Saghro Group in the Ougnat Massif (Morocco), an evidence for a continuous Cadomian basin along the northern West African Craton

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Tectonics, Tectonophysics

Saghro Group in the Ougnat Massif (Morocco), an evidence for a continuous Cadomian basin along the northern West African Craton

Andre´ Michard

^a,

*, Abderrahmane Soulaimani

^b

, Hassan Ouanaimi

^c

, Youssef Raddi

^d,e

, Lahsen Aı¨t Brahim

^d

, Ech-Cherki Rjimati

^e

,

Lahssen Baidder

^f

, Omar Saddiqi

^f

aUniversite´ Paris-Sud(Orsay),10,ruedesJeuˆneurs,75002Paris,France

bDepartmentofGeology,FacultyofSciencesSemlalia,CadiAyyadUniversity,P.O.Box2390,Marrakech,Morocco

cE´colenormalesupe´rieure,CadiAyyadUniversity,BPS2400,Marrakech,Morocco

dDepartment‘‘SciencesdelaTerre’’,FacultyofSciencesAgdal,MohamedVUniversity,BP1014,RabatAgdal,Morocco

eDirectiondelage´ologie,ministe`redel’E´nergieetdesMines,RabatInstitute,B.P.6208,HautAgdal,Rabat,Morocco

fGeosciencesLaboratory,FacultyofSciencesAı¨nChock,HassanIIUniversity,BP5366Maaˆrif,Casablanca,Morocco

1. Introduction

The Neoproterozoic Pan-African Belt surrounds the Paleoproterozoic West African Craton (WAC) along its northernandeasternborders,i.e.,intheAnti-Atlasandthe C.R.Geosciencexxx(2017)xxx–xxx

* Correspondingauthor.

E-mailaddress:andremichard@orange.fr(A.Michard).

ARTICLE INFO

Articlehistory:

Received11January2017

Acceptedafterrevision31January2017 Availableonlinexxx

HandledbyIsabelleManighetti

Keywords:

Pan-African Cadomian Anti-Atlas Ougarta Hoggar

ABSTRACT

TheSaghroGroup(SG)isafolded,low-gradevolcano-sedimentaryseriesupto8kmthick thatcropsoutwithinandtothenorthofthePan-Africansuturezoneinthecentraland easternAnti-Atlas.HerewedescribetheSGoftheOugnatinliersthatareexposedinthe easternmostAnti-Atlasbeneaththeunconformable,LateEdiacaranOuarzazateGroup (OZG) volcanic rocks. The Ougnat SG mostly consists of volcaniclastic greywackes accumulated in a peritidal-to-shallow basin. The basin inﬁlling was deformed by NNE-trending, mostly upright folds with axial-planar slaty cleavage and low-grade metamorphism. The deformed SG rocks were intruded by the 550 Ma Mellab hypovolcanicgranodiorite.ThelatteralsocrosscutsthelowestOZGrocksthataredated to574–571MainthewesternSaghroregion.TheSGrocksthatformtheSirouaandSaghro inliershaveanoldestageof620–610Maandwerefoldedat610–580Maattheonsetof theCadomianorogenicevents.WeshowthattheSGrocksaresimilartothe‘‘Se´rieverte’’

(SV)rocksthatareexposedintheOugartaandwesternHoggareastofthePan-African suture.WeinferthattheSGandSVrocksaccumulatedinasame,continuousbasinthat wasboundingtheWestAfricanCratontothenorthandtheeast.Thisstronglysubsiding basin formed close to a volcanic arc and was folded during the last Pan-African synmetamorphicevents.Foldorientationandageoffoldingdifferhoweveralongtheedge oftheWestAfricanCraton.TheorogenicgreywackesthatformtheremnantsoftheSG-SV basinthusconstituteapreciousrecordofthediachronicCadomianevents.l.alongthe WestAfricanCratonnorthernmargin.

C 2017Acade´miedessciences.PublishedbyElsevierMassonSAS.Allrightsreserved.

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CRAS2A-3336;No.ofPages10

ContentslistsavailableatScienceDirect

Comptes Rendus Geoscience

w ww . sc i e nce d i re ct . co m

http://dx.doi.org/10.1016/j.crte.2017.01.001

1631-0713/^C2017Acade´miedessciences.PublishedbyElsevierMassonSAS.Allrightsreserved.

