Flexure and seismicity across the oceancontinent transition in the Gulf of Cadiz

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  • Journal of Geodynamics 47 (2009) 119129

    Contents lists available at ScienceDirect

    Journal of Geodynamics

    journa l homepage: ht tp : / /www.e lsev ie

    Flexure enGulf of

    Maria C.a CIMA-FCMA,b UIED-FCT, Un

    a r t i c l

    Article history:Received 8 NoReceived in reAccepted 15 Ju

    Keywords:FlexureStrengthOceancontSeismicityGulf of Cadi

    loading cstresat we

    modelling along a previously studied (Fernandez, M., Marzn, I., Torn, M., 2004. Lithospheric transitionfrom the Variscan Iberian Massif to the Jurassic oceanic crust of the Central Atlantic. Tectonophysics, 386,

    1. Introd

    The Guthe seawAfrica-Eu(Fig. 1). Itseveral prrelated tocreationconvergethe Oligo(3) the wMiocene,basin, wh

    In proapproximup of a p

    CorrespE-mail a

    0264-3707/doi:10.1016inent transition

    z

    97115) vertical section of the lithosphere, approximately perpendicular to the Africa-Eurasia conver-gence. We nd that the exural stresses are focussed in the oceancontinent transition, within a zoneapproximately 150km wide, between the base of the continental slope and the Horseshoe Abyssal Plain.We show that the exural stresses are mainly supported by the upper mantle and predict their values fortwo different thermal scenarios. The compositional layering in the crust is shown to play an importantrole in the focussing of the strain energy along the crust/mantle interface. Finally, we observe that there isa correlation between the modelled strain energy and the earthquake distribution. The maximum com-pressive stress difference can be as much as 65% of the strength in compression. The maximum inuenceis observed at 10km depth near the Horseshoe Abyssal Plain. We conclude that exural stresses aloneare not enough to cause rupture or yielding in the Gulf of Cadiz. However, like plate boundary forces andinherited mechanical weaknesses, they need to be incorporated when assessing seismic hazard in thisregion.

    2008 Elsevier Ltd. All rights reserved.

    uction

    lf of Cadiz, including the Algarve continental margin andard continuation of the Guadalquivir Basin, frames therasia plate boundary to the west of the Gibraltar Straits structure is the result of a complex evolution involvingocesses (e.g. Grcia et al., 2003): (1) extensional processesPangea rifting and Atlantic opening which led to the

    of a passive margin in the western part of the Gulf; (2)nce between the African and Eurasian plates that, sincecene, dominates the structural and tectonic setting; andestward movement of the Alboran domain during theresponsible for the Betics and the Guadalquivir forelandich caused the emplacement of allochthonous terrains.cess (1), rifting along the southern Iberian margin startedately in the Early Jurassic (190Ma) resulting in the break-reviously existing large carbonate and clastic shelf (e.g.

    onding author. Tel.: +351 289800938; fax: +351 289800069.ddress: mcneves@ualg.pt (M.C. Neves).

    Andeweg, 2002). Active rifting changed to post-rift during the LateJurassic (160Ma) with related thermal subsidence lasting untilthe Cretaceous (Vera, 1988 in Andeweg, 2002). The early evolu-tion of the margin and the limits of the oceanic and continentaldomains in the Gulf of Cadiz remain unclear. Wide-angle and seis-mic reection data, as well as gravity data, indicate a continentaldomain beneath the central Gulf of Cadiz, with Moho depths vary-ing between 30km near the coastline to 20km offshore, and anoceanic domain in the region of the Horseshoe Abyssal Plain, withMoho depths of 12km (e.g. Purdy, 1975; Gonzlez-Fernndez etal., 2001; Fernandez et al., 2004). However, the location of theoceancontinent transition is controversial, with some authorsarguing that a portion of oceanic lithosphere, formed during theTethys opening, is still present in a presumed fore arc region in thecentral part of the Gulf (e.g. Maldonado et al., 1999; Gutscher et al.,2002). In the cross-section of the lithosphere we consider in thisstudy, the oceancontinent transition occurs in the contact areabetween transition zones 1 and 2 in Fig. 1.

    Process (2), the convergence between the Africa and the Eurasiaplates, has been one of the most important sources of stress notonly in the Gulf of Cadiz but also all over Iberia (Andeweg, 2002).

    $ see front matter 2008 Elsevier Ltd. All rights reserved./j.jog.2008.07.002and seismicity across the oceancontinCadiz

    Nevesa,, Rui G.M. Nevesb

    Universidade do Algarve, Campus de Gambelas, 8000 Faro, Portugaliversidade Nova de Lisboa, Monte da Caparica, 2829 Caparica, Portugal

    e i n f o

    vember 2007vised form 30 June 2008ly 2008

    a b s t r a c t

    In theGulf ofCadiz thewater/sedimentlithosphere are sources of vertical loadgate the relation between the bendingof deformation and seismicity. For thr .com/ locate / jog

    t transition in the

    andthedensity contrastsbetween thecontinental andoceanicausing exure. The main objective of this work is to investi-ses associated with exural isostasy and the observed patterncombine a strength analysis and nite element numerical

  • 120 M.C. Neves, R.G.M. Neves / Journal of Geodynamics 47 (2009) 119129

    Fig. 1. Locatioital data (htt(http://www.i(http://www.sand the OceanSVF: S. Vicenttransition 1, tr

