Multiphased tectonic evolution of the Central Algerian margin from combined wide-angle and

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Multiphased tectonic evolution of the Central Algerian
margin from combined wide-angle and reflection seismic
data off Tipaza,Algeria
Ang´elique Leprˆetre, Frauke Klingelhoefer, David Graindorge, Philippe
Schnurle, Marie-Odile Beslier, Karim Yelles, Jacques D´everch`ere, Rabah
Bracene
To cite this version:
Ang´elique Leprˆetre, Frauke Klingelhoefer, David Graindorge, Philippe Schnurle, Marie-Odile
Beslier, et al.. Multiphased tectonic evolution of the Central Algerian margin from com-
bined wide-angle and reflection seismic data off Tipaza,Algeria. Journal of Geophysical Re-
search, American Geophysical Union, 2013, 118, pp.3899-3916. <10.1002/jgrb.50318>. <insu-
00915216>
HAL Id: insu-00915216
https://hal-insu.archives-ouvertes.fr/insu-00915216
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JOURNALOFGEOPHYSICALRESEARCH:SOLIDEARTH,VOL.118,3899–3916,doi:10.1002/jgrb.50318,2013
Multiphased tectonic evolution of the Central Algerian margin
from combined wide-angle and reflection seismic data off
Tipaza, Algeria
A.Leprêtre,1 F.Klingelhoefer,2 D.Graindorge,1 P.Schnurle,2 M.O.Beslier,3 K.Yelles,4
J.Déverchère,1 andR.Bracene5
Received23October2012;revised4July2013;accepted30July2013;published26August2013.
[1] TheoriginoftheAlgerianmarginremainsoneofthekeyquestionsstilldiscussedin
theWesternMediterraneansea,duetotheimprecisenatureandkinematicsofthe
associatedbasinduringtheNeogene.Forthefirsttime,thedeepstructureofthe
MaghrebianmarginwasexploredduringtheSPIRALseismicsurvey.Inthiswork,we
presentaN-StransectoffTipaza(westofAlgiers),aplacewherethemarginbroadens
duetoatopographichigh(Khayr-al-DinBank).Newdeeppenetrationseismicprofiles
allowustoimagethesedimentarysequenceintheAlgerianbasinandthecrustalstructure
atthecontinent-oceanboundary.Modelingofthewide-angledatashowsthinningofthe
basement,frommorethan15kminthecontinentaluppermargintoonly5–6kmof
oceanic-typebasementintheAlgerianbasin,andrevealsaverynarroworabsent
transitionalzone.Analysisofthedeepstructureofthemarginindicatesfeaturesinherited
fromitscomplexevolution:(1)anoceanic-typecrustinthedeepbasin,(2)similarities
withmarginsformedinatransform-typesetting,(3)aprogressivedeepeningofthewhole
sedimentarycover,andthethickeningofthePlio-Quaternarysedimentsatthemarginfoot,
coevalwith(4)adownwardflexureofthebasementinthebasin.Thesefeaturesarguefor
amultiphasedevolutionofthemargin,including(1)anearlystageofriftingand/or
spreading,(2)alatetranscurrentepisoderelatedtothewestwardmigrationoftheAlboran
domain,and(3)adiffusePlio-Quaternarycompressionalreactivationofthemargin.
Citation: Leprêtre,A., F.Klingelhoefer, D. Graindorge,P. Schnurle,M. O.Beslier, K.Yelles, J.Déverchère, and R.Bracene
(2013),MultiphasedtectonicevolutionoftheCentralAlgerianmarginfromcombinedwide-angleandreflectionseismicdataoff
Tipaza,Algeria,J.Geophys.Res.SolidEarth,118,3899–3916,doi:10.1002/jgrb.50318.
1. Introduction [e.g., Bown and White, 1994; Louden and Chian, 1999;
Geoffroy, 2005; Ziegler and Cloetingh, 2004]. Neverthe-
[2] Research on continental passive margins is largely
less, during rifting and/or subsequent oceanic spreading
focused on the understanding of rifting processes and
stages, the above factors, together with the geodynamical
mechanisms of lithosphere thinning leading to continen-
setting, may evolve, conferring to the margins a com-
tal breakup and spreading. The way in which continen-
plex structure. In addition, rifting in back-arc basins might
tal margins exhibit various structural styles according to
be different in some points from cratonic rifting, though
the setting of rifting, inheritance, magmatic supply, and/or
the mechanics of actual fracturing of the continental crust
mantle conditions during their formation is well studied
remainssimilar.Themaindifferencebetweenback-arcand
cratonic rifting is the presence of a subducting slab in the
1Domainesocéaniques,UMR6538,InstitutUniversitaireEuropéende mantle beneath the back-arc basin [Currie and Hyndman,
laMer,UniversitédeBretagneOccidentale,Plouzané,France. 2006;DunnandMartinez,2011].