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westernHoggar,respectively(Fig.1A;Caby,2003;Ganade deAraujoetal.,2014;Gasquetetal.,2008).Thegeologyof theAnti-AtlassegmentoftheNeoproterozoicBeltreveals thatnorth-dipping subductionzoneswereactive in the regionbetweenca.750Maand570Maandevolvedinto dominantstrike-slipmotionbytheend oftheir activity (Blein et al.,2014a;El Hadiet al.,2010; Thomas etal., 2004).InthewesternHoggarregion,thePan-AfricanBeltis characterizedbyseveralsubductionzonesdippingeither westoreast,andactivebetweenca.900Maand580Ma (Boschetal.,2016;Caby,2003).IntheinterveningOugarta Belt, the Neoproterozoic folded basement crops out intermittentlybeneaththeLateEdiacaran(580–540Ma) coverseries(BouimaandMezghache,2002; Cabyetal., 2010; Dostal et al., 2002; Fabre, 1976, 1978, 2005), suggestingcontinuitybetweentheAnti-Atlasandwestern Hoggarsegmentsofthebelt.TheerodedPan-Africanbelt anditsLateEdiacaran–Paleozoiccoverweresubsequently affected by Variscan and Atlasic deformations, which allowedthePan-Africanbasementtocropoutasuplifted inliers(‘‘boutonnie`res’’)intheAnti-Atlas(Fig.1B),Ougarta andwesternHoggar(Smithetal.,2006).

Inthelastdecade,theknowledgeoftheAnti-AtlasPan- AfricanBelt hasbeengreatlyimprovedthroughdetailed mappinganddating(mainlyU–PbzirconSHRIMP).Theage oftheophioliteandoceanicarcunitsthatareexposedin theSiroua–BouAzzersuturezone(Fig.1B)hasbeenfound tobecloseto760–700Ma,andtheiremplacementdatedat ca.660–640Ma(Bleinetal.,2014b;ElHadietal.,2010;

Inglis etal.,2005; Soulaimaniet al.,2013;Triantafyllou etal.,2016;Walshetal.,2012).Likewise,thelow-grade platformunitsintheforelandofthePan-Africansouthof thesuturezone(quartzitesandcarbonatesoftheTaghdout Group), which were previously ascribed to the Tonian, havebeeneventuallyshowntobeLatePaleoproterozoic (basedonacross-cuttingmaﬁcsilldatedc.1710Ma[U–Pb baddeleyite;Ikenneetal.,2016]and163934Ma[U–Pb SHRIMP;AitLahnaetal.,2016]).Incontrast,theyoungest foldedseriesofthePan-Africaninliers,namelytheSaghro, Bou Salda and Tiddiline groups (Thomas et al., 2004) havebeenpoorlystudied.Theiragehasbeenestimatedat ca. 620–610Ma (Abatietal., 2010;Hefferanetal., 2012;

Lie´geoisetal.,2006,inGasquetetal.,2008),yetonfewdata.

However,theSaghroGroup(SG)isaca.8-km-thickseries thatextendsover300kmoflengthfromtheSirouatothe SaghroandOugnatmassifsintheeasternAnti-Atlas,andthe availableagesaretoofewtoproperlycharacterizeitsage.

The present paper focuses on the SG inliers of the Ougnat Massif, which we analyze through detailed mapping,andﬁeld observationsandmeasurements.We describe the lithostratigraphy and the structure of the Ougnatinliers,thencomparethemwiththeclassicalSG inliersintheSaghroMassif.Wethencomparetheentire setofSGrockstothe‘‘Se´rieverte’’(SV)formationsthat crop outfromsoutheastern OugartatoWesternHoggar.

The ca. 6000-m-thick, volcaniclastic SV series was deﬁned by Caby (1970-1983), mentioned by Fabre (1976,p.24),andlabeled‘‘PharusianII’’byBertrandand Fig.1.A.StructuralcontextofthenorthernWAC.PASZ:Pan-Africansuturezone;SAF:SouthAtlasfault.(1):thelowestlevelsofthesouthernReguibat ShieldcoverareMeso-toEarlyNeoproterozoicinage(Rooneyetal.,2010).B.MapofthecentralandeasternAnti-AtlasProterozoicinliers(afterGasquet etal.,2008,modiﬁed).C.SketchgeologicalmapoftheOugnatmassif(afterDestombesandHollard,1988).Theasteriskmarksthedualunconformityontop oftheSG.

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Caby(1978).RecentdescriptionscanbefoundinBouima andMezghache(2002),Dostaletal.(2002)andCabyetal.

(2010).OurcomparisonoftheSGandSVseriessuggests thatbothweredeformedduringtwodiachronic,structur- allydistinctlatePan-African(Cadomians.l.)eventsalong thenorthernandeasternmarginsoftheWAC.

