Tectonophysics, 213 (1992) 227-234

Elsevier Science Publishers B.V., Amsterdam

Chronology and geodynamic setting of Cretaceous-Cenozoicrifting in West and Central Africa

R. Guirauda, R.M. Binks b, J.D. Fairhead band M. Wilson ba Faculte des Sciences, 33 rue Louis Pasteur, 84000 Avignon and CGG, Uniu. Montpel/ier II, 34095 Montpellier Cedex 5, France

b Department of Earth Sciences, University of Leeds, Leeds LS2 9fT, UK

(Received October 7,1991; revised version accepted December 3,1991)

ABSTRACT

Guiraud, R., Binks, R.M., Fairhead, J.D. and Wilson, M., 1992. Chronology and geodynamic setting of Cretaceous-Ceno-zoic rifting in West and Central Africa. In: P.A. Ziegler (Editor), Geodynamics of Rifting, Volume II. Case HistoryStudies on Rifts: North and South America and Africa. Tectonophysics, 213: 227-234.

The development of the Early Cretaceous to Palaeogene West and Central African rift system, which extends fromNigeria (Benue trough) to Kenya (Anza trough), can be related to the build-up of intraplate tensional stresses during thebreak up of Gondwana, which caused reactivation of pre-existing zones of lithospheric weakness. Repeated changes in theintraplate stress regime of Africa are reflected by phases of crustal extension alternating with episodes of compression.Many of these events can be correlated with changes in rates of seafloor spreading in the Central and South Atlantic oceans,as reflected in flowline patterns. The West and Central African rifts can be considered as typical 'passive' rifts which evolvedin response to the build-up of intraplate stresses. However, the St. Helena hot spot appears to have been located beneaththe Equatorial plate boundary at approximately 120 Ma and may have played an important role in weakening thelithosphere during extension.

Introduction

This summary aims at describing the variousrifting phases which affected West and CentralAfricaduring Mesozoic and Cenozoic times. It isbasedon papers presented in the West and Cen-tral African section of this volume. We define

eight major tectonic and magmatic events andconsider the relationships between them and thegeodynamicevolution of the African plate; more-over, we attempt to show how rifting and oro-genic phases, magmatism, thermal subsidence,uplift and basin inversion fit into an overall pic-ture (Fig. 1).

Correspondence to: R. Guiraud, Universite d'Avignon, Faculte

des Sciences, Laboratoire de Geologie Dynamique et Ap-

pliquee, 33 rue Louis Pasteur, 84000 Avignon, France.

Elsevier Science Publishers B.V.

Permo-Triassic initial rifting episodes related tobreak up of Gondwana

The initial break up of Gondwana began dur-ing the Permo-Carboniferous and Triassic. Rift-ing concentrated on widely separated areas,namely on the eastern margin of Africa, along theaxis of the future Central Atlantic ocean and inthe Tethys domain. All these areas are character-ized by rift-related magmatic events. In SoutheastAfrica, the onset of Karoo rifting is dated as LateCarboniferous and Permian (Lambiase, 1989); astrong Early Jurassic magmatic event occurredduring the 190-200 Ma time interval (Dingle etaI., 1983) and preceded the Late Jurassic split ofGondwana into a western and an eastern half.

Along the northwestern margin of Africa, riftingcommenced during the Middle Triassic and cul-minated, at the transition from the Triassic to the

227

228 R. GUIRAUD ET AL T

AGES RIFTING EPISODES OROGENIC MAIN MAGMATICPHASES EVENTS

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CRETACEOUS-CENOZOIC RIF

Jurassic, with the intndykes and sills. Such inSpain to Liberia; theyTaoudenni Basin wherof about 200 Ma (H.

pers. commun., 1991).Central Atlantic dombetween Africa and f\175 Ma (Favre and StMediterranean Tethmenced during the

rapidly westward durmagmatism peaked iAlgeria during the Isic, prior to Mid-Jmtween North Africa1992a). Within Afric:assic rifting include(Reyre, 1984) and tPan African suturewhere Permian-Jursions occur (Lefranc

