Why did Arabia separate from Africa? Insights from3-D laboratory experiments

N. Bellahsen a;b;�, C. Faccenna c, F. Funiciello c, J.M. Daniel a, L. Jolivet b

a Division Ge¤ologie-Ge¤ochimie, Institut Franc&ais du Pe¤trole, Rueil Malmaison, Franceb Laboratoire de Tectonique, Universite¤ Pierre et Marie Curie, Paris, France

c Dipartimento di Scienze Geologiche, Universita' degli Studi ‘Roma Tre’, Rome, Italy

Received 6 April 2003; received in revised form 8 September 2003; accepted 10 September 2003

Abstract

We have performed 3-D scaled lithospheric experiments to investigate the role of the gravitational force exerted bya subducting slab on the deformation of the subducting plate itself. Experiments have been constructed using a densesilicone putty plate, to simulate a thin viscous lithosphere, floating in the middle of a large box filled with glucosesyrup, simulating the upper mantle. We examine three different plate configurations: (i) subduction of a uniformoceanic plate, (ii) subduction of an oceanic^continental plate system and, (iii) subduction of a more complex oceanic^continental system simulating the asymmetric Africa^Eurasia system. Each model has been performed with andwithout the presence of a circular weak zone inside the subducting plate to test the near-surface weakening effect of aplume activity. Our results show that a subducting plate can deform in its interior only if the force distribution varieslaterally along the subduction zone, i.e. by the asymmetrical entrance of continental material along the trench. Inparticular, extensional deformation of the plate occurs when a portion of the subduction zone is locked by thecollisional process. The results of this study can be used to analyze the formation of the Arabian plate. We found thatintraplate stresses, similar to those that generated the Africa^Arabia break-up, can be related to the Neogeneevolution of the northern convergent margin of the African plate, where a lateral change from collision(Mediterranean and Bitlis) to active subduction (Makran) has been described. Second, intraplate stress and strainlocalization are favored by the presence of a weakness zone, such as the one generated by the Afar plume, producing apattern of extensional deformation belts resembling the Red Sea^Gulf of Aden rift system.: 2003 Elsevier B.V. All rights reserved.

Keywords: Arabia; Gulf of Aden; Red Sea; Afar hot spot; Arabia^Eurasia plate boundary

1. Introduction

The initiation of rifting, the break-up of conti-nents, and the formation of new plates are stillenigmatic processes. In some cases, the close tem-poral relationships between magmatism and con-tinental rifting suggest that the action of hotplumes favors the break-up of continents [1], as

0012-821X / 03 / $ ^ see front matter : 2003 Elsevier B.V. All rights reserved.doi:10.1016/S0012-821X(03)00516-8

* Corresponding author. Present address: Department ofGeological and Environmental Sciences, Stanford University,Stanford, CA 94305-2115, USA. Tel. : +1-650-723-4788x2;Fax: +1-650-725-0979.E-mail addresses: nicolasb@pangea.stanford.edu,

nicolas.bellahsen@lgs.jussieu.fr (N. Bellahsen).

EPSL 6847 4-11-03

Earth and Planetary Science Letters 216 (2003) 365^381

R

Available online at www.sciencedirect.com

www.elsevier.com/locate/epsl

suggested for the Gondwanaland break-up [2].However, the activity of some hot spots did notlead to continental break-up, such as in the Sibe-rian Traps [3]. Moreover, a considerable delay canoccur between the onset of a plume activity andthe onset of spreading (e.g. South African andCentral Atlantic traps discussed by Hill [3] andCourtillot et al. [1]). This suggests that favorableboundary conditions in the form of far-¢eld ex-tensional stresses are also required to lead to platefragmentation. These far-¢eld extensional stressesare classically attributed to subduction processes.For example, the distribution and the orientationof ridges more or less re£ecting the shape of sub-duction zones [4] suggest that slab pull forcesrelated to subducted lithosphere can be guidedthrough the slab^plate system and transmittedover large distances within the plate. In thiscase, it is likely that strain will localize in regionsof pre-existing weakness such as older suturezones [5].Lithosphere/mantle plume interactions result in

global weakening of the lithosphere (especially byerosion at the base of the lithosphere) and surfacedoming (see Saunders et al. [6] for an exhaustivesynthesis of plume/lithosphere interactions andreferences), and thus may also localize the defor-mation. Geodynamic models shed greater light onthis problem by showing that the supercontinentalcycle (collision followed by break-up) can be re-produced over a time scale of 108 yr by the com-bined action of subducting plate and mantle up-welling. Their mutual contribution is largely afunction of both the wavelength of the convectionpattern and the source of internal heat generation[7]. However, over a shorter time scale, it is notclear how convection processes (subductions andplumes) produce a stress level able to overcomethe strength of plates.The formation of the Arabian plate (Fig. 1)

represents a spectacular example where, overshort time scales, the mutual interaction betweenbody (hot-spot related) and far-¢eld forces can beevaluated. Authors who investigated the forcesnecessary to break up the Africa plate recognizedthe importance of both the Afar hot spot and theTethyan subduction, still active beneath Makran[1,8^11]. The main paradox for the separation of

the Arabian plate is that the formation of the newextensional plate boundary to the south occurredduring the collision and the indentation of theplate into Eurasia to the north. We performed aset of 3-D laboratory experiments to explore therole of the subduction^collision process, speci¢-cally addressing the case of Arabia^Africa sepa-ration.

2. Geodynamic framework of the Arabian plate

The plate boundaries of the Arabian Plate (Fig.1) are detailed successively. The northern colli-sion^subduction zone (Bitlis^Zagros^Makran) isthe result of the last stages of evolution of theTethyan subduction zone, which dates back tothe beginning of the Mesozoic [12,13]. The Meso-zoic evolution of this Tethyan subduction systemwas punctuated by episodes of ridge subductionand by the generation of a new trench, obductingoceanic material on the southern basin shoulders(see Searle and Cox [14] for a discussion). Thecomplete consumption of the (Neo-)Tethyanocean and related collision occurred only in theCenozoic with a progressive migration of the col-lision towards the east (Fig. 2): the African^Ara-bian platform collided with Eurasia, ¢rst in Tur-key around 40 Ma [15] and later in Iran from 20Ma [13]. Beneath the Makran trench, the distri-bution of deep earthquakes [16] and arc volca-nism [17] shows that oceanic subduction is stillactive today. The slab has a clear signature evenin the seismic tomography images (see ¢gure 11ain Bijwaard et al. [18]), and the length of the slabcan be estimated at more than 1300 km. Thistectonic evolution reveals an eastward propaga-tion of the collisional event, and suggests thatthe Tethyan ocean was narrowing westward [12](Fig. 2).The formation of the Arabian plate and its

break-up from Africa initiated at about 30^35Ma (Fig. 2), with the formation of the Red Sea^Gulf of Aden rift system, centered in the Afarregion. Volcanism and uplift can be related tothe presence of a hot spot below East Africawhose trace on the surface dates back to 45 Ma,with a peak of activity around 30 Ma ([19,20]

EPSL 6847 4-11-03

N. Bellahsen et al. / Earth and Planetary Science Letters 216 (2003) 365^381366

Fig. 1. Tectonic framework of the Arabian plate. Direction and value of relative velocities (empty arrows) are from Reilinger etal. [77] for the convergent northern margin, from Le Pichon and Gaulier [29] for the Red Sea, from Jestin et al. [76] for the Gulfof Aden, from Quennel [30], and Le Pichon and Gaulier [29] for the Levant fracture zone, and from Argus and Gordon [78] forthe Owen fracture zone. The absolute velocities (plain arrows) are from Argus and Gordon [78]. The geometry of the Owen frac-ture zone, and the Arabia^Somalia^India triple junction is from Fournier et al. [79].

