R. B. Mcconnell Streatwic\, Streat Near Hassocks, Sussex, United Kingdo Geological Development of the Rift System of Eastern

R. B. McCONNELL Streatwic\, Streat near Hassocks, Sussex, United Kingdo

Geological Development of the

Rift System of Eastern Africa

ABSTRACT canism of continental type affects the northern part of the eastern rift lineament, but this The active East African Rift System forms a restricted volcanic sector contrasts strongly belt of anastomosing faults extending 4,000 km with the major nonvolcanic portion of the rift south-southwest from the junction of the Red system. The lack of spreading of the rifts may Sea and Gulf of Aden to the Zambezi River. be due to the compression of the African'Plate The presence of certain ancient features, in- between the spreading Mid-Atlantic and Mid- cluding the Great Dyke of Rhodesia and an Indian Ocean Ridges. alignment of Bushveld-type igneous complexes, Long fault lineaments are features of the indicates that the system formerly extended as earth's crust, and a review shows that intra- an infracrustal lineament another 1,500 km continental rift valleys of East African type south-southwest down the eastern half of occur where such lineaments must cut through Africa to the Orange River. The rift system is particularly resistant cratonized massifs, form- entirely intracontinental, and formed in the ing wide zones of fracturing and shearing with ancient African Precambrian platform with blastomylonites, flaser gneisses, and migmatites whose 3-b.y, geological history it is intimately in the plastic infracrust. The mobile belts of the associated. The Cenozoic rift faults follow system may thus be compared with orogenies in mobile belts moulded upon ancient shields and which plastic deformation and migmatization formed during at least seven major orogenic at deep levels are succeeded by isostatic uplift, cycles affecting pre-Silurian "assemblages" but the linear Cenozoic arches thus produced in comprising complex rock groups characterized East Africa are affected by typical rift valley by structural and metamorphic similarities but faulting at the brittle surface levels and are including units of greatly differing ages. An thus genetically associated with infracrustal analysis of major structural features moulded Precambrian structures and continued high on the Tanganyika Shield indicates that the heat flow while reflecting supracrustal mechan- belts may have originated at about 2.7 b.y. by ics. The lineaments do not always follow fold dextral transcurrent movement between the belts but may cut obliquely across their grain. ancient shields, but horizontal movement was Although the most superficial rift faulting is subsequently impeded by cratonization and steeply normal and antithetic, observations replaced by vertical displacement. suggest that the deeper faults are vertical, and The nature of the rifted belts indicates con- it is proposed that subsurface plasticity and ex- trol by mantle mechanisms and repeated re- pansion due to heat flow is a causative factor in activation, so the term perennial deep linea- the sinking of strips of cool surface rock in a ment is proposed. Comparison with other intra- shield environment. continental rift systems and preliminary geophysical results in East Africa strongly suggest INTRODUCTION lithospheric states similar to those underlying The early explorers Livingstone, Burton, the mid-ocean ridges, implying high heat flow Speke, and Stanley, while searching for the restricted to the lineaments in a generally cool sources of the Nile and Congo Rivers, noted continental environment. Geological mapping the remarkable fjordlike character of Lakes proves, however, that no formation of new Nyasa, Tanganyika, and Albert. Douville re- oceanic crust has taken place in spite of the lated the great scarps of northern Ethiopia to great age of the lineaments. Profuse rift vol- the Jordan Valley-Red Sea system. It was not

Geological Society of America Bulletin, v. 83, p. 2549-2572, 6 figs., September 1972 2549

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until von Teleki had discovered Lake Rudolf in Mapping in southwest Tanzania led to the 1887, however, that the true course of the hypothesis that the rift system had originated main rift (Briiche) system of East Africa was as a series of fundamental faults, associated with outlined by Suess (1891) as stretching from metamorphism and granitization in an early Ethiopia through Kenya to Lake Manyara and Precambrian Ubendian orogeny (McConnell, then jumping to Lake Nyasa and extending 1950, 1951). The Cenozoic rift faults would southward through the Shire valley to the then be the latest rejuvenations, displaying Zambezi. In 1893, J. W. Gregory visited Kenya vertical faulting in the rigid surface zones of (then British East Africa) and gave the first the crust but genetically related to earlier detailed description of "The Great Rift infracrustal dislocations, and rift volcanism Valley" (1896), also describing the Western might be due to renewed magmatic activity in Rift Valley. Bailey Willis (1936) stressed that the deep crust. the rift valleys were merely features of the Dixey (1956) supported the notion that the broad uplift of the East African plateau and pattern for the later rift faulting originated in always associated with the formation of arches. Precambrian times. He regarded the develop- Whereas formation of the rift valleys by the ment of the whole of eastern Africa as being downfaulting of the keystone of an arch by related since Jurassic times to the uplift of a tension, as proposed originally for the Rhine- swell concomitant with the downwarping of a graben, had been supported by Gregory and Mozambique geosyncline which affected the many other geologists, Willis developed the whole of Africa from the Mozambique Channel compression hypothesis, introduced by Way- to Somalia. Brock (1955) and Dixey (1959) land (1930), that the floors of the rift valleys have emphasized the vertical character of were depressed by reverse faults disguised by supracrustal rift tectonics. later slumping of the steep walls. Bullard (1936) Recent emphasis in rift valley studies interpreted gravity measurements across East (UMC/UNESCO, 1965) on the Cenozoic and Africa, showing considerable negative Bouguer Holocene rifting and volcanism in the northern anomalies over the rift valleys, as indicating half of the rift system has been accentuated by compression. There thus developed two op- the discovery of the world-wide mid-ocean posed schools of thought, one attributing the ridge system with central rift valleys (Heezen, formation of rift valleys sensu stricto (see Ap- 1969), which closely resemble those of East pendix) to compression, while most geologists Africa in morphology. A revolution in geo- regarded them as the result of tension; in this logical thinking has given rise to a global view paper I will offer evidence indicating that the of tectonics and much interest has centered on evolution of the rift system has been long and the northern sector of the East African Rift complicated, and dependent on many factors System as representing an easily accessible other than simple tension or compression. example of rifting. In previous papers (Mc- E. O. Teale (1936) was probably the first to Connell, 1967, 1969a, 1970) the conception of suspect that the East African Rift System the Precambrian origin of the East African might have had an early origin, as he noted Rift System and its genetic association with that certain Karroo basins in Tanzania were ancient mobile zones has been brought up to aligned parallel to the rift system. Geologists date, and evidence has been presented (Mc- have also noted that some rift faults were Connell, 1969b) that intracontinental rift parallel to Precambrian features, but assumed zones in the Guiana (South America) and West that the later faulting had simply followed African Shields may project into the ancestral older zones of weakness. Dixey (1939) showed axes of the spreading and drifting apart of the that many of the features of the Malawi trough American, Eurasian, and African crustal plates. dated from the early Cretaceous. He later As many authors have described the northern emphasized the importance of erosion in the volcanic sector of the East African Rift System, formation of Cenozoic and Recent rift valley this paper deals mainly with sectors in which scarps and suggested that the major part was the older rocks are visible, and its purpose is to due to a "far earlier set of features, probably in present briefly the fundamental geological the main of post-Karroo or Jurassic age" evidence for the Precambrian origin of the (Dixey, 1946, p. 342). He also showed that the basic pattern of the whole rift system of eastern rift faults in Zambia were intimately associated Africa, as shown in Figure 1, and for the man- with Karroo structures. ner of its evolution as follows: (a) The East

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African Rift System proper between the Red The junction of the eastern and western Sea and the Zambezi River.(Fig. 2). Most of branches is obscured by the Rungwe Volcanics the faults of this system are still intermittently of Pleistocene to Holocene age, but the Rukwa active, (b) A southern sector that includes the Rift Valley (Figs. 3 and 4) continues south of rift valleys and faults surrounding the Rhodesian the junction (Bloomfield, 1968), and paired Shield, the branch of dislocations turning rift faults can be traced, with some zigzags, to southwestward along the Damaride trend, and the Zambezi River at 18° S. (Fig. 2). The the belt in southern Africa of geological features southern portion forms the Shire Rift Valley, continuing the main north-northeast axis of the accentuated by the erosion of the Shire River, rift system from the Zambezi to the Orange and continues to the south of the Zambezi by River. the narrow Urema subsurface graben to the EAST AFRICAN RIFT SYSTEM coast at Beira (International Tectonic Map of Africa, 1968). Southwest of Beira, the western The outline of the East African Rift System margin of the Urema graben continues in the proper extends about 4,000 km in a general post-Karroo faulting which bounds the Rho- south-southwesterly direction down the eastern desian Shield and the post-Jurassic monocline side of Africa from the Red Sea to Beira (Fig. which takes the Lebombo lavas of Karroo age 2) (Gregory, 1921; Dixey, 1956; McConnell, beneath the Inhambane basin (Dixey, 1956; 1967). Haughton, 1969, p. 373) and forms the eastern From southeastern Tanzania, a system of margin of the Rhodesian and Kaapvaal Shields. block faulting extends roughly north-northeast The course of the eastern rift system does from Lake Malawi into southeastern Kenya. It not end at the Mbeya node (9° S.) where it is defines a series of Karroo basins separated by sharply cut across by the dominant Rukwa uplifted basement blocks such as the Mahenge Rift faulting, but continues to the south- massif and the Uluguru Mountains (Fig. 3). southwest along the Luangwa Rift Valley. This system is related to the main rift system It is cut off at 13° to 14° S. by a dislocation zone (McConnell, 1951) but will not be considered and continues to the west-southwest along the in detail here because it is complicated by the Luano Rift north of Lusaka (Swardt and others, presence of strong downwarping and faulting 1965). Between the Shire and Luano Rifts, associated with the formation of the coastal the east-trending Middle Zambezi valley forms geosyncline studied by Kent and others (1971). a deep trough excavated by the river in Karroo This system of block faulting appears to con- rocks and bounded to the south by the steep tinue to the north-northeast along the straight fault scarp of the northern boundary of the line of the eastern coast of Kenya and Somalia; Rhodesian Shield. The Luangwa Rift con- to the south it bends southwest (Ruhuhu tinues aligned with the northeast-rifted Kariba graben, Figs. 3 and 4) and may continue across section of the Middle Zambezi (Vail, 1967), Lake Malawi by dislocations in Zambia (Swardt and the projections of this trend in the and others, 1965). Ghanzi ridge, south of the Okavango swamps, Plan of the Rift System and the northeast-trending faults mapped in North of the parallel 10° S. two branches of the Makarikari region of the Kalahari basin the main rift system are clearly distinguishable: appear to indicate that this dislocation forms an eastern branch starting from the junction the northwestern boundary of the Rhodesian near Mbeya1 (Figs. 2 and 3) and running north- Shield. The Great Dyke of Rhodesia continues northeasterly around the southern margin of the general trend of the Gregory Rift System the Tanganyika Shield through the Gregory (McConnell, 1951). Vail (1967) has also shown and Ethiopian Rift Valleys to the southern that the Rhodesian Shield is surrounded by apex of the Afar triangle; and a western branch post-Karroo faults. In the Precambrian Lim- that extends from the Mbeya junction to popo Belt (Mason, 1969) an east-trending rift Uganda in a series of straight segments con- valley limits the shield to the south. nected by nodes where successive fault direc- Geological Background tions intersect. The segments of the western branch form a broad arc between Mbeya and The geological background to the East the Aswa dislocation zone (Fig. 2). African Rift System is schematically shown in Figure 2. Five major cycles of pre-Silurian 1 Pronounced Umbeya. tectonism, metamorphism, and magmatism are

