Flexural behaviour of the north Algerian margin and tectonic implications

12 Pages·2017·8.01 MB·English

Flexural behaviour of the north Algerian margin and
tectonic implications
Lamine Hamai, Carole Petit, Abdeslem Abtout, Abdelkarim Yelles-Chaouche,
Jacques D´everch`ere
To cite this version:
Lamine Hamai, Carole Petit, Abdeslem Abtout, Abdelkarim Yelles-Chaouche, Jacques
D´everch`ere. Flexural behaviour of the north Algerian margin and tectonic implications.
Geophysical Journal International, Oxford University Press (OUP), 2015, 201, pp.1426-1436.
<10.1093/gji/ggv098>. <insu-01142064>
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Geophysical Journal International
Geophys.J.Int.(2015)201,1426–1436 doi:10.1093/gji/ggv098
GJIGeodynamicsandtectonics
Flexural behaviour of the north Algerian margin and tectonic
implications
Lamine Hamai,1,2 Carole Petit,1 Abdeslem Abtout,2 Abdelkarim Yelles-Chaouche2
and Jacques De´verche`re3
1Geoazur,CNRS-UNS-IRD,250rueAlbertEinstein,F-06560Valbonne-SophiaAntipolis,France.E-mail:[email protected]
2CentredeRecherchesenAstronomieAstrophysiqueetGe´ophysique,Routedel’Observatoire,BP63,Algiers,Algeria
3DomainesOceaniques,InstitutEurope´endelaMer,CNRS-UBO,TechnopoˆleBrest-Iroise,PlaceNicolasCopernic,F-29280Plouzane´,France
Accepted2015February24.Received2015February23;inoriginalform2014October15
SUMMARY
TheAlgerianmarginformedthroughback-arcopeningoftheAlgerianbasin(Mediterranean
Sea)resultingfromtheroll-backoftheTethyanslab.Recentgeophysicaldataacquiredalong D
o
the Algerian margin showed evidence of active or recent compressive deformation in the w
n
basin due to the ongoing Africa–Eurasia convergence. Published data from four wide-angle lo
a
d
seismicprofileshaveallowedimagingthedeepstructureoftheAlgerianmarginanditsadjacent e
d
basins.Inthisstudy,weconvertedthesevelocitymodelsintodensitymodels,thenintoisostatic fro
m
anomalies. This allowed us to image an isostatic disequilibrium (relative to a local isostasy h
model)reachingamaximumamplitudeatthemargintoe.Convertingisostaticanomaliesinto ttp
MohodepthvariationsshowsthattheMohoextractedfromwide-angleseismicdataisdeeper ://gji.o
than the one predicted by a local isostasy model in the oceanic domain, and shallower than x
fo
itinthecontinentaldomain.Theseanomaliescanbeinterpretedbyoppositeflexuresoftwo rd
jo
platesseparatedbyaplateboundarylocatedclosetothemargintoe.Weuseafiniteelement u
rn
model to simulate the lithospheric flexure. The amplitude of the equivalent vertical Moho als
deflectionislargerinthecentralpartofthestudyarea(6–7km)thanontheeasternmostand .org
westernmostprofiles(3km).Theeffectiveelasticthicknessusedtobestmatchthecomputed b/
y
deflectionisalwaysextremelylow(alwayslessthan10km)andprobablyreflectstherelatively gu
e
lowstrengthofthelithosphereclosetotheplateboundary.Comparisonwithotherwide-angle st o
seismic profiles across an active and a passive margin show that the North Algerian margin n A
datistphleayssouisthoesrtantitcipaonfomthaelioecsecalno-sceontotintheanttotrfaannsiaticotniv,epomsasrigbliyn.inFdinicaalltyin,gpltahtaetflaefxourrmeeisrphaigshsievset pril 5, 2
margindetachmentisreactivatedasacrustalscalereversefaultpre-datingafuturesubduction. 01
5
Key words: Gravity anomalies and Earth structure; Continental margins: convergent;
Dynamicsoflithosphereandmantle;Lithosphericflexure;Crustalstructure.
2004,andreferencestherein):inducedorspontaneous,depending
1 INTRODUCTION
onwhethertheyaredrivenbythefar-fieldtectonicplatemotionsor
Whereasoceanicsubductionisoneofthemostimportantprocesses bylocalgravitationalinstabilities.Inallcases,mostnumericalmod-
of plate tectonics, understanding how and where it begins is still elsofsubductioninitiationrequireapre-existingweaknesszonein
amatterofdebate,mostlybecauseexamplesofincipientoceanic whichdeformationislocated(Gerya2011,andreferencestherein).
subductionworldwidearescarce(e.g.Gerya2011,andreferences However,theactualstructuresofthisweakzone,itsorigin,andits
therein). The transition from passive to active margin can result relationshipwiththeearlystagesofcompressionaldeformationare
from various phenomena, including buoyancy contrasts, thermal stillunclear.Decipheringthestructureofarecentlyinvertedpas-
instability of the oceanic lithosphere, differential surface loading siveortransformmargincanthushelpunderstandinghowinduced
duetoerosionanddeposition,bodyforcesorotherparameters(e.g. subductioninitiatesandlocalizes.
Gurnis 1992; Faccenna et al. 1999; Regenauer-Lieb et al. 2001; The Algerian basin is a young oceanic basin (∼20 Ma or less,
Niu et al. 2003 ; Zhuet al. 2009). Commonly, subduction initi- Schettino & Turco 2006, 2011; van Hinsbergen et al. 2014) un-
ation mechanisms are assumed to fall into two categories (Stern dergoingcompressiveboundaryconditionsduetoAfrica–Eurasia
1426 (cid:3)C TheAuthors2015.PublishedbyOxfordUniversityPressonbehalfofTheRoyalAstronomicalSociety.
