The Middle–Late Jurassic to earliest Cretaceous fold belts of the Yanshanian orogen in North China remain enigmatic with respect to their coeval deformation histories and possible relationship to the contemporaneous Cordilleran-style margin of eastern Asia. We present geological mapping, structural data, and a >400-km-long, strike-perpendicular balanced cross section for the Taihang-Luliangshan fold belt exposed in the late Cenozoic central Shanxi Rift. The northeast-southwest–trending Taihang-Luliangshan fold belt consists of long-wavelength folds (∼35–110 km) with ∼1–9 km of structural relief cored by Archean and Paleoproterozoic metamorphic and igneous basement rocks. The fold belt accommodated ≥11 km of northwest-southeast shortening between the Taihangshan fault, bounding the North China Plain, in the east and the Ordos Basin in the west. Geological mapping in the Xizhoushan, a northeast-southwest–oriented range within the larger Taihangshan mountain belt, reveals two major basement-cored folds: (1) the Xizhou syncline, with an axial trace that extends for ∼100 km and is characterized by a steep to overturned forelimb consistent with a southeast sense of vergence, and (2) the Hutuo River anticline, which exposes Archean–Paleoproterozoic rocks in its core that are unconformably overlain by shallowly dipping (<∼20°) Lower Paleozoic rocks. In the Luliangshan, Mesozoic structures include the Luliang anticline, the largest recognized anticline in the region, the Ningjing syncline, which preserves a complete section of Paleozoic to Upper Jurassic strata, and the Wuzhai anticline; together, these folds are characterized by a wavelength of ∼45–50 km. Shortening in the Taihang-Luliangshan fold belt is estimated to have occurred between ca. 160 Ma and 135 Ma, based on the age of the youngest deformed Upper Jurassic rocks in the Ningjing syncline, previously published low-temperature thermochronology, and regional correlations to better-studied Yanshanian fold belts. The timing of basement-involved deformation in the Taihang-Luliangshan fold belt, which formed >1000 km from the nearest plate margin, corresponds with the termination of arc magmatism along the eastern margin of Asia, implying a potential linkage to the kinematics of the westward-subducting Izanagi (paleo-Pacific) plate.
The Middle–Late Jurassic to earliest Cretaceous Yanshanian orogeny of North China (see reviews by Zhang et al., 2008; Dong et al., 2008; Zhang et al., 2014) is recognized from contractional and transpressional fold-and-thrust belts in the Tanlu fault zone and North China Plain (Zhang et al., 2015), the Yanshan and western hills of Beijing (e.g., Davis et al., 2001), the ranges bordering the northern and western Ordos Basin (Darby and Ritts, 2007; Davis and Darby, 2010; Huang et al., 2015), and the Taihang-Luliangshan of the central Shanxi Province (this study; Fig. 1). The mechanisms invoked for Yanshanian shortening are debated and include the influence of far-field deformation in response to closure of the Mongol-Okhostk suture (Fig. 1A; e.g., Davis et al., 2001), terrane amalgamation in Tibet (e.g., Darby and Ritts, 2007), intracontinental subduction processes (Faure et al., 2012), and/or westward subduction of the Izanagi (i.e., paleo-Pacific) plate beneath the eastern margin of Asia (e.g., Yang and Dong, 2018). However, none of these models accurately depicts the structural geology of the Taihang-Luliangshan fold belt and the way in which it fits into a unified regional kinematic model for the Yanshanian orogeny. Moreover, existing models of Yanshanian deformation and contemporaneous magmatic processes (e.g., Zhang et al., 2014) do not satisfactorily discuss the magmatic history of the Jurassic eastern Asian magmatic arc and its potential relationship to the kinematics of the subducting Izanagi plate and intracontinental deformation in North China.
This study presents new geological mapping, structural data, and a regional balanced cross section of the Taihang-Luliangshan fold belt. We focus on describing the principal Yanshanian structural features of the central Shanxi Province, north of the city of Taiyuan (Fig. 2), to reveal the style of contractional deformation in the region and to discuss this deformation in the context of the regional kinematics of the Yanshanian orogenic event and its possible relationship to the eastern Asian Cordilleran-style margin.
Regional Geological Setting
The geology investigated in this study is located in the Shanxi Province between the city of Taiyuan and the Hengshan (“shan” = mountains; Fig. 2). The pre-Cenozoic geology is exposed in the footwall blocks of the Shanxi Rift, a Miocene to present rift system located between the North China Plain in the east and the Ordos Basin in the west (Fig. 1; Xu and Ma, 1992; Xu et al., 1993; Zhang et al., 1998, 2003; Shi et al., 2015; Middleton et al., 2017). The Taihangshan and Luliangshan define the eastern and western rift shoulders of the Shanxi Rift, respectively (Figs. 1 and 2). The eastern Taihangshan is subdivided into several subsidiary ranges that trend northeast-southwest, parallel to their respective range-front normal faults (e.g., Xizhoushan and Wutaishan). The Luliangshan is a large (∼19,000 km2) northeast-southwest–trending range with its northern boundary at the Hengshan (Fig. 2). These ranges expose Archean–Paleoproterozoic metamorphic and igneous basement rocks, low-grade metamorphic Proterozoic sedimentary rocks, and Paleozoic through Upper Jurassic strata (SBGMR, 1989; Fig. 2). The rift grabens and half-grabens of the Shanxi Rift north of Taiyuan are bounded by active, range-front normal faults and are filled with ∼1–2 km of Pliocene–Quaternary sediment (e.g., Xu et al., 1993).
Middle Jurassic–Early Cretaceous Shortening
Middle–Late Jurassic to Early Cretaceous shortening was accommodated by fold-and-thrust belt structures along the Tanlu fault zone, Bohai Bay, the Yanshan and the western hills of Beijing, and the circum-Ordos region (Fig. 1). Shortening during this time is classically associated with the Yanshanian deformational episode (e.g., Davis et al., 2001), which followed earlier phases of orogenesis in North China associated with the late Paleozoic–Triassic closure of the Xilamulin and Solonker sutures (e.g., Wang and Liu, 1986; Zhang et al., 2009c; Eizenhöfer et al., 2014) and the Triassic to Early Jurassic collision between the North and South China blocks along the Dabie-Sulu suture (Fig. 1; e.g., Hacker et al., 2000).