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generally recognized in Africa; they have been remarkable feature have now been dated in the defined by Cahen and Snelling (1966) and by 2,800 to 2,500 m.y. range (Allsopp, 1965) and Clifford (1970) and appear to have occurred may represent an early manifestation of the within the same broad time spans throughout rift system which has not been reactivated. Africa, although Cahen (1970) has shown that The Tanganyika Shield terminates to the the Kibaran and Katangan cycles may overlap east at the margin of the Mozambique Belt in time. The Mozambique Belt, described (Hepworth and Kennerley, 1970), but its below, consists largely of units tectonized in limits to the south and west, confused by earlier Precambrian cycles which have been migmatization and by a widespread lateritic reactivated in Katangan (Mozambiquian) time cover, may be shown by a line of positive with an almost universal resetting of radio- isostatic anomaly (Masson Smith, 1965) which metric clocks. follows the course of the Dodoman schist belt Dorsal of Eastern Africa. A wide meridional (see below). The Holocene kimberlitic volcanic belt down the eastern side of the African plat- craters of the Igwisi Hills (Dawson, 1970, p. form was consolidated in Precambrian time and 327) are situated on a gravity high on this line consists of normal continental crust. This belt (Fig. 3). is crossed by mobile zones and so cannot be Dodoman, Nyanzian-Kavirondian, and called a shield (see Appendix), but it does form Equivalent Assemblages. Closely associated a "dorsal" elongated south-southwest con- with the ancient granitoid shields are assem- sisting of high plateaus and mountainous areas blages of metasedimentary and metavolcanic of Precambrian rocks, with a few shallow basins rocks (Fig. 2) which generally form schist and of continental Paleozoic and later sedimentary greenstone belts of low metamorphic grade, and volcanic rocks. The dorsal plunges to the but pass into migmatite, banded gneiss, and east beneath the marine Phanerozoic coastal other gneissose rocks in the amphibolite or sediments and to the west beneath four con- granulite facies. These rocks are all older than tinental sedimentary basins of the African a late Archean orogeny (2,700 to 2,300 m.y.) platform: the Nile, the Congo, Barotseland, and may be much older (Vail and Dodson, and the Kalahari, each separated by less 1969). The ancient assemblages of East Africa conspicuous basement ribs directed more or bear close resemblances in lithologic composi- less at right angles to the East African dorsal. tion, gold content, metamorphism, structural Granitoid Shields. Ancient granitoid shields setting, and apparent radiometric ages to the (Figs. 1 and 2) appear to control the direction metavolcanic and metasedimentary greenstone of the Precambrian mobile zones. They also and schist belts of the Rhodesian and Kaapvaal affect the later dislocations; for example, the Shields (Shackleton, 1970). Tanganyika Shield deflects the rift system into Ubendian Assemblage and Its Equivalents. eastern and western branches. The Rhodesian An outstanding feature of Figures 1 and 2 is the Shield is closely surrounded by post-Karroo northwest-trending Ubendide erogenic belt fault troughs (Vail, 1967), but it also appears to which extends for over 500 km from northern have diverted the course of the Luangwa and Malawi to Zaire (Congo Republic) west of Malawi Rift lineaments. Lake Tanganyika. Recent work (Cannon and The great age of the granitoid shields is others, 1969) has shown that the Ubendian being established by the latest geochronological orogeny was the dominant factor in the tec- work, which gives dates as old as 3,300 m.y. tonics and metamorphism of northern Malawi. for the Rhodesian Shield (Vail and Dodson, To the east, the strike of the belt is parallel to 1969), and 3,200 m.y. for the Kaapvaal Shield the great rift scarps by which the Livingstone (Anhaeusser and others, 1969). A similar age is Mountains rise from Lake Malawi (Fig. 4); to suggested for the Tanganyika and West Nile- the west, however, the Ubendide structures are Uganda Shields (Shackleton, 1970), but max- oblique to the faulted margin of the lake. imum dates are difficult to establish, although To the north and northwest of Lake Tan- 3,000 m.y. is probably a minimum (Cahen and ganyika, the Ubendides are continuous with Snelling, 1966; Spooner and others, 1970). the Rusizian (Cahen and Snelling, 1966) and The Great Dyke, which is in line with the are also correlated with the Buganda-Toro main East African rift direction, cuts across the assemblage of Uganda shown in Figure 6. Both Rhodesian Shield. The mafic rocks forming this groups of rocks have apparent radiometric ages

in the 2,000 to 1,600 m.y. range, but may be the northeast of Lake Malawi, although it older (Shackleton, 1970). The Buganda-Toro strikes northwest parallel to the Ubendide Belt assemblage continues west of the rift valley in and the Rukwa-Malawi Rift Valley, is also the Kibalian orogenic belt (Cahen and Snelling, accepted as of Kibaran age (Cahen and Snelling, 1966, p. 56), which gives comparable dates. In 1966, p. 87). Uganda an approximately east-trending line Katangide and Damaride Belts. The (Macdonald, 1969) separates the Buganda- Katangide Belt is the best known of the Pre- Toro from the dominantly granitoid rocks to cambrian units in Africa, and its stratigraphy the north; in part this limit is an unconformity, has been thoroughly described in Katanga as evidenced by basal conglomerate and arenite, (Cahen and Lepersonne, 1967) and Zambia. but it is also a structural front marking the These sedimentary rocks overlie unconform- northern limit of a strongly tectonized east- ably the Kibaran assemblage and comprise trending belt which crosses the Ruwenzori widespread tabular areas, as well as the Lufilian Mountains and is perhaps continued in the arc (Fig. 1) folded between 750 and 620 m.y. boundary between the Kibalian and the West with post-tectonic phases up to 450 m.y. ago Nile assemblages west of Lake Albert. (Cahen and Snelling, 1966). Kibaride and Irumide Belts. The Kibaride In South West Africa the Damarides belt is one of the major features of eastern (Martin, 1961) emerge from the sediments Africa and, in contrast with the older Precam- of the Barotse Basin and have been shown to be brian belts, it is well known stratigraphically continuous with the Katangides by Clifford as well as structurally (Cahen, 1970). It extends (1967) who distinguishes two upper Pro- north-northeast from Katanga through the terozoic-lower Paleozoic orogenic episodes in Burundian Belt (Fig. 2) to join the Karagwe- southern Africa, namely the Katangan (660 to Ankolean of Uganda where it swings to the 580 m.y.) and the Damaran (550 to 450 m.y.). west. It unconformably overlies the Rusizian In Katanga, Zambia, and South West Africa and the Buganda-Toro. The trends of the belt the Katangan-Damaran episodes were accom- have little in common with the rift system panied by the reactivation of older Precam- except in Katanga where the Upemba post- brian basement (Swardt and others, 1965). Miocene north-northeast-trending fault scarp An important contribution to understanding and downwarped trough is believed to form the African platform was made by Kennedy part of the rift system. Cahen and Snelling (1964), who recognized three ancient units, (1966) place the activity of the Kibarides the Kalahari, Congo, and West African cratons, between 1,290 and 850 m.y., but the latter established as the result of basement differ- date is late orogenic, and perhaps a range of entiation during a "Pan-African" tectono- 1,300 to 1,000 m.y. would be of more general thermal episode dated about 500 m.y., which significance. corresponds to the Damaride and Katangide In eastern Zambia a metamorphosed sedi- fold belts and to widespread zones of basement metary group known as the Irumide Belt of reactivation in East Africa. Kibaran age (Cahen and Snelling, 1966, p. 86) Mozambique Belt. In southern Africa an follows a northwest to west-northwest direction assemblage of upper Proterozoic folded rocks and overlies an earlier metamorphic group of and tectonized basement known as the meridional trend (Ackermann and Forster, Mozambique Belt (Holmes, 1951) skirts the 1960). The general strike of this belt follows Rhodesian Shield and extends northward from that of the Luano and Luangwa Rift Valleys the Limpopo Belt (Figs. 1 and 2) through which contain a Karroo sedimentary fill Mozambique (Oberholzer, 1968), eastern Zam- (Drysdall and Weller, 1966) deposited in a bia, southern Malawi, eastern Tanzania (the pre-Karroo rift valley, the boundaries of which Usagaran System of Quennell and others, have been rejuvenated in Cenozoic times. 1956), Kenya, the eastern fringe of Uganda, In northern Malawi the arenaceous Mafingi and across Ethiopia. This belt presents many beds are probably related to the Irumides difficult problems, and it is only necessary to (Cannon and others, 1969) and also reflect the treat it in a general way in this paper as an strike of the eastern rift system north of the assemblage which may be partly Archean but Mbeya node. The Ukinga Group (Fig. 4) to has been tectonized and reactivated at sucessive

