Geology and geophysics of the Rukwa Rift, East Africa

Abstract

Exploration in the Rukwa rift, using gravity and seismic reflection surveys, fieldwork, and drilling has defined the structure and stratigraphy of the basin in greater detail than any other part of the Western rift. The stratigraphy comprises Precambrian basement, Karroo sandstones, shales and coals, upper Miocene red beds, and Miocene‐Recent lacustrine and fluvial sediments. During Miocene‐Recent rifting the greatest sediment input apparently came from axial fluvial systems flowing from the northwest and southeast. The southwestern area experienced alternating shallow lacustrine and fluviodeltaic conditions during the Miocene‐Recent. Cenozoic age rift structures have a dominant NW‐SE and a subordinate N‐S trend. The NW‐SE trend tends to follow a Precambrian basement and later Karroo structural trend. NE‐SW seismic lines indicate up to 10 km extension of the Tertiary section in a direction oblique to the probable E‐W regional extension direction. In the southeastern portion of the basin both Karroo and Tertiary‐Quaternary sediments expand into the Lupa fault zone, reaching thicknesses of up to 3 and 7 km, respectively. Tertiary‐Quaternary sediment thicknesses decrease northwestward, accompanied by a decrease in the amount of extension and a broadening of the basin as extension is transferred to the Lake Tanganyika rift.

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

“…These results are in good agreement with previous modeling of apatite fission track and length parameters from samples collected in the Cherangani Hills and the basement rocks SE of Lake Turkana, which revealed 60 to 70°C of cooling between approximately 60 and 50 Ma [Foster and Gleadow, 1996;Spiegel et al, 2007]. This cooling appears to have been coeval with renewed extension and tectonic subsidence in the Anza Rift [i.e., Morley et al, 1999b;Bosworth and Morley, 1994;Morley, 2002], which is inferred to have been associated with flexural upwarping of rift flanks in that region [Foster and Gleadow, 1996].…”

“…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]. As neither currently available geophysical, geological, nor thermo-chronological data suggest similar coeval Paleogene extension processes in the central and southern sectors of the Kenya Rift, it appears that early Cenozoic rifting in Kenya was focused in the greater Turkana region [i.e., Morley et al, 1992;Foster and Gleadow, 1992;Foster and Gleadow, 1996;Morley, 1999;Spiegel et al, 2007], the Anza Rift, where Mesozoic extensional faults were reactivated [i.e., Morley et al, 1999b;Bosworth and Morley, 1994], and regions as far south as the transition between the central and northern Kenya rifts (this study).…”

Pyroxenite xenoliths from Marsabit (Northern Kenya): evidence for different magmatic events in the lithospheric mantle and interaction between peridotite and pyroxenite

“…In our earlier study (Kaeser et al 2006), we concluded that the porphyroclastic peridotite xenoliths essentially reflect pervasive deformation associated with decompression and cooling in the context of multiple rifting episodes to which the lithospheric mantle in this region was subjected (Anza Graben and Kenya rift; e.g., Winn et al 1993;Morley et al 2006). Thermobarometric data were interpreted in terms of a P-T path characterised by three successive stages ( Fig.…”

“…2a-c) indicating that they are not cognate to their host lava. The occurrence of deformed cumulates from alkaline melts can be explained by the long-time magmatic activity in the region of Marsabit, starting in Mesozoic-Paleogene times (i.e., diabase intrusion associated with the development of the Anza Graben; Winn et al 1993;Morley et al 2006), until the development of the Marsabit shield volcano. The xenolith-bearing basanites and alkali basalts represent the youngest volcanic products (1.8-0.5 Ma; Brotzu et al 1984;Key et al 1987) and thus potentially sampled mafic lithologies accumulated earlier.…”

Deformation and sedimentary evolution of the Lake Albert Rift (Uganda, East African Rift System)

“…In the Anza Rift (Morley et al, 1999a), this corresponds to the end of the subsidence of the basin under a strike-slip tectonic regime characterized by a major unconformity and the last marine flooding in the area.…”