Beginning ca. 165–160 Ma, the southern Tanlu fault accommodated left-lateral slip as part of a transpressional strain regime (Wang, 2006). This transpressional regime is attributed to fault reactivation of the antecedent north-northeast–striking Tanlu fault (Zhu et al., 2009; Zhao et al., 2016b) during oblique convergence between North China and the Izanagi plate (Zhu et al., 2005; Zhang et al., 2015). Other north-northeast–striking fault zones in the North China Plain (e.g., Cangxian fault; Fig. 1) may have similarly been reactivated as transpressional structures during the Middle Jurassic–Early Cretaceous (e.g., Li et al., 2012). Subsurface borehole data from the North China Plain confirm the presence of Upper Jurassic–Lower Cretaceous alluvial and fluvial sedimentary sections, which may be related to northwest-directed, thrust-bounded basins (Li et al., 2017).
In the Yanshan and western hills of Beijing, a Middle–Late Jurassic to Early Cretaceous fold-and-thrust belt developed along northwest- and southeast-directed thrust faults (e.g., Davis et al., 2001; Zhang et al., 2014). Geological, detrital provenance, and geochronologic evidence indicates that fold belt formation associated with the Yanshanian orogeny initiated by, or shortly before, ca. 160 Ma (e.g., Davis et al., 1998, 2009; Yang et al., 2006), and shortening was coeval and cospatial with magmatism (e.g., Davis et al., 2001) and deposition of synorogenic volcanic rocks (e.g., Liu et al., 2006; Fu et al., 2018). Yanshanian thrust faults are characterized as thin skinned in style, with displacements of >25–30 km and detachments principally along rheologically weak units in Proterozoic strata (Davis et al., 2001). These Late Jurassic to Early Cretaceous thrust faults offset east-west–striking Late Triassic–Early Jurassic (>180 Ma) basement-involved faults and folds (Chen, 1998; Davis et al., 2001; Liu et al., 2012; Wang et al., 2013; Li et al., 2016). Contraction in the Yanshan fold belt largely ceased by ca. 137 Ma, the age of andesite flows unconformably overlying folded Lower Cretaceous strata (Yang et al., 2006).
In the circum-Ordos region, Late Jurassic–Early Cretaceous thrust faults are exposed in the Daqingshan, Langshan, Helanshan, and Zhuozishan (Fig. 1), with strike orientations that are subparallel to the periphery of the Ordos Basin. Thrust fault orientations may have been controlled by a change in the mechanical properties of the crust at the periphery of the Ordos Basin (e.g., Darby and Ritts, 2002).
In the Daqingshan, thrust faults generally strike east-northeast, are doubly vergent, and offset Upper Jurassic and Lower Cretaceous continental strata; the final episode of shortening is constrained by ca. 136–133 Ma syncontractional basalt flows (Davis and Darby, 2010). In the Langshan, northeast-southwest–striking thrust faults juxtapose Precambrian basement and late Paleozoic–Triassic granitoids over syncontractional Upper Jurassic–Lower Cretaceous strata (Darby and Ritts, 2007). In the Helanshan, approximately north-south–striking thrust faults offset Archean basement rocks and Paleozoic–Mesozoic strata, and these are correlated to the same fold belt exposed in the Zhuozishan, where a large east-vergent, basement-cored anticline is exposed (Fig. 1; Darby and Ritts, 2002; Huang et al., 2015). Shortening commencing during the Late Jurassic in the Helanshan and Zhuozishan is based on folded Upper Jurassic and Lower Cretaceous strata in angular unconformity over older Paleozoic–Mesozoic strata (Huang et al., 2015) and Late Jurassic–Early Cretaceous apatite and zircon fission-track ages (Zhao et al., 2007). However, east-west shortening in the western Ordos region may have started as early as the Early–Middle Jurassic (Darby and Ritts, 2002).
Deformation between ca. 165 and 135 Ma in North China is commonly attributed to far-field strain related to a complex interplay of compression associated with the amalgamation of Eurasia, including closure of the Mongol-Okhostk Ocean and terrane suturing in Tibet, and west-dipping subduction of oceanic lithosphere along the eastern Asian margin (e.g., Zhang et al., 2008; Dong et al., 2008). The favored mechanisms commonly depend on the location of the observed deformation and include: (1) shortening in the Yanshan associated with closure along the Mongol-Okhotsk suture in Mongolia and southeastern Russia (Fig. 1A), but with coeval magmatism attributed to Izanagi plate subduction (Davis et al., 2001); (2) collision of the Lhasa terrane in Tibet with the southern margin of Eurasia leading to shortening in the Langshan (Darby and Ritts, 2007); (3) subduction of the Izanagi plate and contemporaneous lithospheric subduction of the North China block beneath the Alashan block accommodated by pure shear shortening in the Taihangshan and Helanshan and right-lateral transpression in the Yinshan-Yanshan belt (Faure et al., 2012); and (4) Izanagi plate convergence resulting in the northeast-southwest fold belts within the Taihangshan and Helanshan (Darby and Ritts, 2002; Yang and Dong, 2018).
The geological mapping and structural analysis part of this study is based on a reconnaissance of the Luliangshan and mapping at ∼1:100,000 to ∼1:64,000 scale in the Xizhoushan. The regional structural geology was investigated through mapping contact relations between basement rocks and Paleozoic–Mesozoic strata, and structural analysis of minor and regional folds. The SBGMR (1989) geological map and accompanying stratigraphic descriptions were used to identify stratigraphic contacts in the field to generate a generalized stratigraphic column (Fig. 3). A regional balanced cross section was constructed using 2D Move by Midland Valley (https://www.mve.com/software/move) and is based on previously mapped relationships (SBGMR, 1989; HBGMR, 1991; CAGS, 2002) and field data acquired during this study.
