Morphosedimentary response of rivers crossing multiple fault-controlled subsiding areas: field evidence and laboratory experiments

Downstream changes of fluvial styles and related grain size triggered by localised tectonically-induced changes in riverbed gradient are still poorly understood, especially in terms of their impact on the accumulation of alluvial successions. In this study, we analyse the morpho-sedimentary response of rivers crossing multiple fault-controlled subsiding areas, by using field data from the age-constrained, fluvial deposits of the Pleistocene Dandiero Basin (Eritrea) to create scaled, controlled laboratory experiments performed at the Eurotank Stratigraphic Analogue Modelling Facility (Utrecht University, NL). With this experimental series, we quantified the impacts of degradational/aggradational fluvial dynamics showing that stream bed degradation occurs upstream of subsiding depocenters following the localised increase in river slope. Following a tectonic-induced decrease in river slope, aggradation occurs downstream of the fault zones, and marked in-channel aggradation promotes the branching of major river trunks into minor channels and the development of unchannelised tabular bodies. Experiments also show that highly subsiding areas promote the accumulation of fine-grained deposits, but their accumulation zones shift downstream following localised bed aggradation. We show that where multiple subsiding areas occur along a river, localised depocenters separated by degradational areas occur, causing general starvation in the downstream subsiding reaches, where lacustrine deposition became common. These findings suggest that the role of active faults could have been significantly overlooked when studying how changes in allogenic forcings impact alluvial strata. The results obtained in this study offer a solid basis for creating a predictive model for facies distribution in river dynamics, providing insights into detecting neotectonic signatures in active rivers and identifying tectonic imprints on ancient fluvial successions.