Crustal structure of the eastern Algerian continental margin and adjacent deep basin

27 Pages·2015·19.08 MB·English

Geophysical Journal International
Geophys.J.Int.(2015)201,1912–1938 doi:10.1093/gji/ggv102
GJIGeodynamicsandtectonics
Crustal structure of the eastern Algerian continental margin
and adjacent deep basin: implications for late Cenozoic geodynamic
evolution of the western Mediterranean
B. Bouyahiaoui,1 F. Sage,2 A. Abtout,1 F. Klingelhoefer,3 K. Yelles-Chaouche,1
P. Schnu¨rle,3 A. Marok,4 J. De´verche`re,5 M. Arab,6 A. Galve2 and J.Y. Collot2
1CentredeRechercheenAstronomie,AstrophysiqueetGe´ophysique(CRAAG),BP63Bouzare´ah16340Alger,Alge´rie.E-mail:[email protected]
2UMPC,UNSA,CNRS,IRD,Ge´oazur,250,AvenueAlbertEinstein,F-06560Valbonne,France
3InstitutFranc¸aisdeRecherchepourl’ExploitationdelaMer(IFREMER),ZIdelaPointedeDiable,F-29280Plouzane´,France
4DepartmentofEarthandUniverseSciences,UniversityofTlemcen,BP119,13000Tlemcen,Algeria
5Universite´deBrest(UBO),CNRSUMR6538DomainesOce´aniques,InstitutUniversitaireEurope´endelaMer,F-29280Plouzane´,France
6SonatrachExploration,AlgerianNationalOilCompany,Algeria
Accepted2015February26.Received2015February25;inoriginalform2014June30 D
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SUMMARY de
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WedeterminethedeepstructureoftheeasternAlgerianbasinanditssouthernmargininthe fro
Annaba region (easternmost Algeria), to better constrain the plate kinematic reconstruction m
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in this region. This study is based on new geophysical data collected during the SPIRAL ttp
cruisein2009,whichincludedawide-angle,240-km-long,onshore–offshoreseismicprofile, ://g
multichannel seismic reflection lines and gravity and magnetic data, complemented by the ji.o
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available geophysical data for the study area. The analysis and modelling of the wide-angle ford
seismic data including refracted and reflected arrival travel times, and integrated with the jo
u
multichannel seismic reflection lines, reveal the detailed structure of an ocean-to-continent rna
transition.Inthedeepbasin,thereisan∼5.5-km-thickoceaniccrustthatiscomposedoftwo ls.o
rg
layers.TheupperlayerofthecrustisdefinedbyahighvelocitygradientandP-wavevelocities a/
between4.8and6.0kms−1,fromthetoptothebottom.Thelowercrustisdefinedbyalower t IF
velocity gradient and P-wave velocity between 6.0 and 7.1km s−1. The Poisson ratio in the RE
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lower crust deduced from S-wave modelling is 0.28, which indicates that the lower crust is E
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composed mainly of gabbros. Below the continental edge, a typical continental crust with o
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P-wave velocities between 5.2 and 7.0km s−1, from the top to the bottom, shows a gradual M
a
seawardthinningof∼15kmoveran∼35-kmdistance.Thisthinningisregularlydistributed y 2
7
betweentheupperandlowercrusts,anditcharacterizesariftedmargin,whichhasresultedfrom , 2
0
backarcextensionattherearoftheKabylianblock,hererepresentedbytheEdoughMassifat 1
5
theshoreline.Abovethecontinentalbasement,an∼2-km-thick,pre-Messiniansedimentlayer
withacomplexinternalstructureisinterpretedasallochthonousnappesofflyschbackthrusted
onthemarginduringthecollisionofKabyliawiththeAfricanmargin.Thecrustalstructure,
moreover, provides evidence for Miocene emplacement of magmatic intrusions in both the
deep basin and the continental margin. Based on the crustal structure, we propose that the
eastern Algerian basin opened during the southeastward migration of the European forearc
beforethecollision,alongaNW–SEelongatedspreadingcentrethatranperpendiculartothe
subduction trend. Such an atypical geometry is explained by the diverging directions of the
subductionrollbackduringthebackarcopening:eastwardfortheCorsica–Sardiniablock,and
southward for the Kabylian blocks. This geometry of the forearc can be interpreted as the
surfaceexpressionofaslabtearatdepth,whichisresponsibleforatypicalmagmatisminthe
overlyingbackarcoceanicbasin.
Keywords: Tomography;Compositionoftheoceaniccrust;Continentalmargins;divergent;
Africa.
1912 (cid:3)C TheAuthors2015.PublishedbyOxfordUniversityPressonbehalfofTheRoyalAstronomicalSociety
DeepstructureoftheeasternAlgerianmargin 1913
2001; Rosenbaum & Lister 2004a), or opened during the Oligo-
1 INTRODUCTION
Miocene,suchastheLiguro-Provenc¸albasinortheAlgerianbasin
The Mediterranean Sea at present forms a single, 2.5 × 106km2 (e.g.Bouillin1986;Faccennaetal.2001;Rosenbaumetal.2002;
basinthatissurroundedbycontinents.However,infurtherdetail,it Gattaccecaetal.2007;Billietal.2011;Sageetal.2011;Fig.1a).