2DepartmentofMarineGeosciences,Ifremer,Plouzané,France. [3] The North African Algerian passive continentalmar-
3UniversitédeNiceSophia-Antipolis,CNRS(UMR7329),Observa- gin results from back-arc opening of the Western Mediter-
toiredelaCôtedAzur,Valbonne,France.
ranean basin during Oligo-Miocene times [Schettino and
4CentredeRechercheenAstronomieAstrophysiqueetGéophysique,
Algiers,Algeria. Turco,2006].Whiletheopeninghistoriesoftheneighboring
5SONATRACHExploration,Boumerdès,Algeria. basins (Liguro-Provençal and Tyrrhenian basins, Figure 1)
are fairly well understood today [Gueguen et al., 1998;
Corresponding author: A. Leprêtre, Domaines océaniques, UMR
Jolivet and Faccenna, 2000; Rosenbaum et al., 2002], the
6538, Institut Universitaire Européen de la Mer, Université de
Bretagne Occidentale, place Nicolas Copernic, 29280 Plouzané, France. mechanisms for the opening of the Algerian basin are still
([email protected]) controversial regarding (1) the rifting processes (asymmet-
ric, symmetric), (2) the rate and direction of opening, and
©2013.AmericanGeophysicalUnion.AllRightsReserved.
2169-9313/13/10.1002/jgrb.50318 (3) the postrift evolution of its southern margin. This is
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LEPRÊTREETAL.:CENTRALALGERIANMARGINEVOLUTION
Figure 1. Present-day tectonic map of the Western Mediterranean area. Bathymetry and topography
are from ETOPO1 1min Global relief (www.ngdc.noaa.gov). Major tectonics feature are from Frizon
de Lamotte et al. [2000] and Billi et al. [2011]. The white arrow shows velocity of the African plate
relativetothestableEuropeanplate(5mm/yr)fromGPSmeasurements[NocquetandCalais,2004].
TheredrectanglemarksthelocationoftheregiondisplayedinFigure2.ThetwoinsetsshowtheWestern
Mediterraneansetting(a)at35Maand(b)at18Ma,simplifiedandmodifiedfromLonerganandWhite
[1997],withthemigrationoftheInternalZonesbehindtheTethyansubductionfrontandtheassociated
back-arcopeningoftheAlgerianbasin.Theredcrossshowstheapproximativepositionofourstudyarea.
C:Corsica,S:Sardinia,GK:GreatKabylia,LK:LesserKabylia,Pe:Pelorian,Ca:Calabria,Al:Alboran,
B:Betic,R:Rif.,M.E:MazarronEscarpment.
especially due to the lack of knowledgeon the deep geom- physique; and Directorate-General for Scientific Research
etry of the basin and surrounding margins. The Algerian andTechnologicalDevelopment)andFrenchResearchorga-
margin is now one of the few examples of a margin that nizations (Centre National de la Recherche Scientifique,
experiencedatectonicinversion,resultingintherecentand InstitutFrançaisdeRecherchepourl’ExploitationdelaMer
actual compressional field [Serpelloni et al., 2007] attested (Ifremer), Institut de Recherche pour le Développement,
bytheseismicityaswellastectonicandkinematicevidences anduniversities).Specifically,newwide-angleseismictran-
[Yellesetal.,2009]. sects,togetherwithcoincidentmultichannelseismic(MCS)
[4] Because of its setting, the Central Algerian margin dataprovidethefirstconstraintsonthemargin’sdeepstruc-
(Tipazaregion,westofAlgiers)isakeyareaforattempting ture,onthenatureoftheocean-continenttransition(OCT),
thereconstructionoftectonicevolutioninthissouthernpart and the associated Algerian basin, as well as on the recent
oftheWesternMediterraneanseaandforunderstandingthe compressive reactivation at crustal scale. In this study we
modification of passive margins by reactivation processes. present first results from a deep seismic transect across
Furthermore, it is the only place where a large-scale tilted the Central Algerian margin based on forward modeling of
block,calledtheKhayr-al-DinBankandinheritedfromthe wide-angleseismicdataandacoincidentmultichannelseis-
rifting stage, is proposed to be present [El Robrini, 1986; micprofileandcompareitwithothermarginsoftheWestern
Domzig et al., 2006; Yelles et al., 2009; Strzerzynski et al., MediterraneanseaandtheAtlanticocean.
