Knickpoint morphotectonics of the Middle Shire River basin: Implications for the evolution of rift interaction zones

Abstract

Tectonic and paleo‐environmental reconstructions of rift evolution typically rely on the interpretation of sedimentary sequences, but this is rarely possible in early‐stage rifts where sediment volumes are low. To overcome this challenge, we use geomorphology to investigate landscape evolution and the role of different forcing mechanisms during basin development. Here, we focus on the humid Middle Shire River basin, located within the zone of progressive interaction and linkage between the southern Malawi Rift and Shire Rift Zone, East Africa. We used a digital elevation model to map knickpoints and knickpoint morphologies in the Middle Shire River basin and examined the relationships with pre‐rift and syn‐rift structures within the rift interaction zone. The main axial stream, Shire River, descends steeply, 372 m over a 50 km distance, across exposed metamorphic basement along the rift floor, exhibiting a strongly disequilibrated longitudinal elevation profile with both ‘mobile’ and ‘fixed’ knickpoints. In particular, we identify two clusters of mobile knickpoints, which we interpret as associated with baselevel fall events at the downstream end of the exposed basement that triggered knickpoint migration through the fluvial network since at least the Mid. Pleistocene. We infer that after the integration of the axial stream across the Middle Shire Basin, the knickpoints migrate upstream in response to fault‐related subsidence in the Shire Rift Zone. Conversely, the fixed knickpoints are interpreted to reflect local differential bedrock erodibility at lithologic contacts or basement‐hosted fault scarps along the basin floor. The results suggest that Middle Shire basin opening, associated with rift linkage, is likely a recent event (at least Mid. Pleistocene) relative to the Late Oligocene activation of Cenozoic rifting in the East African Rift's Western Branch. These findings support the hypothesis that the Western Branch developed from the gradual propagation, linkage and coalescence of initially nucleated distinct rift basins.

Geomorphic expressions of active rifting reflect the role of structural inheritance: A new model for the evolution of the Shanxi Rift, North China

“…Two significant zones in the evolution of the Shanxi Rift are the two RIZs: Shilingguan and Lingshi that form between the major subbasins. Both zones are generally more elevated than the surrounding basins making them potential sediment sources (Gawthorpe and Leeder, 1993) which is also shown by their patchy thin sediment fill compared to the major basins (Xu and Ma, 1992) RIZs are classified in their geometrical organisation (underlapping or overlapping, parallel, oblique or orthogonal) and may involve two or more distinct rift segments (Kolawole et al, 2022). The Shilingguan RIZ (Fig.…”

“…In subaerial, exposed active rifts geomorphology and surface expression of faults is a commonly used tool that has been successfully employed in regions such as the Basin and Range (Jackson and Leeder, 1994;Densmore et al, 2003;Densmore et al, 2004), the Apennines (Whittaker et al, 2008;Geurts et al, 2020) the Gulf of Corinth (Leeder and Jackson, 1993;Goldsworthy and Jackson, 2000) or the East African Rift (Erbello et al, 2022;Dulanya et al, 2022). These geomorphic approaches are varied and include studying the drainage evolution, the topographic response to faulting or using rivers to track the transient uplift rate.…”

“…is the drainage between both segments connected). They come with distinct morphological responses and have relevance to the seismic hazard so in the following we will classify the two RIZs based onKolawole et al (2022) classification scheme and asses the response of the geomorphic indices. https://doi.org/10.5194/egusphere-2023-2563 Preprint.…”