Mesozoic-Cenozoic Basin Evolution-reforming And Basin-mountain Coupling In Southern Ordos Basin

Sedimentary basin and orogen are the basic structural units. They are coexistence as a contradictory unity with opposites, because they often form and evolve in uniform structural and dynamic framework. We must reveal orogenic process from studying basin and explore the formation mechanism of the basin from studying orogen. So that we can reconstruct the relationships with basin-mountain coupling systems and dynamic transform mechanism in different periods. In this way we discuss the formation of continental orogen and sedimentary basin, geodynamic evolution mechanism and migration of substances would be more effective and in-deep.With the three principles (including integrity, development and synthesis) of basin analysis as the guidance, the research methods of reforming basin, reconstruction of the original basin boundary and coupling relationship of basin-mountain has been used. Considering regional tectonic framework, tectonic events and characteristics of prototype basin in primary period of basin evolution, the author use geological analysis, mineral dating method, provenance analysis method and restoration of eroded thickness to investigate the evolution and reforming process of southern Ordos basin in Mesozoic-Cenozoic and basin-mountain coupling relationship in two periods.Based on a large amount of mineral age data, the basin evolution was divided into four periods:Late Triassic Yanchang period, middle Jurassic Yan'an period, Zhiluo-Anding period and early Cretaceous. Since the late Cretaceous the reforming process of basin can be divided into five stages: late Cretaceous-Paleocene, Eocene-Oligocene, early and middle Miocene and late Miocene-Pliocene and Quaternary. Through reconstructing the paleo-topography of the four major forming periods, and original sedimentary basin boundaries of southern basin was initially identified, which indicated the southern boundary may extend to the North-Qinling Mountain in Yanchang period that was the peak development of Ordos basin, then it move to north gradually in Jurassic and located in Weihe basin even northern Qinling Mountain in early Cretaceous. The east boundary was in Hebei and Anhui province in Yanchang period. Then it also contracted in Jurassic and moved westward to the west of Taihang mountain area in Zhiluo-Anding period. Lvliangshan mountain roughly controlled the boundary in early Cretaceous, while the southwest boundary was always near the large faults at the western Liupan Mountain with little change in different period.In late Triassic Yanchang period which is the initial stage of basin-mountain coupling in Mesozoic, foreland basin characters developed in southern basin and had significant coupling relationship with Qinling orogen when the tectonic background is active. The Qinling orogen almost take control of the provenance of southern basin, so during Chang 7-Chang 1 member, the provenance from south and north convergence at Huanxian-Huachi-Huangling area. Early and middle Jurassic, depression basin developed and the basin moved to the north obviously. Late Jurassic-early to early Cretaceous, intracontinental orogenic movement of Qinling orogen was very strong and the thrust nappe was obvious. So the southern basin had the unique feature of intracontinental foreland basin which was mainly molasse formation. Middle and late of early Cretaceous, depression basin developed in extension tectonic setting. The development of Cenozoic Weihe fault has tight connection with the extension, uplifting and faulting of Qinling orogen in K2-E1 under the total compression system. South of Ordos basin began to couple with mountain for the second period. So bilateral mountain (northern Qinling Mountain and northern Weihe uplift) not only provide sufficient provenance but also take control of deposition and subsidence of Weihe basin and make it outward expanding in the form of warped fault. The study of two periods of basin-mountain coupling in Mesozoic-Cenozoic not only get the internal of basin post-reforming and reconstructing original sedimentary basin boundaries which are regional dynamic background and source but also use reforming and restoring methods to research evolution of structural dynamics in the study area, which has the important scientific significance and innovation.