iscomposedofacomplexmosaicofsub-basinsthatshowdifferent TheMediterraneanbasinis,moreover,surroundedbyorogenicbelts
agesanddifferentevolutionarystages,whichmarkouttheAfrica– thatattesttotheCenozoicclosureoftheoceansthatwerepreviously
Europeanconvergentboundary.Whilesomeofthesesub-basinsare partoftheMediterraneandomain.Rapidtransitionintimeandin
ancientoceansthatclosedalongactivesubductionzones,suchas space from basin opening at the back of the subduction zones to
theeasternMediterraneanbasin(e.g.Dercourtetal.1986;Wortel collisionzoneshascharacterizedtheAfrican–Eurasianborderfor
&Spakman2000;Jolivetetal.2003),someothersarestillopening, atleast50Ma(e.g.Jolivetetal.2003),whichledtothepresent-
suchastheTyrrhenianbasin(e.g.Gueguenetal.1998;Sartorietal. dayMediterraneanconfiguration.Understandingthespatiotemporal
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Figure1. (a)MagneticanomaliesfromGaldeano&Rossignol(1977)superimposedonthemapoftheWesternMediterraneanarea.Whitelines:locationof
thefivewide-angleseismicprofilesacquiredduringSPIRALcruise.WA-An,Annabaprofile(thisstudy);WA-Ji,Jijelprofile;WA-Ka,GreatKabyliaprofile;
WA-Ti,TipazaprofileandWA-Mo,Mostaganemprofile.Redline:locationofthewide-angleseismicprofilefromtheEuropeanGeotraverseproject(Peirce&
Barton1992).Whiteframe:locationoftheinsetthatdisplaysregularlyorganizedNW–SEmagneticanomaliesinthecentralpartoftheeasternAlgerianbasin
(fromSchettino&Turco2006).Blackframe:locationofthecentraleasternAlgerianbasinasdisplayedinFig.2.(b)WesternMediterraneansettingat35Ma,
modifiedfromGelabertetal.(2002),indicatingthelocationoftheEuropeanforearc(inyellow),beforetheAlgerianbasinopening.DuringtheAlgerianand
Tyrrhenianbasinformation,theEuropeanforearcwasfragmentedandmigratedtothesouthandtotheeasttoreachitspresentdayposition(shownin(a)),
duetosubductionrollback(seetextfordetailsandreferences).Thenamesoftheforearcfragmentsweregivenaccordingtotheirpresent-daylocation,andare
fromwesttoeast:R,Rif;B,Betics;GK,GreatKabylia;LK,LesserKabylia;Pe,Peloritani;Ca,Calabria.TheyformtheAlKaPeCablockdefinedbyBouillin
(1986),withAl,Alboran=R+B;Ka,Kabylia=GK+PK;Pe,PeloritaniandCa,Calabria.
1914 B.Bouyahiaouietal.
evolution of the Mediterranean basins is important, to be able to studieshaveproposedthatthedeepbasinhasresultedfromdelami-
betterunraveltheprocessofbackarcopeningandtheevolutionof nationofacontinentallithosphere(Roureetal.2012),moststudies
complexconvergentplateboundaries. have considered the eastern basin as an oceanic domain that was
Numerouskinematicmodelshavebeenproposedtoexplainthe formedbybackarcaccretion,boundedtothesouthbyitscontinental
current structure of the western Mediterranean basin that have margin(Alvarezetal.1974;Cohen1980;Lonergan&White1997;
mostly been based on magnetic data (Schettino & Turco 2006; Doglionietal.1999;FrizondeLamotteetal.2000;Jolivet&Fac-
Gattacceca et al. 2007), crustal structures of the basin and ad- cenna2000;Faccennaetal.2001;Gelabertetal.2002;Rosenbaum
jacent margins (Mauffret et al. 2004), geological observations of etal.2002;Mauffretetal.2004;Schettino&Turco2006).Accord-
theMediterraneancollisionzone(Lustrinoetal.2011;Carminati ingtothesemodels,theeasternAlgerianmarginmightinsteadbe
etal.2012),kinematicconstraints(Gueguenetal.1998;Jolivet& consideredariftedstretched(e.g.Gueguenetal.1998;Jolivet&
Faccenna2000)andtheintegrationofdifferenttypesofdata(e.g. Faccenna2000),ortranscurrent(e.g.Mauffretetal.2004;Schettino
tectonic, seismological, geodetic, tomographic, seismic reflection &Turco2006),continentalmarginsegment,withimplicationson
data;Billietal.2011).Allofthekinematicreconstructionmodels thewaytheslabescapedtotheeast.
agree that in an initial stage before the opening of the Algerian Theaimofthisstudywastodeterminethecrustalstructureof
basin,theAlboran,Kabylia,PeloritaniandCalabriablocks,which theeasternAlgerianbasinanditssouthernmarginintheAnnaba
weredefinedastheAlKaPeCablock(Bouillin1986),werepartof region (Fig. 2a), to better constrain the kinematic models of the
theEuropeansouthernmarginuntil∼23Ma(Alvarezetal.1974; easternAlgerianbasinandtheeastwardmigrationoftheslab.This
Cohen 1980; Bouillin 1986; Lonergan & White 1997 ; Verge´s & study is based on: (1) new data acquired in 2009 during the Sis-
Saba`t1999;FrizondeLamotteetal.2000;Rosenbaumetal.2002; mique Profonde et Investigation Re´gionale au Nord de l’Alge´rie
Mauffretetal.2004;Billietal.2011).Thekinematicreconstruc- (SPIRAL)cruise,whichincludeda240-km-longonshore-offshore
tionmodelsalsoagreethatinafinalstage,theAlKaPeCAblocks wide-angleseismicprofile,deep-penetratingmultichannelseismic D
accreted by collision along the northern African border, ∼16–18 (MCS)linesandgravimetricandmagneticdata(Fig.2a);and(2) ow
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Ma (Fig. 1a, R, GK, LK) (Monie´ et al. 1984; Saadallah & Caby additionalavailabledatathatincludedmultibeambathymetryand lo
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1996;Lonergan&White1997;Faccennaetal.2001;Roureetal. high-resolutionseismicdata,industrialdeep-penetratingMCSdata, de
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2012),thecollisionzonebeingunderlainbyadetached,northward andcomplementarygravimetricandmagneticdata(Fig.2a). fro
dippingslab(e.g.vanHinsbergenetal.2014). m
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Mostauthorsproposethatbetweenthesetwostages,theAlge- ttp
rsilaanbb(Aaslivnareevzolevteadl.in19re7l4a;tiConohtoena1r9o8ll0b;acLkonperorgcaenss&ofWthheitTee1th9y9a7n; 2 GEOLOGICAL SETTING ://gji.o