FlexuralbehaviourofthenorthAlgerianmargin 1427
Figure 1. Topography, bathymetry by ETOPO1 1-min global relief (www.ngdc.noaa.gov) and seismicity of the Algerian margin (epicentres and focal
mechanisms).SolidlinesindicatethelocationoftheSPIRALwide-angleprofiles.InsetshowsamoreregionaltectonicsketchoftheMediterraneanRegion D
o
(modifiedafterBarrieretal.2004). w
n
lo
a
convergenceatarateoflessthan∼1cmyr−1(Deweyetal.1989; de
Sboerupnedlaloryniceotndailt.io2n0s07fa;vCorarsmubindautciteiotnali.nc2e0p1t2io)n. Tahloensge tkhieneNmoarttihc 2HISSUTMORMYARY OF GEODYNAMICAL d from
h
African margin (Cloetingh et al. 1982, 1989). Northern Algeria, The Western Mediterranean Sea belongs to the Alpine orogenic ttp
locatedattheAfrica–EurasiaPlateboundarysouthoftheAlgerian system and is located near the boundary of Africa and Eurasia ://g
basin,currentlyundergoesaslowcompressionaldeformationonits plates.ThewesternMediterraneanbasindepictsaunique,complex ji.o
x
continentalmargin(Auzendeetal.1972;FrizondeLamotteetal. structure,withitssteepmountainrangesandlargecontinentalisland fo
2011),asevidencedbyactiveseismicityrecordedbothonlandand blockswhichseparateseveralsub-basins(Fig.1). rdjo
u
atsea(Domzigetal.2006;Yelles-Chaoucheetal.2006;Kherroubi During the Liassic, the opening of the central Atlantic caused rn
a
et al. 2009; Kherroubi 2011). This recent compression (of which a sinistral strike-slip motion between Africa and Iberia, creating ls
.o
theonsetisdatedbetween10and2Ma)betweenEurasiaandAfrica a transformzone passingthroughthe Gibraltar strait(Roca et al. rg
hasbeenproposedearlier(e.g.LePichon1968;Talwanietal.1969; 2004).Approximatelyatthesametime,thedislocationofPangea b/
y
Caputoetal.1970;McKenzieetal.1970),howeverfromGibraltar ledtotheopeningoftheAlpineTethys(Irving1977,2004;Muttoni gu
e
toSicily,theprecisegeometryoftheplateboundaryisstillpoorly et al. 1996; Frizon de Lamotte et al. 1991, 2011). The northern st o
defined,becausethedeformationisstilldiffuseandnotyetlocal- TethyanpassivemarginisbornatthesouthernedgeoftheAlKa- n
A
ized on a well-defined plate interface. The North African margin PeCa(AlboranKabyliaPeloritanandCalabria,Bouillin1986)con- p
inAlgeriamaythereforerepresentatransitionalstagebetweenac- tinentalblocksystem,itselfattachedtotheSardiniaandBalearic ril 5
tiveandpassivemarginsettings(Lonergan&White1997;Gueguen proto-islands.Inthe Late Cretaceous (84Ma, e.g. Gelabert et al. , 2
0
etal.1998;Rosenbaumetal.2002a;Faccennaetal.2004;Mauffret 2002),themotionoftheAfricanplatechangedduetotheopening 15
etal.2004;Jolivetetal.2006,2009;Yellesetal.2009;Carminati oftheSouthAtlantic,causingtheonsetofconvergencebetweenEu-
etal.2012).Ifthiswasthecase,thenthelithosphereshoulddisplay ropeandAfrica(Olivetetal.1982;Deweyetal.1989;Ricou1996;
an isostatic signal close to that of mature subduction zones, that Rosenbaumetal.2002a;Cavazzaetal.2004;Schettino&Turco
is, a progressive downward bending of the oceanic plate towards 2011). This new stress regime initiated the closure of the Tethys
thecontinentalmarginthatisnotduetoa‘normal’platedeflection Ocean, which was accommodated in the Maghreb by northward
underitsownload. subductionandopeningoftheAlgerianbackarcbasinbetweensta-
Inthisstudy,weuserecentlypublished2-Dvelocitymodelscom- bleEurasiaandAlKaPeCamicrocontinents.Thisstageendedwith
putedfromwide-angleseismicdataoftheSPIRAL2009surveyon thecollisionbetweenthecontinentalAlKaPeCablocksandAfrica
theNorthAlgerianmargin(Aidietal.2013;Lepreˆtreetal.2013; (Lonergan&White1997;Gueguenetal.1998;Rocaetal.1999;
Mihoubietal.2014;Bouyahiaouietal.2015).Thevelocitystruc- Verges & Sabat 1999; Frizon de Lamotte et al. 2000; Faccenna
tureisconvertedintoadensitystructureandanisostaticanalysis et al. 2001 ; Roca2001; Rosenbaum et al. 2002a; Mauffret et al.
isperformedbycomparingmeasured(frommergeddatasets)and 2004). These continental blocks, initially belonging to the Euro-
computedgravityanomalies.Isostaticanomaliesaremodelledus- peanmargin(Bouillin1986),brokeupintoseveralsub-blocksthat
ingabrokenelasticplatemodel.Fromtheseresults,weaim(1)to migrated southward and formed the internal zones of the Alpine
testwhetheraflexuraleffectisindeeddetectablealongtheAlgerian south-Mediterranean belt after their collision with North Africa.
margin, (2) if so, to evidence the along-strike changes of flexure Atwo-stagekinematicmodelofopeningoftheAlgerianbasinhas
and of mechanical properties of the oceanic lithosphere and (3) beenproposed,withafirstN–Sopeningduetothesouthwardmigra-
todiscusstheiroriginbycomparisonwithothertypesofmargins tionoftheAlKaPeCablocksat35Myr,followedbyanEWopening
