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Tectonophysics 444

A top to bottom lithospheric study of Africa and Arabia

Michael E. Pasyanos a,⁎, Andrew A. Nyblade b

a Earth Science Division, Lawrence Livermore National Laboratory, Livermore, California, USA 94551b Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania, USA 16802

Received 8 November 2006; received in revised form 14 June 2007; accepted 11 July 2007Available online 22 August 2007

Abstract

We study the lithospheric structure of Africa, Arabia and adjacent oceanic regions with fundamental-mode surface waves over abroad period range. Including group velocities with periods shorter than 35 s allows us to examine shallower features than previousstudies of the whole continent. In the process, we have developed a crustal thickness map of Africa. Main features include crustalthickness increases under the West African, Congo, and Kalahari cratons. We find crustal thinning under Mesozoic and Cenozoicrifts, including the Benue Trough, Red Sea, and East, Central, and West African rift systems, along with less abrupt crustalthickness changes at passive continental margins. We also find crustal thickness differences in North Africa between the WestAfrican Craton and East Saharan Shield. Crustal shear wave velocities are generally faster in oceanic regions and cratons, andslower in more recent crust and in active and remnant orogenic regions. Deeper structure, related to the thickness of cratons andmodern rifting, is generally consistent with previous work. Under cratons we find thick lithosphere and fast upper mantle velocities,while under rifts we find thinned lithosphere and slower upper mantle velocities. However, we also find the lack of a thick cratonickeel beneath the central portion of the Congo Craton. There are no consistent effects in areas classified as hotspots, indicating thatthere seem to be numerous origins for these features. Finally, it appears that the African Superswell, which is responsible for highelevation and uplift over large portions of Africa, has had a significantly different impact (as indicated by features such astemperature, time of influence, etc.) in the north and the south. This is consistent with episodic activity at shallow depths, which iswell-expressed in northeastern Africa and Arabia today.© 2007 Elsevier B.V. All rights reserved.

Keywords: Crust; Moho; Upper mantle; Group velocity; Lithosphere; Congo Craton

1. Introduction

In general, the seismic structure of Africa and Arabiaremains poorly studied, owing largely to the sparsedistribution of seismic stations and to the existence oflarge aseismic areas on the African and Arabian plates.

⁎ Corresponding author. Lawrence Livermore National Laboratory,P.O. Box 808, L-205, Livermore, CA 94551-0808, USA. Tel.: +1 925423 6835; fax: +1 925 423 4077.

E-mail address: pasyanos1@llnl.gov (M.E. Pasyanos).

0040-1951/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.tecto.2007.07.008

Studies using temporary deployments of seismometershave been carried out in a number of geographiclocalities, for example around the Cenozoic rifts andplateaus in Ethiopia, Kenya and Tanzania, and acrossthe shields in Arabia and southern Africa, providingdetailed seismic images of the crust and upper mantle.However, for many parts of Africa and Arabia little isknown about the seismic structure of the lithosphereand, for these regions, what little we know aboutstructure comes primarily from global and continental-scale tomography studies.

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The earliest tomography studies of mantle structure(Dziewonski, 1984) revealed thick, seismically fast“keels” under cratonic regions of Africa, and slow struc-ture elsewhere. More recent studies have provided refinedimages of the cratonic keels (e.g. Ritsema et al., 1999;Ritsema and van Heijst, 2000; King and Ritsema, 2000;Grand, 2002; Priestley et al., 2006), as well as of anom-alously slow upper mantle structure under eastern Africaand parts of the Arabian Peninsula (Debayle et al., 2001;

Simmons et al., 2007). Many of these studies have reliedon surface wave data to provide spatial coverage becauseof the sparse seismic station coverage mentioned above,and have variable resolution depending mainly on thedensity of crossing ray paths and the frequency content ofthe surface waves used. For crustal structure, continental-scale models for Africa have been provided as a part ofglobalmodels (e.g. CRUST5.1;Mooney et al., 1998). Thisis in contrast to crustal thickness maps developed for

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Europe (Meissner et al., 1987), Asia (Kunin et al., 1987),and North America (Das and Nolet, 1998).

In this study, we have attempted to improve uponprevious continental-scale models of crust and uppermantle structure by developing a new velocity modelusing group velocity measurements of fundamental-model Rayleigh and Love waves between periods of 7and 100 s. Our model differs from previously publishedmodels in several ways, including the density of raycoverage in some localities, the period range of thegroup velocities, and the modeling algorithm. Impor-tantly, our model is the first continental-scale modeldeveloped using dispersion measurements at periodsless than 40 s, measurements that provide good con-straints on crust and uppermost mantle structure.

Because our data set does not contain dispersionmeasurements at periods greater than 100 s, our modelprovides limited resolution of sub-lithospheric mantlestructure. Consequently, we focus the discussion of ourmodel in this paper on structure at crustal and lithosphericmantle depths. Our model is broadly consistent withresults of previous studies inmanyways, but it also showssome new features. After describing the new model andcomparing it with previous continental-scale models, weexamine in more detail the structure of the lithospherickeels under the Archean cratons.

2. Background

In this section, we provide a brief review of the majortectonic features of Africa and Arabia that are relevant tothe discussion of our new model, in particular, Archeanblocks, Mesozoic and Cenozoic rifts, and major sedi-mentary basins (Fig. 1). For comprehensive reviews ofAfrican geology and tectonics we refer the reader toCahen et al. (1984) and Burke (1996). The tectonics ofAfrica (Fig. 1) are unusual in the sense that, in general,the structure of the continent is very old and that tectonicactivity more recent than the Paleozoic occurs only inlimited regions. In fact, of all the continents, Africa has

Fig. 1. Tectonic map of Africa. Platform and shield areas are indicated by the singthickness contours (graylines). Plate boundaries are indicated by the thick blacklinJunction,Alp=Alps,AP=ArabianPlatform,Ar=ArabianShield,AM=AtlasMtsSea, Can = Canary Is., Car = Carpathians, Cas = Caspian Sea, C = Caucasus, CAComores I., Con=CongoBasin, CC=CongoCraton,D=Darfour,DKB=DamarE.Mediterranean, ES=E.SharanShield, Eth =Ethiopian Plateau, Eto =EtoshaBasIon = Ionian Basin, Ind = Indian Ocean, IP = Iranian Plateau, IR = Indian Ridge, KRift, Li = Limpopo Belt, LV = Lake Victoria, Mad = Madagascar, MC = Mt.CamNamaqua-Natal Belt, Nu=Nubian Shield, Per = PersianGulf, R =Reunion, RS=RBasin, Tan=TanzaniaCraton, Ta=TaoudenniBasin, Tib=Tibesti, Tri =Tristan daV = Vema Seamount, Wal =Walvis Ridge, WAC =W.African Craton, WAR =WCraton, Zim = Zimbabwe Craton.

the highest percentage of exposed Precambrian crust(Goodwin, 1996).

