When continents rift to form new ocean basins, the rifting is sometimes accompanied by massive igneous activity. We show that the production of magmatically active rifted margins and the effusion of flood basalts onto the adjacent continents can be explained by a simple model of rifting above a thermal anomaly in the underlying mantle. The igneous rocks are generated by decompression melting of hot asthenospheric mantle as it rises passively beneath the stretched and thinned lithosphere. Mantle plumes generate regions beneath the lithosphere typically 2000 km in diameter with temperatures raised 100–200°C above normal. These relatively small mantle temperature increases are sufficient to cause the generation of huge quantities of melt by decompression: an increase of 100°C above normal doubles the amount of melt whilst a 200°C increase can quadruple it. In the first part of this paper we develop our model to predict the effects of melt generation for varying amounts of stretching with a range of mantle temperatures. The melt generated by decompression migrates rapidly upward, until it is either extruded as basalt flows or intruded into or beneath the crust. Addition of large quantities of new igneous rock to the crust considerably modifies the subsidence in rifted regions. Stretching by a factor of 5 above normal temperature mantle produces immediate subsidence of more than 2 km in order to maintain isostatic equilibrium. If the mantle is 150°C or more hotter than normal, the same amount of stretching results in uplift above sea level. Melt generated from abnormally hot mantle is more magnesian rich than that produced from normal temperature mantle. This causes an increase in seismic velocity of the igneous rocks emplaced in the crust, from typically 6.8 km/s for normal mantle temperatures to 7.2 km/s or higher. There is a concomitant density increase. In the second part of the paper we review volcanic continental margins and flood basalt provinces globally and show that they are always related to the thermal anomaly created by a nearby mantle plume. Our model of melt generation in passively upwelling mantle beneath rifting continental lithosphere can explain all the major rift-related igneous provinces. These include the Tertiary igneous provinces of Britain and Greenland and the associated volcanic continental margins caused by opening of the North Atlantic in the presence of the Iceland plume; the Parana and parts of the Karoo flood basalts together with volcanic continental margins generated when the South Atlantic opened; the Deccan flood basalts of India and the Seychelles-Saya da Malha volcanic province created when the Seychelles split off India above the Reunion hot spot; the Ethiopian and Yemen Traps created by rifting of the Red Sea and Gulf of Aden region above the Afar hot spot; and the oldest and probably originally the largest flood basalt province of the Karoo produced when Gondwana split apart. New continental splits do not always occur above thermal anomalies in the mantle caused by plumes, but when they do, huge quantities of igneous material are added to the continental crust. This is an important method of increasing the volume of the continental crust through geologic time.

Magmatism at rift zones: The generation of volcanic continental margins and flood basalts

Around the globe, and in a variety of settings including active and inactive mountain belts, increases in sedimentation rates as well as in grain sizes of sediments were recorded at approximately 2-4 Myr ago, implying increased erosion rates. A change in climate represents the only process that is globally synchronous and can potentially account for the widespread increase in erosion and sedimentation, but no single process-like a lowering of sea levels or expanded glaciation-can explain increases in sedimentation in all environments, encompassing continental margins and interiors, and tropical as well as higher latitudes. We suggest that climate affected erosion mainly by the transition from a period of climate stability, in which landscapes had attained equilibrium configurations, to a time of frequent and abrupt changes in temperature, precipitation and vegetation, which prevented fluvial and glacial systems from establishing equilibrium states.

Increased sedimentation rates and grain sizes 2-4 Myr ago due to the influence of climate change on erosion rates.