2010].
[5] Among the major unsolved questions, we would like
to address the following points: (1) What is the nature 2. GeologicalSetting
and thickness of the crust underlying the Algerian basin?
(2) Where is the ocean-continent transition, and what is its 2.1. TheAlgerianMargin
origin? (3) What is the nature and deep geometry of the [7] TheAlgerianmargincorrespondstotheplatebound-
Khayr-al-Din Bank? (4) Is there evidence for deep mark- arybetweentheEuropeanandAfricanplates.Itisbounded
ers of margin reactivation? (5) What are the implications to the north by the Algerian basin and to the south by an
oftheseresultsonmodelsfortheevolutionoftheAlgerian Alpine-type belt called Maghrebides (known as the “Tell”
basinanditssouthernmargin? north Algeria, Figure 1), resulting from the subduction and
[6] Inordertounravelthedeepgeometryandstructuresof closure of the Tethyan ocean under the European plate
theMaghrebianmarginbothonshoreandoffshore,theSPI- inMiocenetimes[Auzendeetal.,1973;FrizondeLamotte
RAL (Sismique Profonde et Investigations Régionales en etal.,2000].
Algérie)projectwaslaunchedinSeptember2009incollab- [8] TheevolutionoftheAlgerianmarginiscloselyrelated
oration between Algerian scientific institutions (Sonatrach; to the rollback of the Tethyan slab and the related back-
CentredeRechercheenAstronomie,AstrophysiqueetGeo- arcopeningoftheWesternMediterraneanbasins(Figure1).
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LEPRÊTREETAL.:CENTRALALGERIANMARGINEVOLUTION
Figure 2. Location of the wide-angle seismic profile in the Bou Ismail Bay, sector of Tipaza. Ocean
bottom seismometer (OBS) positions are marked by red circles and the land stations by blue triangles.
The numbers of the stations shown in Figure 6 are shaded. The multichannel seismic profile Spi06 is
indicatedwithawhiteline.ThegeologicalandtectonicframeworkonlandisextractedfromYellesetal.
[2009].KADF:Khayr-al-Dinfault,CF:Chenouafault,SF:Sahelfault,SA:Sahelanticline,Al:Algiers
massif,Ch:Chenouamassif.
ThereisageneralconsensusthattheAlgero-Provençalbasin etal.,1998;Gelabertetal.,2002;LonerganandWhite,1997
openedduringlateOligocene-earlyMiocenetimesinaback- andSchettinoandTurco,2006].Whatevermodelchosen,the
arc position behind the Tethyan subduction zone [Jolivet westernmostAlgerianmargincanbeassumedtorepresenta
andFaccenna,2000;Gelabertetal.,2002;Speranzaetal., purelystrike-sliptypemargin[Domzigetal.,2006],having
2002]. In the early Miocene, the stretching of the Euro- formedasaSTEP-faultsystem(subduction-transformedge
peanplateisassumedtohavecausedthedrifting,spreading, propagator,GoversandWortel[2005]).
and finally the collision of parts of a continental block, the [10] TheAlgerianmarginandbasinarethenmarkedbya
Internal Zones, called AlKaPeCa (for Alboran, Kabylies, majorsalinitycrisisduringMessiniantimes,whichaffected
Peloritan, and Calabria; Bouillin [1986]), with the African the whole Western Mediterranean domain and surrounding
continent (Figure 1a). Currently, those Internal blocks are margins (5.96–5.32Ma, Hsu et al. [1973]; Gautier et al.
scattered around the Western Mediterranean basin, part of [1994]; Krijgsman et al. [1999]). This event resulted in the
themhavingbeenaccretedalongtheAlgerianmargin,such progressive closure of the connection between the Atlantic
astheKabylianblocks(Figure1). Ocean and the Mediterranean Sea, responsible for a total
[9] Models of the opening of the Algerian basin remain sea level fall of more than 1500m [Ryan and Cita, 1978].
controversial regarding the kinematics and nature of the It led to an intense erosion of the basin margins, the accu-
margins:(1)Someauthorspromoteanopeningofthisbasin mulation of erosional products in the downslope domain
attherearofadoublesubductiontowardthewest(Alboran) [SavoyeandPiper,1991;Sageetal.,2005],andthedeposi-
and the east (Calabria) [Malinverno and Ryan, 1986; tionofthethickevaporiticMessiniansequencesinthedeep
Lonergan and White, 1997], after the Kabylian collision Mediterranean basin [Montadert et al., 1970; Hsu et al.,
withtheAfricanplate(18Ma),resultinginadominantE-W 1973;Lofietal.,2011]responsibleformarkedsalttectonics.
