Mesozoic intraplate deformation of the central North China Craton: Mechanism and tectonic setting

Journal Pre-proofs Mesozoic intraplate deformation of the central North China Craton: Mechanism and tectonic setting Jin Zhang, Junfeng Qu, Beihang Zhang, Heng Zhao, Pengfei Niu, Shuo Zhao, Jie Hui, Long Yun, Fengjun Nie, Yannan Wang PII: DOI: Reference:

S1367-9120(20)30050-X https://doi.org/10.1016/j.jseaes.2020.104269 JAES 104269

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Journal of Asian Earth Sciences

Received Date: Revised Date: Accepted Date:

9 May 2019 4 February 2020 4 February 2020

Please cite this article as: Zhang, J., Qu, J., Zhang, B., Zhao, H., Niu, P., Zhao, S., Hui, J., Yun, L., Nie, F., Wang, Y., Mesozoic intraplate deformation of the central North China Craton: Mechanism and tectonic setting, Journal of Asian Earth Sciences (2020), doi: https://doi.org/10.1016/j.jseaes.2020.104269

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Mesozoic intraplate deformation of the central North China Craton: Mechanism and tectonic setting

Jin Zhang1, Junfeng Qu1, Beihang Zhang1, Heng Zhao1, Pengfei Niu1, Shuo Zhao1, Jie Hui2, Long Yun3, Fengjun Nie4, Yannan Wang5

1. Key Laboratory of Deep-Earth Dynamics of Ministry of Natural Resources, Institute of Geology, Chinese Academy of Geological Sciences, Beijing, 100037, China 2. University of Chinese Academy of Sciences, Beijing, 100049, China 3. Beijing Research Institute of Uranium Geology, Beijing, 100029, China 4. East China University of Technology, Nanchang, Jiangxi, 330013, China 5. Hebei University of Engineering; School of Earth Science and Engineering, Handan, 056038, Hebei Province, China

Abstract The North China Craton (NCC) is fundamentally characterized by Mesozoic intraplate deformation controlled by preexisting basement fabrics and far-field stress fields. In this study, a structural analysis of typical outcrops and representative sections was conducted to investigate the Mesozoic deformation in the central NCC. Our analysis indicates that this Mesozoic deformation is a typical example of thick-skinned tectonics and that the central NCC experienced two stages of deformation during this period. The first stage was driven by nearly east-west maximum compressional stress during the Late Jurassic, which led to the development of north-south-trending structures in the central NCC, such as the Lüliangshan range and Qinshui Basin. The

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second stage involved NNW-SSE maximum compressional stress during the Late Cretaceous, which resulted in the development of east-west-trending structures and the activation of large sinistral strike-slip faults, such as the Huoshan-Fushan fault. The key mechanism for the deformation during the Late Jurassic was the low-angle subduction of the Paleo-Pacific Plate beneath the NCC, which also laid the foundation for the later destruction of the craton during the Cretaceous. Moreover, a Late Cretaceous collision between the southeastern Eurasian continent and an unknown block or oceanic plateau may have resulted in the subsequent deformation of the NCC. Keywords: North China Craton, basement-involved structure, intraplate deformation, Mesozoic, far-field effect

1. Introduction During the Mesozoic, the North China Craton (NCC) experienced strong tectonic events, including the notable Yanshanian Movement, which has been studied for almost 100 years (Wong, 1927; Zhao, 1990; Zheng et al., 1998; Davis et al., 1998, 2002, 2001; Meng et al., 2003; Zhao et al., 2004a, 2004b; Wang et al., 2011; Lin et al., 2013; Zhu, et al., 2015; Dong et al., 2015 and references therein; Clinkscales and Kapp, 2019). Recently, the Mesozoic tectonics of the NCC have become a major research topic with regard to its destruction (Zhang et al., 2011; Zhang, 2012; Zhu et al., 2011, 2017 and references therein; Cao et al., 2015, 2018; Wu et al., 2019), during which marked intraplate deformation occurred within the craton, especially in its central and eastern parts and marginal regions, such as the Yinshan-Yanshan and eastern Liaoning regions (Chen, 1998; Zheng et al., 1998; Davis et al.,

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1998, 2002; Zhang et al., 2004, 2012; Liu et al., 2005, 2007; Meng et al., 2014; Li et al., 2016; Qiu et al., 2019; Kong et al., 2019), the Ordos Basin (Zhao et al., 2019) and the western Ordos region (Darby and Ritts, 2002; Zhang et al., 2008, 2014; Faure et al., 2012). However, few systematic studies have been conducted on the Mesozoic deformation in the central NCC (i.e., entire Shanxi, western Hebei, southern Inner Mongolia and northern Henan provinces), although the outcrops are excellent (Wang, 1925; Liao et al., 2007; Zhang et al., 2008; Zhang, 2012). Most of the central NCC is located in Shanxi Province, which we consider to be representative of the central NCC; hence, the fieldwork constituting this study was performed in Shanxi Province (Figs. 1 and 2). During the Mesozoic, a series of NNE- and ENE-trending basement-involved folds and thrust faults developed in the Paleoproterozoic Trans-North China Orogenic Belt (TNCOB) in the Shanxi region. These basement-involved folds are typical thick-skinned structures. However, the geometry, kinematics, mechanism, timing of these structures and the tectonic settings in which they formed remain unclear; furthermore, why and how the NCC was deformed are still debated (Xu et al., 1998; Sun et al., 2004; Yang et al., 2006; Liao et al., 2007; Zhang et al., 2008; Zhang, 2012; Wang et al., 2010; Faure et al., 2012; Kusky et al., 2014; Clinkscales and Kapp, 2019). Numerous events during the late Mesozoic influenced the evolution of the NCC, namely, the closure of the Mongol-Okhotsk Ocean to the north of the NCC (Zorin, 1999), tectonic events in the Tethys regime to the southwest (Şengör and Natal’in, 1996), and the westward subduction of the Paleo-Pacific Plate (Maruyama et al., 1997; Wessel and Kroenke, 2008; Faure et al., 2012; Müller et al., 2016; Zhu et al.,

3

2017; Wu et al., 2019; Li et al., 2019). However, the impacts of different tectonic regimes on the NCC remain unresolved. In this study, structural mapping and analysis were performed to acquire detailed structural data from key outcrops and along representative sections of Mesozoic structures (Fig. 1). In addition, we carefully extracted all existing structural data from the region to construct a new set of maps and sections for the central NCC. Deep structural profiles constrained by surface geological features and geophysical data are described, and the geometry, kinematics, mechanism, timing and tectonic setting of the basement-involved folds in the central NCC are discussed based on these data. 2. Geological background 2.1 Regional Geological Background The central NCC (the region between the Taihangshan range to the east and the Lüliangshan range to the west) is the main component of the north-south-trending TNCOB, which is approximately 1500 km long and 100-300 km wide and formed in response to the collision between the Eastern and Western Blocks (inset in Fig. 1) (Zhao et al., 2005). Many north-south or northeast-trending high-grade Precambrian metamorphic rocks crop out in this orogenic belt. However, the timing of this orogeny remains controversial; for example, the Paleoproterozoic (Zhao et al., 2005) and Neoarchean (Zhai et al., 2000; Kusky, 2011) have both been proposed. The basement of the central NCC consists of several Paleoproterozoic units that formed during orogenesis, including the Wutai Complex, Hengshan Complex, Fuping Complex, Jiehekou Group, Lüliang Group, Taiyueshan Group, and Huoshan Group, most of which are regional metamorphic rocks, 4

with some reaching the granulite facies. The main rock types are tonalite– trondhjemite–granodiorite (TTG), amphibolite gneiss, magnetite quartzite, and marble (Geng et al., 2000; Zhao et al., 2005). The NCC formed after the Paleoproterozoic (1.8 Ga), and sedimentary strata representing a stable setting were later deposited on top of it (Zhao and Carwood, 2012). The Meso- to Neoproterozoic strata within the NCC consist of neritic clastic deposits in the lower part with a gradual change to limestone and dolomite in the upper part. No late Neoproterozoic deposits have been found in this region. Thick-bedded Cambrian-Ordovician sandstone and limestone unconformably cover older sedimentary strata and gneisses of different ages (BGMRSP, 1989). During the Middle Ordovician and early Carboniferous, the region was uplifted, possibly by the southward subduction of the Paleo-Asian Ocean beneath the NCC (Zhao, 1990). During the Carboniferous-Permian, coal-bearing clastic strata of marine-terrigenous facies were deposited unconformably atop early Paleozoic strata in the central NCC (BGMRSP, 1989). During the Mesozoic, thick fluvial and lacustrine sediments were deposited in the NCC (Wang, 1925; Zhao, 1990; Meng et al., 2014; Wei et al., 2017). In the Ordos Basin to the west, the Triassic and early Jurassic strata are mainly red thick-bedded sandstone and mudstone. Thick coalbeds were deposited during the Middle Jurassic, and Upper Jurassic and Lower Cretaceous strata, including fluvial and eolian deposits, were deposited in arid environments. Few Upper Cretaceous deposits have been found in the central and western NCC (BGMRSP, 1989). Tectonically, the central NCC also experienced marked intraplate deformation events during the Mesozoic; the

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main

tectonic

framework

of

the

central

NCC,

including

nearly

north-south-trending faults and folds, which are now expressed as mountain ranges (Figs. 1 and 2), developed during this period (Wang, 1925; Zhang et al., 2006, 2007, 2011; Liao et al., 2007; Cao et al., 2015, 2018; Clinkscales and Kapp, 2019; Li et al., 2019). Subsequently, a series of northeast-trending rift basins (the Fenwei Graben) developed in the central NCC during the late Cenozoic, possibly in response to the far-field effects of the Indo-Eurasian collision (RGAFSOMSSB, 1988; Zhang et al., 1998). 2.2 Major Tectonic Units The main structures in the central NCC related to Mesozoic deformation strike nearly north-south or nearly east-west. The main north-south-trending structures include (from west to east) the Lishi fault, Lüliangshan anticline, eastern Lüliangshan fault, Huoshan-Fushan fault, Qinshui Basin and Taihangshan fault (Figs. 1 and 2). The main east-west-trending structures include the Hengshan range, Wutaishan range and Xizhoushan range in northeastern Shanxi and the southern Lüliangshan and Zhongtiaoshan ranges in southern Shanxi (Figs. 1 and 2). These major tectonic units are described from west to east and from north to south in the following sections. 2.2.1 Nearly North-South-Trending Structures (1) Lishi Fault The north-south-striking Lishi fault is located to the west of the Lüliangshan range (Fig. 1). Many studies have suggested that the Lishi fault extends northwards to the east of the Yellow River (for example, Zhao et al., 1990) and can be divided into northern and southern segments. The northern segment consists of four NNE-trending thrust faults to the north of Lishi city, 6

each of which dips to the northwest at 60-80° and cuts Precambrian metamorphic rocks. The southern segment extends into the Fenwei Graben at the southern end of the Lüliangshan range (BGMRSP, 1989) and consists of several north-south-trending high-angle (70-85°) east-dipping thrust faults. The hanging wall of these thrust faults contains the Lüliangshan anticline, while the footwall consists of a north-south-trending syncline comprising vertical Cambrian, Ordovician, Carboniferous, Permian and Triassic sedimentary rocks. Traditionally, the Lishi fault is considered to be approximately 270 km long and 1-10 km wide (BGMRSP, 1989). Recent studies have argued that the Kouquan fault to the west of the Cenozoic Datong subbasin represents the northern segment of the Lishi fault and that the entire Lishi fault is therefore approximately 600-700 km long (Zhao and Zhang, 2011) (Figs. 1 and 2). (2) Lüliangshan Anticline The Lüliangshan anticline is located in western Shanxi, and trends north-south in the south and NNE-SSW in the north. The anticline is approximately 400 km long and situated at 1500-2830 m above sea level (Fig. 1). The basement of the Lüliangshan anticline consists of the Paleoproterozoic Yejishan Group, Lüliang Group, and Jiehekou Group with Neoarchean to Paleoproterozoic granitic gneiss in the core (Zhao et al., 2008; Liu et al., 2011). The overlying strata consist of Mesoproterozoic Hangaoshan Group conglomerates, Cambrian-Ordovician limestone, and Carboniferous-Permian clastic rocks and limestone (Fig. 1). The Carboniferous and Permian strata are mainly distributed in the center of the southern Lüliangshan region, while the Cambrian-Ordovician limestone crops out mainly in the two limbs of the anticline (Fig. 1).