Controversies regarding the correlation of erosion cycles within southern Africa are traced through some earlier contributions; many of these reflect a failure to apply rigorous field criteria to problems of mapping. The analysis of sections linking key coastal deposits to planation remnants in the interior is used to extrapolate field observations over wider areas. The resulting map of erosion surfaces, and contour plans of elevations on the more important of these, form the basis of the paper. A consistent datum is provided by the oldest of the surfaces (the African) through the diagnostic deep weathering profiles and duricrust cappings associated with it. The Great Escarpment was formed following continental rifting, and its survival up to the present as a major topographic feature is, in large measure, a function of the high elevations consequent upon the central position of southern Africa in Gondwanaland prior to this event. There is conclusive evidence for the co-existence of surfaces of the same age at different elevations inland of and below the Great Escarpment. This vertical separation can be ascribed to the existence, from early times, of separate base-levels for the plateau and coastal areas. The onshore evidence of erosion surfaces is correlated with recent data on offshore sedimentation, and reveals that a single cycle of erosion (interrupted by minor tectonic interludes) prevailed from the time of rifting to the early Miocene. By the end of this period a gentle pediplain (the African surface) extended across most of southern Africa at elevations of 500-600 m. Most erosion and scarp recession occurred during the earlier part of this interval and produced thick late Jurassic and Cretaceous sedimentary sequences, but shelf sedimentation declined during the Tertiary and had virtually ceased by the Oligocene, when interior planation had advanced to a stage where sediment supply to most rivers was minimal. Modest renewed uplift of 150-300 m in the Miocene tilted the continent slightly to the west and initiated a new (Post-African J) landscape cycle. This was accompanied by renewed offshore sedimentation, although at lower rates than during the Cretaceous. The cycle was terminated near the end of the Pliocene, and its relatively short duration resulted in imperfect planation in most areas to levels of no more than 100-300 m below the African surface. A second uplift of major proportions at the end of the Pliocene raised the eastern interior of the sub-continent by as much as 900 m, although much smaller movements characterized the western areas and interior axes of uplift. Major monoclinal warping resulted in the southeastern hinterland. The ensuing Post-African II cycle is manifested chiefly in deep incision of the coastal hinterland and downcutting along major rivers of the interior. Earlier surfaces were severely deformed and dissected, especially in the south-east of the country. Planation was limited to a few areas close to the coast. The resulting sedimentation is evident mainly in the offshore deltas of major rivers owing to the late development of vigorous coastal currents, but a major increase is reflected throughout the deeper ocean basins. The successive uplifts to which the continent was subject produced major changes in drainage patterns, although the evidence for pre-Miocene distributions is tenuous in many areas. The preservation of discrete erosion surfaces with separating escarpments over considerable periods not only confirms the episodic nature of these uplifts, but indicates a dominance of backwearing over down wearing in the geomorphic evolution of the sub-continent. The mechanisms controlling the uplift of southern Africa and similar passive continental margins are of considerable importance in the light of an increasing awareness that many elevated areas within the continents can be ascribed to such movements. Some thoughts are offered towards the formulation of a tectonic model which takes account of the evolutionary scheme now established for southern Africa.

Geomorphic evolution of Southern Africa since the Mesozoic

The East African rift system

Reconstructions of the relative positions of the India and Eurasia plates, using recently revised histories of movement between India and Somalia and between North America and Eurasia and of the opening of the East African Rift, show that India's convergence rate with Eurasia slowed by more than 40% between 20 and 10 Ma. Much evidence suggests that beginning in that interval, the Tibetan Plateau grew outward rapidly and that radially oriented compressive strain in the area surrounding Tibet increased. An abrupt increase in the mean elevation of the plateau provides a simple explanation for all of these changes. Elementary calculations show that removal of mantle lithosphere from beneath Tibet, or from just part of it, would lead to both a modest increase in the mean elevation of the plateau of ~1 km and a substantial change in the balance of forces per unit length applied to the India and Eurasia plates.

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Slowing of India's convergence with Eurasia since 20 Ma and its implications for Tibetan mantle dynamics

Geology of the Eastern Rift System of Africa

Twenty-two new K-Ar dates allow reorganization of the volcano-stratigraphy of the Kedong–Kinangop region on the east side of the central Kenya rift for the most recent 4 Ma, during which time most of the structural features evolved. Four principal phases of volcanic eruption were: widespread Kinangop tuff ash flows (3.7–3.4 Ma), Limuru flood trachytes (2.0–1.8 Ma), basalts and flood trachytes of the rift floor (1.65–0.9 Ma), Quaternary salic caldera volcanoes (0.4–0 Ma). K-Ar dating allows recognition of the Cobb Mountain normal polarity event at about 1.18 Ma, and dating of the Kedong flood at about 0.1 Ma. Major faulting occurred at 4–3 Ma, converting an early half-graben to a graben. Fault zones then migrated inwards progressively forming step-fault platforms at 3–2 Ma and 1.7 Ma, such that the graben narrowed with time. Extensive minor faulting of the rift floor occurred during the most recent 0.8 Ma. The average extension rate across the central rift was 3 mm a −1 for the last 7 Ma, and is close to plate tectonic estimates. Individual fault strain rates are estimated at 0.1–0.4 mm a −1 . Faulting was episodic rather than continuous, and some episodes affected both sides of the rift. Volumes of magma erupted during 4–0 Ma require generation of at least 1.2 × 10 5 km 3 of primary magma in the 200 km length of the south–central rift if fractional crystallization is the dominant differentiation mechanism. This volume implies mantle upflow either as an asthenosphere ‘plume’, or as the result of lithosphere extension. The tectonic and magmatic sequence of events is proposed to result from lithosphere thinning, emplacement of large magma bodies in or near the lower crust, regional uplifts and superimposed limited lateral separation of the lithosphere as a result of plate motions.

B. H. Baker

Papers

Geology of the Eastern Rift System of Africa

Stratigraphy, geochronology and volcano-tectonic evolution of the Kedong–Naivasha–Kinangop region, Gregory Rift Valley, Kenya