Neocomian-early All

A second stage 0sition from the Lat'ceous, affected laT!South and Equatorand Central Africaextend from NigeJ(Anza trough), ancfrom Tunisia to Egthis volume; ChaJGenik, 1992, this .1992, this volume),River Basin (Nammargin, in the DrBasin (western SUI

synchronous. Howand in the WCARbasins may have 1ing the Hauterivisome instances «(

stages of rifting (Cretaceous. Thes

are probably reIpropagation of CI

CRETACEOUS-CENOZOIC RIFTING IN WEST AND CENTRAL AFRICA

Jurassic, with the intrusion of extensive doleritedykesand sills. Such intrusions are reported fromSpain to Liberia; they occupy a large area in theTaoudenni Basin where they yield 39Ar_40Aragesof about 200 Ma (H. Bertrand and G. Feraud,pers. commun., 1991). Rifting terminated in theCentral Atlantic domain with crustal separationbetween Africa and North America around 180-175Ma (Favre and Stampfli, 1992). In the centralMediterranean Tethys domain, rifting com-menced during the Permian and propagatedrapidly westward during the Triassic; rift-relatedmagmatism peaked in the area of Morocco andAlgeria during the Late Triassic/earliest Juras-sic, prior to Mid-Jurassic crustal separation be-tween North Africa and Europe (Ziegler, 1988,1992a).Within Africa, areas of minor Permo- Tri-assic rifting include the on-shore Gabon basin(Reyre, 1984) and the Tezzofi trough along thePan African suture, southwest of the Hoggar,where Permian-Jurassic alkaline syenitic intru-sionsoccur (Lefranc and Guiraud, 1990).

Neocomian-early Aptian rifting episode

A second stage of rifting, initiated at the tran-sition from the Late Jurassic to the Early Creta-ceous, affected large areas, including the proto-South and Equatorial Atlantic margins, the Westand Central African Rift Systems (WCARS)whichextend from Nigeria (Benue trough) to Kenya(Anza trough), and the northern African marginfrom Tunisia to Egypt (Binks and Fairhead, 1992,this volume; Chang et al., 1992, this volume;Genik, 1992, this volume; Guiraud and Maurin,1992,this volume). Onset of rifting in the OrangeRiver Basin (Namibian margin), along the Beninmargin, in the Upper Benue and in the MugladBasin (western Sudan) appears to be more or lesssynchronous. However, along the Atlantic Marginand in the WCARSdevelopment of some riftedbasins may have begun somewhat later (e.g. dur-ing the Hauterivian in northern Cameroon). Insome instances (e.g. Congo-Gabon margin) twostages of rifting are recognized during the EarlyCretaceous. These Early Cretaceous rifting eventsare probably related to progressive northwardpropagation of crustal separation between Africa

229

and South America and seafloor spreading in theSouth Atlantic, including the break down of theWalvis-Sao Paulo Ridge barrier. The end of thisrifting stage is marked in the South Atlantic by avery early Aptian strong phase of magmatic activ-ity which affected the Namibia-Angolan andBrazilian sectors of the South Atlantic margins.Eruption of tholeiitic Parana (Brazil) andEtendeka (Namibia) flood basalts has been re-lated to activity of a deep mantle hot spot be-neath the newly developing plate boundary(O'Connor and Duncan, 1990).

Magmatism also occurred along the BrazilianEquatorial Atlantic margin and in northernCameroon, as evident by the emplacement ofhigh level intrusives, ranging in composition fromalkaline to tholeiitic basalts and their differenti-

ates (Wilson and Guiraud, 1992, this volume).Initiation of this phase of magmatic activity maybe linked to the development of the St. Helenahot spot (O'Connor and Duncan, 1990).

Rift geometries and structural data indicatethat starting from the Neocomian onward threeor possibly four internally rigid continental'blocks' began to separate within the Africanplate, namely the Western, the Austral and theArabian-Nubian blocks (Guiraud and Maurin,1992, this volume), with the latter possibly subdi-vided into a Central and Eastern block (Genik,1992, this volume). The boundaries of each ofthese blocks correspond to zones of crustal weak-ness (e.g. mylonite zones) which were inheritedfrom the Pan-African orogeny. Guiraud andMaurin (1992, this volume) favour a northwardmovement of the Arabian-Nubian block with re-

spect to the Western and the Austral blocks,whereas Genik (1992, this volume) and Binks andFairhead (1992, this volume) favour a morenortheasterly movement of the Arabian-Nubianblock. Binks and Fairhead (1992, this volume)suggest that the Central and South Atlantic riftsdeveloped independently of each other duringthis early stage of rifting and seafloor spreadingand that they coalesced only later into one riftsystem. Differential movements between Northand South America and Africa were taken up byshearing in the area of the Equatorial fracturezone, the Caribbean and within Africa. Definition

230

of an accurate location for the early opening poleof the South Atlantic has proved to be difficult. Abetter resolution of the fracture zones close to

the continental margins by new seismic reflection,aeromagnetic and gravity data, will be shortlyavailable and will provide further constraints onthe position of this stage pole.

