CRUSTAL STRUCTURE BENEATH THE KENYA RIFT FROM AXIAL PROFILE DATA

Modelling of the KRISP 90 axial line data shows that major crustal thi nning occurs along the axis of the Kenya Rift from Moho depths of 35 k m in the south beneath the Kenya Dome in the vicinity of Lake Naivasha to 20 km in the north beneath Lake Turkana. Low P-n velocities of 7.5 -7.7 km/s are found beneath the whole of the axial line. The results i ndicate that crustal extension increases to the north and that the low P-n velocities are probably caused by magma (partial melt) rising fro m below and being trapped in the uppermost kilometres of the mantle. A long the axial line, the rift infill consisting of volcanics and a min or amount of sediments varies in thickness from zero where Precambrian crystalline basement highs occur to 5-6 km beneath the lakes Turkana and Naivasha. Analysis of the P-g phase shows that the upper crystalli ne crust has velocities of 6.1-6.3 km/s. Bearing in mind the Cainozoic volcanism associated with the rift, these velocities most probably re present Precambrian basement intruded by small amounts of igneous mate rial. The boundary between the upper and lower crusts occurs at about 10 km depth beneath the northern part of the rift and 15 km depth bene ath the southern part of the rift. The upper part of the lower crust h as velocities of 6.4-6.5 km/s. The basal crustal layer which varies in thickness from a maximum of 2 km in the north to around 9 km in the s outh has a velocity of about 6.8 km/s.