Steady rifting in northern Kenya inferred from deformed Holocene lake shorelines of the Suguta and Turkana basins.

Abstract

A comparison of deformation rates in active rifts over different temporal scales may help to decipher variations in their structural evolution, controlling mechanisms, and evolution of sedimentary environments through time. Here we use deformed lake shorelines in the Suguta and Turkana basins in northern Kenya as strain markers to estimate deformation rates at the 103–104 yr time scale and compare them with rates spanning 101–107 yr. Both basins are internally drained today, but until 7 to 5 kyr lake levels were 300 and 100 m higher, respectively, maintained by the elevation of overflow sills connecting them with the Nile drainage. Protracted high lake levels resulted in formation of a maximum highstand shoreline — a distinct geomorphic feature virtually continuous for several tens of kilometers. We surveyed the elevation of this geomorphic marker at 45 sites along > 100 km of the rift, and use the overflow sills as vertical datum. Thin-shell elastic and thermomechanical models for this region predict up to ~ 10 m of rapid isostatic rebound associated with lake-level falls lasting until ~ 2 kyr ago. Holocene cumulative throw rates along four rift-normal profiles are 6.8–8.5 mm/yr, or 7.5–9.6 mm/yr if isostatic rebound is considered. Assuming fault dips of 55–65°, inferred from seismic reflection profiles, we obtained extension rates of 3.2–6 mm/yr (including uncertainties in field measurements, fault dips, and ages), or 3.5–6.7 mm/yr considering rebound. Our estimates are consistent, within uncertainties, with extension rates of 4–5.1 mm/yr predicted by a modern plate-kinematic model and plate reconstructions since 3.2 Myr. The Holocene strain rate of 10− 15 s− 1 is similar to estimates on the ~ 106 yr scale, but over an order of magnitude higher than on the ~ 107 yr scale. This is coherent with continuous localization and narrowing of the plate boundary, implying that the lithospheric blocks limiting the Kenya Rift are relatively rigid. Increasing strain rate under steady extension rate suggests that, as the magnitude of extension and crustal thinning increases, the role of regional processes such as weakening by volcanism becomes dominant over far-field plate tectonics controlling the breakup process and the transition from continental rifting to oceanic spreading.