openingbetween16and8MabehindtheGibraltararcroll- The Messinian units form a good temporal seismic marker,
backandAlboranblockmigrationtowardthewest[Mauffret easilyrecognizableintheMediterraneanarea.
etal.,2004].ThewestwardmigrationoftheAlboranblock [11] The Plio-Quaternary period was then characterized
[Mauffret et al., 2004] would have induced a left-lateral bythetectonicinversionoftheAlgerianmargin.Thismajor
deformationalongtheWesternandCentralAlgerianmargins tectonic episode is still in progress and contributes to the
and right-lateral deformation along the Balearic Promon- general structure of the Algerian margin. Recent kinematic
tory [Camerlenghi et al., 2009] (Figure 1b). (2) Other studiesindicateapresent-dayshorteningassociatedwiththe
authors propose an older NW-SE opening of the Algerian NW-SE[Stichetal.,2006]convergencebetweentheAfrican
basinbehindtheretreatingofasubductionzonetowardthe andEuropeanplatesofabout5–6mm/yratthelongitudeof
S-SE, whereas no significant displacement (i.e., less Algiers[NocquetandCalais,2004](Figure1).Asignificant
than200km)oftheAlboranblockisconsidered[Gueguen part (between 1.6 and 2.7mm/yr) of the deformation may
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LEPRÊTREETAL.:CENTRALALGERIANMARGINEVOLUTION
Figure3. High-resolutionseismicprofilesfromtheMARADJAcruise(2003)acrosstheKhayr-al-Din
Bank(modifiedafterYellesetal.[2009](verticalexaggeration:9).Thepositionofeachlineisindicated
by red lines on the map, and the deep seismic profile SPIRAL Spi06 is indicated by the black line. (a)
Seismicsectionshowingthesteepnorthernslopeofthebanctowardthedeepbasin,andthesedimentary
sequenceatthetopofthebancandatitsfoot.(b)Seismicsectionacrossthecentralpartofthebanc.The
inset shows the compressive bulge at the foot of the margin identified by Yelles et al. [2009], which is
assumedtoberelatedtothepresenceofasouthdippingblindthrustbeneaththeKADB.
currently occur offshore Algeria (Serpelloni et al. [2007]; AlgiersInternalmassifstothewestandtheeast,respectively
Meghraoui et al. [1996], respectively), and even further (ChandAl,Figure2),andtothenorthbythedeepAlgerian
north in the SE Iberian margin [Maillard and Mauffret, basin.
2013]. [14] The KADB was recently investigated using mor-
[12] Contractional deformation is supported by the exis- phological and high-resolution seismic data (MARADJA
tence of dominantly reverse-type fault plane solutions in 2003 and 2005 cruises [Déverchère et al., 2005; Domzig
the present seismicity, as exemplified by earthquakes of et al., 2006, Yelles et al., 2009; Strzerzynski et al., 2010])
Chenoua(M =6.0,1989[Bounifetal.,2003]),AinBenian and is interpreted as a tilted block inherited from the rift-
w
(M =5.7, 1996), and Boumerdès (M =6.8, 2003 [Delouis ing of the Algero-Provençal basin, as first suggested by El
w w
et al., 2004]) (Figure 2). In recent papers, authors describe Robrini [1986]. It is also assumed to represent relics of the
active offshore structures as folds and south dipping blind Kabylian basement, originally part of the Internal Zones,
thrusts both east and west of Algiers [Domzig et al., 2006; and the offshore extension of the Chenoua and Algiers
Déverchère et al., 2005; Yelles et al., 2009; Strzerzynski internalmassifswhichoutcroponshore[Domzigetal.,2006;
et al., 2010], one of them being tentatively related to the Strzerzynskietal.,2010](Figure2).
destructive Boumerdès earthquake in May 2003 and attest- [15] Evidence for recent compression is indicated by
ing to the recent compressional reactivation of the margin the presence of an asymmetric 100m high bulge at the
(Figure2).TheinversionoftheAlgerianmarginremainsan foot of the slope visible in the morphology and imaged
active process, and the North African margin could repre- by seismic reflection profiles from the MARADJA cruise
sentanearlystageofincipientsubduction,asfirstsuggested (Figure 3b). The asymmetric steeper northern flank of the
by Auzende et al. [1972] and later by Yelles et al. [2009] bulgeandtheassociatedbasementupliftcouldbecontrolled
and Strzerzynski et al. [2010], based on recent studies con- byanactivesouthdippingthrustsystemlocatedbeneaththe
ductedontheKhayr-al-DinBank,abathymetrichighinthe Khayr-al-Din Bank, although this structure is not directly
region of Tipaza (Figure 2). This area is thus assumed to imaged[Domzigetal.,2006;Yellesetal.,2009](Figure3b).