    Large-scaleIberia, withbeen shownand southekinematicsEurasia cona rate of ap2007; Nocqgence is essas evidenceent orientatthrusts andet al., 2003;1999; Zitellet al., 2004the seismicited to an En map of the study area showing the modelled prole (thick solid line). Shadedp://www.ngdc.noaa.gov/mgg/gebco/gebco.html), with contour interval at 500m. Episc.ac.uk) complemented with data from the IM catalogue between 1970 and 2000. Focal meismology.harvard.edu/projects/CMT/). Structural domains, in decreasing grey intensity fic domain taken from Medialdea et al. (2004). Faults traced after Terrinha et al. (submiFault, MPF: Marqus de Pombal fault, PSF: Pereira de Sousa fault, GB: Gorringe Bank, CPansition 2 and continental) according to the crustal structure and seismicity distribution

    lithosphere andupper crustal foldingdistributed acrossdominant wavelengths of 250km and 50km, hasto have resulted from shortening at both the northern

    rn margins of Iberia (Cloetingh et al., 2002). The platecomputed fromGPSdata shows thatpresent-dayAfrica-vergence occurs in a NWSE to WNWESE direction atproximately 45mm/year (e.g. Fernandes et al., 2003,uet and Calais, 2004). In the Gulf of Cadiz this conver-entially accommodated by diffuse brittle deformationd by widely developed tectonic structures, with differ-ions andkinematics: predominantlyNNESSWstrikingWNW/ESE trendingdextral strike-slip faults (e.g. GrciaSartori et al., 1994; Pinheiro et al., 1996; Hayward et al.,ini et al., 2001, 2004; Terrinha et al., 2003; Medialdea). Other evidence of this diffuse brittle deformation isity in the area. Earthquake epicentres are mainly lim--W trending area about 150km wide and ranging from

    7W to 12

    150km (e.gdepths (

  • M.C. Neves, R.G.M. Neves / Journal of Geodynamics 47 (2009) 119129 121

    exural effects may also arise due to the lateral density varia-tions created by the co-existence of the oceanic and continentaldomains.

    Howeveignored theequilibriumet al. (2004ously the gto estimatelithosphereIt was alsoet al. (2002order approthe Gulf ofses about kindicationsing their faucannot resostresses.

    In this ware threefoand assessof Cadiz; focal prole(2) investigtribution; fFernandezalong the sthis regionstresses andinduced byearthquakewith the nuexure andity.

    2. Crustal

    The lithoject of numhazard riskles of theand wide-ayears withBIGSETS98,these projecthe crust antion of nearIAM data rcentral parttinental ba(Gonzlez-Ffrom a dept1012km inless than 7ket al., 1994;of the crustaccepted onmodelling a1999; Grcidomain ofthere is no c

    The sectresented in

    San Vincent (CSV) it coincides with seismic reection prole IAM3,already studied in great detail by several authors (e.g. Gonzlezet al., 1996; Tortella et al., 1997). In the SAP the crustal structure

    d ondeat is bion drictlythethe g

    ardinhe sethe

    er par parspa

    inter840conding otwo

    iddleed bere Pkm/8.2he r

    sediariabsternfromnarythe

    400chaogion..0 km/s inso foSAPnortg bohrust dedea ee amkm)Medre al2). Tg Wonsted f

    mici

    seisc Ca020n) forsmicthinr, previous numerical models in the Gulf of Cadiz havestrength of the lithosphere and assumed local isostatic. This assumption was made in the study by Fernandez) who used a nite element code to solve simultane-eopotential, lithostatic and heat transport equationsthe rock parameters and thermal properties of thealong a prole across the oceancontinent transition.made by Jimnez-Munt et al. (2001) and Negredo

    ) who used the thin sheet approach to provide rst-ximations of the strain and strain rate distribution inCadiz. These efforts allowed the testing of hypothe-inematic poles and boundary conditions and providedon the long-term seismic hazard of faults by estimat-lt slip rates. Nevertheless, this 2D horizontal approachlve vertical variations of strength and neglects exural

    ork we aim to address these issues. Our main goalsld: (1) provide an estimate of the bending stresseshow important exural isostasy may be in the Gulfr this we model the Fernandez et al. (2004) verti-

    cutting across the continental and oceanic domains;ate the role of a layered rheology in the stress dis-or this we use the crustal structure documented byet al. (2004) to infer the vertical strength variationsame prole and estimate the mechanical thickness in; and (3) explore the relation between the bendingthe seismicity. Recognising that the bending stresses

    exural loading might be one of the possible causes ofs (e.g. Watts, 2001), we combine the strength analysismerical modelling to investigate the relation betweenthe observed pattern of deformation and seismic-

    structure

    spheric structure in the Gulf of Cadiz has been the sub-erous seismic campaigns justied in part by the seismicin this region. In addition to the seismic reection pro-oil industry, many near-vertical reection, refractionngle reection proles have been acquired in recentin the scope of projects such as RIFANO92, IAM93,TASYO-2000, SISMAR2001 and VOLTAIRE2002. Amongts IAM93was pioneer in revealing the deep structure ofd mantle (Banda et al., 1995). The combined interpreta--vertical reection and refraction/wide-angle reectionst suggested that the crust underlying the eastern andof the Gulf of Cadiz is of continental type, with a con-

