Deformation constraints of oceanic subduction and closure: A case study of the Enger Us ophiolitic mélange and its foreland in the middle section of the Central Asian Orogenic Belt

The subduction and closure of the Paleo-Asian Ocean led to the formation of the Central Asian Orogenic Belt (CAOB), but the detailed direction of subduction, results of collision, and subsequent intracontinental deformation have not been effectively distinguished in the CAOB. This study takes the Enger Us ophiolitic mélange in the southern margin of the CAOB, which is currently one of the best exposed ophiolitic mélanges in the orogenic belt, as an example. Through detailed mapping, the internal composition and structures of the mélange were determined. The S-C structures and ductile shear deformation showed that the ancient ocean obliquely subducted left-laterally to the south-southwest, leading to strain partition. This is the first report of strain partition during the subduction of oceanic plate in the central and eastern CAOB. The left-lateral shearing was mainly distributed in the southern part of the mélange, while the west-northwest−east-southeast−trending shortening structures mainly formed in the central and northern parts. The overall north-south shortening deformation of the mélange, together with the Late Permian deformation to the north of the mélange constitutes a foreland fold and thrust system caused by the collision. The southern margin of the middle CAOB underwent three stages of deformation: late Paleozoic subduction and accretion, Late Permian collision, and latest Permian−Early Triassic intracontinental shearing. The closure of the Paleo-Asian Ocean at the southern margin of the CAOB occurred in the Late Permian. The mapping of ophiolitic mélanges can effectively distinguish subduction, collision, and intracontinental deformation, and reconstruct the history of ocean and continent transitions.