构造掀斜主导的断陷湖盆缓坡层序“源—汇”正演模拟定量研究

断陷湖盆缓坡带的构造掀斜作用对其地层叠置样式和沉积体系展布具有重要的控制作用。通过基于“源—汇”系统理论的地层正演模拟研究方法,在三级层序地层格架下（以2 Myr为总时长）对断陷盆地缓坡带构造掀斜型层序地层发育样式及其相关地貌演化、沉积物通量变化进行了定量表征。以恒定的绝对湖平面为研究前提,共建立了高/低沉积物供给的背景下4种不同的地层正演模型。两种不同缓坡带掀斜初始位移模型的设定反映了掀斜点和沉积滨线的相对位置,以便于不同掀斜层序样式对比分析。研究表明,掀斜支点位于滨线之上的模型常形成湖侵和正常湖退,而掀斜支点位于滨线之下的模型常形成强制湖退。滨线迁移轨迹和层序叠置样式的不一致性贯穿整个层序发育过程,是沉积物通量及滨线迁移共同作用的结果。该模型中沉积物通量的变化直接响应于源区地貌演变,滨线迁移则受控于新增可容空间和沉积物供给的相互关系。这一系列构造掀斜型层序模型及其相关成因解释对断陷盆地层序地层格架及其叠置样式的精细解剖具有重要的指导意义。该模拟研究对断陷湖盆的高频湖平面变化特征也有较为合理的解释（如海洋同位素3期以来的贝加尔湖）。然而本次正演模拟所设置的初始地貌较为单一,在相同构造、气候条件下和不同沉积物供给背景下所形成地层模型无法涵盖所有缓坡带层序地层类型。由此可见,基于“源—汇”系统思想探讨断陷盆地层序地层多样性的相关模拟工作仍然任重道远。

Source-to-sink Quantitative Stratigraphic Forward Modeling on the Tilted Hanging-wall Sequence Architecture of a Tectonically-driven Lacustrine Rift Basin

Tilted hanging-wall dipslope margins associated with extensional tectonism exert a fundamental control on the stratigraphic and facies distribution in tectonically-active half-graben basins. A source-to-sink （S2S） based three-dimensional （3D） stratigraphic forward modelling （SFM） code is used in this study to quantify the third-order sequence architecture of the tilted hanging-wall margin and its relation to varying sediment supply and catchment geomorphological change in the catchment area over a period of 2 Myr. Four model runs were constrained by the relatively high or low sediment-supply condition, as well as the fixed absolute lake level. Two displacement models showing the relative location between fulcrum and shoreline have been incorporated for comparison. The modeling shows that the systems tracts of a third-order tilted sequence differ, even within the same displacement model. Lake transgression and normal regression occur when the fulcrum is located above the shoreline, whereas forced lake regression may occur when the fulcrum is located below the shoreline. The coupling and decoupling interactions of shoreline trajectory and stratigraphic stacking pattern are presented for the entire sequence. These are formed by the variable sediment flux in response to the geomorphic evolution of the catchment area and shoreline migration, with the interplay of the rate of accommodation and sedimentation. The stratigraphic models and a plausible interpretation have shed a light on the anatomy of the sequence architecture and stratigraphic stacking patterns of a tectonically-active rift basin, and may also be well applied to the interpretation of high-frequency lake level changes in some rift lakes, such as Lake Baikal since marine isotope Stage 3 （MIS-3）. However, the initial catchment geomorphological condition set for the numerical model contributes to only part of the diverse sediment-supply models, even within the same climate and tectonic forcings. This indicates that more work is required to explore the diversity of rift sequence architectures from a S2S perspective.