Emplacement mechanisms of a dyke swarm across the brittle-ductile transition and the geodynamic implications for magma-rich margins

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Emplacement mechanisms of a dyke swarm across the

brittle-ductile transition and the geodynamic

implications for magma-rich margins

Hans Jørgen Kjøll, Galland Olivier, Loic Labrousse, Torgeir B. Andersen

To cite this version:

Hans Jørgen Kjøll, Galland Olivier, Loic Labrousse, Torgeir B. Andersen.

Emplacement

mech-anisms of a dyke swarm across the brittle-ductile transition and the geodynamic implications

for magma-rich margins.

Earth and Planetary Science Letters, Elsevier, 2019, 518, pp.223-235.

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Earth and Planetary Science Letters 518 (2019) 223–235

Contents lists available atScienceDirect

Earth

and

Planetary

Science

Letters

www.elsevier.com/locate/epsl

Emplacement

mechanisms

of

a

dyke

swarm

across

the

brittle-ductile

transition

and

the

geodynamic

implications

for

magma-rich

margins

Hans

Jørgen Kjøll

a

,

∗

, Olivier Galland

b

,

Loic Labrousse

c

,

Torgeir

B. Andersen

a

a_The_Centre_for_Earth_Evolution_and_Dynamics_(CEED),_University_of_Oslo,_PO_Box₁₀₄₇_Blindern,_NO-0316_Oslo,_Norway b_Physics_of_Geological_Processes,_the_NJORD_Centre,_Department_of_Geosciences,_University_of_Oslo,_Norway

c_Sorbonne_Université,_CNRS-INSU,_Institut_des_Sciences_de_la_Terre_Paris,_ISTeP_UMR_7193,_F-75005_Paris,_France

a

r

t

i

c

l

e

i

n

f

o

a

b

s

t

r

a

c

t

Articlehistory:

Received18January2019

Receivedinrevisedform7April2019 Accepted9April2019

Availableonline22May2019 Editor: T.A.Mather

Keywords:

dykeemplacementmechanisms brittle-ductiletransition magma-richriftedmargin structuralreconstruction

IgneousdykesarethemainmagmatransportpathwaysthroughtheEarth’scrust,andtheyareconsidered tocontributetotectonicextensioninvolcanicrifts.Dykesaretypicallyconsideredtoresultfrombrittle fracturing,even in the ductilecrust. A common assumptionis thatdyke orientation is controlledby tectonicstresses, such thatdykesin riftsare expected tobe verticaland perpendicular toextension. Here we report on detailed field observations of a spectacularly well-exposed dyke swarmto show thatdykeswerenotsystematicallyemplacedbypurelybrittleprocessesandthatdykeorientationmay differfromthedominanttectonicstressorientations.Thedykecomplexformednearthebrittle-ductile transition duringopening ofthe IapetusOceanand is nowexposedin theScandinavian Caledonides. Distinctdykemorphologiesrelatedtodifferentemplacementmechanismshasbeenrecognized:1)Brittle dykesthat exhibitstraightcontacts withthe host rock,sharptips, en-echelonsegments withbridges exhibiting angularfragments; 2) Brittle-ductile dykesthat exhibitundulating contacts, rounded tips, ductile foldingin thehost rock and contemporaneous brittleand ductile features;3) Ductile “dykes” thatexhibitroundedshapesand minglingbetweenthe softductile hostrock andtheintrudingmafic magma.Thebrittledykesexhibittwodistinctorientationsseparatedbyc.30◦thataremutually cross-cutting,suggestingthatthedykeswamdidnotconsistofonlyverticalsheetsperpendiculartoregional extension,as expectedinrifts.We wereabletouse thewell-exposedhost rock layersas markersto performakinematicrestorationtoquantifytheaveragestrainaccommodatingtheemplacementofthe dykecomplex:itaccommodatedfor>100%extension,butcounter-intuitivelyitalsoaccommodatedfor 27% crustal thickening. We infer that the magmainflux rate was higherthan the tectonicstretching rate, implying that magma was emplaced forcefully, as supported by field observations.Finally, our observationssuggestthatthefastemplacementofthedykeswarmtriggeredarapidshallowingofthe brittle-ductiletransition,andleadtoaconsiderableweakeningofthecrust.Theinterpretationspresented herecould potentiallyhavelargeimplications forsurfacetopographyandseismicityinactiveriftsand volcanicareasaroundtheworld.

