include (1) 2468 Ma, (2) 1847 Ma, (3) 924 Ma, (4) 426 Ma, and (5) 282 Ma. To express age groups concisely, the above five main groups are referred to as follows: group 2468, group 1847, group 924, group 426, and group 282 (Fig. 9A). Although the group 924 and group 2468 are less significant in the composite plot in view of zircon proportion (Fig. 9A), they cannot be ignored because they act as major age groups in individual samples, such as the Xinjing sample. On the basis of the U-Pb age heterogeneity and sample localities, we divide the WOB into three sub-regions (Figs. 8 and 9A). 5.2.1. Southern region The southern region (region C in Fig. 8) is located in the southwest of the WOB. It is defined as the region in front of the QiQinOB and roughly subparallel to the strike of the Cenozoic Liupanshan (Fig. 8). This region is evidenced by two samples in front of the QinOB (Fig. 8); while one sample in the Jingyuan section from the northeastern margin of the QiOB acts as a control sample in the

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following discussion. Detrital zircons in this region are characterized by a dominant age peak of group 426 and two subordinate age peaks of Triassic group and group 924 (Figs. 8 and 9A). Unique to this sub-region, the group 426 is the dominant age group. 5.2.2. Middle region The middle region (region B in Fig. 8) is located in the southerncentral area of the WOB, at the tectonic junction between the AB, QiOB, and Ordos Basin, as evidenced by five samples from different sections. It is separated from the northern region at roughly 38 N, corresponding to the famous “38 N tectonic belt”, which is a complex and long-lived belt developing in North China (Zheng and Wang, 1995; Qiu, 2011; Li et al., 2012). Results show that this region can be further subdivided into two sub-regions according to the age compositions. One sub-region includes four samples mainly consisting of detrital zircons of group 282 and group 1847, along with two minor age peaks of group 2468 and group 426. The group 1847 is the most significant age component in this sub-region. The other sub-region is located in the Weiningbeishan area, a junction region between the AB and QiOB (Zhang et al., 2000; Ai et al., 2011). The Xinjing sample in this sub-region predominantly includes zircons of the group 924 and group 2468, with some less important late Neoproterozoic and Mesoproterozoic zircons, which is quite different from the age compositions of the other four samples in the middle region (Fig. 9A). 5.2.3. Northern region The northern region (region A in Fig. 8) is located in the northern-central part of the WOB, as evidenced by four samples. Three samples from the Ciyaobao and Helanshan sections are very special because they exclusively contain zircons of the group 282, while the other age groups are negligible and can be ignored (Guo, 2010; Zhao et al., 2015). One sample in the Zhuozishan section is distinct from the three samples as it has two additional age groups that peak at 2468 Ma and 1847 Ma, suggesting additional provenances for this sample. As a result, the Zhuozishan area is alone classified as a sub-region (Fig. 9A). Two samples in the Langshan area acted as control samples (Wang, 2013) (Fig. 8).

6. Discussion 6.1. Primary provenance 6.1.1. The southern region Samples in this part contain detrital zircons with a dominant age peak of group 426, and two subordinate age peaks of Triassic group and group 924 (Fig. 9A). Early Paleozoic igneous rocks spread widely in the Qi-QinOB (Fig. 3) and Early Paleozoic detrital zircons also occupy a large proportion in the Paleozoic sedimentary samples in front of the Qi-QinOB (Fig. 4). Thus, these two sources can both be the derivations for the detrital zircons of group 426 in the southern region. Zircon grains of the group 924 have several derivations. On the one hand, Neoproterozoic igneous rocks in the AB and Qi-QinOB could serve as their origins (Geng and Zhou, 2010; Pei et al., 2012; Gao et al., 2013b; Song et al., 2017) (Fig. 3); on the other hand, the Paleozoic sedimentary rocks fed by the AB and Qi-QinOB could also supply the Neoproterozoic detrital zircons (Fig. 4). However, these Neoproterozoic detrital zircons in the Yan’an

Figure 8. Kernel Density Estimate (KDE) and histograms for the Yan’an Formation in the WOB and adjacent units. According to the age populations, the WOB are divided into three sub-regions: Aenorthern region; Bemiddle region, Cesouthern region. Two samples (1 and 2) in the Langshan and one sample (12) from the northeastern margin of the QiOB are also shown as the control samples (in green). The samples from the Langshan are not marked on this geological map but are included in Fig. 3. Abbr.: JeJurassic, TeTriassic, UPeUpper Paleozoic, LPeLower Paleozoic, UPteUpper Proterozoic, MPteMiddle Proterozoic, LPteLower Proterozoic.

