African Rift tectonics and sedimentation, an introduction

Abstract

Rifts are one of the most spectacular features of global morphology. Within oceans they separate plates as new oceanic crust is created. Within continents they may form deep valleys such as the Rhine graben, within which runs one of the world's busiest waterways. They are often floored by deep lakes, such as Lake Baikal, the deepest lake in the world today whose floor lies 1700 m below the surface of the lake. Ancient rifts are the sites of petroleum accumulations beneath the margins of the Atlantic, in the North Sea and in China, of coals and oil shales and of minerals, including phosphates, barite, Cu-Pb-Zn sulphides and uranium (Robbins 1983). Of all the world's rifts none can match in scale and diversity the Great Rift System, which runs for over 7000 km and includes the East African Rift System and its extension through the Red Sea into the Dead Sea, to form a unique feature of global geology. Its study is important not only for what it can tell about the nature and origin of present-day rifts, the thermal, magmatic and tectonic processes which gave rise to them, climatic changes, and sedimentary, particularly lacustrine and volcaniclastic, processes. It is also essential for the understanding of the processes which formed passive continental margins, all of which are underlain by rifted sedimentary basins, and failed rifts whether they formed within continents or at the junction of continents and oceans. Early workers on rifts were impressed by the wide valleys, 40-50 km across, filled by young alluvial and lacustrine sediments and bordered by steep escarpments rising in some cases a few hundred metres, in others up to 3-4000 m above the valley floor. Many rifts are the sites of volcanic activity, sometimes of rather unusual composition. These unusual volcanic source rocks may in turn be the cause of peculiar lake water chemistries, many rift lakes containing unique suites of evaporitic minerals. Although at one time some people considered rifts to have been formed by compression (rampvalley hypothesis), discussion today revolves principally around the extent to which rift valleys are extensional features or are the result of oblique-slip movements between two laterally moving blocks. There is no doubt now that some rift valleys are the result of strike-slip movement. The Dead Sea/ Gulf of Elat rift lies along a lineament that separates two lithospheric plates and, as Arabia moves northwards relative to the Palestinian (Mediterranean) block and faults curve or sidestep each other, extensional pull-apart basins are formed, such as the Dead Sea whose surface lies further below sea level than any other place in the world. A feature of these strike-slip rift valleys is that the extensional valleys pass, along the rift, into mountainous zones of compression and uplift. Rapid erosion from these mountains leads to substantial sediment supply for the basins from within the fault system itself. Ancient strike-slip rift basins are therefore characterized by very thick, rapidly accumulated basin fills with areas of contemporaneous compressional tectonics and unconformities not far away. Rift valley basins that are extensional features on a regional scale tend to sink rather more slowly, have less sediment entering them, and show no evidence of synchronous compressional tectonics in their neighbourhood. Cross-sections through them show normal faulting, probably listric in form. The main controversy with regard to extensional rift valleys is whether magmatic activity in rifts reflects an underlying heat source such as a convection plume in the mantle or hot spot, the rifting then being the consequence of expansion and uplift brought about by thermal activity, or whether the magmatic activity is merely the result of regional or local extensional stresses which have passively allowed magma to come to the surface. Some rifts that penetrate continents are clearly failed rifts or aulacogens that began as one arm of a triple junction. As ocean floor spreading developed along two arms, one arm was left as a 'failed rift' in the sense that new oceanic crust was not created. Perhaps the best known is the Benuc trough within which the Niger delta developed, However, the northern (Viking) graben of the North Sea has been claimed to be one (Whiteman et al. 1975), and so has that part of the East African Rift known as the Afar triangle where thc Red Sea and the Gulf of Aden Rifts successfully drifted, whilst the Afar triangle 'failed'. Molnar & Tapponnier (1975) have argued that other rifts are the result of continental collision, in particular that the impact of India with Asia caused not only major lateral displacement along