Rifting of Africa and pattern of mantle convection beneath the African plate

Tectonophysics, 215 (1992) 35-53 Elsevier Science Publishers B.V., Amsterdam

35

Rifting of Africa and pattern of mantle convection beneath the African plate N. Pavoni Institut für Geophysik, ETH Hönggerberg, CH-8093 Zürich, Switzerland (Received October 4, 1991; revised version accepted December 20,1991)

ABSTRACT Pavoni, N., 1992. Rifting of Africa and pattern of mantle convection beneath the African plate. In: P.A. Ziegler (Editor), Geodynamics of Rifting, Volume III. Thematic Discussions. Tectonophysics,215: 35-53. The Mesozoic and Cenozoic tectonic setting of the African continent is characterized by an extensional stress regime, as is evident ffom the evolution of the West, Central and East African rift systems, the opening of the Indian and Atlantic oceans and the development of Tethys domain, as well as from the results of geophysical investigations. The most prominent feature of the Bouguer gravity anomaly map of Africa is a great belt of negative Bouguer anomalies which extends across the entire African continent from Ethiopia in the East to Benguela in the West. In conjunction with the Red Sea spreading zone this belt appears to encircle a "centre" in Cameroon. It probably corresponds to a distinct zone of lithospheric thinning and extension. The entire African continent is characterized by higher-than-normal elevations. It is located inside the African residual geoid high. Earthquake fault-plane solutions confirm the extensional character of the present tectonic regime of eastern and southern Africa where normal faulting predominates. The average orientation of T axes is N - S in South Africa, NW-SE in Zambia, Mozambique and Madagascar, E - W in Kenya and Ethiopia, and NE-SW in the Red Sea region. From north to south, the orientation of T axes rotates systematically in a clockwise sense by 130° and describes a radial pattern centred on equatorial West Africa. This indicates that the African lithosphere has been subject to continent-wide radial extension around a hypothetical West African "spreading centre", referred to as the African Spreading Centre A (ASC-A), which coincides with the geographical centre of the African plate. Shear traction exerted by the convecting mantle on the base of the lithosphere is considered to be a major driving force of plate movements. In the case of Africa, the continent-wide regularities in the pattern of lithospheric extension, the elevated position of the continent, as well as the evolution of the mid-oceanic ridge system surrounding Africa, provide distinct boundary conditions for the pattern of large-scale mantle flow beneath the African plate. It is proposed that relatively warm and less dense mantle material rises below Africa, forming a single mega-plume or a number of plumes. At the base of the African lithosphere, the ascending material diverges and flows radially away from the centre of ascent, corresponding to the ASC-A. This ascending flow forms part of a very large mantle convection cell which occupies the Eurafrican hemisphere. The descending branch of this convection cell is located at a distance of 80-90° from the ASC-A. The mid-oceanic ridges surrounding the African continent are located between the upwelling and the downwelling branches of this convection cell, at a distance of 50-60° from the ASC-A. Upper mantle flow below the sea-floor spreading axes is uni-directional and horizontal and is directed away from the ASC-A.

Introduction The geotectonic setting of the African continent is in several aspects unique. Since Triassic times an extensional regime has governed the

Correspondence to: N. Pavoni, Institut für Geophysik, ETHZ Hönggerberg, CH-8093 Zürich, Switzerland.

tectonic evolution of most of this continent; an exception is the area of the northwest African Atlas Mountains, which is not addressed in the present study. The long history of African crustal extension is documented by the widespread occurrence of volcanism and intraplate rifting, diverging plate movements and its present seismicity. Africa and the adjacent parts of the North Atlantic and Indian Oceans display higher-than-

0040-1951/92/$05.00 © 1992 - Elsevier Science Publishers B.V. i.V. All rights reserved

36

N. PAVONI

normal elevations (Cazenave et al., 1989; Anderson, 1989). Africa is located in the central part of a large geoid high, referred to as the African geoid high. Africa is surrounded in the West, South and East by active sea-floor spreading ridges. This arrangement of mid-oceanic ridges, together with the concentric growth pattern of the African plate and the present stress state of the continent set special boundary conditions for the relation between sea-floor spreading and mantle flow patterns. In a global tectonic framework, the African plate represents the counterpart of the Pacific plate (Pavoni, 1981; 1991). Pattern of tension axis orientations The present tectonic activity in East Africa is well documented by a pronounced belt of seismic-

ity which extends from the Afar region in Northern Ethiopia to the extreme south of the African continent (e.g., Fairhead and Girdler, 1971, 1972; Brown and Girdler, 1980; Fairhead and Stuart, 1982; Shudofsky, 1985; Kebede 1989; Simkin et al., 1989). A most comprehensive, systematic investigation of fault-plane solutions of intermediate-sized earthquakes in eastern and southern Africa, using Rayleigh-wave inversion and body wave modelling techniques, has been performed by Shudofsky (1985). A catalogue of some 140 East African events with magnitudes rab > 4.7 was compiled for the period January 1964 to June 1978. From this catalogue 23 earthquakes, which produced good signals at a suitable number of WWSSN stations, were selected by Shudofsky for an analysis of their source mechanism. Each solution is represented in detail with all the observa-

TABLE 1 List of T axes of fault-plane solutions, represented in Figs, la and b Event number and date

Latitude (°)

Longitude (Έ)

Body wave magnitude

Focal depth (km)

Azimuth T axis

Ref.

a 1972 Jun 28 b 1969 Mar 31 c 1967 Mar 13 1 1971 Nov 13 2 1977Jul.08 3 1964 May 07 4 1977 Dec 15 5 1972 Feb 13 6 1967 Oct 14 7 1966 Mar 20 8 1966 May 17 9 1966 Mar 21 10 1974 Apr 25 11 1966 Oct 05 12 1977 Jul 06 13 1975 Mar 26 14 1976 Sep 19 15 1966 May 06 16 1968 May 15 17 1968 Dec 02 18 1972 Dec 18 19 1975 Apr 04 20 1975 Feb 15 21 1976 Jul 01 22 1969 Sep 29 23 1970 Apr 14