1. Introduction

Dykes,andigneoussheet intrusionsingeneral,are fundamen-talmagmapathwaysthroughtheEarth’scrust.Theiremplacement is controlled by a complex set of factors, such ascrustal stress, crustal heterogeneities,topographic loading andmagma viscosity (e.g.Spacapanetal.,2017; Kavanagh,2018; HallsandFahrig,1987; Magee et al., 2016). Most models consider dykes as hydrofrac-tures propagating as tensile cracks (mode I) in a brittle fashion withopeningperpendicular totheleastprincipal stress,implying

*

Correspondingauthor.

E-mailaddress:h.j.kjoll@geo.uio.no(H.J. Kjøll).

that dykes emplaced in rifts are expected to be vertical and ac-commodatecrustalextension (Keiretal.,2011).Becausetherates for dyke emplacement are much higher than the tectonic strain rates,dyke emplacement is assumedtobe a brittleprocess even in theductile crust (e.g. Rivalta etal., 2015; Sassieret al., 2009; White etal., 2011). The low seismicity in thelower crust in ac-tive volcanic rifts, however, questionsthe purely brittle propaga-tionofdykesindeepcrustal levels(e.g.Ágústsdóttiretal., 2016; Legrand et al., 2002; White and McCausland, 2016) and suggest that ductile mechanisms may play a pivotal role in dyke em-placement.In addition,ﬁeld observations (Spacapanetal., 2017), numericalmodelling(WeinbergandRegenauer-Lieb,2010), labora-toryexperiments(Bertelsenetal.,2018) andtheory(Rubin, 1993)

https://doi.org/10.1016/j.epsl.2019.04.016

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showthatdykesandsheetintrusionscanbeemplacedbya combi-nationofbrittleandductiledeformation ofthehostrock,namely viscoelasticﬁngeringorductile fracturing.Towhat extentductile deformationaccommodatestheemplacementofdykesinthe duc-tilecrustisstillpoorlyknown.

Andersoniantheorypredicts thattheemplacement ofdykesis controlled at a ﬁrst order by far-ﬁeldcrustal stresses (Anderson,

1936; Nakamura, 1977), implying that tectonicstresses dominate over stresses induced by magmatism. Such a scenario is likely when the magma inﬂux rate is lower than the tectonic stretch-ing rate. Fieldevidence,however,showsthat magma inﬂuxrates atcentralvolcanoes inriftscan belarge enoughtoproducelocal magmaticstressesthat overcometectonic stresses,leading tothe emplacement of cone sheet swarms (e.g. Burchardt et al., 2013; PasquareandTibaldi,2007).InLargeIgneousProvinces,magmatic ratescanbe extremeatlithosphericscale(e.g.up to0.78km3_/yr

fortheKarooBasin;Svensenetal., 2012),buttheeffectsofsuch highinﬂuxrateonmagmaemplacementmechanismsandcrustal deformationhavenotbeeninvestigated.Furthermore,thethermal footprint of magmatism related to such major events has been showntopotentiallysigniﬁcantlyweakenthecrust (Danielsetal.,

2014; Kjølletal., 2019).Suchweakeningstronglydependsonthe balancebetweenthetectonicstretchingrateandthemagmainﬂux rate(Daniels et al., 2014). However, geodynamicmodels of con-tinental breakup associated with LargeIgneous Provinces do not accountforthethermo-mechanicalimpactofthemagmatism(e.g. Allken etal., 2012). This suggeststhat the effects of magmatism on the rheological structure of thecrust andthe tectonic evolu-tionofmagma-richriftedmarginsneedtobeaddressedandbetter documentedin ordertounderstandthe activeprocessesthat can beinferredfromremotesensing andgeophysics,deepwithinrifts (e.g.Bastowetal.,2011).

In this paper, we report on detailed ﬁeld observations of a spectacularly exposed dyke swarm emplaced at mid-crustal lev-els in the magma-rich rifted margin related to a large igneous province that developed during the breakup of the palaeoconti-nentsBalticaandLaurentia,

∼

605Maago.Ourobservationsallow ustoreveal (1)thecomplexbrittle/ductile emplacementofdykes indeepcrustallevels,(2)magmaemplacementmechanismand re-latedcrustaldeformationassociatedwithhighmagmainﬂuxrates, and(3)upwardmigrationofthebrittle-ductiletransition(BDT) be-causeofmagmatism.

