The effect of lateral tectonic tilting on fluviodeltaic surficial and stratal asymmetries: experiment and theory

Tectonic influence on deltas has long been recognized for its importance in morphodynamic and stratigraphic development. Here, we explore the control of lateral tectonic tilting on a prograding fluviodeltaic system through six laboratory experiments with a range of tilting rates. Basement tilting was applied along an axis that bisects the centre of the experimental delta, which forced uplift on one half of the basin and subsidence on the opposite half. In the experiments with lower tilting rates, the delta advanced faster in the direction of uplift due to the decline in relative base level. This slow uplift created truncated stratigraphic intervals that were dominated by active channel cut and fill. On the opposite side where subsidence occurred, the shoreline still prograded, but with decreased rates, while the delta topset was deposited thicker, alternating packages of fine and coarse sediments. The fluvial system was active uniformly across the delta in these slow tilting runs and produced asymmetry in shoreline planform geometries. In the experiments with higher titling rates, deposition quickly ceased on the uplift side and stacked conformable sequences of delta lobes on the subsidence side. The result was an overall lack of progradation in all directions. Progressively greater tilting rates used in these high tilting runs yielded steering of channels towards the direction with higher subsidence and developed even more asymmetrical stratal patterns. Characteristic tectonic and channel timescales applied to the experimental conditions prove to be good predictors of the fluviodeltaic planform and stratigraphic asymmetries. The deltaic asymmetry for the Ganges–Brahmaputra (G–B) system is largely comparable to the experiments with timescale ratios similar to those estimated for the G–B system.