Tectonophysics, 213 (1992) 141-151

Elsevier Science Publishers B.V., Amsterdam

A plate tectonic setting for Mesozoic rifts of Westand Central Africa

R.M. Binks and J.D. FairheadDepartment of Earth Sciences, University of Leeds, Leeds, LS29fT, UK

(Received May 24, 1991; revised version accepted November 28, 199])

ABSTRACT

Binks, R.M. and Fairhead, J.D., 1992. A plate tectonic setting for 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 andAfrica. Tectonophysics, 213: 141-151.

Africa, by virtue of its central position within Gondwana, has recorded much of the complex history of plate interactionswhich have progressively fragmented this supercontinent since the Early Mesozoic. Continental reconstructions reveal botha temporal and spatial relationship between the development of the continental margins of Africa and the formation ofrifted sedimentary basins deep within the African continent. The multi-stage opening of the Atlantic Ocean and associatedrifting in West and Central Africa provide one of the most impressive examples of ocean-continent tectonic interactions.During the Early Cretaceous the onset of rifting along the future margins of the South Atlantic is contemporaneous withintra-continental rifting generating both strike-slip and extensional basins within West and Central Africa. The syn-rift phaseof intra-continental basin development continued until the Santonian, by which time Africa and South America had beenphysicallyseparated for approximately 10 Ma. Except for minor rifting during the Senonian and Tertiary, the short-livedphase of deformation at about 80 Ma marks the transition into the post-rift or "sag" phase of basin development. Thisdeformational event can be correlated with a period of plate motion changes, recognised from fracture zone geometries,seen in both the Central and South Atlantic oceans. Using present day stress analogies, Cretaceous rifting in Africa canprovide a means of indicating the regional palaeostress directions within Africa at this time.

Introduction

Since the Late Palaeozoic, the progressivefragmentation of Gondwana has led to not onlymajorbasin formation along the present-day con-tinental margins but also to distinct phases ofintra-continental rift basin development withinthe African continent. In this paper, we distin-guishthree major phases of basin development:(1)prior to 180 Ma, (2) 130-80 Ma and (3) 20-0Maand focus in on the second phase.

Correspondenceto: J.D. Fairhead, Department of Earth Sci-ences, University of Leeds, Leeds LS2 9JT, UK.

Phase 1

This phase of basin development is associatedwith the breakup of Gondwana which com-menced in the Middle Jurassic with the openingof the Central Atlantic (Klitgord and Schouten,1986). There were no associated major intra-con-tinental basins developed in North West Africacoincident with this phase of opening, probablydue to the location of the stable West African

cratonic block, although Permo-Triassic age riftscan be found in Morocco and East and SouthAfrica. By the Middle Jurassic, the Indian Oceanhad started to open with the drift of Madagascar,India and Antarctica away from Africa. Thisbreakup is preceded by a significant phase of

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

141

142

rR.M. BINKS AND J.D. FAIRHEAD PLATE TECTONIC SETIING

JII.

The most recent IAfrican continent be

intra-continental riftithe extensive KarooAfrica (Fig. la). Thdevelop in the Permutilizing the weak zcbile belts which sunSouthern Africa. Pre

ern Africa clearly ibelts continue to beplate stresses as na(Fairhead and HendeBy -180 Ma, whe

crustal material be~margin of Africa, acbasins had ceased alsedimentation in rephase of basin subsic

Phase 2

1

II1

II

From the Late Ju

wards, rifting was tathe proto-South Atlamenced it's separaticis coincident with ou

tin ental rifting, whicthe West and Centra

1992, this volume; Ithis volume). Continby the emplacementis dated at -130 ~

(Rabinowitz and Lal

progressively youngtequatorial region ha(by -119 Ma (Masc

opening of the Equ:and South Atlanticpendently from one 2able stress buildingThis stress was dis1

(Pindell and DeweyAfrica (Fair head, I'1991) and northeastthis volume).

