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Late Mesozoic development of the southern Qinling-Dabieshan foreland fold-thrust belt, Central China, and its role in continent-continent collision
Shaofeng Liu, Wangpeng Li, Kai Wang, Tao Qian, Chengxin Jiang
PII: S0040-1951(15)00059-1 DOI: doi: 10.1016/j.tecto.2015.01.015 Reference: TECTO 126530
To appear in: Tectonophysics
Received date: 24 July 2014 Revised date: 25 November 2014 Accepted date: 11 January 2015
Please cite this article as: Liu, Shaofeng, Li, Wangpeng, Wang, Kai, Qian, Tao, Jiang, Chengxin, Late Mesozoic development of the southern Qinling-Dabieshan foreland fold- thrust belt, Central China, and its role in continent-continent collision, Tectonophysics (2015), doi: 10.1016/j.tecto.2015.01.015
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Late Mesozoic development of the southern Qinling-Dabieshan foreland fold-thrust belt, Central China, and its role in continent-continent collision
Shaofeng Liu*, Wangpeng Li, Kai Wang, Tao Qian, and Chengxin Jiang
State Key Laboratory of Geological Processes and Mineral Resources and College of Geosciences and Resources, China University of Geosciences (Beijing), Beijing 100083, China
Abstract
The southern Qinling-Dabieshan foreland fold-thrust belt is a key region in which to investigate the continental-continental collision between North China and South
China. We provide structural evidence of a diachronous dextral strike-slip thrusting deformation from the Middle Triassic in the east to the late Late Triassic in the west, which indicates a northwestward oblique subduction and suturing of the South China plate under the North China-Qinling-Dabieshan plate. We also identify a later phase of intracontinentalACCEPTED deformation that included MANUSCRIPT an orthogonal intracontinental collision and south-vergent thrusting during the Early and Middle Jurassic, indentation of
South China into the Qinling-Dabieshan Orogen and arc-shaped extrusions of the southern Qinling-Dabieshan foreland fold-thrust belt from the Late Jurassic to the
Cretaceous. Our results reveal that the long-term intracontinental collision rotated
* Corresponding author at: College of Geosciences and Resources, China University of Geosciences, Beijing 100083, China. Tel./fax: +86 10 82321159. E-mail address: [email protected] (S.F. Liu).
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clockwise in a northwest-southeast-vergent direction in the Late Triassic to northeast-east-southwest-west-vergent in the Late Cretaceous. This collision may have been driven by a north-vergent subduction of the Meso-Tethys Ocean in the south, west-vergent subduction of the Izanagi plate in the east, and south-vergent compression of the Eurasia plate in the north.
Graphical abstract
Highlights
ACCEPTED MANUSCRIPT
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Integration of foreland structure marks orthogonal intracontinental collision.
Intracontinental indentation resulted in structural differentiation and extrusion.
Long-term intracontinental collision rotated clockwise.
Keywords: Southern Qinling-Dabieshan foreland fold-thrust belt; Dextral strike-slip thrusting; Arc-shaped thrust belt; Clockwise rotation; Indentation.
1. Introduction
The Qinling-Dabieshan Orogen was formed by the collision of the South China plate with the North China plate along two north-dipping subduction zones. The
Shangdan suture developed in the north during the Late Paleozoic, and the Mianlue suture formed to its south in the Early Mesozoic. These two sutures are exposed in northern Qinling–Dabieshan along the Shangxian-Danfeng fault (F6) and in southern
Qinling–Dabieshan along the Chengkou-Xiangfan fault (F1) (Zhang et al., 2001) (Fig.
1). The major continental fragments that compose central China, which include (from north to south)ACCEPTED the North China plate, Qinling MANUSCRIPT–Dabieshan micro-plate, and South
China plate, originated from two episodes of rifting during the Late Precambrian and
Devonian. The Shangdan and Mianlue Oceans separated these crustal fragments from north to south. With the closure of the Shangdan oceanic basin, the northern margin of
South China was completely separated from the Qinling–Dabieshan micro-plate to the north by the Mianlue suture (Liu and Zhang, 1999; Zhang et al., 2001; Liu et al.,
2005). The Mianlue suture zone represents the closure of the Mianlue Ocean, which
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separated the previously amalgamated North China-Qinling-Dabieshan plate from the
South China plate in the northeastern branch of the Paleo-Tethys Ocean in Late
Paleozoic time (Zhang et al., 2001; Zhang et al., 2004; Liu et al., 2005). After the ocean closed during the Late Triassic, the southern Qinling-Dabieshan and northern
Yangtze foreland along the suture zone underwent long-term suturing, intracontinental shortening deformation and long-distance subduction of northern
South China under the North China-Qinling-Dabieshan plate. The destruction and burial of the collisional suture zone makes detailed analysis of the deformation mechanism difficult, and the processes of intracontinental deformation along the suture zone must be further unraveled. The southern Qinling-Dabieshan foreland fold-thrust belt (SQDB), which developed south of the Mianlue suture zone, provides an opportunity to reconstruct the details of the collision and reveal its geodynamics. In this paper, we perform a detailed structural deformation analysis of the foreland fold-thrust belt to reconstruct the evolution of the collision between the South China plate and Qinling-Dabieshan micro-plate from the Late Triassic to the Tertiary and reveal its uniqueACCEPTED process of structural deformation. MANUSCRIPT
2. Regional setting
The SQDB extends between the towns of Songpan and Wuxue along the northern margin of the Yangtze block in South China (Fig. 1). The coupled northern
Yangtze foreland basin (NYZB) developed in front of the fold-thrust belt and includes the Sichuan basin, Dangyang basin, and Southeast Hubei basin (Liu et al., 2005). The
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fold-thrust belt and the basin make up a foreland structural system. The
Chengkou-Xiangfan fault (F1) in the fold-thrust belt is an important boundary between the Qinling-Dabieshan micro-plate and Yangtze block (or South China plate)
(Fig. 1) and locally buries older deformational features, including the Mianlue suture and the SQDB, under its thrust nappe (Liu et al., 2005). The Xuefengshan fold-thrust belt is also located along the northern margin of the Yangtze block and is south of the foreland structural system. Both the SQDB and Xuefengshan fold-thrust belt were formed during the collision between South China and North China-Qinling-Dabieshan during the Mesozoic, but they belong to different structural domains. The former was formed during the Qinling-Dabieshan orogeny (Liu et al., 2005), whereas the latter was part of the South China Intracontinental Orogenic belt (Zhang et al., 2013).
The Mianlue suture zone is a composite tectonic zone that formed from the
Qinling-Dabieshan subduction-collision suture and was superimposed by Mesozoic and Cenozoic intracontinental structures (Zhang et al., 2001). Most parts of this suture zone are cut at a depth below the Qinling-Dabieshan micro-plate by the
Chengkou-XiangfanACCEPTED fault thrust (F1) (Zhang MANUSCRIPT et al., 2001), and its development and extension have been documented by Li and Sun (1996), Li et al. (1996), Lai et al.
