Article,
A crustal and uppermost mantle cross-sectional model of the Kenya Rift derived from seismic and gravity data
Tectonophysics, 236 (1-4): 217--250 (Sep 30, 1994)
DOI: 10.1016/0040-1951(94)90178-3
Abstract
The most significant implication of a combined seismic and gravity
model derived from the data along the KRISP 90 cross-rift line on
the Equator concerns the compensation mechanism for the uplifted
East African plateau. The seismic model shows that despite an elevation
difference of 400 m the thickness of the crust at both ends of the
profile is approximately the same at about 34 km b.s.l. A regional
gradient needs to be added to the gravity field calculated from the
seismic model to agree with the observed. This regional is consistent
with Ebinger et al.'s (1989) proposal that the plateau is being dynamically
supported by convective processes in the mantle. The principal features
revealed by the seismic modelling are: (1) the presence of the thick
graben infill, and in particular a very deep (8 km b.s.l.) low-velocity
body beneath the Kerio Valley adjoining the rift's western boundary,
the Elgeyo fault; (2) the correlation between seismic velocities
in the upper crust and different Precambrian (Archaean and Pan-African
Mozambique orogenic belt) crustal packages identified along the profile;
(3) a thickening of the crust beneath both margins of the rift; (4)
the apparent symmetry in crustal thinning mirroring the surface distribution
of Tertiary volcanics and sediments, suggesting a pure shear mechanism
of crustal extension; (5) the presence of a wide, anomalously low-velocity
(7.6-7.8 km/s) body at the base of the crust identified along the
axial line and which it has been argued must include partial melt.
This may be continuous with a deep low-velocity zone beneath the
rift previously identified from the study of teleseismic residuals;
and (6) the presence of an intra-mantle reflector at a depth of approximately
55 km beneath the western margin of the rift.
