Multiple phases of deformation in the southern Helanshan tectonic Belt, northern China

The Southern Helanshan Tectonic Belt (SHTB) is surrounded by the Alxa Massif, the Ordos Basin and the Tibetan Plateau, and its Phanerozoic structures records crucial Phanerozoic deformation characteristics for studying the multi-phased intracontinental deformation processes among these neighboring units. Combined with regional geology, our structural analysis reveals that the SHTB has experienced five phases of shortening deformation during the Phanerozoic. The first phase (D-1) is represented by E-W-trending overturned folds in the Cambrian and Ordovician strata. These folds resulted from two episodes of N-S shortening during the Late Cambrian-Early Ordovician and Late Ordovician-Early Devonian respectively, which were attributed to the closure of the Qilian ocean. The second phase (D-2) is characterized by E-W-trending open folds involving the Upper Devonian strata, which was the products of N-S shortening caused by the assembly of the Qilian ocean and the North China Craton (NCC) at the end of Devonian. The third phase (D-3) is indicated by some WNW-trending Jura-type folds in the Carboniferous strata, formed by NNE-SSW shortening during the Late Triassic-Early Jurassic related to the collision between the NCC and the South China Block. The fourth phase (D-4) is inferred by NNE-trending shortening structures in the Jurassic and its underlying strata, generated by WNW-ESE shortening in the Late Jurassic echoing the Paleo-Pacific subduction. The fifth stage (D-5) is characterized by the NE-SW shortening in the Late Miocene, leading to the formation of many NW-SE striking thrust faults in the Miocene and its underlying strata, because the outward expansion of the Tibetan Plateau.