Phase 3

PLATE TECTONIC SETTING FOR WEST AND CENTRAL AFRICA

intra-continental rifting and sedimentation withinthe extensive Karoo basins of East and SouthernAfrica (Fig. la). These Karoo basins began todevelop in the Permo-Triassic (Lambiase, 1989),utilizing the weak zones of the Pan-African mo-bile belts which surround the cratonic blocks ofSouthern Africa. Present day seismicity in South-ern Africa clearly indicates that these mobilebelts continue to be weak crustal zones releasingplate stresses as narrow zones of earthquakes(Fairhead and Henderson, 1977; Daly et aI., 1989).By -180 Ma, when emplacement of oceaniccrustal material began along this southeasternmargin of Africa, active rifting within the Karoobasins had ceased although there was continuedsedimentation in response to a thermal "sag"phase of basin subsidence (McKenzie, 1978).

Phase2

From the Late Jurassic-Early Cretaceous on-wards, rifting was taking place along the line ofthe proto-South Atlantic, as South America com-menced it's separation from Africa (Fig. Ib). Thisis coincident with our second phase of intra-con-tinental rifting, which led to the development ofthe West and Central African rift systems (Genik,1992, this volume; Guiraud and Maurin, 1992,this volume). Continental separation, as definedby the emplacement of oceanic crustal material,is dated at -130 Ma for the Cape Basin area(Rabinowitz and LaBrecque, 1979) and becomesprogressively younger northwards so that theequatorial region had only just begun to separateby -119 Ma (Mascle et aI., 1986). Prior to theopening of the Equatorial Atlantic, the Centraland South Atlantic oceans were opening inde-pendently from one another resulting in consider-able stress building up in the equatorial region.This stress was dissipated into the Caribbean(Pindell and Dewey, 1982), West and CentralMrica (Fairhead, 1988b; Fairhead and Binks,1991)and northeast Brazil (Chang et aI., 1992,this volume).

Phase3

The most recent phase of rifting to affect theMrican continent began in the Late Eocene and

143

generated the structures of the Red Sea, Gulf ofAden and East African Rift System (Fig. lc;Rosendahl et aI., 1992, this volume).

These three periods of rifting and breakup ofGondwana illustrate an intimate relationship be-tween the plate tectonic evolution of the oceanicbasins and continental tectonics, with a majorphase of intra-continental rifting preceding eachstage of break-up. Although many basins con-tinue to deepen due to thermal subsidence, activerifting generally ceases around the time thatoceanic crustal material begins to be emplacedalong the line of crustal separation associatedwith that stage of development. All three phasesof intra-continental rifting described above uti-lize, wherever possible, weak crust of the Pan-African mobile zones. This paper focuses on theCretaceous development of the West and CentralAfrican rift systems and the associated opening ofthe South Atlantic Ocean, highlighting the evi-dence available which can relate these events.

Kinematic studies in the Central Atlantic

(Klitgord and Schouten, 1986) have shown longperiods of smoothly varying plate motion inter-rupted by short periods of rapid changes whichinclude events dated at -130 and -80 Ma. For

the South Atlantic, analyses of fracture zone ge-ometries suggest a similar opening history (Candeet aI., 1988; Fairhead, 1988a; Fairhead and Binks,1991), although the -80 Ma event is less wellpronounced than in the Central Atlantic. Theflowline event at - 130 Ma ties in well with the

onset of our second phase of African rifting,whilst the -80 Ma event correlates with a periodof deformation seen within many of the Creta-ceous basins of West Africa (Benkhelil, 1982;Benkhelil et aI., 1988; Guiraud et aI., 1987; Genik,1992, this volume; Guiraud and Maurin, 1992,this volume) which will be discussed more fullylater. The spatial correlation between continentaland oceanic tectonics is best illustrated in the

Equatorial Atlantic where most of the major frac-ture zones appear to continue right up to theAfrican margin and in some instances link upwith major continental structures; e.g. the Char-cot fracture zone can be seen to link with the

strike-slip zone of the Benue Trough, in the re-gion of the Niger delta (see Fig. 4). The continen-

144

tal extensions of these fracture zones are associ-

ated with historic seismicity (Sykes, 1978; Am-braseys and Adams, 1986), indicating that thesefracture zones are weak and still able to dissipatestress into the adjacent continental areas.

