Upper mantle seismic velocity structure beneath Tanzania, east Africa: Implications for the stability of cratonic lithosphere

Abstract

Abstract. The assertion of cratonic stability put forward in the model for deep continental structure can be tested by examining upper mantle structure beneath the Tanzania Craton, which lies within a tectonically active region in east Africa. Tomographic inversions of about 1200 teleseismic P and $ travel times indicate that high-velocity lithosphere beneath the Tanzania Craton extends to a depth of at least 200 km and possibly to 300 or 350 km. Based on the thickness of mantle lithosphere beneath Archean cratons elsewhere, it appears that the mantle lithosphere of the Tanzania Craton has not been extensively disrupted by the Cenozoic tectonism in east Africa, thus corroborating the assertion of cratonic stability in the model for deep continental structure. The presence of thick, high-velocity structure beneath the Tanzania Craton implies relatively low temperatures within the cratonic mantle lithosphere, consistent with relatively low surface heat flow. The thick cratonic keel is surrounded by low seismic velocity regions beneath the east African rifts that extend to depths below 400 km. Our models show a shear velocity contrast between the cratonic lithosphere and the uppermost mantle beneath the eastern branch of the rift system of about 5% to 6%, but from resolution experiments we infer that this contrast could be underestimated by as much as a factor of 1.5. We attribute about half of this velocity contrast to the depleted composition of the cratonic keel and the other half to thermal alteration of upper mantle beneath the rifts. Low-density structures that may be required to provide buoyant support for the elevation of the Tanzania Craton must reside at depths greater than about 300-350 km.

Survival of Archean cratonal lithosphere

“…This range may be the unstable result of subtracting two comparable numbers to get mantle heat flow and dividing by this number to get lithospheric thickness. Bank et al, 1998;Ritsema et al, 1998;Simons et al, 1999;Priestley, 1999;Ritsema and van Heist, 2000]. The more reliable studies take explicit account of downward smearing of cratonal features, a well-known artifact of tomographic inversion.…”

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

“…[58] To show regions of lithospheric thinning and upper mantle structure at depth, we overlay the final station estimates upon the results at 200-km depth of an S wave tomographic image [Ritsema et al, 1998] (Figure 9). The APM of Africa appears to be slow, and thus it has been a challenge to constrain the direction.…”

“…[12] Structure deeper in the upper mantle beneath the orogenic belts and craton in Tanzania has been modeled by inverting relative travel times of teleseismic P and S waves [Ritsema et al, 1998] and Rayleigh wave phase velocities [Weeraratne et al, 2003] for upper mantle seismic velocity variations and by geographically stacking receiver functions to image topography on the 410-and 660-km discontinuities . A low-velocity zone is found under the craton at $170 km depth.…”

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.…”