| dc.description.abstract | Lake Magadi area, located in the southern part of the Kenya Rift is a high seismic region
where rifting is still active. The quantitative analysis of the orientation and size
characteristics of the fracture and fault data gathered at different scales from a SPOT
image, aerial photos and in the field within key areas resulted in the following points.
There is a self-similarity in fault and fracture principal directions recognised at the
different telescopic scales. 1 : 125 000 SPOT image, 1 : 76 000 aerial photos, and
outcrop scale fieldwork revealed three important structural orientations. They are N015
± 15 E, N015 ± 15 W, and N090 ± 20 E. The N015 ± 15 E regional direction is the one
which structures this southern segment of the Kenya Rift. It corresponds to normal faults
which size and spacing distribution is asymmetric within the western and eastern parts
of the rift floor that are separated by the axial zone. A decrease of mean length, spacing
and throw from margins towards the rift axis characterises mainly the eastern part of the
rift floor, thus emphasising the described geometrical asymmetry. A north towards south
rift propagation is suggested by the decrease of fault length toward south.
The N015 ± 15 W is found mainly concentrated in a fracturing zone of same orientation.
This structural zone crosses Lake Magadi through its western arm, and has been linked
by previous authors to the famous Aswa-Nandi-Loita shear zone. The presence of this
Precambrian direction in all the geological formations of Magadi area indicates the
continuous reactivation of this weak zone during the Cenozoic and Quaternary rift
deformations, under. the control of the regional extension direction. Previous structural
interpretations of the southern Kenya Rift have assumed a clockwise rotation of the
extension direction to NW-SE during the Quaternary times, However, the results of the
present study suggest that this assumption is not realised yet in Magadi area. Rather,
an E-W to ESE-WNW- extension is still the dominant one as revealed from the
neotectonic joint analysis. This direction is normal to the regional NNE normal fault trend
which structure this segment of the rift, and is coa~for.rned by a dextral oblique-slip component shown on N015 ± 15 W faults. First results from a microseismic survey of
the same study area are consistent with and support this conclusion. | en |