2. Geologicalsetting

The geological observations presented here are fromtwo dis-tinctregions innorthernNorwayandSweden,referred toas Cor-rovarreandSarek,respectively (Fig.1). InCorrovarre,thestudied outcrops are from a 0.5

×

6 kmlens ina mountainous area. In Sarek,theﬁeldareaconsistsofhighmountainsdeeplydissectedby glaciers,wherehighcliffsexposeexceptionallywell-preservedand large outcrops.Among these we investigatedtwo localities infor-mallynamed‘Favoritkammen’and‘Favorithelleren’bySvenningsen (1995),usingahelicoptertogetoverviewpictures(Fig.1Aand C). The studied dyke complex can be observed along a string of outcropsspreading900kmfromsouthtonorthalongthe Caledo-nianmountainbeltinScandinavia(Fig. 1B). Thedykesaremostly ofbasalticcompositionandshowlittlechemicalvariability(Tegner etal.,2019andreferencestherein).Radiometricdatingshowsthat the dyke complex developed between616 and 597Ma (Kjøll et al., 2019 and references therein). Speciﬁcally, in Sarek and Cor-rovarre,newU-Pbdatingindicatesthattheswarmswereemplaced at608

±

1and 605.7

±

1.8Ma, respectively (Kjøll etal., 2019; Svenningsen,2001). Thedyke complexwas partofamagma-rich rifted margin (Abdelmalak etal., 2015; Kjøll etal., 2019) associ-atedwiththeinteraction betweenamantleplume (Tegneretal.,

2019) and an active rift system that marked the opening of the Iapetus Ocean(Andréassonetal.,1992,1998),separatingthe tec-tonicplatesBalticaandLaurentia(Cawoodetal.,2001).Thedykes wereemplaced inthickpackagesofLateNeoproterozoic sedimen-tary rocks (Svenningsen, 1994a). In Sarek, the dykes’ host rock consists ofevaporitic deposits andcarbonates (100-500m) over-lainbya

>

2500m-thicksuccessionofarkosicsandstonewiththin micaceouslayers(Svenningsen,1994a).AtCorrovarre,mostofthe hostrockconsistofsandstone,withlocaldiscontinuouscarbonate layers (Lindahl et al., 2005; Zwaan and vanRoermund,1990). At both SarekandCorrovarre,geothermobarometryindicatethat the dykes crystallizedat apressure ofc.0.3–0.45 GPa,suggestingan emplacementdepthof

∼

10 km(Kjølletal.,2019),i.e.closetothe BDT fora geothermof30◦C/km.AtCorrovarre, geothermobarom-etry suggestsgranulitefaciesmetamorphic conditionsatthetime ofdykeemplacement(Kjølletal.,2019;ZwaanandvanRoermund,

1990),suggestingthatthegeothermincreasedsigniﬁcantlyduring dykeemplacement.

IntheLateSiluriantoEarlyDevonian,themostdistalpartsof the Baltican margin, which containedthe studied dyke complex, was subducted andsubsequentlyexhumedduring theCaledonian orogeny (Andréassonet al., 1998). Locally, the dyke complex es-caped penetrative Caledonian strain and metamorphism, and re-mained preserved in large tectonic lenses (Svenningsen, 1994a). The studied outcropsare atthe coreoftheseundeformed lenses (Fig.1).Amorecomprehensivedescriptionofthegeologicalsetting canbefoundinthesupplementarymaterial,part1.

3. Geologicalobservations

Given the large size of the studied outcrops, we performed photogrammetric surveysusing an Unmanned Aerial Vehicle (DJI Phantom4)andpicturestakenusingaDSLRcamerawithGPS sen-sor froma helicopter. The georeferenced imageswere processed usingAdobeLightroomtoenhancecontrastsandcolours,and sub-sequently used to compute 3D photogrammetric models, Digital ElevationModels,3D texturedmeshesandorthomosaicswiththe Agisoft PhotoScan Professional software. The 3D mesh generated inPhotoScanwasimportedintheopensourcesoftwareLIME, de-velopedbytheVirtualOutcropGeologyGroup,UniResearchCIPR attheUniversityofBergen,whichallowsstructuralmeasurements ofgeologicalplanes,bypickingthreepointsonthemeasured sur-face (Buckleyetal., 2017).When outcropswere accessible,direct outcrop observations, measurements and descriptions were con-ducted.