Aptian-Albian rifting episode

At the end of the Early Aptian, a new riftingepisode commenced which mainly affected riftslocated along the Equatorial Atlantic margin,within the WCARS(from Benue to southern Chad)and in northern Libya, while the Sudanese rifts,the east Niger Tenere troughs and the Gao troughof eastern Mali also remained active. This riftingphase lasted until Late Albian times. Magmaticactivity was concentrated in the southern parts ofthe Benue Trough and included both alkalineand transitional basalts and their differentiates(Wilson and Guiraud, 1992, this volume).

Structurally, the NW-SE trending rifts evolvedin response to an approximate NE-SW crustalextension, while the E- W/ENE- WSW strikingelongated pull-apart basins developed in re-sponse to dextral (Cameroon to Sudan) and sinis-tral (Niger Delta to Lake Chad) strike-slip move-ments. Essentially orthogonal extensional strainin the basins of the Southern Sudan and East

Niger was taken up along the dextral CentralMrican and the sinistral Benue fracture zones,respectively, which provided a link to the Gulf ofGuinea. Contemporaneous rapid opening of theEquatorial Atlantic Ocean (Mascle et aI., 1988),resulted in linking up of the Central and SouthAtlantic mid-oceanic spreading ridges via theEquatorial fracture zones. Differential rates ofseafloor spreading in the Central and South At-lantic provide an explanation for the rejuvenationof crustal weakness zones in Africa and impor-tant lateral displacements along them (Fairheadand Binks, 1991).

The Santonian (80-85 Ma) compressive event

The stratigraphic record of many Mricanbasins contains an intra-Santonian hiatus,

R. GUIRAUD ET AL.

whereby Campanian sediments unconformablyoverly Coniacian strata or occasionally an olderfolded series. This hiatus can be related to a

regional compressional episode, referred to as'the Santonian event', which affected much of theWCARSfrom the Lower Benue to the Chad-Sudan

border (Ngangom, 1983; Avbovbo et aI., 1986;Benkhelil et aI., 1988). This event is associatedwith folding, conjugate strike-slip faulting, re-verse faulting, often generating transpressionalflower structures, development of local schistosi-ties (Abakaliki anticlinorium), and inversion ofsome of the deepest Early to Middle Cretaceousbasins (Doseo and Doba basins; Genik, 1992, thisvolume). At the same time, the large NW-SEtrending rifts of eastern Niger, Sudan and north-ern Libya continued to develop due to exten-sional faulting. Structural data indicate a generalNW-SE shortening direction, accompanied by adextral movement along the southern Chadstrike-slip fault zone.

The Santonian compressional event can alsobe recognized elsewhere in Mrica, for instance inthe Mandera trough of western Somalia (strikeand structure similar to Benue trough) and par-ticularly along the North African margin(Morocco to the Syrian Arc; Guiraud, 1986). Thisevent is synchronous with a rapid change in thedirection of the Mrican plate motion that is wellestablished by kinematic studies of the Atlanticopening (Klitgord and Schouten, 1986; Fairheadand Binks, 1991; Binks and Fairhead, 1992, thisvolume). This rapid change in the Mrican platemotion entailed the onset of its collisional inter-

action with the European plate (Olivet et aI.,1984; Ziegler, 1988). At the same time, differ-ences in spreading rates between the Central andSouth Atlantic spreading ridges were taken up inMrica along the broad WSW-ENE striking zoneof deformation described in the previous section.This zone extends from the Equatorial Atlanticacross Mrica, with deformation focused in therift zones, located along pre-existing zones oflithospheric weakness. Deformation in the WCARStook place between 85 and 80 Ma; the earlierdate is derived from syntectonic metamorphismin North Mrica and the Western Alps (Guiraud,1986).

-CRETACEOUS-CENOZOIC

The Santonian (

change in the intra.which led to distine

in many of the inl1992, this volume).

