Abstract
The KRISP flank line E converges with the Kenya rift at an angle of
about 45 deg and is approximately parallel to the older Anza graben
to the north. The depth to the basement is almost zero along the
entire onshore part of the profile with higher velocities at the
southeastern end indicative of extensive Precambrian gabbroic intrusions
in the upper crust. The Moho shallows steadily from about 35 km at
the southeastern end of the profile to about 24 km under Lake Turkana.
Even though the Moho rises fairly steadily, there is significant
heterogeneity in the crust above it. This shows that the extension
is unevenly distributed between the upper and the lower crust. The
Moho is laminated and variably reflective. Compared to the KRISP
cross-line D further south, the crust is unexpectedly thin and shows
extension increasing in a northerly direction. This extension is
probably not associated with the Anza and Kenya rifting but with
the profile's position on the slope of the Kenya dome. The indications
are that there is a relatively abrupt change to a 20-km Moho depth
near the Lake Turkana Central shotpoint. This change to a mid-rift
crustal thickness occurs not at the postulated margin at the southeastern
shore of Lake Turkana but at least 50 km further to the northwest.
We suggest that the position of this margin may need to be redefined.
The Pn velocity is quite high at 8.1 km/s. This may indicate either
a cold upper mantle or anisotropy. An upper-mantle reflector has
been identified between 15 and 20 km below the Moho. It dips gently
away from the rift.
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