Fig. 2. Tertiary tectonic evolution of the Africa^Eurasia^Arabia system, at 65, 35^30 and present-day ages (modi¢ed from Der-court et al. [64]). White regions represent continents, light gray represents continental platform, and dark gray represents ocean.Black arrows are absolute velocities [78]. Empty arrows are the direction of extension (references given in the text and the cap-tion of Fig. 1).

EPSL 6847 4-11-03

N. Bellahsen et al. / Earth and Planetary Science Letters 216 (2003) 365^381 367

and references therein). The deformation pat-tern is complex in this region [21^23] as well asthe timing between volcanism events and riftingonset [24,25] and will not be discussed in this pa-per.The Red Sea and the Gulf of Suez (Fig. 1),

which represent the western branch of the riftsystem, rifted at the beginning of the Oligocene[26], possibly reactivating older weakness zones[27]. In the Red Sea, dykes and basaltic £oodswere emplaced during the Oligocene [26] andsyn-rift deposits accumulated from the Oligo-cene (34 Ma) to Miocene (14 Ma) [28]. Oligoceneextensional direction is believed to be aroundN45‡E in the southern Red Sea [29]. During Mio-cene times, the activity of the Gulf of Suez de-creased as it was transferred to the Levant faultzone (Fig. 1), which accommodated a total left-lateral displacement of 107 km [30], since 14 Ma[28].The eastern arm of the rift system is repre-

sented by the Gulf of Aden rift (Fig. 1). Firstsyn-rift sediments are dated to late Eocene^earlyOligocene, around 35 Ma [31,32]. Extension oc-curred in two directions, ¢rst N20‡E and thenN160‡E [33^35]. The rift and ridge global trend(N75‡E) is strongly oblique to both the early syn-rift extension (N20‡E) and the present relativeplate motion (N27‡E). While the segmentationof the fault network seems to be due to this obliq-uity, the origin of the obliquity is still enigmatic.Several mechanisms have been suggested: anoblique rifting due to a pre-existing lithosphericzone of weakness [36], an oblique rift propagationalong such a weakness zone [37], and a propaga-tion from the Carlsberg Ridge towards the Afarhot spot [9]. The Owen fracture zone links theGulf of Aden to the Makran subduction zone,with a dextral movement.The overall picture for the formation of the

Arabian plate is rather well established. The platedevelops as follows: (a) A collisional process getsyounger towards the east, ¢rst in Turkey around40 Ma and later in Iran. Oceanic subduction re-mains active only further east, below Makran.(b) The activity of the Afar hot spot started about45 Ma with a peak of volcanism recorded between35 and 30 Ma. (c) The break-up of Africa and the

formation of rifts in the Gulf of Aden, Red Sea,and Afar started around 35 Ma. Contemporane-ously with the formation of the Arabian plate, theabsolute motion of Africa decreased from about3 cm/yr until 40^30 Ma to the present-day rate ofabout 1 cm/yr [11] because of the Eurasia^Africacollision.While the temporal relationship between colli-

sion^subduction and the extension has alreadybeen noticed [11], the role of the subduction pro-cess for the break-up of Africa has never beentested. Our experiments have been speci¢cally de-signed to explore the connection between the sub-duction dynamics and the deformation of theAfrican plate.

3. Model set-up

3.1. Model de¢nition

In our experiments, we simpli¢ed the plate^slabsystem similarly to a thin viscous sheet approxi-mation [38]. In addition, we have used a linearlyviscous material, whereas laboratory experimentsshow that natural material obeys a creep powerlaw of deformation. In the discussion we willunderline that this assumption will further corrob-orate our results. The mantle in our models hasno thermal convection. We assume that the man-tle £ow is only generated by subduction and wedo not consider the e¡ects of global £ow [39] orlocal background £ow that is not generated by thesubducting slab. We simulate the thermal e¡ectsof oceanic lithosphere aging by imposing a nega-tively buoyant oceanic plate. The imposed level ofnegative buoyancy is preserved throughout theexperiment, neglecting thermal di¡usion andphase changes [40^42]. The high velocity of sub-duction (s 1 cm/yr) recorded in our experimentsjusti¢es the ¢rst assumption, ensuring that ad-vection overcomes conduction (Peclet number([advection of heat]/[conduction of heat])s 20,see Turcotte and Schubert [43]). The upper man-tle^lower mantle boundary is simulated as an im-permeable barrier over a time scale of about 50Myr. This assumption is justi¢ed by the geody-namic models [44^47], where the penetration of

EPSL 6847 4-11-03

N. Bellahsen et al. / Earth and Planetary Science Letters 216 (2003) 365^381368

slab inside the lower mantle is signi¢cantly re-tarded by the combined e¡ect of the endothermicphase transition [48,49] and viscosity increase [50^52].We expressly isolate the e¡ect of the slab pull

force to deform the subducting plate in its interi-or. Thus, we do not include in our system forcesrelated to the active upwelling of material due toplume activity. The e¡ect on the lithosphere of theplume is simpli¢ed by a thinner (0.9 mm equiva-lent to 60 km in nature), lighter and less viscouscircular zone (equivalent to 300 km in diameter)placed inside the plate (Table 1).

3.2. Forces at work

The main driving force in our system is the slabpull, Fsp [53,54], which is a function of the densitycontrast vb between the oceanic lithosphere andthe mantle, the gravity g, the plate thickness h,and the mantle thickness z :

F sp ¼ vbghz ð1Þ

Its e¡ect is simulated by imposing a constantdensity contrast vb of about 100 kg/m3, whichresults in a maximum slab pull force about 1.4 Nm31, which scales in natural systems to 4U1013 Nm31. Continental lithosphere, conversely, is set upto be positively buoyant with respect to theunderlying material (vb=20 kg/m3). Experimen-tal tests and calculations show that the Argandnumber (ratio of gravitational force to lithospher-ic strength) of the continental lithosphere withrespect to glucose syrup is low enough to ensurethat the continental plate remains stable over theexperimental time scale.Resisting forces are mainly related to viscous

dissipation due to bending of the plate at trench(Fb) e.g. [55]. The role of this force has been esti-mated by Becker et al. [56,57] and its value can beestimated experimentally and theoretically by cal-culating the load of subducted material necessaryto initiate the subduction process (about 0.2 Nm31). Sliding along the subduction fault andalong transform faults, the resistance to penetra-tion inside the mantle, and other viscous cou-plings play a subordinate role in our experiments[55,56,58]. To avoid edge e¡ects, our slab^plate

system is isolated in the middle of the box (Fig.3) and the subduction zone is lubricated by hon-ey. Hence, the transform plate boundary and thesubduction zone are considered to be as weak asthe viscosity of the underlying material, and theirstrength should be considered as a lower bound[59].

3.3. Materials

Following the approach used in previous ana-log studies ([60] and references therein), the rhe-ology of the lithosphere^mantle system is ap-proximated by a linear viscous multilayer. Inparticular, we used silicone putty^honey, to ap-proximate the long-term behavior of the slab^mantle system. Silicone putty (Table 1) is a vis-co-elastic material but at experimental strain ratebehaves only viscously [61] as the experimentaltime scale is higher than the Maxwell relaxationtime (about 1 s). The experimental tests show, inaddition, that silicone is a quasi-Newtonian £uidwhere stress increases linearly with strain rate [61].The upper mantle has been modeled by honey,which is a Newtonian low-viscosity and high-den-sity £uid (Table 1).Physical parameters used to scale our laborato-

ry model are listed in Table 1. The scale factor forlength is 1.6U1037 (1 cm in the experiment cor-responds to 60 km). The scale density factor be-tween the oceanic lithosphere and the upper man-tle is 1.07, whereas the viscosity ratio between theslab (Rocean) and the upper mantle (Rmantle) is ¢xedto 460, respecting the range of natural viscositycontrast [50^52]. The buoyancy number F [62]represents the ratio of the driving buoyant tothe resisting viscous forces. The characteristic ve-locity, v, for our models is the velocity of trenchmotion while the governing viscosity, R, is theviscosity of the oceanic lithosphere, and L is thedomain length corresponding to the upper mantle.Taking characteristic numbers for the Earth andthe model given in Table 1, the buoyancy numberturns out to be 0.50 and 0.54, respectively. Con-sidering the imposed scale ratio for length, grav-ity, viscosity and density (Table 1), we can esti-mate that 1 min in the model corresponds toabout 1 Myr in nature.