(1997, 1998), Xu et al. (1998), Dong et al. (1999), and Zhang et al. (2001). The ophiolites that represent the remnants of the paleo-oceanic crust, associated island-arc volcanics and bimodal volcanics crop out in the towns of Mianxian-Lueyang in western Qinling. Structural melanges that consist of ophiolite remnants, island arc volcanic rocks, deepwater and forearc sediments, and basement rocks separated by
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ductile shear belts are locally exposed along the Chengkou-Xiangfan fault in the towns of Suixian and Qichun in southern Dabieshan (F1; Fig. 1). N-MORB and
E-MORB ophiolite and subduction-related volcanics were reported in the Suixian region (Dong et al., 1999). Fragments of the gabbro derived from a depleted asthenosphere, and the andesites formed in an active continental margin setting have been identified from the Qichun area along the Chengkou-Xiangfan fault zone (Lai et al., 2004). These melanges represent evidence of the existence and extension of a
Mianlue oceanic basin between the Qinling-Dabieshan micro-plate and Yangtze block during the Late Paleozoic and Early Triassic, which later disappeared on the southern margin of the Qinling-Dabieshan Orogen (Zhang et al., 2004). Geological and geochemical (including Pb, Sr, and Nd isotopic tracers; Zhang et al., 2001) analyses imply that the oceanic basin was part of the northeastern branch of the Paleo-Tethys during the Late Paleozoic. A passive continental marginal basin developed along the northern Yangtze block (Liu and Zhang, 1999; Zhang et al., 2001). The strata in the
Late Paleozoic Mianlue ocean basin and the Mesozoic foreland basin (Liu and Zhang,
1999) were deformedACCEPTED into the SQDB. MANUSCRIPT
The fold-thrust belt in the Qinling-Dabieshan micro-plate includes the North
Dabashan, Wudangshan, and Dabieshan thrust belts (Fig. 1). The North Dabashan thrust belt is bounded by the northwest-trending Ankang fault (F22) in the north and the southwest-convex Chengkou-Xiangfan fault (F1) in the south. The North
Dabashan and South Dabashan thrust belts formed a typical southwest-convex arc-shaped structural belt at the edges of the Mianlue suture zone between the
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Qinling-Dabieshan micro-plate and Yangtze block. The North Dabashan thrust belt mostly consists of Meso-Neoproterozoic metamorphic rocks and Sinian and Early
Paleozoic strata (Zhang et al., 2001). Studies of sedimentation, the geochemical characteristics and isotope chronology of the volcanic rocks, and the thrust deformation show that North Dabashan underwent three stages of tectonic evolution beginning in the Proterozoic: Cambrian-Middle Devonian extension, Middle Triassic inversion, and Middle Triassic-Early Jurassic thrusting (He et al., 1999). The
Wudangshan nappe is located east of North Dabashan and was thrust southward onto the Lower Paleozoic strata of North Dabashan and SQDB along the Ankang fault (F22) and Chengkou-Xiangfan fault (F1) (Fig. 3C; Zhang et al., 2001; Shi et al., 2012); this represents the top-to-the-south ductile deformation associated with the subduction of the Yangtze block under the Qinling-Dabieshan micro-plate. The antiformal shape of the Wudangshan dome, which is shown by widespread bedding-parallel foliation, might have developed after the south-vergent thrusting and is associated with the exhumation of the high-pressure metamorphic rocks (Zhang et al., 2001). The
Dabieshan thrustACCEPTED belt, which is located atMANUSCRIPT the eastern end of the Qinling-Dabieshan
Orogen, underwent the final stages of continental collision and subsequent shortening.
These final stages continued from the Late Paleozoic through the Early Cretaceous, produced the high-pressure and ultrahigh-pressure metamorphic rocks, and caused their rapid exhumation (Xu et al., 1992; Liu and Zhang, 2013). Geochronologic and metamorphic studies suggest that the ultrahigh-pressure metamorphic rocks formed during the subduction of South China under North China at 220–240 Ma (e.g., Liou et
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al., 1997; Li et al., 1993). Subsequently, the ultrahigh-pressure metamorphic rocks were exhumed in two phases: at 226–219 Ma and 180–170 Ma (e.g., Li et al., 1999;
Li et al., 2001; Liu et al., 2001). In addition, the Dabieshan core underwent another rapid doming episode during the Early Cretaceous (130–110 Ma) (Li et al., 2002).
The Xuefengshan fold-thrust belt is located south of the SQDB (Fig. 1). The belt is arc-shaped and protrudes to the northwest. From the southeast to the northwest, the
Xuefengshan fold-thrust belt is divided into a southern basement uplift sub-belt, a northern compound fold sub-belt and a frontal thin-skinned fold sub-belt (Liu et al.,
2005). The basement uplift is bounded by the Jiangnan thrust fault (F8), and the
Proterozoic basement and Lower Paleozoic stratigraphy were folded and thrusted northward. The folds in the compound fold sub-belt mainly affect Paleozoic and
Lower Mesozoic rocks. The frontal thin-skinned fold sub-belt, which is bounded by the Qiyueshan (F9) and Huayingshan thrust faults (F10), is characterized by tight anticlines and broad synclines and mainly affects Late Paleozoic and Early Mesozoic strata that overlie subsurface detachment faults (Liu et al., 2010). The Xuefengshan fold-thrust beltACCEPTED is deformed and pinches outMANUSCRIPT to the east because of the southward thrusting of the SQDB (Fig. 1).
3. Structural zonation and styles
The SQDB is divided into the northern sub-belt, southern sub-belt, and frontal sub-belt by the Chengkou-Xiangfan fault (F1), Zhenba-Jingshan fault (F3), and
Dahe-Qiaoting fault (18) in the Micangshan region; the Tiexi-Wuxi fault (F4) in the
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South Dabashan region; and the Tongchenghe fault (F11) in the southern Dabieshan region (Fig. 1). The SQDB is linked to the fold-thrust belt in the Qinling by the
Chengkou-Xiangfan fault (F1) in the north and transferred to the Xuefengshan fold-thrust belt along an inferred structural line from Yichang to Daye. The northern sub-belt extends continuously along the southern margin of the Qinling-Dabieshan
Orogen, but the southern and frontal sub-belts are separated by the
Hannan-Micangshan and Huangling basement culminations (Fig. 1) with a clear structural difference along the strike. The southern and frontal sub-belts in the southern Dabieshan and the South Dabashan regions, together with the structures that are overprinted on the northern sub-belt and the Qinling-Dabieshan, form the southern
Dabieshan arc-shaped structure (Dai et al., 2000) and the Dabashan arc-shaped structure (Zhang et al., 2001), respectively.
3.1. The northern sub-belt
The northern sub-belt defines an arc between the towns of Songpan and
Chengkou to ACCEPTEDDahongshan and is bounded MANUSCRIPT by the Chengkou-Xiangfan fault (F1) in the north and Beichuan-Yinxiu fault (F20) and Zhenba-Jingshan fault (F3) in the south
(Fig. 1). This sub-belt is overthrusted by the south-vergent North Dabashan and
Dabieshan thrust nappes in the Qinling-Dabieshan micro-plate, narrows in the
Dabashan, and is covered in southern Dabieshan (Liu et al., 2003; 2005; Li et al.,
2010).