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(3) Eastern Lüliangshan Faults The eastern Lüliangshan faults consist of (from north to south) the Kouquan fault, Ximafang-Chunjingwa fault, Huyanshan fault and Luoyunshan fault (Figs. 1 and 2). The Kouquan fault is an almost vertical top-to-the-west thrust fault (Fig. 1) that thrusts Paleoproterozoic granitic gneiss over Middle Jurassic sandstone. The Ximafang-Chunjingwa fault constitutes the western boundary of the Jingle-Ningwu syncline (Figs. 1 and 2). In the southern segment,

this

fault

thrusts

Paleoproterozoic

granite

eastwards

over

Cambrian-Ordovician limestone, whereas in the northern segment, this fault becomes a top-to-the-west, thin-skinned thrust in the cover layers of the syncline. The Huyanshan fault is also a top-to-the-east thrust that thrusts Paleoproterozoic garnet-bearing granitic gneiss over Cambrian-Ordovician limestone. The Luoyunshan thrust fault represents the eastern boundary of the southern Lüliangshan anticline (Fig. 1) and represents a typical thin-skinned thrust fault, which is different from the other thrust faults in the eastern Lüliangshan region. Some fault-related folds are present in the hanging wall of the top-to-the-west Luoyunshan fault (Liu et al., 1985), which is cut by Cenozoic normal faults to the east. (4) Huoshan-Fushan Fault The Huoshan-Fushan fault consists of the Huoshan fault to the north and the Fushan fault to the south. The north-south-trending Huoshan fault, which is approximately 120 km long, signifies the western boundary of the Huoshan range. Since the Mesozoic, multiple phases of activity have occurred along the Huoshan fault, with sinistral transpression resulting in a throw exceeding 1000 m (BGMRSP, 1989) but an unknown amount of horizontal displacement.

8

During the late Cenozoic, normal faults developed parallel to the preexisting thrust faults, with thick sediments being deposited in the Fenwei Graben to the west since the Pliocene; in addition, the Huoshan range in the footwall to the east underwent intense uplift and tilting. In contrast, the NNE-trending Fushan fault, which is more than 60 km long, controls the western boundary of the Qinshui Basin and cuts Permian and Triassic clastic rocks (Fig. 1). This fault formed during the late Mesozoic as a thrust fault but was reactivated as a normal fault that cut Pleistocene red siltstone during the late Cenozoic (Xu et al., 1993). (5) Huoshan Basement-Involved Anticline The Huoshan anticline is a domal structure with a north-south axis that is slightly longer than its east-west axis (Fig. 1). Only the eastern half of the Huoshan anticline is preserved because of cutting by later sinistral faults and Cenozoic normal faults (Fig. 1). The core of the Huoshan anticline consists of Paleoproterozoic granitic gneiss (i.e., the Taiyueshan Group). The cover sedimentary rocks are Cambrian-Ordovician sandstone and limestone, Carboniferous-Permian coal-bearing sandstone and limestone and Triassic terrestrial clastic rocks. (6) Qinshui Basin The Qinshui Basin is a large NNE-trending Mesozoic syncline (Figs. 1 and 2). The Huoshan is located to the west of this basin, and the Taihangshan is located to the east, while the northern margin of the basin is cut by Cenozoic normal faults associated with the Fenwei Graben. Paleozoic strata dip into the basin and crop out along the basin margins, and the core of the basin is covered by red Triassic strata. Although the Qinshui Basin is a syncline, its

9

elevation is similar to that of the Taihangshan range to the east. (7) Taihangshan Fault The NNE-striking Taihangshan fault, which is approximately 350 km long and 1-8 km wide (Cao and Guan, 1997), forms the boundary between the Taihangshan range to the east and the Qinshui Basin to the west (Figs. 1 and 2).

The

fault

cuts

Cambrian-Ordovician

Precambrian limestone

and

gneiss

and

sandstone;

sedimentary

rocks;

Carboniferous-Permian

limestone, bauxite, sandstone and shale; and Triassic-Jurassic sandstone and mudstone (BGMRSP, 1989). 2.2.2 Nearly East-West-Trending Structures (1) Hengshan and Wutai Ranges The Hengshan range (Figs. 1 and 2) consists mainly of Paleoproterozoic high-pressure granulite and gneiss (Wei et al., 2014), and patches of thick-bedded Cambrian limestone crop out in the northern part of the range, while the Wutai range consists of Paleoproterozoic granite and greenstone. The Paleoproterozoic Zhujiafang ductile shear zone is located between the Hengshan range to the north and the Wutai range to the south (Faure et al., 2007). (2) Xizhoushan Range The

Xizhoushan

Cambrian-Ordovician

range

(Figs.

thick-bedded

1

and

2)

limestone

consists and

mainly

features

of a

top-to-the-southeast thrust along the northern foothills (Clinkscales and Kapp, 2019). (3) Southern Lüliangshan Range The east-west-trending Xiweikou thrust fault developed in the southern

10

Lüliangshan range (Figs. 1 and 2) and thrusts Paleoproterozoic granitic gneiss northwards over Cambrian-Permian limestone. The displacement of the fault, which formed during the Late Jurassic, is more than 100 m (BGMRSP, 1989). (4) Zhongtiaoshan Range The ENE-striking Zhongtiaoshan range is located in southern Shanxi (Figs. 1 and 2) and consists primarily of the Neoarchean Sushui Complex to the north and Meso- to Neoproterozoic volcanic-sedimentary rocks and early Paleozoic clastics and limestone to the south. A large vertical and east-west-trending fault in the Zhongtiaoshan range thrusts the Sushui Complex over Cambrian sedimentary rocks. To the north of the Zhongtiaoshan range, the Yuncheng subbasin of the Cenozoic Fenwei Graben contains Cenozoic sediments up to 3-5 km thick (RGAFSOMSSB, 1988). Active east-west-trending normal faults are present between the range and the basin. 3. Field methods A geological survey was carried out between 2016 and 2018. We constructed geological profiles at different scales across the study region and analyzed a variety of previous studies, geological maps, satellite images, and digital elevation data (Fig. 2). The orientations of fault planes, slickenlines and bedding/foliations/lineations were recorded across the entire study area; if possible, multiple measurements of the orientations of slickenlines and faults were collected. Sense-of-slip fault indicators, such as Riedel shears, S-C fabrics, mineral fibers and asymmetric folds, were also recorded. The fault data were used to calculate fault plane solutions where slickenlines were recorded. The FaultKin (5.2) and Stereonet 8 applications created by R. W. Allmendinger of Cornell University were used to calculate the 11

fault plane solutions and to visualize and analyze the structural data, respectively. Many fault structures in the study area have inherited existing planes (faults, foliations, joints, etc.), but local factors, such as postfaulting rotation, multiphase deformation, and spatial strain heterogeneity, cannot be ruled out; consequently, some results are inconsistent . However, because of the clear crosscutting relationships, we can distinguish two deformation events. Our results generally show event similarities across the entire region, although at any given spot, measurements of the stress field may not be identical. This discrepancy may be due to local factors and/or inherited characteristics. We refer to Marrett and Allmendinger (1990) for a discussion and critical evaluation of this method. 4. Structural characteristics 4.1 Nearly North-South-Trending Structures 4.1.1 Southern Lishi Fault The southern Lishi fault strikes north-south, dips to the east at 70-80°, and has a fault zone 30-300 m wide. The hanging wall of the southern Lishi fault is the main component of the Lüliangshan range. Tectonic lenses, fault gouges, fault breccias and local cleavages are observed within the fault zone. Paleoproterozoic migmatite was thrust westwards over Ordovician limestone in Huangtu, Xi County (Fig. 3), and the Ordovician limestone formed an overturned north-south-striking syncline (Fig. 3B). The field observations revealed that the thrust fault consists of a series of small, densely arranged thrust faults that are rooted in the gneiss within the basement (Fig. 3A). Early Paleozoic sandstone and limestone cover the gneiss with an angular unconformity. The marker layer (the red early Cambrian Huoshan sandstone) 12

in the footwall and hanging wall indicates that the displacement along the thrust fault was limited, creating a typical basement-involved fold in the hanging wall (Fig. 3A). The field observations also showed that a series of densely distributed and steeply dipping joints developed in the migmatitic gneiss (the Lüliang Group) in the basement (Fig. 3C), that many slickenlines formed on the joints in the hinges of the folds, and that the faults are parallel to the

joints

(Figs.

3D

and

3E).