2.1. Northern and Central Africa

The Precambrian tectonic history of northern andcentral Africa, very briefly, includes major craton-forming events in the Archean, followed by a complexhistory of mountain building and rifting that culminatedwith the Pan-African Orogeny during the formation ofGondwanaland. Archean crust is exposed in theMan andReguibat blocks within the West African Shield, withinthe Congo Shield in Gabon, Cameroon, and the CentralAfrican Republic, and in the Uweinat block in southernEgypt and northern Sudan. Cenozoic tectonic structureof northern Africa is dominated by convergence withEurasia. In northwest Africa, this expresses itself in theAtlas Mts. orogeny. In the Western Mediterranean, theAfrican Plate subducts along the Calabrian, Hellenic,and Cyprean arcs. North Africa is also dotted with sev-eral hotspots including the Hoggar in southern Algeria,Tibesti in northern Chad, and Darfour in western Sudan.Offshore, hotspots are found associated with the Canaryand Cape Verde Islands. In addition, Mesozoic rift basinsformed across parts of north-central and central Africa(the Central African Rift System) during the opening ofthe Atlantic in the Jurassic.

2.2. Eastern Africa and Arabia

Eastern Africa is where much of the Cenozoic tec-tonic activity is found within Africa. The Red Sea Rifthas split the Nubian and Arabian Shields; the Gulf ofAden separates the Arabian and Somali Plates; and theEast African Rift is starting to separate the Somali Platefrom the rest of Africa. These three rift zones form atriple junction in the Afar. This area also sits at thenorthern edge of the African Superswell, an upliftedregion spanning much of eastern and southern Africacharacterized by high residual topography (Nyblade and

le and double hatched lines. Deep basins are noted by the 2.5 km sedimentes. The African Superswell is outlined by the dashed lines. Af = Afar Triple,Asc=Ascension I.,Atl =AtlanticOcean,BT=BenueTrough,BS=BlackB = C.African Belt, CAR = C.African Rift, CFB = Cape Fold Belt, Com =-KatangaBelt, DSR=DeadSeaRift, E=Etna, EAR=E.AfricanRift, EM=in,GA=Gulf ofAden,GG=Gulf ofGuinea,Ho=Hoggar, Ill = Illizi Basin,B =Kheiss Belt, KV =Kaapvaal Craton, Ka =Kalahari Basin, Ke =Kenyaeroon, M = Mesopotamian Foredeep, Moz = Mozambique Basin, NN =ed SeaRift, Sey = Seychelles, Si = Sirt Basin, SH=St.Helena, So =SomaliCunha, TP=TurkishPlateau,Ukr =UkranianShield,Uw=UweinatBlock,.African Rift, WM =W.Mediterranean, Zag = Zagros Mts., Zam = Zambia

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Robinson, 1994). Rifting to the south continues alongthe Eastern and Western Branches of the East AfricanRift System into southern Africa.

The Arabian Peninsula is separating fromAfrica alongthe Red Sea and Gulf of Aden rifts. Northwest movementof the Arabian Plate is expressed as convergence in theZagros orogeny, elevation of the Turkish and IranianPlateaus, and left-lateral movement along the Dead SeaRift andEastAnatolian Fault Zone. The large sedimentarybasins of the Persian Gulf and Mesopotamian Foredeeplie at the base of the Zagros.

2.3. Southern Africa

In southern Africa, two major Archean blocks arefound, one in the southern Democratic Republic ofCongo (D.R.C.) and extending into northern Angola,and the Kalahari Craton. The Kalahari Craton occupiesmuch of southern Africa and is comprised of severalmajor provinces. Trending from north to south, these arethe Zimbabwe Craton, the Limpopo Belt, and theKaapvaal Craton. The Archean blocks in the D.R.C. andAngola form the southern extent of the Congo Craton.Many Proterozoic mobile belts, which formed duringmultiple orogenic events, surround the Congo andKalahari Cratons. At the southern tip of the continent, ayounger (Proterozoic–Paleozoic) orogenic eventformed the Cape Fold Belt as part of the Gondwanideorogen with proto-South America and proto-Antarctica.

The Phanerozoic history of southern Africa is markedby the development of the Karoo rifts and basins, manyof which are associated with the breakup of Gondwana-land. In the Mesozoic and Cenozoic, several of thehotspots formed offshore southern Africa, including theSt. Helena and Ascension Island hotspots to the west,and the Comores and Reunion Island hotspots to the east.Starting during the Jurassic, Madagascar along withIndia, Antarctica and Australia, split off from the SomaliCoast and traveled south before first separating fromAntarctica and Australia (∼130 Ma), and then India(∼90 Ma) during the Cretaceous.

2.4. Rifts and basins

The Mesozoic and Cenozoic rifts within Africa havelargely formed in Proterozoic mobile belts surroundingthe Archean blocks. For example, the Benue Trough andthe rest of the Central African Rift System formed to thenorth of Archean blocks in Cameroon and the CentralAfrican Republic that are part of the Congo Craton. TheMesozoic rifts in north-central Africa formed to the eastof the West African Shield, the two branches of the East

African Rift System developed around the margins ofthe Tanzania Craton, and many of the Karoo rifts insouthern Africa formed around the eastern, northern,and western sides of the Zimbabwe Craton.