In the northern Longmenshan and Hannan regions, the northern sub-belt is
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characterized by imbricate, south- or southeast-vergent thrusting of Meso- and
Neoproterozoic and Early Paleozoic strata (Fig. 2). From the northern Longmenshan to the Hannan regions, the structural trend changes from the northeast to northeast-east. The Bikou basement uplift, which is manifested as south- or south-southeast-vergent folds and thrusts, is bounded by the northeast-trending
Qinchuan fault (F5) to the south and east-west-trending Chengkou-Xiangfan fault (F1) to the north and forms a wedge-shaped structural zone that narrows to the east and widens to the west. Therefore, the fold and thrust structures in the Hannan, northern
Longmenshan, and Songpan regions were all parts of the northern sub-belt of the
SQDB and form a ―wedge-shaped‖ structural zone.
The northern sub-belt continuously extends across the Hannan region to South
Dabashan and Dahongshan. The thrust belt consists of imbricated thrust sheets of
Neoproterozoic, Early Paleozoic, and a few Permian to Lower Triassic strata. The main thrust faults, including the Chengkou-Xiangfan fault (F1), Pingba fault (F2), and
Zhenba-Jingshan fault (F3), are typically detached through the base of the
NeoproterozoicACCEPTED (Sinian) metamorphic conglomerate-sandstoneMANUSCRIPT successions, and the thrust sheets have been tightly folded and faulted (Fig. 3A, B, and C). Several northeast- or southwest-dipping thrust fault zones exposed near the town of Chengkou in South Dabashan define two flower structures (Figs. 3A and 4A and B). Based on seismic-reflection data (Dong et al., 2013), each flower structure merges downward into single south-vergent, north-dipping thrust faults, the Pingba fault and Zihuang fault, which form crustal-scale thrust wedges. These deformation zones are
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characterized by ca. 3-km-wide penetrative-distributed asymmetric plunging folds involving Upper Sinian-Lower Paleozoic limestone, and foliations of rotated porphyroclasts in the Neoproterozoic phyllite, and bounded by strike-slipping thrust faults. The structural elements within the deformation zones are consistent with that of a transpressional deformation with a dextral strike-slip component (Fig. 4A and B; Liu et al., submitted; Li et al., 2014).
This transpressional structure was also demonstrated by N-S-trending high-angle dextral strike-slip faults and sheath folds in the Gaochuan terrane within the western
Chengkou-Xiangfan fault zone to the west of the town of Ziyang (Hu et al., 2012). At the eastern end of the Chengkou-Xiangfan fault zone in southern Dabieshan, the early stage of deformation is dominated by southeast-vergent recumbent folds and thrusts in the Middle Triassic blueschist units (Li et al., 2010), which formed under the same kinematic process as the dextral strike-slip thrust. The south-southeast- or southeast-vergent thrust in the westernmost part of the northern sub-belt in the
Hannan and northern Longmenshan was the extension of the dextral strike-slip thrust structure to theACCEPTED west, and both suggest MANUSCRIPT northwest-southeast-directed compression in present-day coordinates.
3.2. The southern Dabieshan structural belt
The southern Dabieshan arc-shaped structural belt is located between the towns of Yichang and Daye (Fig. 1). This belt pinches out to the west along the Huangling dome. To the east, the thrust belt is buried by thrusted Dabieshan basement near the
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city of Wuxue (Liu et al., 2003). To the south, as the southern Dabieshan arc-shaped belt thrust southward and the Xuefengshan fold-thrust belt expanded northwestward, the two belts overlapped one another (Liu et al., 2005; Li et al., 2010).
The southern Dabieshan belt is characterized by folds and thrusts that involved
Silurian-Triassic strata in its northern part and Late Triassic-Middle Jurassic foreland basin deposits in its southern part. The field investigations and seismic interpretation revealed that the southern Dabieshan belt includes three imbricated, south-vergent thrust faults—the Tongchenghe fault (F11), Jinmen fault (F12), and Hanshui fault
(F13) (Fig. 3D)—which were deformed into a south-southwest-convex arc. These thrust faults are mostly unconformably covered by Late Cretaceous to Tertiary deposits of the rift basin. This rifting was formed by subsequent reversed normal faulting of the thrust faults at its basement (Fig. 3D). The north-dipping normal faults controlled the deposits in the half-grabens. In front of the southern sub-belt, the
Tongchenghe thrust fault (F11) cut through the north-vergent Gong’an (F16) and
Jianli thrust faults (F17) in the Xuefengshan fold-thrust belt across section S-7 (Fig.
3D), which suggestsACCEPTED that the frontal faults MANUSCRIPT of the Xuefengshan fold-thrust belt were active early. However, opposite structural sequences have also been reported near the towns of Daye and Wuxue (Liu et al., 2003). The north-vergent Xuefengshan fold-thrust belt initiated at approximately the same time as, and continued later than, the southern Dabieshan belt. Based on the deep seismic profile across the Dabieshan, the Chengkou-Xiangfan fault (F1) can be traced from the surface to the Moho as a north-dipping zone. The Dabieshan metamorphic terrane thrust southward and
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covered the entire SQDB. The SQDB and the Xuefengshan fold-thrust belt converged and were thrust under the Dabieshan thrust front (Li et al., 2010; Dong et al., 2004).
These data show that the area from Jinzhou to Daye was an interfering and converging zone of both thrust belts, and the Dabieshan metamorphic terrane was a south-verging thrust nappe that formed as a result of later-stage thrusting of the
Chengkou-Xiangfan fault. To the west of this zone, the Yichang region is the frontal sub-belt of the SQDB, and it is characterized by gently dipping (less than 10 degrees) monoclinic Paleozoic to Middle Triassic strata that formed due to the obstruction of the Huangling basement uplift.