EPSL 6847 4-11-03

N. Bellahsen et al. / Earth and Planetary Science Letters 216 (2003) 365^381 369

3.4. Experimental procedure

The multilayered system was arranged in asquare plexiglass tank (Fig. 3) (20 cm high, 80cm long, and 80 cm wide). A total number of20 di¡erent experiments (performed two to sixtimes) were performed using variable combina-tions of thickness, viscosities, densities of the plateand mantle and di¡erent boundary conditions.Each experiment was monitored using a sequenceof photographs taken in time intervals (from 1 to4 min) in the lateral and top view. Trench retreat,plate motion, amount of subduction and dip ofthe slab were measured using image-processingtools. An initial regular grid drawn on the plateallows us to automatically calculate the ¢nite andcumulative displacement ¢eld of each single nodeof the grid along the x and y direction at chosentime intervals. The deformation of each elementof the grid in time is then calculated using thedisplacement ¢eld and used to calculate the major

(e1) and minor (e2) axes of the strain ellipse, theirrelative strike (orientation of e1) and their di¡er-ence in length (elongation of the strain ellipsee13e2). In addition, extension along sections isgiven in percentages by normalizing the di¡eren-tial length (¢nal3initial) by the initial length ofthe section.The subduction process is arbitrarily initiated

by forcing the plate tip inside the syrup up to alevel (about 2.5 cm, corresponding to about 150km in nature) where the gravitational force over-comes the resisting forces (Fig. 3a).

4. Results

We select four representative 3-D laboratoryexperiments from a larger set of 20, to analyzethe role of the subduction^collision processes inthe development of a new extensional plateboundary in the subducting plate. We show di¡er-

Table 1List of physical parameters of the experiment and their corresponding natural values

Parameters Model Nature

g Gravitational acceleration m s32 9.81 9.81Thicknesshocean Oceanic lithosphere thickness m 0.012 70000hcont Continental lithosphere thickness 0.017 100000z Upper mantle 0.11 660000

Thickness ratio (hocean/z) 0.10 0.10Thickness ratio (hcont/z) 0.15 0.15Length scale factor lmodel/lnature = 1.6U1037

Densitybocean Oceanic lithosphere kg m33 1480 3300bcont Continental lithosphere 1360 3000bmantle Upper mantle 1380 3220

Density ratio (bocean/bmantle) 1.07 1.02Density contrast (vb=bocean3bmantle) 100 80

ViscosityRocean Oceanic lithosphere Pa s 2U105 4U1023

Rcont Continental lithosphere 1.4U105 1023

Rmantle Upper mantle 460 4U1021

Viscosity ratio (Rocean/Rmantle) 430 100Viscosity ratio (Rocean/Rcont) 1.4 4

Characteristic timet (Tnature/tmodel =Rnbmgmlm/Rmbngnln) s 60 (1 min) 3U1013 (1 Myr)Buoyancy numberF (F=L2gvb/Rv) 0.50 0.54

Measurements of the viscosity of the silicone and the honey have been performed using a concentric-cylinder (Couette) viscosime-ter at room temperature of 22‡C.

EPSL 6847 4-11-03

N. Bellahsen et al. / Earth and Planetary Science Letters 216 (2003) 365^381370

ent plate con¢gurations: (i) subduction of a uni-form oceanic plate, (ii) subduction of an oceanic^continental plate system, and (iii) subduction of amore complex oceanic^continental system simu-lating the asymmetric Africa^Eurasia system atthe beginning of the Tertiary. Each model experi-ment has been performed with or without thepresence of upper plate and the circular weakzone inside the subducting plate.We summarize the behavior of each experiment

using diagrams in which axes are dimensionlessparameters representative of time (x-axis) andlength variations (y-axis: amount of subduction,trench retreat, plate motion). The time is normal-ized by the time necessary for the slab tip to reachto bottom of the convective layer. This character-istic time is calculated using the formula in [55,56,58]. They show that the slab tip depth can berelated to the initial slab depth and exponentiallyto the time (see [55,56,58] for the demonstration).The time t necessary to reach the bottom of theconvective layer can then be expressed as:

t ¼ lnðz1=z0Þh2W=ðvbgr3Þ ð2Þ

where z0 and z1 are the initial slab tip depths andwhen it reaches the bottom, respectively. h is theslab thickness, W its viscosity, vb the density con-trast between the slab and the mantle, g the grav-ity acceleration and r the radius of curvature ofthe slab (here 4.5 cm). This characteristic time isobtained as equal to 8 min (which is consistent

with our results, see below). The length dimensionis normalized by the depth of convection, i.e.11 cm.

4.1. Oceanic plate system

In experiment 1 (Fig. 3b), we test the behaviorof a uniform oceanic plate subducting in theupper mantle (Fig. 4a). In the experiment, we rec-ognize a typical sequence of events. In the ¢rstphase (0^0.75 non-dimensional time), the platesinks into the mantle and the trench starts to re-treat. The velocity and the dip of subduction in-crease progressively with the slab length (Fig. 4a).In particular, the slab velocity (as well as theamount of trench retreat and plate motion) hasbeen predicted by the balance of slab pull and theresistance of the slab to bending at trench [56,58].The second phase occurs when the slab ap-proaches the transition zone around a non-dimen-sional time of 0.75. The tip of the slab folds atdepth and, simultaneously, the trend of subduc-tion changes to a forward migration. The trenchadvances during a non-dimensional time of about1 while the plate accelerates. As a kinematic con-sequence, the slab attains a reclined U-shape (Fig.4a). During the third phase, the system reaches asteady-state con¢guration within the experimentaltime. The open U closes due to the hinge collapseand the dip of the slab stabilizes at 60‡. Simulta-neously, the trench starts to retreat again at a

Fig. 3. Model set-up. (a) Oblique view of the experimental set-up. The lithosphere plate (1.2 cm thick) £oats in the middle of theplexiglass box ¢lled up with honey. (b) Surface view of experiments 1 and 2. The plate of experiment 1 is composed only of anoceanic layer, whereas experiment 2 is composed of a continental^oceanic layer. Each experiment has been performed includingor not the weakness zone (dashed circle). (c) Top view of experiments 3 and 4. Experiment 4 di¡ers from experiment 3 by thepresence of a weakness zone (dashed circle) inside the continental plate. Experiment description uses a geographic coordinate sys-tem marked by the north (arrow).

EPSL 6847 4-11-03

N. Bellahsen et al. / Earth and Planetary Science Letters 216 (2003) 365^381 371

constant velocity while the plate motion slowsdown.This experiment highlights the strong depen-

dence of the plate and trench motion on the be-havior of the slab at depth. In this case, the force

produced by subduction is entirely transmitted tothe plate and controls the plate and trench migra-tion. However, the plate does not deform in itsinterior (Fig. 4b), even if a weakness zone is lo-cated in its center.