2. Geologicalsetting

The Saghro and Ougnat massifs constitute isolated exposures of a 30-km-large, ENE-trending Proterozoic basement antiformwhoseaxisis partiallyburied under folded Paleozoic deposits between Tineghir and Alnif (Fig.1B).FoldingofthePaleozoicseriesresultsfromthe Variscancompressionassociatedwiththe‘‘MesetaOroge- ny’’ (e.g., Baidder et al., 2016; Michard et al., 2010;

Soulaimanietal.,2014),whereastheantiformalstructure derives from the subsequent Atlas Orogeny (Frizon de Lamotteetal.,2008,withreferencestherein).TheSaghro andOugnatmassifsextendnorthofthePan-Africansuture zone, which is marked by the Siroua and Bou Azzer ophiolites and oceanic arc units (Blein et al., 2014b;

Hefferanetal.,2000;Leblanc,1981;Saquaqueetal.,1989;

Soulaimani et al., 2006). The ‘‘Anti-Atlas Major Fault’’

(AAMF; Choubert, 1947, 1963; Leblanc, 1975) follows approximatelythefrontoftheobductedunits.TheAnti- Atlasinlierssouth oftheAAMFbelongtothedeformed margin of the WAC (Ennih et al., 2001; Gasquet et al., 2008),whereastheSaghro-OugnatunitsbelongtothePan- African orogenic system, likely thrust over the north- dipping margin of theWAC (Ennih and Lie´geois,2001, 2003, 2008; Gasquet et al., 2008). The Saghro-Ougnat terrainsarecomposedofsmallSGinliersunconformably overlain by sub-horizontal volcanic (dominantly felsic) andclasticformationsthatbelongtotheca.570–540Ma, Late Ediacaran Ouarzazate Group (OZG; Gasquet et al., 2005, 2008; Thomas et al., 2004; Walsh et al., 2012).

Numerousgranodioritehypovolcanicplutonsandfelsicor maﬁcdykesintrudetheSGterrainsandthelowerpartof theOZG(Baidadaetal.,2016;Cheilletzetal.,2002;Errami etal.,2009;Youbietal.,2013).

TheOugnatMassifincludestwoSGinliers,‘‘Mellab’’to thenorthand‘‘Ighrane’’tothesouth,separatedbythe‘‘Tizi n’Izemstrip’’ofCambrianandOrdovicianstrata(Fig.1C).

TheseinliersexposeoutcropsoftheSGformationsandof the Mellabhypovolcanic granodioritedated to54726 (Rb–Sr;Mrinietal.,1992),whichintrudestheSGseries.Both inliers are unconformablyoverlainbyLateEdiacaranOZG rocksorbyLower–MiddleCambriansandstones,themselves followed upward by Ordovician to Devonian formations (DestombesandHollard,1988).TheOZGbeginswithcoarse breccias,thenincludestwoseriesofvolcanicrocksseparated by a discontinuous conglomeratic formation (Abia et al., 2003;Paile,1983;Raddi,2014;Raddietal.,2006,2012).The majorunconformityatthebottomoftheOZGrevealsthatthe metamorphicandfoldedSGterrainswereexhumedatthat time(ca.570Ma)uptothesurface.Thisexhumationlikely occurredinacontextofextensionalfaultingassociatedwith post-orogenic collapse and/or incipientcontinental rifting (Gasquetet al., 2008; Soulaimani etal., 2014). The more moderate unconformity that exists at the bottom of the

shallowwaterCambriandepositslikelyrevealsfurtherstages intheriftingevolutionfurthertothewest,whichresultedin theopeningof theIapetus, andthen ofthe Rheicoceans (Nanceetal.,2012).

3. Methods

Thisworkisbasedon detailedgeologicalmappingof theBouadil and southern Goulmimaareas at a scaleof 1:50,000 (Raddi et al., 2006, 2012), combined with structuralanalysisinthefield.Weadditionallyperformed remotesatelliteimageanalysistofurtherinvestigatethe geologyoftherocksandtherelationsbetweenthevarious tectonic structures that cut the surface. The thermal conditions that prevailed during the deformation of the SG rocksare defined through the recognition of specific structural features at the outcrop scale (fold geometry, axial-planecleavage)and attheopticalmicroscope scale (microstructuresandmineralassociationsinthinsections).

4. Results

4.1. Lithostratigraphy

We built a stratigraphic column across the Ighrane inlierbetweenTizin’IzemandIghrane(Fig.2).TheMellab inlierexposesmainlytheupperpartofthiscolumn.From the base of the stratigraphy upwards, the SG series containsrhythmicallyalternatingcentimeter-todecime- ter-thick ﬁne-grained sandstones, arkoses, greywackes, andquartzwackesthatareinterbeddedwithblackshales.