haverecordedallthetectonicepisodesthathaveaffectedthe In north Algeria, structures inherited from the Miocene
Algerianmargin,fromtherollbackoftheTethyanslabtothe phasearegenerallycharacterizedbyasouthwardvergence,
recentcompressionalreactivationofthemargin. liketheMiocenesutureborderingtheInternalZonestothe
south, whereas newly formed reverse structures from the
2.2. SectoroftheKhayr-al-DinBank(KADB) reactivation exhibit an opposite northward vergence [Yelles
[13] West of Algiers, the continental shelf significantly etal.,2009;Déverchèreetal.,2005](Figure2).
widensandformsthebathymetrichighoftheKhayr-al-Din
Bank (KADB, Figure 2). This structure extends over
80km in a roughly E-W direction and 45km in a N-S 3. SeismicData
direction,overlookingthedeepbasinofabout2000mdepth
(Figure 2). The northern KADB limit shows a steep slope 3.1. DataAcquisition
with a basinward dip of about 12ı (Figure 3a). It is bor- [16] A wide-angle seismic profile close to Tipaza and
dered onshore by the Sahel structure, the Chenoua and the coincident MCS cross-section Spi06 are presented in
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LEPRÊTREETAL.:CENTRALALGERIANMARGINEVOLUTION
channels.MCSandOBSdatawererecordedwithasample
rateof4ms.
3.2. MultichannelSeismic(MCS)DataandProcessing
[18] The SPIRAL seismic sources were chosen to image
deep targets, such as the top and the base of the crust,
the OCT, and the deep rooting of the structures. There-
fore, the deep penetrating and low-frequency MCS data
set is complementary to the high-resolution and superfi-
cialdataacquiredduringtheMARADJAcruises(2003and
2005) and was used to image deep structures underneath
the salt layer (Figures 4 and 5). A first quality control was
undertaken on groups of traces using the SISPEED soft-
ware, and further processing of the MCS data was then
performedusingtheGEOCLUSTERsoftware.Theprocess-
ingsequenceincludedexternalandinternalmutes,spherical
divergence correction, bandpass filtering (3–5–95–105Hz),
anddynamiccorrections.Twoconsecutivevelocityanalyses
wereconductedevery200CMP(commonmidpoint)leading
to the final stack. The last processing step was the appli-
cation of a frequency-wavenumber migration on the data
using a constant 1550km/s water velocity. The Spi06 pro-
file exhibits a higher resolution than the Spi25 profile, due
tothehigherfrequenciesofthesource.Therefore,theMCS
interpretation presented in this paper is based on the Spi06
profile, whereas interfaces from MCS data integrated dur-
Figure 4. (a) Stratigraphic units identified on the line ingtheforwardmodeling(seethenextsection)werepicked
Spi06inthedeepAlgerianbasin,(b)theircorrelationswith from the Spi25 profile to avoid even minimal differences
velocities from the velocity forward modeling (this study), in time and/or space between MCS and wide-angle arrival
andwith(c)high-resolutionseismicreflectiondatafromthe times.Thewide-angleseismicdataaidsgeologicalinterpre-
MARADJAcruise(profilea,Figure3). tation of the crust by providing deeper and complementary
informationsuchasP-wavevelocities(Vp)onthestructure
ofthemargin.
this work (Figure 2). Deep seismic data were acquired
duringtheSPIRALcruiseconductedontheR/VL’Atalante 3.3. SeismicVelocityModelingoftheWide-Angle
(IFREMER) in October–November 2009. Thirty-nine SeismicData
four-component ocean bottom seismometers (OBS) spaced [19] The refraction data were modeled using forward
at3kmintervalweredeployedalongthe120kmNNW-SSE modeling technique, taking into account first as well as
profileacrossthewesternAlgerianmargin(BouIsmailbay) secondary arrivals from OBS and land stations, and reflec-
offshore,and23landstationswerealsodeployed,extending torspickedfromthecoincidentmultichannelseismicsection
the marine transect by 110km on land. The seismic profile (Figure7).