    sement formed by Precambrian and Palaeozoic rocksernndez et al., 2001). The Moho gradually shallowsh of 3032km in the continental margin of SW Iberia tothe Horseshoe Abyssal Plain (HAP), reaching values ofm in the Seine Abyssal Plain (SAP) (Purdy, 1975; SartoriMatias, 1996; Gonzlez et al., 1998). The oceanic naturein theHAP, the SAP and in theGorringe Bank is generallythe grounds of seismic interpretation, gravity anomalynd bottom sampling (e.g. Purdy, 1975; Hayward et al.,a et al., 2003). In the region between the continentalthe Gulf of Cadiz and the oceanic domain of the HAPlear evidence of crust type.ion of the lithosphere considered in this study is rep-Fig. 2. Between the eastern end of the HAP and Cape

    is baseMedialZone, irefractthan stporatedeneers.

    Regalong taroundan uppa lowedomainhas ancrust/2(3) a seconsistsists ofand macterizCSV. H6.26.4and 8.1sition tand 2.

    Theing a vthe eain ageQuaterAmongaboutfor itsthis refrom 23.7 kmsion al

    Thewith aaffectinthese tsole ouMedialas larg(10601999;folds aal., 200trendinifestatisubmit

    3. Seis

    TheSeismifor 197Bulletithe seitionwithe work of Purdy (1975), Sartori et al. (1994) andet al. (2004). Inland of CSV, in the South Portugueseased on the interpretation of seismic wide-angle andata by Matias (1996) and Gonzlez et al. (1998). Ratherfollowing the seismically dened interfaces, we incor-

    results of the modelling of Fernandez et al. (2004) toeometry and physical properties of the different lay-

    g the basement, four main zones can be identiedction of Fig. 2. From SW to NE: (1) an oceanic domainSAP where the crust, about 7km thick, is divided intort with P-wave velocities in the range 4.15.6 km/s andt with velocities of about 6.3 km/s; (2) a transitionalnning from around km-300 to km-150 where the crustmediate composition/density between that of oceanickg/m3 and that of upper continental crust/2740kg/m3;transitional domain ranging fromkm-150kmtokm-50f stretched continental lithosphere. This domain con-layers with seismic velocities analogous to the uppercontinental crust; and (4) a continental crust char-

    y three layers that thickens to 30km within 50km of-wave velocities are 5.26.1 km/s in the upper crust,s in the middle crust, 6.76.9 km/s in the lower crustkm/s in the mantle. We call the oceancontinent tran-egion near km-150 lying between transitional zones 1

    mentary cover is continuous from the SAP to CSV hav-le thickness that reaches its maximum (2 s TWT) inHAP. It consists of ve stratigraphic units, spanningupper Jurassic-lower Aptian at the base to Miocene-at the top (Tortella et al., 1997; Medialdea et al., 2004).se units the allochthonous body of the Gulf of Cadiz,500ms TWT thick in the HAP, is particularly relevanttic character and role in the geodynamic evolution ofP-wave velocities in the depositional sequence range/s in the post-Miocene marine sediments at the top towhat is interpreted as the Mesozoic carbonate succes-und in the Algarve margin outcrops.and theHAPare characterisedby active thrust tectonics,hwestward verging thrust system trending NNE-SSWth the sediment cover and the basement. Some of

    t faults are observed as internal crustal reectors thatveloping seaoor elevations (e.g. Terrinha et al., 2003;t al., 2004). Shortening in the region is also expressedplitude (up to 800m) and intermediate wavelengthfolds of the sediments and basement (Hayward et al.,

    ialdea et al., 2004). Similar intermediate wavelengthso observed within the Iberia Peninsula (Cloetingh ethe NNESSW thrust system is crosscut by lineamentsNWESE that have been interpreted as seaoor man-of deep right-lateral strike-slip faults (Terrinha et al.,or publication).

    ty

    micity data used in this study is taken from the IMtalogue of Continental Portugal and adjacent region00, and from the ISC catalogue (http://www.isc.ac.uk/the period from 1964 to the present. As shown in Fig. 1

    ity from 12W to 7W is distributed in an E-W direc-a band of approximately 150km.Within this band three

  • 122 M.C. Neves, R.G.M. Neves / Journal of Geodynamics 47 (2009) 119129

    Fig. 2. Structu e extekg/m3. OC: oc ntal caccording to th

    regions of ccan be recoruns fromregion of ththeHAP (29have small

    Earthquoceanic andoceanic lithmantle so tare considethe explanamore contromic at these

    Focal meing, with tGrimison anThe strike-swith right-lconsideringwith a NNW(1), close tand Eurasiashow the sahypothesisThere is nostress is stesuggestinging at shallo(Stich et al.,

    Fig. 3 shconsideredwide bandGorringe Baquakes at 1database asCarrilho, peshown in thaxis in Fig.below sea l

    Lookingzones: (1) adomain calllarger earth(3) another

    , marf theuggecrustmagnesenonch-50

    t theentalg a

    he stte the-sli

    olog

    form

    onstCadt is bper

    creepbrit

    trengof fare of the crust along the prole studied by Fernandez et al. (2004) showing also theanic crust, TC: transitional crust, UC: upper continental crust, MC: middle continee seismicity distribution (in Fig. 3).