The

development

of

the

typical

basement-involved fold displayed in Fig. 2 may have been related to the reactivation of joints due to nearly east-west compression (Fig. 3E). In Jinjiachuan (west of Linfen city), the Lishi fault is a wide fault zone (approximately 300 m wide) with thick fault breccias (Fig. 3F). Tectonic lenses in the fault breccias are all composed of Ordovician limestone. The interior structures, such as small duplexes along the fault, indicate that early deformation involved top-to-the-west thrusting (Fig. 3G) and that the hanging wall is a fault-propagation fold composed of Ordovician limestone (this fold may represent the upper portion of a basement-involved fold) (Fig. 3F). The fault was cut by later sinistral strike-slip faults (Fig. 3F). Early thrusting driven by nearly east-west compression thrust the Ordovician limestone westwards over the Carboniferous limestone (Fig. 3F). Moreover, Carboniferous clastic rocks (the Bengxi Formation) and limestone (the Taiyuan Formation) directly cover the Ordovician limestone with a disconformity in the NCC, suggesting that the displacement along the Lishi fault in this region was very limited. 4.1.2 Northern Lishi Fault The northern Lishi fault consists of four secondary NNE-trending thrust faults. The main fault planes commonly developed along surfaces between

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different but parallel strata (Fig. 1). In addition to the Lüliang Group, Cambrian-Ordovician limestone and Paleoproterozoic supracrustal rocks (such as the Yejishan Group) are cut by this fault. The Northern Lishi Fault dips to the west at 60-80°, which is different from the southern Lishi fault, and meter-scale outcrops indicate top-to-the-east thrusting (Fig. 4). Thick fault breccias (Fig. 4B) with vertical fabrics developed in the fault zone, and back-thrust faults resulted in the formation of large box folds in the overlying strata. Generally, the northern Lishi fault is parallel to the foliation of the gneiss in the basement, and the hanging wall of the Lishi fault contains a series of basement-involved folds. 4.1.3 Lüliangshan Basement-Involved Fold The Lüliangshan range is a large anticline (Figs. 1 and 2); because the metamorphic rocks in the Lüliangshan range have been involved in folding, this anticline is a basement-involved anticline. The above descriptions show that the southern Lishi fault formed from preexisting joints in migmatitic gneiss and that the sedimentary cover was folded into a west-verging anticline (Fig. 3). In the central Lüliangshan range, the Paleoproterozoic granitic gneiss is widely distributed. The granitic gneiss has experienced considerable deformation, as observed in the Gongyang area, and a series of high-angle thrust faults developed in the basement (Fig. 5A). Because of these dense faults, the granitic gneiss is intensely fractured. These faults mainly strike nearly north-south (Fig. 5B) and are parallel to the fold hinge and foliations but dip in the direction opposite to that of the western boundary fault of the southern Lüliangshan range (i.e., the Lishi fault). In the central area, top-to-the-east thrust faults developed in the basement, and many S-C fabrics

14

developed in the main fault zone (Figs. 5C and 5D). Fault plane solutions show NW-SE

compression

(Fig.

5E)

and

indicate

that

the

overlying

Cambrian-Ordovician limestone experienced only folding (Fig. 5A). Faults that cut the gneiss in the basement (the Lüliang Group) crop out in the Zhike area in the central Lüliangshan range and cut Cambrian limestone and red sandstone, which unconformably overlie the gneiss (Fig. 5F). All these faults take advantage of preexisting foliations in the basement gneiss (Figs. 5F and 5G); the formation of the Lüliangshan anticline was therefore related to the reactivation of foliations in the basement gneiss. The fault plane solution in the Zhike area indicates nearly east-west compression (Fig. 5H). Near Lishi city, the Lüliangshan is an anticline that verges to the east (Fig. 5I). The thrust fault in the eastern limb, which is parallel to the foliation of the gneiss in the core (Fig. 5I), connects with the Zhike fault to the south (Fig. 5F). In summary, the development of the Lüliangshan basement-involved anticline and its related structures was controlled by basement structures. 4.1.4 Ximafang-Chunjingwa Fault The Ximafang-Chunjingwa fault constitutes the western boundary of the Jingle-Ningwu syncline (Figs. 1 and 2). The southern segment consists of the west-dipping Ximafang fault, and the northern segment comprises the east-dipping

Chunjingwa

fault.

The

Jingle-Ningwu

syncline

is

a

NE-SW-trending asymmetrical syncline that is approximately 160 km long and 20 km wide with a steep or overturned northwestern limb and a shallowly dipping southeastern limb (Fig. 6A). The youngest strata in the syncline compose the Middle Jurassic Datong Formation. The Cambrian-Ordovician limestone and Carboniferous-Triassic clastic rocks in the area are also folded.

15

Additionally, the metamorphic sandstone, conglomerates and quartzite of the Paleoproterozoic Hutuo Group crop out to the west of the southern portion of the syncline (Fig. 6A), while Precambrian granite and lower Paleozoic clastic rocks and limestone crop out to the west of the northern part of the syncline (Fig. 6B). Along the Yushuwan profile in the southern segment (Fig. 6A), the eastern fault is parallel to the bedding of the sandstone in the Hutuo Group; the hanging wall is a syncline composed of the Hutuo Group, while the footwall is a syncline composed of Paleozoic-Mesozoic strata with an overturned northwestern limb (Fig. 6A). Farther northward in the Xinpu area, Paleozoic-Mesozoic strata cover the Hutuo Group with an angular unconformity; no known faults exist between these units (Fig. 6B). In contrast, faults that developed from preexisting joints are present in the Hutuo Group and granite to the west (Figs. 6C and 6D). A set of conjugate joints exists in the Hutuo Group and Paleoproterozoic granite (Fig. 6C); most of these joints strike north-south, including a group of westward-dipping joints that were reactivated as thrust faults (Fig. 6D). These phenomena indicate that preexisting joints in the basement controlled the development of both the fold in the Yushuwan area and the monocline in the Xinpu area. Previous geological mapping farther northwards along the western boundary of the Jingle-Ningwu syncline did not identify the Ximafang fault. However, a thin-skinned thrust fault (i.e., the Chunjingwa fault) was discovered to the east in the Dongzhai area , where the mountain range to the west of the Jingle-Ningwu syncline is known as the Luyashan range (Figs. 1 and 2), which consists predominantly of Paleoproterozoic coarse-grained charnockite

16

(2172-2032 Ma) (Yang and Santosh, 2015). Nearly horizontal early Cambrian thick-bedded Huoshan sandstones crop out at the top of the range and unconformably cover the charnockite. Away from the Jingle-Ningwu syncline to the west, the charnockite is massive, with gneissosity dipping to the west. Only horizontal unloading joints and nonsystematic joints are present in the charnockite. However, farther to the east, monoclines such as that in Fig. 7 formed in the cover layer. A group of thrust faults developed along the gneissosity, and S-C fabrics formed between these parallel thrust faults (Fig. 7A). In addition, systematic west-dipping joints also developed, and certain faults exploited these joints, forming a broad deformation zone (Figs. 7G and 7H). In addition to these basement thrust faults, small-scale thrust faults with their related structures such as ramp-anticlines also develop in the cover layers (Fig. 7B), which are also parallel to most thrust faults in the basement. Near the Jingle-Ningwu syncline, the early Paleozoic strata dip to the east, forming a monocline (Figs. 7C and 7D). Beneath this monocline, the charnockite experienced strong brittle deformation (Fig. 7D). In this location, in addition to two sets of NW-dipping joints in the charnockite, a series of parallel thrust faults developed along the gneissosity (Figs. 7A and 7F). Beyond the monocline, certain faults took advantage of the preexisting gneissosity, but these faults are sporadically distributed. Beneath the monocline, however, faults exploiting the preexisting gneissosity are very common. Hence, the formation of this monocline may have been closely related to these dense thrust faults. In fact, the charnockite is massive to the west with progressively weaker deformation, and the charnockite is intensely deformed only beneath the monocline of early Paleozoic strata (Fig. 7D). Therefore, we conclude that

17

the Luyashan range to the west of the Jingle-Ningwu syncline is a large basement-involved anticline (Fig. 7D) and that the reactivation of the preexisting gneissosity in the basement controlled the formation of this anticline. The Chunjingwa fault in the Dongzhai area formed between the early and late Paleozoic strata in the western limb of the Jingle-Ningwu syncline (Fig. 8). This fault has been proposed to be the western boundary of the Jingle-Ningwu syncline, similar to the Ximafang fault to the south (Wang, 1925; BGMRSP, 1988). The hanging wall of the Chunjingwa fault consists of thick-bedded Middle Ordovician limestone, while the footwall consists of late Carboniferous coal-bearing strata, and no basement rocks were observed along the fault (Fig. 8). The direction of movement of the hanging wall of the Chunjingwa fault is opposite to that of both the thrusting in the basement to the west of Dongzhai (Fig. 7) and the Ximafang fault to the south. Notably, the fault plane solution indicates NW-SE compression, which may be similar to the stress field that created the basement-involved anticline (Fig. 8A). During the formation of a basement-involved anticline, thin-skinned structures with an opposite sense of offset relative to the main basement thrust fault commonly form in the triangular region in front of the forelimb of the fold due to space constraints (Erslev, 2005). Therefore, the Chunjingwa fault is not an extension of the Ximafang fault to the south but rather a subordinate structure associated with the basement-involved anticline controlled by the Ximafang fault. 4.1.5 Kouquan Fault The Kouquan fault represents the western boundary of the Datong subbasin and is an active normal fault that currently dips to the southeast

18

(Figs. 1 and 2). However, this fault was a top-to-the-northwest thrust fault during the Mesozoic. Along the northern section of the fault, the hanging wall consists of Paleoproterozoic granitic gneiss, while the western footwall consists of Cambrian medium-bedded limestone and Middle Jurassic coal-bearing strata (Fig. 9A). Near the thrust fault, the strata in the footwall are vertical or even overturned but rapidly become horizontal to the west (Fig. 9A). Slickenlines on the bedding planes in the footwall that are nearly parallel to the movement direction of the main thrust fault indicate flexural slip folding. In the hanging wall, a series of steep southeast-dipping thrust faults developed in the Paleoproterozoic granitic gneiss, whereas two sets of joints developed in the gneiss farther to the east (Fig. 9F). One set of joints later became thrust faults. The faults in the basement are dense, and the displacements of individual thrust faults may not have been large because the strain was distributed onto these dense thrust faults; thus, the deformation of the basement is not obvious. These deformation phenomena indicate that the Mesozoic deformation in the Datong area involved the basement (Fig. 9A). The southeastern limb of the anticline was already broken and became the basement of the Cenozoic Datong subbasin. Because the foliations of the granitic gneiss dip at low angles, the foliations were not easily reactivated in later tectonic events. In the Datong area, the structures that control the formation of the basement-involved folds are preexisting joints, but the timing of the formation of these joints is unknown. Multistage faulting occurred in the basement near the Kouquan fault in Datong. These densely distributed faults with straight fault planes commonly

19

control the orientations of valleys. Two directions of slickenlines have been found: an earlier direction indicating thrusting and a later direction indicating sinistral oblique thrusting. Fault plane solutions indicate that early thrusting was caused by NW-SE compression, while nearly north-south compression resulted in later oblique sinistral thrusting (Figs. 9C, 9D and 9E). In the southern section of the fault in the Emaokou area to the south of Datong, the main Kouquan thrust fault consists of three secondary thrust faults (Fig. 9G). Among these faults, one thrust dips to the west with overturned Cambrian limestone in the hanging wall and Permian coal-bearing sandstone in the footwall (Fig. 9G). To the east, the hanging wall of the Kouquan thrust fault is cut by a sinistral strike-slip fault of unknown age (Fig. 9H) and a Cenozoic high-angle normal fault. The hanging wall of the present-day Emaokou thrust is a recumbent fold composed of lower-middle Cambrian limestone (Fig. 9G). This fold contacts nearly horizontal Carboniferous limestone across a fault, which was described as a top-to-the-east thrust by BGMRSP (1989); nevertheless, the shear indicators on the fault plane indicate a top-to-the-west thrust. Two high-angle faults developed to the east and were interpreted to be west-directed thrust faults (BGMRSP, 1989). However, obvious sinistral slickenlines are present on one of these two fault planes, and this sinistral fault also cuts the entire Emaokou thrust (Figs. 9G and 9H); therefore, this fault was active later. The geometry of the Emaokou thrust indicates that it may signify the western limb of a large box fold (Figs. 9G and 9J). A series of thrust faults and a nearly horizontal thrust are boundary faults that constrain the deformation of the anticline (Fig. 9J), similar to the trishear model (Erslev, 1991). Further