3.1. ObservationsinSarekarea

Thesedimentarylayeringofthehostrockissub-vertical, show-inglargerotationoftheexposedcrustalblockwithunknownfinite rotations.Nevertheless,therelativeorientationsofthedykeswith respect toeach other,andwithrespectto thesedimentary layer-ingoftheirhostrock,aremostlypreservedandprimary,giventhat thereisnosignificantinternaltectonicdeformationinthestudied sub-areas.Cross-bedding, upwardsfiningsequencesanderosional surfaces, all indicate right-way-up toward the south both at the Favoritkammen andFavorithellerenlocalities (Fig.2; Svenningsen,

1994b),showingthatthecurrentsub-horizontalcross-section rep-resents a verticalproﬁle through the crust. Thisimplies that the Favorithelleren outcropwas deeper thanthe Favoritkammen out-crop (Fig. 1) since no major shear zone was detected between them. These exceptional exposures allow detailed descriptions of dyke shapes and strain recorded in the host rock in a continu-ousverticalsectionforalmost2km.Structuralmappingalongthe entireproﬁlehighlightsthepresenceofbrittle,ductileand brittle-ductile structures related to dykes and their emplacement. The followingsectionsreportontheserespectivestructures.

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H.J. Kjøll et al. / Earth and Planetary Science Letters 518 (2019) 223–235 225

Fig. 1. A)GeneralizedgeologicalmapofSarekafterSvenningsen (1995).Fieldareasareindicatedbyblackrectangles.Notethatbeddingislocallyoverturnedonthenorth limbofthesyncline.Whitelinesindicateorientationofdykes.BlackstarindicateslocationofthegeologistsinFig.1C.B)Simplifiedgeologicalmapofthecentraland northernScandes.White(andgreen)areasarerocksaffectedbyCaledoniandeformation.Greenareasindicatetheextentofthevariablymetamorphosedpre-Caledonian marginofBaltica.PinkisbasementunaffectedbyCaledoniandeformationandmetamorphism.C.PhotographofthesummitoftheFavoritkammencliff(geologistsforscale highlightedbyarrow),inSarek.Theonlyaccesswaswithahelicopter.ThispictureshowstheremotenessandinaccessibilityofmanyofthekeyfieldlocalitiesinSarek NationalPark,northofthepolarcircleinSweden.(Forinterpretationofthecoloursinthefigure(s),thereaderisreferredtothewebversionofthisarticle.)

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Fig. 2. Overviewphotographsofthe“Favoritkammen”locality,a200mtall,verticalcliffwallinSarekNationalpark,northernSweden.Sedimentarylayeringisindicatedby blackdashedlines.A)Overviewoftheentire“Favoritkammen”,c.1.5kmlong,continuousoutcropthatshowbothbrittle,ductileandbrittle-ductilestructuresrelatedto thedykeemplacement.Sometopographycausessomedistortionofthecompositeorthomosaic.Blackrectanglesshowlocationofrespectiveﬁgures.D:Dolerite,A: Arkose. B) DetailedcompositephotographofasmallpartofFavoritkammen,wheredykesarecolourcodedaccordingtoorientation.Notethatthedykeswiththedifferent orien-tationsaremutuallycross-cuttingi.e.Da(orange)cutsDb(green)andviceversa.Inaddition,atseverallocalitiesthedykesbendandchangedirection(e.g.Fig.3AandD).

Stereonet#1showsrawdataacquiredfrom3Doutcropmodel.Notethatthedykesformtwogroups,oneE-dippingandoneSSE-dipping.Stereonet#2showsthepresent av-eragedykeorientationforthetwodykegroupsandthebedding,denotedDa,DbandS0,respectively.Stereonet#3showsaveragedykeorientationsforthetwodykegroups whenbeddingisrotatedtohorizontal.Wherecross-cuttingrelationshipscanbediscernedtheyarecolourcodedaccordingtothestereonets.