    oncentrated events, which seem to be SWNE oriented,gnised: the region of the Gorringe Bank, the region thatCSV along the S. Vicente Canyon to the HAP, and thee Guadalquivir Bank. The largest events concentrate in.2.1969,Ms=8and12.2.2007,Mw=6.1)but themajorityto moderate magnitudes (Ms 4)top of the crust and in the upper mantle beneath theshelf. These events have known fault plane solutions

    dominant strike-slip faulting regime. Extension paral-eepest topography due to lateral density variations cane strike-slip regime here, i.e. modify pure compressionp (Andeweg, 2002).

    y

    ation laws

    rain the mechanical structure of the lithosphere in theiz we use the concept of yield strength envelope. Thisased on the assumption that at low conning pressuresatures fracturing is predominant, while at high temper-deformationmechanismsdominate in the lithosphere.tle regime the critical stress difference at failure (theth) is given by the Coulomb frictional law, assuming

    vourable orientation and negligible cohesion (Sibson, g z(1 ) (1)3 is the maximum stress difference, is a constanthe fault type and frictional coefcient, is the aver-of rocks above depth z, and is the pore uid factor.

    nce of information regarding the pore uid factor werostatic conditions (=0.4). A friction coefcient of 0.75r most rock types gives =3.0 in compression (thrustd =0.75 in extension (normal faulting).ctile regime, deformation is assumed to followapower-tion creep equation (Ranalli, 1995),

    B

    )1/nexp

    (A

    nRT

    )(2)

    e strain rate (s1),R is theuniversal ideal gas constant,Tte temperature,A is the creep activation enthalpy andBaterial creep parameters. The yield strength envelope is

  • M.C. Neves, R.G.M. Neves / Journal of Geodynamics 47 (2009) 119129 123

    Fig. 3. Cross-s ight grsubtraction of ide wEvents with m re disdominated by inateand the contin cent (

    built assumis given byas a rst-orsharp.

    WealsoaThis is an avday shortenaveraged ovand also wiinclude the(Jimnez-M

    The rheothe densitythat correladominant lial., 1996). Apetrologicaand densitirheologies w

    The mathe upper culite in theadopted hyate in mosttectono-thecrust we as

    4.2. Temper

    EstimateSouth Portubetween 60a continentow is evemeasuremeindicating a1998; Verzhmodelling yvalue is notlithospherean age of ringe Bank.

    Given thstructed thby xing between thfor oceanicPlate Coolin

    25kthet conayerrGT3C+Me upantleuctud offnd thed inpre

    olempref Cadprodcontthegranhereeaniin thduch rerengtged

    rhepe isToadh-dehownection showing the vertical seismicity distribution along the modelled prole. The lthe seawater-depth. The focal depths of the earthquakes that fall within a 75km wagnitude greater than 4 are displayed as larger grey circles. Four different zones alarge earthquakes in the Horseshoe Abyssal Plain (see Fig. 1), transition zone 1 domental domain characterised by an essentially aseismic mantle inland of Cape St. Vin

    ing that at each depth the maximum stress differencethe minimum of Eqs. (1) and (2). We further assume,der approximation, that the brittleductile transition is

    ssumethat thepresent-daybulk strain rate is1016 s1.erage value consistent with measurements of present-ing rates of 45mm/year (e.g. Fernandes et al., 2003)er an area of horizontal deformation of about 150km,th numerical modelling results in the Gulf of Cadiz thatcontribution of both fault slip and inelastic deformationunt et al., 2001; Negredo et al., 2002).logical structure along the prole was derived fromand seismic velocity model (Fig. 2) following studieste seismic velocitiesmeasured in the laboratory and thethology (e.g. Christensen and Mooney, 1995; Okaya etlthough a unique correlation does not exist, a rst-orderl classication derived from seismic P-wave velocitieses is listed in Table 1. Creep parameters for the severalere taken from Afonso and Ranalli (2004).

    in lithologic units in the continent are quartzite inrust, felsic granulite in the middle crust, mac gran-lower crust and peridotite in the mantle. We have

    drated rheologies because these seem to be appropri-continental environments affected by post-Paleozoicrmal events (Afonso and Ranalli, 2004). For the oceanicsumed the wet diabase deformation law.

    ature and strength

    s of surface heat ow in SW Iberia are scarce. In theguese Zone the few measurements available rangeand 70mW/m2 (Fernandez et al., 1998), sowe assumedal surface heat ow of 64mW/m2. Offshore the heat

    ness =1margin1D heathree-ltion. Focrust (Uused thand mmal strstripperates aare list

    Thethe prfor coGulf owouldIn theers in(felsictrast, tand octiallybrittlestrengteral stunchanered.

