Analysis Of Subsidence History And Anomalous Subsidence In The Southern Songliao Basin

To study the history of subsidence in the southern Songliao Basin and understand the formation mechanisms and evolution of this basin, this paper takes the drilling data and seismic profiles in Songliao basin as the research object, uses the theoretical methods and technological means of basin analysis, basin simulation and so on, to obtain the information of the subsidence and the subsidence rate of different periods,based on the previous study of the history of basin subsidence and structural evolution process. We divide the history of basin structural evolution according to rate of basin subsidence for several episodes. Next, we separate the post-rift anomalous subsidence in the south Songliao basin applying backstripping and strain rate inversion methods,and explore the relation between the subduction of paleo-pacific plate and the anomalous subsidence in study area. Anomalous subsidence is defined as the difference between theoretical and observed post-rift subsidence.The Songliao Basin exhibits the double-layered geologic structure of a rift depression within the deeper layer that is related to syn-rift subsidence and the overlying layer associated with post-rift subsidence, and the upper post-rift subsidence was much thicker than the syn-rift subsidence that differ from traditional rift basin model. The lower Cretaceous syn-rift stage could be divide to three episodes based on the subsidence rate. The rate in the southern part of study area increased but the northern part decreased through the syn-rift stage, which could be related to the intensity of basement rifting. In the post-rift stage, the subsidence rate accelerated abnormally differ from the Mckenzie’s rift basin model, and the process of post-rift subsidence could be divide to five episodes according to subsidence rate. The quantitative differentiation of subsidence across the Songliao Basin during the Cretaceous period revealed that the tectonic subsidence rate increased abnormally during the post-rift stage. In addition, the rapid subsidence corresponds to a relatively high strain rate in the crust. The anomalous subsidence was deeper than the normal thermal subsidence in the Songliao Basin, and the associated deposits unevenly overlie the entire basin, with a maximum thickness of 800 m in the southern basin.The long-wavelength anomalous subsidence of the Songliao Basin is characterized by an initial rapid subsidence in the west that subsequently migrated to the east and then back to the west, forming a trough-shaped subsidence profile. The rate of anomalous subsidence initially increased to a maximum(23m/Ma) at the mid-point of the period before decreasing. The deficiency of post-rift subsidence in Cretaceous rift basins in Yanshan belt might be the result of regional uplift. The Luanping Basin, one of basins in Yanshan area, experienced an anomalous uplift of 300 m-400 m.The Cretaceous anomalous post-rift subsidence in the southern Songliao Basin was primarily induced by down drag pressure of subducting western Pacific slab and deep mantle flow under Songliao Basin. The basement faulting in Songliao Basin during Early Cretaceous and normal faulting in the post rift stage also make contribution to anomalous post-rift subsidence. The anomalous subsidence in Songliao basin and the anomalous uplift in Yanshan belt might record the changes in the dynamic topography generated by deep mantle abnormalities that are induced by the subducting Izanagi plate beneath Eurasian plate.