Summary
Most magmatic continental rifts are characterized by surficial expression of magma, which explains the role played by magma in softening the lithosphere in extensional environments, enhancing rifting. The Malawi Rift forms the southern limit of the Western Branch of the East African Rift System (EARS). The Malawi Rift, though showing geomorphic features of a rift, does not show any volcanism except at its northern tip where the rift is older. This has raised the question of the need for magma in rift initiation. Determining the thermal structure beneath the Malawi rift could provide an insight into the mechanism of strain localisation during the rift initiation. The purpose of this study is to determine the thermal structures beneath the Malawi rift through 2D power density spectral analysis of aeromagnetic data collected over Malawi, and how they are related to strain localisation during rift initiation. The thermal structure was determined from heat flow values derived from the Curie point depth (CPD) of the Malawi rift. CPD of the Malawi Rift range from 17.8 to 27.5 km.
The estimated heat flow values range between 52 and 81 mW m -2. Elevated heat flow values (>75 mW m-2) occur in the north, center and south of the rift. Areas of elevated heat flow values are interpreted as due to the presents of partial melts and mantle fluids.
Introduction
Buck [2004, 2006] and Ebinger [2005] through numerical modelling highlighted the importance of magma in weakening the lithosphere in extensional environments, thereby enhancing rifting. Buck [2004, 2006] suggested that dike intrusion in the lithosphere is able to reduce the yield stress by an order of magnitude leading to strong strain localization within the magmatic segments. The prevalence of magmatic and volcanic centres within the Afar, Mid Ethiopian rift and the Eastern branch supports the Buck model. However the western branch of the EARS although showing geomorphic features of a rift such as welldeveloped border faults, and asymmetric half grabens is considered a dry rift and largely amagmatic along most of its length, except at isolated locations at the tips of the rift basins (e.g. Rungwe volcanic field at the tip of the Malawi rift).This has raised the question of the need of magma from elevated asthenosphere in rift initiation. In order to understand the mechanisms of strain localization during rift initiation in amagmatic rifts, it is necessary to consider other possible geodynamic processes that can facilitate rifting.
