Zusammenfassung
On the basis of a test experiment in 1985 (KRISP 85) an integrated
seismic-refraction/teleseismic survey (KRISP 90) was undertaken to
study the deep structure beneath the Kenya rift down to depths of
100-150 km. This paper summarizes the highlights of KRISP 90 as reported
in this volume and discusses their broad implications as well as
the structure of the Kenya rift in the general framework of other
continental rifts. Major scientific goals of this phase of KRISP
were to reveal the detailed crustal and upper mantle structure under
the Kenya rift, to study the relationship between mantle updoming
and the development of sedimentary basins and other shallow structures
within the rift, to understand the role of the Kenya rift within
the Afro-Arabian rift system and within a global perspective and
to elucidate fundamental questions such as the mode and mechanism
of continental rifting. The KRISP results clearly demonstrate that
the Kenya rift is associated with sharply defined lithospheric thinning
and very low upper mantle velocities down to depths of over 150 km.
In the south-central portion of the rift, the lithospheric mantle
has been thinned much more than the crust. To the north, high-velocity
layers detected in the upper mantle appear to require the presence
of anistropy in the form of the alignment of olivine crystals. Major
axial variations in structure were also discovered, which correlate
very well with variations in the amount of extension, the physiographic
width of the rift valley, the regional topography and the regional
gravity anomalies. Similar relationships are particularly well documented
in the Rio Grande rift. To the extent that truly comparable data
sets are available, the Kenya rift shares many features with other
rift zones. For example, crustal structure under the Kenya, Rio Grande
and Baikal rifts and the Rhine Graben is generally symmetrically
centered on the rift valleys. However, the Kenya rift is distinctive,
but not unique, in terms of the amount of volcanism. This volcanic
activity would suggest large-scale modification of the crust by magmatism.
Although there is evidence of underplating in the form of a relatively
high-velocity lower crustal layer, there are no major seismic velocity
anomalies in the middle and upper crust which would suggest pervasive
magmatism. This apparent lack of major modification is an enigma
which requires further study.
Nutzer