The present day relationship between platemotion and intra-plate stress regimes, determinedfrom focal mechanisms, in situ stress measure-ments and borehole breakout studies, has beenstudied by Zoback et a!. (1989) as part of theWorld Stress Map Project. They observe that themaximum horizontal stress is subparallel to thedirection of plate motion suggesting that irrespec-tive of the varied individual stresses affecting anarea (e.g. tectonic stresses resulting from plateboundary forces and flexure and localised stressesdue to topography, erosion, etc.), the net balanceof forces driving a plate also dominates the plateinterior. It would therefore be expected that thegeometry of rifted basins could providepalaeostress direction indicators for the plate tec-tonic processes acting at such times. An alterna-tive way of viewing this relationship is to considerthat stage poles which describe the progressiveopening of an ocean basin, also provide the meansof mapping the relative regional stress directionsexisting at that time.

R.M. BINKS AND J.D. FAIRHEAD

Rift systems of West and Central Africa

The Cretaceous rift systems of West and Cen-tral Africa extend for over 4000 km from Nigerianorthwards into Niger and Libya and eastwardsthrough southern Chad into Sudan and Kenya.The Cretaceous rifts are, like the Karoo rifts,located within Pan-African zones of lithosphericweakness (Daly et a!., 1989) and exhibit bothstrike-slip and extensional basin geometries (Fig.2). The Mesozoic rifts can be divided into twosub-systems: the West African and the CentralAfrican, each of which consists of a set of strike-slip fractures which penetrate deep into theAfrican continent before terminating by trans-forming motions along them into the develop-ment of extensional basins orientated perpendic-ular to the direction of strike-slip motion.

The West African sub-system extends from theGulf of Guinea northeastward along the sinistralstrike-slip "wrench" zone of the Benue Troughwhich splits in northeastern Nigeria into twobranches: the Yola rift striking east intoCameroon and the Gongola rift striking northeastinto the Lake Chad region. The Gongola branchhas been shown from geophysical studies (Louis,1970; Fairhead and Green, 1989) to join in the

E::J Pre-Cretaceous rocks

o Post-Cretaceous rocks

10::0:-:<1Cretaceous rift basins

+ Cretaceous stress field (Inferred)

Fig. 2. Geometry of the rift systems of West and Central Africa based on Fairhead (1988a) and Guiraud and Maurin (1992, thisvolume).

PLATE TECTONIC SETfINC

Lake Chad area wi1Niger which are oricBenue Trough (NN~basins have been sh.to be affected by a nlineament which Cft17°N. North of thisbasins are orientate,dying out in southenset of NNE faults cthrough the Tibesti jbasins of Libya.

The Central Africilar geometry, with ~slip zone showing if.(Ngangom, 1983; CNarrow "pull-apart"length of the strike-:Cameroon through :tral African RepubMovement along thinto a series of N'which extend into

(McHargue et a!., 1Cretaceous rifting nrift structures whichby the Cenozoic Eas

Each of the sub-:bounded sub-basinsnature; thus the strvariation from one a

have been publishe,the better knownbas

(e.g. Burke, 1976;P,Cratchley et a!., 198/aI., 1986; Maurin etSchull, 1988)while sstructural setting 01basins can be foune

1992, this volume;volume; Guiraud aniThe Cretaceous ba:

thick sedimentary seLake Chad area amin excess of 10 km1988; Genik, 1992,1values between 4 anet a!., 1963; Cratchl

PLATE TECfONIC SEllING FOR WEST AND CENTRAL AFRICA

Lake Chad area with the rift basins of eastern

Niger which are orientated perpendicular to theBenue Trough (NNW-SSE). These eastern Nigerbasins have been shown by Guiraud et ai. (987)to be affected by a major N080° to N090°-strikinglineament which crosses the basins at latitude

17°N. North of this lineament the eastern Nigerbasins are orientated in a NNW-SSE direction,dyingout in southern Algeria. In the same area, aset of NNE faults could possibly provide a linkthrough the Tibesti area to the Murzuk and Sirtebasins of Libya.

The Central African sub-system exhibits a sim-ilar geometry, with a major ENE-oriented strike-slip zone showing in this case dextral movement(Ngangom, 1983; Cornacchia and Oars, 1983).Narrow "puIl-apart" basins are located along thelength of the strike-slip zone which extends fromCameroon through southern Chad and the Cen-tral African Republic into west-central Sudan.Movement along this fault system is dissipatedinto a series of NW-SE orientated rift basins

which extend into southern Sudan and Kenya(McHargue et aI., 1992, this volume). In Kenya,Cretaceous rifting rejuvenates the Jurassic Anzarift structures which have, in turn, been cross-cutby the Cenozoic East African rift system.

