Crust and upper mantle structure in East Africa: Implications for the origin of Cenozoic rifting and volcanism and the formation of magmatic rifted margins

Abstract

Crust and upper mantle structure in East Africa, together with the tectonic history of the region, is used to evaluate geodynamic processes commonly associated with the formation of magmatic rifted margins. Cenozoic rifting and volcanism in East Africa represent the earliest stage in the development of a rifted continental margin, and East Africa is one of the few places where geodynamic processes may be active that could lead to the development of a magmatic rifted margin. The Precambrian tectonic framework of East Africa is characterized by an Archean craton (Tanzania Craton) surrounded by Proterozoic mobile belts. An extensive nonmagmatic rift system associated with the separation of Madagascar from Africa developed in the mobile belts during the Permian-Cretaceous. In the Cenozoic, two rift branches (Western Rift and Eastern Rift) formed in the mobile belts. Volcanism is present in both branches, but most of it is concentrated in the Eastern Rift. Crustal structure away from the Cenozoic rifts is typical for Precambrian crust. Similarly, uppermost mantle structure across East Africa does not appear to have been altered, except beneath the rift valleys. Deeper in the upper mantle a thick (~200 km) lithospheric keel is found under the Tanzania Craton, and a broad (200-400 km wide) thermal anomaly extending to a depth of at least 400 km is beneath the Eastern Rift. Several models cannot fully account for the tectonic history of East Africa and/or the structure of the crust and upper mantle, including edge ×ow in the convecting mantle around the keels of Archean cratons, broad (> 500 km wide) thermal upwellings originating in the lower mantle, a relatively stationary plume head, and a plume head that ×ows outward along topography on the underside of the lithosphere. Consequently, these models are not strong candidates for the origin of volcanism and rifting in East Africa. Models with two or more plume heads, however, can explain the relevant observations, and therefore a multiple plume head explanation for the rifting and volcanism in East Africa is favored. These µndings further call into question the viability of nonplume models for the formation of magmatic rifted margins, particularly models invoking edge ×ow in the convecting mantle around cratonic keels.

Rift induced delamination of mantle lithosphere and crustal uplift: a new mechanism for explaining Rwenzori Mountains’ extreme elevation?

“…A simple estimation of the Lake Albert basin age can be done by dividing the maximum sediment thickness of about 4 km by the actual observed sedimentation rate of 0.27 mm/year giving Nyblade (1997Nyblade ( , 2002 and Koehn et al (2008a, b) Int J Earth Sci (Geol Rundsch) (2010) 99: 1511-1524 1513 14 Ma (Schlüter 1997), but the rate probably was episodic (Bauer et al 2008). Karner et al (2000) determined a cumulative extension of 6-16 km, division by the actual drift rate of 2.1 mm/year (Stamps et al 2008) results in a range from 3 to 8 Ma, but again the rate surely was not constant.…”

“…2) (Nyblade 2002;Chorowicz 2005). Kampunzu et al (1998) propose an along axis southward propagation of the WR, based on a first cycle of volcanism at 11 Ma in the Virunga region (North Kivu), at 10 Ma in Bukavu (South Kivu) and at 9 Ma in Rungwe (near the triple junction, southern end of WR).…”

On the relationship between extension and anisotropy: Constraints from shear wave splitting across the East African Plateau

“…Early studies used seismic refraction data and observations from teleseismic and regional earthquakes to examine crust and mantle structure. These studies yielded 40-48 km Moho depth estimates beneath unrifted crust and 20-to 32-km depths under the rift valleys (see review by Nyblade [2002] and references therein). We discuss more recent results below.…”

“…[5] The Tanzania craton is bordered to the south and southwest by the Paleoproterozoic Ubendian Belt, a southeast striking strike-slip shear belt of granulites and amphibolites deformed during the Ubendian orogeny (2100-2025 Myr ago) [Lenoir et al, 1994; Theunissen et Tanzania Network, 1999-2002 stations (KMBO, MBAR, and NAI), and Kenya Rift International Seismic Project (KRISP) 1990 Network [Gao et al, 1997]. The inset map in the upper right corner shows the general Precambrian structural trends in the mobile belts and craton [Holmes, 1951;Cahen et al, 1984;Shackleton, 1986] (see also Figure 8) and the approximate boundaries of the East African Plateau (dotted line).…”

Long lasting epeirogenic uplift from mantle plumes and the origin of the Southern African Plateau

“…A region of low seismic wave speeds has been imaged in the upper mantle beneath much of East Africa, providing strong evidence for thermally perturbed structure [e.g., Fuchs et al, 1977, and references therein; Ritsema et al, 1998;Nyblade et al, 2000;Ritsema and van Heijst, 2000;Debayle et al, 2001;Nyblade, 2002;Weerarantne et al, 2003]. Numerous hot spots, manifest as seamount chains, ridges and rises, are found within the southeastern Atlantic Ocean basin, and provide evidence of lithospheric reheating.…”