Evolution and provenance of the Xuefeng intracontinental tectonic system in South China: Constraints from detrital zircon fission track thermochronology

Accepted Manuscript Full length article Evolution and provenance of the Xuefeng Intracontinental Tectonic System in South China: constraints from detrital zircon fission track thermochronology Chen Zheng, Changhai Xu, Manfred R. Brix, Zuyi Zhou PII: DOI: Reference:

S1367-9120(19)30072-0 https://doi.org/10.1016/j.jseaes.2019.02.012 JAES 3790

To appear in:

Journal of Asian Earth Sciences

Received Date: Revised Date: Accepted Date:

10 October 2018 20 February 2019 21 February 2019

Please cite this article as: Zheng, C., Xu, C., Brix, M.R., Zhou, Z., Evolution and provenance of the Xuefeng Intracontinental Tectonic System in South China: constraints from detrital zircon fission track thermochronology, Journal of Asian Earth Sciences (2019), doi: https://doi.org/10.1016/j.jseaes.2019.02.012

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Evolution and provenance of the Xuefeng Intracontinental Tectonic System in South China: constraints from detrital zircon fission track thermochronology

Chen Zhenga, Changhai Xu a,*, Manfred R. Brixb, Zuyi Zhoua a

State Key Laboratory of Marine Geology and School of Ocean and Earth Science, Tongji University, Shanghai 200092, China.

b

Faculty of geosciences and Institute of Geology, Mineralogy, and Geophysics, Ruhr-Universität Bochum, Bochum 44780, Germany.

Correspondence information: Changhai Xu, State Key Laboratory of Marine Geology, School of Ocean and Earth Science, Tongji University, Shanghai 200092, China, [email protected], +2165982358.

Evolution and provenance of the Xuefeng Intracontinental Tectonic System in South China: constraints from detrital zircon fission track thermochronology Chen Zhenga, Changhai Xu a,*, Manfred R. Brixb, Zuyi Zhoua a

State Key Laboratory of Marine Geology and School of Ocean and Earth

Science, Tongji University, Shanghai 200092, China. b

Faculty of geosciences and Institute of Geology, Mineralogy, and Geophysics,

Ruhr-Universität Bochum, Bochum 44780, Germany. Abstract The Xuefeng Intracontinental Tectonic System (XITS) is a large-scale compressional tectonic system in the central South China block. It is characterized by northeast-trending fold-thrust belts involving the Xuefeng zone, the Chevron syncline fold-thrust zone, the Chevron anticline fold-thrust zone and the Central Sichuan basin. This paper reports detrital zircon fission track (ZFT) analyses of the sandstones sampled from Neoproterozoic to Middle Jurassic sequences, and further discusses their relation with low-temperature tectonic evolution and sediment provenance. The totally annealed ZFT data of 452 to 377 Ma from the Neoproterozoic and Devonian sandstones were related to an Early Paleozoic low-temperature exhumation of the Jiangnan orogen due to intracontinental orogeny in the South China block. Those annealed ZFT components from Triassic to Jurassic sandstones, 2

grouped 236-189 Ma, 189-142 Ma, and 142-124 Ma decreasingly from east to west structurally imply a northwestward progressively squeezing by the Xuefeng intracontinental orogeny as a result of the Paleo-Pacific subduction. The unannealed age components (411 Ma, 362-377 Ma, 270-216 Ma, and 209-183 Ma) of the Upper Triassic to Middle Jurassic sandstones, are comparable to the multistage tectonic evolution of the Qinling orogen through subduction, accretion and collision and are therefore indicative of sample potential provenance. Keywords: zircon fission track; tectonic evolution; sediment provenance; Xuefeng Intracontinental Tectonic System; South China block. 1. Introduction The Xuefeng Intracontinental Tectonic System (XITS) as a typical Mesozoic compressional deformation system largely occupies the central South China block between the Qinling orogen to the north, the Sichuan basin to the west and the Jiangnan orogen to the east (Fig. 1, Yan et al., 2009; Li et al., 2012), and represents a key area for understanding tectonic propagation related to the decollement from Xuefeng Mountain due to the far-field subduction of Paleo-Pacific plate in the Mesozoic (Li and Li, 2007; Wang et al., 2010). Studies on the fold-thrust belts in the XITS has been covered extensively by using stratigraphy (Chu et al., 2102a; Li et al., 2012), structural geology (Yan et al., 2003, 2016; Wang et al., 2012), geochronology (Deng et al., 2013; Wang et al., 2013), seismic interpretation (Li et al., 2015; Li et al., 3

2018) and other methods. The proposed dynamics is related to multilayer decollement, such as westward nappe model (Yan et al., 2003), kink folds model (Ding et al., 2007), horizontal contraction model (Bai et al., 2015), sandbox model (Yan et al., 2016), etc. Due to the deformational process of fold-thrust belts mainly as a part of low-temperature evolution, some researchers commonly use apatite fission track (AFT) and (U-Th)/He dating, followed by thermal history modelling to constrain the cooling and exhumation of sedimentary outcrop rocks through 90~120 °C (Wagner and Van den Haute, 1992), and 55~80 °C (Farley, 2000). Mei et al. (2010) reported the timing of rapid cooling of fold-thrust belts decreased westward from 165 Ma, 154 Ma, 145 Ma, 136 Ma, 120 Ma, 115 Ma, to 95 Ma (Fig. 1). Meanwhile, Yuan et al. (2010) obtained an age decrease of exhumation westward from 137 Ma, 97 Ma, to 56 Ma. Shi et al. (2016) found the timing of intense uplift ranging from 130 to 75Ma, while Shen et al. (2009) and Zhu et al. (2018) reported a rapid exhumation in 100-70Ma in the northeastern Sichuan basin. Previous studies have identified the timing of deformational process in general as Mesozoic, but this still remains debatable varying from Triassic to Cretaceous among different units (Yan et al., 2003; Hu et al., 2009; Shen et al., 2009; Mei et al., 2010; Shi et al., 2016). We here select zircon fission track (ZFT) data widely from Neoproterozoic to Jurassic outcropped sandstones with a higher partial annealing zone to trace the progressive deformation within the XITS, which has not been paid 4

