Abstract
The Kenya Rift is an active continental rift that has developed since
the Late Oligocene. Although a thermal origin for the rifting episode
is indicated by the scale of volcanism and its relative timing with
uplift and faulting, the influence of pre-existing lithospheric structural
controls is poorly understood. The interpretation of a 430-km-long
seismic refraction and gravity line across the southern part of the
Kenya Rift shows that the rift is developed across a transition zone,
thought to represent the sheared Proterozoic boundary between the
Archaean Nyanza Craton and the mobile Mozambique Belt. This zone
of weakness has been exploited by the recent thermal rifting event.
The Moho is at a depth of 33 km beneath the Archaean craton in the
western part of the profile, and 40 km beneath the Mozambique Belt
in the east. A few kilometres of localised crustal thinning has developed
across the transition from thin to thick crust. At the surface, brittle
faulting has formed an asymmetric rift basin 3.6 km deep, filled
with low-velocity volcanic rocks. Basement velocities show a transition
across the same area from low velocities (6.0 km/s) in the Archaean,
to high velocities (6.35 km/s) in the Proterozoic. Mid-crustal layers
show no deformation that can be attributed to the rifting event.
Poorly constrained upper mantle velocities of 7.8 km/s beneath the
southern rift confirm the continuation of the axial low-velocity
zone imaged in previous seismic experiments. This is interpreted
as the effect of small degrees of partial melt caused by elevated
mantle temperatures. Gravity modelling suggests a contribution to
the Bouguer anomaly from below the Moho, invoking the need for deep
density contrasts. The regional gravity gradient necessary to model
the Bouguer anomaly is used as supporting evidence for mantle-plume
type circulation beneath the uplifted East African Plateau to the
west of the Kenya Rift.
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