Each of the sub-systems contains many fault-bounded sub-basins, mostly of a "puIl apart"nature; thus the stratigraphy exhibits significantvariation from one area to another. Many papershave been published, giving detailed studies ofthe better known basins such as the Benue Trough(e.g. Burke, 1976; Petters, 1981; Benkhelil, 1982;Cratchley et aI., 1984; Peterson, 1986; Avbovbo etaI., 1986; Maurin et aI., 1986; Popoff et aI., 1986;SchuIl,1988) while summaries of stratigraphy andstructural setting of hitherto not yet describedbasins can be found in this volume (see Genik,1992, this volume; McHargue et aI., 1992, thisvolume; Guiraud and Maurin, 1992, this volume).The Cretaceous basins frequently contain verythick sedimentary sequences. For example, in theLake Chad area and parts of Sudan, thicknessesin excess of 10 km have been recorded (SchuIl,1988;Genik, 1992, this volume) while elsewherevalues between 4 and 7 km are common (Carteret aI., 1963; Cratchley et aI., 1984; Genik, 1992,

145

this volume). Regional post-rift subsidence (ther-mal sag) has resulted in most of the rift systembeing covered with up to 4 km of Tertiary andRecent sediments (Avbovbo et aI., 1986). Theoldest sediments found within the rifts of Westand Central Africa are Neocomian-Barremian inage and are contained in narrow fault-boundedtroughs in northern Cameroon and southern Chad(Genik, 1992, this volume; Guiraud and Maurin,1992, this volume). Compared to the rift systemof East Africa, the rift systems of West andCentral African are associated with only minormagmatic activity of which the volcanics of theBenue Trough are the best known. Transitionalalkaline basalts from the Upper Benue Troughhave been dated at 127:t 6 Ma (Baudin, 1986)and basal volcanics have been found in the Bima

Hills with an age of 147:t 7 Ma (Popoff et aI.,1983). By the Mid-Albian 004 Ma) marine sedi-ments were deposited in the Benue Trough (Be-nkheIiI, 1986; Guiraud and Maurin, 1991). Thismarine incursion is contemporaneous with thegeneration of oceanic crust in the equatorial partsof the Atlantic (MascIe et aI., 1986); this is inagreement with the northward propagation ofseafloor spreading in the South Atlantic Ocean.

Rifting and sedimentation continued, in a vari-ety of tectonic settings, within the West and Cen-tral African systems throughout the Cretaceous.Prior to the Santonian, the Benue Trough showssinistral transtensional movement which dissi-

pates into the "pure" extensional basins of east-ern Niger, while the Central African sub-systemexhibits dextral movement, dissipating into theextensional basins of Sudan and Kenya. The finalseparation of the African and South Americancontinents occurred at -105 Ma (MascIe et aI.,1988) but the thermal development of the intra-continental rifts continued until the Santonian

when an important phase of basin inversion tookplace along the Benue Trough (Benkhelil, 1982;Benkhelil et aI., 1988).

This Santonian deformational event at -80Ma is widely recognised in West and CentralAfrica as a short-lived period of compressionproducing folds paraIlel to the axis of the BenueTrough, Nigeria (Avbovbo et aI., 1986; Benkhelilet aI., 1988; Guiraud et aI., 1987; Genik, 1992,

146

this volume; Guiraud and Maurin, 1992, this vol-ume) and NE-SW transpressional anticlinalstructures in Niger and Chad (Genik, 1992, thisvolume). The compressional event, which is re-sponsible for generation of these structures, iscontemporaneous with widespread dextral strike-slip movements along the Central African sub-system, as evident by seismically imaged "flower"structures (Genik, 1992, this volume). Contempo-raneous development of the Sudan basins, char-acterized by continued sedimentation and crustalextension, is consistent with this dextral move-ment along the Central African shear zone(Schull, 1988; McHargue et aI., 1992, this volume).After the -80 Ma event, major tectonic move-ments within the Central Africa sub-systems ap-pear to have ceased (Fairhead, 1988b) althoughthe basins continued to sag and deepen in re-sponse to post-rift thermal relaxation of the litho-sphere, resulting in the accumulation of thick

(a)o

-'00

:+--RIFT-+:, I~m

200 Distance (km)

... -'00- (b);;e0-CCII< 'i -'40.....CI"eg,-::>oCD

R.M. BINKS AND J.D. FAIRHEAD

Tertiary to Recent sediments covering most ofthe Cretaceous rifts.

