Elsevier

Tectonophysics

Volume 801, 20 February 2021, 228713
Tectonophysics

Middle Jurassic orogeny in the northern North China block

https://doi.org/10.1016/j.tecto.2020.228713Get rights and content

Highlights

  • Middle Jurassic orogeny was a strong shortening event in the northern North China block.

  • Shortening occurred in a short timespan from ~170 to 165 Ma.

  • Thrusting and folding are thin-skinned and verge predominantly northwestward.

  • Flat subduction of the paleo-Pacific plate might trigger the Middle Jurassic orogeny.

Abstract

Multiple shortening took place during the Mesozoic in the periphery of the North China block (NCB), a major tectonic grain of East Asia, but no consensus exists as to the timing, intensity, and causes of individual events. This work focuses on the first phase shortening in Jurassic, which represents the initiation of Yanshanian orogeny, by investigating the structural evolution of the Yanshan belt in the middle segment of the northern NCB. The first-phase shortening used to be regarded as a weak compressive event for it is recorded by parallel or low-angle unconformities between Middle and Upper Jurassic strata in some places. Our recent field investigations, however, identified some pronounced angular unconformities separating the Upper Jurassic strata from the underlying highly folded units in the northern Yanshan belt. The new observations demonstrate that the first-phase shortening was actually a strong contractional event. The co-existence of parallel and angular unconformities in the southern and northern Yanshan belt can be explained by different thrusting processes. It is argued that lateral variations of initial basement dips must have played a crucial role in controlling structural evolution of the overlying pre-Middle Jurassic sedimentary succession. It is shown that the first-phase thrusting and folding system was thin-skinned and predominantly verge northwestward. Deformational intensity also weakened toward the north and northwest. The first-phase shortening is constrained to occur in a short time span from 170 to 165 Ma based on new radiometric ages of strata above and below the unconformity, and presumably took place simultaneously in the northern NCB. The first-phase shortening was likely driven by horizontal tectonic push from the southeast. It is suggested that low-angle subduction of the paleo-Pacific plate might be responsible for the Middle Jurassic orogeny in the northern NCB and other parts of East Asia.

Introduction

The North China block (NCB), also called the North China craton, had kept its tectonic stability as a whole from the Mesoproterozoic to early Mesozoic although its peripheral zones experienced strong crustal deformation because of sequential terrane accretions on the north and the NCB–SCB collision on the south (Cui et al., 2000; Li, 1994, Li, 2013; Liu et al., 2003, Liu et al., 2013; Meng et al., 2019; Zhao et al., 2010). Late Mesozoic saw a period when the NCB was affected by episodic contraction and extension (Davis et al., 2001; Li et al., 2004; Ma et al., 2002; Meng et al., 2019; G. Zhu et al., 2018). Destabilization of the NCB is attributed primarily to Early Cretaceous lithospheric rifting induced by high-angle subduction of the western paleo-Pacific plate (cf. Fan et al., 2000; Zhu et al., 2012). Two shortening events preceded the Early Cretaceous extension, as recorded by two regional angular unconformities in the Mesozoic successions in the northern NCB. The two unconformities separate the Upper Jurassic from the Lower-Middle Jurassic and Lower Cretaceous sequences, respectively (Meng, 2017). Wong (1927) coined the term “the Yanshanian Movements” to specify the intracontinental orogenesis in the Yanshan belt in the northern NCB.