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Figure 9. (A) Composite age-distribution curves for the detrital zircon grains of the Yan’an Formation from each region and the whole WOB. The U-Pb age spans of potential source rocks are derived from Figs. 3 and 4 and the references therein. The gray lever of the age bar represents different age proportions: 100% gray stands for the dominant age group, 50% for the subordinate age group, and 10% gray for the insignificant age group. Division of sample location: NWOBenorthern WOB, MWOBemiddle WOB, SWOBesouthern WOB. (B) Paleocurrents of the Yan’an Formation in the WOB. Partial data are collected from Zhang et al. (2012) and Wang (2013).

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Formation are more likely to be sourced from the Qi-QinOB. Reasons are as follows: (1) the southern region is geographically closer to the Qi-QinOB; (2) the Yan’an Formation of the middle region, geographically between the southern region and the AB, contains negligible Neoproterozoic zircon grains (Figs. 7 and 8), which implies that Neoproterozoic basements of the AB may not have been exhumed to provide Neoproterozoic detrital zircons; and (3) the proportions of the Paleozoic and Neoproterozoic zircons in the Yan’an Formation of the southern region show great similarity to those from the Qi-QinOB (Figs. 3 and 9A). Younger Indosinian (Triassic) zircon grains define a sharp age spike on the composite plot for the southern region (Fig. 8), indicating that Triassic granitoid rocks in the QinOB (Fig. 3) began to be exhumed and eroded in the Jurassic. Since the Triassic strata currently expose widely in the southern region of the WOB (Fig. 1C), the underlying Paleozoic successions might be not unroofed to surface during the Jurassic. Thus, the sediments of the Yan’an Formation were mainly derived from the igneous rocks in the Qi-QinOB. Besides, the CL images of the detrital zircons in the southern region show clear features of oscillatory zones (Fig. 6), indicative of igneous origins. Moreover, measurements in the Ankou section indicate that the paleocurrents are primarily eastesoutheastenortheastward (Fig. 9B), verifying the primary provenance from the Qi-QinOB. 6.1.2. The northern region In this region, zircon grains in the Yan’an Formation mostly fall in the age range of 260e290 Ma; these Early Permian zircons define a single age peak of group 282 in the age-distribution curves (Figs. 8 and 9A). The widespread Early Permian igneous rocks in the AB and the YS (Fig. 3) might be the primary origins for these detrital zircons. The age compositions of the two samples in the Langshan area within the YS verify this assumption, which also dominantly contain the detrital zircons of group 282 (Fig. 8). Since the Qi-QinOB contains negligible Late Paleozoic magmatic rocks (Fig. 3) (Zhang et al., 2016b), detrital zircons of group 282 in the Yan’an Formation of the northern WOB are supposed to be mainly derived from the YS and/or the AB. Besides the age group 282, the sample in the Zhuozishan section also contains other two apparent age clusters of the group 1847 and group 2468. Metamorphic basements in the YS are characterized by age peaks at 1847 Ma and 2468 Ma (Fig. 3), so they can account for