Thepsammiticunitsdominatethelowersection,andﬁner grainedpeliticrocksbecomemorecommonintheupper section. Pelitic rocks are interbedded with arkose and quartzwackebedsupto30–40cminthickness(Fig.3A).

The arkose and quartzwacke beds display hummocky cross-stratiﬁcation(HCS)structures(Fig.3B),whereasthe ﬁne-grainedsandstonescontainplanarcross-bedding.In places,intraformationalslumpfoldsareobserved(Fig.3C) locallyassociatedwithsandydykesinsiltybeds(Fig.3D).

Altogether, these attest to a rapid deposition of wet turbiditicsediments.NearIghrane,ameter-thickamphib- olite bed likely derived from some maﬁc volcanic or pyroclastic bed is intercalatedwithin thequartz-biotite hornfelsderivedfromtheSGsedimentsnexttotheMellab granodioritecontact.

4.2. Petrography

Under the microscope, thecoarse lithologies display lithoclastsofquartz,quartz+plagioclase,andquartz+K- feldsparfromgraniticand/orvolcanic,felsictoandesitic sources.Intheﬁne-grainedlithologies,aslatycleavageS1 (tectonicfoliation)canbeobserved;it is obliquetothe bedding and cut by tension gashes that are ﬁlled with quartz and calcite. This suggests that pressure solution occurredduringfolding(Fig.SM1)undertemperaturesin therangeof250–2808C,whichistheboundarybetween anchizone and lowermost greenschist-facies metamorphism(Verdeletal.,2011;Wood,1974).However,fresh

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andpoorlyorientedbiotitelamellaearealsopresentinthe outcropsadjacenttotheMellabvillage(Fig.SM1C).Since exposuresoftheMellabgranodioriteexist2kmwestand 2.5kmnortheastoftheMellabvillage,thesebiotitesmight attesttothethermalimprintoftheMellabintrusion.Inthe proximalaureoleofthegranodioriteintrusivebody,theSG lithologiesaretransformedintobiotite-richschists,whose

foliation is affected by high-temperature folds with polygonalarcsofundeformedmicas(Fig.SM1D).

4.3. Structure

The structures observed at the outcrop scale are of threetypes,whichsuggestthattheyhaverecordedthree Fig.2.StratigraphiccolumnoftheOugnatMassif.Doublearrows:potential‘‘de´collement’’level.

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superimposed deformational events. The ﬁrst event is recorded by the synsedimentary intraformational folds anddisharmonicfoldingdescribedabove(Fig.3C,D).This event wasmost likely a gravity sliding of incompletely consolidated sedimentsalong abasinslope. We labeled thisearlyevent‘‘D0’’.

The second deformation event is evidenced by the widespread axial-planar foliation ‘‘S1’’ (Figs. 3A and 4;

Figs.SM2,SM3).Welabeledit‘‘D1’’.Thetectonicfoliation isaslatycleavageobliquetothebedding,developedinthe pelites and siltites and associated with the low-grade greenschist-faciesrecrystallizationsdescribedabove.The D1 foldsare either observed or inferred fromthe S0/S1

relationships.Theyaretypicallyuprightorvergingtothe NW(Fig.SM2A)orSE(Fig.SM2B).Theiraxialplanestrend dominantly NNE–SSW next to Mellab but they rotate progressively toNE–SWin thesoutheastern partofthe Ighraneinlier. Fold axes plunge byless than 308tothe northeastortothesouthwest.

IntheIghraneinlierabout2kmsouthofTizin’Izem,a NE-striking,morethan15-km-longfaultthatsplaysinthe south separates two units of folded SG formations (Fig.SM4).This‘‘F0–1’’fault(Fig.4)isconﬁnedintheSG formationsandmighthaveatwofoldoriginlinkedtothe D0andD1events.

Finally,inthecontactaureoleoftheMellabgranodio- ritenorthof Ighrane(Fig.SM5), a late synmetamorphic compressionalevent‘‘D2’’isrecognized.Thiseventcaused thetiltingoftheearlierfoldsandtherefoldingoftheearly metamorphicfoliationunderhornfelsfaciesconditions.