worldwide. withtheformationofoceaniccrustalongNStrendingaccretionary
1428 L.Hamaietal.
D
o
w
n
lo
a
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Figure 2. Free air anomaly map of northern Algeria. Solid lines mark the SPIRAL wide-angle seismic profiles (TI = Tipaza, KA = Greater Kabylia, ed
JI=Jijel,AN=Annaba). fro
m
h
ttp
://g
centresbetween16and8Myr(Mauffretetal.2004;Mauffret2007; attachments.Thisdifferencewasaddedtotheoldvaluestobring ji.o
vanHinsbergenetal.2014).Accordingtothishypothesis,thelate themtothesamereferenceellipsoidastheabsolutemeasurements. xfo
stageofopeningofthebasinisassociatedwiththewestwardmigra- Inasecondstep,outlierswereeliminatedbyasystematicsearch rd
jo
tionoftheGibraltarArcandeastwardmigrationoftheCalabrian of peaks outside the mean distribution, centred on a square of urn
arc,andoccurredafterthecollisionoftheKabylianmassifswith 100 km × 100 km. These treatments provided a self-consistent als
North Africa. Magmatism attributed to slab detachment occurred free-air anomaly map attached to the absolute gravity reference .org
firstintheLanghien(∼15Ma)atthecentralAlgeriancoastandthen bases. b/
y
spreadtothewestandeast,supportingthehypothesisofopposite Theresultingfreeairanomalymapshowsseveralpositiveanoma- g
u
migrationoftheCalabrianandGibraltarslabtears(Carminatietal. liesonthenorthernofcontinentalmargin,andnegativeanomalies es
1998;Mauryetal.2000;Savelli2002;Spakman&Wortel2004). representingthesedimentarybasins(Fig.2).Thelatterarenamely, t on
Nowadays,thestillongoingAfrica–Europeconvergenceisrecorded thebackarcMitidjabasinsnearAlgiers,theTiziOuzoubasinand A
p
inAlgeria,bothinthecoastalbasinsonland(e.g.theChelifand thesmallbasinsofGuelmaandSeybouseintheEastnearAnnaba. ril 5
Mitidjabasins)andintheoffshore,withcharacteristicfold-thrust Positiveanomaliesonthecontinentaldomainarelimitedtothenorth , 2
0
structures (Boudiaf 1996 ; De´verche`re et al. 2005; Domzig et al. bynegativeanomaliesrepresentingthelimitofthemargin.Some 1
5
2006; Yelles-Chaouche et al. 2006; Kherroubi et al. 2009; Yelles positive anomalies however extend offshore, in the Kheir Eddine
etal.2009). bankinAlgiers,andtheEdoughmassifnearAnnaba.
Freeairanomalyprofileswerethendrawnalongfourwide-angle
seismic profiles of the SPIRAL survey (Tipaza, Greater Kabylia,
3 AVAILABLE DATA SETS Jijel and Annaba profiles from west to east, Fig. 2), in order to
In order to quantitatively assess the mechanical properties at the provide a first-order check on the velocity model determined by
plateboundary,severalterrestrialandmarinegravitydatasetsfrom forward inversion of wide-angle data (Aidi et al. 2013 ; Leperˆtre
the‘BureauGravime´triqueInternational(BGI,InternationalGravi- etal.2013;Mihoubietal.2014;Bouyahiaouietal.2015).
metric Bureau 2012), http://bgi.obs-mip.fr’ database were used.
Thisdatabaseconsistsofirregularlyspacedpointsissuefromsome
4 METHODS
of survey, attached to different gravity bases (Bouyahiaoui et al.
2011).Itsaccuracyisestimatedat1mGalandthemeasurements For each of the four profiles, the final velocity model computed
areattachedtothe1930Potsdamnetwork(Idres&Aifa1995). fromwide-angleseismicforwardmodellingwasconvertedintoa
InordertousetheBGIdata,itwasessentialtobringalldatasur- regular5km×1kmvelocityVgridusingalinearinterpolation
veytothesamelevelofreference(forthedatatobehomogeneous). method.Weconvertitintoadensityρ profileusingtwodifferent
Forthispurpose,weusedthenewgravitymeasurementsacquired velocity–densityconversionrulesinordertotestthesensitivityof
in the framework of missions CRAAG at places where BGI data ourmodelstothevelocity–densityconversionfactor.
existed(Abtoutetal.2014).Comparisonofolddatatonewmea- Wefirstusethreedifferentvelocity–densityconversionsforthe
surementsallowedustocomputeasystematicoffsetduetodifferent sediments,uppercrustanddeepcrustandmantle,usingHamilton
FlexuralbehaviourofthenorthAlgerianmargin 1429
(1978),Carlson&Herrick(1990)andBirch(1961)laws,suchas,
respectively:
ρ =1+1.18(V −1.5)0.22
6.0
ρ =3.61−
V
dV
B = . (1)
dρ
FortheBirchlaw,wechooseaBfactorof2.8andanintegration
constantof–1.45inordertoretrievecommonlyaccepteddensity
Figure3. Setupoftheflexuralmodels.Teistheeffectiveelasticthickness
valuesof1030and3300kgm−3forthewateranduppermostmantle,
inkm.Thedeflection(w)issettozeroattheextremitiesoftheplates,and
respectively.ThesecondconversionmodelusestheBirchlawforall imposedattheplatejunction.
crustandmantlerocks.Thevelocity–densityconversionisdifferent
forthesaltlayer,forwhichthedensityisof2200kgm−3forseismic sumoftheelasticthicknessesofthecrustandmantle.Iftheyare
velocitiesrangingbetween3.9and4.1kms−1.Thedensitymodels mechanicallydecoupled,Tewrites(Burov&Diament1995):
arethenusedtocalculateatheoreticalgravimetricanomalythatcan (cid:3) (cid:4)
Te= h3 +h3 1/3, (6)
becomparedtothemeasuredone. mant crust
Wethentestiftheprofilesareinlocalisostaticequilibriuminan whereh andh arethecrustandmantleelasticthicknesses,
crust mant
Airysense.Ifthiswasthecase,thentheweightofeachverticalcol- respectively.