3.1.1. Brittlestructures

The dykes at the Favoritkammen cliff appear as relatively straight segments(Fig. 2 andFig.3).The contactsare very sharp andregular.Locally, gentleundulationsofthecontactsarevisible (Fig. 3A). Only a few dyke tips have beenidentified inthe area. Theyappear as very sharptips terminating wedge-shaped dykes thataresignificantlythinnerthantheother dykes(Fig. 3C). These thinner dykesalso exhibit regular andstraight contacts withthe host rock. Local jogs/deflections are common along the strike of the dykes where the dykes cross-cut thin micaceous layers be-tween quartzitebeds (Fig. 3C,inset). It is noticeablethat even if thehostrockisstronglylayered,nosill,i.e.layer-parallelintrusion, isobserved.Wheredykesegmentsarearrangedinen-echelon pat-tern,bridges develop betweensegments. Generally, thesebridges

are brokenwithirregularlyshaped pieces ofhostrockﬂoatingin thedykewhereitsteps(Fig.3D).

Outcrop-scale observations at Favorithelleren show that most dykesexhibit chilled margins(Fig.3B. Whenboth dyke wallsare in contactwiththe sedimentaryhostrock,dykesdisplay double-sidedchilledmargins.Stackeddykesandsheeteddykes(Fig.2and Fig.3),withamorecomplexchilledmargindistribution,havealso beenobserved.

The complete exposure at Favoritkammen and Favorithelleren allowedareconstructionoftheentireoutcroponvirtual3D mod-els, which were used for systematic measurements of 212 dyke thicknesses aswell asthe orientation of some ofthe dykes and the strike/dipof thebedding inthe hostrock(Fig. 2andFig. 3). Note that the dyke thicknesses are measured on the3D models,

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Fig. 3. Overviewﬁgureofareaswherethedykeemplacementprimarilyiscontrolledbybrittlemechanisms.A)Complexcross-cuttingrelationshipsbetweenthetwo orien-tationsofdykesdisplayedinFig.2a.Thecentraldyke,highlightedwitharedarrow,displaysadramaticbendofc.80◦fromDatoDb.Blackarrowindicatestranstensional

openingofadyke.Openingvectorsaredrawnaswhitearrows.Theopeningisconstrainedtothesandstonelayerswithminimalopeningintheargillaceousdomains be-tweenthepuresandstonelayers.Blackdashedlinesindicatebeddinginmeta-arkose.B)Chilledmarginatcontactbetweentwodykes.Whitedashedlineindicateschilled margin.C)Thin,dyketippropagatedorthogonaltobedding.Somewhataffectedbyheterogeneitiesinthehostrockasseenintheinsert.Herethedykemakedanabrupt shiftindirectionattheendofthesandstonebed,butthefracturepropagatedstraightbeforebendingin(dashedlineinzoom)D)Twodykeswithsharpanddramaticbends. Orientationis measureddirectlyin3Dmodelandplottedinstereonet.Joints,orthogonaltothedykemargincanbeobservedinbothdykesmarkedbybluelines.Black dashedlinesindicatebeddinginmeta-arkose.Notepartiallybrokenbridgewithangularedges.

such that they are estimatesofthe true thicknesses,not the ap-parent thickness on the cliff. The measurement was taken away from tips and where the dyke segments were assumed to dis-play an average thickness. The measured dyke thicknesses range from0.2 m to 18 m, the mean value being 5.2 m and the me-dian being 4.5 m (Fig. 4). Fig. 4 displays the histogram of the dykethickness, which exhibitsasignificantly right-skewed distri-bution. Following Krumbholz et al. (2014), we fitted our dataset witha Weibull probability function, the fittingparameters being listed inFig. 4. Notethat all measured dykespresented here in-trudearkoseandnot theunderlyingcarbonate, meaningthat the thicknessdistributionisnot affectedbydistincthostrock litholo-gies.

The spectacular outcrop at Favoritkammen shows that the

dykes are not all parallel. The methodical measurements of the dyke orientationsshow two dominanttrends,one dipping to the ENE(orientationDa,orangeonFig.2andFig.3)andonedipping

to the south (orientation Db, green on Fig. 2 and Fig. 3). These

twomain orientationsare separatedby anaverageacuteangleof ca.30◦.Daissub-orthogonaltobedding(83◦ precisely) whileDb

showsan acuteangleof53◦ (Fig.2A). Weobserve mutual cross-cuttingrelationbetweentheDaandDb dykepopulations,meaning

thatthetwopopulationsarecontemporaneous(Fig.2andFig.3A). In addition, we locally observe sharp bends between long dyke segments of distinct orientations, highlighting that a single dyke can havesegments withorientation Da andother segments with

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Fig. 4. TheprobabilitydensityfunctionofthedykesfollowsaWeibulldistribution.