    Theenvelo2003).strengtnario sn less constrained with very few available heat ownts in the region of the Horseshoe Abyssal Plain (HAP)n average value of 5715mW/m2 (Fernandez et al.,bitsky and Zolotarev, 1989). The Fernandez et al. (2004)ielded a value of about 40mW/m2 in the HAP but thisconsistent with the asymptotic geotherm for oceanicmore than120Maold. (Hayward et al. (1999) suggested152Ma for the oceanic lithosphere in the nearby Gor-)e scarcity of surface heat owmeasurementswe recon-e temperature structure along the modelled proleve geotherms (GT1 to GT5) and linearly interpolatingen (Fig. 4a). GT4 (=GT5) is the asymptotic geothermlithosphere 120Ma old, computed according to thegModel (mantle temperature =1330 C and plate thick-

    between geCowie et altinental geosurface heaT2 is the gaccording tageof theocpart of the Gmay perhapeffect of theand strain rthe brittlereduction oNeverthelesresides mosey region displays the crustal geometry, without sediments, after theindow in either side of the prole are projected on the cross-section.tinguished: a practically aseismic oceanic domain, transition zone 2d by small magnitude earthquakes clustering at the base of the crust,at km 0).

    m). In the transition zones and in the continentalgeotherms were computed by solving the steady stateduction equation with radiogenic heat production forlithospheric models of varying thickness and composi-at km-150 the three layers consideredwere sediments,C average, see Fig. 2) and mantle. For GT2 and GT1 we

    per crust, the combinedmid- and lower crust (MC+LC),. Note that the sediments are included in the ther-re computation for their blanketing effect but they arethe numerical model (Section 5). The heat productionermal conductivities for the several rock compositionsTable 1 andwere taken fromAfonso and Ranalli (2004).sent-day strength envelopes for different sites alongare shown in Fig. 4a. We only show the envelopesssion because this is the dominant regime in theiz. An extensional regime, which can exist locally,uce slightly deeper cut-off depths of brittle strength.inent there is mechanical layering with ductile lay-crust, particularly at the bottom of the middle

    ulite) and lower (mac granulite) crust. In con-are no ductile interleaved layers in the transition

    c zones. There, the upper crust strength is essen-e brittle domain with a slight deepening of the

    tile transition oceanwards. However, most of thesides in the mantle and there are no pronounced lat-h variations. These are general features that remainwhen other sensible composition options are consid-

    ological structure adopted to construct the strengthnot free from ambiguities (e.g. Ranalli, 2003; Burov,dress theeffect of another temperature structureon thepth distribution we considered the hypothetical sce-in Fig. 4b.A cosinebell functionwasused to interpolate

    otherms T1 and T2 (Fig. 4b), inspired on the work of. (2005) who used a similar procedure. T1 is the con-therm computed as in Fig. 4a assuming a continentalt ow of 64mW/m2 and a surface temperature of 15 C.eotherm for oceanic lithosphere 10Ma old computedo the Plate Cooling Model. Taking into account that theeanic lithosphere at the endof the rifting in thewesternulf of Cadiz was 30Ma, the thermal structure in Fig. 4bs resemble that of the active rifting stage. To isolate thetemperature we assumed a present-day composition

    ate. In this hypothetical scenario the cut-off depth ofstrength shallows oceanwards producing a signicantf strength in the transition and oceanic zones (Fig. 4b).s, the strength in the transitional andoceanic zones stilltly in the mantle.

  • 124 M.C. Neves, R.G.M. Neves / Journal of Geodynamics 47 (2009) 119129

    Table 1Thermal and mechanical material parameters

    Sediments UCC (wet quartzite) MCC (wet felsic granulite) LCC (wet mac granulite) OC (wet diabase) TC (wet quartzite) Mantle (wet peridotite)

    Q (Wm3) 1.2 1.4 0.4 0.4 0.4 0.006K (Wm1 K1) 2.3 2.5 2.1 2.1 2.1 3.0 (kgm3) 2400 2740 2800 2950 2840 2800 3300A (MPan s1) 3.2104 8.0103 1.4104 2.0104 3.2104 2.0103B (kJmol1) 154 243 445 260 154 471n 2.3 3.1 4.2 3.4 2.3 4.0E (Pa) 0.71011 0.71011 0.71011 0.71011 0.71011 0.71011 0.25 0.25 0.25 0.25 0.25 0.25

    Q, volumetric heat production rate; K, thermal conductivity; , density; A, E and n, material creep parameters; E, Youngs modulus; , Poissons ratio. Data from Afonso andRanalli (2004) and Fernandez et al. (2004).

    5. Flexural modelling

    Vertical loads in isostatic equilibriumproduceexure andbend-ing when the lithosphere has non-zero strength. However, theimportance of exural isostasy to the overall deformation has notbeen duly emphasized in previous studies of the Gulf of Cadiz. Toaddress this problem we used an elasto-visco-plastic 2D nite ele-ment modelling approach and analysed the deformation in termsof stress and strain distribution with depth.

    5.1. Numerical procedure

    To perform the elasto-plastic simulations we used a modiedversion of the FEVPLIB nite element program that was describedin detail by Bott (1997). This package incorporates elasto-plasticdeformatiomethod anproduced b(e.g. Bott, 1yield strengpower-lawtemperaturlution is nthe excess

    condition for plastic yielding obeys the MohrCoulomb crite-rion.

    The nite element grid that represents the vertical section ofthe lithosphere is 600km long and 140km deep. Quadrilateralelements with eight nodes form a regular grid with maximum res-olution of 1 km0.5 km in the topmost 30km. The plane strainhypothesis, suitable to studyexure in twodimensions, is assumed.As boundary conditions we x the SW end of the model in thehorizontal direction, but let it be free to move in the vertical direc-tion. Other boundaries are free to move in both directions. Isostaticboundary conditions are applied at the surface to simulate isostaticrestoring forces.