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the Paleoproterozoic zircon grains in the Yan’an Formation of this section. The measurements indicate that paleocurrents in the northern region vary substantially (Fig. 9B) (Zhang, 2010). Especially in the Helanshan and Ciyaobao sections, paleocurrents can be in all directions. However, the eastward and southeastward paleocurrents are dominant (Fig. 9B). 6.1.3. The middle region For the samples of the sub-region 1, one main age cluster is the Late Paleoproterozoic group that peaks at ca. 1847 Ma (Fig. 9A). However, this age group is barely present as a dominant age assemblage in the Yan’an Formation of other parts, except in the Zhuozishan section (Figs. 8 and 9A). Late Paleoproterozoic crystalline rocks in the age range of 1650e2000 Ma now expose in almost every periphery tectonic unit, including the khondalite terrane in the YS (e.g., Wan et al., 2009; Jiao and Guo, 2011; Jiao et al., 2013), the metamorphic crystalline basement in the eastern AB (Geng and Zhou, 2010), and the igneous and metamorphic outcrops in the eastern QiOB (Gao et al., 2013b; Xu et al., 2014; You et al., 2014). Gao et al. (2013b) argued that these rocks were the evidence of a unified tectonic-thermal event and the reactivity of the Helan aulacogen rift. It is noteworthy that there are also some metamorphic outcrops aging from 2.0 Ga to 1.8 Ga in the Helanshan and Zhuozishan in the northern WOB (e.g., Yin et al., 2009, 2011) (Fig. 1B). These Paleoproterozoic metamorphic rocks seemingly present a more plausible origin for the zircon grains of group 1847 in the middle WOB because they are closer. However, the uplifting time is a factor that should also be considered when applying the U-Pb ages in provenance analysis (Gehrels et al., 2011; Thomas, 2011). According to the data of apatite fission tracks, Zhao et al. (2007) argued that the earliest uplifting time for the Helanshan was the Late Jurassic, whereas the stratigraphic age of the Yan’an Formation is the EarlyeMiddle Jurassic, older than its uplifting time. In addition, the samples of the Yan’an Formation from the Helanshan section do not contain much late Paleoproterozoic detrital zircons (Fig. 8). Even though the sample from the Zhuozishan section contains the group 1847 (Fig. 8), their proportion is much smaller than those from the farther southern region. These phenomena suggest that the khondalite terrane at Helanshan and Zhuozishan was not exhumed during the Early Jurassic as the potential provenance for the detrital zircons of group 1847.

Figure 10. (A) Comparison of the age composition of the Yan’an Formation in middle sub-region 1 with the Triassic and Upper Paleozoic strata in the southern WOB, illustrating clear similarity in ages. (B) Comparison of the age composition of the Yan’an Formation in the Xinjing section with the pre-Jurassic strata in the northern WOB. Note that the curves in the lower part are superimposed. Darker colors indicate a higher frequency of ages.

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The widespread presence of the late Paleoproterozoic source rocks in the surrounding tectonic units makes it difficult to distinguish its provenance. As for the other three subordinate age groups (Fig. 9A), the zircon grains of group 426 are most likely to be derived from the Qi-QinOB (Yang et al., 2006; Pei et al., 2007; Wang et al., 2009a,b) or the AB (Dan et al., 2016) (Figs. 3 and 9A), whereas those of the groups 282 and 2468 are more likely to be from the AB (Zhou and Yu, 1989; Geng and Zhou, 2012; Zhang et al., 2012, 2015; Dan et al., 2014) or the YS (Luo et al., 2010; Zhai, 2010, 2011) (Figs. 3 and 9A). A mixture of provenance from the southwest and north seems a reasonable interpretation to explain the age compositions of the Yan’an Formation in the middle sub-region 1. The U-Pb composition of the detrital zircons from the middle sub-region 2 is quite different from those observed at other sections. Their age distributions of the zircon grains are very similar to those of the AB (Fig. 4). This suggests that the sediments of the Yan’an Formation at the Xinjing section have affinities with the AB.

