Triassic Nappe in the Central Part of the Southern Central Asian Orogenic Belt (Ejinaq, NW China): Evidence from Structural Analysis and Geothermochronology

Abstract

The thrust nappe played an important role in the Mesozoic tectonic evolution of the middle part of the Central Asian Orogenic Belt (CAOB). However, the timing, structural style and kinematic processes of the thrust nappe remain controversial, particularly the detail of the thrust nappe in the Guaizihu region (110 km east of Ejinaq). In this study, we investigate new field mapping, seismic sections, geochronology and low-temperature thermochronometric dating to provide constraints on the history of this thrust nappe in the Chaheilingashun area (northwestern Guaizihu region). The field mapping, seismic sections and structural analysis reveal that the autochthonous system had developed a series of strong fold structures in the upper Permian strata. The allochthonous system mainly contains Devonian monzogranite (U-Pb age, ranges from 386.7 to 389.0 Ma) and Meso-Neoproterozoic schists (the maximum depositional age, ~880 Ma), which were thrust upon the upper Permian strata during Middle to Late Triassic. Based on similar rocks, geochronological dating and the Yagan thrust, we suggest that the postulated root zone of this allochthon might have originated from the Huhetaoergai area (40-60 km northwest of the study area). The geochronological results reveal that the lower age limit of this thrust nappe is constrained by the Lower-Middle Triassic syntectonic sediments (tuffaceous sandstone, ~247 Ma), which is the sedimentary response of the fold structure.,The timing of the termination of this thrust nappe is defined by the cooling age ( 40 Ar/ 39 Ar data, 217-211 Ma) of the Devonian monzogranite and Meso-Neoproterozoic schists. Thus, we consider this thrust event in the study area to potentially have occurred in the period from 247 Ma to 211 Ma, which may represent the tectonic response to the closure of the Paleo-Asian Ocean.

Early Cretaceous Thrust and Nappe Tectonics in North Qilian Shan, Northern Tibetan Plateau: Evidence from Field Mapping, Geochronology, and Deep Structural Analysis

“…2a). In map view, the NQTS could be divided into three belts, i.e., the root, middle, and front belts (Chen et al, 2019b;Su et al, 2023). South to the Sunan Town, the NQT is considered as the root belt of the thrust and nappe system, while its hanging wall in the North Qilian Shan is regarded as the rear of the system.…”

Differential deformation mechanism of E–W‐trending Weiningbeishan fold‐thrust belt from central Ningxia, NW China

“…However, the formation of the E–W‐trending folds in the Zhuozishan and Shitanjing areas during the Middle–Late Triassic is probably related to the closure of the Paleo‐Asian Ocean to the north (Su et al, 2023; Yu et al, 2023; Zhang, Li, et al, 2009). For the same reason, some nappe structures were formed in the southern Central Asian Orogenic Belt (Su et al, 2023; Yu et al, 2023) during the Middle–Late Triassic.…”

“…After the closure of the North Qilian Ocean during the Late Ordovician-Early Silurian (Chen et al, 2022;Song et al, 2013;Xu et al, 2015;Yu et al, 2021) and the collision between the Central Qilian and Alxa blocks during the Silurian-Middle Devonian, the WFTB and its surrounding areas likely experienced post-orogenic extension during the Late Palaeozoic (Huo et al, 1989;Li, 2006;Song et al, 2013;Zhang, 2019). During the Middle-Late Triassic, due to combined effect of the collision between the South China and North China plates to the south (Dong et al, 2011;Li, 2006;Yang & Dong, 2020;Yin & Nie, 1993;Zhang et al, 2001 and the closure of the Paleo-Asian Ocean to the north (Song et al, 2021;Su et al, 2023;Yu et al, 2023;Zhang, Li, et al, 2009), a large number of the E-W-trending folds and thrust faults were formed in the WFTB and its surrounding areas. In the later period, the WFTB and its surrounding areas were modified by Yanshanian and Himalayan intracontinental deformation to a certain extent (BGMN, 1990;Cheng, 2019;Craddock et al, 2011;Guo et al, 2015Guo et al, , 2016Huo et al, 1989;Li, 1999Li, , 2006Min et al, 2023;Shi et al, 2006;Yang & Dong, 2020;Zhang, 2007;Zhang et al, 1991Zhang et al, , 2010Zhang & Dong, 2019;Zheng et al, 2006).…”

“…This nearly N-S compression in the Middle-Late Triassic is probably related to the collision between the South China and North China plates and the closure of the Mianlue Ocean to the south. However, the formation of the E-W-trending folds in the Zhuozishan and Shitanjing areas during the Middle-Late Triassic is probably related to the closure of the Paleo-Asian Ocean to the north (Su et al, 2023;Yu et al, 2023;Zhang, Li, et al, 2009). For the same reason, some nappe structures were formed in the southern Central Asian Orogenic Belt (Su et al, 2023;Yu et al, 2023) During the Middle Jurassic-early Early Cretaceous, the multiplate convergence tectonic system was forming in East Asia (Zhang & Dong, 2019).…”

“…However, the formation of the E-W-trending folds in the Zhuozishan and Shitanjing areas during the Middle-Late Triassic is probably related to the closure of the Paleo-Asian Ocean to the north (Su et al, 2023;Yu et al, 2023;Zhang, Li, et al, 2009). For the same reason, some nappe structures were formed in the southern Central Asian Orogenic Belt (Su et al, 2023;Yu et al, 2023) During the Middle Jurassic-early Early Cretaceous, the multiplate convergence tectonic system was forming in East Asia (Zhang & Dong, 2019). In the Daqingshan area of the northern margin of Ordos Basin, NNW/N-directed thrust sheet was developed during the Late Jurassic-Early Cretaceous (Darby et al, 2001;Zheng et al, 1998), which is probably related to the closure of the Mongol-Okhotsk Ocean.…”