BASEMENT ROCKS AND STRATIGRAPHIC DESCRIPTIONS
Neoarchean–Paleoproterozoic Basement Rocks and Hutuo Group
Neoarchean–Paleoproterozoic tonalite-trondhjemite-granodiorite (TTG) orthogneisses, migmatites, and granitoid intrusions comprise the dominant basement lithologies of the central Shanxi Rift and North China craton (e.g., Zhao et al., 2005; Faure et al., 2007; Zhao et al., 2008; Trap et al., 2009; Kusky et al., 2016). They include ca. 2500 Ma orthogneisses, granitoids, and metavolcanic sequences (Zhao et al., 2002; Kröner et al., 2005a, 2005b; Li et al., 2010), and lesser ca. 2100 Ma and ca. 1870 Ma granitoids and gneisses (Wilde et al., 2005; Trap et al., 2009; Zhang et al., 2009a). Basement rocks in the Luliangshan consist principally of TTG orthogneisses (Fig. 4A), although a variety of granitoids (Fig. 4B) and other metavolcanic rocks are recognized (Trap et al., 2009). Basement rocks in the southwestern Xizhoushan consist of weakly to nonfoliated syenogranite of a distinctive pink and weathered red color (Fig. 4C), which likely correlates to the younger suite of ca. 2100 Ma or 1800 Ma Paleoproterozoic plutons. In contrast, basement rocks in the central Xizhoushan are dominantly TTG orthogneisses and amphibolites (Fig. 2; e.g., SBGMR, 1989; Li et al., 2010).
Igneous activity, deformation and metamorphism, and sedimentation between ca. 1900 and 1800 Ma are attributed to collision between the Western (Ordos) and Eastern blocks of the North China craton along the Trans–North China suture (Fig. 1; Kröner et al., 2006; Zhang et al., 2007; Trap et al., 2009; Zhang et al., 2009a; Li et al., 2010). This suturing event led to the development of the Trans–North China orogen (e.g., Faure et al., 2007) and a foreland basin represented by the Paleoproterozoic Hutuo Group in the central Shanxi region (Li et al., 2010). The Hutuo Group is dominated by greenschist-facies metasedimentary rocks (Faure et al., 2007) that unconformably overlie Neoarchean–Paleoproterozoic basement rocks in the southern Wutaishan and the northeastern Xizhoushan (Figs. 2 and 3). The Hutuo Group in the Xizhoushan includes interbedded phyllite, quartzite (Fig. 4D), marble, and slate.
Cambrian–Ordovician strata unconformably overlie the Hutuo Group and older basement rocks (Fig. 4E). The Cambrian section consists of mixed siliciclastic-carbonate facies (e.g., Meng et al., 1997). The lowermost section in the Xizhoushan and Luliangshan (Fig. 2) commonly consists of basal conglomerates, with clast lithologies consistent with derivation from basement or Hutuo Group rocks (Fig. 4F), and the section fines upward into fine- to medium-grained sandstone and red micaceous siltstone (Fig. 4G). The lowermost siliciclastic-dominant section grades into thin-bedded to medium-bedded (∼0.5–1 m) oolitic limestone and carbonate facies. The mixed siliciclastic-carbonate Cambrian section grades upward into the carbonate-rich Ordovician section, dominated by medium- to thick-bedded micritic and oolitic wackestone to grainstone limestone facies deposited as part of the regional Paleozoic North China carbonate platform (Meng et al., 1997).
Cambrian–Ordovician strata are disconformably overlain by Carboniferous–Permian strata (SBGMR, 1989). The lower part of the Upper Paleozoic section consists of medium- to thin-bedded oolitic and micritic carbonate facies, which grade upward into a mixed carbonate and siliciclastic-rich section (Fig. 4H) interbedded with coal. Upper Paleozoic strata are interpreted as marginal- to shallow-marine and nonmarine deltaic and fluvial rocks and can be broadly correlated from the Ordos Basin (Lan et al., 2016) to the Taihangshan (e.g., exposures in the Qinshui syncline; Fig. 1; Shao et al., 2015).
Triassic–Jurassic rocks overlie the Upper Paleozoic section and comprise a continental siliciclastic section of dominantly sandstone, siltstone, and mudstone (Figs. 3 and 4I). Jurassic strata in the Ningjing syncline—also called the Ningwu-Jingle Basin—consist of Lower Jurassic alluvial-fluvial-deltaic facies, overlain by deltaic-lacustrine strata with interbedded coal seams, and an upper fluvial-eolian section (e.g., Xu et al., 2019). The Jurassic section is interbedded with ca. 179 Ma (Li et al., 2014) and ca. 160 Ma (Xu et al., 2019) tuffs in its lower and upper part, respectively. The uppermost tuff is overlain by ∼200 m of fluvial and eolian facies of the Tianchihe Formation (Figs. 3 and 4I). Workers have proposed that by ca. 168 Ma, the Ningjing syncline (i.e., Ningwu-Jingle Basin) was a synformal depocenter isolated from the Ordos Basin (Li et al., 2014). However, this interpretation is inconsistent with the conclusions of other workers, who argue that the fluvial facies of the Tianchihe Formation were deposited when the Ordos Basin was formerly integrated (Xu et al., 2019) and extended east into the Luliangshan (e.g., Liu et al., 2015). In contrast, correlative sections to the uppermost eolian facies of the Tianchihe Formation are not recognized in the Ordos Basin (Xu et al., 2019). Therefore, it remains unclear when the Ningjing syncline developed into an isolated depocenter as a result of Yanshanian folding and if the uppermost Jurassic rocks of the Ningjing syncline represent preshortening or early syncontractional deposits.