Throughout Africa, there are a large number of sedi-mentary basins. Using the system of Clifford (1986), thebasins can be classified as interior basins (large basinswithin stable continental shield) like the Taoudenni andEtosha Basins, marginal sag basins (basins located at thecontinental edge) such as the Somali and MozambiqueBasins, rift basins like the Benue Trough and East Africanand Sirt Basins, and composite and complex basins, suchas the Cuvette Centrale and Illizi Basins (Schlüter, 2006).The Cuvette Centrale is of particular interest, because itcovers the interior ofwhat is often referred to as the greaterCongo Craton. Under the Cuvette Centrale, the Archeanblocks in the Central African Republic, Cameroon, andGabon to the north and west are thought to connect withthe Archean blocks in the southern D.R.C. and northernAngola to form perhaps the largest Archean craton inAfrica (e.g. Kampunzu and Popoff, 1991). However, aNeo-Proterozoic rift has been imaged under the CuvetteCentral (Daly et al., 1991), suggesting that there might notbe any Archean crust under the sedimentary cover.

2.5. African Superswell

Another significant influence for the continent is theAfrican Superswell (Nyblade and Robinson, 1994). Thislarge feature, as noted previously, consists of elevatedportions of the African continent along a swath runningfrom south of Africa to the middle of the Arabian Penin-sula, and whose origin may be connected to deep lowermantle processes (Ritsema et al., 1999). Besides the highdynamic topography, visible features of the superswellinclude major volcanic episodes, elevated heat flow inmobile belts of southern Africa (Nyblade and Robinson,1994), and rifting and volcanism in eastern Africa andthe Afar region. Global tomography models (e.g. van derHilst et al., 1997; Grand, 2002; Simmons et al., 2007)have persistently found low velocities in the lowermantle beneath southern Africa, and both global andregional tomography models show low velocities in theupper mantle beneath eastern Africa and the ArabianShield (e.g., Achauer et al., 1994; Slack et al., 1994;Ritsema et al., 1999; Bastow et al., 2005; Benoit et al.,2006b; Park and Nyblade, 2006; Park et al., 2007).

3. Data and measurements

Our main source for station coverage of the regioncomes from stations in theGlobal SeismicNetwork (GSN),

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which provides coverage both in Africa andArabia, as wellas offshore. Other contributing FDSN stations includethose from the GEOSCOPE network, the MEDNETnetwork, and other U.S. networks. Coverage of severalregions has been significantly improved with PASSCALdeployments in Tanzania (Nyblade et al., 1996), SaudiArabia (Vernon et al., 1996), Ethiopia and Kenya (Nybladeand Langston, 2002), South Africa (Nguuri et al., 2001),and is continuing with a deployment in Cameroon (Tibiet al., 2005). We have included data from the one openstation (CM18) in the Cameroon deployment. In the case ofother PASSCAL deployments (i.e. eastern Turkey, Tanza-nia, South Africa) sometimes only subsets of the completestation list were used. This is because, with surface waves,resources are best spent making measurements at widelydispersed stations rather than at closely spaced stations in asmall region. Data from several stations in the MIDSEAdeployment (van der Lee et al., 2001) improved coverage inNorth Africa. In some instances, particularly in the MiddleEast, we made use of data from local seismic networksincluding those in Jordan, Kuwait, Iraq, Libya, and theUnited Arab Emirates. A map showing stations where wehave made at least 10 Rayleigh wave dispersion measure-ments at 50 s period is shown in Fig. 2a.

Fig. 2. a) Station map for Africa and Arabia showing stations (marked by reb) Path map of surface wave dispersion measurements, shown for 50 s Rayleiand blue lines indicate paths.

Using these stations, we have made surface wavedispersion measurements for tens of thousands of pathsacross Africa and Arabia. The measurements employedhere represent a five-fold increase in the number of pathsthan an earlier study which covered North Africa andportions of the Middle East (Pasyanos and Walter, 2002).Surface wave path coverage is shown in Fig. 2b.Seismicity along the mid-Atlantic rift, Indian ridge, EastAfrican Rift, Red Sea, Gulf of Aden, and along the Tethyscollision zone provides the bulk of events for dispersionanalysis. Overall, we have excellent path coverage of theregion. Clearly, however, we have better coverage (greaterpath density and more crossing paths) in North Africa andArabia than for southern Africa and the south Atlantic.Path coverage is generally poorer for Love waves thanRayleigh waves, and the number of paths in the regiondrops off significantly at the shortest periods (≤10 s).

Surface wave group velocities for the region aredetermined using seismic tomography. We use the latestupdates of the group velocity maps of Pasyanos (2005).The inversion uses a 1° by 1° equal area grid, and aconjugate gradient method is used to solve for lateralvariations in group velocity. A variable smoothnesstechnique is used to improve the resolution of the model.

d triangles) which have been used to make dispersion measurements.gh waves. Yellow circles indicate events, red triangles indicate stations,

Fig. 3. Group velocities (in km/s) for 20 s and 60 s Rayleigh waves. Slow velocities are indicated in red and fast velocities in blue.

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Model resolution is also significantly improved over the2002 study that covered North Africa. Resolutions inwell-sampled regions at intermediate periods (20–40 s)approach 1°, while the resolution of periods sensitive tomantle depths can be as good as 2°. The maps providegroup velocities of Love and Rayleigh waves across theregion for periods from 7 to 100 s.

Surface wave tomography maps for two periods areshown in Fig. 3. The first is for 20 s Rayleigh waves(Fig. 3a), which are generally sensitive to shallowstructure. Fast group velocities at this period correspondto thin, oceanic crust, not only in the Atlantic and IndianOceans, but also in the Western Mediterranean Sea, RedSea, and Gulf of Aden. Slow group velocities corre-spond to large sedimentary basins as illustrated with theEastern Mediterranean, Black Sea, North and SouthCaspian Basins, and Persian Gulf. Within Africa itself,slow velocities are associated with the Illizi basin inAlgeria and Tunis, the Taoudenni Basin in Mauritaniaand Mali, the basins in Somalia, and the CuvetteCentrale (Central Basin) of the Congo Basin. Offshore

basins do not appear as slow features on this mapbecause of the counter-effect of thin underlying crust.

At 60 s, the Rayleigh waves are sensitive to deep crustand upper mantle structure. In Fig. 3b, slow velocities areassociated with either slow upper mantle velocities orthick crust (i.e. Zagros Mts.). Plate boundaries, inparticular, stand out and include both divergent bound-aries, such as the mid-Atlantic ridge, Indian ridge, RedSea rift, and Benue Trough, and convergent boundaries,such as the Tethys Belt. Fast velocities stand out undercontinental cratons and old oceanic crust. In East Africa,the two branches of the East African Rift System can beseen encircling the fast Tanzania Craton.

