ORIENTATION OF PRECAMBRIAN STRUCTURES AND THE FORMATION OF BORDER FAULTS IN THE LAKE MALOMBE AREA, SOUTHERN MALAWI RIFT

Continental rifting is an important plate tectonic process that creates resources, seismic hazards, and new ocean basins. Previous studies have indicated that preexisting structures may influence the formation of new faults in rift basins. The Lake Malombe area in the southern Malawi Rift (in the East African Rift System) consists of two border faults that separate a graben from foliated Precambrian rocks. The purpose of this research is to see the effect of the orientation of Precambrian structures on the formation of new faults by rifting in the Lake Malombe area. This study used Shuttle Radar Topography Mission, Digital Elevation Model (DEM) data, structural mapping, and aeromagnetic data to investigate the correlation in orientation between the older and younger structures in the area. In addition, E-W transects were taken to produce ten topographic profiles. The structure map from the DEM shows the N-S striking borders faults around the lake, as well as foliations and joints on the Precambrian exposures. The topographic profiles show that border faults define a graben, which is occupied in part by Lake Malombe. Results of meso-scale mapping indicate foliations that are orientated NE-SW on the eastern border fault and NW-SE on the western border fault, at angles up to 50° from the fault. There are a range of dips, but the majority of the foliations dip steeply. Meso-scale joints strike nearly N-S on both the east and the west side, with mostly vertical dips. The aeromagnetic maps show magnetic anomalies interpreted as foliations. Broad and diffused magnetic anomalies suggest that larger displacement has occurred on the western border fault than on the eastern border fault where distinct foliation patterns are observed. Meso-scale foliations close to the western border fault have a high likelihood of reactivation based on their orientation with respect to their current stress regime. In contrast less foliations have a high likelihood of reactivation next to the eastern border fault. This difference in orientation may explain why there is more apparent larger displacement on the western border fault and supports the hypothesis that the orientation of preexisting foliations have an influence on the formation of new rifts faults.