POSSIBLE CRUSTAL AND SUB-CONTINENTAL LITHOSPHERIC MANTLE DECOUPLING BENEATH THE MALAWI RIFT

The Neogene Malawi Rift forms the southern terminus of the Western Branch of the East African Rift System (EARS). Geodetic studies suggest higher extension rates in the north leading to the assertion that the rift is unzipping from north to south as illustrated by the angular velocity of the Rovuma plate relative to the Nubian plate that decreases from 2.2 mm/year in the north to 1.5 mm/year in the south. We used the World Gravity Map (WGM2012) satellite gravity data and the two-dimensional (2D) power-density spectrum method to investigate the along strike variability in crustal and sub-continental lithospheric mantle (SCLM) structure beneath the rift. We identified a density discontinuity at shallow depths interpreted to be the Moho at depths of 31‒45 km and a deeper discontinuity at depths of 64‒124 km interpreted to represent either the mid-lithospheric discontinuity (MLD) or the lithosphere-asthenosphere boundary (LAB). We observe the following: (1) there is no consistent pattern of N-S elongated crustal thinning following the surface expression of the rift. In the north the crustal thickness ranges between 38 and 45 km beneath the Rungwe Volcanic Province (RVP). The thickness of the crust immediately south of the RVP varies between34 and 37 km. Within the central part of the rift, a ~125 km wide E-W tending, zone of ~41.5 km thick crust underlies the rift. The northern boundary of this zone is marked by the Precambrian Mwembeshi and Macologe shear zones interpreted as major lithospheric boundaries between differing Precambrian terranes. South of this zone, the crust thins to ~35 km as the rift approaches its southern termination against the NW-trending Precambrian Zambezi orogenic belt and the NW-trending Karoo-aged Shire graben. (2) Different from the crustal thickness heterogeneity, a localized zone of an elevated MLD or LAB is found to underlie the entire length of the rift stretching from its northern to southern tips. The depth of this density-contrast boundary is as shallow as ~64 km beneath the rift and it is deeper than 100 km beneath the Precambrian terranes reaching in places ~124 km. This indicates a uniform stretching of the SCLM by a factor of 1.5 to 1.8 beneath the entire length of the rift. Our results point to a possible decoupling of the crust from the SCLM during the early stages of the development of the Malawi Rift.