umnshouldbethesameallalongtheprofile.Foreachlongitudinal A finite element formulation is used to model the lithospheric
profilenode,theweightP(x)ofthecolumnofcrustandmantleare flexureattheplateboundary(Kwon&Bang2000).Theprofilesare D
o
computedsuchas: discretizedintotwodistinctplatesforthecontinentalandoceanic wn
(cid:2)Z parts,whichallowustoapplyoppositeflexuresonbothdomains. loa
d
P(x)= gρ(x,z)h, (2) The two plates can be separated by a transition zone of variable ed
z=0 widthwherenoflexureiscomputed.Thistransitionzoneisafree fro
m
Wherehistheheightofthegridelement(constant),gthegravi- slip boundary, that is, each plate can move independently from h
tdcaoetemioppneuastlteapdcocaineloltenorgaftttihhoeenpparnroodfifiZllee)is.bRytheseluacbtoitvrmaecpwteinenisggahtaitnoananvdoeemrpaatghleie(tstao(cid:3)ktehPne(axcso)tmhare-e ttohhfeesoectiohsnemrtiioncneensh.taWoltsedhooamnvealaifnnodciu.ssTelehdseosudwresfletleulcdtcyiooonnnstatrhnaedinoetchdeeadnpuilceaptetaorcttubhreevcaaltauucsrkee ttp://gji.ox
putedcolumnweights.Theseanomaliescanbeinterpretedasdevia- oppositelytotheirpointofjunction(x=0)aresettozero,andthe ford
tionsfromthelocalisostaticequilibriumintheAirysense.Positive amountofdeflectioncomputedfromtheequivalentMohodeflection jou
deflection means thatthe lithosphereis heavier than predicted by isappliedatbothendsofeachplate(x=end),suchas(Fig.3): rna
auantpcwArauirsrdtya-fllteylexpvueerelms.roedseull,tinwghiicnhacManohboeuipnltiefrtparnedtemdafnotrleinesmtapnlcaeceamseannt wd2(0w)=0 byls.org/
=0 g
Weconvertthese‘weightanomalies’inanequivalentdeflection dx2(0) ues
oftheMoho(cid:3)hmohowith: w =(cid:3)h . (7) t on
(cid:3)wtivhheemlryoe.ho(cid:3)ρ=mhma−onh(cid:3)do iρPsc(tρhamreer−etfhoρerceu),pthpeerammoanutnlteoafnddecflreucsttiodnentshiatitessh,oreusldpe(b3ce)- Tonh(eeendim)sic(cid:5)(cid:6)safilctu(ilmnaotkehodm(egn)dib)veetnwbeyenthtehermcosmvapluutee:dflexureandthemodelled April 5, 201
substractedfromtheobservedMohotobringittoanisostatically (cid:6)(cid:7)1 (cid:2)N 5
rms= (wobs−wcalc)2, (8)
compensated(‘unflexed’)position.Wethenmodelthiscalculated N i i
deflectionusingathinelasticplatemodelandatrialanderrorap- i=0
proach.Inthismodel,theplatebendsundertheeffectofvertical whereNisthenumberofpoints,wobs istheobservedflexureand
loadsand/ormoments,whichcausetheobservedplatedeflection. wcalcistheflexurecalculatedusingthefiniteelementmethod.Inall
Here,allverticalloadsduetodensitychangeswithinthecrustare ourmodels,theeffectiveelasticthicknessisassumedconstant.The
already taken into account by the isostatic analysis, and we seek mainreasonisthattheseismicprofilesareshortcomparedtothe
modelling an ‘additional’ Moho deflection due to external plate characteristicwavelengthofflexuralresponse.Asaconsequence,
bendingforces.Theflexurewofathinelasticplatein1-Dunderan weareabletodetermineonlytheeffectiveelasticthicknessofthe
appliedloadqdependsonitsrigidityDsuchas: lithosphereclosetotheplateboundaries,whichisprobablymuch
d4w lowerthantheoverallplaterigidity(becauseoflocalplateweaken-
Ddx4 +(ρm−ρc)gw=q(x), (4) ingduetotectonicdeformationandbendingstresses).Introducing
aplatevariablerigidityonsuchshortprofileswouldratherhelpus
whereDisthelithosphererigidityconnectedtotheeffectiveelastic
fittinganoisethanatruesignalintheplateflexure.
thicknessTe:
E.Te3
D= (5)
12.(1−ν2) 5 MODELLING RESULTS
With E and v the Young’s modulus and the Poisson coefficient, Wepresentinthissectiontheresultsofgravity,isostasyandflex-
respectively.Inamechanicallycouplelithosphere,Teissimplythe ure modelling of each profile from west to east (Figs 4–7 , and
1430 L.Hamaietal.
D
o
w
n
lo
a
d
e
d
Figure4. (a)TopographyandbathymetryfortheTipazaprofile.(b)Free-airanomaly.Dottedredlinecorrespondstomeasuredgravity.Solidgreenandblue fro
m
linesarepredictedfree-airanomaliesusingdifferentparametersforthesediments,theuppercrust,andthelowercrustandmantle(Conversion1)orusingonly
h
Birch’slaw(conversion2),respectively.(c)DensitymodelcalculatedbyconvertingtheseismicvelocitiesintodensitieswithBirch’slaw(seismicvelocities ttp
afterLepreˆtreetal.2013).(d)2-DflexuralmodeloftheTipazaprofile.Greenandbluelinesarethecomputeddeflectioncorrespondingtothetwodifferent ://g
velocity–densityconversions,andblacklineisthemodelleddeflection.Thickverticallinesdelimitatethetransitionzonebetweenthetwoflexedplates. ji.o
x
fo
rd
jo
u
rn
a
ls
.o
rg
b/
y
g
u
e
s
t o
n
A
p
ril 5
, 2
0
1
5
Figure5. SamelegendasFig.4fortheGreaterKabyliaprofile.SeismicvelocitiesafterAidietal.(2013).
SupportingInformation1–4).Foreachprofile,thevelocity/density crust,andlowercrustandmantle,respectively,givesalowergravity
conversionprovidesslightlydifferentresultsdependingonthecon- anomaly(byabout10–50mGal)andaloweramplitudeofequiva-
versionmethod:thefirstmethodwhichusesHamilton(1978),Carl- lentdeflection(0.5–1km)thanthesecondmethodwhichusesthe
son&Herrick(1990)andBirch(1961)lawsforthesediments,upper Birchlawonly.Despitesomediscrepancies,thesecondconversion
FlexuralbehaviourofthenorthAlgerianmargin 1431
D
o
w
n
lo
a
d
e
d
Figure6. SamelegendasFig.4fortheJijelprofile.SeismicvelocitiesafterMihoubietal.(2014). fro
m
h
ttp
://g
ji.o
x
fo
rd
jo
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a
ls
.o
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b/
y
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s
t o
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A
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ril 5
, 2
0
1
5
Figure7. SamelegendasFig.4fortheAnnabaprofile.SeismicvelocitiesafterBouyahiaouietal.(2015).Discrepanciesbetweenbothmethodscanbeusedas
anestimateofthemeanerrorontheresultingequivalentdeflection(i.e.0.1–1km).Sensitivitytestsallowedustoestimateanuncertaintyinthemeaneffective
elasticthicknessofabout1km(Table1).