αistheshapeparameterandβisthescaleparameterofthedistribution.n denotes

thenumberofmeasureddykes.x andσ denotethemeanandstandarddeviation, respectively.Measurementshavebeendonedirectlyonthe3Dmodelwherethe exposeddykesegmentappearstodisplayanaveragethickness.

On the cliff of Favoritkammen, the host rock of the dykes is remarkablywellexposed andwell preservedfromtheCaledonian deformation andregional metamorphism.The sedimentary layer-ingisvisiblethroughout,formingmassivebedsthatrangein thick-nessfromafew tensofcentimetrestoseveralmetres(Fig. 2and Fig.3). Remarkably, thesedimentary layers exhibit very little de-formation, except gentle bending (Fig. 3C and D). We correlated thelayersequencesonbothsidesofthedykesbyutilizing charac-teristic markersin themeta-sediments, such that itwas possible toreconstructthekinematicsassociatedwiththeemplacementof each dyke. Such reconstruction showsthat the mode of opening ofmostdykesisobliquetothedykewalls, implyingmixedmode I andmode II fracturing (Fig. 3A). Furthermore,this leads to an apparentoffsetofearlydykescross-cutbylaterdykes(Fig.3A).

Basedontheseobservations,weperformeda2Dstrain restora-tionoftheFavoritkammenoutcrop,inordertoquantifythestrain induced by the emplacement of the dyke complex (Fig. 5). The restorationmethodisdescribedinsupplementarymaterial,part2. Thecalculatedstrain ellipsededucedfromeigenvectorsofthe2D strain ﬁeld tensorindicates that theintrusion ofthe dykesis re-sponsibleforanoverall 2Ddilatancyof147%,whichisconsistent withvolume addition due to magma injection (Fig. 5). However, thismeasureddilatancyisnotisotropic,asillustratedbythe non-equidimensional calculated strain ellipse (Fig. 5): the maximum stretching ( Xmax) is 194%, while the minimum stretching ( Xmin)

is127%.We observethattheaxisofmaximumstretching is sub-perpendicularto thedirectionofthedyke familyDb,andoblique

to thedirectionof thedyke family Da.We note aswell that the

maximumstretchingdirectionisobliquetothebedding.

3.1.2. Brittle-ductilestructures

At the base of the Favoritkammen cliff (Fig. 1A and 2B), the shapes and structuresof the dykes are different than those ob-served inthe upperparts ofthe cliff(Fig. 2B). Contacts between the dolerite and the host rock are still sharp, but they are not planar,andtheyoftendisplaywavyboundariesandcomplex mor-phologiessuch aspinch-and-swellstructures(Fig. 6A). The intru-sionsalsoexhibitsigniﬁcantthicknessvariations(Fig.6A).Thehost rock layers appear much more deformed (Fig. 6A). The outcrop ofFig. 6B provides very precise relationships between successive dykesandthehostrock.There,anearlylargedyke wasemplaced (Dolerite#1inFig.6B), withathinwedge-shapedoffshoot sheet characteristicofbrittlefracturing (highlightedbywhitearrowsin Fig.6B).Thethinoffshootiscross-cutbyalaterthickerdyke (Do-lerite#2 inFig.6B), which hasan en-echelon conﬁgurationwith twobranches,separatedbyabrokenbridgewithbothsharp angu-larandroundedcontactswiththehostrock.Thehostrocklayering as well as the thin offshoot from dolerite #1, below the lower branch of dolerite #2 is planar, whereas the thin offshoot dyke

Fig. 5. Diagramshowingestimatesofinversestrainellipserelatedtodyke emplace-mentintwodimensions.Pre-intrusiongeometrywasreconstructedbycorrelating host-rockmarkersintheorthorectiﬁedimageofFig.3A.Thesemarkerswereused torestorebackthehost-rockpolygoncentroidstoestimatehomogeneousstrain el-lipse.Thismethoddoesnotaccountforanyoutofplanemotion.

above dolerite #2, as well as the host rock layering, are folded. NotethattheoutcropsofFig.6AandBareataverysimilar strati-graphic levelasthose ofFig. 3(see locationsin Fig. 2), withthe samehostrocklithology(arkosicsandstone).