20

deformation of the anticline caused the overturning of the strata between these faults and caused the early Paleozoic and late Paleozoic strata to come into contact with each other. The fault plane solution of the later strike-slip fault indicates north-south compression (Fig. 9I). 4.1.6 Huyanshan Fault The Huyanshan fault, which trends north-south or NNE (Fig. 1), extends northwards through Jiaocheng and Loufan and connects with the eastern margin faults of the Jingle-Ningwu syncline (Fig. 1). The fault is cut by the Fenwei Graben to the south, it may connect with the Huoshan fault to the south. In the north, the hanging wall to the west consists of Paleoproterozoic migmatitic gneiss, while the footwall consists of early Paleozoic limestone and late Paleozoic coal-bearing strata that dip to the east, some of which are even overturned. Although no faults have been found between the Cambrian strata and the Precambrian gneiss, many small-scale thrust faults developed in the gneiss within the basement near the boundary (Fig. 10C). The activation of these faults caused the deformation of the gneiss in the basement and resulted in the formation of a monocline in the Cambrian strata. Many dip-slip slickenlines developed on these small-scale thrust faults (Fig. 10B), and their fault plane solution indicates NW-SE compression (Fig. 10A). In the cover layers, a large normal fault formed between the Cambrian-Ordovician limestone in the footwall and the Carboniferous-Permian coal-bearing strata in the hanging wall (Fig. 11), and later sinistral strike-slip movement was superimposed onto previous normal faulting (Fig. 11E). These normal faults are parallel to the bedding of the Middle Ordovician limestone in the footwall (Fig. 11), and the displacement on the normal faults is small, as

21

evidenced by the fact that the Carboniferous-Permian coal-bearing strata in the study region were deposited directly atop the Middle Ordovician limestone. Therefore, we argue that these normal faults may have formed when the cover layers

were

rotated

and

steepened

during

the

formation

of

the

basement-involved anticline. The fault plane solution of the later sinistral strike-slip faulting indicates a component of NW-SE compression (Fig. 11). 4.1.7 Luoyunshan Fault The eastern boundary of the southern Lüliangshan range is the Luoyunshan thrust fault (Fig. 1). This top-to-the-west thrust fault is a typical ramp-flat-style fault in medium-bedded Cambrian limestone; no basement is involved. The hanging wall moved westwards, and décollement folds formed in front of the fault. 4.1.8 Huoshan Basement-Involved Fold The Huoshan anticline is a fold with Precambrian basement as the core. Two sets of perpendicular joints occur in the core of the Huoshan anticline: one set strikes east-west, and the other set strikes north-south (Figs. 12A and 12B). All the joints in both sets are vertical in the core and become shallow in the limbs. In the core of the anticline, slickenlines are present on many joints, indicating that these joints were later reactivated as faults. (1) Nearly East-West-Trending Joints The east-west-trending joints in the core dip to the south or north at high angles (Fig. 12B). The timing and mechanism of the formation of these east-west-trending

joints

are

unknown.

In

addition,

many

nearly

east-west-trending thrust faults with various dip angles developed in the core of the anticline, some of which may have taken advantage of the joints, and

22

many slickenlines developed on the joint planes. The thrust faults are densely distributed, but the displacement on individual faults is limited. Some of these thrust faults cut through the unconformity between the gneiss and early Paleozoic sandstone and limestone (Fig. 12C). In the core of the Huoshan anticline,

these

east-west-trending

joints

or

thrust

faults

cut

the

north-south-trending joints (Fig. 12B), and they are much longer and feature more consistent strike directions than the north-south trending joints. (2) Nearly North-South-Trending Joints These joints dip at different angles (Fig. 12A) and are much shorter than the east-west-trending joints due to being cut by the east-west-trending joints and thrust faults (Fig. 12B). Slickenlines are also present on these north-south-trending joints, indicating later reactivation. The fault plane solution of these reactivated joints indicates NW-SE compression (Fig. 12A). In addition, a series of north-south-trending thrust faults developed in the basement gneiss (Fig. 12E). Many thrust faults originated from previous joints or took advantage of joints (Figs. 12E and 12F); the displacement of these thrust faults was not large and was similar to that of the east-west-trending joints. These basement faults may have resulted in the development of the north-south-trending Huoshan anticline. The basement deformation characteristics of the Huoshan anticline described above indicate that this anticline may be similar to both the Lüliangshan basement-involved anticline (Fig. 5) and the Luyashan basement-involved anticline in Dongzhai (Fig. 7). The dome-like Huoshan anticline may have resulted from the superposition of two perpendicular folds. The east-west-trending anticline was caused by the reactivation of

23

east-west-trending joints and thrust faults, most of which are essentially parallel to the joints; the fault plane solutions of these thrust faults indicate that north-south compression resulted in the development of the east-west-trending anticline (Figs. 12B and 12C). Similar to the reactivation of the previously described east-west-trending joints and thrusting, the reactivation of north-south-trending joints and thrusting in the basement caused the development of the north-south-trending anticline. The north-south-trending joints and thrust faults are cut by the east-west-trending joints and faults (Fig. 12B); therefore, the north-south-trending anticline is older than the east-west-trending anticline. 4.1.9 Huoshan-Fushan Fault The Huoshan fault cuts the Huoshan anticline (Figs. 1 and 2). Early thrusting moved the Paleoproterozoic gneiss westwards over early-middle Cambrian sandstone and limestone. The footwall of the thrust fault, which dips to the east and features thick fault breccias, was deformed into a closed syncline with a vertical to overturned eastern limb. In the central segment near Xingtang Temple, the Huoshan fault is a large, steeply west-dipping strike-slip fault that strikes north-south and exhibits a flower structure in profile (Fig. 13A). The fault is parallel to the gneissosity of the Taiyueshan Group in the foothills (Fig. 13A). The hanging wall consists of nearly horizontal lower-middle Cambrian thick-bedded limestone (Fig. 13A), and thick fault breccias (8-10 m) developed along the fault (Fig. 13B). Nearly horizontal slickenlines (Fig. 13C) with many domino structures and S-C fabrics near the main fault indicate sinistral strike-slip movement (Fig. 13D). Horizontal slickenlines developed on the fault plane, and their fault plane solution

24

indicates NW-SE compression (Fig. 13A). In the southern segment near Guangsheng Temple, the Huoshan fault mainly strikes NE-SW (Fig. 13E), and at least two phases of activity, including sinistral strike-slip faulting and normal faulting, have occurred. The strike-slip fault with thick fault breccias (5-20 m) is almost vertical and exhibits a flower structure in profile (Fig. 13E), and nearly horizontal slickenlines are developed on the main fault plane. The fault developed mainly in lower-middle Cambrian limestone, and the shear indicators indicate sinistral strike-slip movement. A series of high-angle normal faults developed to the west of this strike-slip fault (Fig. 13E). Moreover, to the west of this strike-slip fault, the late Cenozoic Fenwei Graben continues to experience normal faulting (Xu et al., 1993; 2018), and north-south-trending normal faults near Guangsheng Temple are parallel to the active normal faults along the eastern margin of the Linfeng subbasin. Seismicity along these faults is also evidenced by the occurrence of a paleo-earthquake (M=8) near Guangsheng Temple in 1303 (Xu et al., 1993; 2018). Therefore, we suggest that the normal faults near Guangsheng Temple are Cenozoic in age and that sinistral strike-slip faulting may have occurred earlier. Early thrusting and later sinistral strike-slip faulting occurred on the Fushan fault to the south. In its early stage, the Fushan fault was a top-to-the-northwest thrust fault, and a series of NNE-SSW-trending fault-related folds developed in the hanging wall. The presence of conjugate joints in the hinge of an anticline indicates NW-SE compression, which is almost identical to the fault plane solutions for thrusting in the region (see below). All these folds were later cut by a nearly vertical sinistral strike-slip

25

fault. The hanging wall of the fault to the west consists of gray feldspathic quartz sandstone from the late Permian Shangshihezi Formation, and the footwall consists of purple sandstones from the Middle Triassic Ermaying Formation (Fig. 14). Slickenlines developed on the fault planes during both stages, and those on the later fault planes are nearly horizontal (Fig. 14). The fault plane solutions indicate that NW-SE compression caused early thrusting, and nearly north-south compression resulted in later sinistral strike-slip faulting (Fig. 14), similar to the patterns observed in other regions (see below). 4.1.10 Taihangshan Fault The trace of the Taihangshan fault is very clear in both field and satellite images and strikes predominantly NNE-SSW (Figs. 1 and 2). A series of SE-verging fault-propagation folds composed of Paleozoic limestone and sandstone developed along the Taihangshan fault, especially along the southern segment. Many slickenlines developed on the bedding planes because of flexural slip folding. To the east of Changzhi city, the Taihangshan fault is more than 40 m wide and features a series of secondary thrust faults (Fig. 15), some of which exploited an unconformity (corresponding to a paleoweathering crust) between Carboniferous

argillite

and

Ordovician

limestone.

The

SE-directed

Taihangshan thrust fault resulted in SE-verging folds, and both of these structures were cut by a later sinistral strike-slip fault (Fig. 15B). The later sinistral strike-slip fault is vertical, and its fault plane solution indicates NW-SE compression (Fig. 15C). Multistage activation of the Taihangshan fault was also discovered in the Zuoquan area to the north. The earlier event involved top-to-the-southeast thrusting, while the later event involved strike-slip