    5.2. Applied loads

    devemodepartstionatinensed

    nodaove

    ds, so

    Fig. 4. Cross-sisotherm inter(see text). (B) Amay representand T2 (presenn with a nite yield strength using the viscoplasticd has been used to model stress and displacementsy anomalous density in a variety of tectonic settings997; Zhang and Bott, 2000; Neves et al., 2004). Theth is computed at each depth assuming brittle andcreep deformation depending on temperature. Thee structure is specied as input and the thermal evo-ot modelled. When the yield strength is exceeded,stress is removed using a time stepping scheme. The

    Theof thein twogravitaby con

    Thedirectare remupwarection showing the calculated strength envelops in compression for selected sites along tval. (A) Present-day strength and temperature distribution. The GT1 to GT5 indicate the ln hypothetical thermal scenario and the corresponding strength proles used to evaluatean early stage of the Algarves margin rifting. A cosine bell function was used to interpolatt-day continental geotherm).lopment of topography is theexural isostatic responsel to the vertical loads. The vertical loads are separated: (1) the weight of the sediments and water and (2) thel body forces due to the lateral density contrasts causedtal stretching.iment and water load is applied instantaneously asl forces (Fig. 5a). The seawater and sediment layersd and all density interfaces lying underneath shiftedthat the top of the basement is initially at the sur-he prole and the estimated temperature eld contoured at a 400 Cocation of the geotherms used to estimate the temperature structurethe effect of the temperature on the modelling results. This scenarioe the temperature between geotherm T1 (10Ma oceanic lithosphere)

  • M.C. Neves, R.G.M. Neves / Journal of Geodynamics 47 (2009) 119129 125

    Fig. 5. Loads a sedimapplied at the waterupwards. (C) T subtrstructure show

    face of theforces are indensitydepdensitydep7km thick a3300kg/m3

    subtractingstructure (Fforces andforces and sconcave uptive to the omargin rela

    5.3. Modell

    The loaddistributionlithospheretial stress anseismicity ithe differentical plane ousing this dand followi

    al ohori

    s thafored mat a nfaulormaper

    ergyatio

    Fig. 6. The debathymetry supplied to the numerical model. (A) The dashed line represents the weight of thesurface as direct nodal forces. (B) Restored cross-section of the crystalline crust. Thehe density anomalies that generate internal gravitational body forces are obtained byn above.

    nite element model (Fig. 5b). The gravitational bodycorporated as density anomalies relative to a referenceth prole at the SW end of the model. The referenceth prole comprises two layers: normal oceanic crustnd uniform mantle below, with densities of 2840 and, respectively. The density anomalies are obtained bythe reference depthdensity prole from the densityig. 5c). Negative density anomalies generate buoyancyuplift, while positive anomalies generate downwardubsidence. While the sediment and water load produceward exure, buoyancy of the continental crust rela-

    as normtical orimpliemodelssign antion th(thrustsion (nenergythe endeformceanic crust/mantle causes the uplift of the continentaltive to the ocean basin (Fig. 6).

    ing results

    s in Fig. 5 have been applied to the two strength-depths shown in Fig. 4. The resulting exural response of theis now examined in terms of the patterns of differen-d strain energy. The relation of thesewith the observed

    s discussed in the next section. The differential stress isce between the horizontal and vertical stress in the ver-f the model. The conditions for failure can be describedifference. According to Andersons theory of faultingng Sibson (1974) the faulting regimes can be classied

    ations withimportant i

    Thediffein Fig. 7 for tferential strand is typicat relativelyoccur at theconcentratethetical scerange correexural stretial stressesbetweenkm

    ection of the top surface of the model in response to the topographic and internal gravitapported by exure (grey solid line). The solid dark grey line is the actual bathymetry refeent layer. The solid line is the sum of the water and sediment loadsand sediment layers were removed and all density interfaces shiftedacting the reference depthdensity prole (see text) from the density

    r as thrust if the maximum compressive stress is ver-zontal, respectively. Since the plane strain formulationt the out-of-plane stress is the intermediate stress, oursee a normal or thrust fault regime depending on theagnitude of the differential stress. We use the conven-egative differential stress corresponds to compression

    t regime) and a positive differential stress to an exten-l fault regime). The density of the strain energy (strain

    unit of volume hereafter shortly called strain energy) iscontained in a material as a consequence of its elasticn (Ranalli, 1995). The strain energy state, and its vari-

    time, reect the intensity of crustal activity and is anndex of earthquake potential.rential stress and strain energydue toexure are shownhe actual andhypothetical scenarios. Thepattern of dif-ess reects the downward deection of the lithosphereal of concave elastic bending: negative stresses occurshallow levels due to contraction and positive valuesbottomdue to extension. Vertically, the largest stressesat a depth of approximately 1040km in the hypo-

    nario and at 1050km in the actual state. This depthsponds to the elastic core that is capable of supportingsses on a long time scale. Laterally, the largest differen-occur where the plate curvature reaches its maximum,-200andkm-50.Moreover, there isnosignicantprop-

    tional loads (dotted black line) ts the wavelengths (>150km) of therenced to the depth of the ocean basin at the SW end of the prole.

  • 126 M.C. Neves, R.G.M. Neves / Journal of Geodynamics 47 (2009) 119129

    Fig. 7. Close uadjacent regiothetical (Fig. 4correspondingplotted for the

    agation ofbending issive stressetemperaturthe actual sdipping to tlateral temp

    The pattferential stareas. CompMoho, remato deepen arelease of thto occur in renergy accuthe Moho lidependencein Fig. 8. Theis quite diffincludes coshows thatsituation.

