Structural characteristics of the Saozishan Formation in the Hongshishan area of the Gansu Beishan and its dynamic background

The Saozishan Formation is developed in the Gansu Beishan area, formed in the Early Carboniferous, as an important part of the Hongshishan ophiolitic mélange, which records and preserves the important information about the formation and evolution of the Hongshishan ocean, also an important window to recover the Late Paleozoic paleogeographic environment and property of the ocean basin. According to the field geological investigation, the strata were strongly deformed, and it is very difficult to recognize the primary sedimentary sequence and also to study on the original depositional environment. Therefore, based on the identification of the material composition of the Saozishan Formation, this study determines its deformation sequences through detailed structural analysis, and restores its original sedimentary bedding by excluding later structural modifications. It is shown that the Saozishan Formation has undergone three periods of deformation, in which the first period of deformation is characterized by top-to SSE thrusting with left-lateral strike-slip motion, possibly as the products of the northward subduction of the oceanic crust in the early period; the second period of deformation is characterized by NNE−SSW compression with right-lateral strike-slip motion, possibly as the products of the collision stage; the third period of deformation shows NE−SW trending left-lateral strike-slip motion. Combined with the deformation and metamorphic characteristics, the Saozishan Formation shows characteristics of middle-shallow level deformation, the composition of the Saozishan Formation shows an obvious regularity in the north-south spatial distribution, indicating that the volcanic-sedimentary characteristics gradually matured towards the south of the rift basin. Together with the Carboniferous to Permian arc igneous rocks on the north side of the Hongshishan-Pengboshan fault, the dynamic characteristics of the northward subduction of the Hongshishan ancient ocean basin are revealed, which also provides geological evidence for further study of the tectonic evolution of the southern margin of the Central Asian Orogenic Belt.