Dceongnlaio2n0i0e0t;aFla.c1c9e9n9n;aFertizaol.n2d0e01L;aGmeoltatbeeerttaelt.a2l0.20000;2Jo;lRivoeste&nbaFuacm- 2.1 KinematicreconstructionoftheeasternAlgerianbasin xford
jo
etal.2002;Mauffretetal.2004;Schettino&Turco2006;vanHins- TheeasternAlgerianbasinformsa15000km2basinthatisbounded urn
bergen et al. 2014) enhanced by slab tears to the west and to the bytheeasternAlgerianmargintothesouth,theSardiniaChannel als
east,ingoodagreementwithtomographicstudies(Fig.1a;Spak- totheeast,theLiguro-Provencalbasintothenorth,andthewestern .org
manetal.1993;Carminatietal.1998;Piromallo&Morelli2003; Algerianbasintothewest(Fig.1a).Itsoceanicnatureissuggested a/
Spakman&Wortel2004;Bezadaetal.2013;Palomerasetal.2014; byan∼400-km-wideand∼200-km-longsetofmagneticanoma- t IF
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Thurneretal.2014),howeverthetimingandthegeometryofthe lies that trend N–S to NW–SW as they diverge southward within E
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southwardEuropeanforearcmigrationarestillamatterofdebate. theeasternAlgerianbasin(Fig.1a,insert;Galdeano&Rossignol E
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To the west, the westward rapid slab rollback under the Alboran 1977; Schettino & Turco 2006). Indeed, these are the only clear, o
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SeawouldberesponsiblefortheMioceneopeningofthewestern regularlyorganized,magneticanomaliesthathavebeenidentified M
Algerianbasin(Re´haultetal.1984;Gueguenetal.1998;Jolivet& inthewesternMediterraneandomain.Inotherpartsofthebasin, ay
2
Faccenna2000;Verge´s&Ferna`ndez2012;Medaourietal.2014; disorganizedmagneticanomalieshavebeeninterpretedasthere- 7
vanHinsbergenetal.2014).Here,thewestwardvergingslabwould sultofirregularandsporadicaccretionprocessesatthebackofthe , 20
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stillbeattachedtothesurfacelithospherebeneaththeBeticandRif subduction zones, based on the atypical structure of the oceanic 5
belts,butwouldbedetachedundertheAlboran(Palomerasetal. crust (Contrucci et al. 2001; Rollet et al. 2002). In contrast, the
2014;Thurneretal.2014),wherethelithosphericmantlemightbe setofregularanomaliesobservedwithintheeasternAlgerianbasin
drivenintotheasthenosphericmantle(Bezadaetal.2013;Palom- suggests that a steady oceanic accretion occurred in this part of
erasetal.2014;Thurneretal.2014).Recentstudiesdevotedtothe the basin. The timing and geometries proposed for the formation
crustalstructureofthewesternAlgerianmarginandtheadjacent ofthispartofthebasin,however,varyfromonemodeltoanother,
basinhaveproposedthatthesurfaceexpressionoftheslabtearin mainly because the age of these anomalies is still not known. In
thisareaisa‘STEP’fault(subduction-transformedgepropagator; the simplest model, the Algerian basin opened at the back of the
Govers&Wortel2005)thatisedgingthewesternAlgerianbasinto AlKaPeCablocks,asitmigratedtothesouthwithanarcuateshape
thesouth(Verge´s&Ferna`ndez2012;Medaourietal.2014;Badji (Gueguenetal.1998;Jolivet&Faccenna2000;Gelabertetal.2002;
etal.2015)andfollowstheslabretreatparalleltothecontinent– Rosenbaumetal.2002;Rosenbaum&Lister2004b;Michardetal.
oceanboundary(Medaourietal.2014;Badjietal.2015). 2006;Schettino&Turco2006).Thesetofregularanomalieswas
Totheeast,thesubductionisstillactiveundertheopeningTyrrhe- explainedbyanoceanicaccretionstagethatresultedfromthecoeval
nian basin (e.g. Jolivet & Faccenna 2000; Rosenbaum & Lister divergentsouthwarddisplacementoftheKabylies,andtheCorso–
2004a;Rosenbaumetal.2008;Gallaisetal.2013).Thewaythe Sardinian block toward the east (Gelabert et al. 2002), possibly
slab migrated from the Kabylian area to its present-day position along a complex spreading system that involved transform faults
isnotclear,mainlybecausetheevolutionoftheeasternAlgerian andtriplejunctions(Schettino&Turco2006).
basinremainspoorlyunderstood.Inparticular,littleisknownabout Other models have proposed a two-stage oceanic opening. In
its crustal structure, and that of its southern margin. While some these two-stage models, the south and southeast migration of the
DeepstructureoftheeasternAlgerianmargin 1915
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Figure2. (a)Locationofthedatasetusedinthisstudy,superimposedonthebathymetricmapprovidedbyMaradja2/Samracruise(2005)(Kherroubietal. M
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2009)andbyETOPO11-minglobalrelief(www.ngdc.noaa.gov)intheoffshoredomain,andontheeasternAlgeriangeologicalmapintheonshoredomain R
(Vila1978).ThegeologicalmapshowsthedifferentstructuralunitsofthestudyareaincludingtheInternalzones(LesserKabylia,Fig.1,LK)andExternal on
zones(Tell).Ed,Edoughmassif;CDF,CapdeFer;An,Annababasin;Mm,Melle`gueMountainsandDi,Diapirarea.Reddots,OBSposition(numberedGHxx); M
a
Triangles,landstationposition(numberedSPxx);Redtriangles,stationswhichrecordedusabledata.Blacktriangles:stationsrecordedonlynoise.Whitelines: y 2
locationofthemultichannelseismiclines,magneticandgravitydataacquiredduringSPIRALcruise.Blacklines:locationoftheseismicreflection,gravityand 7, 2
magneticdataprovidedbySonatrach.Yellowlines:high-resolutionseismicprofilesacquiredduringMaradja2/Samracruise(2005).Redframe:locationofthe 0
1
areadisplayedin(b).Whitedottedline(OCT):ocean-continenttransitionalzoneafterthisstudy.(b)BathymetricmapofthestudyareafromMaradja2/Samra 5
cruise(2005)(Kherroubietal.2009)andfromETOPO11-minglobalrelief(www.ngdc.noaa.gov).CDF,capdeFer,Ed,Edoughmassif.
