Abstract
The crustal and uppermost-mantle structure of major units of the Afro-Arabian
rift system has been consecutively investigated by seismic-refraction
surveys in the Jordan-Dead Sea rift, the Red Sea, the Afar depression
and the East African rift of Kenya. With the exception of the Jordan-Dead
Sea transform, the entire Afro-Arabian rift system is underlain by
anomalous mantle with Pn-velocities less than 8 km/s, while under
the rift flanks the velocity is clearly equal to or above 8.0 km/s.
Various styles of rifting have been found. Oceanic crust floors the
axial trough of the southern Red Sea rift, thinned continental crust
underlies the margins of the Red Sea as the Afar depression and the
northern Kenya rift. On the other hand, 30-35-km-thick continental
crust is found both under the Jordan-Dead Sea rift, where strike-slip
rifting is active and thinning towards the Mediterranean occurs,
and under the central Kenya rift, where updoming is apparently the
controlling feature. While the transition from thinned continental
to 5-6-km-thick oceanic crust in the centre of the Red Sea appears
to be more gradual, the transition from rift-related structure to
undisturbed continental crust of 40 +- 5 km thickness is mostly rather
abrupt. The seismic data indicate various stages of rifting evidenced
by different styles of crustal strucre and they imply the presence
of heated uppermost-mantle under most parts of the rift system, possibly
related to plume activity. Local volcanism may disrupt and/or underplate
the crust in places, altering in particular the structure of the
lower crust. Progressive thickening of the rifted crust away from
the oceanized centres in the southern Red Sea and Gulf of Aden towards
north and south may be viewed as an evolutionary sequence which,
however, may be difficult to explain when viewing the Afro-Arabian
rift system as an active rift controlled by plume activity.
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