6.2. Recycled provenance for the sediments in the middle WOB Interestingly, the detrital-zircon age distributions of the Yan’an Formation samples in the middle sub-region 1 and the Upper PaleozoiceTriassic samples in the WOB (samples of D in Fig. 4) are broadly similar (Fig. 10A); the same phenomenon can also be observed between the Xinjing sample in the sub-region 2 and the Lower Paleozoic samples in the northern WOB (samples of B in Fig. 4) (Fig. 10B). Thus, the pre-Jurassic sediments in the WOB provide an alternative explanation for the provenance of the Yan’an Formation in the middle region. Here, besides further interpretation of the U-Pb ages of detrital zircons, we also present other lines of evidence to discuss the possibility of multi-cycle sedimentation. Generally, a source area can consist of several rock components with different ages, but these components cannot be exhumed at all times. Thus, specific uplifting time should be taken into account and age comparisons cannot be restricted to the present age distributions of source areas (Dickinson and Gehrels, 2009; Gehrels et al., 2011; Thomas, 2011). Detrital zircons in coeval strata close to the source areas can provide direct information about which age components had been exhumed and were available to provide detritus. Regarding the dominant detrital zircons of group 1847 in the middle sub-region 1, the surrounding tectonic units cannot account for all their derivations. The reasons are as follows: (1) the exclusive occurrence of detrital zircons of group 282 in the two samples of the Langshan section and the three samples of the northern WOB suggests that the late Early Permian granites spread widely in the AB and YS during the Jurassic (Figs. 8 and 9A), while the Paleoproterozoic basements of ca. 1847 Ma age might not extensively expose; (2) samples with prominent age cluster of group 426 from the southern WOB imply that the early Paleozoic granites dominated in the QinOB; and (3) the age composition of the sample (the Jingyuan section) in the northeastern margin of the QiOB demonstrates that the Early Paleozoic granites also spread widely in the QiOB (Fig. 8). All of these phenomena suggest that the late Paleoproterozoic rock components were not widely outcropped in the adjacent tectonic units during the Jurassic; otherwise, the ca. 1850 Ma zircons would have accounted for a much larger proportion in the above proximal samples. Instead, the abundant detrital zircons of group 1847 in the Triassic and late Paleozoic strata might serve as the main derivations for the Yan’an Formation of the middle sub-region 1 (Figs. 4 and 10A). Because of thick sedimentary cover of the Upper Paleozoic and Triassic strata, which are at least 2000 m thick (Huo et al., 1989), the Lower Paleozoic strata had little chance to provide detritus to the Yan’an Formation of the middle sub-region 1.

Another evidence that indicates the occurrence of multi-cycling sedimentation for the sediments in the middle WOB is the reworked fossils. In the middle sub-region 1, the lower part of these coal-bearing strata contains large amounts of Late Triassic sporopollen, some of which are the characteristic fossils of the Upper Triassic Yanchang Formation in the interior of the Ordos Basin (Deng and Li, 1998; Fu and Yuan, 1998). The occurrence of these sporopollen once led to the re-definition of the stratigraphic age of this sequence as Late Triassic (Deng and Li, 1998; Fu and Yuan, 1998). Nevertheless, in recent years, the data of apatite fission tracks (Song et al., 2013a) and presence of younger zircons (188e200 Ma) (Guo et al., 2017) in these strata once again constrain their ages to the EarlyeMiddle Jurassic. Moreover, only Triassic sporopollen occur in the coal-bearing strata; representative plant fossils of Triassic age are absent (Bai et al., 2010). Reworked fossils have been widely reported over the last 50 years around the world (e.g., Bless and Streel, 1976; McLean, 1995; Riding et al., 2000; Harper and Collen, 2002; Lopes et al., 2014; Donovan and Fearnhead, 2015) and often lead to confusion regarding the definition of stratigraphic ages. However, their presence can inversely demonstrate recycled provenance from the underlying strata. The fragile plant fossils may be destroyed during the process of retransportation while the more durable sporopollen can stand and were subsequently preserved in the new strata. In addition, compositional analysis of the Yan’an Formation in the middle sub-region 1 indicates that sediments are mostly derived from sedimentary strata and subordinate volcanic rocks, in part metamorphosed, and exposed to erosion by the orogenic uplift of fold belts and thrust (Dickinson and Suczek, 1979; Dickinson et al., 1983) (Fig. 5A). The occurrence of carbonate debris rocks is also in favor of sedimentary provenance. The carbonate components in clastic limestone and sandstone (Fig. 5B) have been generally underestimated or neglected because of their chemical and physical weakness and subsequent modification by diagenesis after sedimentation (Zuffa, 1980). However, their existence as detrital grains in sandstones indicates an intrabasinal origin and proximal accumulation because carbonate rock fragments are susceptible to breakage during transportation and deposition (Dickinson and Suczek, 1979; Mack, 1984; Ingersoll, 1987). Therefore, carbonate debris in the Yan’an Formation in the middle region (Fig. 5B) suggest that detritus may be recycled from the proximal highlands comprising the carbonate rocks. The Paleozoic strata in the WOB are abundant of carbonate rocks (Huo et al., 1989). It is noteworthy that the Yan’an Formation in the Xinjing section of the middle sub-region 2 is quite distinct from the coeval deposits at the other sections in the WOB (Fig. 2). The thickness of the Yan’an Formation in the Xinjing section is generally less than 100 m, and more than 60% of its deposits are composed of conglomerates and coarse-grained sediments. The conglomerate clasts are mainly brownish-red quartz sandstones, greyish-green meta-sandstones, phyllite, and gray limestone, which are interpreted to be directly derived from the Xiangshan Group, Devonian and Carboniferous strata (Huo et al., 1989) in the Weiningbeishan. Some of the conglomerates are 10e50 cm in length and reserve the primary bedding structures of sandstones, reflecting proximal accumulation and seemingly suggesting marginal deposition. According to the well-connected sections, the Yan’an Formation in the Xinjing area pinches out over very short distances toward the two ends of the sections (Ningxia Geology and Mineral Survey Institute, 1987; Guo, 2015). These phenomena imply a drainage-closed and independent intermontane basin. Thus, we infer that the Weiningbeishan was still at high relief and sediments of the Yan’an Formation on it were recycled from the adjacent uplifted sedimentary strata. In conclusion, the U-Pb age comparison, the occurrence of the reworded sporopollen and carbonate debris and the sandstone