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periods and so has acquired common structural nucleus of great age, the Tanganyika Shield, and metamorphic characteristics. The great around which Precambrian mobile belts had majority of radiometric dates in the belt fall been moulded during a period of dextral stress between 900 and 450 m.y., roughly equivalent on a general north-northeast-trending axis to the Katangan cycle (Cahen and Snelling, (Fig. 3, inset a). This pattern was deduced from 1966), and it is an important constituent of the the observation that the compressional axes in zone of Pan-African reactivation. Shackleton the Ubendide Belt in southwest Tanzania and (1967) postulates that the belt is polycyclic and in the Pare-Usambara Mountains in the north- perhaps represents two tectonothermal episodes east (Fig. 3) were directed northwest-southeast, prior to the intrusion of granites in Tanzania whereas tension on northeast-southwest axes dated about 1,035 m.y., with the widespread appeared to be represented by the Lake Eyasi imprint of the Pan-African episode occurring Rift and parallel splay faults en echelon along the still later. eastern margin of the shield. It was also ob- The belt is of importance to the study of the served that the Precambrian mobile belts were rift system because it runs parallel to the eastern followed by Cenozoic rift valleys and scarps, branch of the system from the Zambezi to but that the Precambrian structures had Afar, and early cataclastic zones along the rift formed deep in the earth's crust, whereas the faults give radiometric ages in the Katangan rift faulting was a manifestation of arching and range. It is bounded to the west by a Mozam- fracturing in the rigid surface crust due to bique Front (Fig. 1), generally dividing it from morphogenic uplift along the mobile belts early shields. To the east, it dips below the (Wegmann, 1963; McConnell, 1970). This Phanerozoic coastal sediments. structural pattern indicates dextral movements Rift Volcanism. The Cenozoic rift volcanic of the mobile belts which had split around the rocks, which are so prominent in the easily pre-existing nucleus of the Tanganyika Shield. accessible portions of the rift system in Ethiopia Figure 3 is a new compilation which verifies, and Kenya, are limited to a relatively small with some modifications, the structural picture sector of the East African Rift System. They presented in 1951, which is supported by the are very widespread in Ethiopia, where the following recent work: system meets the spreading oceanic features of 1. Confirmation of the great age of the the Red Sea and the Gulf of Aden, but extend Tanganyika Shield (Hepworth and Kennerley, southward in a band closely associated with the 1970). eastern rift only as far as 4° S. Cenozoic and 2. Confirmation of cataclastic belts in Ubende Holocene volcanic rocks are absent from the with transcurrent movement (Fig. 3, inset c) Western Rift except for the very local occur- by Sutton and others (1954), and Sutton and rences which are associated with nodal points. Watson (1959). The nature and age of the rift volcanic rocks 3. Continuation to the southeast of cata- have been recently described by Baker and clastic belts in the Mbeya area (Brown, 1962). Wohlenberg (1971) for Kenya, by Mohr 4. Overfolding and thrusting on northwest- (1967) and Cass (1970) for Ethiopia, and sum- trending axes in Ukinga farther to the south- med up by King (1970) and Baker and others east (Harpum, 1958). (1972) for the whole of East Africa, so that 5. Proof of overthrusting of Precambrian age further comment is not necessary here. The in the Pare Mountains (Bagnall, 1962). rift volcanic rocks are important in establishing 6. More accurate mapping of northeast- mantle conditions beneath the system, but trending normal splay faulting of the eastern they obscure the Precambrian basement so that margin of the Tanganyika Shield to the south a full picture of the behavior of the infracrustal of the Gregory Rift Valley and discovery of a zones beneath the rifts must be sought in other polyphase dolerite dyke swarm parallel to these areas. faults and in part of Ubendian age (Vail, 1970, p. 344). 7. West-northwest-trending axes of folding MAJOR STRUCTURAL FEATURES and stretching in the Mozambique Belt, OF TANZANIA accompanied by powerful cataclasis and north- An analysis of major structural features of east normal faulting in the margin of the Tanzania (McConnell, 1951) suggested that Tanganyika Shield (Hepworth and Kennerley, the central granitoid complex (Fig. 2) was a 1970), which can be interpreted as dextral

movement of the mobile zone to the south 1954; Sutton and Watson, 1959; Coetzee, deflected by the protruding margin of the 1963) is of a northwest band of high grade shield (Fig. 3, inset b). gneisses about 150 km wide with many zones It is suggested that the dextral stress along of migmatite and intense Precambrian infra- the rift lineament arose from differential move- crustal cataclasis crushed between the Zam- ment between the main African platform and bian cratonic block and the Tanganyika Shield. the eastern margin formed by the Somalia and This mobile belt has been impressed with the Mozambique blocks. Extensive dextral faulting powerful Ubendian orogeny, and McConnell could only have taken place in Late Archean (1951, 1970) has suggested that it is an excep- (ca. 2,700 m.y.) time, although limited dextral tionally clear example of the formation of movement continued during successive tec- Cenozoic rift valleys genetically associated with tonic rejuvenations up to the Mozambiquian, an early Precambrian mobile zone. Post- after which morphogenesis and vertical rift Miocene rift faults closely follow shear zones faulting became dominant. An illustration of marked by blastomylonites, phyllonites, mig- this structure-time relation is seen in the matites, and lenses of mobilized granite. Suc- Gregory Rift Valley, a mainly north-northeast- cessive stages in the evolution of the rift system trending lineament, which dies out as such from its origin as a Precambrian mobile zone south of the Kenya-Tanzania border, where its deep in the earth's crust to the formation of strike carries it into the rigid, heavily craton- the post-Miocene rift valleys are here exposed ized Tanganyika Shield which it cannot pene- by arching and consequent deep erosion. Later trate. Precambrian dextral movement along mapping (for example, Sutton and others, this lineament is indicated by items 6 and 7 1954, 1959; Brown, 1962; Coetzee, 1963) has above, but the traces of early dislocations north confirmed the presence of the remarkable of 3° S. are buried under Cenozoic volcanic Precambrian migmatitic and cataclastic belts rocks which, with the contemporary uplift of coinciding with the course of this portion of the the Kenya dome (Saggerson and Baker, 1965), Western Rift Valley. testify to the continuance in time of a rift Two periods of metamorphism have been lineament. Between Tanzania and Kenya the distinguished (McConnell, 1950), one of a Gregory lineament is crossed at an acute angle regional character followed by migmatization by the Mozambique Front, and beyond the culminating in the emplacement of anatectic east-trending Kavirondo Rift, Sanders (1965) granites such as the Kate granite (Fig. 4). has described the front at the Nandi fault Sutton and others (1954) also describe two escarpment (Fig. 3) as highly tectonized and phases of metamorphism: an earlier period complicated by a combination of southwest- dominated by the granulitic facies and in- ward thrusting against the West Nile-Uganda cluding charnockitic rocks, and a later phase Shield with dextral transcurrent movement. largely in the amphibolite facies and accom- East of Dodoma, the margin of the shield panied by migmatization. It is therefore now swings to the southwest, and this is reflected in suggested that these rocks form a polycyclic a line of almost continuous fault scarps which assemblage, tectonized first in a late Archean extends 400 km in a southwesterly direction orogeny and later reactivated in an Ubendian to Mbeya and overlooks the valley of the orogeny which imposed its isotopic dates of Ruaha River. 2,000 to 1,600 m.y. The evidence on which the preceding geo- Mbeya and Ukinga. A node of great signif- logical outline is based comes from several areas icance in the whole rift system occurs at Mbeya now to be described in which the association of in southern Tanzania (Figs. 2, 3, and 4) where Precambrian structures with Cenozoic rift the Western Rift Valley crosses the general faulting is particularly well seen. meridional trend of the Gregory Rift System and is marked by the 30-km-wide pile of the Southwestern Tanzania Pliocene to Holocene Rungwe Volcanics Ubende and Ufipa. The Ubendides (Fig. 4) (Harkin, 1960). The high-grade Ubendian of southwestern Tanzania extend northwest gneisses, with cataclastic zones and migmatites, from the northern basin of Lake Tanganyika continue the southeasterly strike of the Rukwa to northern Malawi. The general picture Rift Valley and form a wide belt in the (McConnell, 1950, 1969a; Sutton and others, northern province of Malawi (Cannon and

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others, 1969); on the east side of the lake they impressed by dominant Ubendian dates and form a highly characteristic belt (Harpum, then by pegmatitic and other activity, in- 1958), reappearing from beneath the Rungwe cluding the Ukingan episode which represents Volcanics in Ukinga to the north of Lake the Kibaran. The Bukoban sedimentary basin Malawi, and the impressive southwest-facing of Katangan age is affected by folding which scarp of the Livingstone Mountains continues causes associated intraformational unconform- the strike of the northeastern Rukwa scarp ities, which increase in importance toward the almost exactly, associated with zones of cata- Lake Tanganyika Rift. Post-Karroo (Jurassic?) clasis similar to those in Mbeya Mountain and Lower Cretaceous rift faulting was (Brown, 1962). The geology of Ukinga is described from northern Malawi by Dixey particularly well known through the Kipengere (1939, 1956) and is confirmed by the dating map sheet of Harpum (1958). Here, con- (Cahen and Snelling, 1966) of carbonatites tinuing the northwest-trending strike, the injected along fault planes (Brown, 1964; Ubendian gneisses, including the Upangwa Coetzee, 1963) and by other igneous activity. meta-anorthosite complex, are overlain uncon- Fault mylonites along the rift system are also formably by the Ukinga group of slates, probably of this age. Finally, the great supra- phyllites, and quartzites, and the whole com- crustal arching and fracturing of the late plex is strongly over-folded and thrust to the Cenozoic produced the northern basin of Lake northeast. Unconformably overlying this com- Malawi, the Rukwa Rift Valley, and the plex is the Buanji (Katangan) group of con- northern and southern basins of Lake Tan- tinental, dominantly tabular sediments, locally ganyika. The division of the Western Rift into overfolded by a revival of compression from parallel graben systems is marked by the Ufipa the southwest. A postorogenic granite cutting and Kungwe horst blocks (McConnell, 1950) the Ukingan rocks and unconformably overlain and the high western rim of Lake Tanganyika by the Buanji has a radiometric age of 1,300 as shown in Figure 4. m.y. (Cahen and Snelling, 1966, p. 87), dating Each reactivation was accompanied by high the Ukingan orogeny as Kibaride. Thus in heat flow with consequent expansion and lower- Ukinga the Ubendides have undergone intense ing of density along the lineament, causing orogeny in Ubendian time with injection of isostatic imbalance leading to morphogenic anorthosites, another powerful orogeny in the uplift (McConnell, 1970). Uplift resulting Kibaran accompanied by migmatization and from repeated tectonothermal activity is anatectic granites, and a third milder compres- demonstrated by the oblique crossing of the sion in Katangan time; all three with ap- Kibarides by the Ubendides near northern Lake proximately the same major stress direction. Tanganyika (Figs. 2 and 4). This would normal- Time-Structure Relations. Geographically ly suggest that the Ubendide was the younger the Ruaha Fault (Fig. 4 and above) appears to fold belt, but stratigraphy, metamorphic grade, be continued to the southwest of the Ubendian and radiometric dating clearly indicate that it Belt by the Luangwa Rift Valley in Zambia, is older, and the simplest explanation for this and there are supporting geological indications. structural paradox is that the dominant In the Lupa mining district, to the north of Ubendian orogeny overlies a lineament in Mbeya, shear zones parallel to the Ruaha Fault which exceptional heat flow has led to melting, cross the dominant Ubendide strike, and faults migmatization, and granitization, with con- and photo-lineations striking north-northeast sequent reduction in density/depth ratio lead- have been mapped south of Mbeya (Macfar- ing to great isostatic uplift. Even the roots of lane, 1966), as shown in Figure 4. The Rungwe the Kibaride chain have thus been removed Volcanics therefore mark the crossing of two of by the erosion of this morphogenic arch and the the major rift system lineaments, and their great deeps of Lake Tanganyika. 700 m below petrology as described in detail by Harkin sea level in the southern basin (Capart, 1952), (1960) should be of great interest in the inter- and the altitudes of the rift margins (up to pretation of the infracrustal geology. 1,500 m above the African plateau) at the The whole Ubendide Belt is a typical rift present day may be due to the exceptional system mobile zone which has been reactivated character of this lineament. Clifford (1967, p. during successive orogenic cycles (see also 62) notes the recording in the Kibaride fold Mizutani and Yairi, 1968). Dating probably belt of adjacent Rwanda and southern Kivu of from a late Archean cycle, the belt was later Katangan ages representing a "thermal node"

