Modeling the ups and downs of continental rifting: Feedbacks between normal faulting, flexural isostasy and erosion

Understanding the development of intraplate mountain ranges is essential for linking lithospheric deformation to uplift/subsidence, long-term exhumation, and hence the distribution of topography. One key process driving the formation of elevated relief in these settings is continental rifting, where tectonic extension, normal faulting, and flexural isostasy interact to generate central graben structures and elevated rift flanks. These flanks are marked by asymmetric erosion which in turn leads to drainage network reorganization, drainage divide mobility, and rift flank retreat. However, the relative contribution of normal faulting and flexural isostasy to drainage network reorganization remains poorly understood, as does the role of river captures and migrating drainage divides in controlling the spatial distribution of erosion and exhumation.

In this study, we employ the 2D landscape evolution model OpenLEM to investigate the interplay between normal faulting and flexural isostasy during rift flank uplift. Rather than imposing a uniform regional uplift, we use flexural compensation to let uplift emerge dynamically in response to fault-controlled subsidence, tectonic unroofing, and erosion. To ensure realistic drainage organization and sediment routing, a central river is integrated along the graben axis, providing an effective base level and sediment sink throughout model evolution.

First model results show a subsiding central graben structure and uplifting rift flanks, with alluvial fans forming along the boundary. Flexural unloading along the bounding faults induces up to ~600 m of rift flank uplift, generating pronounced topographic asymmetry. This, in turn, leads to asymmetric slopes which promote retrogressive erosion of rivers draining toward the graben, resulting in a lateral migration of the rift flank and the drainage divides. Drainage reorganization occurs through river capture and flow reversals, increasing the contributing area of graben-directed rivers at the expense of outward-draining catchments. Although horizontal fault motion drives lateral graben widening, high erosion rates along the rift flanks dominate, causing progressive flank retreat and a gradual reduction in flank elevation with increasing distance from the graben center.