Geophysical investigation of active continental rifting in southern Kenya

• Main Author: Birt, Christopher Simon
• Published: University of Leicester 1996
• Subjects: 551
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.674485

The history of uplift, volcanism and faulting, and past geophysical studies suggest that extension in the Kenya Rift is actively driven by an upwelling thermal anomaly. The KRISP 94 experiment included a 440 km seismic refraction profile in southern Kenya to investigate the deep structure of the rift, 150 km east of the exposed boundary between the Archean Nyanza craton and the Proterozoic Mozambique orogenic belt. Combined travel-time and gravity interpretations produce an integrated crustal and upper mantle model, showing extensive thrusting of the mobile-belt over the cratonic margin. The rift itself has developed above the buried suture between these units. The asymmetric rift basin (filled with low velocity lavas or sediments) has a maximum depth of 4 km adjacent to the western bounding fault. The high velocity (7.0-7.2 kms-1) lowest crustal layer is modified beneath the rift, possibly by the addition of cumulate layers (products of fractional crystallization). A small amount of crustal thinning (1-2 km) directly beneath the rift axis suggests a pure-shear mechanism at depth. Minimum crustal extension is 4 km, but could be up to 10 km if the whole of the lowest crustal layer is a new magmatic addition. A long-wavelength regional gravity trend is consistent with the presence of a mantle plume beneath the craton to the west. A study of local earthquakes in Tanzania shows that the rift is propagating southwards along the craton margin. Many deep earthquakes (> 20 km) suggest cooler crustal temperatures than in Kenya, and preliminary travel-time tomography shows only minor crustal velocity variations. The results are consistent with rifting in Kenya being driven by an upwelling diapir, originating from a mantle plume beneath the craton. As it has spread, the diapir has been focussed along the suture between the Archean and Proterozoic units, but has not yet produced extensive crustal modification of the youngest part of the rift in Tanzania.