Аннотация
The seismic refraction-wide-angle reflection experiments carried out
in 1985 and 1990 in the Kenya rift (KRISP '85 and KRISP '90) show
major crustal thickness variations both along and across the rift.
Along the rift axis crustal thickness varies from 35 km in the south
beneath the Kenya dome to 20 km in the north beneath the Turkana
region. Due to the distribution of crustal thickness beneath the
rift flanks, it can be stated that the major amount of variation
in crustal thickness along the rift axis is due to the Tertiary rifting
episode. The northwards decrease in crustal thickness can be correlated
with changes in surface topography (northwards decrease), rift width
(northwards increase), surface estimates of extension (5-10 km in
the south and 35-40 km in the north) and Bouguer gravity, the regional
northwards increase of which can be explained entirely by the change
in crustal thickness. Below the 750 km long axial rift profile, uppermost
mantle Pn velocities are low, being 7.5-7.7 km/s. However, under
the northern part of the rift two layers with velocities of 8.1 km/s
and 8.3 km/s are embedded in the low-velocity mantle material at
40-45 km and 60-65 km depth, respectively. In contrast, the wide-angle
data show that beneath the Kenya dome, in the southern part of the
rift, low mantle velocities occur down to at least 65 km depth. This
mantle velocity structure is indicative of the depth to the onset
of melting being at least 65 km beneath the northern part of the
rift and thus not being shallower than the depth (45-50 km) to the
onset of melting under the Kenya dome to the south. A profile across
the rift north of the Kenya dome at the latitude of Lake Baringo
shows that the low uppermost mantle Pn velocity of 7.5-7.7 km/s and
crustal thinning of 5-10 km is confined to below the surface expression
of the rift. An abrupt change in Moho depths and Pn velocities occurs
as the rift boundaries are crossed. Beneath the rift flanks, normal
Pn velocities of 8.0-8.2 km/s occur. The presence of hot mantle material
beneath the Kenya dome since the onset of volcanism here at 15-20
Ma is still compatible with the abrupt change in mantle P-wave velocities
as the rift boundaries are crossed. Petrological interpretation of
the seismic velocities indicates a few (up to 5) percent basaltic
melt in the mantle below the rift except in the two layers with velocities
greater than 8.0 km/s under the northern part of the rift where some
crystal orientation (anisotropy) is necessary. Below about 45-50
km depth beneath the southern part of the rift the magma could exist
as in situ partial melt. The above results, taken together with results
from teleseismic studies, petrology and surface geology, indicate
anomalously hot mantle material appearing below the present site
of the Kenya rift at about 20-30 Ma. The active uprising of this
anomalously hot mantle material since this time has given rise to
widespread volcanism along the whole length of the rift and has modified
the crust beneath the rift by mafic igneous underplating and intrusion,
especially into the basal crustal layer. Accompanying the uprise
of the anomalously hot mantle material minor crustal extension (5-10
km) has occurred beneath the Kenya dome in the southern part of the
rift where crustal thickness is large (35 km). Under the Turkana
region in the northern part of the rift, a greater amount of extension
(35-40 km) has taken place and the crustal thickness is small (20
km), although the depth to the onset of melting under the northern
part of the rift is, if anything, greater than under the southern
part of the rift.
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