27.70N 27.62N 19.79N 11.03N 11.08N 03.90S 04.80S 04.50S 03.32S 00.81N 00.76N 00.80N 01.UN 00.02N 06.62S 05.34S 11.08S 15.72S 15.91S 14.01S 16.71S 21.24S 16.47S 29.51S 33.09S 33.17S

33.80E 33.91E 38.82E 39.71E 39.62E 34.92E 34.92E 34.14E 38.19E 29.90E 29.95E 29.61E 30.05E 29.94E 29.59E 30.13E 32.84E 34.59E 26.16E 23.82E 28.07E 45.13E 41.45E 25.17E 19.52E 19.47E

5.5 6.1 5.6 5.1 5.0 6.2 5.2 5.0 5.1 6.0 5.5 5.3 4.9 5.3 5.2 5.0 5.7 5.3 5.7 5.9 5.3 5.3 5.2 5.9 5.6 5.4

6 6 2 14 19 28 12 6 10 29 7 7 10 23 14 16 25 17 27 13 7 11 12 6 30 10

27.0 53.0 47.0 84.0 70.5 156.0 101.0 51.0 84.5 134.0 122.0 134.0 96.0 80.5 65.0 105.5 108.5 136.0 118.0 110.5 123.0 113.0 122.0 176.0 176.0 192.0

4 1,4 4 3 3 2,3 3 3 3 3 2,3 3 3 3 3 3 3 3 3 2,3 3 3 3 3 2,3 3

References: 1 = McKenzie et al., 1970; 2 = Fairhead and Girdler, 1972; 3 = Shudofsky, 1985; 4 = Huang and Solomon, 1987.

RIFTING AND MANTLE CONVECTION BENEATH THE AFRICAN PLATE

tional data. Focal depths are estimated from the depth of minimum residuals of Rayleigh-wave moment tensor inversions. Shudofsky groups the 23 events into eight regions. Each region is represented in a separate figure showing the epicentral location, focal depth and the fault-plane solution, as well as a simplified structural map. This allows an interpretation of the focal mechanisms in terms of the local structural setting. The wide distribution of epicentres, the pattern of focal depths, the types of focal mechanisms and the regularities in the orientations of tension axes suggest that a large-scale tectonic process is involved in the continental rifting of East Africa. Fault-plane solutions of intermediate-sized earthquakes in East Africa are mostly of the normal faulting type. The trend and type of neotectonic faulting at the surface and the type of focal mechanism of earthquakes is consistent, for instance, along the fault system on the bordering between the Afar region and the Ethiopian plateau to the west, in the southern Kenya rift, in the Ruwenzori region, in the Lake Tanganyika area, and in the Luangwa and Lake Malawi rifts (Shudofsky, 1985). This indicates that the tectonic state of stress giving rise to the current seismicity, is very similar to the stress state responsible for the initiation of rifting. It is of interest to note that the source mechanisms of earthquakes in regions that have not experienced recent rift faulting, or where neotectonic faulting is not yet known, also reveal orientations of tensional axes that are similar to those in neighbouring rifted regions. Evidently, the extensional stress state is also operative in these "unrifted" regions. In order to arrive at a statement regarding the large-scale pattern of the present crustal stress field in East Africa, an investigation of the orientation of the tension axes (T axes) is of primary interest. In the present study the orientations of T axes of all the fault-plane solutions published by Shudofsky (1985), as well as the orientations of T axes of three additional fault-plane solutions of earthquakes caused by normal faulting, located in the axial zone of the Red Sea rift, are considered. They are listed in Table 1 and shown in Fig. 1. The events 1-23 are numbered according to the listing of Shudofsky; a, b and c represent the

37

events in the Red Sea region. The generalized orientations of tension axes are N-S in South Africa, NW-SE in Zambia, Mozambique and Madagascar, E-W in Kenya and Ethiopia and NE-SW in the Red Sea region. A regular pattern of the present large-scale tectonic stress field of East Africa is, thus, clearly reflected by the orientation of the T axes. This stress field is not restricted to the continent but is also evident in the oceanic lithosphere of the Mozambique Channel (event 20) and in Madagascar (event 19). This suggests that a large-scale tectonic process, which reaches far beyond the continent of Africa, is responsible for the present stress state of East Africa. Several attempts have been made to describe the present stress field in and around East Africa and to subdivide it, according to its orientation, into rigid block movements. In this way the NESW tensional stress field in the Red Sea region, which underlies the opening of the Red Sea rift, is considered to be distinctly separate from the E-W to ESE-WNW oriented tensional stress field which characterizes East Africa (e.g., Fairhead and Girdler, 1971; Kebede, 1989). The stress field of southernmost Africa, characterized by N-S oriented T axes, is regarded by Shudofsky (1985) as differing completely from the stress regime of East Africa. In the following a new interpretation of the geometry of the stress field of East Africa is presented, whereby it shall be demonstrated that the tensional stresses in East Africa, as shown by the orientations of T axes in Fig. la, display a large-scale radial pattern. The aim is to define the location of the centre A T of this radial pattern which best fits the given orientations of the available T axes. In determining the location of this centre a graphical pattern recognition procedure was adopted. A polar coordinate system, drawn on a transparent sheet, was laid over the map giving the T axes in eastern and southern Africa and in the Red Sea region (Fig. la). This overlay was then carefully moved into a position that best fitted the pattern of the T axes, and the origin of the coordinates located on the map. A series of repeated tests placed the origin of the well fitting polar coordinate systems into an area

38

N. PAVONI

Fig. 1. (a) Distribution and orientation of T axes of fault-plane solutions of intermediate-sized earthquakes in South and East Africa and Madagascar. Numbering after Shudofsky (1985). See Table 1. (b) Radial pattern, best fitting the T axis orientations. AT is the centre of the polar coordinate system, and is located at 10Έ/Ο°Ν.

in western equatorial Africa that covers about 12 X 12° and that is centred near 10Έ/0°Ν. This location has been chosen as the position of the centre A T (Fig. lb). It can be shown that a radial system centred at A T fits the pattern of T axes much better, i.e., with a much smaller standard deviation, than any "parallel" system. Even for a selected group of T axes (events 1-20) it can be shown that the standard deviation of the fit with the radial system centred at A T is smaller than the standard deviation of the fit with any parallel system, proposed on rigid block movements. From south to north the orientations of tension axes display a gradual counter-clockwise rotation of 130° (Fig. lb).