closer attention regionally in the previous studies. Furthermore, some ZFT data offer the possibility to characterize the denudation history of the potential provenances, in combination with the research results on sediment provenances of the XITS through the detrital zircon U-Pb analysis (Luo et al., 2014; Zhang et al., 2015a; Shao et al., 2016) and tectonic evolutions of the adjacent areas. 2. Geologic settings The XITS, a 1300-km-wide intracontinental tectonics which characterized by NE-trending fold-thrust belts, regionally covers the Xuefeng zone (XFZ), the Chevron syncline fold-thrust zone (CSZ), the Chevron anticline fold-thrust zone (CAZ), and the Central Sichuan basin (CSB). These units are confined by the Cili-Baojing Fault, the Qiyueshan Fault, and the Huayingshan Fault, respectively (Fig. 1, Yan et al., 2003; Li et al., 2012). The XITS has a basement of Proterozoic epimetamorphic sandy-argillaceous detrital sediments which are overlain by a cover sequence of Paleozoic to Middle Triassic marine sediments and Late Triassic to Cretaceous terrestrial clastic sediments (Fig. 1, Yan et al., 2003). The Xuefeng zone as a part of the Jiangnan orogen mainly exposes Proterozoic sedimentary, igneous, and metamorphic rocks. The widely exposed Banxi Group consists of sandstones, siltstones, shales, pelites, tillites, and limestones (Shi et al., 2016). Igneous rocks yielding zircon U-Pb ages from 930 to 825 Ma (e.g., Wang et al., 2007; Li et al., 2009; Zhao et al., 2011; Yao et al., 2014; Su et al., 2018) indicate that the South China block 5

formed in the Early Neoproterozoic through the collision of the Yangtze and Cathaysia blocks along the Jiangshan-Shaoxing suture zone (Charvet, 2013; Zhao, 2015; Cawood et al., 2017; Wang et al., 2018). The zones of the Xuefeng Intracontinental Tectonic System from the Chevron syncline fold-thrust zone, the Chevron anticline fold-thrust zone to the Central Sichuan basin display typical structural styles in which lack of magmatic and metamorphic activities since Mesozoic (Fig. 2). The CSZ is intensely folded and faulted and characterized by thick-skinned widely-spaced synclines in Permian to Triassic sequences coupled with box anticlines in Sinian to Ordovician rocks (Yan et al., 2003). The Sinian strata are composed of

dolostones,

limestones,

phosphorites,

marls,

siltstones,

pelites,

conglomerates, and moraines. The Cambrian strata are grey slates, shales, sandstones, conglomerates, and limestones interbedded with dolostones. The Ordovician strata are characterized by limestones interlayered with shales and dolostones. Thick-bedded Lower and Middle Silurian strata are greyish-green argillaceous siltstones, sandy mudstones, and limestones. Middle and Upper Devonian rocks are mainly sandstones, shales, and limestones. Permian strata are predominated by carbonate-rich rocks (Hunan BGMR, 1997; Yan et al.,

2003).

The

CAZ

is

characterized,

however,

with

thin-skinned

widely-spaced anticlines (mainly narrow Triassic with little Palaeozoic strata) intervened by box synclines (wide Jurassic sequences) (Hu et al., 2007). Most of thrust faults are associated with the cores of anticlines. The Triassic strata 6

contain grey limestone layers intercalated with marls, calcareous shales, and dolostones. Jurassic strata include sandstones, silty mudstones, argillaceous siltstones, shales, and conglomerates (Hubei BGMR, 1996; Sichuan BGMR, 1997). The less deformed CSB has gentle folds, scattered axis of folds, and rare faults. These zones from CSZ, CAZ to CSB are as a result of the westward,

diachronous,

progressive

decollement

combined

with

northwest-directed thrusting mainly in Jurassic to Early Cretaceous (Yan et al., 2003; Jin et al., 2009). Extensive studies have been carried out on the Phanerozoic tectonic evolution of the South China block, including Early Paleozoic (Caledonian), Triassic (Indosinian), and Jurassic-Cretaceous (Yanshanian) tectonothermal events (Li and Li, 2007; Charvet et al., 2010; Wang et al., 2013; Li et al., 2016; Shu et al., 2015; Qiu et al., 2016). The Caledonian intracontinental orogeny was recognized due to a regional unconformity between Middle Devonian and Silurian strata. In addition, the geochemical, zircon U-Pb, and