26

movement. 4.2 Nearly East-West-Trending Structures 4.2.1 The Hengshan Range in Northeastern Shanxi The Hengshan range (Figs. 1 and 2) trends NE-SW, and a Cenozoic extensional basin developed to the northwest of the range. At present, the Hengshan range is tilted to the southeast because of normal faulting along its northwestern foothills. Along the southern foothills, the Paleoproterozoic east-west-trending Zhujiafang strike-slip fault was reactivated as a thrust fault with thick fault breccias (Figs. 16A, 16B and 16C); lower Paleozoic strata in the Hengshan were folded because of this later thrusting. In the interior of the Hengshan range (the Hunyuan Basin), the deformation of the lower Paleozoic strata reflects two events. Later deformation resulted in NE-SW-trending thrust faults and folds with long wavelengths; the main thrust event occurred along a top-to-the-southeast fault. The

earlier

deformation

in

the

Hengshan

range

involved

nearly

north-south-trending thrusting and related folding (Figs. 16D and 16E); the faults

from

this

stage

cut

Cambrian-Ordovician

limestone,

Carboniferous-Permian limestone and sandstone and Middle Jurassic sandstone but were covered by Early Cretaceous strata (Fig. 17). Thus, the earlier deformation may have developed during the Late Jurassic, while the later NE-SW-trending thrust faults likely developed after the Late Jurassic; however, no direct evidence of superimposition was observed. 4.2.2 The Xizhoushan Range in Northeastern Shanxi A top-to-the-southeast thrust developed in the Xizhoushan range (Figs. 1, 2 and 18), and this thrust fault system was then cut by active high-angle 27

normal faults along the northwestern foothills of the range (Fig. 18A). Paleoproterozoic granite was thrust over Cambrian limestone and sandstone at the southeastern end of the Xizhoushan range (Fig. 18). A series of NE-SW-trending folds that verge to the southeast were observed in the Mesoproterozoic stromatolite-bearing dolomite in the hanging wall. Klippes of Mesoproterozoic dolomite over Cambrian limestone have also been reported (BGMRSP, 1989). Growth strata and normal faults are present in lower Cambrian thin-bedded limestone. These normal faults trend NE-SW after structural reconstruction and are parallel to the strike of later Xizhoushan thrust faults, which suggests that the earlier structures may have controlled the development of the later structures. 4.2.3 The Southern Lüliangshan Range in Southern Shanxi An active Cenozoic normal fault separates the southern Lüliangshan range to the north from the east-west-trending Yuncheng subbasin to the south (Figs. 1 and 2). To the north of the normal fault, a north-directed thrust fault moved Paleoproterozoic granitic gneiss over Cambrian-Ordovician limestone and interbedded clastic rocks (Figs. 19A and 19B). This thrust fault extends southwestwards into the southern Ordos Basin, whereas red Cambrian shale unconformably covers the gneiss to the east (Fig. 19A). In Xiweikou, the Cambrian limestone in the footwall of the north-directed thrust is almost vertical to overturned near the thrust but rapidly becomes horizontal to the north and forms an asymmetrical syncline (Fig. 19B). The hanging wall consists of red granitic gneiss. Many derived secondary fractures are present in this gneiss and are nearly parallel to the main thrust fault (Figs. 19B, 19C and 19D). This gneiss also experienced intense shearing, with shear

28

bands oriented parallel to the fault plane. Because Cambrian strata unconformably cover this granitic gneiss, the displacement of the Xiweikou thrust was limited, with a total displacement of no more than 1000 m. The fault plane solution of the main thrust fault indicates north-south compression (Fig. 19B). The deformation of the cover layers in Xiweikou is similar to that of Emaokou (Fig. 9); hence, we suggest a similar mechanism involving a fault-propagation anticline related to the top-to-the-north thrust (Fig. 19E) To the north of the Guzheng area, a thrust fault also developed between the shale and sandstone of the lower Cambrian Mantou Formation (footwall) and Paleoproterozoic granitic gneiss (hanging wall). This fault zone with thick breccias is more than 100 m wide and features a series of top-to-the-north secondary thrust faults and back-thrust faults (Fig. 20). The hanging wall was intensely eroded because of these faults. Multiple stages of fault activity have been found; the earlier stage involved dextral strike-slip faulting (Figs. 20D and 20E), where the associated strike-slip fault dips at a shallow angle (Figs. 20D and 20E), has nearly horizontal slickenlines (Fig. 20D) and was cut by a later top-to-the-north thrust fault. Slickenlines developed in the dip direction on the main fault plane and indicate north-south compression (Fig. 20A). Many secondary fractures or thrust faults developed in the granitic gneiss in the hanging wall (Fig. 20A), and these faults are generally parallel to the main fault (Fig. 20A). All these observations indicate that the granitic gneiss experienced intense shearing. The Xiweikou thrust fault continues southwestwards into the southern Ordos Block, where medium-thick-bedded limestone in the lower Cambrian Zhangxia Formation was thrust northwestwards over limestone in the upper

29

Carboniferous Taiyuan Formation. A series of S-C fabrics in the fault zone indicate northwestward thrusting. The thrust fault is steep, and the fault plane solution indicates NW-SE compression. However, a period of early dextral strike-slip faulting is indicated by the horizontal slickenlines on the fault plane, which are cut by slickenlines produced by later thrusting. The fault plane solution indicates nearly east-west compression during the earlier deformation event (see below). The above phenomena of the Xiweikou thrust indicate a basement-involved fold caused by the northward-directed thrust in the southern Lüliangshan range. The core of the fold consists of Paleoproterozoic granitic gneiss. In the Fenwei Graben to the south of Linfen, several small, isolated hills contain Paleozoic strata or Mesozoic magmatic intrusions, such as the Jiufengding range to the east of Xiangfen and the Erfengshan range to the east of Linfen (Fig. 19A). The Jiufengding range consists mainly of upper Permian sandstones intruded by Mesozoic syenite, monzodiorite, and aegirine-augite syenite (Fig. 21A). These Permian sandstones are intensely deformed in this region and exhibit dense axial plane cleavages (Fig. 21B). The intersection lineation between the cleavage and bedding indicates a nearly north-south-trending fold hinge plunging to the north at a shallow angle (Fig. 21B). Some of these cleavages subsequently became normal faults, and a series of bookshelf structures formed at different scales (Fig. 21B). The relationship among the cleavages, bedding and bookshelf structures caused by bedding-parallel shearing indicates that the outcropping Permian strata belong to the normal limb of a west-verging recumbent anticline (Fig. 21B). This anticline, its related cleavages and later normal faults were all cut by a

30

series of north-south-trending sinistral strike-slip faults (Fig. 21B), most of which dip to the east and exhibit horizontal slickenlines. The fault plane solution indicates NW-SE compression (Fig. 21B). In addition to the two-stage deformation of the Permian sandstone, these north-south-trending strike-slip faults cut syenite to the north (Fig. 21C). Steep valleys are common along these north-south-trending faults. Horizontal slickenlines also developed on the fault planes, and the fault plane solution indicates NW-SE compression (Fig. 21C), similar to the solution calculated from the faults that cut the Permian strata (Fig. 21B). To the south, the syenite also experienced strong deformation (Fig. 21D). A series of NE-SW-trending cleavages and thrust faults developed in the syenite (Fig. 21D), and the zone of dense cleavages is more than 1000 m wide. The characteristics of the Permian sandstone in Xiangfen County indicate two obvious deformation events. Earlier deformation involved west-verging isoclinal folds that strike north-south, and these folds were cut by nearly north-south-trending sinistral strike-slip faults; this crosscutting relationship is also present along the Huoshan fault, Taihangshan fault and Lishi fault. The earlier folds in Shanxi developed during the Late Jurassic, but the timing of the later strike-slip faulting has not been well constrained, as is discussed later in this article. 4.2.4 The Zhongtiaoshan Range in Southern Shanxi Most fabrics in the Zhongtiaoshan range (Figs. 1 and 2) strike nearly east-west (Fig. 22A). This range consists of the Neoarchean Sushui Complex to the north and Neoproterozoic-early Paleozoic volcanic and sedimentary rocks to the south (Fig. 22A). Additionally, an east-west-trending syncline that

31

consists of Neoproterozoic-early Paleozoic sedimentary strata developed in the southern portion of the range. A south-dipping high-angle “normal fault” developed in the center of the Zhongtiaoshan range between the Sushui Complex to the north and the purple Neoproterozoic volcanic rocks to the south (Fig. 22E). The Sushui Complex in the footwall is strongly deformed; the fault plane is parallel to the foliations in the Sushui Complex and features many dip-slip slickenlines. The fault zone of this “normal fault” is more than 20 m wide and contains fault breccias. A series of small thrust faults is present in the purple volcanic rocks in the hanging wall, and all these small faults merge into the main fault and disappear farther south (Fig. 22E). Extensional basins developed to the north and south of the Zhongtiaoshan range; the mountain range is tilted towards the north, with the southern slope being steeper than the northern slope (Fig. 22B). The preexisting steep thrust faults may have been tilted to the north during the Cenozoic, making them appear to be normal faults. An east-west-trending syncline developed in the southern portion of the mountain range, while the northern portion of the Zhongtiaoshan range may be a basement-involved anticline. The main fault dips towards the north, and the core of the anticline consists primarily of the Neoarchean Sushui Complex, similar to other Mesozoic basement-involved folds throughout Shanxi. The main component of the anticline has been destroyed by the development of the Cenozoic Yuncheng subbasin (Fig. 22B). In addition, many nearly east-west-trending thrust faults are present in the Sushui Complex and the Zhongtiaoshan range; each thrust fault is parallel to the foliations in the Sushui Complex (Fig. 22C). We

therefore

infer

that

the

development

32

of

the

Zhongtiaoshan

basement-involved folds was controlled by earlier foliations in the Sushui Complex. In the northeastern Zhongtiaoshan range, the fabrics change from nearly east-west to NE-SW, and the strike of the fault also changes accordingly (Fig. 22A). The kinematics of the fault also change with the strike: the sinistral strike-slip component of the fault becomes more important, as indicated by the flower structures present in cross sections. Furthermore, the fault plane solutions indicate NW-SE compression (Fig. 22D). 5. Discussion 5.1 Mechanism Responsible for the Basement-Involved Folds in Shanxi Mesozoic basement-involved folds are widely distributed in Shanxi from the Kouquanshan range near Datong in the north to the Zhongtiaoshan range in the south and from the Lüliangshan range in the west to the Huoshan range in the center. Although the formation of the NCC is still under debate, Zhao et al. (2005) suggested that the formation of the NCC occurred at 1.85 Ga. However, Kusky (2011) suggested that the formation of the NCC did not occur until or slightly after 2.3 Ga. Others argued it formed at the end of the Archean by arc-continent collisions (∼2.5 Ga) (Zhai et al., 2000; Zhai and Santosh, 2011). Nevertheless, regardless of when and how the NCC developed, most models agree that the TNCOB and its predominant fabrics in the central NCC trend mainly north-south in the south and NE-SW in the north (Zhao et al., 2005; Faure et al., 2007). Many factors controlled the development of the basement-involved folds in Shanxi. The cores of the anticlines in the Lüliangshan, Huoshan and other ranges all experienced intense deformation, generating many faults with 33

different orientations and kinematics, joints and foliations with different origins. However, no obvious folding resembling that which deformed the sedimentary cover can be identified in the basement. Although folds of various scales are present in the basement, most of these folds likely formed during the Paleoproterozoic orogeny in the central NCC. Later deformation in the basement would have been distributed among dense joints, foliations, faults and other fractures in the basement when these planar structures were reactivated under suitable stress fields. The basement rocks are heavily fractured and easily eroded because of this strong deformation. The Huoshan anticline and the Kouquan fault (anticline) were controlled by the reactivation of joints in the basement. The Lüliangshan anticline, including the Luyashan and Zhongtiaoshan basement-involved folds, was controlled by the reactivation of preexisting foliations in the gneiss, and the hanging wall (anticline) of the Ximafang fault was controlled by preexisting faults. Notably, most of the faults in the basement, except for the main faults, do not penetrate into the overlying sedimentary cover. With the deformation of the basement in Shanxi, the sedimentary cover was also deformed accordingly. A series of folds, such as the Lüliangshan anticline, developed in the sedimentary cover. These folds show that the deformation was focused primarily within a narrow zone and typically formed monoclines; as a result, the strata in the hanging walls often rapidly become horizontal. In profile, the main deformation region of the basement-involved anticlines, such as the Emaokou anticline and Luyashan anticline in the Dongzhai area, was focused within a triangular region in front of the main fault. The large east-west-trending anticline from the Zhongtiaoshan range in the

34

south to the southern Lüliangshan range in the north may represent a similar case. According to the structures in Xiweikou to the north, we suggest that the region of the present-day Yuncheng subbasin was a large box anticline during the

Mesozoic,

with

deformation

focused

on

the

northward-

and

southward-directed thrust faults along the northern and southern limbs of the anticline, respectively. The core of the anticline was destroyed by normal faulting during the Cenozoic; only the faults and deformed sedimentary cover strata in the footwalls of these faults are preserved. This anticline may have formed in response to nearly north-south compression, which may have resulted from relative sinistral shearing between the Ordos Block and blocks to the east. All these basement-involved anticlines may have formed through trishear fault-propagation folding (Erslev, 1991). Slickenlines that are perpendicular to fold hinges on bedding planes, intraformational asymmetrical folds, small faults, and back-thrust faults, such as the Chunjingwa fault, indicate that the sedimentary cover was folded by flexural folding. In addition to the above deformation of the sedimentary cover related to basement deformation, typical thin-skinned thrust faults and related folds also developed in the central NCC. For example, the Luoyunshan thrust fault in the southern Lüliangshan developed in Cambrian-Ordovician limestone and does not contain any basement material. The thrust fault of the western Jingle-Ningwu syncline (the Chunjingwa fault) is also a thin-skinned structure (Fig. 8). However, the coeval development of thick- and thin-skinned structures in Shanxi is not contradictory; the Chunjingwa fault is the back-thrust of the Luyashan basement-involved anticline.