Gravity anomalies and crustal thinning The Bouguer gravity anomaly map of Africa displays a series of large-scale anomalies. The most prominent structure is the great arcuate belt of negative Bouguer anomalies which extends across the entire African continent from Ethiopia in the East to Benguela in the West. Girdler (1975) and Brown and Girdler (1980) discuss and present a quantitative interpretation of the "great negative Bouguer anomaly". They present good reasons for assuming that this gravity anomaly belt marks a zone of lithospheric thinning and extension, and that massive replacement of the lithosphere by slightly less dense asthenosphere

39

RIFTING A N D M A N T L E C O N V E C T I O N B E N E A T H T H E A F R I C A N P L A T E

1

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occurs beneath this belt which also covers a major part of the East African rift system. This interpretation accords with observed P wave delays, which are related to the belt of negative Bouguer gravity anomalies (Fairhead and Girdler, 1972), with low S and P wave velocities beneath the western and eastern branches of the East African rift system (Nolet and Mueller, 1982) and low Q values (higher attenuation of seismic waves) beneath the Gregory rift in Kenya (Halderman and Davis, 1991). From the Afar region in northeastern Africa, the axis of maximum lithospheric thinning, as derived from Bouguer gravity anomalies, trends southwards along the eastern branch of the East African rift, crosses the western branch between Lake Tanganyika and Lake Malawi, bends gradu-

ally towards the west in northern Zambia, extends further west along the broad watershed between the Congo and Sambesi/Okawango river systems, and reaches the Atlantic coast south of Benguela (Fig. 2). To the North of the Afar depression (Mohr, 1970, 1987) the Red Sea region forms the direct continuation of this belt. From a geotectonic point of view, a connection of the great negative Bouguer anomaly belt with the Red Sea region seems reasonable as both represent well-defined zones of lithospheric extension. Together they make up a major, arcuate zone of lithospheric extension and thinning, which extends from the Gulf of Suez to the Atlantic coast of southern Angola and encircles a hypothetical centre A G that is located in northern Cameroon at about 8°N/12°E (Fig. 2).

40

N. PAVONI

20°W

C0°H\

50°E

30°

20°

10°

0°

20°

30°

40°SL·

MERCATOR PROJECTION Fig. 2. Lithospheric thinning beneath Africa, derived from Bouguer gravity anomalies (after Brown and Girdler 1980). The axis of maximum thinning has been extended into the spreading axis of the Red Sea (thick dashes). Lithospheric extension to produce the thinning is expected to be oriented at right angles to the axis of maximum thinning. The extension displays a radial pattern centred on An.

A direct connection between the Red Sea region and the East African rift system is also demonstrated by the pattern of post-Cenomanian uplift (Sahagian 1988, his fig. 4) reaching 3 km. A southwestern extension of the uplift seems to trend across Southern Africa toward the Walvis Ridge. It follows the watershed between the Congo and Sambesi/Okawango river systems, forming a saddle as it passes through western Zambia and eastern Angola, and rises to almost 3 km in western Angola and Namibia (Sahagian, 1988). Deviatoric tensional stresses, causing lithospheric extension and thinning, are likely to be

oriented at right angles to the axis of the great negative Bouguer anomaly. From Angola in the west to the Gulf of Suez in the northeast such gravity-derived stress orientations display a radial pattern, centred at A G (Fig. 2). This pattern can be readily compared with the radial pattern derived from the orientation of T axes of fault-plane solutions (Fig. lb). Origin, growth and shape of the African plate The African plate came into existence during the Mesozoic as a consequence of the MidJurassic break-up of Pangea and the Late Juras-

RIFTING AND MANTLE CONVECTION BENEATH THE AFRICAN PLATE

sic-Early Cretaceous disintegration of Gondwana (Binks and Fairhead, 1992; Ziegler 1990, 1992, this volume). The opening of the Atlantic and Indian oceans was accompanied by the closure of the Mesozoic Tethys Ocean. The study of these mega-tectonic interrelations and the increasing amount of geological and geophysical data allowed a rather detailed reconstruction of the evolution of the African plate and of the different stages of the Pangea and Gondwana break-up. Undoubtedly, the Late Mesozoic and Cenozoic plate reorganizations, which underlay the disintegration of these mega-continents, are manifestations of very large-scale changes in the mantle convection systems. It is not the aim of this paper to describe the details of the Gondwana break-up history. A concise account and further references are given by Ziegler (1990). The main interest of this work is to focus on the pattern of motions of North and South America, Antarctica, India and Australia relative to the African continent, and on the growth of the African plate, in order to arrive at an understanding of the large-scale geometry and kinematics of the Gondwana break-up and at some statements about the possibly underlying geodynamic processes. During the Late Mesozoic opening of the Atlantic and Indian oceans, the North American plate moved towards the northwest relative to the African plate, whereas the South American plate moved towards the west, the Antarctic plate towards the south, the Australian plate generally towards the east and the Indian plate towards the northeast. Figure 3 shows an early stage of seafloor spreading at about 120 Ma, just prior to the separation of South America from Africa and illustrates the divergent pattern of deviatoric tension in Africa, as indicated by the continent-wide network of graben structures (Guiraud and Maurin, 1992). Divergent movements of the lithosphere are evident by crustal separation between Africa, North America and East Gondwana, followed by early opening of the Central Atlantic and Indian oceans. An intimate interaction between sea-floor spreading and intra-continental rifting is evident (Fairhead and Binks, 1991; Binks and Fairhead, 1992; Guiraud and Maurin, 1992). In a continent, activity along fault and fracture

41

Fig. 3. Intracontinental rifting in Africa at about 120 Ma ago, prior to the separation of South America from Africa. The network of rifts within Africa (Fairhead and Binks, 1991) and the drift of neighbouring continents away from Africa indicate a large-scale divergence of lithospheric plates. A marks the centre of lithospheric divergence.