40

Ar/39Ar

geochronological data of metamorphic and granitic rocks revealed that the metamorphism and magmatism took place between 460 and 400 Ma (Faure, et al., 2009; Charvet et al., 2010, 2013; Wang et al., 2013; Li et al., 2016; Yan et al., 2017; Sun et al., 2018). Thus, the Early Paleozoic deformation was initiated as early as Middle Ordovician. Although many scholars are more inclined to accept the intraplate model, the subduction process and driving force still remain ambiguous. The Indosinian orogeny, occurred within an 7

intraplate setting, built the tectonic framework of the South China block (Shu et al., 2008; Chu et al., 2012a, 2012b). The NW-SE and ~N-S significant crustal shortening and widespread deformation were associated with the flat subduction of the Paleo-Pacific plate, and the continental collisions not only between the Indochina and South China blocks but also between the North China and South China blocks (Zhou et al., 2006; Chen et al. 2011; Xu et al., 2011; Wang et al., 2013; Qiu et al., 2014; Song et al., 2017). The Yanshanian movement was composed of a Middle-Late Jurassic crustal contraction event and a Cretaceous crustal extension event (Yan et al., 2003; Ding et al., 2007; Shu et al., 2009). Generally, the earlier NE trending fold-thrust belts were attributed to northwestward subduction of the Paleo-Pacific plate (Li and Li, 2007). The competing driving force of later extension can be summarized into slab rollback of the Paleo-Pacific plate, slab foundering and delamination of the subducted slab, and intracontinental lithospheric extension (Li et al., 2016, and references therein). The Qinling orogen, located in the north of the XITS, is a giant and composite orogen between the North China block and the South China block (Fig. 1). This orogen, composed of North Qinling block and South Qinling block, in general underwent five-stage tectonic evolution proposed by most researchers: (i) subduction of the Early-Middle Paleozoic Shangdan oceanic crust beneath the North Qinling block, (ii) an Early Devonian-Early Triassic continental convergence changed from North Qinling block and North China 8

block to South China block and North Qinling block with opening of the Mianlue Ocean, (iii) subduction of Mianlue oceanic crust beneath the South Qinling block in the Permian, (iv) a Triassic collision between South China block and South Qinling block along the Mianlue suture, and finally (v) an intracontinental orogeny in Qinling orogen (Xu et al., 2000; Wu and Zheng, 2013; Dong and Santosh, 2016; Yan et al., 2018). 3. Sample collection, Preparation and Data processing This study collected 21 sandstone samples from the outcrops along a northwestward profile through the XITS (Fig. 2). The two oldest samples (SC02, SC18) are from the Neoproterozoic Wuqiangxi Formation in the XFZ, one sample (SC22) is Middle Devonian in the CSZ, and other 18 samples from the CSZ, CAZ, and CSB are Mesozoic consisting of Middle Triassic Badong Formation, Upper Triassic Xujiahe Formation, and Middle Jurassic Shaximiao Formation (Table 1). The analyses of detrital zircon fission track (DZFT) were carried out by the external-detector method (Wagner and Van den Haute, 1992) usefully for identification of age components (Kowallis et al., 1986). Both zircon separation and sample preparation were performed in State Key Laboratory of Marine Geology of Tongji University (China). Zircons from each sample with the weight of 8-10kg were separated by conventional heavy mineral separation techniques. Rocks were crushed by jaw-crusher, screened by sieve shaker, then washed and dried, finally the 75 to 9

250μm grain portion was retained. After that we used magnetic separator to remove the magnetic minerals and heavy liquid (bromoform, diiodomethane) to purify zircon. Zircon grains from the samples were finally selected by hand-picking under an optical binocular microscope with fiber-optic lights. Zircon grains from the samples and standards (Fish Canyon Tuff & Mt. Dromedary Banatite) were arranged by hand and embedded into Teflon. In order to detect age populations, 2 or 3 mounts per sample were made. Internal surfaces of zircons were revealed by emery paper of 1000 and 2400 mesh size, then polished by diamond paste of 3 μm, 1 μm, and 1/4 μm, respectively. Sample mounts were etched in an eutectic melt of NaOH & KOH (1 : 1 mol) at a temperature of 210 °C for the time range of 8~30h to fully reveal latent spontaneous fission tracks. Both zircon mounts and standard glasses were attached with muscovite external detectors (low-uranium mica) and stacked inside irradiation Polyethylene TRIGA tubes, where dosimeter standard glasses (CN-2) were placed at the top, middle and bottom for measuring the neutron fluence. Two sample tubes were irradiated with total thermal neutron fluence of 1.0×1015ncm-2 at the TRIGA reactor in Oregon State University and cooled in lead containers for 6 weeks. The muscovite external detectors were removed and etched in 40% HF at 20°C for 40 minutes to fully reveal the induced tracks. The last step consists of track counting, personal-ζ determination, detrital zircon fission track age calculation and age components separated. The 10

