The lithospheric structure of the Kenya Rift as revealed by wide-angle seismic measurements

Abstract

The Kenya Rift International Seismic Project (KRISP) seismic refraction-wideangle reflection experiments carried out between 1985 and 1994 show abrupt changes in Moho depths and Pn phase velocities as the rift boundaries are crossed. Beneath the rift flanks, normal Pn phase velocities of 8.0-8.3 kms -1 are observed, except for the Chyulu Hills volcanic field, east of the rift, where it is 7.9-8.0kms -I . Also to the east, some of the thickest crust (38-44km) encountered so far beneath Kenya has been observed over a distance of c. 300 km. However, beneath the surface expression of the rift itself, the uppermost mantle velocity of the Pn phase is anomalously low at 7.5-7.8 km s -1 throughout its length.Beneath the rift itself, there are major differences in crustal thickness, extension and upper mantle velocity structure between the north and the south. Beneath the section from the centre of the Kenya Dome southwards, where the extension is estimated to be 5-10km, the crust is thinned by c. 10 km to a thickness of 35 km, and the narrow low-velocity zone in the mantle extends to a depth of at least 65 km. However, in the north beneath Turkana, where the extension is 35-40km, the crust is only c. 20km thick and two layers with velocities of 8.1 and 8.3 km s -l are embedded in the low velocity mantle material at depths of 40-45 km and 60-65 km. This mantle velocity structure indicates that the depth to the onset of melting is at least 65 km beneath the northern part of the rift and is thus not shallower than the corresponding depth (45-50km) in the south. These results, taken together with those from teleseismic studies, petrology and surface geology, have been used to deduce that anomalously hot mantle material appeared below the present site of the Kenya Rift c. 20-30 Ma ago. This led to widespread volcanism along the whole length of the rift and modification of the underlying crust by mafic igneous underplating and intrusion.

Continent–ocean-transitions: Review, and a new tectono-magmatic model of the Vøring Plateau, NE Atlantic

“…Upper crustal asymmetry also appears to be important near the Afar hot-spot, but in those areas pure-shear deformation appears to be dominant on the lithospheric scale (Khan et al, 1999;Hébert et al, 2001). Such large-scale structures are unresolved by our data, but by reference to the results from the Aden area we speculate that pure-shear may have dominated on the lithospheric scale on the Vøring Margin as well.…”

“…These authors noted that most of the larger NW striking fault systems exhibit asymmetric profiles, and discussed their origin in terms of initiation points where slip starts, and barriers for fault propagation. Although upper crustal asymmetry appears to be important in the area, pure-shear deformation appears to be dominant on the lithospheric scale, at least in the Kenya Rift (Khan et al, 1999) and in the Gulf of Aden (Hébert et al, 2001).…”

“…30 Ma in the northern part of the Kenya Rift, a segment within the East African Rift System (Khan et al, 1999). A magmatic event occurring between 23 and 14 Myr ago, was probably responsible for a phase of magmatic underplating and intrusions in the southern part of the rift (Hay et al, 1995).…”

“…The upper part of the Kenya rift is primarily thinned by simple-shear along normal faults, in some cases leading to almost 10 km deep asymmetric basins (Khan et al, 1999). Predominantly continentward-dipping normal faults related to the stretching of the southern Red Sea and associated with the emplacement of flood basalts, have been documented in Yemen (Geoffroy et al, 1998b).…”

Why is Africa rifting?

“…The EAR has been the focus of a number of geophysical investigations of the rift system for many decades -for example, the early refraction work in Ethiopia (Berckhemer et al 1975) and the KRISP experiment in Kenya (Khan et al 1999). The Ethiopia Afar Geoscientific Lithospheric Experiment (Maguire et al 2003) and the Ethiopia Kenya Broadband Seismic Experiment (Nyblade & Langston 2002) marked the start of a series of deployments of broadband seismometers across the Horn of Africa.…”