35

5.2 Mesozoic Crustal Structures in Shanxi As indicated above, the Late Jurassic structures in the study region are mainly basement-involved structures similar to those of Laramide structures in western North America. The central NCC was already part of a craton but was deformed, despite the well-known fact that cratons are hard to deform. Thus, the crustal structure is key information for studying the involvement of the craton in its deformation. Zhang et al. (2007) studied the deformation in the central NCC and argued that most of the basement-involved folds are related to a large top-to-the-west ramp-flat-style thrust fault in the middle crust and that the Mesozoic deformation in the central NCC can be attributed to the westward subduction of the Paleo-Pacific Plate to the east. However, Wei et al. (2017) argued an east-west-trending pure shear mechanism for the Mesozoic structures in Shanxi. Abundant geophysical monitoring has been performed in the central NCC to study its destruction and has provided detailed information regarding the deep structures in the crust and even the upper mantle (Zheng et al., 2009; Tang et al., 2010). Because eastern China experienced many strong tectonothermal events during the Mesozoic and Cenozoic, the deep structures have been substantially modified. However, some traces of early fabrics may have been preserved (Zheng et al., 2009). In this study, two NW-SE-trending crustal profiles were constructed across the central NCC to constrain the crustal structure therein. These profiles are nearly parallel to two previous geophysical profiles (Tang et al., 2010), along which the shallow structures were measured at a scale of 1:200000 in this study. Another north-south crustal profile was also constructed in the Eastern

36

Block of the NCC (Fig. 23). The deep crustal structures were constrained by recent teleseismic receiver functions (Tang et al., 2010). Because the mantle is uplifted mainly beneath the axis of the Fenwei Graben and the mantle beneath the Lüliangshan range to the west seems completely or almost undisturbed, as indicated by Tang et al. (2010) and Chen et al. (2009), the uplift of the mantle beneath the Fenwei Graben during the Cenozoic was not considered in this interpretation. The southern profile extends through the Ordos Block, the Lüliangshan anticline, the Huoshan anticline, the Qinshui Basin and the Taihangshan range (Figs. 23E and 23F). A low-velocity zone is observed in southern Shanxi at depths of 25-30 km (Tang et al., 2010) and is interpreted to be a décollement thrust fault in the basement. This décollement connects upwards with three local low-velocity zones, which are the Lishi fault to the west, the Huoshan fault in the center and the Taihangshan fault to the east (Figs. 23E and 23F). These three thrust faults in the basement bound two top-to-the-west thrust systems, and basement rocks crop out along or near these faults. Between the regions of basement uplift are tectonic basins, namely, the Qinshui Basin to the east and the basins along the eastern front of the Lüliangshan range to the west (Figs. 23E and 23F). The profile shows that the Taihangshan range to the east has a similar elevation above sea level as the Qinshui Basin to the west (Figs. 23E and 23F). The thick strata from the mid-Proterozoic to the lower Paleozoic are mainly horizontal, except in regions close to the main faults, and a Cenozoic extensional basin (the Bohai Bay basin) exists to the east of the Taihangshan range. The northern profile traverses the Ordos Block, the Lüliangshan range

37

(the Luyashan), the Jingle-Ningwu syncline, and the Xizhoushan range (Figs. 23C and 23D). This profile features a large SE-directed thrust (i.e., the Lüliangshan fault). The frontal fault of this thrust may be the boundary between the Ordos Block and the blocks to the east. This thrust detaches at depths of approximately 20-25 km in a low-velocity zone, as indicated by a teleseismic receiver function study (Tang et al., 2010). Top-to-the-southeast thrust faults are present to the southeast in the Xizhoushan range; however, no clear data show a relationship between the structures in the Xizhoushan range and those to the northwest along the profile. Field evidence indicates that the segment of the Lüliangshan range along the profile developed early (i.e., the Late Jurassic) and that the Xizhoushan range may have developed later (possibly after the Early Cretaceous). The north-south-trending profile in eastern Shanxi cuts across the Hengshan range, the Xizhoushan range (Wutaishan), and the Qinshui Basin (Figs. 23A and 23B). The entire profile is located in the Eastern Block of the NCC. This profile shows that marked basement-involved deformation occurred in the northern segment, as indicated by a series of top-to-the-southeast thrust faults and associated back-thrust faults, and that the Qinshui Basin to the south

may

be

a

syncline

corresponding

to

the

south-verging

basement-involved anticline to the north. The crustal structures along these two east-west-trending profiles indicate that the upper-middle crust of the central NCC was involved in the deformation, but no large top-to-the-west ramp-flat-style thrust faults have been found beneath the many basement-involved anticlines, as argued by Zhang et al. (2007).

38

5.3 Comparison with Laramide Structures in Western North America As known, typical basement-involved structures developed in the interior of North American Plate during the Laramide Orogeny (Brown, 1988; Bump, 2003), which is similar to the intraplate thick-skinned deformation in the central NCC not only in deformation styles but also in the mechanism. Most studies on basement-involved folds have focused on the Laramide orogenic belt in western North America and the Central Andes in South America (Jordan and Allmendinger, 1986; Erslev, 1993, 2005; Stone, 1993; Schmid et al., 1993; Narr and Suppe, 1994; Mitra and Mount, 1998; Garcı´a and Davis, 2004). These studies generally argued for the existence of horizontal compression in the development of these folds. However, the mechanisms responsible for these basement-involved structures are complicated; that is, no single mechanism can explain all basement-involved folds (Brown, 1988). The basement is folded similarly to the cover in some cases (Schmidt et al., 1993; Garcı´a and Davis, 2004), but in most cases, the deformation of the basement involves primarily faulting by cataclasis (Mitra and Frost, 1981) and is therefore different from the folding of the sedimentary cover (Otteman and Snoke, 2005). Various views are held regarding the faulting processes in the basement, including (1) deformation within a fault zone (Mitra and Mount, 1998); (2) deformation distributed among many parallel faults (Spang and Evans, 1988); and (3) deformation within a triangular region in front of the main fault, as described by the trishear model (Erslve, 1991; Bump, 2003). Additionally, many studies indicated that numerous faults in the basement reactivated early planar structures but that newly formed faults can also be observed (Huntoon, 1993; Marshak et al., 2000; Otteman and Snoke, 2005).

39

Many of the phenomena in the study region discussed above indicate that the reactivation of early fabrics may have controlled the development of Mesozoic folds. Most fabrics such as the schistosity, gneissic schistosity in the Precambrian metamorphic rocks formed during the Paleoproterozoic or Neoarchean orogeny (Zhao et al., 2005; Kusky, 2011). A similar conclusion was obtained by Li et al. (2013, 2015), and the inferred processes are similar to those related to the Laramide structures in western North America (Marshak et al., 2000). The main faulting process is distributed deformation among many parallel faults, which is also similar to the deformation pattern of some Laramide structures (Spang and Evans, 1988). Concerning the crustal structures, detachment layers are found in the middle crust of the central NCC with anticlines comprising thin sedimentary layers verging to the east or west (Fig. 23); these detachments are similar to those found within Laramide structures in western North America (Brown, 1988; Marshak et al., 2000). In summary, the similar deep structures, outcrops and

deformation

mechanism

indicated

above

all

show

that

the

basement-involved anticlines in the central NCC are analogs of those produced by Laramide orogenesis in western North America. 5.4 Timing of the Mesozoic Basement-Involved Folds in Shanxi The above descriptions show that the Mesozoic folds in Shanxi, including the Lüliangshan, Huoshan, and Zhongtiaoshan ranges, appear to be related to basement deformation. However, whether these folds developed during the same period remains to be discussed. Previous studies have argued that all basement-involved folds developed during the Late Jurassic-Early Cretaceous (BGMRSP, 1989; Zhang et al., 2007; Cao et al., 2015, 2018; Clinkscales and 40

Kapp, 2019). Few previous studies have identified the two tectonic events leading to the thrusting and folding in Shanxi during the Mesozoic. However, a crosscutting relationship between the two deformation events was found during the fieldwork conducted for this research in many different areas of Shanxi. Along the southern Lishi fault to the west of the Lüliangshan range, an earlier top-to-the-west thrust fault was cut by a later vertical sinistral strike-slip fault. Along the Kouquan fault in the Emaokou area, an earlier top-to-the-west thrust fault and its related fold were similarly cut by a later sinistral strike-slip fault. Additionally, similar phenomena were found along the Huoshan fault, the Fushan fault and the Taihangshan fault to the east and the Huyanshan fault to the east of the Lüliangshan anticline in central Shanxi. In Xiangfen County, the earlier folds that resulted from top-to-the-west thrusting were cut by later sinistral strike-slip faults. In addition, the nearly north-south-trending folds in the eastern Hengshan range involving Middle Jurassic strata were cut by later east-west-trending thrust faults. On the eastern limb of the north-south-trending Huoshan anticline, Type 1 superimposed folds (Ramsay, 1967) with a basin-and-dome pattern formed in the Paleozoic strata (Figs. 1 and 24). The superimposed folds involve Carboniferous-Permian limestone and sandstones (Fig. 24), and the earlier folds are north-south-trending secondary anticlines and synclines with wavelengths of 2-3 km in the eastern limb of the Huoshan anticline. A set of nearly east-west trending normal faults cut these early folds (Fig. 24). However, in the field, all these faults are inverted thrust faults or cut early thrust faults. Parallel to these faults, folds formed with a wavelength of approximately

