Investigating the Influence of Pre-Existing Basement Structures on the Propagation of the Malawi Rift using SRTM, RADARSAT, and Aeromagnetic Data

Abstract:

The Malawi rift is a Neogene, amagmatic rift located where the Western Branch of the East Africa Rift System (EARS) terminates. In more mature rifts, magmatism is frequently recognized as a driving factor in rift propagation; however, the amagmatic nature of the Malawi rift permits investigation into the relationship between pre-existing structures and current rift propagation, without the influence of magmatism. To map surface structures, we used Shuttle Radar Topography Mission (SRTM) Digital Elevation Model (DEM) and RADARSAT imagery over southern Malawi. To process the SRTM data, we applied edge enhancing filters and derivatives, and extracted topographic profiles to examine scarp height and minimum vertical exposed displacement. We mapped morphologically-defined structures by filtering the RADARSAT imagery using an enhanced lee filter to reduce noise and a Laplacian filter for edge enhancement. To examine Precambrian basement structures, we filtered aeromagnetic data using vertical and horizontal derivatives, tilt derivative, and analytic signal to create magnetic anomaly maps. Surface mapping indicated three primary trends in the southern Malawi rift: NW-SE (dominant), NE-SW, both of which are most likely the remnants of Mesozoic Karoo rifting, and a NNE-SSW trend seen in Neogene rifting. The Precambrian basement structural mapping also reveals three primary trends: WNW-ESE, NE-SW, and NW-SE. Ductile deformation causes the dominant basement fabric to change, switching polarity as the rift propagated southward from NE-SW orientation to NW-SE and WNW-ESE orientations, and back to a NE-SW orientation. In general, the surficial structures follow this trend. In some areas, however, the more recent rifting cut across pre-existing basement structures, possibly due to rheological heterogeneities or selective strain partitioning. Nonetheless, pre-existing basement structures played a critical role in strain localization and fault propagation in Malawi. However, sometimes other geological factors might have influenced structural orientation.