3.3. The Micangshan fold-thrust belt and its western extension in the Longmenshan
The nearly east-west-trending Micangshan fold-thrust belt in the southern and frontal sub-belts of the SQDB was cut by the Zhenba-Jingshan fault (F3) and
Tiexi-Wuxi fault (F4) in the east and gradually transitioned to form the foreland and frontal sub-belts in the Longmenshan in the west (Fig. 1). The Micangshan belt is characterizedACCEPTED by the frontal thrust of the MANUSCRIPT Dahe-Qiaoting fault (F18), which involves
Meso-Proterozoic to Paleozoic strata in the southern sub-belt and a passive roof duplex structure in the frontal sub-belt. The Paleozoic-Early Triassic strata in the southern sub-belt were deformed as broad and gentle folds. However, in the frontal region, the Sinian and Early Paleozoic strata were involved in fault-propagation folds, and the lower strata were uplifted (Fig. 2C). The passive roof duplex structure in the frontal sub-belt includes a north-vergent passive roof thrust fault within the
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Lower-Middle Triassic strata and south-vergent imbricate link thrusts that involve
Proterozoic basement rocks (Fig. 2D). The duplex thrust caused folding of the overlying Mesozoic to Cretaceous strata above the roof thrust fault. In contrast to the
Micangshan belt, the northern Longmenshan developed by southeast-vergent reverse folds and thrusts that involved Paleozoic through Late Triassic strata in the southern
(foreland) sub-belt (Fig. 2A). These northeast- or north-northeast-trending structures, which extended northeastward, were superimposed on the Hannan and Micangshan belts and cut off the Zhenba-Jingshan fault (F3) and Proterozoic basement rocks with sinistral strike-slip motions (Fig. 1). In the northern part of Guangyuan-Yanzibian section (S-1’; Fig. 2B), additional north-northeast-trending folds and sinistral strike-slip faults were superimposed on the northeast-east-trending folds and thrust faults. These north-northeast-trending structures did not cut through the frontal sub-belt in the Micangshan, which may suggest that these structures formed earlier than the passive duplex structures in the frontal sub-belt. In the frontal sub-belt of the northern Longmenshan, the passive roof duplex is developed in only the Guangyuan region, whichACCEPTED is the western extension of MANUSCRIPT the structures in the Micangshan (Fig. 2A).
3.4. The Dabashan structural belt
The thrust belts in South Dabashan were deformed to form a west-southwest-protruding arc-shaped structure. When this arc-shaped structure formed and how it was superimposed on early-stage structures are the key questions to understanding the evolution of South Dabashan.
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The Chengkou-Xiangfan fault (F1) in the northern sub-belt is presently an arc-shaped fault zone. The northwest-trending thrust faults in North Dabashan were cut and terminated by the Chengkou-Xiangfan fault in the limbs of the Dabashan arc.
The folds and thrusts within the northern sub-belt, which extend continuously both eastward and westward to the Dahongshan and Hannan, respectively, were obliquely overthrusted by the Chengkou-Xiangfan fault. This evidence suggests that the arc-shaped thrust of the Chengkou-Xiangfan fault represents the later stage of deformation. However, the southern and frontal sub-belts in the outer arc structure bend and are parallel to the major thrust faults of the Chengkou-Xiangfan fault (F1) and Zhenba-Jingshan fault (F3). The principal compressive strain axes were radially distributed in the outer belts (Zhang et al., 2010). Therefore, the arc-shaped structure in the Dabashan formed by the superimposed deformation of the earlier northern sub-belt.
In addition to the arc-shaped reactivated thrust of the Chengkou-Xiangfan fault in the northern sub-belt, a sinistral strike-slip zone is exposed along the eastern limb of the DabashanACCEPTED arc. This zone intersects MANUSCRIPT with the Chengkou-Xiangfan fault at its western end and is parallel to the Chengkou-Xiangfan fault at its eastern end (Fig. 4).
At the town of Zongbao, two types of plunging folds with different dip directions of the hinges were recognized in the Neoproterozoic (Sinian) dolomites: those with northeast-northeast-east-plunging hinges and those with north-northwest-plunging hinges (Fig. 4C). However, some hinges of the former group were folded and superposed by the latter group. The two groups represent dextral strike-slip and
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sinistral strike-slip features, respectively, and the sinistral strike-slip features formed after the dextral features. In the town of Fengxi, a ca. 100-m-wide typical sinistral strike-slip zone is well exposed in the Cambrian limestone, siliceous, and mudstone rocks and is characterized by penetrative-distributed, asymmetrically plunging folds
(Fig. 4D). The fold hinges generally dip to the northwest, and the shear fracture planes dip to the northeast. This structural deformation zone extends eastward to south of
Gangou (Fig. 4E), Yerengou (Fig. 4F), and south of the Dahongshan (Fig. 4G), where it is ca. 2-3 km wide in Silurian siltstone and sandstone, Cambrian limestone, and
Ordovician limestone, respectively. The kinematic indicators of the S-C fabrics (Fig.
4G), rotated porphyroclasts (Fig. 5C), and asymmetrically plunging folds (Fig. 5D, E, and F) demonstrate the sinistral strike-slip thrust motion along the zone to the south of the Chengkou-Xiangfan fault. The structural relationship between this sinistral strike-slip belt and the arc-shaped Chengkou-Xiangfan fault suggests that the strike-slip motion was related to the southwest-westward extrusion and thrusting of
North Dabashan.
In contrastACCEPTED to the structures of the MANUSCRIPT northern sub-belt, the southern and frontal sub-belts in the Dabashan arc-shaped structure are characterized by duplex and imbricate thrust structures that involved the Paleozoic to Mesozoic sedimentary strata
(Fig. 3A, B, and C; Fig. 6). Because of unroofing difference in depth, different types of structures are exposed from west to east. In the eastern part of the arc-shaped structure to the west of the Huangling dome, the Proterozoic (including Sinian) basement rocks are nearly horizontal, and the Cambrian-Ordovician strata are tightly
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folded above their basal detachment fault (Fig. 3C). The middle part to the south of the town of Chengkou developed as a result of lineation folds and south-vergent imbricate thrust faults that involved Paleozoic to Early Triassic strata. Seismic interpretation data indicate that the thrust faults link to the detachment thrust (Fig. 3B).
The western part retains the complete duplex structure (Fig. 3A). Seismic interpretation (Fig. 6) and field work results show two layers of structures that are divided by floor and roof thrusts that detached through the top Sinian and upper
Lower Triassic strata, respectively. The upper structural layer, which is exposed, developed by chevron anticlines and box synclines that involved the Middle Triassic to Jurassic strata. The lower layer is characterized by south-vergent imbricate thrusts that involved the Paleozoic strata, and the thrust faults link the floor thrust to the roof thrust of the duplex. Therefore, a complete duplex structure at different depths is exposed in the southern and frontal sub-belts from east to west.
4. Deformational sequences and ages
The SQDBACCEPTED underwent multiple MANUSCRIPT stages of structural deformation and superimposition. Four deformational events—D1, D2, D3, and D4—are identified based on 40Ar-39Ar dating, unconformities, structural superimposition relationships, and the relationships of stratigraphy to structural deformation, which represent the syn- and post-collision evolution in the SQDB.
4.1. D1 deformation
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Regional structural analyses show that the syn-collisional structures of the D1 deformation are mostly located along the northern sub-belt (Table 1). This deformation is marked by thrusting and thrusting with a dextral strike-slip component
(Figs. 4 and 5). At the western part of the northern sub-belt, the Early Jurassic conglomerate that is tilted and folded unconformably overlies the deformed Silurian and Permian to Triassic strata in the northern Longmenshan (Fig. 2A). In the town of
Mianxian in the northern Hannan region, a wedge-top basin unconformably lies above the south-vergent thrust belt (Fig. 2C). Detrital zircon U-Pb dating of sandstone samples from the base of the stratigraphic sequence (Supplementary Information; Fig.
