Structural framework and denudation history of the flanks of the Kenya and Anza Rifts, East Africa

Abstract

The Kenya Rift in central Kenya (between ∼1° and 2° north latitude) is flanked by vast areas of Proterozoic crystalline rock where traditional stratigraphic markers useful for constraining Mesozoic to Recent structures are absent. Apatite fission track age and length data from about 100 samples collected from (1) relief profiles in north‐south trending mountain ranges, and (2) east‐west transects between mountain ranges constrain the structural framework of Cretaceous through Tertiary faulting in these flanking areas. Data from the profiles reveal regionally consistent age/elevation trends characterized by three distinct intervals, of 600 to 1200 m altitude, within which the apatite apparent ages are nearly concordant. The three isochronous age/elevation intervals yield apparent ages of ∼180, ∼115, and ∼65 Ma and record times of cooling starting at ≥220, 140–120, and 70–60 Ma. We interpret the cooling to be related to episodes of relatively rapid denudation separated in time by periods of slower exhumation. The isochronous intervals of crust are regionally consistent and relative offsets between the intervals allow fault displacements and block tilts to be estimated, when used in conjunction with the variation of fission track age and length along the transects. The fault block geometry revealed by the displaced isochronous layers indicates that normal faults related to rifting, with relative displacements >1 km, extend at least 100 km east of the Kenya Rift Valley. The data indicate a consistent direction of block tilting for the segment of the rift studied, which is similar to the asymmetric extension observed for other sections of the East Africa Rift System and other narrow rifts. The regional block faulting in this area probably occurred during early to middle Tertiary time associated with the late stages of extension in the Anza Rift and/or the early stages of extension of the Kenya Rift, in a setting similar to the present morphology of northern Tanzania. Therefore focusing of faulting within a central rift valley in Kenya appears to have followed a phase of regional extension with minor total strain.

Cenozoic extension in the Kenya Rift from low‐temperature thermochronology: Links to diachronous spatiotemporal evolution of rifting in East Africa

“…Similar results were reported by Foster and Gleadow [1996] and Spiegel et al [2007], with very slow cooling along both the western (Cherangani Hills) and eastern (Ndoto Mountains) rift shoulders from the Eocene through the middle Miocene.…”

“…Foster and Gleadow, 1996;Spiegel et al, 2007], the samples remained at a relatively constant position below the surface (~3 km). This thermal history is quite analogous to the Grand Canyon [Fox and Schuster, 2014], suggesting that AHe results from there are applicable to our study.…”

“…However, apatite (U-Th)/He and fission-track data from the western rift flank (Cherangani Hills of northern Kenya) and from the presentday eastern rift flank southeast of Lake Turkana (Figs. 1 and 2) only reveal clear episodes of cooling during the late Cretaceous to Paleocene and during the late Neogene [Foster and Gleadow, 1996;Spiegel et al, 2007].…”

“…In the greater Turkana region and the western riftshoulder areas of northern Kenya (Fig. 1), these investigations have provided helpful constraints on the regional onset of Mesozoic and early Cenozoic rifting [Foster and Gleadow, 1996;Spiegel et al, 2007]. However, the spatial extent of the related tectonothermal events, the formation of older rift basins underlying the late Cenozoic rifts, and the role of their structures in influencing Miocene to Recent rifting is unclear.…”

“…Second constraint boxes were set to ~150 to 30°C at 40 to 20 Ma for HeFTy and for QTQ, based on the AFT ages, sedimentary evidence in the hanging wall, and volcanic flows on top of the footwall. Third constraint boxes were set to 220 to 160°C at 75 to 40 Ma, based on an early to middle Tertiary regional extension with minor strain, which is also associated with the latest stages of extension of the Anza Rift in the north [Foster and Gleadow, 1996]. Again, HeFTy models always end with a constraint at the present-day surface temperature, ranging from 25 to 15°C, and QTQt models terminate at present-day temperatures ranging from 0° to 50°C The second reheating model used similar constraints as the monotonic model, but with slight changes in the time-temperature limits of the boxes.…”