method gives generally a best fit to observed gravity data (espe- thegravitysignalbutnottheseismicvelocitymodelonthe2-Dpro-
ciallyabetterfittotheamplitudeoftheobservedgravityanomaly) file,ortodeepermantleanomaliessometimessuggested(Cavazza
thanthefirstone,suggestingthattheBirchlawprovidesasatisfy- etal.2004;Spakman&Wortel2004).Onbothprofilesindeed,the
ingvelocity–densityconversioninourcasestudy(Figs4b,5b,6b observed free-air anomaly signal is tilted counter-clockwise with
and 7b). Whereas the fit to gravity data is satisfying for Tipaza respect to the modelled ones, suggesting the presence of a long-
andAnnabaprofiles,itisnotverygoodforthetwocentralprofiles wavelength3-Dtrendinthegravitysignal.Asamatteroffact,the
(GreaterKabyliaandJijel).Thisisprobablyduetosideeffectsofthe Birchlawpredictsafree-airanomalythatisofthesameamplitude
GreaterKabyliamassiflocatedclosetothisprofile,whichaffects as the observed one (i.e. about 150 mGal), whereas the free-air
1432 L.Hamaietal.
Table1. Flexuralparametersforthe4modelledprofiles,withtheamplitudeoftheimposeddeflection
attheoceanicplateboundaryandthebest-fittingeffectiveelasticthicknessdeterminedfromrms
analysis.
Profile Tipaza GreaterKabylia Jijel Annaba
Amplitudeofdownwardoceanicflexure(km) 2±1 6.1±0.5 4.5±0.5 2±1
Best-fittingTe(km) 6±1 7±1 6±1 3±1
Rmsconversion1(km) 0.33 0.80 0.45 0.72
Rmsconversion2(km) 0.23 0.48 0.31 0.36
anomalypredictedbythefirstconversionmethodisofmuchlower 5.4 Annabaprofile
amplitude(lessthan100mGal).Forthisreason,wechosetokeep
The computed deflection along this profile shows a slight over-
thesecondconversionmethod(Birch’slawonly)forallprofiles.
compensation(downwardflexure)ontheoceanicpart,increasing
towardsthefootofthemargin,andanundercompensationonthe
continental domain (Fig. 7c). We set the parameters of the conti-
5.1 Tipazaprofile
nental plate to an elastic thickness of 10 km and a deflection of
Thecomputeddeflectionshowsaclearundercompensation(mass –2.8 km, whereas for the oceanic plate, we set the deflection at
excess)onthecontinentalpartandovercompensation(massdeficit) 2kmandtesteddifferentTevaluesbetween2and7km(Table1).
beneath the oceanic domain (Fig. 4c). Moreover, departure from Theelasticthicknesswhichgivesthebestfitbetweenobservedand
local equilibrium increases exponentially when approaching the modelled data is 3 km. The width of the transition zone in this
ocean–continent transition, both on the continental and oceanic profileis25km(betweenkm-90andkm-115).
parts.Fortheflexuralmodel,theparametersofthecontinentalplate D
o
w
are set to an elastic thickness of 10 km, because of the lack of n
constraintsonthecrustalstructure,andadeflectionof–1.2kmis 5.5 Comparisonwithactiveandpassivemarginsettings loa
d
iamtp2o.2sekdmatatthethpelaptleateende.nFdoarnthdewoececaanliccuplaltaetde,tthheedrmeflsefcotirosneivsesraelt BplyitucdoemopfatrhinegdoFwignsw4a–r7d,bietnadpipnegaorsftthheatotcheeanwicavlietlheonsgpthhearendinatmhe- ed from
elasticlayersofdifferentthicknessesbetween3and7km(Table1). Algerianbasinarelargerforthetwocentralprofiles(4–6km)than h
Tmhoedeelllaesdticdathtaiciksn6es±sw1hikcmh.gTivheestwhiedbthesotffitthbeettwraenesnitioobnsezrovneed(ain.ed. fortheeasternandwesternones(1–2km).Inordertoassesswhether ttp://g
betweenthecontinentalandoceanicplates)inthisprofileis20km theobservedlithospheredeflectionismorerepresentativeofanac- ji.o
tiveorpassivemarginsetting,wecompareourresultsontheGreater x
(betweenkm-40andkm-20). fo
Kabyliaprofile(consideredhereasdepictingacharacteristicflex- rd
uralprofile)totwootherwide-angleseismicprofileacquiredona jou
typical passive margin, in the Atlantic offshore central Morocco, rna
5.2 GreaterKabyliaprofile ls
and across the Sumatra passive margin (Fig. 8, Contrucci et al. .o
Like for the Tipaza profile, the deflection model (Fig. 5c) shows 2004;Klingelhoeferetal.2009,2010). brg/
overcompensationontheoceaniccrust;however,theisostaticsignal ExactlythesameprocessingasforAlgerianprofileswasapplied y g
is much less clear on the continental part. We set the parameters inordertoobtain‘weight’anomalieswhicharethenconvertedinto ue
s
of the continental plate to an elastic thickness of 10 km and a anequivalentMohodeflection.Thepassivemarginprofileacross t o
n
deflectionat1.2kmisimposedatthepointlocatedat150kmfrom centralMoroccodisplaysanirregularpatternofpositiveandneg- A
p
tthheeobceegainnnicinpglaotef,twheepserotfithlee,dbeufltetchtiiosnreamt6ainksmuanncdonwsetracianlecdu.laFteodr aptoivsietei,sothsetaptilcataendoemflaelcietiso,nnoiscwleealrlddeeflpiecctteidononistvhiesiSbulem.aOtrnatphreoofiple- ril 5, 2
thermsforseveralelasticlayersofdifferentthicknessesbetween 01
5
4and10km(Table1).Theelasticthicknesswhichgivesthebest
fitbetweenobservedandmodelleddatais7±1km.Thewidthof
the transition zone in this profile is 20 km (between km-130 and
km-150).