S1) redefined the entire stratigraphic sequence in the basin to extend from the Norian
Stage of the Late Triassic to the Early and Middle Jurassic. The 40Ar/39Ar plateau ages of the biotites and sericites separated from the syn-tectonically formed mica-quartz schist with well-developed foliation and lineation in the Mianlue suture belt and
Qingchuan fault are 201.93±0.71 Ma and 207.8±3.22 Ma, respectively
(Supplementary Information; Fig. S2A and B; Wang et al., 2011a). These results suggest that ACCEPTED the syn-collisional deformation MANUSCRIPT in the Hannan belt began at the latest during the early Late Triassic and that the thrust deformation propagated southeastwards from the hinterland back to the Songpan region (Weislogel et al., 2010) to the foreland in the northern Longmenshan from the middle Late Triassic to the earliest Jurassic (Table 1).
The 40Ar/39Ar preferred ages of sericite found in the Neoproterozoic phyllite at the town of Dazhou, near Chengkou, deformed by the Pingba thrust belt (F2) (Fig. 4B)
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are 245.38±3.26 Ma and 191.13±2.59 Ma (Fig. S2C). The two preferred ages may represent the initial syn-collisional strike-slip thrust deformation age and later post-collisional overprinting, respectively. The former syn-collisional deformation age is also strongly supported by the evidence that the Late Triassic conglomerate and sandstone unconformably overlies the deformed vertical Early Triassic limestone
(Figs. 4B and 7A; Table 1). The high-pressure and ultrahigh-pressure metamorphism in the Dabieshan, which is constrained at 244-236 Ma and ~230-220 Ma, respectively
(Ames et al., 1993; Hacker et al., 1998; Li et al., 2000; Liu et al., 2004), and the age of the early, top-to-SE, upward extrusion of the high-pressure and ultrahigh-pressure metamorphic rocks, which ranges from 241 to 231 Ma (Li et al., 2010; 2011; see the
Ar-Ar age in Fig. 1), indicate Middle Triassic ages for the continental collision in the eastern part of the study area (Table 1; Liu et al., 2003). Thus, we interpret these data to indicate that the combination of thrusting and dextral strike-slip faulting migrated westward from the Middle Triassic to early Late Triassic from southern Dabieshan in the east to northern Hannan in the west.
A sedimentologicalACCEPTED analysis indicates MANUSCRIPT that the Jurassic strata that were deposited in front of the Zhenba-Jingshan fault unconformably overlie the Middle Triassic,
Silurian, and different levels of the underlying Upper Triassic strata in the
Micangshan, South Dabashan, and southern Dabieshan regions (Figs. 7C, 8A, and 9).
This pattern suggests that tectonic tilting and folding occurred at the thrust front before the Early Jurassic. All of this evidence suggests a westward time-transgressive deformation from the Middle Triassic in the east to the latest Triassic in the west (D1),
19 ACCEPTED MANUSCRIPT
which is interpreted to represent syn-collisional deformation.
4.2. D2 deformation
The D2 deformation is characterized by southeast- or south-vergent propagation, folding and thrusting in the northern sub-belt and a regional unconformity between the Late Jurassic through Early Cretaceous stratigraphy and the underlying
Early-Middle Jurassic strata along the northern margin of the Sichuan Basin (Fig. 6).
The folds and thrusts involved with the locally distributed Late Triassic strata (Fig. 7B) and 40Ar/39Ar weighted mean plateau age of 191.13±2.59 Ma (Fig. S2C) along the
Pingba thrust fault zone (F2) indicate that the northern sub-belt continuously underwent Early-Middle Jurassic structural overprinting and deformation that migrated to the south.
Paleogeographic reconstructions indicate that an Early-Middle Jurassic foreland basin formed in front of the northern sub-belt of the SQDB (Liu et al., 2005). These basin deposits unconformably overlie the northern Longmenshan in front of the
Beichuan-YingxiuACCEPTED fault (F20) and the MANUSCRIPT Micangshan, South Dabashan, and southern
Dabieshan regions in front of the Zhenba-Jingshan fault (F3) (Fig. 8A; Fig. 9). The local Early-Middle Jurassic deposits within the northern sub-belt near the towns of
Mianxian and Xixiang, for example, were deposited in the wedge-top depozone (Fig.
9). The reconstructed early Jurassic gravel alluvial fans located in the northern
Longmenshan-Micangshan, South Dabashan, and Xiangfan were deposited in front of the northern sub-belt. These mega-fan deposits were a response to the initiation of the
20 ACCEPTED MANUSCRIPT
D2 deformation in the SQDB. The present-day distribution was caused by overthrusts of the SQDB or erosion in later stages, and the alluvial fans were cut off by the South
Dabashan arc-shaped thrust and the Longmenshan thrust. This evidence suggests that the D2 thrust deformation in the northern sub-belt mostly occurred during the
Early-Middle Jurassic.
4.2. D3 deformation
The D3 deformation was characterized by southeast- or south-vergent propagation, folding and thrusting in the southern sub-belt in the northern
Longmenshan, Micangshan, and southern Dabieshan. The folded and faulted
Early-Middle Jurassic strata in the northern Longmenshan, Micangshan, and southern
Dabieshan belts and rapid pre-Late Cretaceous cooling/denudation in the Micangshan fold-thrust belt (Tian et al., 2012) suggest the influence of thrust deformation on the deposition in the Late Jurassic-Early Cretaceous foredeep of the northwestern Sichuan basin (Liu et al., 2005) and the Early Cretaceous foredeep in front of the southern
Dabieshan arc-shapedACCEPTED thrust belt (Fig. MANUSCRIPT 8B) (Liu et al., 2010). This evidence demonstrates that the southern sub-belt was mainly deformed during the Late Jurassic and Early Cretaceous.
Field structural analysis shows that the northwestern part of the southern
Dabieshan belt extends and is superimposed across the northern sub-belt (Fig. 10). In the Baokang region, northwest-trending folds are superimposed on or transform the nearly east-west-trending folds in the northern sub-belt, and the small
21 ACCEPTED MANUSCRIPT
north-northeast-trending folds are also superimposed on the northwest-trending folds.
South of the Zhenba-Jingshan fault (F3), younger east-west-trending folds and overturned folds are superimposed on the northwest- and northeast-trending folds.
Therefore, four stages of deformation can be identified here. The first stage structures are the nearly east-west-trending folds and thrusts that might have formed during the
Late Triassic to Middle Jurassic in the northern sub-belt (D1-2 deformation), and the second stage involved the superimposition of the northwest-trending folds, which represent the component of the southern sub-belt in the southern Dabieshan arc-shaped structure and formed during the Late Jurassic and Early Cretaceous (D3 deformation). The third stage of deformation might have been related to the uplift of the Huangling dome during the latest Early Cretaceous (Shen et al., 2012), and the fourth stage of deformation was induced by southward expansion of South Dabashan in the southern sub-belt. This superimposition relationship suggests that the development of the southern Dabieshan fold-thrust belt was a separate structural event after the development of the northern sub-belt, and its propagation direction changed to southwestwardACCEPTED to the east of the Huangling MANUSCRIPT basement dome during the Late Jurassic and Early Cretaceous.
