THE MORPHOTECTONIC EVOLUTION OF RIFT-MARGIN MOUNTAINS IN CENTRAL KENYA - CONSTRAINTS FROM APATITE FISSION-TRACK THERMOCHRONOLOGY

Passive margin mountains and rift flank uplifts are common features of extended continental areas. Apatite fission-track thermochronology of Proterozoic rocks from mountain ranges east of the Kenya Rift in central Kenya reveals episodes of rapid cooling and denudation, in Early Cretaceous (130-110 Ma), Late Cretaceous-Paleocene (70-60 Ma), and possibly Miocene (approximately 10 Ma) times. During the Early Cretaceous approximately 1.2-2.0 km of denudation occurred in this area, and evidence for this is preserved in the Karisia Hills. The Late Cretaceous-Paleocene episode is well documented from escarpments along the western margin of the Anza Rift including the Mathews Range. Detailed fission-track age profiles up these escarpments indicate that 2.6-3.0 km of crust was denuded from this area in < 5 Ma. These periods of rapid denudation probably contributed to the deposition of thick sedimentary sequences in the adjacent Anza Rift. A third period of cooling during the Miocene is proposed on the basis of forward modeling of the apatite age and track length data from the Mathews Range samples. Denudation associated with Miocene cooling probably only amounted to approximately 500 m in most areas. The timing of the three denudational episodes can be correlated with intracratonic deformational events elsewhere in Africa, as well as with periods of extension in the Lamu Embayment, Anza Rift and Kenya Rift. The variation of apatite fission-track age and track length with elevation in the mountain ranges also provides a basis for reconstructing preserved parts of pre-Early Cretaceous and pre-Late Cretaceous apatite partial annealing zones. Offsets of these partial annealing zones indicate that the basement mountain ranges in central Kenya developed as rift margin mountains adjacent to the Anza Rift. Early and Late Cretaceous extension in the Anza Rift caused normal faulting with displacements of approximately 1 km and approximately 3-degrees of westward tilting of crustal blocks along its flanks.