Gravity studies (Browne and Fairhead, 1983;Fairhead, 1988a; Fairhead and Green, 1989) haveshown that the rift systems of West and CentralAfrica are associated with broad (regional) posi-tive Bouguer anomalies located symmetricallyover each basin. These anomalies are typically upto 450 km wide and have amplitudes of up to 80mGal (Fig. 3a; Fairhead and Okereke, 1988).Refraction and reflection seismic studies acrossthe Yola rift indicate that at least 11 km of

crustal thinning has occurred (Stuart et aI., 1985).Three-dimensional gravity modelling of the Be-nue Trough indicates that a crustal zone, 150 kmin width, has been thinned by up to 18 km(Fairhead and Okereke, 1988). Superimposed onthe regional positive gravity anomalies are smallernegative Bouguer anomalies, referred to as resid-ual anomalies (Fairhead and Green, 1989). These

400

Distance (km)

6' 16'10' '4'

Fig. 3. Bouguer gravity anomaly and topography profiles across (a) the middle Benue Trough (after Fairhead, 1986) and (b) theeastern rift of the East African Rift System, Lake Magadi (after Fairhead, 1976). On the Bouguer gravity profiles the solid line

represents observed values and the dotted line is the regional field.

PLATE TECTONIC SETIINC

smaller anomalies,extent and amplitudlity of the rift basirprovided by Exxon'

indicate that the riftAfrica have been Slsubsidence and sed

magmatic activity.TIEast African Rift SYicrustal doming, uplmajor magmatic acigravity signatures 01therefore not surpribroad negative grmAfrican rifts, contr2gravity anomaly obs(West and Central A

From both geologtions it is suggested 1and Central Africa (of the McKenzie (19model. The Early 0mation in this area

along the margins 0This intra-continent,

10 ~

10 S

50,

Fig. 4. Shaded relief imag(oceans) combined with tt

illustrating the relationshBFZ = Bahamas fractureRFZ = Romanche fracturl

PLATE TECfONIC SETTING FOR WEST AND CENTRAL AFRICA

smaller anomalies, which vary in their lateralextent and amplitude, reflect the spatial variabil-ity of the rift basin fill. Exploration well dataprovided by Exxon (Fairhead and Green, 1989)indicate that the rift basins of West and Central

Africa have been subjected to rapid periods ofsubsidence and sedimentation with only minormagmatic activity. This contrasts sharply with theEast African Rift System which is associated withcrustal doming, uplift of the rift margins andmajor magmatic activity (Fairhead, 1986). Thegravity signatures over the two rift systems aretherefore not surprisingly very different with abroad negative gravity anomaly over the EastAfrican rifts, contrasting to the broad positivegravity anomaly observed over the palaeo-rifts ofWest and Central Africa (Fig. 3).

From both geological and geophysical observa-tions it is suggested that the rifted basins of Westand Central Africa are among the best examplesof the McKenzie (1978) passive extensional basinmodel. The Early Cretaceous onset of basin for-mation in this area was associated with riftingalong the margins of the future South Atlantic.This intra-continental rifting phase, which in the

20 S

10~

105

147

Benue Trough lasted for some 30 Ma, is evidentby faulting and sedimentation (initially lacustrine,and later marine) which was accompanied by onlyminor volcanism (Wilson and Guiraud, 1992, thisvolume). Following the Santonian compressionalevent, which terminated rifting in most basins,the region continued to subside in response topost-rift thermal relaxation of the lithosphere("sag" phase of McKenzie's 1978 model). Thelack of shoulder uplift in West and CentralAfrican rifts, together with the geometry of thethermal sag basins, indicates that lithospheric ex-tension was depth-dependent (non-linear; Fair-head, 1986). This is to say that the zone ofextension increases in size (width) with depth or,in other words, that lower crustal thinning oc-curred over a broader zone than upper crustalextension by faulting, and that thinning of thelower part of the lithosphere occurred over aneven broader zone. The East African rifts have

not been subjected to as great a degree of crustalextension as the West African rifts. Crustal exten-sion across the Kenya rift is of the order of15 :t 10 km, increasing northwards towardsEthiopia and decreasing towards the south

Fig.4. Shaded relief image (lit from SE) of Haxby's (1989) 5' x 5' gridded free-air gravity map derived from GeosatjSeasat data(oceans) combined with the Bouguer gravity field over Africa compiled by the African Gravity Project (Fair head and Watts, 1988)illustrating the relationship between the fracture zones of the Equatorial Atlantic and the rift structures of West Africa.