TheoriginalgeometryoftheSGfoldbelthasbeenonly slightlymodiﬁedby theVariscanand thepost-Variscan events, as recorded by the deformation of the Late Ediacaran–Paleozoicseries(i.e.formationofaNE-trending antiform, then activation of the Akerouz normal faults system; Fig. 4). We restored the pre-Late Ediacaran–

Cambriangeometryof thepresent-day cross-section by tiltingtheCambrian stratadown tothehorizontal. The resulting ﬁgure (Fig. SM3C) suggests that the belt was formed by basically upright folds locally affected by longitudinalshearzones(e.g.,F0–1).

5. Discussion

5.1. TheOugnatSGbasin,partoftheAnti-AtlasSGbasin

TheSGsedimentarysequenceintheOugnatMassifis equivalenttopartsoftheSGsequencesdescribedfurther tothewestintheSaghroandtheSiroua(Sirwa)massifs (SMTable 1, left side). Particularly significant are the Fig.3. TypicalSGoutcropsnexttoMellab(A–C)andTizin’Izem(D).Boldarrows:sedimentationpolarity;S0:bedding;S1:slatycleavage.A.Alternatingfine- grainedgradedgreywackes,siltsandblackshales.B.ThickarkosicbedshowingHCSstructureinterleavedinasequenceoffine-grainedbeds.C.Isoclinal synsedimentaryslumpfoldintheverticalizedsequence(A–C).D.Disharmonicsynsedimentaryfoldingassociatedwithbedding-parallel‘‘dećollement’’and openingoffracturefilledwithsand(injectite),suggestingincipientdetachmentofunconsolidatedsedimentsalongthebasinslope.

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dominantgreywackefaciesthatrevealmixedvolcanicand terrigenousinputs; thefrequency of turbidities, debris- flows and slumps, which altogether suggest sediment slidingalongsteepslopestowarddeep-watertroughs;the occurrenceofinterbeddedvolcanicflows(mostlybasalts, but alsoandesites and rhyolites in Siroua)towards the bottom of some of the SG inliers withcalc-alkaline to subalcaline characters and affinities with initial rift tholeiites, continental tholeiites or oceanic island alkali basalts(Fekkaketal.,2000,2003;Thomasetal.,2002).The upper, arkosic part of the Siroua SG rocks suggests a decrease in water depth, likely related to the onset of convergence(Thomasetal.,2004).IntheOugnatMassif, the graded beds are scarce, no flute casts have been observed, and HCS structures are observed, altogether suggestingshallowtoperitidalconditionsofsedimenta- tionduringthelaststagesofthebasinevolution.

TheSGbasecouldbeobservednowhere.However,in theSidiFlah inlier,small serpentinitebodies associated withjaspers andcarbonatesare tectonicallyincludedin theSG(Fekkaketal.,2003),whichsuggeststheproximity of an ophiolite-bearing basement. Ultramaﬁc xenoliths havebeendescribedinthegraniteintrusionsofBouskour andBoumalnethatcrosscuttheSidi FlahandBoumalne inliers, respectively (Fig. 1B; Fekkak et al., 2003, with reference therein). In the western Saghro, ultramaﬁc bouldersareincludedlocallyinthebasalbreccia ofthe OZG (Walsh et al., 2012). Altogether, this favors the hypothesisofapartlydeep-waterbasinformedabovea thinned crust likely at the ocean-continent transition (OCT)(Fekkaketal.,2003).

Lie´geoisetal.(2006)haveestimatedtheageoftheSG sedimentationthroughU–Pbdatingofdetritalzirconsin theKelaatMgounainlieroftheSaghromassif.Theyfounda mixingofPaleoproterozoicandNeoproterozoicages,with agesoftheyoungestgrainsclusteringaround620–610Ma.

Theseyoungeragesconstraintheageofthelateststagesof deposition.ThisisconsistentwiththeTDMmodelageinthe 640–580Matime-spanforpillowbasaltﬂowsinterleaved intheSaghroSG(Erramietal.,2009).Thesourceareasof thegreywackes depositswouldbelocatedinthePaleo- proterozoicbasementofthenorthernWACor/andinthe thennewlyformedPan-Africanbeltwheresupra-subduc- tion,pre-collisionalgranodioriteswouldhaveformedfrom 750Mato614Ma(Hefferanetal.,2000,2002;Thomas et al., 2002). Similar results were obtained on detrital

zircons from the SG rocks of the Siroua massif: the youngestzirconpopulationsclusteraround620–610Ma, whereas the proportion of Eburnian (2 Ga) zircons increases upward (Abati et al., 2010). Based on these results,wesuggestthatthemostprobableageoftheSG series in the Ougnat massif is 620–610 Ma (Early Ediacaran).