STRUCTURAL GEOLOGY OF THE XIZHOUSHAN AND LULIANGSHAN
The principal basement-involved structures in the Taihang-Luliangshan fold belt include, from southeast to northwest, the eastern Taihangshan thrust faults, Qinshui syncline, Hutuo River anticline, Xizhou syncline, Luliang anticline, Ningjing syncline, and Wuzhai anticline (Fig. 2). Reverse faults in the region generally have <3 km of stratigraphic separation and displace basement rocks against Lower Paleozoic strata. Structures southeast of the Xizhoushan and south of the Fen River in the Luliangshan (Fig. 2) are not addressed in detail here but are presumed to be basement-involved structures based on regional geologic map relationships (SBGMR, 1989; HBGMR,1991) and by comparison to the geology investigated in the Xizhoushan and northern Luliangshan. The largest of these structures is the Qinshui syncline in the Taihangshan, which is characterized by a northeast-southwest axial trace that extends for ∼320 km with Upper Paleozoic and Triassic strata in its core, flanked by basement-involved structures (Fig. 2).
The Xizhoushan is bounded to the northwest by the late Cenozoic Xizhoushan normal fault (Figs. 5 and 6). This range-bounding fault has a throw of <2 km, based on the thickness of Pliocene and younger strata in the Xinzhou-Dingxiang Basin (e.g., Xu et al., 1993). The southwestern boundary of the range is defined by a late Cenozoic northwest-southeast–striking right-lateral strike-slip fault (Zhang et al., 1998).
Within the Xizhoushan, Archean–Paleoproterozoic basement rocks are exposed in the core of the southwest-plunging Hutuo River anticline, named here after the Hutuo River, which bisects the northern Xizhoushan (Fig. 5). Lower Paleozoic strata unconformably overlie basement rocks along the limbs of the anticline, except for local exposures of the Hutuo Group in the northern Xizhoushan (SBGMR, 1989). Lower Paleozoic strata in the limbs of the Hutuo River anticline generally dip shallowly. The region of shallowly dipping beds along the margin of the Hutuo River anticline can be traced for tens of kilometers; however, local minor folds with wavelengths of meters to tens of meters include more steeply dipping beds. Structural measurements indicate that the Hutuo River anticline is an open, upright fold plunging 2° to the southwest (Fig. 7).
The northeast-southwest–trending Xizhou syncline is located northwest of the Hutuo River anticline and extends subparallel to the northwestern escarpment of the Xizhoushan for ∼100 km (Fig. 2). Hutuo Group and Lower Paleozoic strata are folded along the axial trace, and local exposures of Upper Paleozoic strata crop out in the synclinal hinge zone (Fig. 5). The northwestern limb is steeply dipping (>60°) to overturned, in contrast to its shallow dipping southeastern limb, indicating a vergence toward the southeast (Fig. 7C). In the southwestern Xizhoushan, a steeply dipping to overturned stratigraphic panel crops out along strike for >5 km (Fig. 5).
The geologic map of the southwestern Xizhoushan clarifies the contact relationships between the crystalline basement, Hutuo Group, and lower Paleozoic strata (Fig. 6). In the southwestern portion of the map area (Fig. 6), Paleozoic strata unconformably overlie basement rocks, are typically steeply dipping (e.g., 70° in Fig. 8A), and contain clasts of basement rocks directly above the basement nonconformity (Fig. 8B). The contact generally strikes northeast-southwest, except where folded. Map and field relationships suggest that bedding-parallel flexural slip occurs at the contact between rigid basement rocks and less competent, fine-grained lowermost Paleozoic strata (Fig. 8) and is most prominent along overturned and folded segments of the basement-stratal contact. Flexural slip displacement along the basement-stratal contact should not be regarded as evidence of a major southeast-vergent thrust fault because the stratigraphic separation is minimal (<∼20 m), and locations of flexural slip can be traced along strike with the basal Paleozoic unconformity.
In the southwest Xizhoushan, Paleozoic strata unconformably overlie basement rocks. To the northeast, the Paleozoic section rests on Hutuo Group rocks along an angular unconformity (Figs. 6C and 8C). The contact between the Hutuo Group and basement rocks is interpreted as a nonconformity based on regional map relationships (SBGMR, 1989). In the Xizhoushan, the Hutuo Group section is discontinuous along strike, and where absent, it is a manifestation of along-strike stratigraphic variability as opposed to fault omission. The Hutuo Group is typically overturned to the northwest.
The Xizhoushan exposes minor folds with typical wavelengths on the order of <1 m to ∼500 m (Figs. 8F and 8G). When placed in context with the regional geology, even the largest of these folds represent minor folds that are parasitic to the regional basement-cored folds. For example, in the northwestern limb of the Xizhou syncline, an anticline-syncline pair with a wavelength of ∼500 m involves the Hutuo Group and overlying Lower Paleozoic strata (Figs. 6 and 8G). Structural measurements from this anticline and syncline pair (Fig. 7D) indicate a northwest-southeast shortening direction, compatible with compiled attitudes from the regional-scale folds. The fold kinematics for other measured minor fold data in the Xizhoushan (Fig. 8F) are also consistent with approximately northwest-southeast shortening.
The northern Luliangshan is located west-northwest of the city of Xinzhou and the Xinzhou-Dingxiang Basin (Fig. 2). The eastern escarpment of the Luliangshan dips gently southeast toward the Xinzhou-Dingxiang Basin and may be offset by an oblique-slip normal fault along its northeastern margin (Fig. 2; e.g., Shi et al., 2015). Graben-fill isopach maps do not show a thick basinal section adjacent to the eastern Luliangshan (Xu and Ma, 1992) that would be indicative of a major normal or strike-slip fault along the eastern Luliangshan. The northwestern boundary of the Luliangshan is bounded by northwest-dipping normal faults; elsewhere, Paleozoic strata dip shallowly westward into the Ordos Basin (Fig. 2).