4.2.3. D4 deformation
The D4 deformation is marked by the formation of the Dabashan arc-shaped structure, which includes newly formed duplex structures within the southern and frontal sub-belts, the southwest-westward extrusion and re-thrusting of North
22 ACCEPTED MANUSCRIPT
Dabashan along the Chengkou-Xiangfan fault, and the sinistral strike-slip thrusting on the eastern limb of the arc structure. The folds and thrusts in the southern sub-belt were superimposed on and modified the eastern Micangshan thrust belt in the west
(Fig. 1) and the northwestern part of the southern Dabieshan thrust belt in the east
(Fig. 10), which suggests that these structures formed after both structural belts during the latest Early Cretaceous and Late Cretaceous. Apatite fission-track dating of four
Late Triassic-Jurassic sandstones from the southern sub-belt (Xu et al., 2010; Li et al.,
2010) and one Sinian siltstone from the northern sub-belt (Shi et al., 2012) in South
Dabashan (locations shown in Fig. 1) indicated two cooling phases of ca. 150-105 Ma
(or 132-81 Ma) and 95-45 Ma (or 81-69 Ma) and ca. 130-90 Ma and < ca. 90 Ma during the Cretaceous, respectively. These cooling phases might be related to the Late
Jurassic-Early Cretaceous and latest Early Cretaceous-Late Cretaceous thrust uplift of the Dabashan arc-shaped belt. The Late Cretaceous cooling event (ca. 100-80 Ma) is also clearly recorded by apatite fission-track dating data in the Ankang thrust belt
(F22) in North Dabashan (locations shown in Fig. 1). In the eastern part of this thrust fault (F22), ACCEPTED the observation that the MANUSCRIPT Upper Cretaceous conglomerate-bearing sandstones unconformably overlie the thrusts and folds in the Cambrian-Silurian strata (Shi et al., 2012) strongly indicates that the thrust events occurred before the
Late Cretaceous. Therefore, the Dabashan arc-shaped belt underwent multiple stages of post-collisional deformation during the Late Jurassic to Cretaceous. The southern and frontal sub-belts formed primarily by post-collisional deformation, and the arc-shaped front migrated by thrusting during the Late Cretaceous (until the Early
23 ACCEPTED MANUSCRIPT
Tertiary). Although the syn-collisional structure was best preserved in the northern sub-belt and North Dabashan, these structural belts were overprinted by post-collisional, west-southwest-vergent thrusting during the formation of the arc-shaped structures.
5. Discussion
The formation and development of the SQDB records the initial subduction and collision of the continental crust along the Mianlue suture zone (Liu et al., 2005).
Geologic and paleomagnetic data indicate that the North China and South China plates may have been separated by an ocean basin that was wide in the west and narrow in the east. This basin represented the Mianlue Ocean, which was centered in the Songpan region and extended east to the Qinling-Dabieshan (Zhao and Coe, 1987;
Liu et al., 2005). This ocean was part of the Paleo-Tethys (Metcalfe, 2006). The closure of the Mianlue Ocean marks the final collision of North China and South
China. The post-collisional deformation along the SQDB demonstrate the intracontinentalACCEPTED response to far-field plate MANUSCRIPT subduction and differentiate compression of continental blocks. Based on the structural evolution of the SQDB, we can analyze the tectonic processes of the collisional structural belt.
5.1. Oblique subduction of the northern Yangtze under the Qinling-Dabieshan Orogen
The collision between North China-Qinling-Dabieshan and South China caused by the closure of the Mianlue Ocean and its relationship to the formation of the
24 ACCEPTED MANUSCRIPT
Dabieshan high-pressure and ultrahigh-pressure metamorphic rocks has been a subject of debate. Three primary models have been proposed for this relationship: rotational collision (Zhao and Coe, 1987; Gilder et al., 1999; Wang et al., 2003), indentational collision (Yin and Nie, 1993), and a remnant ocean basin (Zhou and Graham, 1996;
Weislogel et al., 2010). Based on our observations, we agree most with the latter model, in which oblique closure of the Paleo-Tethys Ocean with a northwest-directed continental collision began along the eastern Qinling-Dabieshan Orogen (Liu et al., submitted; Fig. 11A). As a result, the early and long-lived subduction of the South
China continent beneath Dabieshan provided the impetus for the development and later extrusion of the ultrahigh-pressure metamorphic rocks in that area but not elsewhere. An oblique collision of South China with North China resulted in right-lateral oblique thrusting in the suture belt, and following closure, it continued shortening in the Qinling-Dabieshan Orogen. In this model, continental subduction began first and continued for the longest period of time beneath the Dabieshan. As a result, continental underplating was greatest in the area in which the ultrahigh-pressureACCEPTED metamorphic rocks formedMANUSCRIPT during the Early-Middle Triassic and were then extruded during the Late Triassic (Ames et al., 1993; Hacker et al, 1998; Li et al., 2010). The long-lived, continuous continental subduction allowed these ultrahigh-pressure metamorphic rocks to form and subsequently to be returned to the surface in the Dabieshan and not farther west along the Qinling-Dabieshan Orogen.
The collisional strike-slip thrusting and the coupled foreland basin migrated from southern Dabieshan in the Middle Triassic to the northern Hannan during the Late
25 ACCEPTED MANUSCRIPT
Triassic, and the Mianlue Ocean finally closed during the middle Late Triassic (Liu et al., submitted; Fig. 11A). Because of the northwest-southeast compression, a wedge-shaped thrust belt formed in the Songpan region.
5.2. Orthogonal intracontinental collision, structural differentiation and extrusion due to rotation and indentation
During the Early-Middle Jurassic, the integration of the northern sub-belt and its frontal foreland basin system marks the complete amalgamation of South China and
North China-Qinling-Dabieshan (Liu et al., 2005; Fig. 11B). We interpret that the continental collision continued across all of the northern Yangtze as South China changed its direction of compression and subduction relative to North China (Liu et al., 2005) to nearly north-south through the clockwise rotation of South China.
After the Late Jurassic, the SQDB propagated to the south in different ways along the northern Yangtze. During the Late Jurassic and Early Cretaceous, the SQDB thrust southwestward in the southern Dabieshan and Songpan regions, and the
Dabieshan high-pressure/ultrahigh-pressureACCEPTED MANUSCRIPT terrane and Bikou block were offset by their frontal fold-thrust belts (Fig. 11C). The Dabieshan extruded southeastward
(Wang et al., 2011b), whereas the Bikou block extended southwestward. This structural deformation may have been induced by the clockwise rotation of South
China, continuous shortening, and intracontinental indentation of South China into the
Qinling-Dabieshan belt along the Huangling and Hannan-Micangshan domes (Wang et al., 2003). Under these tectonic conditions, the Qinling-Dabieshan Orogen
26 ACCEPTED MANUSCRIPT
continued to shorten from north to south, and the SQDB expanded south-southwestward in the southern Dabieshan and Songpan regions to form two arc-shaped structures. At nearly the same time, the Xuefengshan fold-thrust belt propagated northwestward and converged obliquely with the southwestward-expanding southern Dabieshan thrust belt in the Yichang and Daye regions (Liu et al., 2005). Controlled by this converged thrusting, the terrestrial foreland basin depocentre migrated westwards to the west of the Huangling dome during the Late Jurassic and Early Cretaceous (Fig. 11C).
