The Importance of Drainage Integration and Fluvial Connectivity within Intra-continental Rift Settings

Tectono-stratigraphic and drainage evolution models within intra-continental extensional settings dominantly focus upon individual sub-basins or half-graben. These models rarely acknowledge that, on a regional scale, extensional basins are compartmentalised into numerous sub-basins which; i) exist at different elevations; ii) subside at different rates; iii) vary in their degree of fluvial connectivity; and (iv) may experience significant shifts between erosional and depositional regimes as drainage networks evolve. This study demonstrates how the process of drainage integration and the degree of fluvial connectivity between adjacent sub-basins, influences landscape evolution and stratigraphic development within syn-rift settings. We employ a three-dimensional numerical model which integrates fluvial and hillslope erosion, fluvial sediment transport, tectonic deformation and clastic deposition. The model simulates two adjacent sub-basins separated by an accommodation zone which acts as a topographic barrier. By varying the fault slip rate between sub-basins we create differential subsidence and topographic gradients. This provokes differing responses from the drainage system. We conduct three experiments, each with progressively higher slip rates in the lower sub-basin. Experiment 1 sees both sub-basins experience a 0.25mm/yr slip rate. The elevation difference between the two basins is minor and both sub-basins remain internally drained and isolated. Experiment 2 increases the slip rate in the lower sub-basin to 0.5mm/yr. As elevation between the two sub-basins increases, streams erode headwardly, across the accommodation zone, capturing the drainage in the upper basin. Experiment 3 further increases the slip rate of the lower basin to 1mm/yr which quickly leads to drainage integration. The results show higher tectonic rates increase topographic gradients, driving drainage capture and increasing sediment flux from catchments. The source-to-sink system shows significant variations when the sub-basins remain isolated (Experiment 1) and when they integrate (Experiments 2 and 3). Drainage integration between the sub-basins causes erosion and sediment bypass in the upper sub-basin, as the fluvial network responds to the new base level set by the lower sub-basin. The lower sub-basin receives a major increase in axial sediment supply which is identifiable in the stratigraphic architecture. Sub-basins with identical tectonic and climatic boundary conditions can have vastly different stratigraphic fills depending on the degree of fluvial connectivity and where in the system they are positioned. This work shows a need to reassess rift basin tectono-stratigraphic models to incorporate the regional context in terms of fluvial connectivity and position relative to adjacent sub-basins.