Tectonic evolution of the northern Kenyan Rift

Abstract

Recent geological fieldwork, radiometric age dating of volcanic rocks, gravity and seismic reflection surveys have considerably refined our understanding of the tectonic evolution of the northern Kenya Rift. These data reveal that deep, half-graben basins up to 7 km thick were initiated west of Lake Turkana probably during Late Oligocene-Early Miocene times. The basins, bounded by easterly dipping faults, trend along the western side of the rift from Lake Turkana to the Elgayo Escarpment-Tugen Hills area. Some rift basins were episodically active from the Oligocene to the Pliocene while others were only active for a few million years. Some sag basins may have developed during periods of rifting quiescence. In the Turkana area the location of faulting gradually shifted eastwards with time. Volcanism both preceded and accompanied rifting. Within the Turkana area, volcanism moved south and east with time, beginning about 33 Ma (probably preceding half graben formation by a few million years) and continued to the present day. Extension along faults is greater in the Turkana area (between 25-40 km) and decreases southwards to probably 5-10 km or less in the southern Kenya rift. This pattern of extension values agrees with deep seismic refraction work (KRISP), which indicates thin crust in the Turkana area (about 20 km) that thickens to about 35 km south of Lake Baringo. The extension values, crustal thinning, age of volcanism and the timing of faulting to the south all point towards a southerly propagation of the rift.

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

“…Our thermochronological results from the Elgeyo and Samburu sites in the northern and central Kenya Rift sectors agree with the well-documented, regionally widespread Paleogene episode of extension in the Turkana region [Morley et al, 1992;Foster and Gleadow, 1996;Morley et al, 1999;Morley, 2002;Tiercelin et al, 2012]. East of the present-day Elgeyo Escarpment, Paleogene normal faulting and coeval sedimentation along the proto-Kerio Basin had been previously inferred based on a pronounced negative Bouguer gravity anomaly, reflecting a thick sedimentary fill, thought to be incompatible with the amount of Neogene extension and tectonic basin subsidence [Morley et al, 1992;Mugisha et al, 1997].…”

“…1a) based on seismic reflection data [Mugisha et al, 1997;Hautot et al, 2000]. The seismic reflection data document an early (as of yet undated) stage of tectonically controlled basin subsidence beneath the Neogene Kerio Basin followed by regional thermal basin subsidence and sedimentation within a sag basin [Morley et al, 1992;Mugisha et al, 1997;Morley, 1999a;Hautot et al, 2000]. Thermo-chronological and geophysical evidence for protracted subsidence in this sag basin is corroborated by regional pinch-outs of fluvial and organic-rich lacustrine sediments exposed along the Elgeyo Escarpment [Morley et al, 1992;Mugisha, 1997;Ego, 1994;Renaut et al, 1999] and the regional extent and thickness of the overlying <14.5 Ma phonolites that cover the present-day eastern and western rift shoulders [Lippard, 1973].…”

“…Geophysical data, stratigraphic information, and dated volcanic lava flows have furnished information on the early rifting history in northern Kenya, which began during the late Mesozoic [Foster and Gleadow, 1992;1996;Morley et al, 1992;Wagner et al 1992;Ebinger, 2000;Tiercelin et al, 2004;2012]. These early episodes of extension affected areas between the Indian Ocean and the Lake Turkana region, leading to the accumulation of thick lacustrine, fluvial, and eolian strata in the NW-SE oriented Cretaceous Anza Rift (Fig.…”

“…These basins span the region between the Ugandan border and Lake Turkana (Fig. 1), and they host sedimentary fills 5 to 8 km thick that include intercalated volcanic rocks [Morley et al, 1992;Tiercelin et al, 2012]. 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.…”

“…7). For example, the Lake Rukwa region of the western branch and the rift basins west of Lake Turkana record an Oligocene onset of rifting [Morley et al, 1992[Morley et al, , 1999Roberts et al, 2012]. Our new data from the Elgeyo Escarpment combined with field observations by Chapman et al [1978] and geophysical data from the Kerio Basin [Mugisha et al 1997] also suggests an earlier onset of extensional processes in this región, while the present-day morphologic characteristics of northern and central Kenya rifts are the result of renewed normal faulting after 12 Ma, coeval with extension recorded in the Central Tanganyika Basin [Chapman et al, 1978;Baker et al, 1988;Strecker et al, 1990], and the Albertine and southern Kenya rifts record the formation of extensional basin fills after approximately 9 and 7 Ma, respectively [Crossley, 1979;Pickford and Senut, 1994;Lezzar et al, 1996].…”