We assess the reliability of the surface wave groupvelocities with uncertainty maps. Uncertainties are cal-culated using a bootstrapping approach described inPasyanos et al. (2001) where multiple inversions areperformed on several realizations of the data and arandomized starting model. The uncertainty map for 20 sRayleigh waves is shown in Fig. 4. As expected from pathcoverage, group velocities across southern Europe, the

Fig. 4. Uncertainties (in km/s) of group velocities for 20 s Rayleighwaves. Low uncertainties are indicated in red and high uncertainties inblue. Uncertainties are generally low in southern Europe, the MiddleEast, and East Africa, and higher in oceanic regions.

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Middle East, and Eastern Africa are very reliablydetermined. Uncertainties are slightly higher, but still low,in the Mediterranean Sea, within most of continentalAfrica, and along the mid-Atlantic and mid-Indian rifts.Results are generally less reliable in the ocean away fromthe rifts and in Madagascar (where measurements from anewGSNstation are not yet included), and the uncertaintiesare high in regions to the south and southwest of Africa.

The resolution of the surface waves is plainly sen-sitive to path coverage. Resolution will also be limitedby the wavelengths of the surface waves. At short pe-riods (b30 s), the wavelengths are less than 1°, so weshould have high resolution for shallow structure (e.g.sediments, upper crust), at least where we have fairlygood path coverage. For intermediate periods (30–60 s),the wavelengths range from 1° to 2° and we will haveresolution on this order for deeper structure like crustalthickness and upper mantle velocity. Wavelengths arelonger than 2° at long periods (N60 s), indicating that

these are probably the best resolutions we can hope toachieve for deeper mantle structure.

4. Methodology

We use the group velocities determined from seismictomography to estimate crustal structure. At each point inour model, we collect the group velocities (and associateduncertainties) of both Love and Rayleigh waves forperiods from 7 to 100 s.We then use a modification of thePasyanos and Walter (2002) grid search to invert for thebest 1-D isotropic velocity model that fits the scaleddispersion data (dispersion scaled by the uncertainty) ateach point. The final model is the one that minimizes themisfit of the scaled group velocities. Like the previousgrid search methodology, we solve for crustal thickness(initial range of 5–45 km), average velocity of thecrystalline crust (range of 5.6–7.0 km/s), and the velocityof the uppermost mantle (range of 7.6–8.2 km/s). Allparameters are allowed to expand beyond their initialrange in the cascading grid search. The sediment profile isfixed based on the sediment model of Laske and Masters(1997). The modification to the search that we make hereis the introduction of a lithospheric lid layer overlying anasthenospheric layer with a high Poisson's ratio (σ=0.29)and accordingly lower shear-wave velocity. This is incontrast to the Poisson's ratio of 0.26 that is used for thecrust and mantle lid. Having the asthenospheric layerallows us to fit the long period surface waves that we areunable to fit with a single layer upper mantle. In the gridsearch, we solve for the thickness of this layer. Startingvalues for lithospheric thickness are derived from thegroup velocity–lithospheric thickness relationship (Eq. 5)developed in Pasyanos (2005). Layered mantle structurebelow the lithosphere is represented as perturbations tothe ak135model (Kennett et al., 1995).We emphasize thatthe primary objective of the modeling is not to determinethe exact layered structure of themantle (which is difficultto derive from surface waves), but rather the overallthickness of high-velocity lithospheric material requiredto fit the long period group velocities.

Uncertainties are calculated by examining the range ofmodels that can fit the data within the uncertainties of thegroup velocity points. Where uncertainties on the groupvelocity data are large, a wide range of models can fit thedata, whereas when the uncertainties are small, only arelatively narrow range ofmodels can. In regionswith goodpath coverage (discussed earlier) the uncertainties oncrustal thickness are about 5 km and uncertainties onvelocities are about 0.10 km/s (Pasyanos andWalter, 2002).Uncertainties are significantly higher in regions with poorpath coverage.

Fig. 5. Examples of grid search for crustal structure from a) West Africa and b) East Africa. In each figure, the panels to the left show the fit of themodel (solid lines indicating Rayleigh waves and dashed lines indicating Love waves) to the tomographic models (symbols; triangles for Rayleighand circles for Love), while the panels to the right indicate the resulting 1-D model.

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Fig. 5 shows examples of the grid search for seismicstructure at two locations in North Africa. In each figure,the panel to the left shows the fit of dispersion predictedby the model to the group velocities derived from seis-

mic tomography, while the panel to the right shows theresulting model. Uncertainties in the group velocities arehighly dependent on period and wavetype. The uncer-tainties are higher for Love waves and at the longest and

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shortest periods, primarily due to poorer path coverage.As illustrated in Fig. 5a for a profile in West Africa, evensimple lithospheric models (shown in the right panel)are able to fit the surface waves, although many ofthe details of the profile such as the sharpness of theMoho discontinuity cannot be resolved by the surfacewaves. In this case, we find the surface waves fit by avelocity profile of 35 km thick crust with thin sediments(1.5 km), very fast upper mantle velocities (4.67 km/s),and a thick lid (which extends in this case down to280 km).

The velocity model that is found for a region in EastAfrica looks very different (Fig. 5b). The dispersioncurves that we see for this region are slower than thosefor West Africa, particularly at the longest periodswhere we see a reduction in the group velocities of theRayleigh waves at periods longer than about 50 s. Onceagain, though, we are able to fit the dispersion data witha simple model, although there appears to be somemisfit between the input sediment profile and the shortperiod Love waves. Here, we see a thin (25 km thick)crust with 4 km sediments and slower upper mantlevelocities. In addition, we find that a thinner lid thick-ness is necessary in order to fit the observed long periodsurface wave data. It also appears that some transverseisotropy with vSHNvSV (not modeled here) might benecessary in order to simultaneously fit both the Loveand Rayleigh wave dispersion.