of Damaran-Katangan activity. This area is rocks of the granulite facies. Areas of migmati- also a node in the Wes-tern Rift where the zation and potash permeation also occur along northwest-trending strike of Ubende meets the structural lines in the main mountain blocks. north-trending strike of the northern basin of Confirmation of the general picture of Lake Tanganyika. Other nodes are marked thrusting to the west is given by Bagnall and by local volcanic centers, but perhaps the others. These movements are illustrated by the underlying thermal activity has found relief Changube thrusts (Fig. 5) which are associated in the uplift of the great Kungwe horst (Fig. 4) with a migmatization "seemingly connected and the exceptional vertical displacement of with the intense shearing and thrusting" the rift floor and the boundary lips, (Bagnall, 1962). Lineations pitching flatly to Pangani Rift Zone: Northern Tanzania the east and northeast were mapped throughout the tilt blocks and were interpreted as "b" The Pangani Rift zone (Fig. 5) is an area of lineations associated with early northeast- great geological and structural interest. The trending fold axes. This interpretation may be floor of the rift valley itself, directed north- questioned, however, as in the Namanga area northwest and at an altitude of 600 to 900 m, is of Kenya to the north-northwest (Fig. 3), dominated on the east by the high west-facing where the Pangani structures emerge from fault scarps of the Pare and Usambara Moun- beneath the huge volcanic mass of Kilimanjaro. tains, rising to 1,800 to 2.100 m and forming Joubert (1957) has mapped similar rocks great tilt blocks sloping eastward to the Umba strongly overfolded to the south-southwest, Plains of Kenya. Some 30 to 50 km to the west, and Pulfrey (in Joubert, 1957) has remarked the high block of the Masai Plains drops to the that the lineation here is usually almost at right rift valley in a curving line of unimpressive east- angles to the trend of the foliation traces, facing fault scarps. An exceptional geological whereas to the northeast, toward the interior feature of the whole region is the constant over- of the Mozambique Belt, the lineations are all dip of foliation and structural planes 20° to parallel to the foliation. Pulfrey suggests that 50° to the east and northeast, which affects "a" lineations at Namanga are associated with nearly equally all three units of the rift system, overthrusting to the west. namely: the western border of the Masai Plains; Further support for the conception of the migmatites of the rift valley floor; and the northeast-southwest compression is found in a great Pare and Usambara tilt blocks. On the reinterpretation of the Mkomazi Fault (Fig. 5) basis of a rapid reconnaissance, it was originally as an overthrust. This fault (Bagnall and others, postulated (McConnell, 1951) that the Pare and 1963) runs from southeast to northwest some Usambara tilt block mountains are ancient 5 km west of the frontal scarp of the Usambara rocks thrust to the west-southwest and south- Mountains and then bends north in the gap west during the Precambrian, thus overriding between the Usambara and South Pare tilt the rocks of the Masai Plains on a sheared zone blocks. The fault is accompanied by a permea- now constituting the migmatized floor of the tion zone dipping 25° to 30° to the northeast rift valley. The North Pare and Usambara below the Usambaras and 15° to 50° eastward in Mountains have since been studied by Bagnall the northerly extension, and the lineations (1962) and others. mapped in these migmatites are parallel to The main rock types of the Pare and those in the mountain blocks. Hence it is Usambara tilt block mountains are all of high- suggested that the rocks now forming the grade metamorphic facies and consist chiefly of Usambara massif overrode those of the South granulites and granulitic gneisses deriving from Pare block along the line of the Mkomazi Fault. a very thick series of metamorphosed pelitic The ages of the successive Precambrian and psammitic sediments with intercalations of tectonothermal events affecting the Pare- carbonaceous and calcareous strata. They have Usambara area are problematic. The Pan- been subjected to complex recumbent folding African episode has imprinted K-Ar dates and horizontal translation so that no strati- (Cahen and Snelling, 1966, Table 4.2), a graphical succession can be established. The rift granulite from the Pare Mountains has given floor, however, is formed by "a complex mobi- a whole-rock Rb-Sr age of 800 to 900 m.y., and lized migmatite series" (Bagnall, 1962) with the another granulite from the center of the Masai regional eastward dip, which appears to have Plains has an age of 730 m.y. (Spooner and developed by metasomatic granitization of others, 1970). These ages reflect the Mozam-

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bique orogeny, but the granulites may well date some 3 km above the general level of the mid- from an earlier period. Tertiary African plateau. No attempt at Suggested Relation to East African Struc- describing the evolution of the African Rift tures. The structural relations described above System can omit consideration of this remark- suggest that the granulitic gneisses of the Pare able uplift. Following early exploration (Scott- and Usambara mountains have been thrust to Elliot and Gregory, 1895; Roccati, 1908; the south-southwest over the rocks of the Michot, 1938), the main source of geological Mozambique Belt of Tanzania during a Pre- information comes from the British Ruwenzori cambrian orogeny. Such a movement could Expedition 1951-1952 organized by W. Q. originate either from the southerly drift of a Kennedy of Leeds University and the Geo- Somalia block or from the resistance of this logical Survey of Uganda, in co-operation with block to the northeasterly drift of the Tan- a Belgian party. A preliminary report on the ganyika Shield. results of this expedition has been published The Precambrian structures were formed at by McConnell (1959), who worked on the considerable depth in the crust. Because the high summits in 1943 and during three belt is associated with migmatization and expeditions in 1951 and 1952. Since 1952 the granitization, its density has diminished and it geological exploration of the mountains has has undergone uplift and morphogenesis. The been continued by Kilembe Mines Ltd. Recent actual rift valley formation is ascribed to these work has also been summarized in the Annual late movements of the superficial crust follow- Reports for 1969, 1970, and 1971 of the ing the deep erosion caused by uplift. The Research Institute of African Geology, Univer- present-day Pare and Usambara mountain sity of Leeds. blocks are therefore regarded as due to Ceno- zoic block faulting genetically associated Geological Setting through posttectonic uplift with the intense This mountain block lies at a double node of Precambrian compressive structures (McCon- structural importance (Fig. 6, inset): (1) it lies nell, 1970) and are essentially parallel to them. at the junction of the north-northeast Lake Bagnall (1962) states that no trace of the great Edward and northeast Lake Albert sectors of frontal fault of the Pare Mountains can be seen, the Western Rift, and (2) the line of the owing to retreat of the fault scarp, but the Buganda-Toro front actually crosses the high position of the fault in the plain is indicated by peaks and divides the mountains into a northern a series of water boreholes penetrating fractured range with dominantly north-northeast-trend- granulite below a deep regolith. The sym- ing strikes and southern ranges with strikes pathetic faults in the mountain block men- forming an arc concave to the north. tioned by Bagnall (1962) are mapped as Ruwenzori constitutes a huge, deeply eroded vertical, and it may be deduced that the major whale-back of Precambrian gneiss and schist Cenozoic rift fault is itself vertical. whose narrow fault-bounded northern nose According to published geological maps of rises out of the Lake Albert rift valley and is Tanzania (see Fig. 2) the Pangani rift structure surmounted by a flattish surface rising gradual- does not reach the Indian Ocean, as suggested ly southward to the high summit surface, prob- by Baker (1970, Fig. 4) and Kent and others ably of early- to mid-Tertiary age (Willis, (1971), indicate that the boundary of the Juras- 1936; McConnell, 1959). The summit surface sic and Cenozoic sediments overlying the base- then descends southward for some 25 km, ment and down-warped toward the Indian deeply etched by river valleys, until it slopes Ocean was not dislocated in the Cenozoic by more sharply to plunge beneath the flats of the Pangani fault zone (Fig. 5). It must there- Lake Edward. The western slope is steep and fore be questioned whether the East Kenya- fault-bounded, overlooking the Semliki Rift Somalia block has undergone the clockwise valley. The eastern slope is more gradual, rotation proposed by Baker (1970, p. 385). representing an upwarp except for the great north-northeast-trending Wasa fault (Fig. 6) RUWENZORI MOUNTAINS: determining the narrow northern nose. The WESTERN UGANDA block has been uplifted as a whole by up- The Ruwenzori Mountains (Figs. 2 and 6) warping and faulting, not in a trap-door fashion consist of a Precambrian block 120 km long sloping east, as has been suggested, since and 50 km wide raised by warping and faulting tabular Buganda-Toro beds on the summit

