Ordovician tectonic shift in the western North China Craton constrained by stratigraphic and geochronological analyses

Abstract

The western North China Craton (W-NCC) comprises the Alxa Terrane in the west and the Ordos Block in the east; they are separated by the Helanshan Tectonic Belt (HTB). There is an extensive debate regarding the significant Ordovician tectonic setting of the W-NCC. Most paleogeographic reconstructions emphasized the formation and rapid subsidence of an aulacogen along the HTB during the Middle-Late Ordovician, whereas paleomagnetic and geochronologic results suggested that the Alxa Terrane and the Ordos Block were independent blocks separated by the HTB. In this study, stratigraphic and geochronologic methods were used to constrain the Ordovician tectonic processes of the W-NCC. Stratigraphic correlations show that the Early Ordovician strata comprise ~500-m-thick tidal flat and lagoon carbonate successions with a progressive eastward onlap, featuring a west-deepening shallow-water carbonate shelf. In contrast, the Late Ordovician strata are composed of ~3,000-m-thick abyssal turbidites in the west and ~400-m-thick shallow-water carbonates in the east, defining an eastward-tapering basin architecture. Early Ordovician detrital zircons with ages of ~2,800-1,700 Ma were derived from the Ordos Block; the Late Ordovician turbidites were sourced from the western Alxa Terrane, based on zircon ages clustered at ~1,000-900 Ma. The petrographic modal composition and zircon age distribution imply a provenance shift from a stable craton to a recycled orogen in the Middle Ordovician. These shifts define a tectonic conversion from a passive continental margin to a foreland basin at ~467 Ma, resulting in the eastward progradation of the turbidite wedge around the HTB, the eastward backstepping of the carbonate platform in the east and the eastward expansion of orogenic thrusting in the western Alxa Terrane. This tectono-sedimentary shift coincided with the advancing subduction of the southern Paleo-Asian Ocean beneath the Alxa Terrane, generating the western Alxa continental arc and the paired retro-arc foredeep in the east under a compressional tectonic regime.

Permian tectonic evolution of the southwestern Ordos Basin, North China: Integrating constraints from sandstone petrology and detrital zircon geochronology

“…The underlying Cambrian and Lower Ordovician carbonate-dominated sequences represent a shallow-water carbonate shelf setting, which was followed by the late Ordovician sedimentation of intraclastic limestones within thick black shale (Sun & Dong, 2019c). This evolving depositional environment documents a deep-water carbonate slope system along the western margin of the Ordos marine platform (Wu et al, 2018;Zhao, Liu, Zhao, Wang, & Zhang, 2017), recording the foreland basin evolution since the Middle Ordovician (Sun & Dong, 2020). In response to the Ordovician to Devonian collision along the CCOS to the south, the southwestern Ordos Basin was uplift and eroded from the end Ordovician to the early Carboniferous, recorded by a regional straightigraphic gap (Chen & Liu, 1995Sun & Dong, 2019c;Wang, Zhang, & Chen, 2001).…”

“…Its western segment discussed in this study is defined by the Taihang Mountain to the east, including the Ordos Basin in the east and the Alxa Massif in the west. A striking tectonic feature of the W‐NCC is the development of the North‐South orientated Helan‐Liupanshan Tectonic Belt (Sun & Dong, 2020; Yang & Dong, 2018; Zhang, Ma, & Ren, 2004; Zhao, Liu, et al, 2020), which demarcates the border intervening the Ordos Basin and the Alxa Massif in the north and the Qilian orogen in the south (Figure 1). Lying in the junction between the Qilian‐Qiling Orogenic Belt and the Ordos Basin is the LPTB, which has been correlated as the southward extension of the western Ordos fold‐thrust zone (Chen, Hou, Tian, Liu, & Zhang, 2001; Tang, Feng, & Li, 1990; Tang, Guo, & Wang, 1988; Zhang, 2002).…”

“…The CCOS comprises the Qinling–Dabie orogen in the east and the Qilian orogen in the west as a consequence of multi‐stage subduction‐collision processes between the south and north China blocks from the Palaeozoic to Triassic times and the subsequent Mesozoic to Cenozoic intracontinental deformations (Dong et al, 2008, 2011, 2015). The HCTB was inherited from complex Precambrian tectonic imprints, and evolved into the margin of the Palaeo‐Asian and Palaeo‐Tethys Oceans in the Palaeozoic (Dong & Santosh, 2016; Sun & Dong, 2019a, 2020). This orogen was intensively overprinted by the following Mesozoic to Cenozoic intracontinental orogenic events, leading to the final construction of the North‐South trending HCTB (Dong, He, et al, 2018; Dong et al, 2019; Yang & Dong, 2018).…”

Middle Ordovician mass‐transport deposits from western Inner Mongolia, China: Mechanisms and implications for basin evolution

“…The first stage in the late Middle Ordovician tectonic transition of the western North China Block involved late Darriwilian regional foundering of the Zhuozishan Formation carbonate platform, possibly as an initial stage of foreland basin development along a convergent continental margin between the North China Block and eastern Gondwana (Wang et al ., 2016; Wang & Wang, 2018; Yang et al ., 2019; Sun & Dong, 2020). The rapid transition from carbonate platform deposits to the ribbon rock and marl strata of the Kelimoli Formation (Fig.…”

Zircon U–Pb Dating of Tuff Layers in the Middle–Upper Triassic Strata of Successions and Sedimentation of Chang 7 Member in the Ordos Basin, China

“…The Yanchang Formation is a significant oil-bearing sequence, including delta, fluvial, and lacustrine facies, which deposits from the Middle Triassic to the Late Triassic. It consists of 10 members, with numbers of Chang 1 to Chang 10 upward, according to rock assemblages (Sun and Dong, 2019a;Sun and Dong, 2019b;Sun and Dong, 2020). A series of tuff layers were found in the Yanchang Formation (Figure 2, outcrop and drill core).…”