crosses the deep basin, the Khayr-al-Din Bank, the Sahel 3.3.1. DataQualityandPreprocessing
structure,andtheMitidjabasin(Figure2). oftheWide-AngleSeismicData
[17] Two different seismic sources were used during the [20] OBSdatawerecorrectedforclock-drift,andseafloor
cruiseinordertoachievetwoobjectives:(1)TheMCSpro- positionswerecalculatedusingthedirectwaterwave.Apre-
file Spi06 coincident with the wide-angle line off Tipaza processing sequence was applied to all data (land stations
presentedinthisstudywasacquiredusingaseismicairgun andOBS)inordertoincreasethesignal-to-noiseratioandto
arrayof13airgunsofvariousvolumes(synchronizedonthe better image far-offset arrivals. This sequence is composed
firstbubble)togenerate2299shotsoflowfrequency(max- ofadeconvolutionwhitening,a3–17HzButterworthfilter,
imum frequency of 70Hz) to allow for deep penetration andanautomaticgaincontrol.
oftheseismicsignal[Avediketal.,1993].Thissourcepro- [21] The OBS data acquired along the Tipaza profile are
videdatotalvolumeof50L,withanintershotof20sleading of good quality, with a better signal-to-noise ratio on the
to 50m spacing. Our objective was to increase the seismic vertical geophone component than on the hydrophone. The
coverage to allow for better processing results. (2) For the OBS sections show clear sedimentary (Ps1, Ps2, Ps3) and
wide-angleacquisition,aseismicairgunarraycomposedof crustalarrivals(Pg1,Pg2),anddeeparrivalsfromtheupper
eight airguns of 16L and two airguns of 9L was used to mantle (Pn) are identifiable (Vp  7.6km/s) up to 50km
generate751low-frequencyshots,synchronizedonthefirst offsetawayfromsomeOBS(Figures6aand6b).Sedimen-
peak.Thissourceprovidedatotalvolumeof146L,withan taryreflections(PsP1,PsP2)aswellasreflectionsfromthe
intershot of 60s leading to 150m spacing. Simultaneously top of the basement (PgP) are clearly observed in the deep
withthewide-angleacquisition,thecoincidentMCSprofile basin. Moho reflections (PmP) are not always easily dis-
Spi25wasacquired.AllMCSprofileswererecorded,using cernibleinthedeepAlgerianbasin,evenafterapplyingthe
the 4.5km streamer of Ifremer, composed of 360 12.5m preprocessingsequence.Fortheforwardmodeling,picking
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LEPRÊTREETAL.:CENTRALALGERIANMARGINEVOLUTION
Figure 5. (a) Time migrated multichannel seismic profile Spi06. (b) A corresponding line drawing
showingthedeepeningofboththetopandthebaseoftheMessinianunitsandseveraldeepreflections.
The inset shows deep reflection of low amplitude probably corresponding to the Moho discontinuity
beneaththeKADBat7.2–7.5s,and(c)sectionSpi06withvelocitiesfromforwardmodeling(Figure7)
convertedintimeunderlain.KADB:Khayr-al-DinBank.Plio-Quaternarysedimentthicknessespresented
insection4.3areestimatedusingtheforwardmodelin“e1”and“e2”(seedetailsinthetext).
uncertaintiesweredefinedforeachphaseusingthemethod phases and multichannel data into the model and, on the
of Zelt [1999], based on the ratio of the amplitude 250ms other hand, to verify that all structures from the forward
before and after the picked arrival. A mean error depend- model are required to fit the data. For the modeling, a
ing on the signal-to-noise ratio was calculated from all the minimum structure for the continental crust was used to
picks,foreachphaseofeachstation,andthenconvertedto successfullyexplainarrivalsatthelandstations.
atraveltimepickingerrors,forarangeinvaluesbetween20 [24] Seismic velocities were modeled using the 2-D ray
and125ms.Phaseswithahighratioarecharacterizedbya tracing software XRAYINVR developed by Zelt and Smith
low uncertainty, whereas phases with a low ratio are char- [1992].Thismodelingused alayer-strippingstrategy,from
acterized by a higher uncertainty. Phases names and picks thetopofthemodeldownward.Thevelocitymodeliscon-
aredetailedinTable1.Wide-angledataacquiredinthedeep structed layer after layer and composed of velocity and
basinareratherhomogeneous,whereasdatarecordedclose interface nodes. Depth and velocities were modeled such
to the coastline show significant lateral variations, proba- as to minimize the difference between the observed arrival
blyinducedbystrongchangesinbathymetryandbylateral timesandthearrivaltimescomputedinthemodel(Figures6
structural variations, especially in the crustal part of the and7).