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ridge west of Fort Portal dip 10° to 20° to the Buganda-Toro schists and quartzites. The west. geologists of Kilembe mine have related these The Lake Albert rift valley is occupied by at structures to those of the Buganda-Toro group least 2,400 m of Kaiso-Kisegi beds (Harris and to the east, and Tanner (1969) has shown that others, 1956) of Miocene to mid-Pleistocene the Butiti (Fig. 6) quartzites are unconform- age (Bishop, 1965), and the basement floor able over the basement and form a syncline slopes to the southwest, reaching a depth of overturned to the north. This appears to con- 1,800 m below the average level of the early- to firm that the central band of amphibolite mid-Tertiary surface. Thus, compared with the forming the Stanley peaks is the core of an adjacent uplifted block of Ruwenzori, this overturned syncline (McConnell, 1959, Plate indicates a total vertical displacement of 6 km, VI), thus indicating a strong northward com- possibly a maximum in the African rift system. pressive force driving the southern east-west It is notable that this great graben and horst folds against the northern gneisses with their system is only accompanied by restricted explo- core of Speke Gneiss. sive volcanism without any appreciable mag- A structural feature of particular importance matic effusion. is the horizontal S-shape sketched by the front of the schist group. The schist and amphibolites Structure in the Butiti syncline appear to be wrenched The geological succession now established on to the northwest parallel to the Ruimi River the Ruwenzori Mountains (McConnell, 1959; as they reach the mountain base in the steeply Tanner, 1969; and others) is shown in Figure 6. dipping Ruimi monocline, and they then com- The "basement complex" is correlated with the plete the horizontal S-shape as a syncline over- Uganda basement (Macdonald, 1969) and may turned to the north and bent around the be of great age, probably older than 2,500 m.y. Mount Speke hub, to be again sharply A date of 1,930 + 60 m.y. (Cahen and Snelling, wrenched to the north as they approach the 1966, Table 7.1) relates the overlying schist western base of the mountains. This structure groups to the Buganda-Toro-Ubendian cycle, pattern represents the thrusting of the Bu- but these rocks may be older and merely ganda-Toro Front against and in part over the imprinted with Ubendian dates. The notion of north-northeast-striking basement migmatites rejuvenation is enhanced by the Kibaran dates and granites of the northern nose. These reported for the related schist and amphibolite ancient banded rocks are swung parallel to the formation by Cahen and Snelling (1966, p. 88 great Wasa fault, which is thus comparable to and Table 9.4), and Pan-African dates have the rejuvenated Precambrian rift structures also been recorded (Tanner, 1971, unpub. of Ubende. Since this lineament is overridden data). by the Buganda-Toro schist belt, it is presumed The northern ridge consists of a series of to be older, probably of late Archean age. banded migmatitic gneisses of the basement The deep-seated and fundamental origin of complex, with a general north-northeast- the Wasa fault is further supported by the fact trending strike broadening southward toward that a dextral transcurrent shear zone near the high peaks to merge into the Speke Gneiss, Kilembe has now been established (R. P. which forms the structural hub of Ruwenzori. Freeman, unpub. data) with a post-Toro dis- It is a tectonized massive granite easily displacement of 8 km which continues exactly the tinguished from the normal basement by such strike of the Wasa fault behind the Buganda- features as rounded basic xenoliths which are Toro front. This shear zone is central to the not found in the surrounding banded gneisses. Lake Edward rift valley, and it probably The Buganda-Toro Front has been moulded on originated as a post-Toro rejuvenation along the Speke Gneiss, and the strikes in the the same mantle lineament of which the ancient southern ranges of the mountain in general Wasa dislocation is an infracrustal manifesta- form an east-trending arc centering on Mount tion. It is significant that the throw of the post- Speke. The structure of the southern portion of Toro shear zone, which fades out to the north, the Ruwenzori Mountains is in sharp contrast matches the dextral northward wrench of the with that of the northern ridge, as it consists of Butiti syncline in the S-fold referred to above basement gneisses folded on approximately and is probably taken up by the wrench. east-west axes including synclines of the Thus the structure of the Ruwenzori Moun-

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tains appears to be an outstanding illustration Possible Dextral Displacement of the Buganda- of the cyclic development of the rift system. Toro Front The earliest movements now recognizable were a dislocation zone parallel to the Wasa The intensive northward thrust of the fault affecting the West Nile-Uganda Shield in Buganda-Toro fold belt in Ruwenzori and the the Late Archean cycle; rejuvenation occurred resulting horizontal S-shape around a project- during the dominantly east-striking Buganda- ing foreland is additional evidence of Precam- Toro (Ubendian) orogeny, which was accen- brian dextral shear stress in the Western Rift tuated in the rift zone by the resistance of Valley. Moreover, the 3-km-wide Namarodo the Speke massif. The Karagwe-Ankolean and Namtin refoliation and mylonite zones (Kibaride) and Pan-African orogenies then (Hepworth, 1964, p. 39), mapped to about 35 impressed themselves, resetting the radiometric km north of Lake Albert (Fig. 2), strike north- clocks, and finally vertical uplift recurred in east oblique to the general strike but parallel the Mesozoic, was accentuated in the mid- to the Lake Albert rift, suggesting either trans- Pleistocene (Bishop and Trendall, 1967), and current movement or vertical displacement continued sporadically until the present day under strong hydrostatic compression. A nar- (Fairhead and Girdler, 1970, Fig. 3). row quartz-diorite gneiss band, mostly less than a kilometer in width, extends for at least 15 km Suggested Origin of the Uplift parallel to these crushed bands. Hepworth In common with other features described in (1964, p. 59) suggests that it was intruded this paper, the uplift of the Ruwenzori Moun- along an associated fracture during the later tain block is attributed to the morphogenesis stages of refoliation. This type of dislocation resulting from isostatic imbalance (McConnell, zone, both older than the Cenozoic rift faulting 1959, p. 265; 1970). This block has been shown and oblique to the local strike, is also found to be the locus of intense compressive forces neighboring the Upper Rhinegraben where it during the Ubendian cycle at a node in the has been attributed to transcurrent movement Western Rift Valley where the Buganda-Toro in Variscan time. has been powerfully thrust against, and prob- Some indication of dextral displacement ably to some extent over, the hub of Speke along the Lake Albert rift lineament in the Gneiss representing the foreland of Uganda early Precambrian may also be seen in the basement. This compression was accompanied tectonic pattern northwest of the lake as shown by migmatization and the formation of syn- in Figure 2 (after Hepworth, 1964; Cahen and genetic granites (G. P. Leedal and G.P.L. Lepersonne, 1967, Fig. 4; and Lepersonne, Walker, upub. data; McConnell, 1959, p. 261), 1972). leading to reduction in density. It has been pointed out that volcanism normally occurs in NORTHERN EXTENSION OF THE the Western Rift Valley only at nodes, that is, RIFT SYSTEM in places characterized by high heat flow where the strikes cross or change direction. At the Kenya and Ethiopia Ruwenzori double node the remarkable recent explosive volcanism described by Holmes and The northern part of the East African Rift Harwood (1932) occurs, indicating great mag- System has been the most carefully studied and matic activity in depth. The Cenozoic uplift described (UMC/UNESCO, 1965; Baker of the Ruwenzori block is therefore associated and others, 1972). In Kenya, where the base- with high heat flow leading to melting and ment near the rift is largely concealed, there expansion. The almost exclusively explosive is no evidence of volcanicity or faulting earlier nature of the eruptions may indicate that the than middle Miocene; the evolution of the intense compression recorded by the Pre- main rift system from that time has been cambrian structures still continues and impedes described by Baker and Wohlenberg (1971) magmatic outflow. That the block can still and the branch Kavirondo rift valley, trending rise freely in response to underlying expansion west toward Lake Victoria by Shackleton and consequent isostatic lift is indicated by (1951). A brief summary of work on the asso- the continuing seismic activity and the rough ciated volcanic rocks has recently been given equilibrium recorded in gravity traverses by by King (1970), who also states that the rift J. M. Brown (1956, unpub. data). faulting is mostly normal and tensional and that

only to this extent are the rifts dilational there is evidence that they are distinct from phenomena. Baker and Wohlenberg (1971) the East African Rift System. show that normal faulting at the northern and southern ends of the Kenya dome can have led Northern Termination of the Western Rift to only about 3 km of crustal distension, al- Valley though distension possibly attaining 10 km is The northern end of the Western Rift was suggested for the center of the dome. described by Lepersonne (1972), Macdonald Of interest to the present study is the Mutito (1965) has reviewed the information available, fault zone near Kitui, 120 km east of Nairobi and King (1970) has briefly described its (Fig. 2), where Sanders (1954) has described a volcanicity. The northeast-trending Lake Al- zone of intense shearing and mylonitization, bert rift valley shallows to the north and with intercalated ultra-basic intrusions, which terminates (Bishop, 1965) against basement took place in the Archean along what is now a rocks of the West Nile-Uganda Shield (Fig. 2) north-northwest-trending fault scarp, and at the foot of the plateau from which the "may account for the local intensity of Victoria Nile cataracts in the Murchison Falls. granitization." Sanders considers that the The Nile continues to the northwest in a wide Archean fault zone has probably been re- plain whose formation and history have been juvenated in lower or middle Pleistocene studied in detail by Hepworth (1964) and times to produce the Mutito escarpment, Macdonald (1963). The depression, due in part generally accepted as a portion of the rift to warping and in part to en echelon faulting, system. bends again to the northeast and east until cut To the east of Lake Rudolf the fault scarps off by the northwest-trending Aswa shear zone diverge in an en echelon pattern toward the at Nimule, a puzzling line of refoliation and well-known north-northeasterly Ethiopian Rift granitization (Hepworth and Macdonald, 1966; Valley recently described by Mohr (1967) as Macdonald, 1969) which lines up to the south- floored by continental crust; the Arabo- west, beyond Mount Elgon, with the Nandi Ethiopian swell was probably uplifted more fault (Fig. 3) of western Kenya. The Western than 2,500 m in the early Tertiary, followed by Rift thus appears to dissipate itself in the the deposition of a thick sequence of basalts resistant West Nile-Uganda Shield as the and tuffs. The development of the rift system Gregory Rift does farther south in the Tan- started in the Miocene along the axis of the up- ganyika Shield. lift, and rift and volcanic activity, including the extrusion of the Quaternary Aden Series of SOUTHERN AFRICA volcanic rocks, have continued to the present. In the center of the northern sector of the Rift Valleys in Southern Africa rift valley, Mohr has described a narrow line of Along the line of the crustal lineament pos- active crustal tension and volcanism termed the tulated by the writer to be the southern projec- Wonji fault belt. The association of tectonics tion of the main axis of the East African Rift and magmatism in the Ethiopian Rift, con- System, there are no Cenozoic rift valleys. sidered as a sector of the Afro-Arabian dome, However, Vail (1967) has mapped a pattern of has been discussed by Cass (1970) and ascribed late and post-Karroo faulting around the to an isolated lithothermal event in the upper Rhodesian Shield (Fig. 2), continuing the mantle. A recent contribution by McKenzie Luangwa rift. The fractures described by Vail and others (1970) will be discussed below. are all associated with Karroo basins, which To the north, the Ethiopian Rift Valley were mainly established in pre-Karroo times merges into the Afar depression and Tazieff (Drysdall and Weller, 1966; Swardt and others, (1970) has shown that its western wall curves 1965, p. 90) and the writer suggests that they into the great north-trending western fault are largely due to the post-Karroo isostatic up- scarp of the depression, towering in places to a lift of the Rhodesian Shield which they sur- height of more than 3,000 m. The eastern wall round. Erosion of the Karroo fill has led to of the rift valley diverges to the northeast to disinterment of pre-Tertiary fault scarps. The form the southern scarp of the Afar triangle, Lebombo monocline of Karroo sediments and rising 1,600 to 2,200 m to the Somalia plateau. lavas has also formed as the result of the uplift The Red Sea, Gulf of Aden, and Afar depres- of the Kaapvaal Shield which it borders to the sion are not described in this paper because east. Vail (1967) has remarked on the paral-