5. Results

By assembling the individual 1-D inversions, we havecreated a 3-D model for Africa and Arabia and use theresults to map regional lithospheric structure. Fig. 6ashows a crustal thickness map for the region. The mostoutstanding features are the significantly thinner crust inoceanic regions, not only the Atlantic and Indian Oceans,but also the Red Sea and Mediterranean Basins (i.e.Western Mediterranean and Ionian Basin). More inter-esting are the variations within oceanic regions and theAfrican continent. In the oceans, we find several in-stances of increases in crustal thickness associated withislands such as Ascension, Tristan du Cunha, andReunion, as well as increases in and around Madagascarand the Mozambique Channel. Within Africa, we findthicker crust (N35 km) in the West African, Congo, andKalahari cratons, in contrast to moderate thicknesses inthe East Saharan Shield (25–35 km). This is primarilycontrolled by intermediate period (∼30 s) group ve-locities which indicate faster group velocities in the EastSaharan Shield than nearby cratons. In general, we finda relatively gradual increase in crustal thickness from

the oceanic edges of the continent to the continentalinterior. Another notable feature is the crustal thinning(b25 km) in the Benue Trough and its northern andeastern extensions.

To the northeast, the model shows a gradual increasealong the Arabian Peninsula from the Red Sea (b10 kmthickness) across the shield and platform to the ZagrosMts. and Iranian Platform (N45 km). It also shows thickcrust in the Turkish Plateau and Eastern European Plat-form, but thin crust in the extended crust of WesternEurope. Along the subduction zone in the eastern Medi-terranean, the slow shear wave velocities due to thepresence of volatiles and partial melting in the mantlewedge were being interpreted as crustal velocities andthe crustal thickness was vastly overestimated. In thecase of several nodes from this region, we needed toalter the range of crustal thickness to be less than 45 km.

Another interesting indicator of regional tectonics isthe map of average crustal shear velocity (Fig. 6b). Herewe typically find faster crust in oceanic regions (Atlantic,Mediterranean, Red Sea, etc.) at least in the well-resolvedoceanic regions at the edges of the continents. In general,we see a correlation between crustal velocities and the ageof the latest thermo-tectonic event, with more recentlyaffected crust (like the extended crust in western Europe)having slower velocities than less-recently affected crust.In particular, we can see slower crust in Cenozoic oroge-nic regions (Atlas Mts., Zagros Mts., Turkish–IranianPlateau), and Madagascar. It also appears that the BenueTrough has slow crustal velocities. In contrast, cratonshave high average crustal velocities. In oceanic regionsfarther away from the continents, we don't see any co-herent pattern, probably due to the poorer path coverageat short periods necessary to robustly estimate thisparameter.

Shear wave velocities in the upper mantle directlybeneath the Moho (Fig. 6c) show dramatic contrastsbetween the cratons (West African, Congo, Ukraine,Kalahari Craton) that are associated with fast velocities,and the rifts (Dead Sea Rift, Red Sea, Gulf of Aden, EastAfrican, mid-Atlantic) associated with slow velocities. Incomparison to these very slow regions, the Benue Troughand its northern and eastern extensions are onlymarginally slow. Upper mantle velocities are quite slownear Madagascar and below Reunion Island and theComores. Another interesting feature are some of thelinear marks in Arabia and the south Atlantic. In bothcases, these correspond to known volcanic features suchas Holocene to recent Arabian volcanism and the Tristanhotspot track. In southern Africa, we find fast uppermantle velocities in the Kalahari Craton, but slowervelocities at the southern edge of the continent. There does

Fig. 6. Crustal and upper mantle structure of Africa. a) Crustal thickness map (in km), b) Average crustal shear velocity map (in km/s), c) Uppermantle shear velocity (Sn) map (in km/s), and d) Input sedimentary thickness map (in km).

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not seem to be any systematic difference in the uppermostmantle velocities of Archean and post-Archean terranes.

Among areas designated as hotspots, we don't findany consistent changes in seismic structure in ourmodel. In regions where the hotspots are associated withplate boundaries (such as Afar and Ascension) theeffects appear to be strong. In a continental setting, theAfar hotspot results in thinned crust, but in an oceanicsetting, the Ascension hotspot has thickened crust. Inboth instances, we see very slow upper mantle velocitiesand thinned lithosphere. The Mt. Cameroon hotspotseems to have a lot in common (thinned crust, slowupper mantle velocities, and thinner lithosphere) withthe continental Afar hotspot. In hotspots away fromplate boundaries (Hoggar, Tibesti, Darfour, Canary,Reunion, Cape Verde), the effect on upper mantle struc-ture appears to be weaker and less consistent. Undermost, we find slow Sn velocities, but this effect is muchstronger under some hotspots than others. In someregions, the depth extent of faster lid velocities appearsto be thinner; in other regions the opposite is seen.

In Arabia, we find thin lithosphere in the extensionzones of the Red Sea and Gulf of Aden. The thinnestlithosphere and slowest mantle velocities are found nearthe Afar Triple Junction. Farther to the northeast, we alsofind thin lithosphere under the Turkish–Iranian Plateau.The Arabian Platform has lithospheric thickness inexcess of 150 km, but thins toward the Zagros Mts. Forcompleteness, we have also included a plot (Fig. 6d) ofthe input sediment model of Laske and Masters (1997).

6. Comparisons

In this section, we compare our model to othermodels in the open literature. We focus mainly oncontinental-scale models, but also make some limitedcomparisons with models from regional or local studies.We begin with the crust, and then move to thelithospheric mantle. Sublithospheric mantle structure isnot discussed, because, as noted previously, our modeldoes not have sufficient resolution to image below thelithosphere. For this study, we define the lithosphere asthe seismic lid. Given the simple layer parameterizationof our model, the velocity of the lid can vary throughoutthe model, but everywhere in the model beneath the lidis a lower velocity region that we associate with theasthenosphere.

6.1. Crustal structure

Crustal structure in our model overall is consistentwith a number of profiles averaged by crustal types

(Fig. 2 in Rudnick and Fountain, 1995; Fig. 2 inMooney et al., 1998; Fig. 12 in Pasyanos and Walter,2002) which show fast crustal velocities in oceanicregions and slow crustal velocities in orogenic zones(including Paleozoic orogens) and extensional regions(but not active rifts). We compare our crustal thicknessmap to the velocities from two a priori geophysicalmodels: CRUST2.0 (Bassin et al., 2000) and 3SMAC(Nataf and Ricard, 1996). Like the CRUST5.1 model(Mooney et al., 1998), CRUST2.0 is based on regionalstudies where available, and on geophysical inferencewhere they are not. Since the regional studies are notavailable for large portions of Africa, the crustalthickness in the CRUST2.0 model in many parts ofAfrica is inferred. We find several major differencesbetween the crustal thickness estimates of CRUST2.0and our model. First, while the crustal thickness in theWest African, Congo, and Kalahari Cratons is thick inboth models, there is a significant difference in the EastSaharan Shield. Our model has crustal thicknessesranging from 25–35 km for this region, whileCRUST2.0 shows thicknesses between 35 and 45 km,leading to significant differences in the southern Sahara(Niger, Chad, Sudan).