Because of the continuous strong nearly north-south-vergent shortening, clockwise rotation of South China, and indentation at the Hannan-Micangshan and
Huangling domes after the Early Cretaceous, the crustal materials within the
Qinling-Dabieshan Orogen extruded west-southwestward to form a new extrusion structure—the Dabashan arc-shaped structure—between the Huangling and
Hannan-Micangshan domes (Fig. 11D). This arc-shaped belt was superimposed on and modified the earlier foreland fold-thrust belt, and the Chengkou-Xiangfan fault
(F1) cut off theACCEPTED North Dabashan thrust MANUSCRIPT belt with sinistral strike-slip motion on the eastern limb of the arc belt. The Songpan arc-shaped belt continued to expand southwestward and controlled the Late Cretaceous-Early Tertiary local foredeep deposits in the southwestern Sichuan basin (Liu et al., 2010). At the same time, the
Xuefengshan belt continuously expanded northwestward and converged with the
Dabashan arc front in the eastern Sichuan basin. Under these tectonic conditions, the duplex structure in front of the Dabashan arc and its frontal basin were uplifted and
27 ACCEPTED MANUSCRIPT
exhumed from east to west. To the east of the Dabashan arc, the Dabieshan and its southern foreland region began to collapse and extend (Liu et al., 2003).
5.3. Geodynamics of continental collision during the Late Mesozoic
The long-term shortening during the post-collision period along the
Qinling-Dabieshan Orogen was inferred to have been induced by the northeast- or north-northeast-vergent subduction of the Meso-Tethys in the south, west- or northwest-vergent subduction of the Izanagi plate in the east, and south- or southeast-vergent compression from the Siberia plate in the north (Ratschbacher et al.,
2003; Seton et al., 2012). Under this tectonic setting, the intracontinental collision rotated clockwise from northwest-southeast-vergent in the Late Triassic to northeast-east-southwest-west-vergent in the Late Cretaceous. With the increasing influence of the roll-back subduction of the Izanagi plate, the eastern part of the
SQDB began to extend and collapse during the Late Cretaceous. However, the western part of the SQDB continued to shorten and extrude to the west-southwest (the
Dabashan) duringACCEPTED the Late Cretaceous andMANUSCRIPT into the Tertiary, and it was driven by the northward subduction of Tethys and rapid southwest migration of Eurasia.
The rotational continental collision model introduced by Bottrill et al. (2014) explained the lateral variation and asynchronous onset of the collision and its influence on the burial and exhumation of subducted continental crust and high-pressure/ultrahigh-pressure rocks in the Norwegian Caledonides. The model of
Bottrill et al. might be an analogue of the rotational collision of the
28 ACCEPTED MANUSCRIPT
Qinling-Dabieshan Orogen, although their model was developed from the syn-collision to the post-collision setting. After complete closure of the Mianlue
Ocean at the Late Triassic, the SQDB underwent a similar asynchronous onset of post-collisional thrust starting in the east and propagating westward. Along with the westward migration of the shortening deformation centre, structural doming and extension with rapid unroofing of the high-pressure and ultrahigh-pressure metamorphic rocks occurred at the eastern end of the Dabieshan core in the Late
Jurassic and Early Cretaceous. Subsequently, rifts were formed across the entire
Dabieshan region in the Late Cretaceous. This structural differentiation along the
SQDB may have been caused by a rotation of the subducting plate around the rotational pole.
Conclusions
(1) The SQDB is divided into northern, southern, and frontal sub-belts in which the
Micangshan, Dabashan, and southern Dabieshan fold-thrust belts are identified
from westACCEPTED to east. Four deformational MANUSCRIPT events—D1, D2, D3, and D4—are
identified and represent the syn- and post-collision evolution of the SQDB.
(2) The northern sub-belt was characterized by a diachronous, dextral strike-slip
thrusting deformation from the Middle Triassic in the east to the late Late Triassic
in the west, which indicates an oblique collision between the North
China-Qinling-Dabieshan and South China plates. This oblique motion resulted in
the development and subsequent extrusion of ultrahigh-pressure metamorphic
29 ACCEPTED MANUSCRIPT
rocks in Dabieshan.
(3) During the Early-Middle Jurassic, the integration of the northern sub-belt and its
frontal foreland basin system marks the complete amalgamation of South China
and North China-Qinling-Dabieshan. After this complete collision, the SQDB
underwent tectonic differentiation along its strike because of the rotation and
intracontinental indentation of South China into the Qinling-Dabieshan Orogen at
the Huangling and Hannan-Micangshan domes. The southern Dabieshan and
Songpan arc-shaped thrust belts extruded south-southwestward during the Late
Jurassic and Early Cretaceous, and the Dabashan arc-shaped thrust belt expanded
west-southwestward during the Late Cretaceous.
(4) The long-term shortening during the post-collision period along the
Qinling-Dabieshan Orogen was induced by the northeast- or
north-northeast-vergent subduction of the Meso-Tethys in the south, westward
subduction of the Izanagi plate in the east, and Eurasian southward compression in
the north. The structural differentiation and collisional vergent variation along the
SQDB duringACCEPTED the Late Mesozoic mayMANUSCRIPT have been caused by a rotation of the
subducting plate around the rotational pole.
Acknowledgements
We gratefully acknowledge the informative discussions with Prof. Guowei
Zhang of Northwest University, China. We gratefully acknowledge the critical and
30 ACCEPTED MANUSCRIPT
constructive comments and suggestions made by Hans Thybo, Roy H. Gabrielsen, and another anonymous reviewer, who provided useful comments that significantly improved the clarity of the scientific material in this paper. This study was supported by the Chinese Natural Science Foundation Grants (41030318, 91114203),
Specialized Research Fund for the Doctoral Program of Higher Education (No.
20130022110002), and National Basic Research Program of China (2011CB808901).
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Figure captions
Fig. 1. Structural map of the southern Qinling-Dabieshan Orogen and northern
Yangtze. The black lines indicate the locations of structural cross sections, and the black dots indicate the sites from which samples were collected for 40Ar/39Ar dating.
NC: North China plate; SC: South China plate; QD: Qinling-Dabieshan micro-plate;
YZ: Yangtze block; CA: Cathaysia block; F1: Chengkou-Xiangfan fault (which buried the Mianlue ACCEPTED suture); F2: Pingba fault; MANUSCRIPT F3: Zhenba-Jingshan thrust fault; F4:
Tiexi-Wuxi thrust fault; F5: Qingchuan thrust fault; F6: Shangxian-Danfeng fault
(Shangdan suture); F7: Tanlu fault; F8: Jiangnan thrust fault; F9: Qiyueshan thrust fault; F10: Huayingshan thrust fault; F11: Tongchenghe fault; F12: Jinmen fault; F13:
Hanshui fault; F14: Wulongquan fault; F15: Yangxin fault; F16: Gong’an fault; F17:
Jianli fault; F18: Dahe-Qiaoting fault; F19: Guangxian-Anxian fault; F20:
Beichuan-Yingxiu fault; F21: Wenxian-Maoxian fault; F22: Ankang fault. Apatite
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fission-track (AFT) data are cited from (1) Li et al. (2010), (2) Shi et al. (2012), and (3)
Xu et al. (2010); Ar-Ar data are cited from (4) Li et al. (2011).