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Figure 11. (A) Deformation intensity of the Late TriassiceEarly Jurassic tectonic event in the WOB evidenced by contact relationships. Structural sketches are modified from Liu (1998). Division of the deformation regions: (a) darker brown represents the compressional front of the QiOB, as evidenced by the absence of Triassic strata and recycled sedimentation; (b) lighter brown represents the minor strain concentration zone of the QiOB, as evidenced by the angular unconformity; (c) blue represents the compressional front of the AB, as evidenced by the angular unconformity; (d) superimposed brown and blue represents the junction zone influenced by both the QiOB and the AB, as evidenced by the absence of Triassic strata and recycled sedimentation. Sections: 1eZhuozishan, 2eGuzhangben, 3eLiushugou, 4eShigouyi, 5eShiganggou, 6eWell Huan 14, 7eQingyang, 8eBinxian, 9eShuichuangou, 10eAnkou, 11eTanshan, 12eBaojishan, 13eXinjing (part of the data collected from Tang et al. (1992) and Bai et al. (2010)). (B) Tectonic evolution of the cross-well profile in the middle region, showing how the pre-Jurassic strata were uplifted and acted as potential sedimentary provenance, providing recycled detritus to the Jurassic strata (modified after Guo et al., 2017).

compositions all demonstrate that multi-cycle sedimentation occurred in the two middle sub-regions during the Yan’an Formation deposition. 6.3. Mechanism for recycled sedimentation Precondition for recycled sedimentation is the existence of highlands involving previous sedimentary deposits. Formation of these highlands is generally related to previous tectonic deformation. The Late TriassiceEarly Jurassic tectonic event in the WOB is most likely to account for the recycled sedimentation of the Middle Jurassic Yan’an Formation. Extensive zircon fission-track ages recorded this event in the WOB (Chen et al., 2007; Zhao et al., 2016). On the basis of field work, the contact relationships between the Yan’an Formation and its underlying strata in the WOB show great variations in different regions (Tang et al., 1992) (Fig. 11A). Highangle unconformity occurred in front of the QiOB and AB (regions b and c in Fig. 11A), while parallel unconformity occurred in the interior of the Ordos Basin (sections 7 and 8 in Fig. 11A), suggesting that the deformation intensity gradually decreased eastward. In the three sections (sections 5, 6 and 11) of the Liupanshan area (roughly corresponding to the region a in Fig. 11A), the Triassic strata are absent and the Yan’an Formation unconformably overlies on the Proterozoic, Ordovician, and Permian strata respectively (Fig. 11A). In fact, drilling data also unravel that the Yan’an Formation overlies on the Middle Triassic strata in the Mahuanggou section and on the Carboniferous strata in the Yaoshan section, another two sections in