AlKaPeCaandCorso-Sardiniablockswasfollowedbyoceanicac- ofthemarginfollowingtheslabtearandtheeastwardmigrationof
cretionalongaNW–SEspreadingcentre,whichledtotheobserved theslabalongtheeasternAlgerianmargin,symmetricaltotheevo-
regular anomaly pattern (Cohen 1980; Mauffret et al. 2004). As lutionproposedforthewesternAlgerianmarginbyMedaourietal.
the Corso-Sardinia block is assumed to be fixed in its present- (2014)andBadjietal.(2015).
day position from 15 to 18 Ma, this implies that an ∼400-km-
wide oceanic domain opened toward the west. This might have
2.2 Geologyoftheonshoredomain
beenaccommodatedeitherbyaN–Sintra-oceanicsubductionlo-
cated at ∼5◦30(cid:5)E (Cohen 1980), or by the western migration of The north Algerian geology derives from the Tethys Ocean clo-
theneo-formedoceanicflooralongatransformzonelocatednorth sure during the Algerian basin opening, and the subsequent col-
of the Algerian margin (Mauffret et al. 2004). In the first case, lage of the AlKaPeCa blocks along the North African margin.
theoceanicaccretionallowedthesouthwestwardmigrationofthe This is marked by the southward thrusting of the internal zones,
LesserKabyliatowardtheAfricanmarginaftertheGreaterKabylia formedbythePalaeozoicEuropeancrystallophyllianbasementof
collage (Cohen 1980). In the second case, the accretion occurred the AlKaPeCa blocks and its Mesozoic carbonated sedimentary
afterthecoevalGreaterandtheLesserKabyliacollages(Mauffret cover,whichisrepresentedbythelimestoneofthe‘DorsaleKabyle’
etal.2004).Thesetwo-stagemodelswhereaccretionoccurredafter (Rivie`reetal.1977;Bouillin1979;Ge´ryetal.1981;Djellit1987;
themaincollisionphasearecompatiblewithaSTEP-faultevolution Bracene2001),overtheexternalzonesformedbytheNorthAfrican
1916 B.Bouyahiaouietal.
margin (Fig. 2a). The North African margin is composed of pa- 3 ACQUISITION, DATA QUALITY
rautochthonousTellianunitsthatincludesedimentsfromTriasto AND PROCESSING
Eocenetimes(Fig.2a),whichlieabovetheAfricanbasementthat
Inthepresentstudy,newgeophysicaldatawasacquiredduringthe
outcropsmorethan180kmsouthoftheshoreline.Betweenthein-
SPIRAL cruise, which was conducted on the R/V Atalante (Ifre-
ternalandexternalzones,thesuturezoneismarkedbyflyschunits
mer) in September–November 2009, along the Algerian margin.
of various ages. These correspond to allochthonous clastic sedi-
Duringthiscruise,oneonshore–offshore,wide-angleseismicpro-
mentdepositedintheclosedTethyanoceanicdomain,andinclude
file(Fig.2a,WA-An),fourMCSlines,andmagneticandgravity
theMassilianandMauritanianflysch,whichhavebeeninterpreted
datawererecordedalongtheeasternmostpartoftheAlgerianmar-
as Cretaceous deposits (Djellit 1987), and the Numidian flysch,
ginandtheadjacentdeepbasin(Fig.2a,Spi18toSpi21).MCSLine
which has been interpreted as Oligo-Miocene deposits (Bouillin
Spi18iscoincidentwiththewide-angleseismicprofileWA-An.
et al. 1970). These allochthonous flysch formations are found on
The additional available data used included: (1) bathymetry
the foreland, although they are also backthrusted on the internal
and high-resolution seismic data acquired in 2005 during the
zones(Fig.2a;Durand-Delga1969).
Maradja2/Samra cruises (Fig. 2a, yellow lines; De´verche`re et al.
Theonshorepartofthestudyareaincludesthedifferentunitsof
2005;Domzigetal.2006;Kherroubietal.2009;Yellesetal.2009);
this Alpine collision zone: along the shoreline, the Edough Mas-
(2) time-migrated industry seismic lines provided by Sonatrach
sif (Fig. 2a, Ed) corresponds to a crystallophyllian basement that
(Fig. 2a, lines L1 to L4) and (3) gravity and magnetic data (pro-
iscomposedofavarietyofNeoproterozoicandPalaeozoicmeta-
videdbyCRAAGandSonatrach;Fig.2a,linesL1–L4).
morphicrock(Laouaretal.2002),whichislocallyoverlainbyal-
lochthonousflysch,andhasintrusionsoferuptiverock(Hilly1962).