systems in response to the build-up of a regional, deviatoric tensional stress regime apparently occurs in steps. The cumulative strain achieved during a certain time span during successive steps has to be considered in its entirety in a statement about the stress state of the affected area. In this respect, the observed deformation of the northern and central parts of the African plate, involving NNE-SSW extension during the Barremian and ENE-WSW extension during the Albian (Guiraud and Maurin, 1992), may not necessarily document a rapid change in the orientation of the continent-wide stress field but may actually reflect a sequential reaction of the lithosphere to one and the same long-lived, large-scale, divergent deviatoric stress regime. The oceanic crust and lithosphere formed during the opening of the Atlantic and Indian oceans not only provide a continuous record of the opening through time, but also preserve traces of the underlying lithospheric plate movements. These traces are visible in the trend of fracture zones offsetting the ridge axes. These fracture zones can be regarded as "flow lines" tracing the move-

42

merits of separating plates. The most complete maps of fracture zones, seamounts and other physiographic features of the Atlantic ocean are provided by the analyses of the gravity field derived from SEASAT altimeter data (Haxby, 1987; Fairhead and Binks, 1990). The pattern of flow lines around the African continent shows a divergence of trends which can be taken as evidence for the divergence of plates away from Africa. Large areas characterized by more or less parallel trending flow lines, such as the floor of the Central Atlantic Ocean (WNWESE trending flow lines) and the floor of the South Atlantic Ocean (WSW-ENE trending flow lines), are separated by triangular zones which are characterized by diverging flow lines; these correspond to the Equatorial and southernmost Atlantic (Fig. 4, dotted areas referred to as "insertions"). The diverging sets of flow lines of the

N. PA VON 1

oceanic lithosphere off Africa, as well as the intra-continental tectonics in Africa, show that the North and South American and East Antarctica plates moved away from Africa. They do not support the notion of symmetrical opening of the Atlantic Ocean with respect to the axis of the mid-Atlantic ridge, which would have involved opposite movements of the adjacent plates relative to the ridge axis. During the opening of the Atlantic Ocean, the axis of the active spreading ridge moved away from Africa at half the rate of the sea-floor spreading (Pavoni, 1969). Divergence of North and South America and East Antarctica from Africa was coupled with progressive separation of these continents from each other and the growth of the respective plates, as well as of the African plate, which apparently remained more or less stationary. In the process of this, the triangular "insertion" areas opening

Fig. 4. Diverging drift of North America, South America and East Antarctica away from Africa (large arrows). Base map after Gahagan et al. (1988). Dotted areas: lithospheric insertions; 1 = Equatorial Atlantic lithospheric insertion; 2 = South Atlantic/Antarctic lithospheric insertion.

43

RIFTING AND MANTLE CONVECTION BENEATH THE AFRICAN PLATE

between the diverging plates (Fig. 4) were filled with newly formed oceanic lithosphere and/or accreted crust and lithosphere, as for instance in Central America, the Scotia arc and WestAntarctica. Figure 5 illustrates the growth of the African plate during the breakup of Pangea and Gondwana; the age of the newly formed oceanic lithosphere is indicated by shading (after Scotese et al., 1988). The isochrons and growth stripes of the oceanic lithosphere surround the African continent in a regular concentric pattern, and display a clear age sequence from older to younger with growing distance from the continent. The growth pattern of the oceanic lithosphere of the African plate resembles the growth rings of a tree-trunk; the growth is directed outwards, away from the continent. The growth gradients from older to

>143.8

118.7-143.8

younger ages, as derived from magnetic sea-floor anomalies, point away from the African continent. They display a clear radial pattern around most of the continent; the only exception being the Mediterranean region. From the Canary basin in the Central Atlantic Ocean to the Somalia basin in the Indian Ocean, the growth directions rotate 240° in a counter-clockwise sense. The roughly circular shape of the African plate is complicated by a major, rectangular "kink" in the trend of the mid-oceanic ridges in the area of the Equatorial Atlantic Ocean. This "kink" is inherited from the configuration of the original break-up axis between Africa and South America (Fig. 3). During the opening of the Atlantic and Indian oceans, the size of the African plate increased, whereby its shape became progressively more circular. Due to its almost circular shape,

84.0-118.7

66.2-84.0

6 6 . 2 - 0 . 0 Ma

Fig. 5. Growth of the African plate (after Scotese et al., 1988). Cylindrical, equidistant projection. Age of oceanic lithosphere is indicated by shading. Note concentric pattern of successive lithospheric growth rings around Africa, whereby growth of the plate is directed away from Africa.

44

N. PAVONI

'Λ',ΊΊ'Λ'Λ'ΛΊ' ' '_;

ί '

> 143.8

• ijl 1 1 1 1 l'l 1 1 1 1

•riVi'ri'iVrrri'

118.7-143.8

84.0-118.7

66.2-84.0

66.2-0.0 Ma

Fig. 6. (a) African plate and sub-lithospheric mantle flow (lines) according to the proposed geotectonic model (Pavoni, 1969, 1981). Cylindrical, equidistant projection. Length of flow vectors is proportional to the horizontal component of sub-lithospheric mantle flow, (b) Azimuthal anisotropy in the upper mantle mapped by long period Rayleigh waves (after Montagner and Tanimoto, 1990). The lines indicate the fast phase velocity orientation, which most likely corresponds to the flow direction (±180°). The length of the lines is proportional to the anisotropy.

the geographical "centre" of the African plate can be determined. Based on the distribution of earthquake epicentres associated with the Carlsberg Ridge, the southwestern branch of the Indian Ocean ridge, and the mid-oceanic ridge of the South and Central Atlantic oceans, the geographical centre of the African plate A P is placed at 10°E/1°N (see also Pavoni, 1981, 1985a,b). A different approach to determining the African plate centre was taken by H. Drewes (pers. commun., 1989); based on the distribution of 387 points located along the African plate boundary the centre A P was located at 7°E/4°N. Although there are minor irregularities in the distribution of points and in spite of the fact that the exact

location of the plate boundary in the Alpine/Mediterranean region is still a matter of debate, additional corrections or adjustments are unlikely to change the position given for the centre A P by more than a few degrees. Thus, the African plate centre A P is located very close to the centre A T at 10Έ/0°Ν, derived from the pattern of T axes orientations of fault-plane solutions, and the centre A G at 12°E/8°N, derived from the extent and shape of the great negative Bouguer anomaly of Africa (compare Figs, lb, 2 and 6a). Within the accuracy of determination, which is of the order of a few degrees, the centres A P , A T and A G coincide closely with each other. In combination they define the African Centre A,