calculated personal ζ-calibration factor (Hurford, 1990) is based on Chen Zheng’s value of 139.4±2.7. Fission tracks were measured using a Zeiss Axio Imager M2m microscope at 1000× in transmitted and reflected light, and a software of Trackworks. Table 1 lists ZFT central ages calculated by 22 to 58 grains per sample with 1σ confidence level for age errors. We use χ2 test statistically to assess the observed spread in grain ages (Galbraith, 1981). Over one-half of our samples fail the χ2 test (<5%), the age components have been thus decomposed by the auto mixture model of the “RadialPlotter” software, shown on the radial plots (Vermeesch, 2009), to reflect contribution of sediment provenances and/or constraints on rock burial, annealing, and exhumation. When pass χ2 test (>5%), the age data are treated as one age component by a calculation of the central age. Commonly, about 50 to 100 grains for each sample are measured statistically to acquire exact results of age components. Though less than 50 dateable grains, most of our samples clearly show statistics of age separation and regularity of age component. 4. Results and interpretation 4.1 Samples from Xuefeng zone Samples SC02 and SC18, thick grey coarse sandstones from the Early Neoproterozoic Wuqiangxi Formation, are collected from XFZ. All measured grains from SC02 have ZFT ages younger than the depositional age (890-834 Ma, Hunan BGMR, 1997) which means that the rock had experienced total annealing. Its ZFT data pass χ2 test (42%), thus a central age of 377±16 Ma is 11

considered for this sample (Table 1) to reveal an exhumation time through the zircon partial annealing zone to the surface after deposition. The sample SC18, all grain ages pass χ2 test (68%) with a central age calculation of 452±25 Ma (Table 1), indicating that this sample was also totally thermally reset after deposition, and well recorded an exhumation time. The results of detrital ZFT ages (377 and 452 Ma) from SC02 and SC18 are a response to the Caledonian orogeny which related with an intraplate continental subduction in the South China block. 4.2 Samples from Chevron syncline fold-thrust zone Devonian and Triassic samples were collected for ZFT studies from the CSZ. Sample SC22, located in the east of the CSZ, is a thick greyish-white fine sandstone from the sequence of Middle Devonian. Its ZFT data pass χ2 test (37%) and yield a central age of 383±19 Ma (Table 1) close to the depositional age (393-383 Ma, Hunan BGMR, 1997). We infer that this sample experienced probable annealing, and its central age registers a cooling time when the rock was exhumed through the zircon partial annealing zone up to surface. Samples SC20, SC21, SC23, SC25 and SC30, stratigraphically belonging to the Middle Triassic Badong Formation, were taken from green-greyish fine sandstone layers along a NW-trending section in the north of the CSZ. ZFT central ages for them are respectively 233±14 Ma, 217±12 Ma, 205±11 Ma, 249±19 Ma and 236±10 Ma (Table 1), close to or slightly younger than their depositional age (245-228 Ma, Hunan BGMR, 1997; Hubei BGMR, 1996). 12

These samples, except for SC25, with the single-grain ages all pass χ2 test (5 to 57 %), and their central ages would be used to constrain times of cooling and exhumation of rocks. The result of SC25 comprises two age populations: a dominant mode (60% data) of 189±22 Ma and a secondary component of 411±63 Ma (40% data, Table 2). We propose that the younger age component comes from slightly-annealed zircons as the time constraint on rock cooling and

exhumation

after deposition,

and the

older

age

portion from

hardly-annealed zircons as the age when the source rocks cooled through the zircon partial annealing zone in exhumation. 4.3 Samples from Chevron anticline fold-thrust zone Samples collected from the northern CAZ are Jurassic and Triassic along a NW-trending profile. SC27, SC57, SC58, SC60, SC61 and SC63 were sampled from red to greyish medium-fine to medium-coarse sandstone layers of the Middle Jurassic Shaximiao Formation. They have ZFT central ages of 194±7 Ma, 188±7 Ma, 188±8 Ma, 167±10 Ma, 181±10 Ma, and 183±8 Ma respectively (Table 1) close to or slightly older than the depositional age (172-161 Ma, Hubei BGMR, 1996; Sichuan BGMR, 1997). ZFT data of the samples, due to their single-grain ages failing χ2 test (0%), are statistically grouped into two age components: one is Middle Jurassic to Early Cretaceous (SC27, 163±19 Ma, 45%; SC57, 146±16 Ma, 30%; SC58, 146±15 Ma, 42%; SC60, 143±15 Ma, 68%; SC61, 154±14 Ma, 67%; SC63, 142±12 Ma, 47%), and the other is Middle Permian to Early Triassic (SC27, 229±24 Ma, 55%; 13

SC57, 216±20 Ma, 70%; SC58, 231±23 Ma, 58%; SC60, 254±42 Ma, 32%; SC61, 270±29 Ma, 33%; SC63, 241±21 Ma, 53%; Table 2). As a result of post-depositional partial annealing, the younger ZFT age component could be helpful to define a cooling time through the partial annealing zone up to surface, during which the Jurassic rocks of the basin were exhumed; whereas the older age component could yield a minimum cooling age when the rocks within the source areas underwent an exhumation through the partial annealing zone. The samples from Upper Triassic Xujiahe Formation (SC56, SC59, SC62, and SC64) are greyish-white medium-coarse sandstones, alternately with the Jurassic samples mentioned above (Fig. 2). Their central ages are 222±9 Ma, 201±11 Ma, 197±11 Ma and 211±7 Ma (Table 1), respectively. The ZFT data of sample SC56 pass χ2-test (15%). Its central age of ca. 222 Ma is slightly older than the depositional age (210-201 Ma, Sichuan BGMR, 1997). It may record the cooling age of potential provenance, however, partially affected by post-depositional burial reheating close to the zircon partial annealing zone. In the remaining samples, the central ages fail χ2 test (0 %), and their ZFT data were thus decomposed into individual age components. The majority (84-66 %) of grains belong to an age population of 189 to 182 Ma essentially reflecting post-depositional cooling and exhumation time of the Triassic strata, while the component (34-16 %) of older grains (377±60 Ma, 362±45 Ma, and 268±29 Ma, Table 2) is useful to constrain the minimum cooling ages of potential provenance. 14