5.2. AttributingtheSGtectono-metamorphicevolutiontothe latePan-African,Cadomianevent

IntheOugnatmassif,theSGsynmetamorphicfolding cannotbeaccuratelydatedduetothelackofrobustU–Pb zirconagesfortheMellabgranodioriteandthelowermost overlyingOZGvolcanics.However,thegeochemistryand hypovolcaniccharactersoftheMellabpluton(Raddi,2014;

Raddietal.,2006,2012)suggestthatitbelongstothecalc- alkalinetohigh-Kgranodioriticsuiteassociatedwiththe OZGvolcanics,andwidelyexposedintheSaghromassif (Erramietal.,2009;Gasquetetal.,2005,2008;Schiavo etal.,2007;Walshetal.,2012).TheMellabgranodiorite actuallycrosscutsatsomeplacesthelowervolcanicsofthe OugnatOZG(TamerzagaandBouNagaformations),andit is unconformably overlain by the Aouja n’Aissa–Iferda ignimbritesoftheupperOZG(Raddi,2014;Raddietal., 2006,2012).Theignimbriticserieshavebeendated(U–Pb SIMS zircon ages) at 5525 Ma in the Ougnat massif (Gasquetetal.,2005),andat550–543MaintheImiterinlier ofeasternSaghro(Fig.1B;Gasquetetal.,2005;Levresseetal., 2004).InthewesternSaghro,theequivalentignimbriteshave yieldedsimilar558–556Maages,whereasothervolcanicsof thelowerpartoftheOZGformationsyielded574–571Ma ages(Walshetal.,2012).

Thelatterageswouldconstitutetheupperlimitforthe ageofSGfoldingintheSaghro-Ougnatarea.However,this estimatemustbecorrectedtakingintoaccountthetime neededtoexhumetheSGlow-grademetamorphicunitsup tothesurface.Basedontheirformationattheanchizone–

epizone transitionclose to250–2808C(sect. 4.2), and supposingastandardgeotherm(308C/km),weinferthat theSGfoldshavebeenexhumedfromadepthof8–9km.

Thisexhumationbeganinanorogenic,collisionalcontext andwentonduringtheLateEdiacaranmagmaticactivity andthecoevalextensionintheAnti-Atlasdomain(Blein etal.,2014a;Gasquetetal.,2008).Supposingaveryfast exhumation rateof1mm/yr(ascurrently observedin Fig.4.Schematiccross-sectionoftheOugnatMassif.F0–1:Precambrianfault(D0and/orD1deformationepisodes).SeeSupplementaryMaterialforlocation andinterpretation.

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someorogenicoractiveriftcontexts;e.g.,Baueretal.,2010;

Glotzbachetal.,2008),weinferthattheSGexhumation might have lasted about 8–9 Myr. Considering a more common, although fast exhumation rateof 0.3mm/yr would suggest an exhumation duration of more than 25Myr.Thereforeaconservativeestimateoftheageofthe foldingoftheSGformationsmaybe610–580Ma.

5.3. FromtheAnti-AtlasSGtotheOugarta–WesternHoggar SV:anewproposalandimplications

The 610–580Ma SG fold belt records the latest compressional and/or transpressional, synmetamorphic eventofthePan-AfricanorogeniccycleintheAnti-Atlas.

This event postdates the major Pan-African events

Fig.5. A.FoldtrendsintheSGformationsoftheSaghroandOugnatmassifsshownbyschematicstereograms(lowerhemisphere)ofthedominantstrike anddipoftheaxial-planarslatycleavage.1:Fekkaketal.,2001.2:Fekkaketal.,2003.3:Fekkaketal.,2002.4:Ouguiretal.,1996.5:Raddietal.,2012.6:

Raddi,2014.B.CorrelationswiththesouthernOugartainlier(SebkhaelMellah),AdrarandBledElMasmassifsandOuallen(Ahnet)northwesternHoggar area.7:BouimaandMezghache,2002;Dostaletal.,2002;Cabyetal.,2010.8:Haddoum,2009;Cabyetal.,2010.9:Caby,1970-1983,2003;Haddoum, 2009.GeologicalbackgroundaftertheGeologicalmapofAfrica,scale1:10million,CommissionfortheGeologicalMapoftheWorld(2016),andBoschetal.

(2016)fortheHoggarmassif.IT/PhT/TT:Iskel/Pharusian/TassendjanetTerrane.SVoutcropsfromOugartatowesternHoggarafterCaby(personalcomm., 2016).Paleo-toMesoproterozoicageoftheearliestpost-EburniansedimentsintheWACdomainafterRooneyetal.(2010),Ikenneetal.(2016)andAit Lahnaetal.(2016).