45

RIFTING AND MANTLE CONVECTION BENEATH THE AFRICAN PLATE

Fig. 6. (continued).

which is located at approximately 10°E/0°N (Figs. 3, 6 and 7). Geotectonic model Evolution and the present tectonic stress state of the African plate provide distinct boundary conditions for plate driving mechanisms. The large-scale geometrical regularities observed in the pattern of the present stress field, the arcuate shape of area of Cenozoic lithospheric thinning in Africa, the dispersal pattern of continents during the breakup of Pangea and Gondwana and the ensuing growth of the African plate can be interpreted as manifestations of one and the same large-scale mantle process which acted on the base of the African plate. This geotectonic phenomenon must be very large and have plate-wide dimensions. It should display a distinctly radial pattern, centred on the African Centre A, located at 10Έ/0°Ν. More-

over, this process must have been operating since the Early Mesozoic onset of the Pangea and Gondwana break-up. A geotectonic model explaining the evolution and present tectonic stress state of the African plate has to take into consideration these three boundary conditions. Two important assumptions are made, namely that: (1) generation, tectonic transport, and subduction of the oceanic lithosphere are manifestations of thermal convection in the Earth's mantle; (2) shear traction exerted by the convecting mantle on the base of the lithosphere is a major plate driving force. With these boundary conditions and assumptions the following geotectonic model has been proposed (Pavoni, 1969, 1981, 1985a). Beneath Africa relatively warm and less dense asthenospheric material rises in the form of a mega-plume or a cluster of plumes. The upwelling mantle material is forced to diverge at the base of the

118.7-143.8 84.0-118.7 66.2-84.0 6 6 . 2 - 0 . 0 Ma Fig. 7. Simultaneous growth of the African and Pacific plates during the last 180 Ma (after Scotese et al., 1988). Cylindrical, equid istant projection. A = African pole at 10°E/0°N. P = Pacific pole at 170°W/0°N.

>143.8

RIFTING AND MANTLE CONVECTION BENEATH THE AFRICAN PLATE

African lithosphere and flows laterally away from the centre of ascent, or in other words, radially away from the African upwelling centre A. The cylindrically upwelling mantle flow forms part of a very large, torus-like mantle convection cell in the Eurafrican hemisphere that reaches far beyond the African plate. The downwelling branches of this convection system are located at a distance of 70-90° from the African Centre A. The oceanic ridges surrounding the African continent are located in a position between the upwelling and downwelling branches, at 50-60° distance from the African Centre A. Below the sea-floor spreading axes, upper mantle flow is unidirectional, horizontal and directed away from the African Centre A (Fig. 8). Horizontal divergence of the upwelling mantle flow beneath the African lithosphere explains the observed large-scale divergent movement of lithospheric plates away from the African plate after the break-up of Pangea and Gondwana. It also explains continued, divergent, intraplate rifting of the African continent during the Cretaceous and Cenozoic, as is evidenced by a network of interre-

47

lated graben structures and persistent magmatic activity (Wilson and Guiraud, 1992). Moreover, it explains the continent-wide radial pattern of present deviatoric tensional stresses. All these phenomena are manifestations of what may be referred to as the African lithospheric divergence (ALD). The degree of the coupling between the convecting asthenosphere and the lithosphere is an open question; per unit area the coupling may be rather weak, however, the surface of interaction is large. Viscous drag force is dependent upon the dynamic viscosity of the flow and the morphology (roughness and shape) of the surface of interaction, corresponding to the base of the lithosphere. The drag force is also proportional to the horizontal component of flow velocity relative to the lithosphere. In the direction of increasing horizontal flow velocity, namely in the direction of a positive gradient of sub-lithospheric flow, the overlying lithosphere experiences deviatoric tensional stresses; whereas in the direction of negative gradients of sub-lithospheric flow, the overlying lithosphere experiences compressive de-

Fig. 8. Schematic representation of the bipolar pattern of sub-lithospheric flow produced by axisymmetric bicellular convection in the mantle (redrawn from Pavoni, 1969). Cylindrical, equidistant map projection. Contour lines of continents are indicated. Single lines = divergent and transform boundaries; barbed lines = convergent boundaries. The poles P and A mark the two major centres of cylindrically upwelling mantle flow. They also represent the two major centres of lithospheric divergence. A north-south trending zone of lithospheric convergence and downwelling is located between the two upwelling centres.

48

viatoric stresses (Pavoni, 1969, 1981). Little is known about all these parameters, however. The observation that plate velocities are independent of plate size has been used as a principal argument against the hypothesis that shear traction exerted by the convecting asthenosphere on the base of the lithosphere plays a major role as a plate-moving mechanism (Forsyth and Uyeda, 1975; Cox and Hart, 1986; Kearey and Vine, 1990). In this context it was argued that, if mantle drag forces were indeed operative, the largest plates would be expected to show the greatest drift velocities, since they offered the largest surface onto which this force could act. However, in this argument the possibility of diverging flow beneath a large plate was not considered. The absence of knowledge of the drag forces should not be a reason to neglect the role of active shear traction and to consider solely ridge push, slab pull, trench suction and plateau uplift, balanced by passive drag, as forces acting on plates (Ziegler, 1990, 1992, this volume). In this respect, the evolution and present tectonic stress state of the African plate offer some unique insights. About 80% of the African plate margin is occupied by active sea-floor spreading ridges. If ridge push were the dominant force, one would expect the stress state of Africa to be dominated by major deviatoric compression. However, this is definitely not the case. Figure 1 shows that the African continent and neighbouring regions of the Indian Ocean are affected by