4.4 Samples from Central Sichuan basin Samples SC65, SC66, and SC67, representing the eastern CSB were from medium-fine to medium-coarse red sandstone layers of the Middle Jurassic Shaximiao Formation. The ZFT analyses yielded the central ages of 157±6 Ma, 174±8 Ma, and 182±5 Ma, respectively (Table 1). ZFT data of sample SC67 pass χ2 test (6%), and its central age of ca. 182 Ma older than depositional age (172-161 Ma, Sichuan BGMR, 1997) may record a cooling age associated with exhumation of sediment source rocks. The rest samples with ZFT data fail χ2 test (0%), two age peaks of 124±11 Ma (46% data) and 195±18 Ma (54% data) for SC65, as well as 142±31 Ma (41% data) and 209±44 Ma (59% data) for SC66 (Table 2) were thus calculated by the automatic model. The younger age peaks imply post-depositional cooling of the samples through the zircon partial annealing zone, and the older ones from partially-annealed zircons reveal the minimum cooling ages of potential provenances. 5. Discussion Detrital ZFT analyses of sedimentary rocks usefully constrain cooling, exhumation, and potential provenance. This study combined our detrital ZFT data with other published results to trace the evolution of the XITS together with discussions on potential source areas for the Triassic to Jurassic sediments. The ZFT data for most of our samples failed χ2 test and were decomposed as young and old age components, while those for other samples 15

(passed χ2 test) were statistically grouped as the young and old central ages (Fig. 3). The young age data less than sample depositional ages were interpreted as the result of three tectonic events of the XITS which discretely occurred in Early Paleozoic and Mesozoic. The data older than the depositional ages were comparatively associated with evolution of potential provenances. 5.1 Inferred evolution from detrital ZFT analyses Comparison of the detrital ZFT data (Table 2) indicates that the eastern areas have obviously been affected by a post-depositional thermal overprint where sample zircons almost totally annealed, but the western areas where sample zircons annealed only partially and variously were not sufficiently thermal. As shown in Fig. 3, the age frequency histograms indicate the portions of annealed zircons sampled from various units have a westward decreasing trend, thus the degrees of partial annealing are consistent with observation of progressively differential exhumations from east to west within the XITS. 5.1.1 Relevance to Early Paleozoic intracontinental orogeny The oldest sedimentary rocks examined belong to the Neoproterozoic Wuqiangxi Formation from the XFZ (SC02, SC18) and the Middle Devonian from the CSZ (SC22). Their ZFT central ages respectively record the times of cooling and exhumation of the XFZ and CSZ units (Fig. 4). The Caledonian orogeny, largely evidenced by the exposures of metamorphic (zircon U-Pb, 16

468-420Ma, Wang et al., 2012; Sun et al., 2018) and granitic (zircon U-Pb, 460-400 Ma, Wang et al., 2011; Yan et al., 2017) rocks from the Cathaysia block, by the angular unconformity between pre-Devonian strata and Ordovician strata as well as by folding to thrusting deformation, took place from 460 to 400 Ma, in mechanism interpreted as underthrusting of Cathaysia block northwestward beneath (Charvet et al., 2010), or overthrusting of Cathaysia block northwestward atop the Yangtze block (Li et al., 2010), or possibly in association with the northwestward subduction of the inferred East China Sea block beneath the Cathaysia block (Shu et al., 2015). This ZFT age population (452 to 377 Ma) is proposed as the result of the low-temperature thermal evolution due to the intracontinental orogeny in Early Paleozoic. 5.1.2 Relevance to Mesozoic progressively deformational process Based on the detrital ZFT analyses, the younger age peaks of five sandstone samples (Badong Fm., T 2) within the CSZ have a range of 236 Ma to 189 Ma (Fig. 4). These age peaks could be comparable to the period of Indosinian orogeny, registering low-temperature cooling and exhumation of the eastern XITS. This intracontinental orogeny led to a large-scale fold–thrust system of pre-Triassic strata with metamorphism in the South China block (Shu et al. 2008). It took place at ca. 245-200 Ma, proven by an unconformity regionally between Pre-Mesozoic and Late Triassic strata (Xiao and He, 2005), and zircon U-Pb ages (SHRIMP, LA-ICP-MS) of granites (Chen et al. 2011; Xu et al., 2011; Chu et al., 2012a, b; Song et al., 2017). The Xuefeng 17