Cenomanian -earlydeepening

The occurrence (

(McKenzie, 1978) iNorth and Centralwith a Late Cen

transgression, is sigbroad basins, in execontaining up to 5Termit Basin of eavolume). Accordingof post-rift basin foin response to colithosphere and as'mally disturbed dUJThe isostatic resposubsidence of the e

magmatic activityceases altogether,West and Central,

Good examplesTenere Basin (Eas16 km of marine, (posits (Genik, 19~Basin (W Sudan),N1400E and N-Sthick continental1988; McHargue (Anza Basin (KeIJurassic to Earlytroughs (Mbede, 1~which are superirrto N1600E oriente

ocene and Early E<and Slevin, 1988).veloped in Nigeriathe Benue-Gongoand Gombe-Kerri

)

r1

e'-

nis

IIICldiste.r-

I.,:r-1dmnem.ticheof

.RSiersm

ud,

CRETACEOUS-CENOZOIC RIffING IN WEST AND CENTRAL AFRICA

The Santonian compressive event marked achange in the intra-plate stress regime of Africawhich led to distinct changes in fault geometriesin many of the intracontinental basins (Genik,1992,this volume).

Cenomanian -early Eocene sag phase of basindeepening

The occurrence of a distinct 'thermal sag stage'(McKenzie, 1978) in the evolution of the West,North and Central African rifted basins, coupledwith a Late Cenomanian-Campanian marinetransgression, is significant. During this sag phasebroadbasins, in excess of 150 km wide developed,containing up to 5 km of thick sediments in theTermit Basin of eastern Niger (Genik, 1992, thisvolume).According to the McKenzie (1978) modelof post-rift basin formation, these basins subsidedin response to cooling and contraction of thelithosphere and asthenosphere which were ther-mallydisturbed during the earlier rifting process.The isostatic response to such cooling is flexuralsubsidenceof the crust. In such a thermal settingmagmatic activity decreases rapidly and thenceases altogether, as evident in the basins ofWestand Central Africa.

Good examples of thermal sag basins are theTenere Basin (Eastern Niger) which contains 4 to6 km of marine, continental and lacustrine de-posits (Genik, 1992, this volume), the MugladBasin(W Sudan), which comprises a number ofN1400Eand N-S striking troughs filled with athick continental and lacustrine series (Schull,1988;McHargue et aI., 1992, this volume), theAnza Basin (Kenya), which overlays MiddleJurassic to Early Cretaceous NW-SE trendingtroughs(Mbede, 1987), the Northern Libya basinswhich are superimposed on Cretaceous N1400Eto N1600Eoriented rifts and contain thick Pale-oceneand Early Eocene marine series (Chatellierand Slevin, 1988). Additional flexural basins de-velopedin Nigeria, along the western margins ofthe Benue-Gongola-Bornu fold zone (Anambraand Gombe-Kerri Kerri basins).

231

Intra-Eocene compressional event

Within most of the basins of the WCARSan

intra-Eocene erosional or depositional hiatus isevident. Late Eocene and younger sediments, re-ferred to as 'Continental terminal', rest uncon-formably on the older series (Guiraud, 1986).These series may be folded, particularly in thevicinity of large fracture zones such as theGuinea-Nubia lineaments (Guiraud et aI., 1985)or the west Hoggar Pan-African suture (Bellionand Guiraud, 1988), and also along the northernmargin of Africa, from Morocco to Egypt. Theshortening direction is N1400E to NI600E. Thiscompressional event is related to a major stage inthe collision between the African and Europeanplates, known as the end Lutetian Pyrenean orPyrenean-Atlasic phase (ca. 40 Ma; Guiraud,1986). Similar to the Santonian compressionalevent, the Pyrenean event can also be correlatedwith a change in Atlantic flow-line geometries,reflecting a change in the opening direction ofthe Central Atlantic at about 40 Ma (see fig. 9a inFairhead and Binks, 1991).

Late Eocene-Oligocene rifting episode

Oligocene rifting, well known from East Africaand Western Europe is also evident in the Tenere,Sudan and Anza basins, where thick LateEocene-Oligocene lacustrine and fluviatile se-quences fill N1400E to N-S oriented troughs(Mbede, 1987; Schull, 1988; Genik, 1992, thisvolume; McHargue et aI., 1992, this volume).

