Towards a better understanding of African topography: a review of passive-source seismic studies of the African crust and upper mantle

Abstract

Explaining the cause and support of Africa's varied topography remains a fundamental question for our understanding of the long-term evolution of the continent. As geodynamical modelling becomes more frequently used to investigate this problem, it is important to understand the seismological results that can be incorporated into these models. Crustal thickness estimates are crucial for calculating components of topography that are isostatically compensated. Variations in seismic velocity help constrain variations in subsurface temperature and density and thus buoyancy; measurements of anisotropy can also be used to determine the contribution of the mantle flow field to dynamic topography. In this light, we review the results of passive seismic studies across Africa. At the continental scale there are significant differences in crustal models, meaning large uncertainties in corrections for isostatic topography. In east Africa, multiple seismic experiments have provided firm constraints on crustal and mantle structure. Tomographic images illuminate a broad (c. 500 km wide) low-velocity region in the upper mantle, with possible connection to the African Superplume in the lower mantle. These observations, alongside the variations in radial anisotropy, strongly suggest that the mantle flow field contributes significantly to the uplift of the region. Beneath southern Africa, low velocities are observed near the base of the continental lithosphere; the depth to transition zone discontinuities however suggests that they are not linked to the superplume beneath. It is thus less clear what role the sublithospheric mantle plays in supporting the region's high topography. Many of Africa's secondary topographic features (e.g. Atlas, Hoggar, Bie Dome) are underlain by slow velocities at depths of 100-150 km and are adjacent to rapid changes in lithospheric thickness. Whether these variations in lithospheric structure promote small-scale convection or simply guide the larger-scale mantle flow field remains ambiguous.

Lithospheric low-velocity zones associated with a magmatic segment of the Tanzanian Rift, East Africa

“…13). Finally, the presence of a low-velocity mantle beneath the region is consistent with previous seismological results that demonstrated a marked decrease of surface waves velocity between ∼60 and 100 km depth (Fishwick & Bastow 2011;Weeraratne et al 2003;O'Donnell et al 2013).…”

Use of a high-precision gravity survey to understand the formation of oceanic crust and the role of melt at the southern Red Sea rift in Afar, Ethiopia

“…The results achieved using these different methods are broadly consistent (e.g. Fishwick & Bastow 2011).…”

The development of extension and magmatism in the Red Sea rift of Afar

“…Constraints on crustal structure primarily come from wide-angle seismic studies (Berckhemer et al, 1975;Maguire et al, 2006;Makris and Ginzburg, 1987;Prodehl et al, 1997), teleseismic receiver function studies (Ayele et al, 2004;Dugda et al, 2005;Hammond et al, 2011;Stuart et al, 2006), and from gravity analysis (Redfield et al, 2003;Tessema and Antoine, 2004;Tiberi et al, 2005). Results achieved using these different methods are broadly consistent (e.g., Fishwick and Bastow, 2011).…”