the Liupanshan area (Guo, 2015). These observations demonstrate that the Upper Triassic strata are generally absent in the Liupanshan area. Previous work has suggested that Upper Triassic deposition actually occurred in the whole WOB (Liu, 1998; Liu et al., 2005; Zhao et al., 2006b; Ritts et al., 2009; Bai et al., 2010). The lack of this succession in the Liupanshan area implies an intense Late TriassiceEarly Jurassic event in the middle region of the WOB. This event might account for the uplifting and eroding of the Upper Triassic and/or underlying strata and the forming of highlands inside the WOB. According to the tectonic evolution of the Mesozoic successions in the Wangwa section, the Late Triassic highlands can form as the pre-Jurassic strata thrusted and folded along the previous (?) faults (Fig. 10B). Since the WOB was in a weak intraplate transition zone, there were many deep faults that had the potential to be reactivated as long as regional extension or compression happened (Tang et al., 1992). Some of these Late Triassic highlands could remain in the Jurassic and acted as recycled sedimentary sources for the subsequent deposits (Fig. 10B). In the Weiningbeishan (middle sub-region 2), which is located in the eastern extension of the Hexi Corridor, the Jurassic strata overlie locally on the previous EW-trending folds (Bureau of Geology of the Ningxia Hui Autonomous Region, 1974; Li, 2006). These folds involved the Lower Paleozoic Xiangshan Group, Devonian and Carboniferous strata. The lack of Permian and Triassic strata in these folds suggests a post-Carboniferous and pre-Jurassic event in the Weiningbeishan. Whether regional

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subsidence and deposition occurred in the Weiningbeishan during the Permian-Triassic period remains uncertain. Li (2006) referred these EW-trending folds as the late Triassic deformation according to their structural characteristics. A sample of zircon fission tracks recorded a Late Triassic (206  16 Ma) event in the southern Weiningbeishan (Chen et al., 2007). Therefore, we infer that in the Late Triassic, the Weiningbeishan uplifted and folded rapidly because of the northeastward and southeastward compression. Even in the subsequent Jurassic when regional stretching occurred (Zhao et al., 2000), the Weiningbeishan was still in high relief, only with local subsidence in the Xinjing section surrounded by sedimentary highlands. 6.4. Tectonic implication The above discussions suggest that the Late TriassiceEarly Jurassic deformation occurred extensively in the WOB. Although its deformation traces have been mostly obscured, deformed, and overprinted, this event may be comparable in intensity to the prominent Late Jurassic folding-trusting event in the WOB, because vast Triassic deposits had been eroded in its middle region. Unlike the Late Jurassic event whose deformation mainly concentrated on a NS-trending belt, the Late Triassic most intense deformation occurred in an arc-shaped belt in front of the QiOB (region a in Fig. 11A). Six zircon fission-track samples collected from the northeastern part of the QiOB consistently recorded this late TriassiceEarly Jurassic uplifting events in the belt, as their ages are 190  10 Ma (Proterozoic schist), 212  15 Ma (Carboniferous sandstones), 234  15 Ma (Carboniferous sandstones), 204  23 Ma (Permian sandstones), 196  14 Ma (Triassic sandstones), and 215.6  14.4 Ma (Jurassic sandstones) (article in preparation). The deformation in front of the AB was also very intense, but its intensity is much less than that in front of the QiOB. The intraplate deformation in the WOB and QiOB has been interpreted as the response to the far-field stresses associated with the compression at the boundaries of the Eurasian Plate (e.g., Darby et al., 2001; Darby and Ritts, 2002; Ritts et al., 2009), where Tibetan blocks pieced northward into the southern margin of Eurasian Plate (Enkin et al., 1992; Darby and Ritts, 2002; Ritts et al., 2009; Faure et al., 2012). The QiOB acted as the main indentation that transported the southwesterly far-field stresses to the weak WOB. As a result, thrusts, folds and uplifts occurred extensively in the middle region of the WOB, leaving several residual highlands in the Jurassic. The less intense deformation in the northern WOB may be related to the eastward tectonic escape of the AB (Liu, 1998), responding to the continued collision and amalgamation of the NCC and the Mongolian composite terrane along the PermianeTriassic Solonker suture (Zorin, 1999; Darby et al., 2001). The Weiningbeishan, as in the tectonic junction of the AB and QiOB, underwent the most intense deformation. 7. Conclusions Large amounts of chronological data of detrital zircons from the EarlyeMiddle Jurassic coal-bearing sediments have been presented to study the provenance of the Yan’an Formation in the WOB. The sedimentary domain of the Jurassic WOB can be divided into three parts: northern, middle, and southern. Sediments in the northern and southern regions were primarily derived from the proximal mountains; those in the middle region, however, were partly originated from the PaleozoiceTriassic sedimentary beds. The occurrence of recycled sedimentation during the EarlyeMiddle Jurassic indicates that the Late TriassiceEarly Jurassic tectonic event was very intense in the WOB, especially for the region in front of the QiOB. The QiOB acted as a force indentation, which

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