Farther to the south, our model shows crustal thinningassociatedwith the Benue Trough and continuing throughsouthern Chad and Sudan into Kenya. While theCRUST2.0 model does not have any crustal thinningassociated with these features, a study of crustal structurein southern Sudan and northern Kenya using surfacewaves also finds thinned crust (Benoit et al., 2006a).Another region showing a large difference is the island ofMadagascar. While CRUST2.0 has a crustal thickness ofabout 40 km for this region, our model finds crustalthicknesses of 25–35 km. Reviewing a number of studies,de Wit (2003) suggests a continental crust of 30–42 kmbelow the center ofMadagascar that rapidly thins to about25–27 km along the east coast, along with a lithosphericthickness ranging from 70–130 km.

As mentioned earlier, there seems to be a contrast inthe crustal thickness variations along continentalmargins. While CRUST2.0 has abrupt boundaries, thecrustal thickness model presented here has more gradualcrustal thickness variations between oceanic crust andthe continental margins. This finding is well-illustratedin Libya, Morocco, and Angola.

A comparison of our crustal thickness map to the3SMAC model yields a better match. Like our study, the3SMAC Moho model does not have sharp boundariesright at the continental margins. The model finds crustalthickness increases inNorth Africa and the CongoCraton,but doesn't have any thickening associated with the

Fig. 7. Horizontal cross sections showing the shear-wave velocity variation (in %) from ak135 at a) 100, b) 150, c) 200, and d) 250 km depth. Thefigures correspond well to the upper mantle values from Ritsema and van Heijst (2000).

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39M.E. Pasyanos, A.A. Nyblade / Tectonophysics 444 (2007) 27–44

cratons in southern Africa. There is also some thinningassociated with the Benue Trough, although the thinningdoes not extend into the West and Central African riftsystems. Finally, the crustal thickness for Madagascar in3SMAC (25–35 km) is more in line with our study thanCRUST2.0.

Instead of comparing the crustal structure of ourmodel to the numerous high-resolution models that existfor several regions in our study area (i.e. West CentralAfrica, Tanzania Craton, Ethiopia/Kenya), we have se-lected one of the regions (southern Africa) and comparethe results for this region. In southern Africa, Nguuriet al. (2001) estimated crustal thickness from the south-ern Africa seismic experiment using receiver functions.The authors find moderate thickness crust (∼35–40 km)along the undisturbed Kaapvaal Craton and ZimbabweCraton interspersed with thicker crust (∼45–55 km)along the Bushveld Complex and the Limpopo andNamaqua-Natal mobile belts. In our study, we findsimilar moderate thicknesses in the cratonic nuclei,along with thick crust under the Bushveld Complex andsurrounding Okwa Belt, but find very different crustalthickness estimates in the Namaqua-Natal Mobile Belt.This discrepancy can perhaps be explained by the factthat the crustal thicknesses in the Nguuri et al. (2001)model were constructed by using a cratonic velocitystructure which, the authors admit, might not be validfor off-craton structure or modified cratonic regionssuch as these regions.

6.2. Lithospheric mantle structure

We also compare our uppermost mantle velocity re-sults to the velocities from the CRUST2.0 and 3SMACmodels. Again, the fit to the CRUST2.0 model is ratherpoor. The model has uniform, extremely fast (∼4.7 km/s)upper mantle velocities throughout the African continent,except along the East African Rift system. In contrast, thefit to the 3SMAC model is rather good. Like our results,this model also predicts moderately slower velocitiesoutside of the extremely fast cratons, although the ve-locities are even slower in our model. Overall, it seemsthat the 3SMAC model is the better predictor of seismicstructure in Africa than CRUST2.0.

In the upper mantle beneath southern Africa, we findfast velocities in the Kalahari Craton, but slower ve-locities at the southern edge of the continent. Whilethere is some suggestion that upper mantle velocities inthe region are uniformly high, at least for P-waves(Wright et al., 2003), there is evidence on variations inthe craton. Reporting on a study of Cichowicz andGreen (1992), Durrheim and Mooney (1994) state that

the Kaapvaal Craton has relatively low velocities (mean4.3 km/s) for the crust and uppermost mantle down to80 km depth, consistent with many results from theKaapvaal seismic project (e.g., de Wit et al., 2004 andreferences therein). The fast velocities are consistentwith previous studies of the region (Zhao et al., 1999;James et al., 2001), with the latter also finding slowervelocities in the Cape Fold Belt.

A number of global and continental-scale tomograph-ic models have been published, and most of them showsimilar lithospheric mantle structure beneath Africaand Arabia. Thus, for comparison, we focus on justone model, that from Ritsema and van Heijst (2000),hereafter referred to as RV&H. Fig. 7 shows horizontalslices through our model at 100 km, 150 km, 200 km,and 250 km depth. The depths were chosen to allow adirect comparison to the results of RV&H. At all depths,there is excellent agreement between the two studies,with both having high velocity structures beneath theWest African, Congo andKalahari cratons down to about250 km depth, and low velocity structures observedbeneath the East African, Red Sea, Gulf of Aden, mid-Atlantic and northwestern Indian Ridge.

However, there are some differences between themodels, particularly in central Africa beneath the CongoCraton, in southern Africa, and in the Zagros Mts. ofIran. In our model, there is no pronounced lithospherickeel under the center part of the Cuvette Centrale andhence Congo Craton. Our model shows thick lithospherebeneath the Archean blocks surrounding the CuvetteCentrale but not directly beneath it, whereas the RV&Hmodel show the thickest lithosphere beneath the CuvetteCentrale. In southern Africa, RV&H find more or less acontinuous zone of thick fast lithosphere that extendsfrom the northern edge of the Congo Craton to thesouthern continental edge. Our model shows the samefeature, but the nature of it changes in the vicinity ofNamibia, Botswana, and Zimbabwe, where the KaapvaalCraton abuts with the Congo and Zimbabwe Cratonsalong the Kheiss and Limpopo Belts. The models alsodiffer in the Zagros Mts. of Iran where RV&H find thicklithosphere extending down to 200 km. This is incontrast to the relatively thin lithosphere found in ourmodel.