The three principal folds in the northern Luliangshan are, from southeast to northwest, the Luliang anticline, Ningjing syncline, and Wuzhai anticline (Fig. 2). The southwest-plunging Luliang anticline consists of basement rocks unconformably overlain by Lower Paleozoic strata, which generally exhibit shallow dips in the southeastern limb and moderate to steep dips (∼30°–70°) in the northwestern limb. The northwestern limb is offset by a southeast-dipping normal fault with a slip separation of ∼2.5 km that juxtaposes basement rocks in the footwall against Lower Paleozoic strata in the hanging wall (Fig. 9A). Steep beds of Lower Paleozoic strata in the northwestern limb are interpreted to be offset by a northwest-vergent thrust fault that emplaces lowermost Paleozoic strata against Upper Paleozoic rocks (Fig. 2). This may be the largest thrust fault in the Luliangshan with ∼2.5–3 km of slip based on the cross-section restoration (Fig. 9).
The basement-stratal contact of the Luliang anticline can be traced around its fold axis in the eastern Luliangshan to southwest of the city of Xinzhou (Fig. 2). Regional map relationships (SBGMR, 1989) suggest that the southern limb of this anticline continues along strike into the Xizhoushan. Hence, the Xizhou syncline represents the southeastern syncline pair to the Luliang anticline. This interpretation implies that the Luliang anticline is a broad, regional-scale anticline with its southeastern limb shared with the Xizhou syncline and its northwestern limb shared with the Ningjing syncline (Fig. 9B).
The Ningjing syncline is characterized by steep (>60°) to shallow (<10°) Lower Paleozoic–Jurassic beds, and an open, concentric fold geometry. Limb bedding measurements indicate that the syncline plunges 5° toward the southwest in the northern Luliangshan (Fig. 7E). Regional map relations south of the Fen River suggest that folded Lower Paleozoic strata and basement rocks comprise a northeast-plunging segment of this fold (Fig. 2), consistent with the interpretation of the Ningjing syncline as a doubly plunging syncline. The Luliang-Ningjing fold pair exhibits ∼9 km of structural relief—the largest of all folds in the region (Fig. 9B).
The Wuzhai anticline, named here after the local county name, has a moderate (∼50°) to steep (>60°) southeastern shared limb with the Ningjing syncline (Fig. 2). The nonconformity between basement rocks and overlying Cambrian sandstone dips ∼50° southeast (Fig. 4E), indicating that basement rocks are folded and comprise the core of the Wuzhai anticline. The northwestern limb dips shallowly to the west toward the Ordos Basin (SBGMR, 1989), suggesting a southeast vergence for the Wuzhai anticline. No major contractional structures were recognized from regional map relationships west of the Wuzhai anticline except for a possible monoclinal limb at the edge of the Ordos Basin, which roughly occurs at the location of the Trans–North China suture (Figs. 1 and 9B).
Previously Published Low-Temperature Thermochronology
Apatite and zircon fission-track (AFT, ZFT) studies from the Taihangshan and Luliangshan have defined periods of exhumation contemporaneous with Yanshanian shortening and post-shortening denudation (Cao et al., 2015; Zhao et al., 2016a). Sample locations pertinent to this study are shown on Figure 2.
AFT samples analyzed by Cao et al. (2015) from the core of the Luliang anticline (Fig. 2) generally yielded central ages between 150 and 120 Ma, and associated inverse time-temperature models suggested (1) fast cooling between 170 and 135 Ma followed by (2) slow cooling until ca. 60 Ma, and (3) renewed cooling between 60 and 35 Ma. AFT samples collected from the Xizhoushan (Cao et al., 2015) generally yielded younger central ages of ca. 80–50 Ma, and modeled samples suggested (1) slow to moderate cooling rates (<3 °C/m.y.) before ca. 60–55 Ma and rapid cooling (∼3–4 °C/m.y.) starting ca. 25 Ma. Late Cretaceous and Cenozoic cooling likely represents denudation of the Taihangshan during opening of extensional basins in the North China Plain and Bohai Bay region (e.g., Allen et al., 1997; Ren et al., 2002; Zhang et al., 2003).
Zhao et al. (2016a) analyzed AFT and ZFT samples from the northwestern limb of the Luliang anticline, the Ningjing syncline, and the flanks of the Wuzhai anticline (Fig. 2). AFT samples from Triassic and Jurassic strata in the Ningjing syncline yielded central ages of 70 ± 5 Ma, indicating that the Mesozoic section was sufficiently buried to reset the detrital apatite grains. Reset ages are interpreted as a consequence of burial by Lower Cretaceous strata correlative to strata in the Ordos Basin (Zhao et al., 2016a). AFT and ZFT data from the flanks of the Luliang and Wuzhai anticlines yielded central ages of ca. 130–120 Ma and are interpreted to be associated with initial shortening in the Luliangshan by ca. 120 Ma, immediately after deposition of the hypothesized Lower Cretaceous section (Zhao et al., 2016a).
Cross-Section Assumptions and Deformation Magnitude Estimates
An ∼410-km-long balanced cross section, measured from the Ordos Basin to the Taihangshan fault (Fig. 9), was constructed using stratigraphic thicknesses for Paleozoic–Jurassic strata from the Ningjing syncline (SBGMR, 1989), which were determined from previously mapped contacts and bedding attitudes acquired during this study. Structural and stratigraphic contacts were based on mapping conducted in this study, previously published provincial geological maps from the Shanxi and Hebei Provinces (SBGMR, 1989; HBGMR, 1991), and correlation to previously documented Yanshanian structures in the international literature (e.g., Zhang et al., 2008).
The line of section was drawn perpendicular to subperpendicular to the major fold axes and structural trends in the Taihangshan and Luliangshan. The balanced section assumes that northwest-southeast shortening postdated the deposition of Lower–Upper Jurassic strata (e.g., Liu et al., 2013) in the Ningjing syncline. The basement-involved folds are characterized as parallel folds with constant bedding attitudes applied to the entire stratigraphic column. Where steep to overturned stratigraphic panels are observed (e.g., Xizhoushan), this deformation is interpreted as a result of fault propagation folding associated with a basement-rooted thrust fault (e.g., Erslev and Rogers, 1993). This interpretation is consistent with field observations of outcrop-scale thrust faults and associated fault-propagation folds (Fig. 8F).