Theoriginofthismassifhasbeendebatedforalongtime.Although
3.1 Wide-angleseismicdata
atfirstitwasconsideredaspartoftheAfricanmargin(Vila1980; D
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Bouillin 1986; Frizon de Lamotte et al. 2000 ; Cabyet al. 2001), Thecompleteonshore–offshoreprofileis240kminlength,along w
n
recentstudiesindicatethatitispartoftheEuropeanbasementthat which 42 ocean-bottom seismometers (OBS; Fig. 2a, red circles) lo
a
formstheeastwardextensionoftheLesserKabylia,andthusispart and25landstations(Fig.2a,redandblacktriangles)weredeployed. de
d
of the internal zones (Laouar et al. 2002; Bruguier et al. 2009). The average spacing between the OBS and between the land sta- fro
SouthoftheEdoughMassif,thereisalargesedimentarybasinthat tionswas3and5km,respectively.TheOBSusedwereMicroOBS, m
isformedbytheAfricanMesozoictoCenozoicsedimentarycover, MicroOBS+ and OldOBS from Ifremer (Auffret et al. 2004). http
between the Mellegue Massif and the Saharan flexure, with this The land stations used were of the Geostar-2000, Geodevice and ://g
area characterized by intense Triassic salt diapirism [Fig. 2a, ‘di- Kinemetricstypes. ji.o
apirarea’(Di);Dubourdieu1956;Rouvier1977;ChikhiAouimeur Theseismicsourceusedforthewide-angleprofilewasa146-l xfo
1980; Pertthuisot & Rouvier 1992]. Between the diapir area and airgunarraytunedtothefirstpeak,whichwascomposedofeight rd
jo
theEdoughMassif,theE–Welongated,QuaternaryAnnababasin 16-l airguns and two 9-l airguns. The airgun array generated 807 urn
extendsoveranareaof∼1200km2(Fig.2a,An).TheCretaceousto shotsalongtheprofile,every60s,leadingto150-mshotspacing. als
Oligo-Mioceneflyschconsistsofseveralsheetsthatdeformedand The marine instrument positions were corrected for drift dur- .org
stackedalongthrusts,toforman∼1.5-km-thicksedimentpilewith ingtheirdescenttotheseafloorusingthefirst-arrivaldirectwave. a/
acomplexinternalstructure(Hilly1962).TheNumidianflyschis Clock drift of the internal clock of the seafloor instruments was t IF
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particularlywellrepresentedSEofAnnaba,whereitsoutcropsover correctedlinearly.Onfaroffsets,thesignal-to-noiseratiowasim- E
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3500km2 (Fig. 2a). The accurate location of the Alpine suture is proved by processing the OBS and land station data, to facilitate E
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notknowninthestudyarea.Itisprobablylocatedunderneaththe arrival identification and interpretation. The processing sequence o
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flyschcover,southoftheEdoughMassif,anditprobablyextends included:spectraldeconvolution(whitening),4–18Hzbandpassfil- M
totheeastintheoffshoredomain(Fig.2a). tering(Butterworth),andautomaticgaincontrol. ay
2
Offshore,allOBSexceptone(whichwaslost;GH14)recorded 7
goodqualitydata(e.g.seeFigs3and4).FortheP-wavearrivals,the , 20
1
datashowedhighersignal-to-noiseratioontheverticalgeophone 5
2.3 Geologyoftheoffshoredomain
componentthanonthehydrophoneandhorizontalgeophonecom-
Little is known about the deep crustal structure offshore of the ponents,withcleararrivalsidentifiableuptoa65-kmoffsetinthe
Annabaregion.Indeed,previousstudieshaveessentiallybeenbased deepbasin,anda50-kmoffsetatshallowwaterdepth(Figs3a,b,
onhigh-resolutionseismicreflectiondata,whichresultedindetailed andAppendix1).S-wavearrivalswerealsoobservedonthehori-
imagingoftherecentsedimentarycover.Thesedatashowthatthe zontalgeophoneof18OBSlocatedwithinthedeepbasin(Fig.5b,
margin and the deep basin edge were reactivated by contraction blue dots) for offsets that ranged between ∼15km and ∼40km
duringthePlio-Quaternary(Kherroubietal.2009),withtherecent (Fig.4).TheotherOBSdidnotshowanyS-wave,eitherbecausethe
margininversionindicatedbyolderdeeppenetratingseismicpro- seafloorinstrumentswereequippedwithhydrophoneonly(Fig.5b,
files(Mauffret2007).TheaverageMohodepth,asgivenbygravity blackdots),orbecauseS-wavesdidnotgetgeneratedorwerenot
modelling of the European Plate, is of the order of 25km below detectableonthedatasetduetopoorsignal-to-noiseratio(Fig.5b,
the margin and 15km below the deep basin (Grad et al. 2009). reddots).
EastoftheAnnabaarea,theSardinianChannelisunderlainbya Fortheonshorepartoftheprofile,onlythe13stationslocated
10-km-thick to 25-km-thick continental basement that is overlain within an 80-km range from the shoreline recorded usable data
by a 4-km-thick sedimentary cover, as shown by the wide-angle (Fig. 2a, red triangles). The absence of useful signals from the
seismicdatafromtheEuropeanGeotraverseproject(Peirce&Bar- other stations was probably related to the lack of energy at large
ton1992).ThebasementcropsoutonGaliteIsland(Fig.1a),and offsetand/ortocomplexgeologicstructuresthataffectedthewave
thisisinterpretedaspartoftheinternalzones(Tricartetal.1994; propagation, rather than to dysfunction of the instruments. The
Bouillinetal.1998;Mascleetal.2004;Belayounietal.2010). dataacquiredshowedgoodsignal-to-noiseratio,withidentifiable
DeepstructureoftheeasternAlgerianmargin 1917
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Figure3. Examplesofrepresentativewide-anglesectionsrecordedalongWA-AnprofileandshowingP-wavearrivals.(a)and(b)OBSsections(GH39 .o
andGH26,respectively)locatedwithinthedeepbasin,(c)land-stationsection(SP01)locatedonshore2kmsouthoftheshoreline.Theseseismicsections arg/
cAourtroemspaotnicdGtoaitnheCovnetrrtoicla(lAcGomC)p,ownietnhtare7ckomrdss.−T1hveeylowcietryerdedisupcltaiyoend.Rafetderlidnaetsa:ppricokcsesosfinthgeinficrslutdairnrigv:adlsecuosendvoflourtifiornstwahrriitveanlintogm,Bogutrtaeprhwyo.rthfilter(4–18Hz), t IFR
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arrivalsforsomestations,to∼70kmdistance(Fig.3c,Appendix1). 1973)andthesmoothedNMO-velocitymodel;(7)multipleatten- R o
NoS-waveswereobservedforthesesections. uation using the two complementary methods of surface-related n M
multiple elimination based on the subtraction of a model of the a
y
multiple to the dataset (Berkhout & Verschuur 1997), and a 27
3.2 Multichannelseismicdata parabolic stack radon transform based on the velocity difference , 2
0
1
The SPIRAL MCS data (Fig. 2a, Spi18 to Spi21) were acquired between the multiples and the primaries (Bradshaw & Ng 1987); 5
usinga4.5-km-longstreamer(Ifremer)composedof360channels and(8)Kirchhoffpre-stacktimemigration.Thefinalpre-stacktime
with a 12.5-m interval. The seismic source was an airgun array migrationvelocitymodelwasobtainedafterfiveiterationsofve-
composedof13airgunsofvariousvolumestunedtothefirstbubble locity picking performed on migrated CMPs every 1250 m. The
pulse,togeneratelow-frequencysignalsfordeeppenetration(dom- processed section was plotted after application of a two-window,
inantfrequency,∼25Hz)(Avediketal.1993).Theseismicsource time-variant, Butterworth frequency filter of [3–8–20–30Hz] and
usedatotalvolumeof3040cubicinches(49.8l),withaninter-shot [3–8–20–30Hz],appliedfromtheseabedtothetopoftheacous-
of 20 s, which provided 50-m shot spacing. The MCS data were tic basement, and deeper than the top of the acoustic basement,
recordedwitha4-mssamplerate. respectively(Fig.5).