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tensional stresses. This means that the ridge push forces are overcome by some plate-wide, counteracting forces. One of the forces that can produce regional deviatoric tension is plateau uplift (Bott, 1982). Yet, the distribution of uplifted regions in Africa does not yield the observed continent-wide radial pattern of deviatoric tension. Moreover, the focal mechanisms of earthquakes located off the continent and in Madagascar cannot be explained by plateau uplift in Africa. Trench suction can only play a role along the northern margin of the African plate. Therefore, it is proposed that drag forces exerted by the upwelling and radially outflowing asthenosphere on the base of the African lithosphere, as well as unidirectional horizontal flow beneath the active sea-floor spreading ridges surrounding Africa (Figs. 6a and 9a), balance and overcome the ridge push forces, thus giving rise to the ALD. A positive gradient of the horizontal component of sub-lithospheric mantle flow is assumed to cause lithospheric extension and diverging plate drift. The horizontal mantle flow components are directed radially away from the African Centre A. The extent of asthenospheric flow is plate-wide. The velocity of the horizontal components of sub-lithospheric mantle flow reaches a broad maximum beneath and beyond the active sea-floor spreading ridges surrounding the African plate. Figure 6a shows the growth pattern of the African plate, combined with the radial pattern of sub-lithospheric mantle flow beneath the

Fig. 9. Sea-floor spreading and sub-lithospheric mantle flow beneath active mid-oceanic ridges for transverse ridges (redrawn from Pavoni, 1969). R = active sea-floor spreading ridge; adjacent hatching = newly formed oceanic lithosphere; outlined arrows = velocities of lithospheric plates A and B. (a) Diverging flow model: ascending flow, diverging beneath the ridge, (b) unidirectional flow model: unidirectional flow with positive horizontal velocity gradient. A = leading plate, B = trailing plate. The entire ridge system is gradually displaced in the direction of flow.

RIFTING AND MANTLE CONVECTION BENEATH THE AFRICAN PLATE

African plate, as suggested by the above proposed model. The fundamental mode of the mantle flow, diverging radially away from the African Centre A, is shown by vectors giving the direction of flow away from A; their length is proportional to the horizontal component of flow velocity. Mantle flow and the resultant drag forces are directed at right angles to the trend of the active sea-floor spreading ridges which encircle the African plate. Horizontal flow velocities in the asthenosphere reach their largest values near or beyond the mid-oceanic ridges. According to this model, the net torque exerted onto the African plate by the diverging sub-lithospheric flow is nearly zero. This is compatible with the fact that the African plate underwent only relatively minor displacements during Mesozoic and Cenozoic times as compared to the drift pattern of the North and South American and East Antarctic plates (see Ziegler, 1992, this volume). Investigation of the ALD shows that strongly diverging mantle flow beneath the large African plate and the active drag exerted by flow on the base of the lithosphere must be seriously taken into consideration. In Fig. 6, the postulated pattern of sub-lithospheric mantle flow beneath the African plate is compared with the pattern of azimuthal anisotropy of phase velocities of Rayleigh and Love waves in the upper mantle. Azimuthal anisotropy of seismic velocities is interpreted as representing the orientation of mantle flow, whereby the direction of flow remains unknown (Tanimoto and Anderson, 1985; Montagner and Tanimoto, 1990). Figure 6b shows the pattern of azimuthal anisotropy of phase velocities of Rayleigh waves down to about 400 km depth below the African plate, as mapped by Montagner and Tanimoto (1990). Deep structure, geophysical evidence The simple, large-scale pattern of mantle flow beneath Africa, as proposed by the geotectonic model presented here, illustrates only the basic mode of mantle convection. Local and regional complications in this pattern, caused by the configuration of the African lithosphere and also due

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to the time dependence of the convection system, are to be expected. Africa formed the core of Gondwana, and the thickness of the African continental lithosphere is likely to vary considerably and may in some areas reach values of up to 200 km. Irregularities in thickness and structure of the African lithosphere may influence greatly, on either a regional or local scale, the flow pattern and velocity of the upwelling and diverging mantle material as it reaches the base of the lithosphere and, consequently, also the coupling between the lithosphere and the convecting asthenosphere. Reduced seismic velocities are observed beneath the eastern Sahara, the Red Sea region and the East African rift system, suggesting that, in these regions, ascending mantle flow is accompanied by partial replacement of the lower lithosphere by partly molten, asthenospheric material ascending from deep (> 200 km deep) reservoirs (Brown and Girdler, 1980; Nolet and Mueller, 1982; Hadiouche et al., 1989; Achauer et al., 1990; Ebinger and Karner, 1990; Halderman and Davies, 1991). Reduced P and S wave velocities, indicative of less dense and relatively hot material, are observed in the lower mantle beneath Africa (Dziewonski, 1984; Tanimoto, 1990), suggesting ascending flow in the lower mantle. This is in general agreement with the proposed ascending flow in the upper mantle beneath the African lithosphere, derived from geotectonic investigations (Pavoni, 1985a,b). Whether this upwelling occurs in the form of a single mega-plume, a group of smaller plumes and/or in a broad cylindrical ascending limb of a large convection cell, remains an open question. The shape and the relatively deep-reaching root of the African continental lithosphere may possibly divide the upwelling mantle into several branches. It is possible that the origin and relatively large number of African hotspots is directly related to upwelling of the deep mantle (Pavoni, 1981). The proposed convection system is a time-dependent system. The origin of geotectonic cycles is probably interrelated with the onset, build-up, main phase and decay of such a large-scale convection system (Pavoni, 1981, 1985a, 1988; Ziegler, 1990, 1992, this volume).