The fault offset across the eastern Taihangshan thrust fault is based on regional mapped relationships (SBGMR, 1989) and a projected fault cutoff assuming the minimum required slip. The Taihangshan fault, bounding the eastern margin of the Taihangshan, is assumed to be a southeast-dipping structure (e.g., Yu and Koyi, 2016), and its total offset is schematic and not accounted for in the sequential restoration.
The cross section assumed a uniform thickness for the Paleozoic–Jurassic stratigraphic column (consistent with those shown in Fig. 3). Variations for the thickness of the Hutuo Group were not considered and the Hutuo Group was grouped into the Precambrian for simplicity. Although variations in stratigraphic thicknesses for the Shanxi Province have been reported (SBGMR, 1989), exposures in the limbs of Ningjing syncline offer the only complete Paleozoic–Jurassic section in the immediate region. Also, the cross section did not assume any section overlying the Jurassic. However, this assumption is likely incorrect. AFT dates of 70 ± 5 Ma from Jurassic strata of the Ningjing syncline suggest that the Jurassic section may have been overlain by sufficient younger strata to reset the detrital apatite grains prior to ca. 70 Ma (Zhao et al., 2016a).
A line-length restoration on Yanshanian fold structures was performed by flattening the Paleozoic–Jurassic stratigraphic section and maintaining equal stratigraphic thickness; no flexural isostasy was applied. The datum selected to flatten the section corresponds to the subhorizontal basement-Paleozoic contact in the region between the Wuzhai anticline and Ordos Basin (Fig. 9B). This region is characterized by very shallow dips and an absence of major structures (Fig. 2) and is assumed to have experienced minimal shortening or extension. Basement rocks that occupy a structural position above the basement-Paleozoic datum (in the pre-extensional section) account for a cross-sectional area of ∼698 km2 across 364 km of section. This is consistent with <∼2 km of basement rock uplift above the datum.
The section serves as a first-order assessment on the style and magnitude of shortening across the Taihang-Luliangshan fold belt. The restoration yields a minimum of 11 km of shortening. The cross section only shows bulk shortening from regional faults and folds such that the estimate does not include contributions from internal strain or small-scale structures. However, our conservative estimate is consistent with a basement-involved style of deformation associated with large-wavelengths folds and an absence of major thrust faults duplicating stratigraphic sections as observed in thin-skinned thrust belts (e.g., Lacombe and Bellahsen, 2016).
The cross section shows ∼2 km of postshortening extension, which does not include slip across the Taihangshan fault. Extension is accommodated across two principal normal faults: the Xizhoushan fault and a southeast-dipping normal fault in the Luliangshan that offsets the northwestern forelimb of the Luliang anticline and emplaces basement rocks against Lower Paleozoic strata. Total slip across the Xizhoushan fault is assumed to be ∼2280 m, based on a predicted fault throw of ∼1800 from the thickest section of basin-fill sediment adjacent to the fault trend (Xu et al., 1993) and a fault dip of ∼58° (Fig. 6B). Slip for the southeast-dipping normal fault in the Luliangshan is ∼2.5 km and was determined from map and structural attitude relations.
Regional Comparisons and Timing of Shortening
A comparison of the Taihang-Luliangshan fold belt of the central Shanxi Province to the Middle–Late Jurassic to Early Cretaceous fold belts of the Yanshan and circum-Ordos region reveals several noteworthy distinctions. (1) Synorogenic Upper Jurassic to Lower Cretaceous growth strata or volcanic rocks are not clearly recognized in the Xizhoushan or Luliangshan, except for the possible uppermost <160 Ma section of the Tianchihe Formation (Fig. 3). We interpret the geographically expansive exposures of Archean–Paleoproterozoic rocks in the region (Fig. 2) as representative of a deeply exhumed basement-involved fold belt where synorogenic strata would be absent. (2) There are no recognized Mesozoic plutonic rocks in the Xizhoushan or Luliangshan that crosscut the shortening structures, or allow for robust determinations on timing, such as in the Yanshan (Davis et al., 2001) and Daqingshan (Davis and Darby, 2010). The absence of unambiguous syncontractional clastic rocks and radiometrically constrained crosscutting relationships underscores the importance of examination of thermochronologic data to define the timing and significance of cooling events (e.g., Cao et al., 2015; Zhao et al., 2016a) in relation to the regional structural geology. (3) Contrasting styles of thick- versus thin-skinned deformation are observed between the Yanshan and circum-Ordos region of the Daqingshan, Helanshan, and Taihang-Luliangshan. The Taihang-Luliangshan is characterized by long-wavelength (∼35–110 km) basement-involved folds, and geological relationships suggest that some of these folds are likely fault-propagation folds because of a steep to overturned limb geometry (e.g., Xizhou syncline); however, major thrust faults exposed at the surface that displace basement rocks over Mesozoic strata are not present. Ramp-flat basement-involved thrust geometries, as identified elsewhere in the circum-Ordos region (e.g., Darby and Ritts, 2007), are absent in the investigated region. (4) Extensional metamorphic core complexes of similar type to the Yungmengshan to the north of Beijing (Davis et al., 1996) or in the Daqingshan (Davis et al., 2002) are not present in the central Shanxi region. This is consistent with the interpretation of Early Cretaceous metamorphic core complexes occupying an ∼east-west trending corridor along the northern margin of North China from the Daqingshan and Yanshan in the west (e.g., Davis and Darby, 2010), to the Liaodong and Jiaodong peninsulas in the east (Fig. 1; e.g., Lin et al., 2006; Yang et al., 2007).
The timing for the initiation of shortening in the Taihang-Luliangshan fold belt remains poorly resolved. Shortening in the Luliangshan has been proposed to have begun as late as ca. 120 Ma (Zhao et al., 2016a); however, this interpretation is at odds with regional timing relationships for Yanshanian deformation, which generally argue for earliest shortening by the Middle–Late Jurassic (Zhang et al., 2008; Dong et al., 2008), and it is also inconsistent with a well-recognized transition from contractional to extensional tectonics in North China at ca. 135–130 Ma (e.g., Wu et al., 2005; Davis and Darby, 2010). Modeled time-temperature paths from AFT data from the Luliangshan suggest that shortening may have commenced as early as ca. 170 Ma (Cao et al., 2015), but these published modeled paths tend to show a wide range of uncertainty for pre-Cretaceous cooling paths (Cao et al., 2015) and therefore do not tightly constrain the timing of Yanshanian-related exhumation.