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lelism of the post-Karroo troughs to foliation northeast-trending lineament in the underlying trends in the adjacent Precambrian rocks. This mantle, and, with periodic reactivation, a is not surprising, as the Kariba troughs con- difference in the age of the associated mafic tinue the Irumide and Luangwa trends into intrusives is implicit. the Damaride trend of South West Africa In view of the vast amount of geological (Clifford, 1967), the Middle Zambezi troughs information now available in southern Africa are parallel to the east-trending Zambezi Belt, and recently compiled by Haughton (1969), the eastern troughs are parallel to the Mozam- the writer approaches with great diffidence the bique Belt in eastern Rhodesia, and the ancient question of the proposed southern extension of Limpopo Belt has been revived (Mason, 1969). the main axis of the rift system lineament but A genetic association of the post-Silurian-pre- would draw attention to the following features Carboniferous and the late and post-Karroo (Fig. 1): faulting with the earlier Precambrian mobile 1. The Rhodesian and Kaapvaal Shields belts is suggested according to the mechanism (eratons), as depicted by Anhaeusser and others outlined in this paper. (1969), lie centrally along the.axis. Possibly the main difference between a shield and a Southern Extension of the Rift Lineament craton (see Appendix) is that the former is an area of negative gravity anomaly, a characteris- It has been proposed (McConnell, 1967) that tic of the lineament, which causes the ancient the main axis of the rift system lineament of rocks to emerge as shields. Admittedly the eastern Africa continues south of the Zambezi Kaapvaal craton is not altogether a true shield River by the line of the Great Dyke of in this sense, owing to the presence of later Rhodesia (Figs. 1 and 2), the projection of syneclises, but it does stand up as high ground which bisects the Bushveld Complex and lines corresponding to the High Veld uplift of L. C. up a number of geological features on a north- King (1967, p. 241) in comparison with, for northeasterly strike as far south as the sub- instance, the downwarped Kalahari craton. Karroo Trompsburg Complex (Haughton, 2. The north-northeast-south-southwest 1969, p. 187) just north of the Orange River. alignment of the deep sources of the mafic Cousins (1959) has described this line as the rocks of the Great Dyke (Worst, 1960). "Bushveld chain of intrusives" stretching 3. The bodies of Stormberg lavas at the cross- 1,500 km from the Zambezi to the Orange ing of the Limpopo Belt by the postulated rift River. There are obvious objections to the system lineament. assumption of this north-northeast-trending 4. The Villa Nora norites and red granite lineament. The most obvious being the dom- (Cousins, 1959). inance of a latitudinal trend of Precam- 5. The Stormberg lavas at the crossing of the brian mobile zones such as, from north to south, east-west Bushveld igneous complex by the the Zambezi, the Limpopo, and the Namaqua- postulated lineament. Natal Belts (Nicolaysen, 1962; Vail, 1967; 6. The Johannesburg hub of ancient granite. Clifford, 1967; Crockett and Mason, 1968; 7. The Vredefort dome, with the presence and others). In this connection Haughton of younger granites of Bushveld age and its (1969) has also stressed the east-west elongation positive gravity anomaly suggesting underlying of the mafic rocks of the Bushveld Complex. mafic rocks. Another difficulty is that the norites of the 8. The south-southwest-pointing wedge- Great Dyke have been dated as late Archean shaped termination of the sub-Karroo Wit- (Allsopp, 1965), the Bushveld Complex as a watersrand and Ventersdorp basins (Borchers, whole at about 2,000 m.y. (Nicolaysen and 1961). others, 1958), and the Trompsburg Complex 9. The sub-Karroo Trompsburg Complex: provisionally at about 1,300 m.y. (Davies and indicated by boreholes to be "a layered others, 1970). The nature of deep-seated line- plutonic igneous body 6,000 to 10,000 ft. aments is still not properly understood, and thick with a diameter of about 30 miles" mobile belts are known to cross each other and (Haughton, 1969, p. 187) associated with a red to be constantly reactivated. The dominance granophyric granite said to be indistinguishable of east-west mobility in southern Africa there- from the Bushveld granite of the same type. fore does not exclude the presence of a north- Significantly, this complex is at the node where

the presumed rift-system axis would cross the rift valleys were compressional structures with east-trending Namaqua-Natal Belt (Nicolay- the crustal floor forced down into the denser sen, 1962) and the Lesotho trend of kimberlite substratum. Harris and others (1956) attributed pipes (Crockett and Mason, 1968, Fig. 2). the localized negative anomalies merely to the 10. The north-northeast trend also coincides great thickness of sediments in the grabens with one of the axes which could be drawn which had been proved in boreholes, although through the central cluster of carbonatite plugs this would not apply to Lake Tanganyika. and kimberlite pipes shown by Haughton Gravimetric Surveys of Uganda (Atlas of (1969, Fig. 20). Uganda, Entebbe, 1969) and of Tanzania 11. The principal body of Stormberg (Masson Smith, 1965), have confirmed Bui- sediments and lavas in Lesotho is also elongated lard's 1936 results and Masson Smith (personal north-northeast but to the east of the main axis. commun., 1969) has also attributed the negative 12. Finally, the postulated main axis is parallel anomalies to sedimentary fill. Nevertheless, to the general alignment of the great north- these negative anomalies may be of great signif- northeast dislocation zones in the floor of the icance in view of Mueller's (1970) conclusions Indian Ocean (Heezen, 1969). concerning a somewhat similar anomaly in the Rhinegraben. Dopp (1964) has in fact detected GEOPHYSICAL CHARACTERISTICS OF low-density basal crust at depths of between THE EAST AFRICAN RIFT SYSTEM 17 km and 33 km below the Western Rift in an analysis of three short-range earthquake To visualize the third dimension of the East records, and Sowerbutts (1969) has used African Rift System it is necessary to sum- gravimetric data to construct crustal models marize its geophysical characteristics. The data along the 3° S. parallel indicating either upper fall into three groups: seismic, gravimetric, and mantle or basal crust of low density beneath geothermal, and much new information has the rift system. An east-west crustal section recently become available. near the equator based on gravity data has been Seismicity of the Rift System. The East interpreted by Girdler and others (1969) as African Rift System is a zone of shallow earth- showing an intrusive zone of low-velocity quakes similar to that of the mid-ocean ridge mantle material 1,000 km in width below the system, and Wohlenberg (1969) has described whole rift belt, with attenuation of the litho- the distribution of epicenters from 1958 to spheric plate. 1963, showing that nearly all known tectonic Much recent work has been done in the structures were seismically active. The average Gregory Rift Valley, and a seismic refraction focal depth was 20 km, with few figures line was shot between Lakes Hannington exceeding 40 km. This figure of 20 km, com- (0°40' N., 36° 15' E.) and Rudolf (Griffiths parable with that for the average focal depths and others, 1971) in conjunction with a beneath the Rhinegraben (Mueller, 1970) and gravimetric survey by Khan and Mansfield Lake Baikal (Florensov, 1969), probably indi- (1971). The results of both investigations cates the top of a penetrative upwelling of indicate a lenticular body of material of mantle material beneath the rift lineaments. density 3.15 g/cm3 (P velocity about 7.5 Girdler and others (1969) have recalculated all km/sec) at a depth of 20 km extending to teleseismic events in East Africa in the 1955 to about 60 km, and with a width of about 200 1968 period and confirm close correlation with km, thus resembling the mafic pillow (lilies, rift faults. 1970) beneath the Rhinegraben. The conclusion Gravimetric Anomalies. Bullard (1936) is drawn "... that the mantle low velocity found negative Bouguer anomalies on the East layer or asthenosphere is here penetrating the African plateau correlating with the altitude lithosphere . . . ." (Griffiths and others, 1971, and showed that the platform was isostatically p. 71). A narrow ridge of positive gravity compensated. He discovered that the Lake anomalies with exceptionally high P velocities Albert, Lake Tanganyika, and Lake Rukwa for the upper crust was also noted along the grabens in the Western Rift were the seat of axis of the rift valley from the equator to Lake large, uncompensated negative Bouguer anom- Rudolf, but it is significant that this is a sector alies and concluded that this condition sup- in which rift volcanism is highly developed, ported Wayland's (1930) suggestion that the and the crust may be penetrated by feeder