    6. Discussi

    6.1. Flexure

    The timfrom the tim

    observe a correlation between estimates of elastic plate thicknessand estimates of seismogenic layer thickness. Indeed, in the PacicOcean the earthquakes show a distribution and a focal mechanism

    that seems to be directly connected to the up-warping ofanic lithosphere seawarddeep-sea trenchs (e.g.Watts, 2001;et al., 2007). In the continents this relation is harder toproves been strongly debated (e.g. Maggi et al., 2000; Watts and2003;HandyandBrun,2004).Oneof the reasons for suchdif-is that the continents have been subjected to more complexon histories than the oceans. In addition, the lithospheresry justies the existence of earthquakes which are relatede sequence of geological processes and not with the actualation state (e.g. Muller et al., 1996).obvious that exure is not the only source of stress in theCadiz. Geological, geodetic and focal mechanism data showe region is dominated by NWSE compression associatedeAfrica-Eurasia convergence.Moreover, inheritedmechani-knesses are certainly akey factor in thedistributionof stress.erefore expected that the earthquakes are releasing energyrmation associated with the plate convergence, focussed atedmechanicalweaknesses. So thequestion iswhether ornotcontributes to the actual state of stress. In our opinion Fig. 7that there is a clear correlation between strain energy asso-with exure and the earthquake distribution. Furthermore,s thatinuiThisthe Mn thg trehis tp of the modelling results in the oceancontinent transition zone and

    patterntheoceTassaraand haBurov,cultyevolutimemowith thdeform

    It isGulf ofthat thwith thcalweaIt is thof defoinheritexureshowsciatedit showdisconology.belowbetweedippinzone. Tn. Top two panels: stress difference distribution predicted for hypo-b) and the present-day (Fig. 4a) thermal states. Bottom two panels:density of strain energy. The focal depths of the earthquakes are alsoactual state.

    stresses seawards or landwards, that is, the effect ofrather localized in the transition zone. The compres-s focus just below the base of the crust regardless of thee structure, although they spread over a wider area intate. In contrast, the trend of the region of extension,he NE in the hypothetical scenario, is controlled by theerature gradient.ern of strain energy is very similar to that of the dif-ress being distributed into approximately two mainaring the two scenarios the upper area, adjacent to theins nearly stationary, while the lower area is deducednd rotate towards the horizontal in the actual state. Thee strain energy in the formof earthquakes ismore likelyegions where the strain energy accumulates. The strainmulates in the upper mantle and not in the crust. Thus,mits the upper strain accumulation region. This strongof strain energy on composition is even more evidentstrain pattern produced by a uniformcrustmodel (top)

    erent from that produced by a more realistic model thatmpositional layering in the crust (bottom). This resulta layered rheology seems to be more at its place in this

    on

    and earthquakes

    e scale of seismic energy release is certainly differente scale of elastic plate exure. So, it is extraordinary to

    nitude evenearthquakedeeper towsuggests thhave createreactivated

    It is evidthecontineure. These ebe related toeral densitytopographic

    Fig. 8. Dependposition of thcomposition (ain both cases.t there is a relation between seismicity and mechanicalties connecting zones of contrasting density and rhe-relation is particularly visible in the upper mantle justoho between km-110 and km-70. Another correlation

    e modelling results and the seismicity is the apparentnd to the NE of the distributions along the transitionrend is not only suggested by the cluster of small mag-ts below the Moho but also by the larger magnitudeswith known focalmechanismsolutionswhichbecomeards the NE (e.g. between km-200 and km-100). Thisat the strain energy induced by exure in the past maydpreferential regions ofweakness,which are nowbeingin the actual state of compression.ent that there are other clusters of events, particularly inntal region,whichdonot showanycorrelationwithex-vents may be due to other local processes and/or maythe 3Dnature of the deformation. For instance, the lat-variations associated with the changing trend of steepslopes, from nearly EW along the Algarve coast to NS

    ence of the strain energy distribution on composition. Top: the com-e crust is uniform (wet quartzite). Bottom: the crust has a layereds in Fig. 2 and Table 1). The present-day thermal structure is assumedThe strain energy (U) scale is the same as in Fig. 7.

  • M.C. Neves, R.G.M. Neves / Journal of Geodynamics 47 (2009) 119129 127

    Fig. 9. Estima ed ondepth distribu 0) is sh30km depth

    north of thedeformatiotion (Andewusing a fullmodelling wfocal mecha

    6.2. The str

    It has lozones are olithosphereness in extmechanicalbehind suchWhen the bsion at the Wcrust withact as uidtle peridotiof friction opredicted bpentinites mcontributinreduction oconcentratitransition z

    In this stsition zoneof 0.75 tosequently, talong the p4.41013 Nness of theis 48 and 45

    Only inral rigidity athe mechanmechanicaltic thicknesHaywardetadiscontinuof loading (tloaded 12Mmargin alsoof 35km (Tthe Africa-Eweaknesses

    ancytimaordinto cpar

    h enessive ofn atbritts notst ann the

    clus

    he Gntinconte eed thper

    proaal, it afrom

    essioessesrds t. Sincconsin co

    exurwithtes of the integrated strength (solid line) along the oceancontinent transition, bastion of the modelled stress-difference in the actual state (track at km-150 and km-5the stress difference is compressive and below is tensile.