We tentatively favor the initiation of folding of the Ningjing syncline after deposition of the fluvial-dominated lower section of the Upper Jurassic Tianchihe Formation (Fig. 3). Meandering fluvial deposits of the lower Tianchihe Formation are correlated to more extensive Upper Jurassic strata of the Ordos Basin, which formerly occupied a larger area that included the Luliangshan and presumably predated the formation of the Taihang-Luliangshan fold belt. The age of the base of the Tianchihe Formation is constrained by a 160 ± 1 Ma tuff (Xu et al., 2019). We argue that shortening, development of the Ningjing syncline, and/or topography associated with shortening must have occurred shortly after deposition of this tuff and the overlying fluvial section, but it remains unclear if the uppermost Jurassic eolian facies of the Tianchihe Formation (Xu et al., 2019) represent the earliest syncontractional deposits. However, shortening initiating by ca. 160 Ma is consistent with the timing of other documented structures in the Yanshan fold belt (e.g., Davis et al., 2009) and the circum-Ordos region (e.g., Zhao et al., 2007).
Fission track studies also reveal a slowdown in cooling from the Early to Late Cretaceous (ca. 140–65 Ma) followed by an increase in cooling rates during the Paleocene (e.g., Cao et al., 2015). A slowdown in cooling by the Early Cretaceous is interpreted to represent a significant decrease in erosion rates coincident with a cessation of shortening in the Taihang-Luliangshan fold belt. The later increase in cooling rates overlaps with the age of early extension and basin formation in the North China Plain and Bohai Bay region (e.g., Ren et al., 2002). As such, this cooling signature may represent the erosional exhumation and progressive unroofing of the Taihang-Luliangshan fold belt. Therefore, we place the upper age limit of shortening to the Early Cretaceous, ca. 140–135 Ma, based on the low-temperature thermochronology data and comparison with the upper age limit for shortening in the Daqingshan and Yanshan (see “Middle Jurassic–Early Cretaceous Shortening” section).
Regional Kinematics and Geodynamic Implications
We present a regional kinematic model that accounts for the differences in structural styles across coeval Yanshanian fold belts and the history of arc magmatism along the eastern Asian continental margin (Figs. 10 and 11). A discussion of the magmatic arc of eastern Asia is an important contribution that is rarely discussed in regional syntheses on Yanshanian deformation in North China (e.g., Zhang et al., 2008; Dong et al., 2008; Zhang et al., 2014; Zhai et al., 2016), despite authors commonly linking deformation to the subduction of the Izanagi plate. Our kinematic model divides the Yanshanian Cordilleran-style orogen of North China and the Korean Peninsula into four main domains, including, from southeast to northwest, (1) the Cordilleran-style magmatic arc, (2) an inboard domain of transpression and northwest-southeast shortening, (3) an intracontinental domain of basement-involved northwest-southeast coaxial strain, and (4) the Ordos Basin and its surrounding uplifts (Fig. 11).
The magmatic arc is exemplified by batholithic exposures in the Korean Peninsula. The history of arc magmatism in the Korean Peninsula can be subdivided into three principal stages: (1) a period of calc-alkaline magmatism between ca. 200 and 165 Ma, which swept inboard with time; (2) a magmatic lull between ca. 165 and 120 Ma; and (3) renewed magmatism from ca. 120 to 70 Ma (Kim et al., 2015, 2016). The inboard sweep in magmatism and subsequent 165–120 Ma lull are attributed to progressive shallowing in the subduction angle of the Izanagi plate beneath the Korean Peninsula (Sagong and Kwon, 2005; Kim et al., 2016; Park et al., 2018). This magmatic lull in the Korean Peninsula was coincident with Late Jurassic and earliest Cretaceous volcanic activity in the Yanshan (e.g., Fu et al., 2018, and references therein), suggesting that the locus of magmatism swept inboard into North China. As the Izanagi plate began to rollback starting in the Early Cretaceous, there was a renewal in magmatism by 120 Ma in the Korean Peninsula.
We argue for a causal link between the cessation of arc magmatism at ca. 165 Ma and the initiation of regional Yanshanian shortening between 165 and 160 Ma. This does not preclude the combined influence of shortening associated with closure along the Mongol-Okhotsk suture, intracontinental lithospheric subduction with the Alashan block overriding the Ordos block (Faure et al., 2012), or contraction associated with other far-field effects (e.g., Dong et al., 2008). However, when considering the magmatic lull in the volcanic arc, a coincident increase in the relative plate velocity of the Izanagi plate translating northwest at rates of ∼60 mm/yr to >110 mm/yr during the Late Jurassic to Early Cretaceous (Fig. 11; Müller et al., 2016), and an increase in plate convergence rates (>100 mm/yr by 150 Ma) between the Izanagi plate and North China (Liu et al., 2017), the subduction kinematics of the Izanagi plate are the most compelling first-order mechanism to explain the onset of regional Yanshanian orogenesis and intracontinental shortening.
A domain of transpressional strain was active northwest of the extinct volcanic arc and east of the Taihangshan fault. Plate circuit reconstructions postulate that from 160 Ma to 140 Ma, the trajectory of the Izanagi plate became increasingly oblique to the trace of the Izanagi plate boundary (Fig. 11; Müller et al., 2016). This oblique convergence may have been accommodated by left-lateral slip along preexisting north-northeast–striking faults (e.g., Zhu et al., 2010; Zhao et al., 2016b); however, this transpression may also be explained by a regional northwest-southeast greatest compressive stress (σ1) acting on these preexisting fault lineaments (Fig. 10). Transpression is best exemplified by deformation along the Tanlu and Cangxian fault systems (Fig. 11; e.g., Wang, 2006). Fault reactivation may have also played an important role in the development of the Yanshan fold belt, where pre-Yanshanian east-west faults (e.g., Wang et al., 2013) associated with latest Paleozoic and earliest Mesozoic orogenesis (e.g., Yang et al., 2006) were reactivated beginning in the Middle to Late Jurassic. Reactivation of these preexisting east-west faults in the Yanshan is compatible with right-lateral shear by regional northwest-southeast shortening (Fig. 10; Faure et al., 2012).