TheprocessingsequenceappliedtothedatausedtheGeocluster TheadditionalmultichannelseismiclinesL1toL4ofFig.2awere
software (CGG Veritas package), and included: (1) quality con- acquiredandprocessedin2000byWestern-Geco(Cope2003)for
trolofthedataset;(2)zero-phaseconversionofthesinglebubble Sonatrach.Theseismicsourceconsistedofatunedfirstpeakfroma
wavelet;(3)commonmid-point(CMP)gathering(CMPdistance, sleeveairgunarrayof3000ci(45l)shotatapressureof∼2000psi
6.25 m; coverage, 45-fold); (4) reverse Q-filtering to remove the andtowedat6mindepth(Arabetal.2014;Medaourietal.2014).
nonstationary phase components of the data, and external mute Theshotpointintervalwas25m.Thereceiverwasa480-channel
fortheremovalofthestretchedpartofthesignaloftherefracted (12.5meach),6000-m-longstreamertowedat8mindepth.The
waves;(5)normalmoveout(NMO)velocityanalysesperformedev- processingsampleintervalwas4msforaprocessingrecordlength
ery200CMP(1250m),inthreesuccessiveiterations;(6)spherical of10000ms(Arabetal.2014).Theprocessingsequenceisdetailed
divergence correction based on the compensation laws (Newman inMedaourietal.(2014).
1918 B.Bouyahiaouietal.
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Figure4. ExamplesofrepresentativesectionsrecordedwithinthedeepbasinalongWA-Anprofilefor(a)OBSGH16,(b)OBSGH26and(c)OBSGH33.The , 2
0
blackframesindicatethelocationofthezoomspresentedatthetopofeachseismicsection.ThebluedotspointouttheS-wavetraveltimesusedformodelling. 15
TheSwavescomefromP-waveconversionalongsubsurfaceinterfaces.Thethreeseismicsectionscorrespondtothehorizontalcomponentrecordsofthe
OBSs.Theyweredisplayedafterdataprocessingincluding:deconvolutionwhitening,Butterworthfilter(4–18Hz),AutomaticGainControl(AGC),witha
4.5kms−1velocityreduction.
3.3 Gravityandmagneticdata ThedistancesbetweentheSPIRALprofiles,whichwereoftheor-
derof12km,didnotprovideadequatecoverageforgravityandmag-
Four gravity and magnetic profiles were acquired offshore of the
neticanomalymapping,sothatexistingdataprovidedbySonatrach
AnnabaareaduringtheSPIRALcruisealongtheMCSlines(Fig.2a,
werealsoused(Fig.2a,blacklines).Theseexistingprofileshadan
white lines). The gravity data were recorded with a 25-m inter-
averagelengthof80km.
stationspacing,usingaLockheedMartinBGM-5gravimeter.These
dataweretiedtoabsolutegravityviaareferencepointlocatedin
OranHarbor(Fig.1a)andusingaterrestrialScintrexCG3gravime-
ter.Onegravitymeasurewasrecordedevery10s,afterfiltering.The
4 RESULTS
instrumentdriftwaslinearlycorrectedbetweenties,beforeandafter
thecruise.TheFreeAirandtheBougueranomalieswerecalculated
4.1 Morphology
afterEo¨tvo¨scorrection,with1-mGalprecision.Themagneticdata
wererecordedat150-m(60-s)intervals,usingaSeaSPYmagne- TheeasternmostAlgerianmarginischaracterizedbytwodistinct
tometer,whichmeasuredthetotalintensityofthegeomagneticfield morphological domains, with the location of the wide-angle pro-
withan∼0.2-nTprecision. file marking the limit between these two. West of the wide-angle
DeepstructureoftheeasternAlgerianmargin 1919
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Figure5. Time-migratedMCSprofileSpi18(coincidentwiththewide-angleLineWA-An;seeFig.2aforlocation)withsuperimpositionoftheseismic .org
interpretationoftheline(a),andtheP-wavevelocity–depthmodeldeducedfromforwardmodellingofthewide-angledataconvertedintoatwo-waytraveltime a/
section(b).ThedotsatthetopoftheseismicsectionmarkthelocationoftheOBS(Bluedots:Swavesobservedonthehorizontalgeophone;Reddots:noS t IF
wavedetectedonthehorizontalgeophone;Blackdots:NoSwaverecordedbecausethereceiverswerehydrophonesonly). R
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line (Fig. 2b, the western margin segment), the continental shelf 4.2 Multichannelseismicdatainterpretation M
a
reaches a maximum width of 15km within the Skikda Bay, and y
2
reacheshalfthiswidthwestandeastofthebay,offshoreofCapde 4.2.1 Architectureoftheacousticunits 7, 2
Fer(Fig.2b,CDF).Thecontinentalslopeextendsover18kmand 0
showsaregular,abrupt,8◦averageslope.Thissectorismarkedby Theseismicinterpretationproposedbelowisbasedontheseismic 15
units defined in previous studies conducted on the Algerian mar-
abundanterosivegulliesthatgenerallyrunparalleltotheslopeline.