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A further observation, relevant to the proposed geotectonic model, concerns the position of Africa in the frame of the residual geoid. The African continent displays a higher-than-normal topographic elevation. Africa is located in the central part of the large Atlantic-African residual geoid high (Chase, 1979; Crough and Jurdy, 1980). The African Centre A lies very near to the centre of the residual geoid high (Pavoni, 1983, 1985a,b). The elevated position of the African continent in the central part of this residual geoid high is interpreted as dynamic uplift, which is directly related to the upwelling mantle. Unidirectional versus diverging mantle flow beneath active sea-floor spreading ridges In the foregoing, horizontal flow in the mantle beneath the active mid-oceanic ridges surrounding the African plate has been postulated, whereby the flow is directed away from the African Centre A (Fig. 6a). The concept of unidirectional, horizontal flow in the mantle beneath active spreading ridges is fundamentally important to the proposed model of mantle convection below Africa. It is markedly different from the conventional concept of ascending and diverging mantle flow at spreading ridges (Fig. 9a), as shown in practically all geology and geophysics textbooks and review papers (e.g., Cox and Hart, 1986, pp. 6, 19, 25; Wyllie, 1988; Anderson, 1989, p. 325; Condie, 1989; Peltier, 1989; Fowler, 1990; Kearey and Vine, 1990, pp. 65, 223, 227). In the model presented here, the oceanic ridges are regarded as passive structures and weak zones in the lithosphere (Pavoni, 1969, 1981). In the case of transverse ridges, the lithosphere is being torn apart due to different drift velocities of the lithospheric plates separated by the respective ridge (Fig. 9b). The space opening between the faster-moving "leading" plate A and the slowermoving "trailing" plate B is filled by the passively upwelling asthenosphere. This concept is supported by the geochemical signature of the MORB basalts (for further discussion see Ziegler, 1992, this volume). Nevertheless, the growth of the plates by sea-floor spreading is symmetrical

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about the ridge axis. The ridge axis moves away from the trailing plate at half the rate that the leading plate separates from the trailing plate (Pavoni, 1969). As compared with the conventional, diverging flow model (Fig. 9a), the unidirectional flow model (Fig. 9b) provides a much higher degree of freedom for: (1) the actual position of the ridge with respect to the sub-lithospheric flow; (2) a lateral displacement of the ridge system relative to the African divergence centre; (3) the development of ridge-ridge transform faults; and (4) the formation of triple junctions. Moreover, the unidirectional flow model allows more freedom in the construction of possible large-scale mantle convection patterns. There is no longer the need to correlate the distribution of upwelling and diverging mantle flow directly with the distribution of active sea-floor spreading axes. In any case, the unidirectional flow model allows for a lower spherical harmonic mode pattern of large-scale circulation in the mantle than the diverging flow model. Furthermore, the unidirectional, passive asthenospheric upwelling model can account for the often abrupt termination of activity along established sea-floor spreading axes (Ziegler, 1990, 1992, this volume). In the case of the unidirectional flow model, a certain asymmetry in the temperatures distribution of the lithosphere and mantle on two sides of the mid-oceanic ridge would be expected. Considering Fig. 9b, one would expect relatively higher temperatures in the mantle below the "trailing" plate B than below the "leading" plate A. As a consequence of this cooling and thickening of the oceanic lithosphere should be slower on the "trailing" plate, which is located closer to the actively upwelling branch of the mantle convection system, than on the leading plate. Based on a recent study of the global Earth structure by surface waves, Tanimoto and Zhang (1990) report asymmetric thickening of plates about ridges. On the eastern side of the mid-Atlantic ridge, which is located nearer to the African upwelling centre, reduced phase velocities of Love waves and reduced thicknesses of oceanic lithosphere, as compared to the western side of the ridge, are observed. As such, the results of Tanimoto and

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RIFTING A N D M A N T L E C O N V E C T I O N B E N E A T H T H E A F R I C A N P L A T E

Zhang (1990) support the unidirectional flow model. African lithospheric divergence, ALD, and global tectonics The African plate is one of the Earth's major lithospheric plates. Its development during Mesozoic and Cenozoic times was intimately related with the global processes which governed the break-up of Pangea and Gondwana. An insight into these processes can be gained from plate reconstructions for the last 180 Ma. Such reconstructions are of particular value since they are constrained by the record of the sea-floor magnetic anomalies. For a summary of plate reconstructions outlining the break-up of the Pangean super-continent and the evolution of the modern system of lithospheric plates, the reader is referred to Scotese et al. (1988) and Ziegler (1990, 1992, this volume). The author would like to stress here very briefly one important aspect of the tectonic evolution of the globe during last 180 Ma. This concerns the simultaneous, concordant evolution of the lithosphere in the Pacific and anti-Pacific hemispheres (Pavoni, 1969, 1985a,b, 1991). The African lithospheric divergence (ALD), centred at pole A at 10°E/0°N, has its counterpart in the Pacific lithospheric divergence (PLD), centred at pole P at 170°W/0°N (Fig. 7). ALD and PLD are manifestations of a fundamental hemispherical symmetry, reflecting a bipolarity of geodynamic processes. The Pacific and the African plates are homologous plates (Fig. 8). Recent geophysical investigations show that the same fundamental Pacific/anti-Pacific bipolarity is evident in the spherical harmonic degree and order 2 distribution of lateral heterogeneities of seismic velocity and density in the lower mantle (Dziewonsky, 1984; Pavoni, 1985; Richards et al., 1988), as well as in the form of the residual geoid (Chase, 1979; Crough and Jurdy, 1980; Pavoni, 1983; Hager, 1984). This points to a common origin for the anomalies and the bipolarity. Reduced seismic velocities, indicative of less dense and relatively hot material, are observed in

the lower mantle beneath both the Central Pacific plate and the African plate. Discussion A most remarkable result of the present investigations on Africa is the recognition of a continent-wide and plate-wide radial pattern that is evident from: (1) the distribution and orientation of deviatoric tensional stresses; (2) the arrangement of lithospheric extension along the great negative Bouguer anomaly belt and the Red Sea rift; (3) the diverging block movements, documented by the continued intra-continental rifting in Africa, by the breakup of Gondwana and the subsequent drift of its fragments away from Africa, entailing a radial growth of the African plate. Within the accuracy of determination, the location of the centres of these radial patterns seem to coincide, defining the African Centre A, located at 10°E/0°N. The present stress state of the African continent, the configuration of active sea-floor spreading ridges around Africa and the tectonic evolution of the African plate provide distinct boundary conditions for possible plate driving forces and their relative importance. Ridge push forces exerted on the African plate have to be balanced by some other forces acting on the African plate in order to explain the extensional stress regime which, even at present, dominates the African continent. Shear traction exerted by the convening asthenosphere on the base of the African lithosphere offers an explanation for the phenomena observed. The proposed sub-lithospheric flow is directed away from the African Centre A. In the direction of the mantle flow, the horizontal flow velocity has a positive gradient and reaches a maximum beyond the boundary of the African plate. Acknowledgements I am grateful to the Inter-Union Commission on the Lithosphere, Working Group 3 for inviting