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dykes. A somewhat similar picture of the con- Upper Rhinegraben stitution of the lithosphere is given by Baker and Wohlenberg (1971), but it must be The Upper Rhinegraben is the best known emphasized that it only applies to the in- of the world's intracontinental rift valleys. The tensely volcanized sector of the Gregory Rift graben is at the center of an arch 190 km in Valley. width, the shoulders of which rose as the floor Heat Flow. High heat flow, as indicated in sank. lilies (1970) shows that the graben has the Rhinegraben and Lake Baikal structures, is subsided along normal faults with antithetic postulated in this paper as a dominant factor dips and that the total distension amounts to in the origin of intracontinental as well as 4.8 km since rifting began about 45 m.y. ago. oceanic rift systems, but indications of heat He discusses whether the graben distension is flow from the East African Rift are still con- due to lateral tension as a primary cause, or fined to surface observation of thermal phenom- whether the tension is secondary and the con- ena in the segments affected by volcanism. sequence of another process evolving within the rift itself. The crustal structure beneath the INTRACONTINENTAL RIFT VALLEY Rhinegraben has been investigated by geo- SYSTEMS physical methods (Mueller and others, 1969; The term rift valley (see Appendix) was first Mueller, 1970). An outstanding characteristic applied by Gregory (1896) to the rift valley of is the existence of a subcrustal swell, or pillow, Kenya, and accordingly should only be used of mantle-derived mafic material forming a low for the special type of graben characterized by velocity zone about 25 km below the graben a width averaging around 50 km associated floor, coinciding in width with the rift swell, with crustal lineaments of great length and with and lilies attributes the distension in part to uplifted arches. Many such rift valleys are arching of the crust (Cloos, 1939) aided by known on the globe; the most numerous are gravity sliding on the flanks. Above the mafic those developed in oceanic crust on the great pillow, a sialic low-velocity zone between 10 system of mid-ocean ridges (Heezen, 1969) and 20 km depth has been established by which are the center of the high sea-floor geophysical methods. The focal depth of spreading rates generally regarded as respons- earthquakes beneath the eastern escarpment ible for continental drift. Geological mapping of the Rhinegraben is between 15 and 20 km of the intracontinental rift valleys shows that (Mueller, 1970, p. 33), although under the they are not spreading at any comparable rate graben proper the depths rarely exceed 8 km, but are the site of only minor distension thus suggesting association with the sialic low- amounting, where measurable, to only a few velocity channel. kilometers during the Cenozoic (lilies, 1970; The Rhinegraben forms a segment of the King, 1970; Baker and Wohlenberg, 1971) and Mediterranean-Mjosen line of Stille, figured that they do not contain new oceanic crust, by lilies (1970, Fig. 6). The Rhinegraben itself with the possible exception of the northern follows a Rhenish direction, north-northeast, extremity of the Ethiopian Rift Valley. and cuts across the Vosges-Black Forest craton It is an important fact that all intracon- as well as the northeast-trending Variscan fold tinental rift valleys lie, as far as is known to the belt. On either side of the Cenozoic graben, writer, in cratonic and largely Precambrian however, there are marked Rhenish lineaments crust. The Levantine rift valleys may appear of Hercynian age accompanied by blastomylo- to be an exception, but they lie on the western nites with the intrusion and extrusion of margin of the Arabian Shield and are narrower granitic and gabbroid plutonites, and lower than normal. The famous Upper Rhinegraben Permian extrusives. lilies (1962) interpreted and the Lake Baikal rift systems have been the the lineaments as sinistral wrench faults, over- subject of extensive study, and the brief lain unconformably by Carboniferous, Per- descriptions which follow may help to give mian, or Triassic sediments, and ancestral to a better understanding of intracontinental rift the rift faulting. Originally he regarded the systems in general. Other rift valleys which Rhinegraben as a rejuvenation of the shear might repay further study are the North zones in the Hercynian basement, but he now Savannas rift valley in Guyana, the Perth regards the rifting process (taphrogenesis) as valley in Western Australia, and various rift having evolved independently, merely taking systems in the Brazilian Shield. advantage of an ancient zone of weakness. The

association of alkaline volcanics with the of unconsolidated sediments, while others con- Rhinegraben is well known, as also is the high sider the possibility of an "antiroot." Florensov heat flow (Mueller, 1970). (1969, p. 455) considers that beneath the entire rift zone lies a crust-mantle mixture extending Baikal Rift System up into the lithosphere. The Baikal Cenozoic rift zone has been Exceptionally high heat flow is associated described by Florensov (1969), and a guidebook with the central Baikal rift zone and new prepared for the twelfth AZOPRO session held at determinations (Lubimova and Feldmann, Irkutsk in 1969 describes the geological 1970, Fig. 8) show a normal Precambrian plat- evolution of Pribaikalia (the Baikal region) form heat flow of about 1 Heat Flow Unit over since the early Precambrian. Many features the Irkutskaya (Angara) Plateau rising sharply recall the East African Rift System and may to some 3 HFU over the Baikal rift zone. help in its interpretation. At least seven main stages in the tectonic development of the area ORIGIN OF INTRACONTINENTAL are described by Belitchenko and Khrenov RIFT VALLEYS (1969), namely: Archean; lower, middle, and The modern concept of sea-floor spreading upper Proterozoic; early Caledonian; Mesozoic; with formation of new oceanic crust has and Cenozoic. It is of interest that, as in East focussed attention on the worldwide mid- Africa, the Archean is described as locally ocean ridge system, a great deal of which is metamorphosed to granulite facies with the crowned by a typical rift valley. Recent maps formation of palingenetic and ultrameta- of the system show it passing up the Indian morphic granites and charnockites dated at Ocean, curving into the Gulf of Aden (Laugh- about 2,500 m.y. Depositional, erogenic, and ton, 1966), then bifurcating at the Afar Depres- metamorphic evolution has also followed sion with the Red Sea as one branch and the roughly similar lines up to the Lower Paleozoic, East African Rift System as the other. Heezen and Belitchenko and Krhenov conclude (1969, (1969) has shown the close morphological p. 35): "Thus the brief analysis of the geo- resemblance between the latter and the mid- synclinal stage of development shows con- ocean rifts, and seismological maps also show the vincingly that the Baikal mountain region belt of shallow-focus seismic epicenters, char- ... is one of the striking examples of the acteristic of the mid-ocean ridges, as continuous polycyclic development of the geosynclinal with the East African system. Girdler (1970, systems of the Earth." Fig. 5), however, has shown that the main Regarding the formation of the Baikal magnetic trends of the Gulf of Aden do not depression in the Cenozoic, Belitchenko and curve into the north-northeast-trending Khrenov (1969, p. 37) state: "Thus the effects Ethiopian Rift Valley but continue westward of specific tectonomagmatic activization in across Afar, and Fairhead and Girdler (1970, Pribaikalia have been marked repeatedly and Fig. 8) have shown the same for the line of resulted in the formation of various tectonic seismic epicenters. There is also a fundamental structures. The Pribaikalian volcanic belt was difference between the two systems—the East formed in the mid-Proterozoic, the region of African Rift System is not spreading, although the Baikal-Uchar arch and the depressions seismically active, and does not contain new accompanying it were formed in the Mesozoic oceanic crust (King, 1970). Nevertheless the and the system of the Baikal rift in the evidence described in this paper supports the Cenozoic. These structures . . . present ... as hypothesis that the mantle lineaments under- now becomes quite certain, a definite regularity lying intracontinental rifts are similar to those in the history of the earth's crust develop- which act as centers for sea-floor spreading, but ment." that distension has been impeded by their The Baikal rift zone is complex, and al- geotectonic setting. Since the East African though the central deep rift is about 50 km Rift System lies within the African lithospheric wide, subparallel rift faults extend over a belt plate it is being squeezed between the Mid- some 150 to 200 km wide. The faults at surface Atlantic and Mid-Indian Ocean spreading dip normally at 60° to 70° into the grabens, but ridges, whose continued activity is outlined by the structure is essentially vertical. A negative recent teleseismic observations (Fairhead and gravimetric anomaly over Lake Baikal has been Girdler, 1970, Fig. 3), and so the African line- attributed by some authors to the thick series ament cannot spread. A possible exception is

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the northern extremity of the Ethiopian Rift by the location of the RTiinegraben, which has Valley (McKenzie and others, 1970), but the formed where the great meridional Mediter- distension is probably much less than the 65 km ranean-Mjosen line cuts through the Vosges- postulated. Black Forest massif and the cratonized Variscan The evidence described in this paper sug- fold belt. gests that the lithosphere (crust and upper The supracrustal rift faults, vertical or mantle) is traversed by vertical shear zones, normal, are associated with arching according very ancient and periodically reactivated, for to the mechanism illustrated by Cloos (1939), which the term perennial deep lineament is but there is a basic weakness in this concept, as proposed. These lineaments or deep faults, the great vertical difference in crustal condi- whose nature and origin pose many problems, tions is neglected. The evidence indicates that would be channels for localized heat flow in- the fundamental mechanism is dependent on ducing phase changes or penetration by the high heat flow causing expansion, uparching, underlying liquid or plastic low-velocity layer and extension, and the consequent lowering of (Beloussov, 1966) similar to the penetrative density enables the cold central strip of the arch convection postulated by Elder (1966). All to sink into the hotter underlying migmatitic deep lineaments may not have an origin as crust. An example of how this may occur is ancient as those of eastern Africa, but the suggested by the crustal stratification below rejuvenation of Precambrian mobile belts has the Rhinegraben. The mafic pillow certainly been proposed in connection with the develop- introduces heat, and the presence of the sialic ment of the Atlantic by Sutton (1968) and low velocity channel is interpreted by Hanel McConnell (1969b), and for the Red Sea by (1970) and Mueller (1970, p. 30) as indicating Schurmann (1966). Although rift lineaments the presence of molten or plastic granitic frequently follow the grain of Precambrian fold material between the depths of 10 to 20 km. belts, they are oblique in some cases, and frac- Since the boundary faults of the Rhinegraben ture systems of this length cannot be simply can only be followed by seismic reflection to a due to pre-existing structures in the basement, depth of some 7 km (lilies, 1970, Fig. 5), it as has been suggested by Le Bas (1971), but appears possible that the graben floor has sub- must have formed in response to a definite sided into the less dense material of a molten stress pattern. or very plastic zone. The existence of two cor- The existence of lengthy faults in the earth's responding low-velocity zones beneath the crust is well established, and these appear to be Baikal rift zone is relevant (Lubimova and dominantly transcurrent. These fault zones are Feldmann, 1970, Fig. 19). generally narrow and may be simple shears; the Bailey (1970) has pointed out that the up- writer suggests that when such dislocations are ward movement of water vapor and other forced to cross the more strongly cratonized volatiles will lower the temperature of crustal areas of the crust, they meet with such resist- melting, and that subrift lineaments are likely ance that they form wide zones of shearing and to be zones of mantle degassing with the forma- fracturing manifested at deep crustal levels by tion of volatile-rich residual-type magmas metamorphism, migmatization, and granitiza- characteristic of the rift system. Harris (1970) tion, passing at slightly shallower depth to has also shown in some detail how the East flaser gneiss and blastomylonites, and near the African rift volcanism could be due to an up- surface to cataclasites and rnylonites. These welling of mantle material along narrow dyke- are the belts of Precambrian tectonothermal like channels, the heat source deriving from activity or orogenies mapped, for instance, in radioactive decay in the mantle and comparable Ubende, the Kitui area in Kenya and elsewhere, to penetrative convection (Elder, 1966). The which have subsequently been arched with rising of isolated lithothermal systems in- consequent vertical and normal rift faulting. volving both heat and mass transfer is also As pointed out above, all the intracontinental postulated by Cass (1970, p. 295). rift valleys sensu stricto, as opposed to grabens of different sizes and origins (see Appendix), CONCLUSIONS occur in ancient cratonized platforms and have The rift system of eastern Africa includes a a remarkably uniform width. The crustal complicated pattern of lineaments extending relations of a typical rift valley are illustrated south-southwest for 5,500 km from the Red Sea

