Strain Accommodation of Cenozoic Rifting in the Northern Margin of the Shire Graben, Southern Malawi Rift

The NW-trending Shire Graben in southern Malawi is a Jurassic rift structure that marks the termination of the N-S striking Malawi Rift. Previous studies suggest that structures of the Shire graben accommodate extension of Cenozoic rifting by reactivation of normal faults as strike-slip faults. We conducted fieldwork to document and analyze the kinematics of structures at the northern escarpment of the Shire Graben, defined by the Mwanza and Thyolo faults. The escarpment is composed of Precambrian gneisses, schists, and amphibolites. These rocks have undergone deformation that generated well-developed foliation, folds, shear zones, and boudins. In the Thyolo escarpment, foliations strike NW-SE and NE-SW. Two different types of magma intruded the Precambrian rocks exposed in the Thyolo escarpment. Granodioritic and granitic dikes 0.05-0.3 m thick strike NW-SE and dip 20-40° to the NE. However, 0.03-5.7 m thick basaltic and gabbroic dikes, which strike NE-SW and dip 70-90° to the NW, cut both the felsic dikes and the Precambrian rocks and are exposed in most of the Thyolo escarpment. Precambrian rocks within the Mwanza escarpment have foliations with a dominant NW-SE strike and a SW dip. The foliation is cut by NW-trending and NE-dipping strike slip faults. Few dikes were observed in the Mwanza escarpment area. Based on the data collected from both escarpments, we observe: (1) Dikes, associated to the regional Karoo Jurassic igneous event, dominate in the Thyolo Escarpment, and not in the Mwanza Escarpment; (2) propagation paths of intrusions are not localized along foliation planes in the Precambrian rocks, but rather cut across it; (3) Faulting is rarely observed at the meso-scale in the Thyolo Escarpment, yet it is localized as strike slip faulting of unknown age in the western Mwanza Escarpment; (4) there are no fault surfaces that contain multiple striation directions observed in the study area that would indicate reactivation of faults. These results indicate that there is no meso-scale deformation observed in the Thyolo Escarpment that can be associated with current extension in the Malawi Rift. Strain is most likely accommodated along poorly-developed border faults to the north or along blind faults within the Shire Graben.