An integrated geophysical analysis of the upper crust of the southern Kenya rift

Abstract

SUMMARY Previous interpretations of seismic data collected by the Kenya Rift International Seismic Project (KRISP) experiments indicate the presence of crustal thickening within the rift valley area beneath the Kenya dome, an uplift centred on the southern part of the Kenya rift. North of the dome, these interpretations show thinning of the crust and an increase in crustal extension. To the south near the Kenya/Tanzania border, crustal thinning associated with the rift is modest. Our study was aimed at further investigating crustal structure from this dome southwards via a detailed analysis focused on upper crustal structure. We used results from surface geological mapping, drill hole data from water wells and geothermal exploration wells, KRISP 85 seismic data for a profile across the rift, KRISP 85 and 90 seismic data for a profile along the rift axis and KRISP 94 seismic data for a profile crossing southernmost Kenya to constrain gravity modelling and construction of integrated models of crustal structure. Our integrated analysis produced the following results concerning the structure and evolution of the southern Kenya rift: (1) the graben master faults are consistently located along the western margin of the rift valley, and there is no evidencefor half-graben polarity reversals for a distance of about 300 km; (2) there is no axial (north‐south) crustal symmetry with respect to the apex of the Kenya dome, and the crustal thickness may be as much related to pre-rift crustal type and thickness as it is to crustal thickening and modification by magmatic processes; (3) the pre-existing lithospheric contrast between the Archaean and Proterozoic basement terranes played a significant role in the location and structural geometry of the rift; (4) south of latitude 1uS, low velocities and densities observed under the western flank of the rift probably represent reworked Archaean Tanzanian craton; (5) magmatic modification of the upper crust is modest except near the major Quaternary volcanic centres that produce a series of isolated axial gravity highs; (6) the other element of the axial gravity high is an intrarift horst block that extends along the axis of most of the rift valley.