5.3 Jijelprofile
The computed deflection also shows a clear overcompensation
(downwardflexure)ontheoceanicpart,increasingtowardsthefoot
ofthemargin,likeforthetwoformerprofiles(Fig.6c).Theconti-
nentalpartdoesnotdisplayanyclearisostaticanomalybut,likefor
thepreviousprofiles,thevelocitymodelispoorlyconstrained.We
settheparametersofthecontinentalplatetoanelasticthicknessof
10kmandadeflectionof–1km(km160),whereasfortheoceanic
plate,wesetthedeflectionat4.5kmandtesteddifferentTevalues
between4and10km(Table1).Theelasticthicknesswhichgives
Figure8. Comparisonbetweenthedeflectionprofilesforatypicalpassive
thebestfitbetweenobservedandmodelleddatais8±1km.The marginoffshorecentralMorocco,anactivemarginoffshoreSumatra,and
widthofthetransitionzoneinthisprofileis15km(between165 theGreaterKabyliaprofiles.MoroccoandSumatraprofileswerecomputed
and180km). afterKlingelhoeferetal.(2009,2010).
FlexuralbehaviourofthenorthAlgerianmargin 1433
where it reaches more than 6 km at the trench. This comparison
showsthattheGreaterKabyliaprofileinAlgeria,whereplatede-
flectionisthelargest,hasanisostaticsignatureverysimilartothat
ofatypicalsubductionmargin,althoughnooceanicslabispresently
accommodatingtheAfrica–EuropeconvergenceoffAlgeria.
Figure10. Interpretativecross-sectionofthepresent-daydeformationon
6 DISCUSSION
thenorthAlgerianmargin,involvingapossibledetachmentreactivationon
ByconvertingisostaticanomaliesintoMohodepthvariationsacross topoftheformerOCT,schematicallydrawnbasedonstructureandseismic
foursegmentsofthepresent-dayAlgerianmargin,weshowthatthe interpretationoftheJijelprofile(Mihoubietal.2014).
Moho extracted from wide-angle seismic data is deeper than the
onepredictedbyalocalisostasymodelintheoceanicdomain,and continent transition (OCT) as identified on the velocity models.
shallower than it in the continental domain. These anomalies can IftheOCT,likeonclassicalmagma-poorpassivemargins,results
beinterpretedatfirstorderbyoppositeflexuresoftwoconverging from mantle exhumation along a detachment fault (e.g. Brun &
plates characterized by effective elastic thicknesses (Te) ranging Beslier 1996), then this observation suggests that the margin in-
between3and7kmfortheoceaniclithosphere.Wefindnosystem- version localises on a south-dipping structure that might well be
atictrendinTevariationsisobservedalongthemargin,althoughit theformerdetachmentalongwhichthemantlewasexhumeddur-
isworthnotingthatforthewesternmostprofile,Teistwicelower ingthepassivemarginformation(Fig.10).Whetherthisformation
thanfortheotherones(3kminsteadof6–7km).TheseTevalues was accompanied ornotbythedevelopment ofamargin-parallel
are unexpectedly low for an oceanic lithosphere (Watts & Burov STEP fault (Lepreˆtre et al. 2013; Medaouri et al. 2014 ) is diffi-
2003),evenonly20Myrold.However,ourprofilesarerathershort culttoassessfromthisstudy,asthiskindoffaultcouldalsoplay
D
(∼200km),sowecanonlyreproducetheshortwavelengthcom- theroleofamajorlithosphericweaknessduringmargininversion. o
w
ponentoftheplateflexure,closetotheplateboundary.Therefore, Althoughinaquitedifferentcontext,detachmentreactivationasa nlo
theselowTevaluesaremostlikelyunderestimated.Furthermore, majorthrustduringriftbasininversionhasalsobeendocumentedin ad
e
tchreusset,leoiwthevrabluyeflsemxuayraallsstorersesflesecotrabylotceacltowneicakoerntihnegrmofalthperoocceesasneisc: tThheePnyorretnheAeslg(Leraiganabmriaerllgeinetcaolu.l2d0t1h0u;sJbaemomneeso&ftHheuirsamreaenxsa2m0p1l2e)s. d from
indeed, the collision of the Inner Zones (AlKaPeCa blocks) with ofpassivemargininversionwhereextensionaldetachmentisreac- h
ditnhedetauAcchfermdiceaannmtmiogfaroragtpiipnnogasvtitocelacd.ai1rne9iccMtaiocantihsvai(stsyebeneeePanarrfttoh1lel)o,mwaaeprdrgobincye(asMslaawtuehrriyaclhesthlaaalbs. ttthhiveeatAoendlggeoinriinacgnoumcnopdnreetirsntsheironuntastlainmsgaarogpfirntoht(eoA-Nsuuezboedgnuedcneteieootncaeiln.ant1ei9rc7fad5coe;m,Daef´aivnveorbcuehrlieo`nrwge ttp://gji.ox
fo
2000)andhaslikelyweakenedtheoverlyinglithosphere. etal.2005). rd
The amplitude of downward flexure is slightly different from jou
oneprofiletoanother,dependingontheirpositionalongthecoast. rna
ls
Thesechangescouldbe(atleastpartly)relatedtothedifferencein 7 CONCLUSION .o
geometryofthemarginsegmentsandtheirregionalgeodynamicand Basedonaccuratevelocitymodelsandanewgridofgravitydata, brg/
kfrionmemeaatsitcteovowleustito(ne.tgh.aMtisedliakoeulyripeotlyapl.h2a0se1d4;anvdanstHriiknisnbgelyrgdeinffeetreanlt. theflexureandthemechanicalpropertiesoftheoceaniclithosphere y gue
2014). acrosstheAlgerianmargin,locatedattheboundarybetweenAfrica st o
and Eurasia plates, were modeleld. Our results show that in the n
The different models display a transition zone between A
oceanicdomainoftheAlgerianbasin,the‘weight’ofagivencol- p
cisonatpinperonxtaiml-uaptewlyar2d0ankdmocweiadneica-dnodwisnwsharifdtefldexteodtphleatseosu.tThhi(sofzocnae. uwmhennoafplpitrhooascphhinergettheendfosostyostfetmheatiNcaolrltyhtAofdreiccarneacsoenetxinpeonnteanltmiaallry- ril 5, 2
20km)withrespecttothelocationoftheocean–continenttransi- 01
gin.Conversely,inthecontinentalarea,this‘weight’increaseswhen 5
tionasdepictedbyvelocitymodels(Fig.9).
approachingthemargin,althoughthisisnotstronglyconstrained.
