Tectonic and magmatic processes in the Eastern Branch of the East African Rift and implications for magmatically active continental rifts

Abstract

Magmatism has played a major role in the development of the Eastern Branch of the East African Rift System. Geological, geochemical and geophysical constraints indicate that magmatism and tectonism vary significantly with depth and along strike. In the uppermost crust, faulting, fissuring, block rotation and volcanic construction have been widely documented. However, in the middle to lower crust magmatic intrusion dominates. In some places, the volume of intrusive material generated per kilometer of rift axis equals or exceeds the cross-sectional area created by tectonic extension, thereby maintaining or even increasing the crustal thickness. Elsewhere, magmatism is much less voluminous, and tectonic extension may result in thinning of the crust. Basic and ultrabasic igneous bodies clustered in the crust and uppermost mantle beneath the rift valley probably account for the observed positive axial gravity anomalies and high compressional wave velocities centered there. Incompatible trace-elementratios of Quaternary volcanos suggest that individual magmatic diapirs have intruded the crust and upper mantle of the rift. The diapirs may be fed by a vertical, wedge-like body of asthenosphere with a concentration of partial melt along its apex. In Turkana, where seismic reflection data are available, diapirs corresponding to Quaternary volcanos are located beneath the centers of individual half-graben rift basins. A similar relation between rift basins and Quaternary volcanic centers is inferred for the entire Eastern Branch. The magnitude of extension across the rift units is probably insufficient to cause mantle upwelling and extensive partial melting. Thus, the geometry of rifting at the surface must be controlled by that of the mantle diapirism and related magmatism, not vice versa. The implied style of rifting has important implications for the partitioning of extensional strain through the lithosphere and the constitution of the crust beneath the rift. Aulacogens and rifted continental margins, such as the southern Red Sea and the Norwegian-E. Greenland margins, that formed from magmatically active rifts, may have passed through a similar stage of evolution.