The Gregory Rift

Abstract

The three stages of development of the East African Rift System (EARS), pre-rift, half-graben and full graben, can be related to specific episodes of faulting and volcanism within the Gregory Rift. The pre-rift stage (Late Oligocene–Miocene) initiated formation of the regionally extensive Kenya-Tanzanian Dome, with plateau-style volcanic outpourings and isolated shield volcanoes, e.g. Mount Elgon. Formation of the narrow rift valley in central Kenya commenced during the half-graben stage (Late Miocene–Pliocene) and persisted, particularly in southern Kenya into the full graben stage (Pleistocene). These events triggered extensive plateau-style volcanism which shows a pattern of younging southwards and with the youngest sequences located in the centre of the valley, i.e. as lateral extension of the rift persisted. These events were also accompanied by eruption of discrete volcanic cones throughout central/southern Kenya, albeit they reveal a more chaotic distribution relative to their ages. The extensiveness of volcanism on the Eastern Rift Platform is a notable feature of the Gregory Rift in central/southern Kenya, e.g. Aberdare Range (Miocene), Mount Kenya (Pliocene) and Chyulu Hills (Pleistocene–Holocene). Rifting in northern Tanzania is restricted to the half-graben stage (Pliocene–Pleistocene), an observation consistent with the southward propagation of this branch of the EARS. The rift diverges into three arms within a 200-km-wide, structurally complex area with extensive volcanism. Two of the rifts, including the Natron–Manyara half-graben are a continuation of the full graben in southern Kenya; they peter out southwards. The volcanism of northern Tanzania is divided into an older group (Pliocene–Early Pleistocene), of which the Ngorongoro Volcanic complex is the most well-known example, and a younger group, the latter including the dormant Kibo component of the multicentred Kilimanjaro edifice and the active cones of Mount Meru and Oldoinyo Lengai. Despite the Gregory Rift being dominated by volcanic rocks, localised sedimentary basins occur in the rift valley, e.g. the Lake Manyara basin, and in small warps on the rift platforms, e.g. the Oldupai and Laetoli basins.