Fig. 4. (a) Lateral views of experiment 1 characterized by the subduction of an oceanic plate. (b) Amount of subduction, trenchmotion, and plate motion normalized by the depth of convection versus normalized time of experiment 1. Arrows refer to thefour stages presented in panel a. Plate motion represents the motion of the trailing edge of the plate with respect to the ¢xedbox boundaries. Trench motion represents the subduction zone motion with respect to the box boundaries (negative when trenchadvances towards the upper plate, positive when trench retreats towards the subducting plate). Amount of subduction is calcu-lated by summing up the amount of trench motion, the amount of plate motion, and the initial subducted length. (c) Lateralview of experiment 2 characterized by the subduction of an oceanic^continental plate. (d) Amount of subduction, trench motion,and plate motion versus time of experiment 2. Arrows refer to the four stages presented in panel c, gray arrow indicates the en-trance of continent at trench. Note the faster initial stage of evolution of experiment 2 with respect to experiment 1. This is dueto the di¡erent initial con¢guration of the model set-up characterized by a larger amount of initial subduction of experiment 2.

EPSL 6847 4-11-03

N. Bellahsen et al. / Earth and Planetary Science Letters 216 (2003) 365^381372

4.2. Oceanic^continental plate system

Experiment 2 (Fig. 3b) shows the behavior ofan oceanic^continental plate subducting in theupper mantle (Fig. 4a). The continent^ocean tran-sition in the subducting plate has been orientedparallel to the trench (Fig. 3b). During the sub-duction of the oceanic lithosphere, the behavior ofthe slab is similar to the ¢rst and the second phaseof experiment 1 (Fig. 4b): the slab falls into theupper mantle, accelerating exponentially, and in-creases its dip to reach an overturned con¢gura-tion. When the slab reaches the mantle disconti-nuity, the system decelerates and the trenchadvances. The entrance of the continent at trenchoccurred at a non-dimensional time around 1.25.From this moment, the system evolution di¡ersfrom the previous one as the subduction velocityprogressively slows down and the trench keeps onadvancing until the system stops. Subductionceases when the negative buoyancy becomeszero. It occurs when the continental lithosphereis subducted to depths of about 300^350 km(non-dimensional length of 0.5, non-dimensionaltime around 2). As in the previous experiment, theplate does not present any signi¢cant deformationin its interior even if a weakness zone is insertedin the middle of the plate.

4.3. Complex oceanic^continental plate system

Experiments 3 and 4 (Fig. 2) have been set upin such a manner as to simulate a small portion(about 30%) of the Africa^Tethyan system at thebeginning of the Tertiary. Experiment 3 di¡ersfrom experiment 4 as it does not include the pres-ence of the weakness zone related to the plumeactivity. In both experiments, the ocean^continenttransition in the subducting plate runs from par-allel to oblique to the trench system (Fig. 3c) inorder to simulate the transition from the Mediter-ranean to the Middle East region (Fig. 2). In theMediterranean, the Alpine collisional system fromthe Rif-Betic to the Bitlis has been simpli¢ed by aocean^continent transition parallel to the trenchwith a length of 25 cm (equivalent to 1500 km)(Figs. 2 and 3c). In the Middle East, the increas-ing size of the Tertiary Neo-Tethyan Ocean from

eastern Turkey (about 700 km [63,64]) to Pakistan(ranging from 1100^1400 [63,65] to 1700 km [64])has been modeled by varying the model oceanwidth from 12 (equivalent to 720 km) to 25 cm(equivalent to 1500 km). Both experiments havebeen performed with an overriding continentalplate to simulate the trench locking related tothe entrance of continental material at trench.For convenience, we shall refer hereafter to re-gions of the experiments in terms of geographicaldirections, the northern boundary correspondingto the trench (whose strike is approximatelyN110‡E).Experiment 3 (Fig. 5) is performed with a ho-

mogeneous continental plate, thus excluding thepossible e¡ect of the weakness zone related tothe Afar hot-spot activity. The evolution of thesystem is characterized by a ¢rst phase of accel-erating oceanic subduction followed by a phase ofslowing down which occurs during the entrance ofthe continent in the subduction zone (at a non-dimensional time of 2.5). As already observed inthe previous experiment, the trench advanceswhile the subduction velocity slows down (Fig.5a). Nevertheless, from this moment, the modelevolution di¡ers from the previous experiment(experiment 2). The decrease of the subductionvelocity, in fact, is observed only in the portionof the experiment where continent entered attrench (here referred to as the ‘western portion’).On the other side (‘eastern portion’), the ongoingoceanic subduction respects the path of the veloc-ity observed in experiment 1 (Fig. 5a). As a directconsequence of this con¢guration, the trench ad-vances slowly in the western portion, while it re-treats in the oceanic subduction zone. This ¢rstproduces a small clockwise rotation of the plate(Fig. 5b,c). Afterwards, the ocean also progres-sively closes in the region (east) where the conti-nent^ocean transition is oriented obliquely to thetrench. From this moment, the increasing andprogressive eastward shift of continental subduc-tion also produces a general re-organization ofthe style of subduction in the area where theocean is still subducting, where trench starts ad-vancing. This di¡erential velocity of subduction,increasing eastward, then triggers a small counter-clockwise rotation of the plate (Fig. 5c). However,

EPSL 6847 4-11-03

N. Bellahsen et al. / Earth and Planetary Science Letters 216 (2003) 365^381 373

once the trench is completely locked in the con-tinental western region, the tendency of the plateto rotate is accommodated by deformation of theplate itself. The pattern of deformation of theplate is shown in Fig. 5c. The highest elongationof the strain ellipse is localized at trench, in thetransition from continental to oceanic subduction,reaching a value of 0.25, while in the internalportion of the plate, it attains a value of about0.2. The maximum strain ellipse axes (representedby short lines in Fig. 5c) are generally oriented

perpendicularly to the subduction zone (i.e. ori-ented around N20‡E), slightly turning to a moreoblique trend towards the west. Along the seg-ment a^b (Fig. 5c), extension inside the platedoes not exceed 10%.Experiment 4 is similar to experiment 3, except

for the presence of a circular weak zone thatshould simulate the surface weakening e¡ect ofthe Afar plume (Figs. 3c and 6), neglecting themovement of Africa over the ¢xed plume. Theoverall evolution of the system is similar to the

Fig. 5. (a) Top view of the evolution of experiment 3 at three di¡erent stages. (b) Amount of subduction, trench motion, andplate motion versus time of experiment 3 for the two regions (1, 2) indicated in panel a. Plate motion represents the motion ofthe trailing edge of the plate with respect to the ¢xed box boundaries. Trench motion represents the subduction zone motionwith respect to the box boundaries (negative when trench advances towards the upper plate, positive when trench retreats to-wards the subducting plate). Amount of subduction is calculated by summing up the amount of trench motion and the amountof plate motion. (c) Strain maps: segments represent the orientation of the maximum axis of the strain ellipse, the gray scaleshows the elongation of the strain ellipse (di¡erence between major and minor axes) where darker colors mean higher elongation.(d) Extension in percentage along segment a^b (panel c) calculated by normalizing the di¡erential length (¢nal3initial) by the ini-tial length of the section.

EPSL 6847 4-11-03

N. Bellahsen et al. / Earth and Planetary Science Letters 216 (2003) 365^381374

Fig. 6. (a) Top view of the evolution of experiment 4 at three di¡erent stages. The experiment di¡ers from experiment 3 by thepresence of a weakness zone. (b) Amount of subduction, trench motion, and plate motion versus time of experiment 3 for thetwo regions (1, 2) indicated in panel a. Plate motion represents the motion of the trailing edge of the plate with respect to the¢xed box boundaries. Trench motion represents the subduction zone motion with respect to the box boundaries (negative whentrench advances towards the upper plate, positive when trench retreats towards the subducting plate). Amount of subduction iscalculated by summing up the amount of trench motion and the amount of plate motion. (c) Strain maps: segments representthe orientation of the maximum axis of the strain ellipse, the gray scale shows the elongation of the strain ellipse (di¡erence be-tween major and minor axes) where darker colors mean higher elongation. (d) Extension in percentage along two segments, a^band c^d (panel c) calculated by normalizing the di¡erential length (¢nal3initial) by the initial length of the section. (e) Zoom ofthe northeastern corner of the last stage of experiment 4. The contour lines are highlighted. Note the two deformation belts de-parting from the weak zone. The western one trends about N150‡E and the eastern one trends about N60‡E. In the eastern belt,the direction of extension is strongly oblique to the belt strike, while in the western one, the extension is more perpendicular.

