Apatite fission-track analysis of Kenyan basement rocks: constraints on the thermotectonic evolution of the Kenya dome. A reconnaissance study

Abstract

A total of 43 apatite fission-track ages, together with track length measurements (36 on confined tracks, 7 on projected tracks) are reported from the crystalline basement rocks of Kenya (Pan-African Mozambique Belt and late Archean/ early Proterozoic Nyanza Shield). The ages range from 27 to 319 Ma. Regional relationships between apatite fission-track age and elevation, together with fission-track length information, show that the tectonothermal evolution of the basement in Kenya was not uniform but regionally variable, with a clear relationship to Mesozoic-Recent rifting. Samples from southwest Kenya with ages between 289 and 319 Ma show unimodal confined track length (CTL) frequency distributions with relatively high means (12.75–14.00 μm) and low standard deviations (1.23–1.58 μm). These samples were probably cooled to temperatures of the total stability zone (TSZ) or very close to them around 310 Ma ago. One sample has a significantly lower age of 245 Ma and its CTL distribution is characterized by a lower mean and a higher standard deviation. This suggests partial annealing of fission-tracks during a later thermal event. Samples from the Cherangani Hills were subjected to a thermal event at about 60 Ma B.P., which caused complete annealing in the lowermost parts but only partial annealing in the higher parts of the exposed section. Samples from northwest Kenya have Tertiary ages and broad, partly bimodal CTL distributions, indicating partial annealing during Tertiary reheating, possibly related to Oligocene-Miocene volcanic activity. The basement on the eastern flank of the Kenya Rift shows a pattern of apatite fission-track systematics dominated by a rapid decrease in age away from the Cenozoic Kenya Rift towards Mesozoic-early Tertiary paleorifts (Anza Trough and Lamu Embayment). Fission-track ages near the Kenya Rift vary between 243 and 176 Ma but decrease to minimum values between 110 and 54 Ma adjacent to the palaeorifts. In the same direction, CTL frequency distributions change from mixed or bimodal types with low means (11.78–12.75 μm) and high standard deviations (1.55–2.34 μm) to more or less unimodal types with higher means (13.08–13.95 μm) and lower standard deviations (1.28–1.98 μm). This pattern is interpreted as being the result of partial to complete annealing of fission tracks by thermal events related to Cretaceous-Tertiary rifting in the Anza Trough, and to late Cretaceous intrusive activity along the western margin of the Lamu Embayment. In the Taita Hills (southeast Kenya) an apparent age gradient of about 25 m/Ma may represent a fossil partial annealing zone which was installed after the late Cretaceous event. None of the observed regional apatite age-elevation profiles shows a “break in slope” suggesting that the amount of shoulder uplift during formation of the Kenya Rift was not enough to exhume the pre-uplift total annealing zones.