How normal faults control delta deposition: Insights from analogue modelling

In many natural systems, normal faults induce sedimentation in basins by creating depositional space that is gradually filled by incoming sedimentary infill. In this study, we investigated the response of deltaic systems to normal faulting through a novel analogue modelling approach integrating fluvial and tectonic processes. The models were built in a flume where the engine-driven extension was coeval with a river system controlled by water discharge and sediment feed. The river feeds the tectonically controlled basin where the deltaic lobes form. In the models, we varied engine velocity (i.e., extension/subsidence rate), while keeping the sediment influx and water discharge constant. Faulting of the model sand layers, representing the uppermost crust, is implemented in the flume by a mobile basal sheet, which is pulled from underneath a fixed block at constant velocity. The basin side (i.e., hanging wall) of the main normal fault is filled with water, while a predefined channel guides sediment-rich water towards the basin during early river incision. The river system scaling was done by discharge for the channel dimensions and by sediment mobility number for the sediment transport rate, while the fault slip rates were scaled based on natural fault-controlled basins such as the Roer Valley Graben or the Gulf of Corinth. The difference between natural temporal and spatial scales at which surface and tectonic processes operate was bridged by calculating the ratio between the creation of the accommodation space due to normal fault slip and the average sedimentation rate in the basin. This ratio is calculated for the entire basin and for a single lobe, and is ultimately the key parameter controlling the delta evolution.

The modelling results showed that the active faulting led to progradation and retrogradation of the delta. When the subsidence rate exceeds the sedimentation rate, the delta retrogrades early, and the branching of the delta lobes occurs later. In the model with similar subsidence and sedimentation rates over a lobe, the delta mainly experiences aggradation with several moderate prograding and retrograding cycles. In this situation, there is a minor lateral migration of the delta lobes without branching and significant avulsion. With low subsidence rates, the number of progradation-retrogradation cycles is increasing, the delta progrades farther into the basin, and can cross the conjugate basin-bounding fault(s). Such progradation-retrogradation cycles are often accompanied by local hiatuses, river avulsion, delta lobe branching and lateral migration, which are controlled by an interplay of external forcing and internal delta dynamics.

These findings facilitate understanding of the relationship between tectonics and delta dynamics in natural systems. For instance, due to the slow subsidence and a high sediment supply, the Roer Valley Graben is being overfilled in the early stages, with deltaic formations reaching the other side of the basin before shifting to a late-stage basin-parallel progradation. Contrastingly, the fast subsiding Gulf of Corinth, accompanied by a low sediment supply, has multiple small individual coeval delta lobes, which, presently, do not reach far into the graben and are unable to fill the created accommodation space.