EPSL 6847 4-11-03

N. Bellahsen et al. / Earth and Planetary Science Letters 216 (2003) 365^381 375

one illustrated in experiment 3. However, thepresence of the weak zone produces a di¡erentdistribution of strain intensity, while the orienta-tion of the maximum strain axis is similar to theprevious experiment. Fig. 6c shows that the high-est elongation of the strain ellipse is localized intwo maxima, reaching a value of about 0.25. Oneis centered at the trench in correspondence withthe transition between the ocean and the conti-nent, in analogy with what was already recordedin the previous experiment 3. The other maximumis centered in the weak zone and shows similarvalues of 0.25 of the strain ellipse. In addition,we note the presence of two deformation beltsdeparting from the weak zone itself (Fig. 6c,e).The western one trends about N150‡E and joinsthe weak zone to the trench, just to the west ofthe collision^subduction transition. The easternone trends about N60‡E and joins the weakzone to the eastern border of the plate. Extensionalong two segments, crossing the weak zone (seg-ment a^b, Fig. 6d) and the eastern deformationbelt (segment c^d, Fig. 6d), reaches values of 20^25% and of 10^15%, respectively. The maximumaxis of the strain ellipse (e1) along the easterndeformation belt strikes N20‡E, obliquely to thebelt and sub-perpendicularly to the subductionzone (N110‡E). On the contrary, the maximumaxis of the strain ellipse (e1) along the westerndeformation belt strikes around N50‡E, sub-per-pendicularly to slightly obliquely to the deforma-tion belt and oblique to the trench. The combina-tion of the trend of the deformed zone and thedirection of extension in these zones produces aquite orthogonal extension in the western partand an oblique extension in the eastern part(Fig. 6e), de¢ning a trapezoidal block that sepa-rates from the larger plate moving towards thenorth-northeast.

5. Interpretation and discussion: insights into theArabia^Africa separation

5.1. Interpretation of the experimental results

The four experiments presented here have beenselected to show the role of subduction processes

on the deformation of the subducting plate. In asimple con¢guration (experiment 1), the totalforce (Ftot) generated by the subduction systemcan be obtained while neglecting small termsand considering the only two major forces, theslab pull (Fsp) and the bending resistance (Fb).The value of Ftot after the interaction with the660 km discontinuity can thus be written as:

F tot ¼ F sp3Fb ð3Þ

Fsp is not totally balanced by Fb, and the result-ing force, Ftot, is oriented perpendicularly to thetrench and homogeneously distributed along it.Hence the net pull is totally transferred to thehorizontal part of the plate. The motion of boththe trench and the plate reacts to the changingcon¢guration of the subduction system. As a con-sequence the plate never deforms in its interior,even in the presence of a pre-existing weaknesszone. In the same manner, the entrance of a con-tinent at the trench (experiment 2) produces onlya slowing down and progressive locking of thesystem (when more than 250^350 km entered attrench) due to the progressive decrease of Fsp.These two experiments thus show that internal

deformation of the subducting plate is di⁄cult toreproduce: plates, once created, move coherentlyunder the pulling force of the downwelling slabs.The amount of subduction is completely parti-tioned between trench migration (retreat in theexperiment 1) and plate motion.Experiment 3 shows that internal deformation

of the plate has been obtained only by imposing acomplex initial paleogeographic setting able toproduce a heterogeneous distribution of forcesalong the trench. During the ¢rst phase, a behav-ior analog to that observed in experiment 1 isrecorded. The net pull produced by the subduct-ing oceanic lithosphere is totally transferred to theplate, which, as a consequence, translates perpen-dicularly to the plate margin. The lateral transi-tion and progressive time-space shift from sub-duction to collision is able to cause a gradientin the distribution of the force Ftot. This causesa counterclockwise rotation of the plate towardsthe still e⁄cient subduction zone. Afterwards,once the western portion of the trench in the col-lision zone is completely locked (after 360 km of

EPSL 6847 4-11-03

N. Bellahsen et al. / Earth and Planetary Science Letters 216 (2003) 365^381376

continental subduction and collision with theoverriding plate), the tendency of the plate to ro-tate is inhibited. From this moment, the forcegradient along the trench, from the locked colli-sional boundary to the active oceanic subductionboundary, causes internal deformation of theeastern portion of the plate under the active pull-ing of the eastern subducting oceanic seaway. Lat-eral variations in subduction processes have al-ready been invoked for intraplate deformationand plate fragmentation in the case of the Faral-lon plate [66]. They showed that the lateral varia-tion of the Farallon lithosphere age along thetrench could lead to plate fragmentation due tothe tensional stresses induced by such a subduc-tion beneath South America.Experiment 4 con¢rms that the geometry and

the style of deformation depend upon the imposedpaleogeographical setting. In particular, it showsthat strain localization inside the plate is achievedonly by the presence of a weakness zone. Twonarrow zones of extension (Fig. 6) delimiting ablock pulled inside the subduction zone departfrom this weak zone. The strain ellipse elongationis oriented coherently with the direction of Ftot.

5.2. Discussion of the model assumptions

To summarize, our experiments show that asubducting plate can indeed deform (locking ofa part of the boundary and migration of the ro-tation pole in the plate interior) if the velocity¢eld related to subduction changes laterally. Com-parison between the experimental results and nat-ural examples, of course, should be consideredwith care, taking into account the numerous as-sumptions listed before and brie£y recalled here.First, our experimental run encompasses the

Tertiary evolution of the region, whereas the sub-duction process has been active for a much longertime span. This choice is guided by the fact that inthe late Cretaceous the northern subduction zoneof the Neo-Tethys was temporarily inactivated, asattested by the initiation of a new subductionzone and of the obduction of the Oman ophiolites(see Searle and Cox [14] for a discussion).Second, subduction processes in our experi-

ments are restricted to the upper mantle. This so-

lution can be appropriate when the investigatedprocess occurs over a short time scale (50 Myr)[47].Third, due to experimental limitations, the size

of the model African plate is largely reduced.However, any areal increase should further sup-port our conclusions, as larger plates should o¡era larger resistance to the pull of the slab. Thismechanism can also be enhanced by the presenceof the deep continental roots below central-west-ern and southern Africa that can indeed anchorthe plate to more viscous lower mantle [67].Fourth, the resistance along the plate bound-

aries, both in the collision and in the transformzones [10], has probably been underestimated.Thus, no compressive deformations are generatedin the vicinity of the collision. However, as men-tioned above, the imposed boundary conditionsand the level of slab pull produced by the uppermantle subduction correctly reproduce the slow-ing down of Africa’s absolute motion which hasbeen related to continental subduction and colli-sional process [11,68]. In the model, we observe adecrease in the plate motion during the collisionalprocess (35 Ma) from 2 cm/yr (before the colli-sion) to less than 1 cm/yr (when the collision ismature) at the end of the experiment. The Africanplate e¡ectively slowed down from a velocity of3 cm/yr before the Eurasia^Africa collision to1 cm/yr after the collision [11].Fifth, the use of the Newtonian rheology to

simulate the lithospheric layer is a simpli¢cationof the reality. However, this approximation fur-ther supports our conclusions. In fact, the defor-mation of a Newtonian £uid, like silicone putty,should be far more di¡use than that of a powerlaw material, which is characterized by a strain-softening mechanism. In this sense, the deforma-tion of a natural power-law £uid should be weak-er and more localized than that of the Newtoniansilicone layer, further supporting our arguments.Finally, and most importantly, the thermal ef-

fect and related melt generation are not includedhere. During rifting, these processes are consid-ered fundamental to feed and enhance the exten-sional process to the point that continental litho-sphere breaks up and oceanic lithosphere startsforming at the ridge. For this limitation our mod-

EPSL 6847 4-11-03

N. Bellahsen et al. / Earth and Planetary Science Letters 216 (2003) 365^381 377

el is able to reproduce only the very initial stageof deformation and rifting.

