RIFT BORDER FAULT SEGMENTATION THROUGH ALONG-STRIKE DEFLECTION: EVIDENCE FROM SOUTHERN MALAWI RIFT

We used Shuttle Radar Topography Mission Digital Elevation Model and aeromagnetic data to examine along strike border fault segmentation of the southern extension of the Cenozoic Malawi Rift at 35°E 15°S to 35°45’E 15°45’S. The magma-poor Malawi Rift represents the southernmost extension of the Western Branch of the East African Rift System (EARS). This segment of the rift traverses Mesozoic (Upper Jurassic - Lower Cretaceous) igneous ring complexes (nepheline syenite and syenogranite) and Precambrian crystalline rocks with well-developed foliation (mostly gneisses, granulites, and charnockites). We focused on the interaction of the southeastern border fault of the rift with these inherited upper crustal heterogeneities. This part of the border fault is characterized by an outer NNE-trending discontinuous well-developed escarpment and an inner “zig-zag” less-developed escarpment. At 35°21’E and 15°8’S, the inner border fault interacts with nested igneous ring complexes and deflects along strike departing from parallelism with the preexisting Precambrian structure. At 35°17’E and 15°14’S, the outer border fault follows Precambrian foliation then reactivates a steep margin of a ring complex before it terminates abruptly before reaching the nested ring complexes where it encountered NW-trending Precambrian structures. The border faults were well-developed when they reactivated Precambrian foliation that facilitated strain localization, and they were deflected or terminated when they encountered the ring complexes that presented an anisotropic rheology. Our rift border fault segmentation model can be used to describe rift evolution for other parts of the EARS and demonstrates how anisotropy can govern border fault propagation and segmentation.