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(760–700Ma‘‘Iriri–Tichibabineorogeny’’and670–640Ma

‘‘BouAzzerorogeny’’inthesenseofHefferanetal.,2014), and is supposed to mark the onset of the Cadomian Orogeny,whichlastedfromca.580to550Ma(Hefferan et al., 2014, with references therein; Linnemann et al., 2014).Thepost-SG,pre-OZGopenfoldsthataffecttheBou Saldaand TiddilineformationsareincludedbyHefferan etal.(2014)intheendoftheCadomiancycle.

IntheAnti-Atlas,thetrendoftheSGfoldsisdominantly SW–NE(Fig.5A).Thefoldsappearorientedene´chelonwith respect to the east–west-trending Pan-African suture zonedeﬁnedinthecentralAnti-Atlas(Figs.1B,5B).This architecture suggests that the collision of Avalonia–

Cadomia against the WAC (Ennih and Lie´geois, 2003;

Gasquetetal.,2008;Hefferanetal.,2000)hasbeenoblique withrespecttothemarginoftheWACintheAnti-Atlas, withasoutheastward directionof convergence(Fig.5B, upperpart).Thistranspressionalregimehasactuallybeen evidencedintheIknioungranodioritethatintrudestheSG seriesoftheBoumalneinlier(Fig.1B;ErramiandOlivier, 2012).

TheorientationofthePan-Africansuturezonerotates fromabout east-westin the Anti-Atlas toabout north- southintheAhnet-westernHoggar(Fig.5B;Boschetal., 2016;Caby,2003;Leblanc,1981).Weexaminehereforthe ﬁrst time the southeastward continuation of the SG formationsintotheformationstermed‘‘Se´rieverte’’(SV;

SMTable1),whichextenduptothewesternHoggar.Inthe OugartaBelt,thecoresoftheVariscanfaultedanticlines generally exposemolassic-volcanicseries termed‘‘Se´rie pourpre´e’’equivalenttotheOZG(BouimaandMezghache, 2002;Caby,1970-1983;Cabyetal.,2010;Donzeau,1972;

Fabre, 2005). However, in thesouthern partof thebelt (SebkhaElMellahanticline),andfurthertothesoutheast intheAdrarandBledElMassoutcrops,low-gradefolded terrainssimilartothenorthwesternHoggarSVhavebeen recognized beneath the Cambrian conglomerates and sandstones(Bouima and Mezghache, 2002; Caby et al., 2010;Dostal etal., 2002). The SV(Caby, 1970-1983)or

‘‘PharusianII’’(BertrandandCaby,1978)ofnorthwestern Hoggar (Ouallen in western Ahnet; Fig. 5B) is a thick (>6000m)ﬂysch-likesuccessionofmainlyvolcaniclastic greywackesand conglomerateswithabundantandesitic clasts,depositednexttoanislandarcoranactivemargin (Boschetal.,2016;Caby,2003).Volcanicﬂows(basalts, andesites,rhyolites)areinterbeddedwiththegreywackes.

ThesimilarityoftheOugarta–AhnetSVwiththeAnti-Atlas SGalsoholdsontheirstyleoffolding(uprightfoldswith incipient axial-plane cleavage) and on their associated low-gradegreenschistmetamorphism.FurthertotheSSE intheTassendjanetTerrane,theSVis involvedinwest- vergingnappes,anditsmetamorphicgradeincreasesupto theamphibolite-faciesand anatexis(Boschet al.,2016;

Caby,2003).Inthisregion,theageofthethermalpicof regionalmetamorphism isconstrained byamonazitein situ ²⁰⁷Pb/²⁰⁸Pb ageat 60311Ma (Boschet al., 2002).

However, the peak in pressure conditions is a bit older, 6233Maor610–615Ma(Ar/Aronphengite;Bergeretal., 2014;CabyandMonie´,2003;Jahnetal.,2001).Thezirconage ofultrahigh-pressuremetamorphisminthesuturefurtherto thesouthis608–611Ma(GanadedeAraujoetal.,2014).

Thus,wemayhypothesizethattheSGbasinoftheAnti- Atlas and the SVbasin of the Ougarta–Western Hoggar formedoriginallyaunique,upto6000–8000msubsiding basin that was surrounding the WAC by the end of the Neoproterozoic. The basin was deep in most parts, but includingalsoafewshallowwaterdomains.This‘‘SG–SV Basin’’wasextendingnexttoavolcanicarcthatsourceda huge quantity of orogenic greywackes into the basin.