BFZ = Bahamas fracture zone; GFZ = Guinea fracture zone; FFZ = Four North fracture zone; SPFZ = St. Paul's fracture zone;RFZ = Romanche fracture zone; CFZ = Chain fracture zone; ChFZ = Charcot fracture zone; AFZ = Ascension Island fracture

zone.

hene

148

(Fairhead, 1986); this compares to estimates of 40to 90 km in the West African rift (Browne andFairhead, 1983; Fairhead and Okereke, 1988). Incombination with the highly anomalous (hot) stateof the upper mantle beneath East Africa, this hasresulted in a totally different response of thecrust (Fairhead, 1986).

Opening of the Atlantic Ocean

The development of the Atlantic Ocean beganin the Jurassic with the opening of the CentralAtlantic at -180 Ma (Klitgord and Schouten,1986); this was followed by the opening of theSouth Atlantic in the Early Cretaceous. TheEquatorial Atlantic remained closed until theAptian (- 119 Ma). This entire history of relativeplate movements is preserved in the seafloorspreading pattern as reflected by fracture zonetraces and magnetic anomalies. Since fracturezones delineate spreading centre offsets (trans-forms) through time, they provide an approxi-mate "flowline" trace. Thus, by combining frac-ture zone traces with the age of the oceanic crust,as determined from the geochronology of theseafloor magnetic anomaly pattern, an accuratepicture of plate motions can be obtained. Thegravity map, given in Fig. 4, is derived fromcombining a 5' X 5' gridded Seasat / Geosat dataset (after Haxby, 1988) with the African GravityProject Bouguer (onshore)/free air (offshore)data set (Fairhead and Watts, 1988); it shows theEquatorial Atlantic to be an anomalous area inwhich the fracture zone gravity signatures areunusually strong, even on older oceanic crust, andin many instances can be traced across the entireAtlantic from the west coast of Africa to SouthAmerica and the Caribbean. In the equatorialregion, the mid-ocean ridge crest is dramaticallyoffset by a series of transform faults, including a100 km offset at the Romanche fracture zone.

The Equatorial Atlantic has been defined as thearea bounded by the Bahamas fracture zone tothe north and the Ascension fracture zone to thesouth and is considered to be distinct from theCentral and South Atlantic (Fairhead and Binks,1991). Its northern boundary, to the west of theGuinea plateau, is associated with strong flowline

R.M. BINKS AND J.D. FAIRHEAD

divergence interpreted to be the result of differ-ential opening between the Central and SouthAtlantic oceans (Jones, 1987), while its southernboundary is more diffuse. It is apparent from thegravity map of the area that many of the oceanicfracture zones appear to link up with the majorfracture systems in Africa. For example, theCharcot fracture zone (ChFZ) is aligned with thenorthwest side of the Benue Trough (Fig. 4). Thenorthern and southern boundaries of the Equato-rial Atlantic are considered to be the principalsites for dissipating stresses into the Caribbeanand African continents. Fairhead and Binks

(1991) utilised published stage pole data for theCentral and South Atlantic to assess the effects

of dissipating all the stresses arising from thedifferential opening of the Central and SouthAtlantic oceans along the northern and southernboundary of the Equatorial Atlantic. They con-cluded that between 84 and 0 Ma the geometry ofthe seafloor is consistent with major stress releasealong the northern boundary; however, the corre-lation is not so good for major stress releasealong the southern boundary, except for the pe-riod about 80 Ma when the opening vectors forthe Central and South Atlantic are coincident.

During the Santonian, the fracture systems ofCentral Africa form small circles about the stagepole, thus permitting strike-slip movement alongthe ocean and continental fracture zones to dissi-

pate differential opening between the Centraland South Atlantic. Plate motions older than 84Ma are more difficult to determine due to lesswell defined flowline traces, coupled with theCretaceous period of single positive magnetisa-tion, and are the subject of an ongoing study byone of us (RMB).