to the Orange River, and attaining a width of near-vertical where exhumed by erosion. It is 1,000 km. It is entirely within the Precambrian suggested that rift valleys form when strips of continental platform of Africa and consists of cold continental crust sink into the substratum (a) an active portion north of the Zambezi as its density is lowered by revived heat flow. River, the classical East African Rift System; Intracontinental rift-valley fault systems and (b) a southern portion of eroded Karroo form where major perennial fault lineaments troughs partly reactivated, and a chain of cross ancient resistant cratons, with the con- ancient mafic intrusive masses including the sequent formation of sheared and fractured Great Dyke of Rhodesia and the Bushveld zones of a fairly constant width (30 to 50 km), and Trompsburg Complexes penetrating excep- probably related to the thickness and rigidity of tionally ancient shields. The evidence indicates the crust. that the system originated in the early Precam- The East African rift lineament is indepen- brian and that the Neogene rift fractures are dent of the Red Sea and Gulf of Aden line- genetically associated with the ancient lineaments, but the nature of the underlying litho- aments. sphere appears to resemble that beneath the Study of the deeper crustal levels, where mid-ocean ridges, although this intracon- exposed by erosion, shows that the rift system tinental rift is not subject to comparable lineaments are relatively narrow mobile zones dilation because of the pressure on the African of tectonothermal activity manifested by dis- Plate from the spreading Mid-Atlantic and location, metamorphism, migmatization, and Mid-Indian Ocean ridges. The rift valleys granitization dating back, in part at least, to a found on the mid-ocean ridges form in the late Archean cycle (2,700 to 2,300 m.y. ago), new oceanic crust resulting from sea-floor and have been reactivated during successive spreading and so differ in origin from those in erogenic periods up to the present. They are continental crust. regarded as zones of high heat flow from the The well-known rift volcanism of East mantle with penetrative intrusion from the Africa lies north of 4° S. and is exceptional in low-velocity zone. The lineaments in part this rift system. It may indicate a very slight delimit shields and cratonic blocks older than distension in Ethiopia and northern Kenya due 3,000 m.y., and in part follow orogenic belts, to the Cenozoic opening of the Red Sea and through differing from them in behavior, and Gulf of Aden, but the products are character- locally cutting across the regional grain. It is istically peralkaline in contrast to the general suggested that they represent zones of fund- basaltic and tholeiitic nature of normal mid- amental dislocations in the lithosphere which ocean volcanism. In the remainder of the are subject to frequent rejuvenation and hence African rift system volcanism only occurs at the term perennial deep lineament is proposed nodes where rift strikes intersect or change to designate them. Evidence in Tanzania indi- direction. cates that the lineaments there originated with transcurrent dextral dislocation around an ACKNOWLEDGMENTS ancient Tanganyika Shield. Early movement This paper draws on the work of many other patterns were progressively replaced by vertical geologist. Essential references have been given, displacement due to isostatic adjustments. and others, omitted for space requirements, The arched uplifts of the present-day rift will be found in the papers listed. Special thanks system form along the infracrustal lineaments for helpful discussion and unpublished informa- because these are zones of expansion due to tion are due in particular to J. Lepersonne, and periodic revivals of heat flow, migmatization to J. V. Hepworth, J. H. lilies, J. B. Kennerley. and melting, and hence are zones of low density M. A. Khan, M. J. Le Bas, R. Macdonald, W. and consequent isostatic uplift. Rift valleys I. Manton, J. W. Pallister, R. M. Shackleton, and block faulting, reflecting supracrustal W.T.C. Sowerbutts, and P.W.G. Tanner. mechanics, follow the crest of the arches; Unpublished information on the Ruwenzori distension is limited to a few kilometers and Mountains was kindly provided by the displacement is predominantly vertical. Most management of Kilembe Mines Ltd. through of the recent superficial faults are steeply their consulting geologist G. R. Davis and by normal and antithetic, but faults formed R. P. Freeman. T. N. Clifford and J. V. Watson deeper in the crust are seen to be vertical or helped with the manuscript, and much

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improvement has also followed detailed com- intrusion: Geol. Jour. Spec. Issue no. 2, p. ment made by G. D. Robinson. 177-186. Baker, B. H., 1965, An outline of the geology of the APPENDIX. MEANING OF SELECTED Kenya Rift Valley: Nairobi, Rept. UMC/ TERMS AS ADOPTED IN THIS PAPER UNESCO seminar on the East African Rift System, p. 1-19. Assemblage: Includes groups of metamorphic 1970, The structural pattern of the Afro- rocks in Precambrian orogenic belts which may be Arabian rift system in relation to plate of widely differing ages and difficult to disentangle. tectonics: Royal Soc. London Philos. Trans., Block: A portion of platform appearing rigid in V.A267, p. 383-391. relation to adjacent mobile belts. Differs from a Baker, B. H., and Wohlenberg, J., 1971, Structure shield by containing syneclises of late Proterozoic and evolution of the Kenya Rift Valley: Na- or Phanerozoic rocks (for example, Somalia Block). ture, v. 229, p. 538-542. Craton (H. Stille): Resistant portion of platform Baker, B. H., Mohr, P., and Williams, L.A.J., in which mobile belts have become stabilized (for 1972, Geology of the eastern rift system of example, Kalahari Craton). Roughly equivalent Africa: Geol. Soc. America Spec. Paper 136. to block.. Barrett, W. L., 1969, The stratigraphy and struc- Lineament (W. H. Hobbs): A significant line ture of the southeastern part of the Ruwenzori revealing the hidden architecture of the lithosphere. Mountains: Rept. Research Inst. African Geol. Platform: Large area which has been stable since Univ. Leeds, v. 13, p. 10-13. the early Paleozoic, but may carry dominantly Belitchenko, V. G., and Khrenov, P. N., 1969, A horizontal cover. brief geological essay on Pribaikalia, in Rift valley: Introduced by J. W. Gregory to Pavlovsky, E. V., ed., Geology of Pribaikalia: designate the downfaulted valleys of Africa and Irkutsk, Guidebook 12th AZOPRO Session, Sci. the Near East. It is suggested here that the term Council of the Earth's Crust Inst., p. 5-38. be reserved for the major intracontinental and Beloussov, V. V., 1966, Modern concepts of the mid-ocean features because the term graben (E. structure and development of the Earth's Suess) was used by German miners to designate crust and the upper mantle of continents: Geol. downthrown blocks of any size, dependent on local Soc. London Quart. Jour., v. 122, p. 293-314. supracrustal stresses, and thus covers all down- Bishop, W. W., 1965, Quaternary geology and faulted troughs. geomorphology in the Albertine Rift Valley, Shield (E. Suess): A craton of early Precambrian Uganda, in Wright, H. E., Jr., and Frey, D. G., rocks outcropping as a broad topographic dome eds., International studies on the Quaternary: flanked by younger tabular cover or fold belts. Geol. Soc. America Spec. Paper 84, p. 293-321. REFERENCES CITED Bishop, W. W., and Trendall, A. F., 1967, Erosion- surfaces, tectonics and volcanic activity in Ackermann, E., and Forster, A., 1960, Grundziige Uganda: Geol. Soc. London Quart. Jour., v. der Stratigraphie und Struktur des Irumiden- 122, p. 385-420. Orogens: Internal. Geol. Cong., 21st, Copen- Bloomfield, K., 1968, The pre-Karroo geology of hagen 1960, Pt. 18, p. 182-192. Malawi: Geol. Survey Malawi Mem. 5, 166 p. Allsopp, H. L., 1965, Rb:Sr and K:Ar age measure- Borchers, R., 1961, Exploration of the Witwaters- ments on the Great Dyke of Southern rand System: Geol. Soc. South Africa Trans, Rhodesia: Jour. Geophys. Research, v. 70, p. and Proc., v. 64, p. Ixvii-xcviii. 977-984. Brock, B. B., 1955, Some observations on vertical Anhaeusser, C. R., Mason, R., Viljoen, M. J., and tectonics in Africa: Am. Geophys. Union Viljoen, R. P., 1969, A reappraisal of some Trans., v. 36, p. 1044-1054. aspects of Precambrian shield geology: Geol. Brown, P. E., 1962, The tectonic and metamorphic Soc. America Bull., v. 80, p. 2175-2200. history of the Mbeya region, southwest Bagnall, P. S., 1962, North Pare, quarter degree Tanganyika: Geol. Soc. London Quart. Jour., sheet 73: Tanganyika Geol. Survey Map. v. 118, p. 295-314. Bagnall, P. S., Dundas, D., and Hartley, E. W., 1964, The Songwe scarp carbonatite and asso- 1963, Lushoto, quarter degree sheet 109: ciated feldspathization: Geol. Soc. London Tanganyika Geol. Survey Map. Quart. Jour., v. 120, p. 223-240. Bailey, A. I., 1969, Preliminary report on the Bullard, E. C., 1936, Gravity measurements in geology of the Watamagufu-Bugoye area of East Africa: Royal Soc. London Philos. Trans., the Ruwenzori Mountains: Rept. Research v. A235, p. 445-531. Inst. African Geol. Univ. Leeds, v. 13, p. 13- Cahen, L., 1954, Geologic du Congo Beige: Liege, 14. Vaillant-Carmanne, 577 p. Bailey, D. K., 1970, Volatile flux, heat-focussing 1970, Igneous activity and mineralisation and the generation of magma, in Newall, G., episodes in the evolution of the Kibaride and Rast, N., eds., Mechanism of igneous and Katangide orogenic belts of Central

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