Deep seismic sounding in the Turkana depression, northern Kenya Rift

Abstract

Existing models of the structure and evolution of the Kenya Rift such as pure shear lithospheric extension, extension by simple shear or rift development by diapiric upwelling of an asthenolith, were based upon surface geology and a few geophysical (mainly gravity and seismic) data sets. Since the knowledge of the lithospheric structure plays an important role to distinguish between these different models, the Kenya Rift International Seismic Project was conducted in 1990 in the area of the Kenya Rift. The project involved a detailed multi-fold refraction wide-angle reflection line in the northern Kenya Rift along the western shore of Lake Turkana which is the most prominent feature of the so-called Turkana depression. Under-water explosions were used as sources and a good signal-to-noise ratio was obtained allowing the identification of arrivals from the crust and upper mantle. The most important result of modelling the data is a crustal thickness of 21 km in the area of Lake Turkana, whereas in the area of the Kenya dome the crustal thickness is 34 km. This points to an extension not previously expected in the northern Kenya Rift. The Moho discontinuity appears as a transition zone in the northern part of the profile, whereas in the southern part it changes to a first-order discontinuity. The largest vertical and lateral heterogeneities are observed in the rift infill displaying basins of varying thickness. The upper and lower crust which are separated by the Conrad discontinuity (the velocity changes from 6.2 to 6.4 km/s here) show very little lateral heterogeneity. The P-wave velocity in the upper mantle was modelled to be 7.7 km/s and thus can be distinguished from that in the southern Kenya Rift, where a velocity of 7.5 km/s was observed. The crustal structure of the graben supports a model of a southerly propagation of the rift.