intracontinental orogeny is further associated by Li and Li (2007) with a horizontal far-field stress generation and transmission derived from the flat-slab subduction of the Paleo-Pacific plate. Samples SC59, SC62, and SC64 (Xujiahe Fm., T 3) from the CAZ comprise a younger age population of 189-182 Ma. Other samples (Shaximiao Fm., J2) are indicative of two younger age populations of 163-142 Ma for the CAZ (SC27, SC57, SC58, SC60, SC61, and SC63) and 142-124 Ma for the CSB (SC65 and SC66). These age components are shown in Fig. 4 and elucidate a long-term Yanshanian exhumation of the western XITS. Due to subduction of Paleo-Pacific plate beneath East Asia, this exhumation is possibly in response to far-field compression in the Mesozoic (Shu et al., 2009; Wang et al., 2013). All younger detrital ZFT data generally display four major age groups (Fig. 4) and provide insight into a long-term deformation and cooling history of the XITS. The groups of 236-189 Ma, 189-182 Ma, 163-142 Ma, and 142-124 Ma successively register a low-temperature exhumation from the CSZ, CAZ to CSB, with ages decreasing westward. According to field geology, the stratigraphic contact related to the Xuefeng intracontinental orogeny transited westward gradually from angular unconformity between Middle-Lower Triassic and Upper Triassic strata within the XFZ, between Middle-Lower Jurassic and Middle-Lower Triassic strata within the eastern CSZ, to parallel unconformity and conformity within the western CSZ (Liu et al., 2010). Another angular 18

unconformity due to the Yanshanian orogeny formed between Upper Jurassic and underlying strata within the CAZ, and westward changed to between Cretaceous and underlying strata in the CSB (Mei et al., 2010). These unconformable stratigraphic contacts correspond well to our detrital ZFT age groups in the units. The seismic sections also revealed an intense to weak deformation westward from XFZ to CSB (Ding et al., 2007; Liu et al., 2012; Yan et al., 2003) in accordance with the trend of our detrital ZFT data. Though analyses of AFT thermal histories have constrained the timing of intense exhumation decreased westward from 165 to 95 Ma (Mei et al., 2010; Yuan et al., 2010; Deng et al., 2013), the obtained ZFT data record an early period of intense exhumation from 236-189 Ma, 189-142 Ma to 142-124 Ma westward for CSZ, for CAZ and for CSB, respectively. All these evidences for the XITS are indicative of a continued intraplate orogeny generated a westward progressive deformation in Mesozoic. Its dynamics is associated with northwest-directed squeezing from decollement of Xuefeng zone, possibly caused by a far-field oblique subduction of the Paleo-Pacific Plate (Li and Li, 2007; Shen et al., 2007; Wang et al., 2013; Shi et al., 2016). 5.2 Potential provenance from detrital ZFT analyses This study also has found some detrital zircons retaining the unannealed FT data that usefully offer information on sediment provenance. These age peaks are displayed in Fig. 5 and grouped as ca. 411 Ma from the CSZ, 377-362 Ma, and 270-216 Ma from the CAZ, and 209-183 Ma from the CSB, 19

respectively, to reflect minimum cooling ages of their sediment provenance when through zircon partial annealing zone. Some researchers proposed that Triassic to Jurassic sequences around the northern Sichuan basin sourced mainly from the Qinling orogen and the North China block in terms of detrital zircon U-Pb data (Luo et al., 2014; Zhang et al., 2015a; Shao et al., 2016). The obtained detrital ZFT unannealed data from the sandstones in age also comply with major tectonic events of the Qinling orogenic evolution. An amount of zircon U-Pb dates previously analyzed from igneous rocks (ophiolite, gabbro, and granite) within the Shangdan suture zone have a range of ca. 540-403 Ma (Pei et al., 2007; Dong et al., 2011a; Zhang et al., 2015b). A series of U-Pb ages of 514-400 Ma additionally constrain the time of HP/UHP metamorphism sampled from the Erlangping zone (Cheng et al., 2012; Bader et al., 2013; Tang et al., 2016). Both magmatism and metamorphism in Early Paleozoic

registered

a

northward

subduction

of

Shangdan

Ocean

(Prototethyan oceanic crust) beneath the North Qinling belt. Therefore, the unannealed age peak (ca. 411 Ma) obtained from Middle Triassic Badong Formation could be associated with a low-temperature exhumation due to the Early Paleozoic plate convergence (Dong et al., 2011b; Wu and Zheng, 2013). Subduction of South Qinling belt beneath the North Qinling belt took place from 400 to 250 Ma, with the closure of the Shangdan ocean (Meng and Zhang, 2000). The unannealed age data (377-362Ma) from Upper Triassic Xujiahe Formation

more

likely

correspond 20

to

a

cooling

event

by

the

continent–continent subduction in Late Devonian. Zircon U-Pb data (340-246 Ma) for volcanic rocks from Mianlue suture (Li et al., 2004; Lai and Qin, 2010) and the ages of zircon U-Pb, as well as Ar-Ar, Rb-Sr, and Lu-Hf isochrons (255-213 Ma) from metamorphic rocks of the orogen (Liu et al., 2004, 2008; Cheng et al., 2009, 2010; Zhou et al., 2011) indicate a continuous evolution from northward subduction of the Mianlue oceanic crust beneath the South Qinling belt in 320-250 Ma to oblique subduction of the South China block beneath the South Qinling belt along the Mianlue suture in 255-225 Ma. Our unannealed age data (270-216 Ma) from the sandstones fit well with this tectonic event. It is possibly deduced that the South Qinling areas as sediment provenance supply detritus to build up the Middle Jurassic Shaximiao Formation within the CAZ. After the amalgamation of South China and North China blocks, the Qinling belt experienced an intracontinental orogeny in 210 to 163 Ma (Meng and Zhang, 2000; Hacker et al., 2004; Dong et al., 2013; Qian et al., 2015). This orogeny is manifested by structural (Li et al., 2013a; Dong et al., 2015), sedimentary (Li et al., 2013b), and isotopic dating (Shen et al., 2007; Shi et al., 2012) analyses. It is therefore considered that the provenance of the Middle Jurassic Shaximiao Formation from the CSB recorded 209 to 183 Ma by ZFT, is associated with the Qinling orogen. 6. Conclusions This study presents the detailed ZFT analyses of the pre-Cretaceous 21

