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

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, similar to 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, similar to 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 (40Ar/39Ar 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.