Hence, the plate discontinuity where plate flexure localises at
These weight changes show that the profiles (and hence, the foot
depth(i.e.closetotheMoho)locatesslightlysouthoftheocean–
of the margin) are not in equilibrium in a local sense: if we con-
vert these weight anomalies into variations of Moho depth, then
theoceanicMohobecomesincreasinglydeeper,andthecontinen-
talMohoincreasingly shallower,oneithersideofadiscontinuity
locatedatthemargintoe.Theisostaticsignatureoftheseprofiles
isthustypicalofbrokenplateswithoppositeflexures(e.g.Watts
1981,1992,2001).Theconversionofthe‘weightanomaly’intoan
equivalentflexureoftheMohoshowsthattheMohoisflexeddown-
wardintheoceanicdomainforallthefourmodelledprofiles,and
probablyupwardinthecontinentalareaatleastfortwoofthem.The
amplitudeoftheequivalentdeflectionislargerinthecentralpartof
thestudyarea(4–6km)thanontheeasternmostandwesternmost
profiles(2km).Theeffectiveelasticthicknessusedtobestmatch
Figure9. Maplocationofthetransitionzone(inred)betweenthetwoflexed
thecomputeddeflectionisalwaysextremelylow(alwayslessthan
platescomparedwiththepositionoftheOCT(ocean-continenttransition)
(ingreen)asdefinedbyseismicvelocitiesidentifiedoneachprofile(thinred 10km)andprobablyreflectstherelativelylowstrengthofthelitho-
lines).Theshapeofthetransitionzoneisinterpolatedbetweenthedifferent sphereclosetotheplateboundary.Finally,plateflexurelocalises
profilesfollowingthegeneralmargingeometry. atthesoutherntipoftheOCT,suggestingthattheformerpassive
1434 L.Hamaietal.
margin detachment is reactivated as a crustal-scale reverse fault Cloetingh,S.A.P.L.,Wortel,M.J.R.&Vlaar,N.J.,1982.Evolutionofpassive
pre-datingafuturesubductionoftheNeogeneoceaniclithosphere. continentalmarginsandinitiationofsubductionzones,Nature,297,139–
142.
Cloetingh,S.,Wortel,R.&Vlaar,N.J.,1989.Ontheinitiationofsubduction
ACKNOWLEDGEMENTS zones,Pureappl.Geophys.,129,7–25.
Contrucci,I.,Klingelho¨fer,F.,Perrot,J.,Bartolome,R.,Gutscher,M.A.,
WethanktheteamsoftheSPIRALProject,onlandandatsea,for Sahabi,M.,Malod,J.A.&Re´hault,J.P.,2004.Thecrustalstructureofthe
theirhugeworkthatallowedustocollecttheseismicdatasetusedin NW-Moroccancontinentalmarginfromwide-angleandreflectionseismic
thisstudy.WebenefitedfromtheinputbyFraukeKlingelhoeferfor data,Geophys.J.Int.,159,117–128.
theuseofseismicdataandcomparisonwithothermargins.Editor De´verche`re,J.etal.,2005.ActivethrustfaultingoffshoreBoumerde`s,Al-
Ste´phaneLabrosse,LaurentJolivetandananonymousreviewerare geria,anditsrelationstothe2003Mw6.9earthquake,Geophys.Res.
Lett.,32,doi:10.1029/2004GL021646.
thankedfortheirconstructiveandpositivereviews.
Dewey,J.F.,Helman,M.L.,Turco,E.,Hutron,D.H.W.&Knott,S.D.,1989.
KinematicsofWesternMediterranean,inAlpineTectonics,Vol.45,pp.
265–283,ed.Coward,M.P.,Detrich,D.&Park,R.G.,Geol.Soc.London,
REFERENCES
Spec.Publ.
Abtout, A., Boukerbout, H., Bouyahiaoui, B. & Gibert, D., 2014. Domzig,A.etal.,2006.SearchingfortheAfrica-EurasiaMioceneboundary
Gravimetric evidences of active faults and underground structure onshoreWesternAlgeria(Maradja’03cruise),C.R.Geosci.,338,80–91.
of the Cheliff seismogenic basin (Algeria), J. Afr. Earth Sci., Faccenna,C.,Giardini,D.,Davy,P.&Argentieri,A.,1999.Initiationofsub-
doi:10.1016/j.jafrearsci.2014.02.011. ductionatAtlantic-typemargins:insightsfromlaboratoryexperiments,
Aidi,C.etal.,2013.DeepstructuresoftheAlgeriancontinentalmargininthe J.geophys.Res.,104,2749–2766.
Greatkabylies-Insightsfromwide-angleseismicmodeling,AGU2013. Faccenna,C.,Becker,T.W.,Lucente,F.P.,Jolivet,L.&Rossetti,F.,2001.