sandstones from the XITS to constrain the low-temperature evolution or the potential provenance. The totally annealed ZFT data of 452 to 377 Ma register a low-temperature exhumation of the Jiangnan orogen in response to the Early Paleozoic intracontinental orogeny. The annealed ZFT components are in general grouped as 236-189 Ma (CSZ), 189-142 Ma (CAZ), and 142-124 Ma (CSB). This westward decreasing trend in age is proposed to be a result of progressive northwestward squeezing by the Xuefeng intracontinental orogeny triggered by the Paleo-Pacific subduction in Mesozoic. In addition, the unannealed ZFT components grouped as ca. 411 Ma, 362-377 Ma, 270-216 Ma, and 209-183 Ma possibly record the minimum exhumation ages of the potential provenance which could be related to multistage evolution of the Qinling orogen. Acknowledgments This work was supported by the National Science and Technology Major Project (Grant No.2017ZX05005001-005), the National Natural Science Foundation of China (Grant No.41876045), the Fundamental Research Funds for the Central Universities (Grant No.22120170193), and the project of Sinopec Petroleum Exploration and Production Research Institute (Contract No.G5800-15-ZS-WX030). The authors gratefully acknowledge Dr. Steve Reese for sample irradiation support. Considerable thanks to the technician Xiaofeng Zhao for sample preparation. We very appreciate constructive comments and suggestions from the reviewer. 22

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Fig. 1. Sketch map of the central South China block and adjacent tectonic zones showing the Sichuan basin, major structural zones, faults and research area (adapted from Li et al., 2012). AFT sample locations in small circles are from Mei et al. (2010).

Fig. 2. Outline of the Xuefeng Intracontinental Tectonic System with sample locations. F1: Cili-Baojing fault; F2: Qiyueshan fault; F3: Huayingshan Fault.

Fig. 3. The radial plot diagrams and age frequency histograms of DZFT ages. Single-grain ages of each sample are shown as identically colored circles and line(s). The Solid line represents the age component, the dashed line represents the central age, and the shaded areas mark the depositional age of each sample. The portions of annealed vs. unannealed zircons of the sandstones vary among the units which reveal a decrease trend of the degrees of partial annealing from east to west.

Fig. 4. Summarized annealed DZFT central ages (±1σ) and age components 39

(±2σ). Symbols represent different strata. A decreasing trend in age from XFZ, CSZ, CAZ to CSB can be observed, in accordance with an Early Paleozoic intracontinental orogeny and a Mesozoic northwestward progressively deformed intraplate orogeny.

Fig. 5. Summarized unannealed DZFT central ages (±1σ) and age components (±2σ). Symbols represent different strata. The age peak distribution is related to four stages of Qinling orogeny.

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Table 1 Sample and zircon fission track data. Analyses by external detector method. N=total counted grains; Ns, Ni and Nd = counted tracks (spontaneous, induced, and on dosimeter, respectively); ρ= track densities (×106 tracks cm-2) on grain (ρs), mica (ρi) and dosimeter (ρd). Samples were irradiated in the TRIGA reactor of Oregon State University (USA); Track measurements used 100× dry objective and 10× eyepiece. Ages are calculated using dosimeter glass CN-2 (U=36.5±0.7 ppm) and personal zeta (139.4± 2.7) by multiple analyses of Fish Canyon Tuff (27.8±0.7 Ma; Hurford, 1990) and Mt. Dromedary Banatite (98.7±1.1 Ma; Green, 1985). P(χ2) is the probability for obtaining a χ2 value for υ degrees of freedom, where υ= number of grains-1. Central age is a modal age, weighted for different precisions of individual grains (Galbraith and Laslett, 1993). Spontaneous Induced Dosimeter Central age Sample Period Formation Lithology Latitude Longitude N Pχ2(%) (Ma ± 1σ) ρs Ns ρi Ni ρd Nd SC02 SC18

Neoproterozoic Neoproterozoic

Wuqiangxi Wuqiangxi

sandstone sandstone

SC20 SC21 SC22 SC23 SC25 SC30

Middle Triassic Middle Triassic Middle Devonian Middle Triassic Middle Triassic Middle Triassic

Badong Badong Badong Badong Badong

sandstone sandstone sandstone sandstone sandstone sandstone

SC27 SC57 SC56 SC58 SC59 SC60 SC61 SC62 SC63 SC64

Middle Jurassic Middle Jurassic Upper Triassic Middle Jurassic Upper Triassic Middle Jurassic Middle Jurassic Upper Triassic Middle Jurassic Upper Triassic

Shaximiao Shaximiao Xujiahe Shaximiao Xujiahe Shaximiao Shaximiao Xujiahe Shaximiao Xujiahe

sandstone sandstone sandstone sandstone sandstone sandstone sandstone sandstone sandstone sandstone

