Structural analysis of neotectonic joints at Lake Magadi area, and their relationship with regional faults using remote sensing imagery interpretation

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.