5.3. Insights into Africa^Arabia separation

Despite these oversimpli¢cations, the results ofexperiments 3 and 4 do provide insights into thedynamics of the Arabia^Africa break-up. Twomajor points should be emphasized:1. Experiments show that the rifting process isstrictly connected to and caused by the colli-sional process and trench locking along thewestern portion of the northern plate bound-ary. Similarly, the onset of rifting inside theAfrican plate occurred soon after the onset ofcollision in Turkey. The complete locking ofthe western portion of the trench is the causeof rifting, as suggested by Jolivet and Faccenna[11]. In our model, the extensional stress ¢eldinside the African plate is generated by thedi¡erential evolution and along-strike variationof the northern converging boundary. Thestress ¢eld generated by the remaining activeportion of the trench, further east below Mak-ran, where oceanic lithosphere is still subduct-ing, contrasts with the complete locking of thetrench in the western portion. Once the Africanplate is locked in the west, the pull exerted bythe Makran subduction zone favors the sepa-ration of Arabia from the rest of the plate.This solution can then explain why the riftingand break-up of Africa occurred during thecollisional process along a large part of thenorthern boundary. Finally, our experimentsalso demonstrate that in a simple oceanic sub-duction one should not expect that the sub-ducting plate deforms and we suggest thatcauses of plate break-up should be analyzedtaking into account the 3-D geometry.

2. Our model does not include the convectionprocess generated by the upwelling plume,nor the migration of the plate over a ¢xedplume. Hence, we cannot de¢ne the role ofthe hot spot in the genesis of the tensionalstress ¢eld acting in the Afar region. However,our experiments show that the weakening e¡ectgenerated by the action of the mantle plumeover the lithosphere is fundamental, as it local-

izes stresses inside the plate. Our experimentsshow that the geometry and kinematics of thetwo extending belts (Fig. 6e) are directly re-lated to the location of the Afar weakenedzone with respect to the geometry of thetrench. This solution can indeed justify the ori-entation of the Red Sea and the Gulf of Adenwithout invoking the presence of pre-existinglinear zones of weakness (for the Gulf ofAden, as suggested by Morgan [69] and Berhe[70], and for the Gulf of Suez, as suggested byManighetti et al. [9] and Hubert-Ferrari [71]).Moreover, the results ¢t well the directions ofextension (compare Figs. 2 and 6e). Thus, theGulf of Suez rifting does not have to predatethe Gulf of Aden rifting to produce the ob-served rift geometries; these rifts seem quitesynchronous, as suggested by the data exposedin Section 2. This model does not need a prop-agation of rifting from the eastern boundary[9,72] and corroborates the stratigraphic evi-dences of a synchronous syn-rift base. The rift-ing could have begun quite synchronouslyalong the whole Gulf of Aden, or have propa-gated very fast, between the Afar hot spot andthe Carlsberg Ridge extremities. This ridge ob-viously o¡ers an additional zone of weaknessfavorable to strain localization, although weshow that its presence is not necessary to re-produce the plate boundary geometry.The onset of accretion in the Red Sea, around

5 Ma [73], occurred latter than in the Gulf ofAden, at 18 Ma in the east [74]. The Red Seaopens at an average velocity around 1.5 cm/yr[75] and the Gulf of Aden at 2 cm/yr [76]. Theseobservations are in good agreement with the ana-log models where the African plate is deformedwith internal rotation whose rotation pole is lo-cated in the western part of the plate. Thus, theextension rate increases from west to east, induc-ing an earlier break-up and a higher accretion ratein the east (Gulf of Aden), although the exten-sional event started quite simultaneously.

6. Conclusions

We have performed a set of experiments in the

EPSL 6847 4-11-03

N. Bellahsen et al. / Earth and Planetary Science Letters 216 (2003) 365^381378

laboratory to investigate the role of the subduc-tion processes in the break-up of the subductingplate. Despite the approximation used in the ex-periments, the result of this study can be used toanalyze the formation of the Arabian plate. Wefound that intraplate stresses, which generated theRed Sea^Gulf of Aden rift system, can be relatedto the evolution of the northern convergent mar-gin of the African plate. Two necessary conditionsare recognized. First, the velocity ¢eld along themargin should vary rapidly so that the plate issubmitted to an extensional stress ¢eld. The Ter-tiary paleogeographic pattern reconstructed inprevious studies can indeed be appropriate to sim-ulate this condition. At about 30^35 Ma, the colli-sional margin in the Mediterranean was probablylocked whereas under the eastern Makran margin,oceanic lithosphere was still actively subducting.Second, intraplate strain localization is favored bythe presence of a weakness zone. The position ofthe Afar plume, simulated only in its weakeninge¡ect, not only caused the localization of the ex-tension but also induced the formation of tworifts, the Red Sea and the Gulf of Aden.The Arabian plate separated from Africa at

around 30 Ma because: (1) it was locked in thenorth in the collision zone and pulled in the eastby the subducting slab below the Makran, and (2)because the Afar hot spot had weakened the Afri-can lithosphere.

Acknowledgements

Frantz Maerten, the ¢rst author of the codeDynel used to compute the deformation grids, isthanked for his help and advice. The authorswould like to thank R. Wortel, M. Jellinek, C.Ebinger and an anonymous reviewer for construc-tive remarks and criticisms on an earlier manu-script that greatly improved the quality of thepaper.[RV]

References

[1] V. Courtillot, C. Jaupart, I. Manighetti, P. Tapponnier, J.Besse, On causal links between £ood basalt and conti-

nental breakup, Earth Planet. Sci. Lett. 166 (1999) 177^195.

[2] M. Storey, The role of mantle plumes in continentalbreakup case histories from Gondwanaland, Nature 377(1995) 301^308.

[3] R.I. Hill, Starting plumes and continental break-up, EarthPlanet. Sci. Lett. 104 (1991) 398^416.

[4] Y. Ricard, C. Froidevaux, Stretching instabilites and lith-ospheric boudinage, J. Geophys. Res. 91 (1986) 8314^8324.

[5] J.A. Dunbar, D.S. Sawyer, How preexisting weaknessescontrol the style of continental breakup, J. Geophys. Res.94 (1989) 7278^7292.

[6] A.D. Saunders, M. Storey, R.W. Kent, M.J. Norry, Con-sequences of plume-lithosphere interactions, in: B.C.Storey, T. Alabaster, R.J. Pankhurst (Eds.), Magmatismand the Causes of Continental Break-up, Geol. Soc. Spec.Publ. 68, London, 1992, pp. 41^60.

[7] J.P. Lowman, G.T. Jarvis, E¡ects of mantle heat sourcedistribution on supercontinent stability, J. Geophys. Res.104 (1999) 12733^12746.

[8] H. Zeyen, F. Volker, V. Wehrle, K. Fuchs, S.V. Sobolev,R. Altherr, Styles of continental rifting crust-mantle de-tachement and mantle plume, Tectonophysics 278 (1997)329^352.

[9] I. Manighetti, P. Tapponnier, V. Courtillot, S. Gruszow,P.Y. Gillot, Propagation of rifting along the Arabia-So-malia plate boundary: the Gulfs of Aden and Tadjoura,J. Geophys. Res. 102 (1997) 2681^2710.

