A morphotectonic study of an extensional fault zone in a magma-rich rift: the Baringo Trachyte Fault System, central Kenya Rift

The Baringo Trachyte Fault System is located within the central Kenya Rift and forms part of a N-S-trending linked extensional fault network. This fau lt system bounds to the west the 8 km deep Baringo Basin which itself lies within the axial valley of the central Kenya Rift. It mainly affects a midd le Pleistocene trachytic dome (510 ka), the so-called Baringo Trachyte (BT) , A morphotectonic study of the 10 km long BT master fault and associated d ownthrow geometries provides constraints on the evolution of a magma-type r ift fault system from an initial stage of crack opening through to propagat ion. A model of radial fault growth is proposed in order to account for the longitudinal segmentation of the main fault escarpment from the median par t to the tips. The small-scale half-graben geometry developed in the median high-strain zone is progressively accommodated laterally by both flexure a nd related narrow compensation grabens. The resulting crack swarms are well -developed at the free southern tip zone. Both the spatial distribution of rock-breaking products and their relations to the immediate hangingwall pro vide further evidence for this hypothesis. Well-developed screes and other gravity-driven structures (slumps) preferentially occur along the median pa rt of the Baringo Trachyte Fault Escarpment, probably as earthquake-induced features. The hangingwall fault zone shows an asymmetrical triangular-shap e with a maximum width of about half the length of the main scarp. This zon e of maximum deformation and subsidence appears to be laterally controlled by two major, conjugate, transverse basement discontinuities lying with a c onjugate geometry. Its internal architecture is dominated by antithetic wes terly-dipping normal faults bounding discrete half-grabens, locally infille d by syn-tectonic volcaniclastics. Chronological data on hydrothermal silic a filling open cracks on the BT footwall suggest that the master fault evol ution occurred from 345 to 198 ka, as the possible result of at least four major normal faulting earthquakes. (C) 2000 Elsevier Science B.V. All right s reserved.