Plate tectonic interactions

Early attempts at pre-drift reconstructions forAfrica and South America were simplistic as theyconsided the plates to be rigid, thus suffering nointernal deformation during their breakup (Bul-lard et aI., 1965; Rabinowitz and LaBrecque;1979; Sibuet and Mascle, 1978). Consequently itwas not possible to obtain a satisfactory fit for thewhole length of the South American-African

PLATE TECTONIC SETTINC

margins. It is now .subjected to intra-pibreakup and collisi(account when carl']structions. Geologi(have shown that th

with the rift systemsconsists of two setsfrom the Gulf of Guthe African continellthrough Niger towarwards through sou!Kenya. This geomeinvoke compressiorAfrica as proposed tBy considering the CIber of rigid "block~zones, stage polesoceanic flowline gelcompatible with the (along these intra-c4From comparisons blrifting in Africa theand spatial link betopening of the Atiarthe rift systems withiThe most striking CO]flowline direction at :Schouten, 1986)whicthe onset and termilrifting in West and '

event produced inveBenue Trough, dextr:tral Africa sub-systelextension in the Sudajhas been recognised tdell Sea as markingreorganisation.

Since the Sudanthrough Kenya to th(that the Mesozoic riftalso in part controlledcontrolling the openinplate tectonic setting (is further complicate,northward movement

with Europe (Ziegle]modified the stress fi~

PLATE TEcrONIC SEITING FOR WEST AND CENTRAL AFRICA

1

margins. It is now accepted that continents aresubjected to intra-plate deformation during theirbreakup and collision. This must be taken intoaccount when carrying out palinspastic recon-structions. Geological and geophysical studieshave shown that the fault geometry, associatedwith the rift systems of West and Central Africa,consists of two sets of fractures which emanate

from the Gulf of Guinea and penetrate deep intothe African continent: one set strikes northwards

through Niger towards Libya and the other east-wards through southern Chad into Sudan andKenya. This geometry avoids the necessity toinvoke compressional tectonics in northeastAfrica as proposed by Pindell and Dewey (1982).Byconsidering the continent to consist of a num-ber of rigid "blocks" separated by deformablezones, stage poles must not only satisfy theoceanic flowline geometries but must also becompatible with the observed type of deformationalong these intra-continental "mobile zones".From comparisons between seafloor opening andrifting in Africa there exists a simple temporaland spatial link between the various stages ofopening of the Atlantic and the development ofthe rift systems within West and Central Africa.The most striking correlations are the changes inflowlinedirection at 130 and 80 Ma (Klitgord andSchouten, 1986) which correlate respectively withthe onset and termination of major Cretaceousrifting in West and Central Africa. The 80 Maevent produced inversion along the axis of theBenue Trough, dextral strike-slip along the Cen-tral Africa sub-system and a period of furtherextensionin the Sudan rifts. Elsewhere, this eventhasbeen recognised by Haxby (1989) in the Wed-dell Sea as marking a major period of platereorganisation.

Since the Sudan rifts extend southeastward

through Kenya to the Indian Ocean, it is likelythat the Mesozoic rift development of Africa wasalsoin part controlled by plate tectonic processescontrolling the opening of the Indian Ocean. Theplatetectonic setting of Mesozoic rifting in Africais further complicated by the Late Cretaceousnorthward movement of African and it's collision

with Europe (Ziegler, 1992) which must havemodified the stress field and thus influenced the

149

rift geometries within Africa. The significance ofthese interactions have yet to be determined.

Acknowledgements

RMB gratefully acknowledges financial sup-port from a NERC research studentship and theBasin Analysis of Africa Project (Leeds Univer-sity).

References

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Avbovbo, AA, Ayoola, E.O. and Osahon, GA., 1986. Depo-sitional and structural styles in the Chad Basin of north-eastern Nigeria. Am. Assoc. Pet. Geo!. Bull., 70(12): 1787-1789.

Baudin, P., 1986. Magmatisme mesozo'ique du fosse de laBenoue (Nigeria). Caracteristiques petrologiques et geoc-himiques, signification geodynamique. Memoire en Depota I'Univ. d'Aix-Marseille III.

Benkhelil, J., 1982. Benue Trough and Benue Chain. Geo!.Mag., 119: 155-168.

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