[10] P.T. Meijer, M.J.R. Wortel, Cenozoic dynamics of theAfrican plate with emphasis on the Africa-Eurasia colli-sion, J. Geophys. Res. 104 (1999) 7405^7418.

[11] L. Jolivet, C. Faccenna, Mediterranean extension andthe Africa-Eurasia collision, Tectonics 19 (2000) 1095^1106.

[12] J. Dercourt, L.P. Zonenshain, L.E. Ricou, V.G. Kuzmin,X. Le Pichon, A.L. Knipper, C. Grandjacquet, I.M.Sbortshikov, J. Geyssant, C. Lepvrier, D.H. Pechersky,J. Boulin, J.C. Sibuet, L.A. Savostin, O. Sorokhtin, M.Westphal, M.L. Bazhenov, J.P. Lauer, B. Biju-Duval,Geological evolution of the Tethys belt from the Atlanticto the Pamir since the Lias, Tectonophysics 123 (1986)241^315.

[13] F. Berberian, M. Berberian, Tectono-plutonic episodes inIran, in: H.K. Gupta, F.M. Delany (Eds.), Zagros, HinduKush, Himalaya, Geodynamic Evolution, GeodynamicsSer. 3, Washington, DC, 1981, pp. 5^32.

[14] M.P. Searle, J. Cox, Tectonic setting, origin, and obduc-tion of the Oman ophiolite, Geol. Soc. Am. Bull. 111(1999) 104^122.

[15] M.R. Hempton, Structure and deformation history of theBitlis suture near Lake Hazar, southeastern Turkey, Geol.Soc. Am. Bull. 96 (1985) 233^243.

[16] R.C. Quittmeyer, A.L. Kafka, J.G. Armbruster, Focalmechanisms and depths of earthquakes in central Paki-stan: a tectonic interpretation, J. Geophys. Res. 89(1984) 2459^2470.

EPSL 6847 4-11-03

N. Bellahsen et al. / Earth and Planetary Science Letters 216 (2003) 365^381 379

[17] A.H. Kazmi, M.Q. Jan, Geology and Tectonics of Paki-stan, Graphic Publishers, Karachi, 1997, 554 pp.

[18] H. Bijwaard, W. Spakman, E.R. Engdahl, Closing the gapbetween regional and global travel time tomography,J. Geophys. Res. 103 (1998) 30055^30078.

[19] C.J. Ebinger, N.H. Sleep, Cenozoic magmatism through-out east African resulting from impact of a single plume,Nature 395 (1998) 788^791.

[20] N.H. Kenea, C.J. Ebinger, D.C. Rex, Late Oligocene vol-canism and extension in the southern Red Sea Hills, Su-dan, J. Geol. Soc. London 158 (2001) 285^294.

[21] D. Rouby, T. Souriot, J.P. Brun, P.R. Cobbold, Displace-ments, strains, and rotations within the Afar depression(Djibouti) from restoration in map view, Tectonics 15(1996) 952^965.

[22] I. Manighetti, P. Tapponnier, V. Courtillot, Y. Gallet, E.Jacques, P.Y. Gillot, Strain transfer between discon-nected, propagating rifts in Afar, J. Geophys. Res. 106(2001) 13613^13665.

[23] G. Eagles, R. Gloaguen, C. Ebinger, Kinematics of theDanakil microplate, Earth Planet. Sci. Lett. 203 (2002)607^620.

[24] C.J. Ebinger, T. Yemane, D.J. Harding, S. Tesfaye, S.Kelley, D.C. Rex, Rift de£ection, migration, and propa-gation Linkage of the Ethiopian and Eastern rifts, Africa,Geol. Soc. Am. Bull. 112 (2000) 163^176.

[25] I.A. Utskins, P.R. Renne, E. Wolfenden, J. Baker, D.Ayalew, M. Menzies, Matching conjugate volcanic riftedmargins 40Ar/39Ar chrono-stratigraphy of pre- and syn-rift bimodal £ood volcanism in Ethiopia an Yemen, EarthPlanet. Sci. Lett. 198 (2002) 289^306.

[26] W. Ghebreab, Tectonics of the Red Sea region reassessed,Earth Sci. Rev. 45 (1998) 1^44.

[27] T.H. Dixon, R.J. Stein, I.M. Hussein, Control of Red Seageometry by Precambrian structures, Tectonics 6 (1987)551^571.

[28] H.J. Bayer, H. Ho«tzl, A.R. Jado, B. Roscher, W. Voggen-reiter, Sedimentary and structural evolution of the north-west Arabian Red Sea margin, Tectonophysics 153 (1988)137^151.

[29] X. Le Pichon, J.M. Gaulier, The rotation of Arabia andthe Levant fault system, Tectonophysics 153 (1988) 271^294.

[30] A.M. Quennel, The structural and geomorphic evolutionof the Dead Sea rift, J. Geol. Soc. London 114 (1958) 1^14.

[31] J. Roger, J.P. Platel, C. Cavelier, C.B.D. Grisac, Donne¤esnouvelles sur la stratigraphie et l’histoire ge¤ologique duDhofar (Sultanat d’Oman), Bull. Soc. Ge¤ol. France 5(1989) 265^277.

[32] F. Watchorn, G.J. Nichols, D.W.J. Bosence, Rift relatedsedimentation and stratigraphy, southern Yemen (Gulf ofAden), in: B.H. Purser, D.W.J. Bosence (Eds.), Sedimen-tation and Tectonics of Rift Basins: Red Sea-Gulf ofAden, Chapman and Hall, London, 1998, pp. 165^189.

[33] P. Huchon, F. Jestin, J.M. Cantagrel, J.M. Gaulier, S.A.Khirbash, A. Gafaneh, Extensional deformation in Ye-

men since Oligocene and the Africa-Arabia-Somalia triplejunction, Ann. Tecton. 5 (1991) 141^163.

[34] C. Lepvrier, M. Fournier, T. Berard, J. Roger, Cenozoicextension in coastal Dhofar (southern Oman): Implica-tions on the oblique rifting of the Gulf of Aden, Tecto-nophysics 357 (2002) 279^293.

[35] P. Huchon, K. Khanbari, Rotation of the syn-rift stress¢eld of the northern Gulf of Aden margin, Yemen, Tec-tonophysics 364 (2003) 147^166.

[36] M.O. Withjack, W.R. Jamison, Deformation produced byoblique rifting, Tectonophysics 126 (1986) 99^124.

[37] O. Dauteuil, P. Huchon, F. Quemeneur, T. Souriot, Prop-agation of an oblique spreading centre: the western Gulfof Aden, Tectonophysics 332 (2001) 423^442.

[38] P. England, D. McKenzie, A thin viscous sheet model forcontinental deformation, Geophys. J. R. Astron. Soc. 70(1982) 295^351.

[39] Y. Ricard, C. Doglioni, R. Sabadini, Di¡erential rotationbetween lithosphere and mantle: a consequence of lateralmantle viscosity variations, J. Geophys. Res. 96 (1991)8407^8415.

[40] H.P. Bunge, M.A. Richards, J.R. Baumgardner, A sensi-tivity study of three-dimensional spherical mantle convec-tion at 10 (super 8) Rayleigh number, J. Geophys. Res.102 (1997) 11991^12007.

[41] C. Lithgow-Bertelloni, M.A. Richards, The dynamics ofcenozoic and mesozoic plate motions, Rev. Geophys. 36(1998) 27^78.

[42] M. Tetzla¡, H. Schmeling, The in£uence of olivine meta-stability on deep subduction of oceanic lithosphere, Phys.Earth Planet. Inter. 120 (2000) 29^38.