We have also compared our lithospheric thicknessestimates with thermal studies. In Artemieva andMooney (2001), the authors use heat flow data toestimate temperatures in the upper mantle and use theprojected temperatures to derive lithospheric thickness.While heat flow coverage in Africa is sparse, where theyhave been able to make estimates, the patterns are verysimilar. Both the seismic and heat flow data suggest thick

40 M.E. Pasyanos, A.A. Nyblade / Tectonophysics 444 (2007) 27–44

lithosphere under the West Africa, Congo, and KalahariCratons and thin lithosphere in the East African Riftsystem, extending to Madagascar. Moreover, both

studies find the thickest lithosphere beneath the cratonin West Africa. Where the studies differ is in thelithospheric thickness of the Benue Trough and the

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eastern portion of the Saharan Shield. In both cases, theseismic thicknesses are significantly thinner thanestimates of the thermal thicknesses, which approach200 km depth. In each instance, however, the regions arefar from the location of any heat flowmeasurements usedin the analysis.

7. Discussion

In summary, we have used surface wave group ve-locities to invert for the lithospheric structure of Africaand Arabia, producing data-driven maps of crustal andlithospheric thickness that, in contrast to other maps ofthe whole region, are not derived using geophysicalinference. Deeper features in the model, as characterizedby horizontal slices through the upper mantle, corre-spond well with other studies of the region. Shallowerfeatures like crustal thickness, however, differ signifi-cantly from inference-based models. In particular, wefind significant increases in crustal thickness associatedwith the West African Craton, the Congo Craton,Tanzania Craton, and Kalahari Craton. We also findthinning associated with the East, Central and WestAfrican Rift Systems. No significant variations in crustalthickness appear to be related to hotspots located awayfrom plate boundaries. Crustal velocities are notablyslower in regions of recent tectonism and in orogeniczones both older and more recent.

Shallow upper mantle structure shows velocity varia-tions consistent with tectonic structure. Slow uppermantle velocities are found under the Red Sea, DeadSea Rift, Gulf of Aden, East African Rift, and mid-Atlantic rift. Very fast upper mantle velocities are foundbeneath the West African Craton, Congo Craton andcratons in eastern and southern Africa (Zimbabwe,Tanzania, Kaapvaal). The Benue Trough appears to be adeeper mantle feature only, with thinned crust and thinnedlithosphere, but not particularly anomalous velocities inthe upper mantle directly beneath theMoho discontinuity.Lithospheric structure is also well correlated to cratonicstructure and is very consistent with the results of RV&H.Our model does not resolve well sublithospheric mantlestructure, and is therefore of limited use for addressingquestions such as the origin of hotspots.

Next we describe variations in lithospheric structureacross Africa and Arabia with three vertical depth slices

Fig. 8. Cross-sections traversing the model. Cross-sections show shear-wavelocation of cross-sections shown below, along with a depth slice through the mand Robinson, 1994). The pink regions show the Archean cratons and the grPopoff, 1991). b) Cross-section A–A' extending from the north Atlantic Oceavelocities at the top of the mantle. Cross-section B–B' extending from the soufrom the mid Atlantic Ocean to the Caspian Sea. Abbreviations are the same

through our model (Fig. 8), and use them to comment insome detail on the structure of the lithosphere under theArchean cratons. Layering in the cross-sections is due tolayers of the ak135 model (Kennett et al., 1995), whichhas discontinuities in the mantle at 150, 190, 225, and270 km depth. The first cross-section, A–A' (Fig. 8b),extends from the north Atlantic to the Indian Ocean, andcuts across the West African, Congo, and Tanzaniancratons. All three cratons have both fast upper mantlevelocities and thick lithosphere. However, the TanzaniaCraton is noticeably thinned on both ends where thecross-section crosses the two branches of the EastAfrican Rift, and it has the shallowest lithospheric keelof the three. The slowest upper mantle velocities alongA–A' are found at the southeastern portion of the cross-section and are associated with the Comores Islandhotspot.

The second cross-section (B–B', Fig. 8b) extendsfrom the south Atlantic to the Arabian Plate crossingthe African Superswell. In this cross-section, thicklithosphere associated with the cratons in southernAfrica can be seen, as well as thinned lithosphere andslow upper mantle velocities along the East African Rift.Even away from the rift zone, however, the mantleappears to be slower under the northern portion of thesuperswell than neighboring regions. At the northeastend, the lithosphere thickens again under the ArabianPlatform.

The third cross-section (C–C', Fig. 8b) runs parallelto cross-section B–B', extending from the Atlantic toEurasia, offset to the northwest of profile B–B'. Here,away from the African Superswell, we find lithospherein the Atlantic that thickens with increasing age towardsthe coast, and thick lithosphere under the Congo Cratonand East Saharan Shield, with significantly thickerlithosphere under the western and northwestern portionsof the Congo Craton. Thin lithosphere and slow mantlevelocities can be seen under the Red Sea, Dead Sea Rift,and northern portion of the Arabian Plate and TurkishPlateau.

The outlines of exposedArchean crust inAfrica, whichcan be used to define the nominal extent of the Archeancratons, are shown in Fig. 8a, superimposed on a slicethrough our velocity model at 200 km depth. For the mostpart, the spatial extent of the cratonic keels in our model,identified as regions with thick (∼150–250 km)

velocity variations (in %) from the ak135 model. a) Map showing theodel at 200 km. Dashed lines outline the African Superswell (Nybladeeen regions the locations of Cenozoic volcanic fields (Kampunzu andn to the Indian Ocean. Values along the profile indicate the shear waveth Atlantic Ocean to the Iranian Plateau. Cross-section C–C' extendingas in Fig. 1.

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lithosphere, is consistent with location of mappedArchean crust. There are a few notable differences,however. Firstly, our model shows thick, craton-likelithosphere extending to the southwest of the KaapvaalCraton offshore. There are few crossing ray paths in thispart of our model, and therefore the fast lithosphericstructure of the Kaapvaal Craton (on-shore) is most likelybeing smeared to the southwest off-shore. Thus, the fastlithospheric structure under the southeastern AtlanticOcean is probably a modeling artifact.