OutcropobservationsattheFavorithellerenlocality showsthat the carbonate layers of the host rock exhibit folds near a dyke, withaxialplaneparalleltothedykecontact(Fig. 6C). Centimetre-scalereverseshearzonesaffectcalc-silicatelayersofthehostrock afewcentimetresawayfromthedykewall.Bothfoldsandreverse shear zones accommodate ductile andbrittle shortening perpen-diculartothedykewall.Fig.6Ddisplaysanotherintrusivecontact atFavorithelleren,whichdisplays avery irregular,“blobby”shape showinglocal meltingofthehostrock.The sedimentarylayering isbendedattheintrusioncontact.NotethattheoutcropofFig.6D is located in a higher stratigraphic position than the outcrop of Fig.6C.

3.2. ObservationsfromtheCorrovarrelens

ThetopographyatCorrovarreismuchsmootherandlowerthan atSarek,suchthatthestudiedoutcropsarediscontinuousbecause of vegetation andlocal slope deposits. Detailed outcrop observa-tionshighlightthepresenceofbrittle-ductileandductilestructures

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Fig. 6. Overviewplateshowingevidenceforbrittle-ductiledeformationrelatedtotheemplacementofmaﬁcdykesinSarek.A)Complexgeometricrelationsbetweendykes andbridgesintheFavoritkammenlocality.Dashedblacklinesindicatebeddinginthearkose.Whitearrowpointstoroundedbridgeinthehostrock.B)Showsthetemporal evolutionoftheBDTasdolerite#1showsathin,tapereddykeoffshootwhichintrudedmeta-arkose,highlightedbywhitelineandarrows.Thisthinoffshootissubsequently foldedasdolerite#2isemplaced.Blackdashedlineindicatebeddinginthemeta-arkose.Bluelinesindicatepreservedcolumnarjointinginalaterdyke.C)Carbonatewith strongercalc-silicatelayersshowsmallscalethrustfaultsduplicatingthecalc-silicatelayersaswellasfoldswithaxialplanesparalleltodykecontact.D)Leucosomes(white arrows)formingatthecontactbetweenasedimentaryrockandadolerite.

relatedtothe dykes andtheir emplacement.Purely brittle struc-tureshavenotbeenobserved.

3.2.1. Brittle-ductilestructures

AcliffinthesouthernpartoftheCorrovarrelensdisplays sev-eralmaﬁcdykesemplacedinsimilararkosicsandstoneasthoseof theFavoritkammen outcrop described above (Fig. 7A). The intru-sivecontactsaresharp.Somecontactsarerelativelystraight,whilst othersexhibit more irregularcurvedshapes. A few dyke tipsare visibleattheoutcrop,andthey are allblunt androunded,which is markedly different from the dyke tips observed in Sarek. The sedimentarylayers are draped around thedyke tipand cut by a swarmoffracturesthatradiate fromthetipofthedyke(Fig. 7B), showingthatthehostrockbehavedinabrittleandductilemanner inresponsetotheimposedstressassociatedwiththepropagating dyketip.

3.2.2. Ductilestructures

ThroughoutCorrovarre,we foundnumerous outcropsof maﬁc intrusions exhibiting highly irregular shapes. Fig. 8A displays a string of dolerite pillows within a migmatitic arkose, where migmatization is regional, but there is no evidence for Caledo-niandeformation.ExtensionalCrenulationCleavage(Fig.8B)

struc-tures within the migmatites are cross-cut by the dyke swarm, implyingthatregionalshearingandstretchingwasaccommodated prior to intrusion. We also found other irregular doleritic intru-sions emplaced within marbles.Fig. 8C displays a lens-shape do-leritedrapedbythefoliationinthemarble,resemblingatectonic boudin. The dolerite-marble contact is sharp, with local angular apophyses. The dolerite exhibitsa chilled margin at the contact, the magmatic texture is preserved, and it does not display any evidence of tectonic deformation. The dolerite also has regular columnar jointing perpendicular to the lens contacts (blue lines in Fig.8C). The righttip of thisdolerite body ishighly irregular, withcomplexlobesresemblingminglingstructuresthatarein an-gular contact withthe foliation of the host marble (Fig. 8D). All theseobservationsevidencethattheshapeofthedoleritebody is primary,i.e.ofintrusiveorigin.