T21A-2522.ContinentalRiftsandRiftedMargins,Posters.CONTROL History of subduction and back-arc extension in the Central Mediter-
ID:1794050. ranean,Geophys.J.Int.,145,809–820. D
o
Auzende,J.M.,Olivet,J.L.&Bonnin,J.,1972.Unestructurecompressive Faccenna,C.,Piromallo,C.,Crespo-Blanc,A.,Jolivet,L.&Rossetti,F., w
n
aunorddel’Alge´rie,DeepSeaRes.,19,149–155. 2004.LateralslabdeformationandtheoriginofthewesternMediter- lo
a
Auzende, J.M., Bonnin, J. & Olivet, J.L. 1975. La marge nord-africaine raneanarcs,Tectonics,23,TC1012,doi:10.1029/2002TC001488. de
d
Bacrorinesri,dEe´.r,e´Cehcaommomt-eRmooakrge,eNac.t&iveG,iBorudlla.nSoo,cG..G,2e´0o0l.4F.rG.,e1o7d,y4n8a6m–i4c9m5.apsof FridzeosncdheevLauacmhoetmtee,nDts.,neA´ongde`rnieeusxd,anJ.s&l’AGrcuebze´otiuc,o-Jr.Cifa.,in1:9d9i1sc.uCsisnioe´nmsautirqluees from
theMediterranean:tectonicsandkinematics(1:3.000.000scale),Com- mode`lesge´odynamiques,Bull.Soc.Ge´ol.Fr.,162,611–626. h
Birmpcahirs,tsF2i.o,,n1J.9fgo6er1ot.hpTehhyGes.evoRelleoosgc.,iitc6ya6lo,Mf2c1ao9pm9o–pf2ret2hs2es4ioW.noarllwda(vCeGsMinWro)c.ksto10kilobars, FriTMzohenedtidwteeorLrmaanmaeioanntts,eteT,peDcs.t,ooSfnatihcinset,AB1t9el,azs7ar4b,0uB–il.7dA6i.n1,g.Barnadcegneeo,dRy.n&amMicesrcoifetrh,eEw.,e2s0te0r0n. ttp://gji.ox
Boudiaf, A., 1996. Etude sismotectonique de la re´gion d’Alger et de la Frizon de Lamotte, D., Raulin, C., Mouchot, N., Wrobel-Daveau, J.C., fo
Kabylie(Alge´rie):Utilisationdesmode`lesnume´riquesdeterrain(MNT) Blanpied, C. & Ringenbach, J.C., 2011. The southernmost margin of rdjo
etdelate´le´de´tectionpourlareconnaissancedesstructurestectoniques the Tethys realm during the Mesozoic and Cenozoic: initial geom- urn
actives:contributiona`l’e´valuationdel’ale´asismique,PhDthesis,Uni- etry and timing of the inversion processes, Tectonics, 30, TC3002, als
versite´MontpellierII,274pp. doi:10.1029/2010TC002691. .org
Bouillin,J.P.,1986.LebassinMaghre´bin:uneanciennelimiteentrel’Europe Gelabert,B.,Sabat,F.&Rodriguez-Perea,A.,2002.Anewproposalforthe b/
etl’Afriquea`l’ouestdesAlpes,Bull.Soc.Ge´ol.Fr.,8,547–558. lateCenozoicgeodynamicevolutionofthewesternMediterranean,Terra y g
Bouyahiaoui,B.,Djeddi,M.,Abtout,A.,Boukerbout,H.&Akacem,N., Nova,14,93–100. ue
s
2011.Etudedelacrouˆtearche´ennedumoˆleInOuzzal(HoggarOcci- Gerya,T.,2011.Futuredirectionsinsubductionmodeling,J.Geodyn.,52, t o
dentale)parlame´thodegravime´trique:identificationdessourcesparla 344–378. n A
Boturaynashfioaromuie´,eBe.netoanld.,e2le0t1te5s.Ccorunstitnaulsetsr,uBcutullr.eSoefrtvh.eGeeaosgte.rNnaAt.l,g2e2r,ia2n5m9–a2rg7i4n. Gugeegoudeynn,aEm.,icDevooglluiotinoin,oCf.th&ewFeesrtnearnndMeze,diMte.r,ra1n9e9a8n.,TOenctothneopphoyssti-c2s5,2M98a, pril 5
and adjacent deep basin (western Mediterranean) and implications for 259–269. , 2
0
kinematicreconstructions,Geophys.J.Int.,inpress. Gurnis,M.,1992.Rapidcontinentalsubsidencefollowingtheinitiationand 15
Brun,J.-P.&Beslier,M.-O.,1996.Mantleexhumationatpassivemargins, evolutionofsubduction,Science,255,1556–1558.
Earthplanet.Sci.Lett.,142,161–173. Hamilton, E.L., 1978. Sound velocity-density relations in sea-floor sedi-
Burov,E.B.&Diament,M.,1995.Theeffectiveelasticthickness(Te)ofcon- mentsandrocks,J.acoust.Soc.Am.,63,366–377.
tinentallithosphere:whatdoesitreallymean?J.geophys.Res.,100(B3), Idres,M.&Aifa,T.,1995.Someparameterstoimproveagravitynetwork
3905–3928. accuracy:applicationtothenewreferencebasestationsnetworkofthe
Carlson,R.L.&Herrick,C.N.,1990.Densitiesandporositiesintheoceanic NorthofAlgeria,Bull.Serv.Geol.Algerie,6,79–94.
crustandtheirvariationswithdepthandage,J.geophys.Res.,95,9153– InternationalGravimetricBureau,2012.IAGGeodesist’sHandbook,2012,
9170. J.Geod.,86(10),Springer,doi:10.1007/s00190-012-0584-1.
Carminati, E., Wortel, M.J.R., Spakman, W. & Sabadini, R., 1998. The Irving,E.,1977.DriftofthemajorcontinentalblockssincetheDevonian,
roleofslabdetachmentprocessesintheopeningofthewestern–central Nature,270,304–309.
Mediterraneanbasins:somegeologicalandgeophysicalevidence,Earth Irving,E.,2004.ThecaseforPangeaB,andIntra-PangeanMegashear,in
planet.Sci.Lett.,160,651–665. TimescalesofthePaleomagneticField,GeophysicalMonographSeries
Carminati,E.,Lustrino,M.&Dogliono,C.,2012.Geodynamicevolution 145,pp.13–27,AmericanGeophysicalUnion.
ofthecentralandwesternMediterranean:tectonicsvs.igneouspetrology Jammes,S.&Huismans,R.,2012.Structuralstylesofmountainbuilding:
constraints,Tectonophysics,doi:10.1016/j.tecto.2012.01.026. controls of lithospheric rheologic stratification and extensional inheri-
Caputo, M., Panza, G.F. & Postpischl, D., 1970. Deep structure of the tance,J.geophys.Res.,117,doi:10.1029/2012JB009376.
Mediterraneanbasin,J.geophys.Res.,75,4919–4923. Jolivet, L., Augier, R., Robin, C., Suc, J.P. & Rouchy, J.M., 2006.
Cavazza,W.,Roure,F.,Spakman,W.,Stampfli,G.M.,Ziegler,P.A.&Grp, Lithospheric-scalegeodynamiccontextoftheMessiniansalinitycrisis,
T.P.W.,2004.ThetransmedAtlas:geological-geophysicalfabricofthe Sediment.Geol.,188–189,9–33.
Mediterranean region—final report of the project, Episodes, 27, 244– Jolivet, L., Faccenna, C. & Piromallo, C., 2009. From mantle to crust:
254. stretchingtheMediterranean,Earthplanet.Sci.Lett.,285,198–209.