profile(Figures6aand6b). [25] The set of observed traveltimes, including refracted
[22] Amongthe23landstationsdeployed,only11exhib- and reflected phases, were picked from the 39 OBS and
ited a sufficient quality to allow picking identification of 11 land stations recorded sections. Geometries of the sed-
arrivals. Most of them were located close to the coastline imentary layers were determined from interfaces picked
(Figure 7). Land station sections did not show sedimentary fromtheMCScoincidentline.Theseinterfacesincludethe
arrivalsduetothelargedistancebetweenthestationandthe Messinian erosion surface on the upper margin, as well as
closest shot but only deep arrivals (PmP, Pg) (Figure 6c). the top of the Messinian units, and, where visible, the base
Pnarrivalsfromtheuppermantlewerenotrecordedbythe of the Messinian salt layer in the deep basin (see geologi-
landstations. cal units, Figures 4 and 5). Arrival times picked from the
3.3.2. ForwardModeling MCS data were converted to depth using velocities from
[23] Construction of a forward ray tracing model allows the forward modeling. For these layers, only the veloci-
us on the one hand to include information from reflected ties were adjusted to reduce the misfit between observed
3904
LEPRÊTREETAL.:CENTRALALGERIANMARGINEVOLUTION
andcalculatedtraveltimes.Theforwardmodelingintegrated
28,586picks.
3.3.3. ErrorAnalyses
[26] The quality of the forward model can be quantified
using the fit between predicted arrival times and traveltime
picks.Thecorrespondingmisfitis121msusing93%ofthe
picks. The number of picks, RMS errors, and 2 for each
phase obtained for the final forward model are detailed in
Table1.
[27] Two-point ray tracing between source and receiver
(Figure 8) shows the well-resolved and the unconstrained
areas. Ray coverage for both diving and reflected waves is
generally very good due to the excellent data quality and
closeinstrumentspacing(Figures8aand8b).Allsedimen-
tarylayersarewellsampledbyreflectedandturningraysin
themarinepartofthemodel.Thecrustallayers,theoceanic
Moho,andtheuppermantlearewellsampled.
[28] Resolution is a measure of the number of rays
passing through a region of the model constrained by a
particular velocity node and is therefore dependent on the
node spacing [Zelt, 1999]. If a layer can be modeled with
one single velocity gradient, then the resolution parameter
will be high even in areas which have lower ray cover-
age,as theareaisrelated to onlyonevelocitynode.Nodes
with values greater than 0.5 are considered well resolved
(Figure9).Thevelocitiesthroughoutthemodelshowares-
olution higher than 0.5 except at the southern end of the
model. The resolution decreases at the ends of the model
wherenorayspassthroughthelayersandalsodecreasesat
the very shallow onshore sedimentary layer due to missing
reverseshotsonland.Uppermantlevelocitiesarewellcon-
strainedathigherlevels,howeverlesssoatincreasingdepth
due to fewer rays penetrating into this deeper portion of
themodel.
[29] In order to estimate the velocity and depth uncer-
tainty of the final velocity model, a perturbation analysis
was performed. The depths of key interfaces were var-
ied, and an Ftest was applied to determine if a significant
change between models could be detected. The 95% con-
fidence limit gives an estimate of the depth uncertainty of
theinterface(Figures9and10).Inordertobetterconstrain
uncertaintyattheMoho,boththedepthofthisinterfaceand
velocitiesinthelowercrustallayerwerechangedsystemati-
cally(Figure10).Weobtainonourfinalmodeluncertainties
of+0.3/–0.4kmand˙0.1km/sfortheMohodepthandfor
Figure6. Threeexamplesofrepresentativerecordsections velocitiesinthelowercrust,respectively.Resultsfromthis
(wide-angle data). (a) Seismic section of OBS 06 on the analysis show that our preferred model allows a maximum
upper margin (top), the corresponding ray paths in the for- of picks to be explained, with a minimum resulting misfit
ward model (middle), and the observed traveltime picks between the picked traveltimes and arrivals predicted from
(thick grey lines) and calculated traveltimes (thin black themodeling.Solutionsleadingtobetterfitsexplainalower
lines) in the forward model (bottom). (b) Seismic section numberofpicksandarethuslessreliable.
