An Integrated Study Of Rift Basin Model And Stratigraphic Response To Spatiotemporally Varying Tectonic Forcing

The stratigraphic evolution of basin is an integrated response of external factors such as tectonics,climate and sea/lake level.Many previous studies have explored the controlling effect of sea/lake level change on stratigraphic record,which usually assume that tectonic forcing is a constant value or uniform in space.In fact,the tectonic forcing is spatiotemporally varied.It controls both the denudation rate(sediment yield)from the upstream drainage basin and the accommodation space in the downstream basin.It is reasonable to assume that the tectonic subsidence is relatively uniform and stable in passive margins for marine basin.However,within the continental rift basins which experience complex tectonic history,the tectonic forcing not only change in amplitude and period,but also migrate significantly in space.Therefore,the spatiotemporally varied tectonic forcing plays a key role in controlling the evolution of the whole depositional systems.It is important to clarify the stratigraphic response under three-dimensional tectonic forcing for understanding the tectonic and sedimentary evolution of rift basins.In this thesis,a coupled 3D tectonic-stratigraphic model is established to model the tectonic-sedimentary evolution of rift basins under the background of setting a stable climate forcing and lake level at a medium scale,and the stratigraphic response to spatiotemporally varied tectonic forcing is quantitatively analyzed from the model results.The model results produce a series of typical structural,geomorphological and sequence characteristics of rift basins.This study makes an important supplement to the current sequence stratigraphic analysis and reveal several new insights as follows: 1.There have decouple behavior between the shoreline trajectory and grain front/grain size front(grain front includes gravel front and sand front,which are used to characterize the change of sequence pattern).That is to say,while the shoreline migrates to the landward,the grain front/grain size front migrates to the basinward(the progradational sequence is formed at the rising of relative lake level).It is the results of tectonic-induce topographic rotation that increase sediment transport capacity.This decoupled behavior will particularly common in underfilled basins.2.Within the decoupled situation,the migrated trajectories of grain front are partially positively correlated with migrated subsidence center,which is a result of competition between tectonic forcing and autogenic sedimentary process.3.Spectral analysis shows that low-frequency tectonic signals have greater potential for preservation,while high-frequency tectonic signals are buffered,shredded or mixed with autogenic signals in sedimentary routing systems,which are not usually preserved in sedimentary records.Meanwhile,there is a lag time between tectonic signal and stratigraphic response,which means that stratigraphic response to tectonic signal is not synchronous.4.The model results show the variability of sequence patterns in threedimensional space,especially the significant change in strike direction.These changes are closely related to spatiotemporally varied tectonic forcing which are easily misinterpreted as the change of lake/sea level or climate fluctuations.5.Based on the analysis of seismic data in Nanpu Sag of Bohai Bay Basin,the sedimentary filling process of Nanpu Sag from Shahejie Formation to Dongying Formation was modeled for further evaluating the stratigraphic response to tectonic forcing and the "non-uniqueness" of sequence interpretation.