Abstract:

Hetao arcuate tectonic system lies along the Great Bend of the Huanghe Rever in the middle-west part of Inner Mongolia.It consists of the Langshan range-basin structural zone,Yinshan faulted zone,Hetao Basin and Huanghe faulted zone.They are all arranged parallel to one another in an arcuate form with an apex towards northwest.The foundamental rocks which constitute Hetao arcuate tectonic system are the Late Archeozoic magmatized gneisses and marbles and the Early and Middle Proterozoic schists,slates and quartzites.Rocks of the Late Proterozoic,the Palaeozoic and the Triassic are almost all absent,but a few outcrops of Cam-brian,Ordovician,Upper Carboniferous and Lower Permian have been found in some localities.Those basins situated in the Langshan range-basin structural zone are filled with Jurasic and Cretaceous conglomerates,sandstones and mudstones,but Hetao Basin is mainly filled with Tertiary and Quaternary sediments,including source rocks of the Guyang Formation of Upper Jurassic-Lower Cretaceous and the Linhe Formation of Oligocene.It is believed that Hetao region appeared to be a wide uplift from Late Proterozoic to Early Mesozoic,and Hetao arcuate tectonic system was formed since late Triassic.It is supposed that those structural zones of Hetao arcuate tectonic system caused by shear-compressive stresses were formed firstly when Ordos massif moved towards NW during Late Triassic and Eocene,and when the massif has moved in reverse direction since Eocene,Hetao Basin—the tensil structural element—was then formed by pulling along the former faulted zones.When Ordos massif moved forward,the apex of the arc was subjected to strong normal compressive stresses,while its SE and SW limbs were effected by the right-and left-lateral shear stresses respectively.This is the most possible reason why the range-basin structures on its two limbs of Langshan structural zone are in an echelon pattern in response to the Yinshan faulted zone.Thus we infer that the formation and evolution of Hetao arcuate tectonic system in respect of the regional tectonics may probably be related to the oppo-site moving of the Eurasian plate to Kula plate,Pacific plate and the Indian plate in Mesozoic and Cenozoic periods.