UPLIFT HISTORY ALONG THE MALAWI RIFT THROUGH MORPHO-TECTONIC ANALYSIS

Previous work on the Malawi Rift has shown at least two distinct phases of rifting, switching from an extensional tectonic regime to a strike-slip regime around 400-500 ka. It has been proposed that the rift is opening in a zipper-like motion from north to south, though the impact of the extension on rift flank uplift remains unknown. We used morpho-tectonic analysis of streams in the rift flanks to compare the uplift history from north to south along the Malawi Rift. The main geomorphic indicators we used were the normalized steepness index generated by the Stream Profiler ArcMap extension and knickpoints that were picked by visually examining sharp changes in steepness along drainage basins that feed into Lake Malawi (Nyasa). These indicators cannot constrain uplift rates but can instead show changes in tectonic regimes along the rift. The middle and northern portions of the rift exhibit high normalized steepness values across all studied border faults, indicating fast uplift rates and a distinct onset of rifting to the north. The southern portion of the rift shows varying levels of response to border faults, with some drainage basins showing little change upstream of the border fault and others showing a distinct increase in normalized steepness. The contrast between morpho-tectonic indicators in the northern and southern sections of the rift suggests a more mature and dynamic rifting process to the north. Previous workers in the Malawi Rift have also noted the influence of pre-existing structures on rift development, which may be expressed through these geographic variations in geomorphic response to border faults. Similarly-oriented pre-existing structures may have provided a weakened pathway for incipient rifting in the north, though no such evidence to confirm a similar pattern exists in the south. Future mapping of these structures and their corresponding dips will show if the decrease in uplift rate to the south can be explained by pre-existing structure orientation.