of OBS 26 in the deep basin (top), the corresponding ray [30] In order to additionally test the validity of the for-
pathsintheforwardmodel(middle),andtheobservedtrav- ward velocity model, we may also convert velocity to
eltime picks (thick grey lines) and calculated traveltimes density using an empirical law. This density model is then
(thinblacklines)intheforwardmodel(bottom).(c)Seismic used to generate a predicted gravity anomaly which can be
sectionoflandstation43(top),thecorrespondingraypaths compared with the measured gravity anomaly. The gravity
intheforwardmodel(middle),andtheobservedtraveltime anomalywasmodeledusingthesoftwareGRAVMOD[Zelt
picks(thickgreylines)andcalculatedtraveltimes(thinblack and Smith, 1992] and free-air gravity anomaly data col-
lines)intheforwardmodel(bottom).Thesethreestationsare lected during the SPIRAL cruise. This modeling approach
shadedinFigure2.Alltheexamplescorrespondtotheverti- isbasedontheempiricalrelationshipexistingbetweenseis-
calcomponentrecording,representedwitha6km/svelocity mic velocities and densities proposed by Ludwig et al.
reduction. [1970].Themisfitbetweencalculatedandpredictedgravity
3905
LEPRÊTREETAL.:CENTRALALGERIANMARGINEVOLUTION
Table1. ResidualTraveltimesandChi-SquareErrorsforAllthePhasesfortheTipazaTransect,UsingForwardModeling
Phase Name NumberofPicks RMS[s] 2Error Uncertainties
Water 1893 0.014 0.460 0.020–0.035
SedimentrefractioninthePlio-Quaternaryunit Ps1 748 0.086 1.261 0.020–0.100
SedimentrefractionintheMessinianunits Ps2 1695 0.104 0.761 0.100–0.125
SedimentrefractioninthePresaltunit Ps3 1494 0.132 1.895 0.020–0.125
ReflectionatthetopoftheMessinianunit PsP1 1710 0.072 6.330 0.025–0.050
ReflectionatthebaseoftheMessinianSaltunit PsP2 1263 0.097 2.399 0.100–0.125
Reflectionatthetopofthebasement PgP 1465 0.128 3.256 0.020–0.125
Refractionintheuppercrust Pg1 4562 0.137 1.232 0.125
Refractioninthelowercrust Pg2 6037 0.121 1.456 0.100–0.125
ReflectionattheMoho PmP 5866 0.142 1.403 0.100–0.125
Refractionintheuppermantle Pn 1853 0.127 1.073 0.100–0.125
AllPhases 28586 0.121 1.720
anomaliesisabout15.5mGal,whichrepresentsagoodval- the seismic profile in order to (1) constrain the structure of
idation of our velocity forward model (Figures 7a and 7b). thesedimentarysequenceandthebasementoftheAlgerian
The largest misfit observed, between 15 and 30km model margin and basin off Tipaza and (2) better understand
distance,mightbeduetothe3-DtopographyoftheKADB the kinematic and tectonic history of the Algerian margin.
(Figure7). The main sedimentary and crustal features identified are
describedbelow.
4. Results 4.1. StructureoftheSedimentaryUnits
[31] Forwardmodelingwascarriedoutandcoupledwith [32] While the MARADJA data were limited by their
the interpretation of the MCS data on the marine part of penetration (Figures 3 and 4), the MCS profile SPIRAL
Figure7. ResultsofforwardvelocityandgravitymodelingalongtheTipazaprofile.(a)Resultsofthe
gravitymodeling.RedlinerepresentsgravityanomaliesfromtheSPIRALcruisemeasurement,andthe
dashed black line represents gravity anomalies calculated from conversion of the seismic velocity pre-
dictedbytheforwardmodelingtodensities.(b)Resultsoftheforwardvelocitymodeling,including39
OBSand11landstations.OBSlocationsareindicatedbyredcircles.Locationsofthelandstationsused
inthemodelingareindicatedbyredtriangles.Seismicrecordsfromthelandstationsindicatedbyyellow
trianglesareoftoolowaqualitytobeintegrated.Isovelocitycontoursarerepresentedevery0.25km/s.
Areasunconstrainedbyraytracingareshaded.Redlinesmarkvelocity-depthprofilesshowninFigure11.
(c)Isostaticgravityanomalycalculatedalongthetransectassumingalocalisostaticequilibriumandcon-
stantvaluesof2700and3300kg/m3forcrustandmantledensities,respectively[e.g.,BehnandLin,2000;
Balmino et al., 2012], and a level of compensation at a depth of 15km. Density for the crust is chosen
to be typical of continental crust. The Bouguer gravity anomaly is from the International Gravimetric
Bureau(BGI)(http://bgi.omp.obs-mip.fr/)(seetextfordetails).
3906