BORDER FAULT SEGMENTATION ALONG THE MALAWI RIFT

Border fault segmentation characterizes the early stage of continental rift zones. Previous studies suggest that border faults in the Malawi Rift vary in displacement along a segment, alternate half-graben polarities, and are connected through accommodation zones. This study uses Shuttle Radar Topography Mission (SRTM) Digital Elevation Model (DEM) data and published seismic sections to investigate the along strike variation in border faults along the Malawi Rift. Available SRTM-DEM data allowed more detailed examination of vertical distances, and relationships between Precambrian basement structures and border fault segments than was previously possible. Detailed E-W topographic profiles were extracted from the SRTM-DEM data at a spacing of 1.4 km along the entire length of the Malawi Rift to determine the fault scarp elevations. Seismic cross-sections were used to provide the subsurface structure and displacement of the border faults. Eight border fault segments were identified based on the location of full-grabens, which occur between two half-grabens of opposite polarities. The length of fault segments varied from ~230 to ~45 km. The general trend observed reveals a decrease in fault scarp elevation from ~2 km in the north to ~0.2 km in the south. Three broad regions were identified based on different fault geometries along the rift. The Northern Area shows the overlap model that contains the transition of half-graben to full-graben and to half-graben of opposing polarity. In some places, border faults follow the general structural grain of the basement. In the Central Area, zones of segmentation coincide with Permo-Triassic grabens that strike at high angle to the trend of the rift. There are no well-defined escarpments in this area. The Southern Area exposes basement structure and low elevations, indicative of no subsidence and rift termination. The well-developed overlap model to the north, the less well-defined escarpments in the center, and rift termination to the south suggest that the rift becomes younger to the south in the direction of propagation. Results from this study suggest that the overlap model of border fault segmentation is only well defined in the northern part of the Malawi Rift, and not in other parts where pre-existing structures at high angle occur, and rifting is young and undeveloped.