Differential Deformation And Its Mechanism In Southern Ordos Basin, North China

The Southern Ordos Basin area is located at the transitional zone of the extensional tectonic domain in Eastern China and the contractional tectonic domain in Western China and is separated from Qinling Orogen by Cenozoic Weihe Graben to the South.Such a unique tectonic location make Southern Ordos Basin area become an ideal place to study continental dynamics,intraplate deformation and the coupling of basin and orogen.Under the guidance of tectonic activity theory,with tectonic evolution to be a main line,and based on a deformation-process-based approach,this thesis explores the characteristics and mechanism of differential deformation,structrural evolution process,and the coupling of basin and orogen in Southern Ordos Basin area by field geology,borehole and logging data,seismic interpretation,and low temperature thermochronometry dating techniques.The results show that the Southern Ordos Basin area presents differential deformation and coexistence of thin-and thick-skinned structrures,which is characterized by South-North zonation,segmentation along strike and vertical zonation.There are 5 main unconformities in Southern Ordos Basin,and the nature of every unconformity varies with its location indicating differential uplift prosesses.Since Phanerozoic,the Southern Ordos area has experienced multi-cycle tectonic stages:(1)divergent margins during Cambrian–Early Ordovician;(2)back-arc basin during Middle-Late Ordovician;(3)cratonic basin from Late Carboniferous to Middle Triassic;(4)intraplate remnant cratonic basin during the Late Triassic–Early Cretaceous;and(5)peripheral rift during Cenozoic.In addition,Southern Ordos Basin area experienced 2 vertical-movement-dominated tectonic events,which are Huaiyuan movement in late Early Ordovician and Indosinian movement during the end of Triassic,respectively;and it also experienced 2 horizontal-movement-dominated tectonic events,which are Caledonian movement in late Early Paleozoic and Yanshanian movement in late Early Mesozoic,respectively.Specially,the Yanshanian movement formed the tectonic framework of the Southern Ordos Basin area.Low temperature thermochronometry dating results revealed 4 heterogeneous tectonic-thermal events in Southern Ordos Basin area:(1)tectonic uplifting during the end of Ordovician(450.1Ma);(2)strong tectonic contraction and uplifting during late Late Triassic to Early Jurassic(209.3Ma~169.2Ma);(3)the differential uplifting of Ordos basin and the formation of 1st planation surface during Late Cretaceous to Paleocene(81.5Ma~58.9Ma);and(4)peripheral rifting,tectonic uplifting of the interior basin and the formation of 2nd planation surface during Eocene to Oligocene(49.4Ma~34.7Ma).In particular,the magnitudes of uplifting event during late Late Triassic to late Early Jurassic were weaken northward;whereas the magnitudes of uplifting event during Late Cretaceous were weaken westward.The differential deformation in Southern Ordos Basin was mainly controlled by transformation of tectonic regimes and stress fields,deep faults,boundary condition,and rheology.There was a good fit between the evolution of Southern Ordos Basin area and orogenesis of Qinling during Phanerozoic.