The Taihang-Luliangshan fold belt represents a region of coaxial northwest-southeast shortening (σ1 = ∼315°/135°; Figs. 7 and 10) located ∼1000–1200 km inboard of the coeval Izanagi subduction plate boundary (Fig. 11). There is no clear evidence from the Taihang-Luliangshan fold belt that supports major transcurrent deformation or fault reactivation across east-west–striking lineaments. The major basement-involved folds are characterized by wavelengths of ∼35–110 km and structural reliefs of ∼1–9 km. Analogous Cordilleran-style systems characterized by regions of intracontinental basement-involved deformation include the Late Cretaceous–Paleogene “Laramide” orogenic belt of the Western United States (e.g., Dickinson and Snyder, 1978; Yonkee and Weil, 2015, and references therein). The Laramide orogeny is interpreted to be the result of flat-slab subduction under the North American plate coincident with a lull in arc magmatism and far-field shortening driven by an end load exerted by the flattening slab (e.g., Axen et al., 2018) or stress transmitted from the top of the shallowly subducting slab into the overlying continent (Bird, 1988). Laramide basement-involved uplifts of the Rocky Mountain region are considerably larger than those of the Taihang-Luliangshan. Laramide structures in the Rocky Mountain region are generally characterized by wavelengths of ∼60–300 km, with an average of ∼190 km (Tikoff and Maxson, 2001), and the largest uplifts (e.g., Wind River Arch) are bound by thrust or reverse faults with offsets of ∼15–20 km (e.g., Marshak et al., 2000; Yonkee and Weil, 2015), which place basement rocks over synorogenic strata. In contrast, thrust or reverse faults in the Taihang-Luliangshan have offsets of <∼5 km and do not emplace basement rocks over synorogenic strata. The Laramide uplifts (or monoclines) of the Colorado Plateau (e.g., Davis and Bump, 2009) may serve as a better analog. Both regions are characterized by reverse fault displacements of <∼5 km with Paleozoic and Mesozoic strata folded over faulted basement rocks (Davis and Bump, 2009). In contrast with the Colorado Plateau where Paleozoic and Mesozoic strata are predominantly exposed at the surface, erosion breached the stratigraphic cover of the Taihang-Luliangshan to reveal expansive exposures of the underlying basement.
West of the Taihang-Luliangshan, the Ordos Basin represents an intracratonic basin with a complex history involving Yanshanian and pre-Yanshanian basin subsidence (e.g., Liu et al., 2013; Ritts et al., 2009) and protracted deformation along its periphery (e.g., Darby and Ritts, 2002). Yanshanian northwest-southeast shortening in the Helanshan and Zhuozishan is kinematically compatible with shortening within the Taihang-Luliangshan fold belt; however, the magnitude of shortening in the Helanshan, estimated to be at least ∼30 km (Darby and Ritts, 2002), exceeds the low magnitude of shortening within the Taihang-Luliangshan (∼11 km). This difference is corroborated by the development of a Late Jurassic to Early Cretaceous flexural foredeep to the west (e.g., Zhang et al., 2009b), adjacent to a greater topographic load from the Helanshan and Zhuozishan fold belt (Fig. 11).
The Taihang-Luliangshan fold belt of North China consist of basement-involved folds formed during the Middle–Late Jurassic to earliest Cretaceous Yanshanian orogenic event. Basement-involved folds were formed by northwest-southeast shortening and are characterized by wavelengths on the order of 35–110 km and structural reliefs as high as ∼9 km. Mapping in the central Shanxi Province demonstrated that some of these folds, such as the Xizhou syncline, have axial traces that extend for >100 km and are characterized by prominent overturned limbs with minor parasitic folds as part of the regional-scale structure. The overturned limb geometry of the Xizhou syncline suggests that this fold is a fault-propagation fold associated with a basement-involved thrust that is likely buried in the adjacent rift basin or was extensionally reactivated to form the Xizhoushan fault. We estimate a minimum of ∼11 km of shortening across the Taihang-Luliangshan fold belt.
The Taihang-Luliangshan fold belt formed in an intracontinental setting at a distance of ∼1000 km or more from the trench of the subducting oceanic Izanagi plate. We propose that the principal structural domains of the Yanshanian orogenic belt can be broadly defined as a region of left-lateral transpressional deformation inboard of a Cordilleran-style magmatic arc, right-lateral transpression along the Yanshan belt, and coaxial strain along the eastern and western margins of the rigid Ordos Basin. The timing of shortening in the Taihang-Luliangshan fold belt is unresolved, but we tentatively interpret shortening to have commenced by ca. 160 Ma. This onset of shortening is temporally consistent with a shutoff in arc magmatism along the eastern Asian Cordilleran-style arc by ca. 165 Ma, which was likely associated with the shallowing of the subducting Izanagi plate. We propose that flat-slab subduction is the best explanation for northwest-southeast basement-involved deformation in the Taihang-Luliangshan fold belt and widespread intracontinental shortening in North China.
We would like to thank Lin Ding and Fulong Cai from the Institute of Tibetan Plateau Research, Chinese Academy of the Sciences, for their invaluable logistical support and assistance during our trips to China. We also thank Houqi Wang and Yao Wei for their assistance in the field, which allowed us to complete this research. We also thank two anonymous reviewers for their comments and suggestions, which helped to significantly improve the figures and maps. This research was funded by National Science Foundation Office of International Science and Engineering (OISE) grant 1545859.