Mesozoic multiple tectonic processes led to structural complexity of the northern NCB, as recorded by the superposition of Jurassic fold-thrust systems and Early Cretaceous extensional basins and metamorphic core complexes (Davis et al., 2001; C. Li et al., 2016). The fold-thrust systems in the northern NCB can be divided into three segments from west to east, the Yinshan, Yanshan and Liaoxi belts, respectively (Fig. 1a). Contractional deformations have been extensively investigated, and a variety of tectonic models were proposed to explain the structural development (Chen, 1998; Davis et al., 1998, Davis et al., 2001; Li et al., 2016; Wang et al., 2018; Yan et al., 2006; Zhang et al., 2011; Zheng et al., 2000). Chen (1998) proposed a thick-skinned model to explain the structural evolution of the Yanshan belt in view of the involvement of the Archean-Paleoproterozoic crystalline basement rocks. Davis et al., 1998, Davis et al., 2001 favored thin-skinned fold-thrust tectonics and inferred large-magnitude displacement of thrust sheets from south to north. Li et al. (2016) conducted restoration of contractional structures of the Yanshan belt and argued that out-of-sequence thrusting might have been dominant in Mesozoic deformational processes. Uncertainties remain as to the timing of the onset of the Yanshanian orogeny, duration of individual shortening events, spatial variation of deformational styles, and mechanisms for cyclic shortening in the northern NCB.

The first-phase shortening was thought to have happened prior to ~180 Ma (Davis et al., 2001; C. Li et al., 2016; Zheng et al., 2000), but it was recently shown that thrusting should not have commenced until the Middle Jurassic (Meng et al., 2019). Opinions also dispute on the duration of the first-phase shortening, which was assumed to have persisted from 170 to 135 Ma according to the timing of contractional structure (Dong et al., 2015), from 175 to 165 Ma based on thrust faults and unconformity (C. Li et al., 2016) or from 170 to 165 Ma through regional unconformity and basin evolution process (Meng et al., 2014). We believe that differences in study regions, dating methods and research specializations have led to inconsistent perceptions of the deformation timing. The Yanshan belt displays marked lateral variations in structural styles and deformation intensity, but no consensus has been reached on how the discrepancies were generated (cf. Faure et al., 2012). Different mechanisms were proposed to account for late Mesozoic intracontinental deformation of the NCB, such as flat subduction of the paleo- Pacific plate during Late Jurassic (Zheng et al., 2000; G. Zhu et al., 2018), and far-field effect of the closure of the Mongol-Okhotsk ocean in the north from Jurassic to Early Cretaceous (Davis et al., 1998; Zhao et al., 2004a) and combined effects of multiple terrane accretion from many directions during 170–136 Ma (Dong et al., 2018). In summary, there are still different understandings on the timing, deformation process and dynamic mechanism of the Yanshanian orogeny.

Previous work mostly focused on the young-generation or end-Jurassic shortening because the resulting structures are well preserved (Davis et al., 2001; Yan et al., 2006; Zhang et al., 2006, Zhang et al., 2011). The first-phase structure and deformational processes are comparatively less understood because of strong modification of later tectonic activities. It is often assumed that the first-phase contraction was weak and merely led to long-wavelength folding (Ting, 1929; Wong, 1929; Zhao, 1959; Zhang et al., 2013). This view was largely based on the observations in the southern part of the western Yanshan belt or the Xishan region where the Middle-Upper Jurassic boundaries are parallel or low-angle unconformities (Bao et al., 1983; Wong, 1927; Zhang et al., 2013).

This study concentrates on the first-phase deformation of the Yanshan orogeny in the western Yanshan belt in that complete Jurassic stratigraphic sequences and first-phase structures are well preserved and unexceptionally exposed. The approaches include our new observations of key stratigraphic contacts and their spatial variations, structural analysis of contractional deformations, and geochronologic constraints on crustal shortening through our newly obtained isotope chronology data of the strata above and below regional unconformities. This study focuses on the first-phase fold-thrust tectonics in the western Yanshan belt, but we also investigated coeval shortening in the eastern Yanshan belt to understand deformational processes in the whole northern margin of the NCB. A tectonic model is accordingly advanced to illustrate how distinct deformations were generated in the southern and northern Yanshan belt and what were the possible causes triggering Middle Jurassic orogeny in the northern NCB.