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me to participate in the Geodynamics of Rifting Symposium, from which I have greatly profited. I thank Dr. Ron W. Girdler, Newcastle upon Tyne, for kindly sending me a copy of the DMAAC Bouguer gravity anomaly map of Africa and additional information about the gravity data of Africa. Special thanks are extended to Hermann Drewes, Munich, for calculating the geographical centre of the African plate from the data set of the DGF, and to my colleagues Roy Freeman, Edi Kissling, Bruno Martinelli, John Stamatakos and Peter A. Ziegler for their critical and constructive comments on an earlier version of this manuscript. Contribution No. 697, Institute of Geophysics, ETH-Zürich. References Achauer, U., Glahn, A. and Granet, M., 1990. Structural comparison of the Kenya Rift Valley and the Rhinegraben Rift System based on teleseismic delay time tomography and gravity modelling. Geodynamics of Rifting Symp. (Glion-sur-Montreux 1990), Abstr. ICL-WG3. Anderson, D.L., 1989. Theory of the Earth. Blackwell, Oxford, 366 + xvi p. Bercovici, D., Schubert, G. and Glatzmaier, G.A., 1989. Three-dimensional spherical models of convection in the Earth's mantle. Science, 244: 950-955. Binks, R.M. and Fairhead, J.D. 1992. A plate tectonic setting for the Mesozoic rifts of West and Central Africa. In: P.A. Ziegler (Editor), Geodynamics of Rifting, Volume II. Case History Studies on Rifts: North and South America and Africa. Tectonophysics, 213: 141-151. Bott, M.H.P., 1982. The Interior of the Earth, its Structure, Constitution and Evolution. Arnold, London, 2nd ed., 403 pp. Brown, C. and Girdler, R.W., 1980. Interpretation of African gravity and its implication for the breakup of the continents. J. Geophys. Res., 85: 6443-6455. Casenave, A., Souriau, A. and Dominh, K., 1989. Global coupling of Earth surface topography with hotspots, geoid and mantle heterogeneities. Nature, 340: 54-57. Chase, CG., 1979. Subduction, the geoid, and lower mantle convection. Nature, 282: 464-468. Condie, K.C., 1989. Plate Tectonics and Crustal Evolution. Pergamon, Oxford, 3rd ed., pp. 188, 352. Cox, A. and Hart, R.B., 1986. Plate Tectonics, How it Works. Blackwell, Oxford, 392 + xxiv pp. Crough, S.T. and Jurdy, D.M., 1980. Subducted lithosphere, hotspots, and the geoid. Earth Planet. Sei. Lett., 48: 15-22. Dziewonski, A.M., 1984. Mapping the lower mantle; Determination of lateral heterogeneity in P velocity up to degree and order 6. J. Geophys. Res., 89: 5929-5952.

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53 Scotese, Ch.R., Gahagan, L.M. and Larson, R.L., 1988. Plate tectonic reconstructions of Cretaceous and Cenozoic ocean basins. In: C.R. Scotese and W.W. Sager (Editors), Mesozoic and Cenozoic Plate Reconstructions. Tectonophysics, 155: 27-48. Shudofsky, G.N., 1985. Source mechanisms and focal depths of East African earthquakes using Rayleigh-wave inversion and body-wave modelling. Geophys. J.R. Astron. Soc, 83: 563-614. Simkin, T., Tilling, R.I., Taggart, J.N., Jones, W.J. and Spall, H., 1989. This Dynamic Planet. World map of volcanoes, earthquakes, and plate tectonics. Smithsonian Institution, Washington, DC. Sykes, L.R., 1967. Mechanism of earthquakes and nature of faulting on the mid-oceanic ridges. J. Geophys. Res., 72: 2131-2153. Tanimoto, T., 1990. Long-wavelength S-wave velocity structure throughout the mantle. Geophys. J. Int., 100: 327-336. Tanimoto, T. and Anderson, D.L., 1985. Lateral heterogeneity and azimuthal anisotropy of the upper mantle: Love and Rayleigh waves 100-250 s. J. Geophys. Res., 90: 1842-1858. Tanimoto, T. and Zhang, Y., 1990. Lithospheric thickness and thermal anomalies in the upper mantle inferred from the Love wave data. Geophys. Res. Lett., 17: 2405-2408. Wilson, M. and Guiraud, R., 1992. Magmatism and rifting in western and central Africa, from Late Jurassic to Recent. In: P.A. Ziegler (Editor), Geodynamics of Rifting, Volume II. Case History Studies on Rifts: North and South America and Africa. Tectonophysics, 213: 203-225. Woodhouse, J.H. and Dziewonski, A.M., 1984. Mapping the upper mantle: Three-dimensional modeling of Earth structure by inversion of seismic waveforms. J. Geophys. Res., 89: 5953-5986. Woodhouse, J.H. and Dziewonski, A.M., 1989. Seismic modelling of the Earth's large-scale three-dimensional structure. Philos. Trans. R. Soc. London Ser. A, 328: 291-308. Wyllie, P.J., 1988. Solidus curves, mantle plumes, and magma generation beneath Hawaii. J. Geophys. Res., 93: 41714181. Ziegler, P.A., 1989. Evolution of Laurussia—a Study in Late Palaeozoic Plate Tectonics. Kluwer, Dordrecht, 102 pp. Ziegler, P.A., 1990. Geological Atlas of Western and Central Europe. Shell Int. Pet. Mij., B.V. and Geol. Soc. London, 2nd ed., 239 pp.