Emplacement of the ophiolitic mélanges in the west Karamay area: Implications for the Late Paleozoic tectonic evolution of West Junggar, northwestern China

The Darbut and Karamay ophiolitic mélanges in the west Karamay area at the southeastern margin of West Junggar, southern Altaids, represent the relict fragments of the Paleozoic Junggar ocean basin. The mélanges contain structurally complex tectonic blocks of ultramafic rocks, gabbro, basalt, and chert that are dispersed within an extensively sheared serpentinite matrix. This study presents new field observations, structural analyses, geochronological and geophysical data for the mélanges and the overlying Carboniferous strata. Three stages of deformation (D1–D3) have been identified. D1 is only locally preserved in the mélanges and is characterized by thrust-imbricated faults, recording the subduction and accretion of the paleo-oceanic crust in the Early Carboniferous. D2 mainly consists of map-scale upright folds (F1) in the Carboniferous strata and NE-SW-striking sub-vertical right-lateral shear zones in the mélanges. D3 is characterized mainly by NE-SW-striking transcurrent left-lateral strike-slip faults and “S-shaped” drag folds (F2). The extensive tectonic reworking during D2 and D3 deformations eradicated most of the D1 structures. Our structural observations, combined with geophysical data, suggest that the basement of the west Karamay area consists of trapped oceanic crust or dismembered ophiolitic fragments and that the Darbut and Karamay ophiolitic mélanges were extruded through right-lateral shear zones. Our results and the published data further suggest that the emplacement of the mélanges and the formation of F1 folds represent regional-scale amalgamation processes between different tectonic blocks and/or plates in the Late Carboniferous. These processes resulted in the closure of the West Junggar remnant ocean basin and other sub-oceanic basins in the Paleo-Asian Ocean, leading to the formation of the Kazakhstan orocline. Furthermore, the D3 deformation was associated with anticlockwise rotation of the West Junggar region in a post-collisional setting in the Permian.