Section snippets

Geologic setting

The Yanshan fold-thrust belt has long been recognized as an intracontinental orogen built up by multiple contractional events in the Mesozoic (Cui et al., 2002; Ge, 1989; Guo et al., 2002; Song, 1999; Zhang, 1999, Zhang, 2008). Episodic shortening was evidenced by the superposed deformation and two regional unconformities below and above Upper Jurassic strata (Davis et al., 2001; Meng et al., 2014). The two unconformities are called Unconformity A and Unconformity B (Fig. 2), result from the

Stratigraphy

The northern NCB shares similar late Paleoproterozoic-Paleozoic stratigraphy to that of its interior but displays distinct Mesozoic lithostratigraphic sequences (Meng et al., 2019). The salient feature of Mesozoic successions in the Yanshan belt is the repeated occurrence of volcanic and volcaniclastic intervals, as represented by Lower Jurassic Nandaling basalts, Upper Jurassic Jiulongshan-Tiaojishan andesite and ignimbrite, and Lower Cretaceous Zhangjiakou rhyolite. Proterozoic successions

Unconformities

Several stratigraphic discordances exist in Proterozoic-Mesozoic sequences, marked by either parallel or angular unconformities. The discordant surfaces in the pre-Middle Jurassic successions prove mostly disconformities in nature (Meng et al., 2014), which are present below the Neoproterozoic, Cambrian, Upper Carboniferous, Triassic, and Lower-Middle Jurassic units, respectively (Fig. 2). The making of these disconformities was thought of as the consequence of eustatic change or vertical

Structure

The present structural framework of the Yanshan belt was shaped by the second-phase contraction, which can be readily perceived by the involvement of Upper Jurassic sequences in the fold-thrust belts. Examples include the NE-SW-trending Baihuashan-Tiaojishan synclinorium in the Xishan region (Fig. 3), the NW-verging Jimingshan and Huangyangshan thrust faults that displaced Mesoproterozoic Wumishan dolostone onto the Upper Jurassic Jiulongshan and Tiaojishan formations in the Xuanhua region (

Timing of first-phase shortening

Timing of the first-phase shortening has been a matter of debate. Some researchers considered that conglomeratic facies are an indication of contractional events and can be used to constrain the timing of crustal shortening. The Longmen Formation, which contains many conglomeratic intervals, was thought of as deposits of compressive basins in the Xishan region (Zhao et al., 2002). However, synchronous volcanic and volcaniclastic interlayers throughout the Longmen succession conflict with the

Deformational processes

As mentioned above, the Unconformity A manifests itself as a parallel unconformity in the Xishan region and an angular unconformity in the southern Xuanhua region, and then loses its identity northwards. No satisfactory interpretations have been provided yet to account for the spatial change of the stratigraphic contacts. The making of disconformities is usually attributed to crustal vertical movement, whereas angular unconformities mostly originate from crustal horizontal compression. The

Tectonic implications

It was once regarded that Mesozoic folding and thrusting initiated since the Late Triassic in the Yanshan belt and propagated southward from hinterland to foreland (Li et al., 2016; Wang et al., 2013b). Late Mesozoic shortening was then featured by out-of-sequence thrusting (Li et al., 2016). Meng et al., 2014, Meng et al., 2019, however, demonstrated that thrusting might not start until the Middle Jurassic (Fig. 2). This work shows that the first-phase thrusting was mainly north-directed or

Conclusions

This study arrived at the following conclusions:

  • (1).

    the northern NCB experienced prominent crustal shortening in the late Middle Jurassic, challenging the previous view that the first-phase deformation was mild compared with subsequent contractional events.

  • (2).

    Middle Jurassic structural evolution of the western Yanshan belt was likely controlled by initial tapers of Proterozoic rift sedimentary wedge, with the high-tapered prism forming the dominant single thrust sheet in the south and the low-tapered

Declaration of Competing Interest

None.

Acknowledgments

Guowei Zhang is thanked for the enlightening discussions in the fieldwork. G.-L. Wu is grateful to Diying Huang for his suggestions about regional stratigraphic correlation. This work is supported by the National Key Research and Development Program of China (Grant 2016YFC0600406), the National Natural Science Foundation of China (Grant 41702237), and by the Strategic Priority Research Program (B) of the Chinese Academy of Sciences (XDB18030103).

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