INTEGRATED STUDIES OF THE STRUCTURE AND EVOLUTION OF THE EAST AFRICAN RIFT SYSTEM

The National Science Foundation's Continental Dynamics (CD) program supported a series (1985, 1990, 1994) of integrated investigations that were part of the Kenya Rift International Seismic Project (KRISP). The purposes of this effort were to investigate the structure and evolution of the East African rift (EAR) and to examine the implications of the results for understanding rifting processes on a global basis. The KRISP results clearly demonstrate that the Kenya rift is associated with sharply defined lithospheric thinning and very low upper mantle velocities down to depths of over 150 km. Major axial variations in crustal and uppermost structure were also discovered and correlate well with variations in the amount of extension, the physiographic width of the rift valley, the regional topography, and the regional gravity anomalies. Magmatic activity has certainly modified the crust but not to the extent previously suggested. This modification has taken the form of an underplated layer that constitutes the lowermost crust and local mafic intrusions beneath volcanic centers. More recently, the CD program supported the Ethiopia Afar Geoscientific Lithospheric Experiment (EAGLE) that was undertaken to provide a snapshot of lithospheric break-up above a mantle upwelling at the transition between continental and oceanic rifting. The focus of the project was the northern Main Ethiopian Rift (NMER) that cuts across the uplifted Ethiopian plateau, which is associated with the Eocene–Oligocene Afar flood basalt province. The initial results of the controlled source seismic portion of this effort are surprisingly similar to what was found in Kenya in many respects. For example, the velocity within the mid- and upper crust varies from ~6.1 km/s beneath the rift flanks to ~6.6 km/s locally beneath overlying linear Quaternary axial magmatic centers. In addition, the crustal thickness along the NMER axis varies from ~40 km in the SW to ~26 km in the NE beneath southern Afar. This variation is interpreted as reflecting the transition from rifted continental crust to the south to a crust in the north that could be almost entirely composed of mantle-derived mafic melt.