[43] D.L. Turcotte, G. Schubert, Geodynamics. Applicationsof Continuum Physics to Geological Problems, Wiley,New York, 1982, 450 pp.

[44] S. Zhong, M. Gurnis, Mantle convection with plates andmobile, faulted plate margins, Science 266 (1995) 838^843.

[45] G.F. Davies, Penetration of plates and plumes throughthe mantle transition zone, Earth Planet. Sci. Lett. 133(1995) 507^516.

[46] U.R. Christensen, The in£uence of trench migration onslab penetration into the lower mantle, Earth Planet. Sci.Lett. 140 (1996) 27^39.

[47] J. Ito, S.D. King, The in£uence of thermodynamic for-mulation on simulations of subduction zone geometry andhistory, Geophys. Res. Lett. 25 (1998) 1463^1466.

[48] C.R. Bina, Mantle discontinuities, Rev. Geophys. 29(1991) 783^793.

[49] S.H. Kirby, W.B. Durham, L.A. Stern, Mantle phase-changes and deep-earthquake faulting in subducting lith-osphere, Science 252 (1991) 216^225.

[50] B.H. Hager, Subducted slabs and the geoid: Constraintson mantle rheology and £ow, J. Geophys. Res. 89 (1984)6003^6015.

[51] G.F. Davies, G.F. Richards, Mantle convection, J. Geol.100 (1992) 151^206.

[52] J.X. Mitrovica, A.M. Forte, Radial pro¢le of mantle vis-cosity Results from the joint inversion of convection and

EPSL 6847 4-11-03

N. Bellahsen et al. / Earth and Planetary Science Letters 216 (2003) 365^381380

postglacial rebound observables, J. Geophys. Res. 102(1997) 2751^2769.

[53] D.W. Forsyth, S. Uyeda, On the relative importance ofdriving forces on plate motion, Geophys. J. R. Astron.Soc. 43 (1975) 163^200.

[54] D. McKenzie, The initiation of trenches: a ¢nite ampli-tude instability, in: M. Talwani, W.C. Pitman (Eds.), Is-land Arcs, Deep Sea Trenches and Back Arc Basins,Maurice Ewing Series 1, 1977, pp. 57^61.

[55] C.P. Conrad, B. Hager, E¡ects of plate bending and faultstrength at subduction zones on plate dynamics, J. Geo-phys. Res. 104 (1999) 17551^17571.

[56] T.W. Becker, C. Faccenna, R.J. O’Connell, D. Giardini,The development of slabs in the upper mantle: Insightsfrom numerical and laboratory experiments, J. Geophys.Res. 104 (1999) 15207^15226.

[57] F. Funiciello, C. Faccenna, D. Giardinni, K. Regenauer-Lieb, Dynamics of retreating slabs (part 2): insights from3-D laboratory experiments, J. Geophys. Res. B 108(2003) 10.1029/2001JB000896.

[58] C. Faccenna, F. Funiciello, D. Giardini, F.P. Lucente,Episodic back arc extension during restricted mantle con-vection in the Central Mediterranean, Earth Planet. Sci.Lett. 187 (2001) 105^116.

[59] S.D. King, B.H. Hager, The relationship between platevelocity and trench viscosity in newtonian and power-law subduction calculations, Geophys. Res. Lett. 17(1990) 2409^2412.

[60] C. Faccenna, P. Davy, J.P. Brun, R. Funiciello, D. Giar-dini, M. Mattei, T. Nalpas, The dynamics of back-arcextension: an experimental approach to the opening ofthe Tyrrhenian Sea, Geophys. J. Int. 126 (1996) 781^795.

[61] R. Weijermars, H. Schmeling, Scaling of Newtonian andnon-Newtonian £uid dynamics without inertia for quanti-tative modelling of rock £ow due to gravity (including theconcept of rheological similarity), Phys. Earth Planet. In-ter. 43 (1986) 316^330.

[62] G.A. Houseman, D. Gubbins, Deformation of subductedoceanic lithosphere, Geophys. J. Int. 131 (1997) 535^551.

[63] J. Besse, V. Courtillot, Paleogeographic maps of the con-tinents bordering the Indian Ocean since the early Juras-sic, J. Geophys. Res. 93 (1988) 11791^11808.

[64] J. Dercourt, L.E. Ricou, B. Vrielynck, Atlas Tethys Pa-laeoenvironmental Maps, Gauthier-Villars, Paris, 1993,307 pp.

[65] C.R. Scotese, L.M. Gahagan, R.L. Larson, Plate tectonicreconstructions of the Cretaceous and Cenozoic oceanbasins, Tectonophysics 153 (1988) 27^48.

[66] R. Wortel, S. Cloething, On the origin of the Cocos-Naz-ca spreading center, Geology 9 (1981) 425^430.

[67] S. Zhong, Role of ocean-continent contrast and continen-tal keels on plate motion, net rotation of lithosphere, andthe geoid, J. Geophys. Res. 106 (2001) 703^712.

[68] P.G. Silver, R.M. Russo, C. Lithgow-Bertelloni, Couplingof South American and African plate motion and platedeformation, Science 279 (1998) 60^63.

[69] W.J. Morgan, Hotspot tracks and the early rifting of theAtlantic, Tectonophysics 94 (1983) 123^139.

[70] S.M. Berhe, Geologic and geochronologic constraints onthe evolution of the Red Sea-Gulf of Aden and AfarDepression, J. Afr. Earth Sci. 5 (1986) 101^117.

[71] A. Hubert-Ferrari, G. King, I. Manighetti, R. Armijo, B.Meyer, P. Tapponnier, Long-term elasticity in the conti-nental lithosphere; modelling the Aden Ridge propaga-tion and the Anatolian extrusion process, Geophys. J.Int. 153 (2003) 111^132.

[72] V. Courtillot, R. Armijo, P. Tapponnier, Kinematics ofthe Sinai triple junction and a two-phase model of Ara-bia-Africa rifting, in: M.P. Coward, J.F. Dewey, P.L.Hancock (Eds.), Continental Extensional Tectonics,Geol. Soc. Spec. Publ. 28, London, 1987, pp. 559^573.

[73] F. Martinez, J.R. Cochran, Structure and tectonics ofthe northern Red Sea catching a continental marginbetween rifting and drifting, Tectonophysics 150 (1988)1^32.

[74] E. d’Acremont, S. Leroy, N. Bellahsen, M.O. Beslier, M.Fournier, P. Gente, P. Patriat, J.M. Fleury, The conjugatepassive margins of the eastern Gulf of Aden: Structureand oceanic initiation, Geophys. Res. Abstract 5 (2003)09586.

[75] D. Chu, D.G. Gordon, Current plate motions across theRed Sea, Geophys. J. Int. 135 (1998) 313^328.

[76] F. Jestin, P. Huchon, J.M. Gaulier, The Somalia plateand the East African Rift System: present day kinematics,Geophys. J. Int. 116 (1994) 637^654.

[77] R.E. Reilinger, S.C. McClusky, M.B. Oral, R.W. King,M.N. Toksoz, A.A. Barka, I. Kinik, O. Lenk, I. Sanli,Global Positioning System measurements of present-daycrustal movements in the Arabia-Africa-Eurasia plate col-lision zone, J. Geophys. Res. 102 (1997) 9983^9999.

[78] D.F. Argus, R.G. Gordon, No-net-rotation model of cur-rent plate velocities incorporating plate motion modelNUVEL-1, Geophys. Res. Lett. 18 (1991) 2039^2042.

[79] M. Fournier, P. Patriat, S. Leroy, Reappraisal of theArabia-India-Somalia triple junction kinematics, EarthPlanet. Sci. Lett. 189 (2002) 103^114.

EPSL 6847 4-11-03

N. Bellahsen et al. / Earth and Planetary Science Letters 216 (2003) 365^381 381