On the Timescales of Border Fault Growth: Pleistocene Slip Acceleration and Lateral Border Fault Propagation in the Lower Shire Graben, East Africa

Abstract

Abstract Normal fault systems may grow by the lateral propagation of segments, yet little is known about the timescales over which this may occur and the implications for rift propagation. Footwall bedrock river networks provide a means to delineate the recent fault displacement patterns since erosional landscapes are sensitive recorders of rock uplift histories. Here, we analyze river profiles and map knickpoints in the Ruo River network and several adjacent rivers draining the footwalls of the en‐echelon Thyolo‐Muona‐Camacho border fault system of the Lower Shire Graben, southern East African Rift System. Using parameters calibrated in previous studies, we estimate the knickpoint initiation times and use linear river profile inversions to understand footwall uplift histories. Abrupt increases in bedrock river channel steepness indices approaching the faults suggest a recent acceleration in fault slip rates. Linear inversion of the bedrock river profiles reveals that footwall uplift rates accelerated from ∼0.1 to 0.3 mm/yr at ∼2–3 Ma in the Thyolo‐Muona footwalls. Later, at ∼0.75–1.0 Myr, farther southeast, the Camacho footwall river profiles show an increase in rock uplift rate from ∼0.05 to 0.2 mm/yr, but this signal decays moving southeast. Modern uplift rates along the Thyolo‐Muona section attain 0.33–0.56 mm/yr, and Camacho 0.19–0.29 mm/yr. We propose that within <2 Myr timescale, the border fault grew by an initial (Early Pleistocene) accelerated slip and hard linkage of the already‐nucleated segments, later followed by the southeastward lateral propagation of the system toward the adjacent rift, the Nsanje Graben.