At the same time, a resurgence of magmaticactivity can be observed in many areas, includingthe Central Atlantic margin, the Hoggar, Air andTibesti massifs, the Cameroon line and EastAfrica (Wilson and Guiraud, 1992, this volume).In West and Central Africa these areas of re-

newed volcanic activity are more or less indepen-dent of contemporaneous crustal extension. Mag-matism is alkaline in composition and the pri-mary magmas appear to have been derived froma basal lithosphere mantle source which may havebeen metasomatised during earlier, possibly hotspot related magmatic events (Wilson andGuiraud, 1992, this volume).

232

The synchronism and similar orientation ofthese different rift systems, as well as the resur-gence of magmatic activity within a very largedomain, including Central and East Africa, theRed Sea and Western Europe must be empha-sized. The geodynamic processes underlying thisevent, which has also been mentioned by Ziegler(1992a), are still not clear. In the South Tenere,Muglad and Anza basins, Late Eocene andOligocene resumption of crustal stretching re-flects reactivation of pre-existing rifts under arenewed intra-plate distensive stress regime.

Neogene magmatic activity

Magmatic activity in West, Central and NorthAfrica increased during the Neogene, with vol-canic fields concentrated in Pan-African mobilebelts. Although volcanic activity is in some in-stances closely associated with Cretaceous rifts(e.g. Benue Trough), many volcanic fields layoutside rifted Mesozoic basins and far away fromthe East African Rift system. Uplift of largedomal structures around centres of Neogene vol-canic activity suggests that these overlie localizedmantle upwellings. These domes are associatedwith only minor Neogene tensional faulting (e.g.Hoggar, Tibesti), though they appear to straddlethe intersection of Mesozoic and older fracture

zones. The chemical composition of the extrudedmagmas indicates that they were derived by par-tial melting from a zone at the base of the conti-nental lithosphere (Wilson and Guiraud, 1992,this volume). Geodynamic processes controllingthe Neogene surge in magmatic activity outsidethe East African rift system are still uncertain.General upwelling of the asthenosphere beneaththe African plate has been postulated by Pavoni(1991, 1992) and may be associated with its di-apiric rise into zones of previously attenuatedlithosphere.

Role of subsidence / uplift during rifting

Geological evidence clearly indicates that theCretaceous-Cenozoic rifts of West and CentralAfrica are associated with broad zones of subsi-

R. GUIRAUD ET AL.

dence. During the Cretaceous, these facilitatedthe penetration of marine transgressions deepinto the African continent (Genik, 1992, this vol-ume). In the Benue Trough, sediments can beseen to onlap basement highs flanking the riftzone. In many of the WCARSbasins, an earlier'syn-rift' tectonic subsidence phase can be sepa-rated from a later 'post-rift' thermal sag subsi-dence phase (see Avbovbo et aI., 1986). The sagbasins are centred over the syn-rift basins butoverstep their margins significantly.

Geophysical modelling of the WCARSrift struc-tures, using gravity data (Fairhead, 1986; Fair-head and Okereke, 1987; Fairhead and Green,1989), constrained by seismic refraction and re-flection data (Stuart et aI., 1985), provide roughestimates of the amounts of crustal thinning andextension. Results clearly demonstrate that thezone of lower crustal thinning is considerablywider than the zone of upper crustal extensionalfaulting and that the zone of mantle lithospherethinning extends over an even broader region.The isostatic response of this model explains theoverall subsidence of the rift shoulders during therifting phase of basin development and the flexu-ral sag of the rifted regions for periods exceeding50 Ma after termination of crustal extension.

Although these observations fit Rowley and Sa-hagian's (1986) depth-dependent pure-shearmodel, which represents a modification ofMcKenzie's (1978) uniform stretching model, ex-tremely well, there is still insufficient data toevaluate the relationship between the amount ofupper crustal extension and the degree of lowercrustal and lithospheric attenuation that may in-clude-apart from a mechanical-a thermalcomponent as well (Ziegler, 1992b).

Evidence for basin uplift is restricted to theSantonian and intra-Eocene phases of intraplatecompressional which affected the rift basins ofWCARS.The massive post-Cretaceous uplift ofbroad basement swells (e.g. Adamawa, Darfur,Tibesti, Hoggar and Air), which are associatedwith Cenozoic volcanic fields, may be related togeodynamic processes affecting the sub-litho-spheric mantle of Africa; these processes are notyet fully understood. Much of these uplifts maybe an isostatic response of the crust to the em-

-CRETACEOUS-CENOZOIC

placement of mantlf(Wilson and Guirau(

Conclusions

During the brealwana, a sequence ofcycles affected Af]Permo-Carboniferou

rifts, the Permo-Triaassic Central AtlantCretaceous South A

The Early Cretac(show strong spatialopening of the SOlOcean. In contrast,zoic rift episodescrustal separation lAfrica, reflect the (sional stress regimeresponse to differelspreading in the Coceans.

