ENHANCED LITHOSPHERIC MANTLE THINNING IN THE MELT-POOR MALAWI RIFT

The concept of continental extension generating large topographic features like those in East Africa only predated that of plate tectonics by 18 years. Today, we know continental rifting is integral to the plate tectonics paradigm as the birthplace of new plate boundaries. However, it is poorly understood how intact continental plate breaks apart. Where high temperatures and abundant melts are present, preferential thinning of the lithospheric mantle compared with the crust is observed in both narrow (e.g. Ethiopia) and wide (e.g. Basin and Range) rifts, suggesting mechanical thinning has been augmented by thermochemical erosion of the lithospheric mantle. Here, we show evidence that even in the magma-poor Malawi rift in the southern part of the East Africa Rift system, narrow rifting at the surface is associated with localized preferential thinning of the lithospheric mantle. We use broadband data acquired as part of the Study of Extension and MaGmatism in Malawi aNd Tanzania (SEGMeNT) experiment, including five stations deployed in Lake Malawi (Nyasa) itself, to generate common-conversion-point stacked Sp converted-wave images. The phase interpreted as the lithosphere-asthenosphere boundary is uplifted by 10s of km within a narrow (<100 km) zone beneath Lake Malawi (Nyasa). Lithospheric mantle thinning is much more pronounced than crustal thinning and cannot be accounted for by mechanical stretching alone. However, surface-wave derived shear-wave velocities from this region and thermobarometry from the Rungwe volcanic province north of the Malawi rift preclude the presence of significant melt or anomalously high mantle temperatures. We suggest that the combination of small amounts of melt and pre-existing rheological or compositional heterogeneities enable localized lithospheric corrosion and thinning in this early stage rift, even in the absence of significant amounts of melt. Thinning is localized beneath the Ubendian Belt, which has been associated with subduction as recently as the Neoproterozoic and may be ripe for volatile-induced melting. The most extreme thinning is seen beneath the Central Basin, where the Cenozoic rifting overlies Mesozoic Karoo rifting, suggesting that lithosphere thinned by Karoo rifting had not recovered by the onset of Cenozoic rifting.