gin and within the adjacent deep basin. All of these studies use
Offshore of Cap de Fer, the slope has a 12-km-wide, dome-like,
the Messinian units (as mobile unit [MU] and upper unit [UU])
rounded shape (Fig. 2b, D). East of the wide-angle line (Fig. 2b,
and surfaces (as Messinian erosion surface [MES]) for chrono-
easternmarginsegment),thecontinentalshelffromAnnabaBayto
stratigraphic markers, as they can be recognized easily in the
theTunisianborderiswider(∼25km)(Figs2aandb).Here,the
seismic lines (Auzende 1978 ; De´verche`re et al. 2005; Domzig
slopeisgreater,andoftheorderof35km,anditismarkedbya
2006;Domzigetal.2006;Mauffret2007;Kherroubietal.2009;
slopebreaklocatedmid-slope.Theslopeisgentle(4◦)initsupper
Yellesetal.2009)andtheyhavebeenwelldated(5.96–5.32Ma;
part,whichextendsover25km,andsharplyincreasesatthedeep
Gautieretal.1994;Krijgsmanetal.1999).Wethusdefinedthree
slopetoreach9◦onaverage.
sedimentaryseismicunits,asMessinian(UU-MU),pre-Messinian
Thedeepbasinshowsasmoothmorphologyandauniformdepth
(Pre-MSC)andpost-Messinian(Plio-Quaternary,PQ)units,using
throughout the study area, with a maximum depth of ∼2800 m.
thenomenclatureproposedbyLofietal.(2011a,b)onthebasisof
The seafloor depth gradually decreases east of Line Spi21, while
arecentcompilationofMediterraneanstudies.Itshouldbenoted
approachingtheSardiniaChannel(Fig.2a).Alongthecontinental
that a Messinian lower unit (LU) is observed locally within the
slopeandbetween7◦35(cid:5)Eand8◦00(cid:5)E,a∼2500-m-deep,∼27-km-
Mediterranean Sea below a MU (Lofi et al. 2011a,b ; Arabet al.
long step overhangs the rest of the abyssal plain (Fig. 2b, ST).
2014). However, because the Messinian LU could not be clearly
Thisstepwasinterpretedastheresultofrecentmarginreactivation
identifiedinthelow-resolutiondatasetofthestudy,itwasincluded
(Kherroubietal.2009).
inthePre-MSCunitinourinterpretationoftheseismicdata.
1920 B.Bouyahiaouietal.
Figure6. Time-migratedMCSLineSpi21(seeFig.2aforlocation)andseismicinterpretation.PQ,Plio-Quaternaryunit;UU,upperMessinianunit;MU,
mobileMessinianunit;Pre-MSC,pre-Messinianunit.Inthenorthernpartoftheseismicline,thereflectormarkingthetopofthebasementisweak,andliesata
constantdepthof∼5.8stwtt,whereasitismorecontinuousandshallowerinitssouthernpart.Thischangeinthetopofthebasementmorphologycorresponds Do
w
toachangeintheMessinianunitfacies,fromroughtosmooth. n
lo
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4.2.2 Acousticunitsinthedeepbasin whichwasoftheorderof∼0.6stwtt,hasbeenexplainedbyPlio- ed
Quaternarycontractivedeformationofthemargin(Kherroubietal. fro
AlongthemainMCSprofile(Spi18),whichiscoincidentwiththe m
wide-angleprofile,theMessinianunitsformaclearsetofreflectors 200S9im).ilarfaciesanddistributionoftheshallowestacousticunitsare http
withinthesedimentarycoverobservedbetween4-sand5-stwo-way observedthroughoutthedeepbasinofthestudyarea.Inparticular, ://g
travel times (stwtt) in the deep basin (Fig. 5, Spi18). These units thediscontinuousbasementtopshowsauniformdepthof∼5.8stwtt ji.o
consistoftheUUandtheunderlyingMU.TheUUis∼0.35-stwtt through the deep basin, ∼15km from the slope toe and beyond. xfo
thick,andhasatypicallayeredfaciesthatshowshighamplitude, rd
In the ∼15-km-wide area that bounds the margin, the top of the jo
and high continuity reflectors (Lofi et al. 2011a). Below UU, the u
MUformsfoldsanddiapirs.ThetopoftheMUisobservedat∼4.4 basementhasenhancedcontinuityandsmoothermorphologyasit rna
stwtt,whichismostlythesamedepthaselsewhereinthewestern deepens,toreach6.5stwttto6.9stwtt,asthePre-MSCunitisthicker ls.o
inthisarea.Anexceptionistheeasternmostpartofthearea,where rg
M20e1d1iate).rrWaneeannot(eDaomslizgihgt2c0h0a6n;geKihnertrhoeubMieestsianli.an20u0n9it;fLacoifiesetaanld. sthloepbeastoeem(eFnitgt.o6p).liHesearet,∼it5i.5stshtwusttsohvaellroaw3e5r-tkhmandeislstaenwcheefrreoimntthhee at IFR/
deformation along the seismic line. Close to the margin, the UU E
vicinity of the margin. All along the eastern part of the Algerian M
reflectorsappeartobecontinuousandthesalttectonicsarechar- margin,thechangesinthebasementtopgeometryandfaciestoward ER
amcatergriizne,dnubmyewroeluls-ddeisaipginrsedoffsomldasllaenrdsidzieaipnitresr,rwuphtetrheeasUfUarrferfloemcttohres tahceoumstaircgfinaccieosindceisdceriwbeitdhatbhoevleatfeorraLlicnheanSgpei1i8n.theMessinianunit on M
(uFniigt,.s5o).thHateirtei,stdhieffirceusultlttoisdeaficnoemthpeleexxasctrtuschtaupreeooffththeeMMUesdsiianpiiarns At CMP number ∼8000 along Line Spi18, an ∼15-km-wide ay 27
amongadiscontinuousUUunit.TheseMessinianunits(UU,MU) TrehlieefMiesssfionrimanedanbdypothset-Mtoepssoinfiathneunpirtes-Mbeevseslinoinanthuenrietli(eFfigsi.d5es),. , 201
pinchoutalongthemargintoeat2.6stwttand4.0stwtt,respectively 5
whereas the pre-Messinian reflectors dip gently toward the relief
(Fig.5).