41

1 km (Fig. 24); we interpret these to be fault-related folds. Because this region is close to the late Cenozoic Jinzhong subbasin of the Fenwei Graben and all the normal faults are parallel to the boundary of the Jinzhong subbasin (Fig. 24), we infer that the original thrust faults were inverted in the late Cenozoic in relation to the development of the Fenwei Graben. If this was indeed the case, then the nearly east-west-trending folds and faults are younger than the north-south-trending Huoshan anticline. Because of this clear crosscutting relationship, these basement-involved folds in the central NCC could not have developed coevally and cannot be interpreted as the result of strain partitioning. Instead, the nearly north-south-trending basement-involved folds in Shanxi (such as the Huoshan anticline and Lüliangshan anticline) formed in response to nearly east-west or ENE-WSW compression, as indicated by the abovementioned fault plane solutions;

as

also

indicated

by

fault

plane

solutions,

the

nearly

east-west-trending folds (such as the southern end of the Lüliangshan, Zhongtiaoshan, and Xizhoushan ranges) formed primarily in response to nearly north-south compression. Regarding the earlier stage of deformation, most previous studies indicated that the earlier nearly north-south-trending thrust faults and folds in Shanxi formed during the Late Jurassic, involving Middle Jurassic strata (Zhang et al., 2008; BGMRSP, 1989; Cao et al., 2015; Clinkscales and Kapp, 2019); these structures are covered by Early Cretaceous strata in the Hunyuan Basin within the Hengshan range (Fig. 17). We also performed zircon U/Th-He dating on rocks from the Lüliangshan anticline and obtained an age of ca. 150 Ma (unpublished data). In addition, a rapid heating event occurred in the

42

Jingle-Ningwu Basin during ca. 170-155 Ma with a sudden increase of geothermal temperature from 80-90℃ to 120-125℃, as evidenced by apatite fission track dating (Liu et al., 2008). In contrast, the timing of the later deformation stage (i.e., which formed the north-south-trending sinistral strike-slip faults and east-west-trending folds) remains unknown. However, some lines of evidence constrain the timing of the formation of these structures. In the Jiufengding area to the east of Xiangfen County, later north-south-trending strike-slip faults cut syenite. Although we did not perform isotopic dating on this syenite, previous studies on similar syenites in nearby regions (such as that in the Erfengshan range to the northeast) showed that these syenites range in age from 133 Ma to 128 Ma (zircon U-Pb ages) and originate from the mantle as a result of lithospheric thinning (Huo et al., 2016; Ying et al., 2011). Thus, the north-south-trending sinistral strike-slip faulting in Xiangfen and its vicinity occurred after the Early Cretaceous. In addition, some recent low-temperature thermochronological studies have also identified a Late Cretaceous cooling event in the study region (Cao et al., 2015; Chang et al., 2018; our unpublished data). After the Late Jurassic, the eastern Eurasian Plate was affected by the subduction of the Paleo-Pacific Plate to the east and the final closure of the Mongol-Okhotsk Ocean. During the Early Cretaceous, the NCC experienced the destruction of the craton; as a result, the lithosphere of the eastern NCC, including that in Shanxi, was greatly thinned (Zhu et al., 2017), and mantle-derived intrusions were emplaced in the eastern NCC. Many Early Cretaceous intrusions (130-125 Ma) in Shanxi, such as the Zijingshan alkaline rocks, Huyanshan alkaline rocks, and Erfengshan syenite, are indicative of

43

magmatism during lithospheric thinning in a back-arc region (Ying et al., 2011; Huo et al., 2015). The Jiufengding syenite to the east of Xiangfen is among these Early Cretaceous alkaline rocks, although no accurate isotopic ages have been obtained, and the syenite in this region belongs to the Erfengshan alkaline rocks (Wang et al., 2009). More importantly, the Jiufengding syenite experienced deformation because it was cut by nearly north-south-trending sinistral strike-slip faults and developed intense cleavage. Previous studies have shown that the eastern NCC experienced strong extension during the Early Cretaceous, whereas few compressive structures since the Early Cretaceous have been reported or studied. Consequently, because the deformation of the Jiufengding syenite likely occurred after the Early Cretaceous and the strike-slip faulting in the Jiufengshan syenite may have occurred after the development of the basement-involved folds (such as the Lüliangshan anticline), we suggest that the earlier nearly north-south-trending Huoshan anticline in Shanxi formed during the Late Jurassic and that the nearly east-west-trending thrust faults, basement-involved folds and nearly north-south-trending sinistral strike-slip faults occurred after the Early Cretaceous. 5.5 Tectonic Settings of the Mesozoic Intraplate Deformation of the NCC Because the central NCC was far from any plate boundaries during the Mesozoic, similar to the late Mesozoic Laramide orogenic belt in western North America, the main factors controlling the Mesozoic intraplate deformation of the NCC remain hotly debated. Some studies have suggested that the key factor was the closure of the Mongol-Okhotsk Ocean to the north and subsequent compression (Davis et al., 2001), whereas other authors have 44

pointed to the westward subduction of the Paleo-Pacific Plate (Zhang et al., 2007; Faure et al., 2012; Cao et al., 2015, 2018; Wu et al., 2019; Li et al., 2019). Many tectonic settings have been proposed for the Laramide orogeny, including (1) retroarc thrusting (Price, 1981); (2) orogenic float (Oldow et al.,1990); (3) flat-slab subduction of the Farallon Plate, which caused thickening of the crust (ca. 51 km) (Bird, 1984; Jordan and Allmendinger, 1986); (4) cordilleran collision (Maxson and Tikoff, 1996); and (5) collapse of the Sevier orogenic belt to the west of the Laramide orogenic belt (Livaccari, 1991). To date, no consensus has been reached (English and Johnston, 2004). Although various hypotheses exist, all of these proposed mechanisms are related to subduction or collision along a plate boundary. The present study distinguishes two stages of deformation in the central NCC during the Mesozoic that resulted in the main structural framework of the NCC. A series of large sinistral strike-slip faults, such as the Tanlu, Taihangshan, Huoshan-Fushan and Lishi faults, developed in the central NCC, which may have caused strain partitioning. However, an obvious crosscutting relationship was identified in the field. In addition, the main sinistral strike-slip activity on these faults occurred during the Early Cretaceous (Zhu et al., 2005). The fault plane solutions indicate that the orientation of the earlier maximum compression trended nearly east-west or ENE-WSW, while that of the later maximum compression trended north-south or NNW-SSE. A consensus has been reached regarding the earlier nearly east-west compression stage, which resulted in the development of the Lüliangshan basement-involved anticline during the Late Jurassic (Dong et al., 2007, 2015;

45

Zhang et al., 2007, 2008), when a subduction zone developed along the eastern margin of the Eurasian Plate, although the exact location of this subduction zone is still questioned. If the thick-skinned structures in the Lüliangshan range were caused by the subduction of the Paleo-Pacific Plate (Faure et al., 2012; Clinkscales and Kapp, 2019), then its subduction should have occurred at a low angle (Fig. 25). Indeed, many recent studies have found that the westward subduction of the Paleo-Pacific Plate beneath the Eurasian Plate was shallow (Zhu et al., 2017 and references therein), supporting the speculations based on structural geology investigations. Furthermore, Faure et al. (2012) argued that the western Ordos thrust belt developed because of the westward subduction of the Paleo-Pacific Plate. If the subduction zone of the Paleo-Pacific Plate was located along the north-south-trending Nadanhada ophiolite mélange during the Late Jurassic, then the effects of low-angle subduction may have reached far into the interior of the NCC, potentially extending ca. 2200 km to the western Ordos Basin and ca. 1800 km to the Lüliangshan range. The north-south-trending sinistral strike-slip faults in Shanxi (i.e., the Huoshan-Fushan fault, Taihangshan fault and Lishi fault) developed after the Early Cretaceous because one of these faults (the Huoshan-Fushan fault) cuts the Early Cretaceous syenite. Previous studies suggested that large-scale sinistral transpression in the eastern Eurasian Plate occurred during the Early Cretaceous (ca. 130-120 Ma) as a result of the northwestward subduction of the Paleo-Pacific Plate (Maruyama et al., 1997; Zhu et al., 2011). Some studies also argued that an unknown oceanic plateau/block (such as the western Philippine Block or the Huocike Block) collided with the southeastern

46

Eurasian Plate during the Late Cretaceous (Faure et al., 1989; Yang et al., 2013; Niu et al., 2015). Yang et al. (2013) postulated that the Okhotsk Block continued moving northward along the eastern margin of the Eurasian continent after this collision, further affecting the Eurasian continent; during this process, the Paleo-Pacific Plate subducted at a low angle. At this time, many Early Cretaceous extensional basins in the NCC and its vicinity, such as the Erlian Basin and Ordos Basin, stopped subsiding, possibly in response to this tectonic event. Cenozoic rifting of the southern Fenwei Graben occurred during the Eocene-Oligocene (Zhang et al., 1998); consequently, the fault that cuts the Early Cretaceous Jiufengding syenite formed between the Early Cretaceous and Eocene. Additionally, because the eastern Eurasian Plate underwent compression during the Late Cretaceous, we argue that the fault cutting the syenite is Late Cretaceous in age. The paleo-stress field, as shown by the fault plane solutions, reflects NW-SE-trending compression, suggesting that the late Mesozoic intraplate deformation in the central NCC may have resulted from oblique collision and subsequent shearing along the southeastern margin of the Eurasian Plate. In summary, the Mesozoic evolution of the NCC can be subdivided into three stages. During the Late Jurassic, stress was transferred far into the interior of the NCC by the low-angle subduction of the Paleo-Pacific Plate beneath the NCC. The preexisting fabrics in the basement were reactivated by east-west compression, which resulted in the development of a series of north-south-trending basement-involved folds (Figs. 25 and 26). During low-angle subduction, the dehydration of the subducting oceanic crust and sediments can add water and other volatiles to the overriding continental

47

lithosphere (Sun and Kennett, 2017); this dehydration process can not only reduce the density of the mantle but can also cause water-induced melting and weaken the lithosphere (Humphreys et al., 2003). These processes resulted in the eventual destruction of the NCC (Fig. 25). Hydration from below and the rollback of the Paleo-Pacific Plate disrupted the lithospheric mantle below the NCC; as a result, the entire NCC experienced regional extension along with substantial magmatic intrusion during the Early Cretaceous (Fig. 25). During the Late Cretaceous, a collision between “an unknown oceanic block” (Niu et al., 2015) and the southeastern Eurasian Plate caused the NCC to experience regional transpression, which created a series of north-south-trending sinistral faults and related east-west-trending thrust faults and folds in the interior of the NCC (Figs. 25 and 26). 6. Conclusions The Mesozoic intraplate deformation of the central NCC, which represents a

typical

thick-skinned

tectonic

regime,

was

characterized

by

basement-involved folding and faulting. Preexisting fabrics in the basement controlled the development of these structures. During the Late Jurassic, nearly

east-west

compression

resulted

in

the

development

of

north-south-trending structures, such as the Lüliangshan and Huoshan ranges. During the Late Cretaceous, NNW-SSE compression caused the development of

ENE-WSW-trending

structures.