    Gorringe Bank, can lead to extensional and strike-slipn under predominantly pure compressional deforma-eg, 2002). These hypotheses need further investigation3D modelling approach that is currently under way. 3Dill also allow us to explore the relation between thenisms and the strain/stress-depth distribution.

    ength of the oceancontinent transition

    ng been recognised that oceancontinent transitionften weak relative to normal oceanic and continental. This is expressed by low values of elastic plate thick-ended continental lithosphere, reecting a long-termweakness (e.g. Watts and Stewart, 1998). The reasonsa weak behaviour on long-time scales are still unclear.

    eta stretching factor is greater than 3, models of exten-est Iberia margin predict embrittlement of the whole

    development of crustal penetrating faults which canconduits, allowing serpentinization of the upper man-tes (Prez-Gussiny and Reston, 2001). The coefcientf serpentinite (0.3) is considerably lower than thaty Byerlees law (Escartn et al., 1997). Moreover, ser-ay favour the connement of uids within fault zones,

    g to an increase in the pore uid factor and to a strengthf the whole crust. This would partially explain theon of deformation and seismicity in oceancontinentones subjected to large present-day tectonic stresses.udywehavenot incorporated anyweakness in the tran-, since we have used a constant coefcient of frictionbuild the brittle part of the strength envelope. Con-he integrated strength is predicted to increase gentlyrole, from 3.21013 N/m in the continent (km-50) to/m in theocean (km-250) (Fig. 9). Themechanical thick-lithosphere (depth above which the strength>10MPa)

    discrepness es

    AccenoughWe comstrengtcomprthe baspressioof thealone ithe crustress i

    7. Con

    In tand cooceanproducmodellimatelythis apmateriaratelycompring stras regaquakesplacedThe ma

    The tion,km in the oceanic and transition zones, respectively.the case of a single-layer elastic rheology, high exu-nd small curvatures will the elastic thickness approachical thickness (Watts and Burov, 2003). Otherwise, thethickness provides an upper bound limit for the elas-s. Thus, in the oceanic lithosphere of the Gorringe Bank,al. (1999) found that thegravityanomaly is explainedbyous (broken) elastic platemodel 35kmthick at the timehe lithospherewouldhavebeen140Maoldwhen itwasa ago). Flexural modelling results in the Western Iberiaindicate a present-day elastic layer thickness in excess

    . Cunha, personal communication). Yielding related tourasia convergence and the presence of mechanicalnot considered in this study may therefore explain the

    of the mo The focus

    dicted forindepend

    The bendtle and sconcentraoceanco40km dlateral tem

    The straintle. The lastretchedkm-100 ththe strength envelopes in compression shown at selected sites. Theown in dark grey for comparison with the strength envelopes. Above

    between the elastic thickness and mechanical thick-tes in this region.g to our results the exural bending stresses are notause yielding and reduce the effective elastic thickness.e the stress difference in the actual state (Fig. 7) and thevelope at km-150 and km-50 in Fig. 9. The maximume stress difference generated by exure,which occurs atthe crust at km-150, attains 65% of the strength in com-the same depth (10km). Unless there is a reductionle strength in the oceancontinent transition, exuresufcient to produce brittle failure and earthquakes ind uppermantle. However, its contribution to the overallGulf of Cadiz cannot be disregarded.

    ions

    ulf of Cadiz the density contrast between oceanicental crust is a source of vertical loading at theinent transition. Vertical loads in isostatic equilibriumxure when the lithosphere has non-zero strength. Weeexureonavertical sectionof the lithosphere approx-pendicular to the Africa-Eurasia convergence. Althoughch does not incorporate out-of-plane displacement ofllowedus toanalyse thecontributionof theexure sep-other sources of stress such as the dominant NWSE

    n. Our main objective was to explore how the bend-were related to the observed deformation, especially

    he actual release of strain energy in the form of earth-e the strength is an input of themodelling,wehave alsotraints on the mechanical structure of the lithosphere.nclusions of our study are thus the following:

    al stresses are focussed in the oceancontinent transi-in a 150km wide zone between km-250 and km-50

    delled prole.sing of bending stresses on the transition zone is pre-different thermal scenarios so this result is relatively

    ent of temperature.ing stresses are mainly supported by the upper man-how a bipolar distribution. The compressive stresseste just below the Moho (at 10km depth in thentinent transition). The tensile stresses underneath (atepth) show a dipping trend that is controlled by theperature gradient.energy due to exure also resides in the upper man-rgest accumulation of strain energy occurs below thecontinental lithosphere underneath the Moho. Neare strain energy concentration coincideswith an impor-

  • 128 M.C. Neves, R.G.M. Neves / Journal of Geodynamics 47 (2009) 119129

    tant cluster of earthquakes located between the S. Vincent faultand the Horseshoe fault.

    The observed correlation between the strain energy and theearthquaksignican

    The compin the focuface.

    Flexural sing in theinheritedaccount in

    Acknowled

    We wounando CarriWe also acSmith, 1998to the ALMO

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    Flexure and seismicity across the ocean-continent transition in the Gulf of CadizIntroductionCrustal structureSeismicityRheologyDeformation lawsTemperature and strength

    Flexural modellingNumerical procedureApplied loadsModelling results

    DiscussionFlexure and earthquakesThe strength of the ocean-continent transition

    ConclusionsAcknowledgementsReferences

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