Evolution of the'

system was accomp~of magmatic activi~Cretaceous magmat]latter evolved in cO!the Gulf of Guineaable Mesozoic andoutside the rifted b~

The Early CretaSouth Atlantic and

the compressionalevents and the La

phase of Africa canin 'flowline' geomeChanges in the rateIrelated to changes i

Geophysical studspheric structure ofrifted basins indica]

sponse to large amoindividual rifts are I

major strike-slip fauAfrica provided we:bIe craton and are (

rf

o.f:rl-11

le

te

:>f

of

Lr,~dto0-.ot

aym-

CRETACEOUS-CENOZOICRIFfING IN WEST AND CENTRAL AFRICA

placement of mantle-derived magmas at its base(Wilson and Guiraud, 1992, this volume).

Conclusions

During the break up of Pangea and Gond-wana, a sequence of more or less discrete riftingcycles affected Africa, commencing with thePermo-Carboniferous development of the Karoorifts, the Permo-Triassic Tethys rifts, the Mid-Tri-assic Central Atlantic rift system and the EarlyCretaceous South Atlantic rift system.

The Early Cretaceous rifting episodes of Africashow strong spatial and temporal links with theopening of the South and Equatorial AtlanticOcean. In contrast, end Cretaceous-Early Ceno-zoic rift episodes of Africa, which post-datecrustal separation between South America andAfrica, reflect the development of a new exten-sional stress regime that probably developed inresponse to differences in the rates of seafloorspreading in the Central and South Atlanticoceans.

Evolution of the West and Central African rift

systemwas accompanied by a generally low levelof magmatic activity. An exception is the EarlyCretaceous magmatism of the Benue Trough; thelatter evolved in conjunction with the opening ofthe Gulf of Guinea. However, there is consider-able Mesozoic and Cenozoic magmatic activityoutside the rifted basins of Africa.

The Early Cretaceous rifting phases of theSouth Atlantic and African domains, as well asthe compressional Santonian and intra-Eoceneevents and the Late Eocene-Oligocene riftingphaseof Africa can all be correlated with changesin 'flowline' geometry in the Central Atlantic.Changes in the rates of seafloor spreading can berelated to changes in plate interactions.

Geophysical studies of the crustal and litho-sphericstructure of the West and Central Africanrifted basins indicate that these subsided in re-sponseto large amounts of crustal extension. Theindividualrifts are linked to oceanic domains via

majorstrike-slip fault zones. The rifted basins ofAfrica provided weak zones in an otherwise sta-blecraton and are extremely sensitive to changes

233

in the regional stress regime (inversion, tensionalreactivation).

The depth-dependent, pure-shear lithosphericstretching model of Rowley and Sahagian (1986)appears to account for most of the observationsin the West and Central African rift system.However, during the initial stages of rifting, activ-ity of a deep mantle hot spot (St. Helena) in theEquatorial region presumably played a major rolein weakening the continental lithosphere by initi-ating its partial melting.

Acknowledgments

The authors acknowledge P. Ziegler and J.Dercourt for their constructive reviews and com-ments.

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Compariso:z

a Ros,

ABSTRACT

Rosendahl, B.R., Izones from an

History Studie!

Northwest-souEastern Branches.

and ' pelagic' sedinalong the Eastern

to be an integral

differences, we haMalawi) and one I

The Tanganyik

zones which gene]

alternate basinal ~with some accomrrand the section is rzones.

The Rukwa Rif

in central Lake Tal

tion zone developsLivingstone, Rukwsub-parallels the ir

pre-Cenozoic secti,thickness.

The Turkana R]

by numerous, NW-

zone is oblique tosub-perpendicular.

Igneous activity is u

is dominated by fluThe structural I

pre-rift anistropiespull-apart basins. A

high tendency to a

between the Tangalfrom systematic difdepths of seismic ir

Correspondence to: B.RUSA.

0040-1951/92/$05.00«