Mantle upwellings and convective instabilities revealed by seismic tomography and helium isotope geochemistry beneath eastern Africa

Abstract

International audienceThe relationship between intraplate volcanism and continental tectonics has been investigated for North and East Africa using a high resolution three-dimensional anisotropic tomographic model derived from seismic data of a French experiment “Horn of Africa” and existing broadband data. The joint inversion for seismic velocity and anisotropy of the upper 400 km of the mantle, and geochemical data reveals a complex interaction between mantle upwellings, and lithosphere. Two kinds of mantle upwellings can be distinguished: The first one, the Afar “plume” originates from deeper than 400 km and is characterized by enrichment in primordial 3He and 3He/4He ratios higher than those along mid-ocean ridges (MOR). The second one, associated with other Cenozoic volcanic provinces (Darfur, Tibesti, Hoggar, Cameroon), with 3He/4He ratios similar to, or lower than MOR, is a consequence of shallower upwelling. The presumed asthenospheric convective instabilities are oriented in an east-west direction, resulting from interaction between south-north asthenospheric mantle flow, main plume head and topography on the base of lithosphere

Helium isotopes at Rungwe Volcanic Province, Tanzania, and the origin of East African Plateaux

“…He component along the Kenyan (Eastern) and Western rifts has hitherto led to suggestions of two distinct scales of mantle convectiondeep and from the lower mantle versus shallow involving only the uppermost mantle -supporting the Ethiopia and Kenya domes, respectively [Pik et al, 2006;Montagner et al, 2007]. Our finding of a plume-like 3 He/ 4 He contribution at the southernmost extremity of recent volcanism along the Western Rift removes this constraint, and we suggest that a lower mantle component is pervasive throughout the EARS -from RVP in the south to the Main Ethiopian Rift and Afar in the north -albeit masked at many localities by a veil of crustal He and/or sampling of MORB-like or SCLM He.…”

“…Along the Main Ethiopian Rift and Afar, 3 He/ 4 He ratios are significantly greater than values characteristic of ambient upper mantle [Marty et al, 1996;Scarsi and Craig, 1996] -as sampled by mid-ocean ridge basalt (MORB) [Graham, 2002], and are consistent with a deep (lower) mantle plume origin for convective uplift of the Ethiopia Dome. On the other hand, Kenya Dome 3 He/ 4 He ratios do not exceed the MORB value [Darling et al, 1995;Pik et al, 2006;Hopp et al, 2007;Tedesco et al, 2010] prompting suggestions of a different scale of mantle convection supporting the Kenya plateau, restricted to the uppermost mantle only [Pik et al, 2006;Montagner et al, 2007]. Here, we target lavas and tephra from Rungwe Volcanic Province (RVP), Tanzania at the southernmost extreme of the Kenya Dome and report the first observation of 3 He/ 4 He ratios >MORB south of the Turkana Depression.…”

Upper mantle temperature and the onset of extension and break-up in Afar, Africa

“…We use a formulation that includes the effect of meltweakening and dehydration strengthening (see Supplementary Material; Armitage et al, 2010). Thermal state and non-Newtonian viscous flow are solved in the finite-element code CitCom (Moresi et al, 1996). We use a 2800 km wide by 700 km deep 2-D Cartesian domain containing 512 by 512 equally spaced nodes, providing a resolution of 5.6 by 1.4 km.…”

Deep Earth Structure - Upper Mantle Structure: Global Isotropic and Anisotropic Elastic Tomography

“…The complete anisotropic tomographic technique (including azimuthal anisotropy) has been applied for investigating the upper mantle structure either at a regional scale or at a global scale. The number of regional investigations using surface waves is too large to cite all of them, but we give some examples of anisotropic tomographies for different regions such as the Indian Ocean (Debayle and Lévêque, 1997;Montagner, 1986a;Montagner and Jobert, 1988), the Atlantic Ocean (Mocquet and Romanowicz, 1989;Silveira and Stutzmann, 2001;Silveira et al, 1998), Africa (Debayle et al, 2001;Hadiouche et al, 1989;Montagner et al, 2007;Priestley et al, 2008;Sebai et al, 2006;Sicilia et al, 2008), the Pacific Ocean (Bussy et al, 1993;Ekströ m and Dziewonski, 1998;Isse et al, 2010;Montagner, 2002;Maggi et al, 2006;Nishimura and Forsyth, 1989;Ritzwoller et al, 2004), Antarctica (Roult et al, 1994), Australia (Debayle and Kennett, 2000;Fishwick et al, 2008;Simons et al, 2002), North America (Deschamps et al, 2008;Li et al, 2003;Lin et al, 2010;Marone and Romanowicz, 2007a,b;Yuan and Romanowicz, 2010;Yuan et al, 2011), and Central Asia (Griot et al, 1998a,b;Lebedev and Nolet, 2003;Villaseñ or et al, 2001;Yao et al, 2010). At a global scale, the reader is referred to the anisotropic models Montagner and Tanimoto (1991)…”