Fig. 2. Structural cross sections of the northern Longmenshan and Micangshan fold-thrust belts. A. Baolun-Yingpan section (S-1). The southern part of the section is based on seismic interpretations. B. Guangyuan-Yanzibian section (S-1’). C.
Wangcang-Ningqiang section (S-2). The inset Mianxian section (S-2’) shows the wedge-top basin along the Mianlue suture zone and sample sites for 40Ar/39Ar and
U-Pb dating. D. Seismic interpretation section of the northern Sichuan basin (S-3).
The ages of the units are as follows: Pt, Proterozoic; Pt2-3, Middle to Late Proterozoic;
Pt3, Late Proterozoic; Pt-Z, Proterozoic to Sinian; Z, Sinian (equivalent to Vendian);
Є, Cambrian; Є-O, Cambrian to Ordovician; O, Ordovician; S, Silurian; S-C, Silurian to Carboniferous; D, Devonian; C, Carboniferous; P, Permian; T, Triassic; T1, Lower
Triassic; T1-2, Lower Triassic to Middle Triassic; T2, Middle Triassic; T3, Upper
Triassic; J1, Lower Jurassic; J1-2, Lower and Middle Jurassic; J1b-J2q, Lower
Jurassic BaitianbaACCEPTED Formation to Middle MANUSCRIPT Jurassic Qianfoya Formation; J2s, Middle
Jurassic Shaximiao Formation; J3sn, Upper Jurassic Suining Formation; J3p, Upper
Jurassic Penglaizhen Formation; and K1, Lower Cretaceous. The stereographic projections (lower hemisphere) show the attitudes of the fold hinges and axial planes.
Dotted lines represent angular unconformities, and dashed lines represent disconformities. Standing people in the photos are provided for scale. The other symbols are the same as in Fig. 1. The locations of the structural cross sections are
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shown in Fig. 1.
Fig. 3. Structural cross sections across the Dabashan and southern Dabieshan regions.
A. Wanyuan-Ziyang section (S-4). The interpretation of the deep structures was based on seismic data. B. Wuxi-Zhuxi section (S-5). C. Zigui section (S-6). D. Seismic interpretation section in southern Dabieshan (S-7). The ages of the units are as follows:
Pt2, Middle Proterozoic; D-P, Devonian to Permian; D-T2, Devonian to Middle
Triassic; T3-J2, Upper Triassic to Middle Jurassic; J3, Upper Jurassic; K2-E, Upper
Cretaceous to Tertiary; and N-Q, Neogene to Quaternary. The other symbols are the same as in Figs. 1 and 2. The locations of the structural cross sections are shown in
Fig. 1.
Fig. 4. Structural cross sections and stereographic projections of the plunging folds, rotated clastic porphyroclasts, and S-C fabrics showing deformational features of the strike-slip thrusts. A, Tianba section; B, Dazhou section, in which the site of 40Ar/39Ar sample S1103077ACCEPTED is shown (modified MANUSCRIPT from Liu et al. (submitted)); C, Zongbao section; D, Fengxi section; E, Gangou section; F, Yerengou section; and G, Zhangji section. The black circles in the stereograms represent the attitudes of the planes of shear zones or thrust faults. The insert map shows the locations of the sections along the northern sub-belt of the SQDB. Other lithology symbols are the same as in Figs.
2C and 3A.
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Fig. 5. Photos of plunging folds (A, B, D, E, and F) and rotated clastic porphyroclasts
(C) in shear zones showing deformational features of strike-slip thrusting. The standing people in D and F and pencil in E are provided for scale. The locations of the photos are shown in Fig. 4.
Fig. 6. Seismic profile and its interpretation of the Dabashan arc-shaped structure
(part of S-4). The location is shown in Fig. 1.
Table 1. Evidence for the age of deformation event 1 (D1) in the northern sub-belt of the southern Qinling-Dabieshan foreland fold-thrust belt (SQDB). The hatched pattern represents hiatuses; the red line represents age intervals of ultrahigh-pressure metamorphism (UHP). Fm, Formation; and Mbr, Member. The other symbols are the same as in Figs. 1 and 2.
Fig. 7. Unconformities exposed along the Pingba fault zone in the western town of
Chengkou (A,ACCEPTED B) and the town of Susong MANUSCRIPT in southern Dabieshan (C). The other symbols are the same as in Fig. 2.
Fig. 8. Tectonic paleogeographic maps of the northern Yangtze region. A, Early
Jurassic stage; B, Early Cretaceous stage. The black line shows the location of the stratigraphic cross section. The other symbols are the same as in Fig. 1.
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Fig. 9. Regional Upper Triassic-lowest Jurassic stratigraphy across section S-A in the
Sichuan basin and thrust belt based on measured sections. M, mudstone; S, sandstone;
G, gravel; Fm, Formation; and Mbr, Member. The ages of 216-229 Ma shown in the
Mianxian section indicate the minimum U-Pb detrital zircon ages from samples
S1103110 and S1103107 (Fig. S1) and suggest that the sandstone was deposited in the
Middle Norian Stage of the Late Triassic. The location of section S-A is shown in Fig.
8A.
Fig. 10. Structural map of superposed folds and their stereographic projections (lower hemisphere) in the Nanzhang region. The black circles, red five-pointed stars, and red circles in the stereographic projections represent the fold limbs, fold hinges, and planes perpendicular to the fold hinges, respectively. T1d, Lower Triassic Daye
Formation; and T1j, Lower Triassic Jialingjiang Formation. The location of this map is shown in Fig. 1. The other symbols are the same as in Figs. 1 and 2.
Fig. 11. PaleotectonicACCEPTED reconstructions showing MANUSCRIPT the mechanism for time-transgressive dextral transpressional deformation of the Mianlue suture zone, structural differentiation, and extrusion along the southern Qinling-Dabieshan foreland fold-thrust belt (SQDB) from the Middle Triassic to Cretaceous. MC-HND,
Micangshan-Hannan dome; HLD, Huangling dome; and XFB, Xuefengshan fold-thrust belt. The other symbols are the same as in Fig. 1.
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Table 1 ACCEPTED MANUSCRIPT
Figure 1
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Figure 2
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Figure 3
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ACCEPTED MANUSCRIPT Figure 4
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ACCEPTED MANUSCRIPT Figure 5
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Figure 6 ACCEPTED MANUSCRIPT
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Figure 7
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Figure 8 ACCEPTED MANUSCRIPT
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Figure 9
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Figure 10
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Figure 11
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