SC65 SC66 SC67

Middle Jurassic Middle Jurassic Middle Jurassic

Shaximiao Shaximiao Shaximiao

sandstone sandstone sandstone

Xuefeng zone 28°36'28.62'' 110°55′02.66″ 40 8.24 29°03′07.71″ 110°34′38.00″ 33 11.16 Chevron syncline fold-thrust zone 29°25′37.55″ 110°39′57.72″ 27 9.93 29°22′07.53″ 110°06′25.96″ 30 9.76 29°26′17.41″ 109°50′57.38″ 22 12.19 29°54′34.50″ 110°02′26.44″ 41 14.38 29°38′50.94″ 109°30′05.08″ 38 13.62 29°43′24.38″ 109°04′02.35″ 40 12.17 Chevron anticline fold-thrust zone 30°10′07.59″ 108°47′35.75″ 54 9.68 30°15′17.90″ 108°26′27.36″ 52 7.98 30°19′34.03″ 108°13′09.41″ 27 8.49 30°28′38.58″ 108°02′33.70″ 51 8.68 30°38′44.93″ 107°49′25.71″ 38 8.07 30°38′13.92″ 107°43′08.08″ 43 9.84 30°45′58.36″ 107°28′07.20″ 52 9.23 30°45′47.14″ 107°22′04.86″ 31 9.17 30°45′03.28″ 107°14′48.32″ 57 7.73 30°45′54.87″ 107°07′23.69″ 43 7.32 Central Sichuan basin 30°57′37.18″ 106°57′40.67″ 58 8.02 31°03′38.92″ 106°42′05.45″ 51 8.04 31°12′33.13″ 106°26′03.70″ 54 7.23 41

8220 6044

0.60 0.66

585 354

0.4 0.39

5864 5814

41.62 68.48

377 ± 16 452 ± 25

2584 3566 6732 4573 4267 6760

1.18 1.22 0.88 1.89 1.44 1.40

291 434 463 603 453 801

0.38 0.39 0.39 0.39 0.40 0.41

5675 5738 5765 5817 5897 6028

57.71 12.12 37.15 5.69 0 7.76

233 ± 14 217 ± 12 383 ± 19 205 ± 11 249 ± 19 236 ± 10

16366 17410 7620 8141 10257 4592 7786 9132 19935 18098

1.35 1.18 1.00 1.32 1.01 1.68 1.41 1.17 1.16 0.87

2280 2535 913 1225 1294 777 1163 1155 2929 2182

0.40 0.41 0.39 0.42 0.38 0.42 0.41 0.38 0.41 0.37

5962 6094 5716 6162 5667 6174 6112 5618 6062 5463

0 0 15.39 0.02 0 0 0 0 0 0

194.3 ± 7 188.4 ± 6.5 221.8 ± 8.9 187.6 ± 8.2 201 ± 11 166.5 ± 9.6 181.4 ± 9.7 197 ± 11 183 ± 7.9 210.7 ± 7.2

14715 9617 16691

1.39 1.23 1.10

2624 1489 2502

0.41 0.40 0.40

6012 5963 5913

0 0 6.19

156.8 ± 6.2 174.4 ± 7.7 182.2 ± 4.5

Table 2 Zircon fission track age components of the samples. The peak-fit ages (± 2SE) with the percentage of grains are statistically decomposed by using the automatic model in the ‘RadialPlotter’ software (Vermeesch, 2009); those peak-fit ages without the percentage are central ages in Table 1; Stratigraphic ages come from related regional geological materials. Sample

Stratigraphic age (Ma)

N

SC02 SC18

890-834 890-834

48 33

SC20 SC21 SC22 SC23 SC25 SC30

245-228 245-228 393.3-382.7 245-228 245-228 245-228

27 30 22 41 38 40

SC27 SC57 SC56 SC58 SC59 SC60 SC61 SC62 SC63 SC64

171.6-161.2 171.6-161.2 210-201 171.6-161.2 210-201 171.6-161.2 171.6-161.2 210-201 171.6-161.2 210-201

54 52 27 51 38 43 52 31 57 43

SC65 SC66 SC67

171.6-161.2 171.6-161.2 171.6-161.2

58 51 54

Automatic modelled age components (Ma ± 2σ) Peak1 Peak2 Peak3 (Evolution-related) (Evolution-related) (Provenance-related) Xuefeng zone 377 ± 16 452 ± 25 Chevron syncline fold-thrust zone 233 ± 14 217 ± 12 383 ± 19 205 ± 11 189 ± 22 (60%) 411 ± 63 (40%) 236 ± 10 Chevron anticline fold-thrust zone 163 ± 19 (45%) 229 ± 24 (55%) 146 ± 16 (30%) 216 ± 20 (70%) 222 ± 9 146 ± 15 (42%) 231 ± 23 (58%) 185 ± 17 (82%) 377 ± 60 (18%) 143 ± 15 (68%) 254 ± 42 (32%) 154 ± 14 (67%) 270 ± 29 (33%) 182 ± 17 (84%) 362 ± 45 (16%) 142 ± 12 (47%) 241 ± 21 (53%) 189 ± 18 (66%) 268 ± 29 (34%) Central Sichuan basin 124 ± 11 (46%) 195 ± 18 (54%) 142 ± 31 (41%) 209 ± 44 (59%) 182 ± 5

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Highlights (1) Zircon fission track analyses of the pre-Cretaceous sandstones are reported. (2) ZFT data constrain the low-temperature evolution or sediment provenance. (3) The

westward

decreasing

trend in

age

represents

a

Mesozoic

intracontinental orogeny. (4) Sediments source from the Qinling orogen due to its multistage evolution.

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Graphical abstract

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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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