Discussingthenatureandthelocationofthearcisbeyond thescopeofthiswork.SubsidenceandclosureoftheSG–SV Basin occurred during the last stagesof the Pan-African orogeniccycle,butwithsomealong-strikedifferencesboth inthetimingandthedeformationmode.IntheAnti-Atlas area, the basin subsidedfrom 610–620 Mato 580 Ma andclosedduringtheCadomianOrogeny(sensuHefferan et al.,2014).In theSaharan regions,the onsetoftheSV sedimentationhasnotbeendatedyet,butpostdates680Ma (Cabyetal.,2010),whereasthebasinclosedbefore620–610 Ma. Thebasinevolution thusshowedsome diachronism alongstrike.Thefoldbeltsthatsubsequentlydeformedthe SG–SVseriesalsoshowdifferencesalongthebasinstrike:

thefoldstrendnortheasttoENEintheAnti-Atlas,andNNW tonorth-southintheSaharanregions(Fig.5).Thissuggests a NW–SE convergenceof theAvalonia–Cadomia terranes withrespecttotheWACintheAnti-Atlas,contrastingwith anENE–WSWconvergenceoftheeasternHoggarterranes (LATEAterranes;Lie´geoisetal.,2003)towardtheeastern boundaryoftheWAC.

6. Conclusion

AttheeasterntipoftheAnti-Atlas,theOugnatMassif showslargeoutcropsofthefolded,low-gradevolcaniclastic Saghro Group (SG), beneath the unconformable, Late Ediacaran Ouarzazate Group (OZG) volcanic formations and the Lower Cambrian sandstones. The Ougnat SG correlates with the central part of the 8-km-thick SG volcaniclasticdepositsrecognizedfurtherwestintheSaghro andtheSirouamassifs,wheretheSGorogenicgreywackes havebeendatedat620–610Ma(EarlyEdiacaran).These rocks are associated with the dismembered ophiolite remnants of the Pan-Africansuture zone(Siroua) orare locatednorthoftheophioliticoutcrops(Saghro,Ougnat).

The3-to4-km-thickSGgreywackeseriesoftheOugnat inliers has been deformed by NNE–trending, mostly upright folds with axial-planar slaty cleavage attesting tolow-grademetamorphicconditions.Thefold beltwas exhumeduptothesurfacebeforetheemplacementofthe LateEdiacaranvolcanicsthataredated574–571Mainthe westernSaghro.HencefoldingoftheSGdepositsoccurred at610–580MaattheonsetoftheCadomianevents.The Ougnat SG rocks are intruded by the 550Ma Mellab hypovolcanicgranodioritethatalsocrosscutsthelowest OZGvolcanics.

The base of the SG is nowhere exposed, but the occurrenceofultramaﬁcshasbeendocumentedlocallyin western Saghro, suggesting a possible ocean–continent transitiondomain,nexttoanoceanicoractivemarginarc duringthelaststagesofthePan-Africansutureevolution.

ThegeodynamicsigniﬁcanceoftheSGfoldbeltisbetter understoodthroughthecorrelationsweproposewiththe

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‘‘Se´rieverte’’(SV)rocksthatcropouttothesoutheastofthe OugnatMassifintheOugartaandwesternHoggarregions.

BoththeSGandSVrocksaccumulatednexttoavolcanic arcinastronglysubsidingbasinandwerefoldedduring thelastPan-Africansynmetamorphicevents.Foldorien- tationandageoffoldingaredifferalongthebasinstrike.

TheorogenicgreywackesoftheSG-SVBasinconstitutea relevant record of the diachronic Cadomian events s.l.

along theWACnorthwesternmarginand shouldbethe aimofnewgeologicalresearch.

Acknowledgements

Logisticsupportfor ﬁeld studieswasgranted bythe Ministry ofEnergyand Mines, Waterand Environment, Rabat.WewarmlythankRenaudCabyandKevinHefferan fortheirthoroughandconstructivereviewsthatallowed ustogreatlyimproveourmanuscript.Anotherreportby Jean-Paul Lie´geoishelped us toprecisesomeimportant pointsconcerningthepositionofthePan-Africansuture andthelikelyextensionofthedeformed(meta-)cratonic borderoftheWAC.Thecarefulannotationofthesubmitted manuscript by Isabelle Manighetti and the editorial advicesbyHe´le`nePaquetaresincerelyacknowledged.

AppendixA. Supplementarydata

Supplementarydataassociatedwiththisarticlecanbe found,intheonlineversion,athttp://dx.doi.org/10.1016/j.

crte.2017.01.001.

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