The

low-angle

subduction

of

the

Paleo-Pacific Plate beneath the NCC during the Late Jurassic caused the deformation of the craton. Low-angle subduction may have transferred stress into the interior of the plate and even to the western boundary of the NCC, laying the foundation for the eventual destruction of the NCC. The collision 48

between the southeastern Eurasian continent and an unknown block or oceanic plateau during the Late Cretaceous may have caused the second stage of intraplate deformation in the NCC.

Acknowledgments We thank Prof. Zhang Qinglong from Nanjing University for his unflagging support during this research. This research was funded by the Scientific Special Projects of Chinese Academy of Geological Sciences (No. YYMF201604), the National Key Basic Research Program of China (No. 2015CB453002), and the China Geological Survey (No. DD20160343-16). We also thank the editor of the journal and three anonymous reviewers for their thoughtful comments and constructive suggestions.

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Figure captions

Fig. 1 Geological map of the Shanxi region with a general stratigraphic column. The red lines are the main fault zones discussed in this study, and the locations of the following figures are also indicated.

Fig. 2 Digital elevation model (DEM) of the study area. The data are from the Shuttle Radar Topography Mission (SRTM) GL3 Global 90m product.

Fig. 3 Southern Lishi fault (see Fig. 1 for location). A. Cross section of the Lishi fault in Huangtu, Xi County, including joints in the Paleoproterozoic migmatitic 61

granitic gneiss in the hanging wall and their stereographic projections. B. Stereographic projection of the monocline. C. Joints in the basement. D. Stereographic projections of joints (lower hemisphere equal-area projections, same in the following). E. Fault plane solution of reactivated joints in the basement. F. Section of the Lishi fault in Jinjiachuan, Linfen. G. Early duplex structures and their stereographic projections. H. Fault plane solution of early thrusting.

Fig. 4 Northern Lishi fault (see Fig. 1 for location). A. Line drawing of the Jingle-eastern Linxian section. B. Nearly vertical eastern branch of the Lishi fault. C. Fault plane solution of the eastern branch of the Lishi fault.

Fig. 5 Lüliangshan basement-involved anticline (see Fig. 1 for location). A. Gongyang cross section, Zhongyang County. B. Stereographic projections of thrust faults. C. Small thrusts and duplex structures in the basement. D. Slickenlines on the thrust fault planes. E. Fault plane solution of the thrust faults. F. Thrust fault in the basement of the Lüliangshan anticline, Zhike, Zhongyang County. H. Fault plane solution of the thrust faults. G. Stereographic projections of faults (red) and foliations (black) in the basement rock. I. Sucun cross section.

Fig. 6 Section of the Jingle-Ningwu syncline (see Fig. 1 for location). A. Yuwan section. B. Xinpu section. C. Two sets of joints in Precambrian sandstone in the Xinpu section and their stereographic projections (see location in B). D. Relationship between the joints and faults in the Precambrian sandstone and

62

stereographic projections of joints (see location in B).

Fig. 7 Characteristics of the basement and cover deformation in Dongzhai (see Fig. 1 for location). A. Duplex in granitic gneiss and stereographic projections of foliations in the basement. B. Thrust fault in Cambrian limestone in the cover layer; the fault is parallel to the faults in A with many tension gashes in the gentle anticline over the ramp. C. Outcrops of the Dongzhai thrust fault. D. Profile of the Dongzhai monocline and the locations of photos C and E. F. Relationship among the foliations, joints and faults in the basement. G. Relationship among the joints, foliations and faults in the basement. H. Sketch of Photo G.

Fig. 8 Chunjingwa thrust fault in Dongzhai (see Fig. 1 for location). A. Line drawing of the thrust fault. B. Outcrops of the Chunjingwa thrust fault and its fault plane solution.

Fig. 9 Structures of the Kouquan fault (see Fig. 1 for location). A. Cross section of Yungang, Datong. B. Vertical thrust fault in the basement. C. Fault plane solution of early thrust faults. D. Fault plane solution of later sinistral strike-slip faults. E. Fault plane solution of a thrust fault in the basement. F. Relationship between joints and thrust faults in the basement. F. Joints in the basement and their relationship with the thrust faults in the basement; red dots are joints, and blue dots are thrust faults. G. Cross section of Emaokou. H. Crosscutting relationship between the early thrust fault (red dashed line) and later sinistral strike-slip fault (white dashed line). I. Fault plane solution of a later strike-slip

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fault. J. Possible evolution model of the Emaokou structure.

Fig. 10 Relationship between faults in the basement and sedimentary cover in the eastern foothills of the Lüliangshan to the north of Jiaocheng (see Fig. 1 for location). A. Cross section of the eastern Lüliangshan thrust fault and stereographic projections of faults and their fault plane solutions. B. Slickenlines on the fault plane indicating thrust movement (dashed line indicating slickenlines). C. Oblique view of outcrops of the fault zone.

Fig. 11 Eastern foothill thrust fault of the Lüliangshan and the later strike-slip fault in the northern Jiaochen area (see Fig. 1 for location). A. Line drawing of the fault zone. B. Fault plane solution of early normal faults. C. Fault plane solution of later strike-slip faults. D. Outcrops of early normal faults. E. Two sets of slickenlines on the fault plane.

Fig. 12 Relationship between faults and systematic joints in the core of the Huoshan anticline (see Fig. 1 for location). A. North-south-trending joints and the fault plane solution of reactivated joints. B. Crosscutting relationship between the north-south-trending joints and east-west-trending joints and the fault plane solution of reactivated east-west-trending joints; red arrows show one of the continuous east-west-trending faults cutting a north-south-trending fault indicated by a yellow arrow. C. East-west-trending thrust faults that cut the Cambrian limestone overlying the basement, Shigaoshan, Lingshi. D. Slickenlines on the east-west-trending fault plane in C. E. Relationship between the north-south-trending faults and north-south-trending joints and

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the stereographic projections of joints in the basement. F. Reactivation of preexisting joints as thrust faults.

Fig. 13 Huoshan fault (see Fig. 1 for location). A. General cross section of the fault in the Xingtang Temple area and its fault plane solution. B. Outcrops of fault and fault breccias. C. Fault plane with nearly horizontal slickenlines. D. S-C fabrics indicating sinistral strike-slipping in a fault breccia. E. Line drawing of two sets of fault zones in Guangsheng Temple and the fault plane solution of early sinistral strike-slip faults.

Fig. 14 Fushan fault (see Fig. 1 for location). A. The vertical feature is a strike-slip fault, and the other is a thrust fault. Note that the steep fault plane marked “D” is continuous and planar and cuts the fault marked “C”. B. A later strike-slip fault and location of photo A. C. Early thrust fault plane and its fault plane solution (see A for the location of the photo). D. Later strike-slip fault plane and its fault plane solution (see A for the location of the photo).

Fig. 15 Cross section of the Taihangshan fault and its related fold to the east of Changzhi city (see Fig. 1 for location). A. Line drawing of the structure of the early thrust fault system and its related fold, which was cut by the later sinistral strike-slip fault. B. Nearly vertical strike-slip fault with thick fault breccias cutting the early back-thrust fault of the main thrust fault; the back-thrust is indicated by a red dashed line (see A for the location of the photo). C. Fault plane solution of the later sinistral strike-slip fault.

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Fig. 16 Thrust faults in the Hengshan range (see Fig. 1 for location). A-C. Thrust faults in the southern foothills of the Hengshan range (stereographic projections of the faults in A and B and fault plane solution in C; BIF in B is the abbreviation of banded iron formation). D and E. North-south-trending thrust fault in the Hengshan range and stereographic projections of related faults.

Fig. 17 Geological map of the Hunyuan Basin in the Hengshan range (modified from BGMRSP (1989)) (see Fig. 1 for location).

Fig. 18 Top-to-the-southeast thrust fault cutting basement granite in the Xizhoushan range (see Fig. 1 for location).

Fig. 19 Basement-involved deformation in the southern Lüliangshan range. A. Geological map of the study region (see Fig. 1 for location). B. Sketch of outcrops in Xiweikou to the north of Hejin. C. Stereographic projections of faults in gneiss in the hanging wall. D. Fault plane solution. E. Basement-involved fold model of the southern Lüliangshan range.

Fig. 20 Multistage faulting to the north of Guzheng (see Fig. 1 for location). A-C. Thrust fault between the basement Neoarchean granitic gneiss and Cambrian shale and sandstone and the fault plane solution. D. Fault plane of the early dextral strike-slip fault. E. Early dextral strike-slip fault between the basement Neoarchean granitic gneiss and early Cambrian sandstones.

Fig. 21 Deformation of Permian strata and Early Cretaceous syenite in

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Jiufengding and its vicinity, Xiangfen County. A. Geological map of the region (see Fig. 1 for location). B. Sketch of outcrops of deformed Permian strata, stereographic projection of bedding and cleavages, the fault plane solution, and the inferred deformation region in a recumbent anticline according to the relationship

between

the

bedding

and

cleavages.

C.

Nearly

north-south-trending strike-slip fault cutting syenite to the north with its fault plane solution. D. Deformation characteristics of syenite to the south with a stereographic projection of small faults and cleavages.

Fig. 22 Mesozoic deformation in the Zhongtiaoshan range. A. Geological map of Zhongtiaoshan (see Fig. 1 for location). B. General cross section of the Zhongtiaoshan range; the topographic line of the section is based on the SRTM 90 m digital elevation model. C. Stereographic projection of the gneissosity in the Sushui Complex. D. Fault plane solution in the northeastern segment of the Zhongtiaoshan range. E. Line sketch of the steep fault zone in the central Zhongtiaoshan range.

Fig. 23 Crustal profiles of the Shanxi region and their locations (the topographic lines of the cross section are based on the SRTM 90 m digital elevation model; the crustal structures are based on Tang et al., 2010).

Fig. 24 Type 1 superimposed folds in the Paleozoic strata in the eastern limb of the Huoshan anticline (see Fig. 1 for location).

Fig. 25 Tectonic settings of the intraplate deformation of the NCC during the

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Mesozoic. A. Late Jurassic. B. Early Cretaceous.

Fig. 26 Mesozoic tectonic map of the Shanxi region and fault plane solutions of the two-stage deformation.

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Credit Author Statement Jin Zhang: Conceptualization, Methodology, Investigation, Funding acquisition Junfeng Qu: Investigation, Project administration Beihang Zhang: Investigation, Writing- Original draft preparation Heng Zhao: Visualization, Formal analysis Pengfei Niu: Investigation Shuo Zhao: Investigation Jie Hui: Writing- Reviewing and Editing Long Yun: Investigation Fengjun Nie: Investigation Yannan Wang: Investigation, Formal analysis

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Author Agreement All the authors have been involved with the work and have read though the manuscript; all authors have approved the manuscript and agreed to its submission. No any conflicts of interest are among authors.

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Highlights The central NCC experienced two-stage deformation during the Mesozoic. The Mesozoic deformation of the central NCC was controlled by preexisting basement fabrics. The Late Jurassic deformation was caused by the low-angle subduction of the Paleo-Pacific oceanic plate.

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Declaration of Interest Statement We declare that no any conflicts of interest are among all authors.

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