edges.ThisreliefisalsoobservedalongLineSpi19(seeFig.2afor
AbovetheMessinianunits,thePQunitischaracterizedbyhigh
location),atitsjunctionwithinLineSpi18.
continuity,lowamplitudereflectors.ThePQunitthickensfromthe
centralpartofthedeepbasintothemargintoe,from∼0.3stwttto
∼1stwtt.
4.2.3 Acousticunitsonthecontinentalmargin
BelowtheUUandMUunits,thePre-MSCunitischaracterized
bysub-horizontalstratification,especiallyatthesouthernedgeof AlongLineSpi18,belowthecontinentalslopeandthecontinental
the basin below the bathymetric step (Fig. 5, ST). The Pre-MSC shelf,theMessinianeventischaracterizedbyaMESthattruncates
unitshowsarelativelyconstantthicknessinthecentralpartofthe theunderlyingreflectors(Fig.5).AbovetheMES,thePQunithas
deepbasin(∼1–1.2stwtt),tothenorthernedgeofthebathymetric athicknessofbetween0.1stwttand0.5stwtt.UndertheMES,the
step.Here,thethicknessofthePre-MSCunitsharplyincreases,to upperpartofthePre-MSCunitischaracterizedbysetsofdipping
reach∼1.7stwttbelowthebathymetricstep. reflectorswithoppositedip(Fig.5).Togofurtherintotheseismic
Farfromthemargin,thetopoftheacousticbasementismarked interpretation of the seismic units along the margin, we used the
by an irregular, discontinuous reflector at ∼5.8 stwtt depth. As high-resolutionseismiclinesavailableinthestudyarea,whichshow
this approaches the margin, it shows enhanced continuity and it differentimagesbetweentheeasternandwesternsegmentsofthe
deepensto6.5stwtt(Fig.5).Atthebasinedge,thePre-MSCand studyarea.
MSCunitsarefoldedandupliftedbelowthebathymetricstep.The Alongtheeasternsegmentofthestudyarea,whichismarkedbya
verticalthrowoftheseismicmarkersalongthenorthernstepedge, smoothmorphology(Fig.2b),thehigh-resolutionseismicreflection
DeepstructureoftheeasternAlgerianmargin 1921
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Figure7. Seismicreflectionlinesandseismicinterpretationillustratingthemarginstructureeastofthewide-angleLineWA-An/Spi18(a,b),andwestof 2
7
these(c).(a)Time-migratedMCSprofileL4(seeFig.2aforlocation)providedbySonatrachanditszoomofthislineonthemarginseismicimage(b).The , 2
pre-MessinianunitsFb(bedded)andFc(chaotic)formsubunitsbelowtheMessinianErosionSurface(MES).Eachsubunitshowsitsownreflectivityand 01
5
deformationpatterns.Thesubunitslimitsareusuallysharpandmarkedbyclearunconformities.Fb-FcsubunitslieinunconformityoveradeepersubunitMio,
characterizedbylow-frequencyreflectorsthatappearlessdeformedthanFb-Fcreflectors.(c)Time-migratedMCSprofileL1(seeFig.2aforlocation)provided
bySonatrach.WestofLineWA-An/Spi18,thePQunitisthin,andonlyafewreflectorsarelocallyobservedjustbelowthePQunit.Theserarereflectorsshow
poorlateralcontinuityandchaoticorganization,exceptatthedeepmargin,wherePre-MSCreflectorsareclearlyidentifiedoverathicknessoffewhundreds
ofmillisecondsoftwo-waytraveltime,withanunusuallyhighdipcomparedtothearealocatedeastofthewide-angleLineWA-An.
linesshowthatthePre-MSCunitcanbesplitintothreeunits:Fb, preventsseismicimagingofthesub-unitreflectors(seeforexample
Fc, and Mio (Figs 7a and b). The Mio forms the deepest unit at Fig.7b,forCMPnumbers∼1500to2000,or2200to2600).The
thetopoftheacousticbasementandisoverlaininunconformityby densityoftheavailableseismiclinesisnotsufficienttodefinethe
FbandFc,whichdifferfromeachotheraccordingtotheiracoustic three-dimensional(3D)accuratespatialorganizationoftheFband
facies. The seismic unit Fb is characterized by clear sub-parallel Fcsub-units.Itishoweverclearthattheyarefoldedinthelinesshot
reflectors, whereas Fc is devoid of reflectors, or shows a chaotic both parallel and perpendicular to the margin, and that the sharp
pattern(Fig.7b).TheFbandFcunitscanbedividedintosub-units contactbetweenthefoldedsub-unitsistectonicratherthanerosive
that show lateral extensions of ∼5km to∼12km. Each Fb and (Figs7aandb).Asthenumerousfoldsimplyacontractivetectonic
Fcsub-unitischaracterizedbyitsowndeformationandreflectivity environment,thesecontactsareinterpretedasthrusts,whichleadto
patterns,andthecontactbetweenthesub-unitsissharpandisusually thestackingofthesubunitsoverthicknessesthatlocallyreach∼1.4
markedbyclearunconformities(Fig.7b).Locally,lateralchangesin stwtt(Fig.7a;i.e.∼2800m,accordingtothevelocity–depthmodel).
reflectivityinsidesomeofthesub-unitsaccompanyincreasesinthe The seismic lines show that the Fb and Fc subunits are observed
reflectordip,whichsuggeststhatacomplexinternalorganization belowtheslopeanduptotheshelfbreak,whichislocated∼20km