Interestingly, our model also shows the fast litho-spheric structure of the Zimbabwe Craton extending tothe northeast beneath central and northern Mozambique.A similar northeastward extension of the ZimbabweCraton can be seen in the RV&H model. TheProterozoic mobile belts flanking the northeast side ofthe Zimbabwe Craton may represent reworked Archeancrust, some of which could have been thrust to the westover the margin of the Zimbabwe Craton. Thus, it is notgeologically unrealistic to find thick, fast lithosphereunder central and northern Mozambique along the edgeof the Zimbabwe Craton.

A third difference, and perhaps the most importantone, between published maps of Archean crust in Africa(Fig. 8a) and the location of thick craton-like lithospherein our model is the Congo Craton. The extent of theCongo Craton shown in Fig. 8a is inferred. Archeancrust is exposed around the periphery of the CongoCraton in the Central African Republic and Cameroon tothe north and northwest, in Gabon to the west, and in thesouthern D.R.C. and Angola to the south and southwest(Cahen et al., 1984). The middle of the so-called CongoCraton is covered by up to 9 km of sediments in theCuvette Centrale. Most previous tomographic images ofthe African upper mantle show one large lithospherickeel centered on the Cuvette Centrale, reinforcing thenotion of a single, large craton comprising the Archeanblocks surrounding the Cuvette Centrale that jointogether under the Cuvette Centrale (e.g., Grand et al.,1997; Ritsema et al., 1999; Ritsema and van Heijst,2000; Debayle et al., 2001; Grand, 2002).

However, the origin of basin subsidence in theCuvette Centrale has been linked to a Neoproterozoicfailed rift and subsequent thermal relaxation of thelithosphere (Daly et al., 1991; Daly et al., 1992). ANeoproterozoic rift origin for the Cuvette Centrale sug-gests that the lithosphere under the basin, and con-sequently the region that is commonly assumed to be thecenter of the Congo Craton, may not be typical thick,cold, strong Archean lithosphere. Because the CuvetteCentrale is surrounded by Archean blocks, it is quitepossible that in many published tomographic images the

deep lithospheric keels under these blocks are beingsmeared together, making the spatial extent of thicklithosphere appear much greater than it really is. Thelimited resolution of published tomographic images isclearly illustrated by the fact that these images do notdistinguish between the Tanzania Craton and the CongoCraton, which are separated by the western branch of theEast African rift system.

Our model is different in that it does not show aregion of thick craton-like lithosphere under the centerof the Cuvette Centrale. This can be seen in the depthslice through the model at 200 km (Fig. 8a), or in thevertical section A–A' in Fig. 8b. On the other hand, ourmodel does show thick (200–250 km) lithosphere underthe Archean blocks to the south, west and northwest ofthe Cuvette Centrale (C–C', Fig. 8b). Consequently, ourmodel suggests that the Congo Craton might not be onelarge Archean block, but rather a number of smallerArchean blocks that do not join under the sedimentarycover of the Cuvette Centrale.

In comparison to estimates of lithospheric thicknessfor other Archean cratons (West Africa, Tanzania,Kaapvaal and Zimbabwe) from other studies, our resultsare similar. The thickness of the lithosphere under thecratons inWest Africa and southern Africa (Kaapvaal andZimbabwe) is about 200–250 km. Beneath the TanzaniaCraton, the lithosphere is only 150–200 km thick.

As a final comparison, we point out that our modelshows a small region with lithosphere about 200 kmthick under the Archean Uweinat block in southernEgypt and northern Sudan extending to the southwestinto the region referred to as the East Saharan Shield(ES) on Fig. 1. This region of thick, fast lithosphere canbe seen in both the depth slices (Fig. 8a) and the verticalprofile C–C' in Fig. 8b. This region of thick lithosphereis not apparent on many other published seismic modelsof African upper mantle.

It appears that the African Superswell, which isresponsible for the uplift of eastern and southern Africa,is a shallow seismic feature only beneath the northernportion of the superswell. Seismically, we do not see thisfeature in the south at depths for which we have surfacewave sensitivity. Earlier studies have established that nosuch evidence exists at least to lower mantle depths fromboth P (Zhao et al., 1999; Simon et al., 2002) and Swave studies (Simon et al., 2003). This is consistentwith the “lava lamp” model (Ladbury, 1999) advocatedby Doucouré and de Wit (2003) and supported by otherseismic data (Ritsema et al., 1999; Behn et al., 2004). Inthis model, the superswell has been only episodicallyactive at shallow depths, coinciding with progressingmagmatic events in southern Africa during the Jurassic,

43M.E. Pasyanos, A.A. Nyblade / Tectonophysics 444 (2007) 27–44

in southwest Africa during the Cretaceous, in east Africaduring the Tertiary, and in northeastern Africa today.

Future work will focus on further improving thelateral and depth resolution of this model. Some gainscould be made by incorporating more surface wavemeasurements into the model, especially from poorlysampled regions of the continent. It would also be usefulto put additional constraints on the model with data setsother than surface waves, such as teleseismic receiverfunctions and body wave travel times, particularlyregional travel times that are sensitive to crustal anduppermost mantle structure. Work in this direction hasalready begun for several regions in our model,including the Kenya Rift (Benoit et al., 2006a), theArabian Peninsula (Tkalcic et al., 2006), eastern Turkey(Gok et al., 2007), and Kuwait (Pasyanos et al., 2007).

Acknowledgements

We thank all of the people and groups contributingdata to this study, including IRIS (www.iris.edu). Weparticularly appreciate IRIS's new single “open station”policy for ongoing PASSCAL experiments. MaggieBenoit and Shannon Hazler made surface wavedispersion measurements included in this study. TerriHauk assembled most of the data used in this project.Figures were generated using the Generic MappingTools (GMT) software (Wessel and Smith, 1998). Thiswork was performed under the auspices of the U.S.Department of Energy by University of CaliforniaLawrence Livermore National Laboratory under con-tract number W-7405-ENG-48. This is LLNL contribu-tion UCRL-JRNL-225868. Support to A.A.N. has beenprovided by the National Science Foundation (grantsEAR 993093, 0003424 and OISE 0530062).

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