Flexure and seismicity across the ocean–continent 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

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