Аннотация
Following two previous experiments in 1985 (KRISP 85) and 1989-1990
(KRISP 90), a series of geophysical experiments was undertaken in
1993-1995 (KRISP 94) to study the lithospheric structure of the southern
Kenya rift down to depths of greater than 100 km, with special emphasis
on the Chyulu Hills, a complex of volcanic centres on the eastern
flank of the rift. KRISP 94 involved a teleseismic tomography experiment
of the Chyulu Hills area in July and August 1993, a seismic refraction-wide-angle
reflection survey across southern Kenya from Lake Victoria to the
Indian Ocean in February 1994, seismicity studies of southern Kenya
from 1993 to 1995, a special seismicity study of the Lake Magadi
area in February 1994, a gravity study along the seismic-refraction
lines before and after the seismic-refraction study, and a magnetotelluric
study of southern Kenya in February 1995. Major scientific goals
of the project were to reveal the detailed crustal and upper-mantle
structure under the southern Kenya rift and its flanks for several
100 km to the west and to the east and their evolution, to study
the relationship between deep crustal and uppermost mantle structure,
to learn more on the development of sedimentary basins and volcanic
features on the flanks and its relation to the Kenya rift, to obtain
information on the temperature conditions underneath the rift and
its flanks, to perform a particular integrated and calibrative study
of seismological and petrological data in the Chyulu Hills, and to
understand the processes which are producing extension, uplift, and
extensive magmatism. This report is an introduction to a series of
subsequent papers. It focuses on the technical description of the
main seismic surveys of the KRISP 94 effort and summarizes the key
results. During the teleseismic survey an array of 31 seismographs
was deployed to record teleseismic, regional and local events for
a period of about 3 months from June to August 1993. The elliptical
array covered an area about 150 km (N-S) x 100 km (E-W) and spanned
the central portion of the Chyulu Hills and its surroundings, with
an average station spacing of 10-30 km. The seismic refraction-wide-angle
reflection survey was carried out in a 2-week period in February
1994. It consisted of two profiles: one extending from Lake Victoria
across the western flank and the southernmost Kenya rift at Lake
Magadi, the other extending from Athi River near Nairobi across the
eastern flank of the rift, traversing the Chyulu Hills and terminating
at the Indian Ocean near Mombasa. A total of 204 mobile seismographs,
with an average station interval of about 2 km, recorded the energy
of underwater and borehole explosions to distances of up to 730 km.
Key results are as follows: (1) The crust reaches a maximum thickness
of up to 44 km under the Chyulu Hills. (2) Only a minor upwarping
of the crust-mantle boundary is seen under the rift proper in the
Lake Magadi area. (3) To the west the crust shallows to about 34
km near Lake Victoria, in contrast to the thickening of the crust
further north from the central part of the rift near Lake Baringo
towards the west. (4) There is a steep rise of the Moho east of the
Chyulu Hills towards the Indian Ocean. (5) P-wave velocities in the
uppermost mantle are above 8 km/s except under the rift proper and
under the Neogene volcanic centre of the Chyulu Hills, where the
velocity is 7.9-8.0 km/s. (6) Under the Chyulu Hills, the Moho is
replaced by a gradual crust-mantle transition, and the low velocities
near the crust-mantle boundary extend to greater depths as evidenced
by teleseismic tomography which indicates a velocity decrease of
3-5\%, i.e. from 8.1-8.2 km/s to at least 7.9 km/s. Both effects
are probably caused by the local recent volcanic activity, but cannot
be interpreted as due to plume activity which is assumed to be present
under the Nyanza craton further west. (7) Gravity modelling and first
preliminary results of the magnetotelluric measurements support the
seismic-refraction and tomographic results. Furthermore, under the
western flank the magnetotelluric and gravity data indicate increased
conductivity and decreased density in the uppermost mantle below
60-80 km depth.
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