4. Interpretation

4.1. Emplacementmechanismsofindividualintrusions

MostofthedykesegmentsinSarekexhibitregularandstraight contactswiththehostrockandsharpandthintips(Fig.3Aand D). The thindykes exhibit a clear wedge shape. Inaddition, angular

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Fig. 7. FigureshowingdetailsfromacliffinsouthernCorrovarre.A)Overviewofthecliff.Stereonetshowsstructural dataofthedykesandhostrock.Whitelinesoutline dykes.Noteagraniticvein,cutbymaﬁcdykes.Notealsotheshapeofthedykes,whichexhibitequalthicknessofthedykes,whichabruptlyterminateinablunttip.B) Detail ofadyketipshowsbeddingwraps thetipandfracturescuttingthefoldedbeddingaheadofthetip(whitearrows).

Fig. 8. Overviewplateofthenorthernsegment.A)Maﬁcmagmawhichintrudedasoftmigmatiticarkosicsandstoneformedtortuouscontactsandcomplexlobes.Digital cameraforscaleB)ECCfabricinthemigmatiticarkoseshowslayer-parallelstretchingandintrusionlocalleucosomes(yellowline).C)Doleritemagmatic“boudin”inmarble. Doleriteshowscooling joints(bluelines),c.10-15cmthickchilledmargin,andacontactmetamorphicaureoleseenasadiscolourationofthemarbleatthecontactbetween dykeandmarble.Notehammerforscaleinthecentralleftoftheimage.WhiterectangleshowslocationofpicturebutnotethatthephotographinDistakenatadifferent anglethanC.D)Magmatic“boudin”neck.Whitearrowindicatescontactmetamorphicaureoleasayellow-ishdiscolouration.Blackarrowshowsthecomplexlobesatthe “boudin”neck,resemblingSaffman-Taylorstructures(SaffmanandTaylor,1958),suggestingamagmaticorigin.Notethatthedoleritecutthefoliationinthemarble.

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broken bridges separate dyke segments, where angular clasts of thehostrockareoftenobservedsurroundedbycrystallised maﬁc dyke(Fig.3C).Fromtheseobservationsweinferthatthe emplace-ment of most of the dykes was primarily controlled by brittle fracturing of the host rock, in good agreement with established modelsof dyke emplacement(e.g. Rivaltaet al., 2015). This con-clusion is corroborated by the measured Weibull distribution of thedyke thicknesses, inagreement with theﬁeld measurements ofKrumbholz etal. (2014), whointerpreted suchathickness dis-tributionbybrittlefailureofthecrustwithdistributedweaknesses ofdifferentsizes.

Conversely, other dykesexhibit distinct shapesandassociated structuresrelated to other emplacement mechanisms. The intru-sionsofFig.6AandBatFavoritkammenexhibitirregularandwavy contacts,andtheir hostrockexhibit signiﬁcantductilefolding. In addition, observed broken bridges along these intrusions exhibit highlyirregular,roundedshapes(Fig.6AandB).The detailed ob-servationsatFavorithelleren(Fig.6C)highlightcoevalductile fold-ing andbrittle faulting of the hostrock layers neara dyke wall. Finally,thedykeofFig.7atCorrovarreexhibitsparallelwallsand ablunt tip,andboth ductiledoming andbrittle fracturing ofthe host rock ahead of the tip. It is important to note that the ob-served ductile deformation is not related to Caledonian regional tectonics,as(1) thebrittle/ductile features ofFig. 6Cand Fig.7B are restricted to the vicinity of the intrusions, and (2) the duc-tiledeformationvisibleinFig.6B onlyaffectstheearly dykesbut not the later ones. We infer from these observations that these intrusionswere emplacedbyviscoelasticfracturingorviscoelastic ﬁngering(Bertelsenetal.,2018).

The local brittle/ductile deformation on Fig. 6C is related to shorteningperpendiculartothedykewall.Inaddition,theductile bending,andassociatedfracturing,aheadofthetipofthedykeon Fig.7Bsuggestthatthedyke tipwaspushingitshostrockahead. Theseobservationsshow that theemplacement ofthe magmain theductile-brittle crust isa forceful process, such that the over-pressured magma deforms its host rock to createits own space, even ina rifting setting.To accommodate the increasing magma volume,thehostrockdeformseitherbybrittlefailure,ductileﬂow, orbothatthesametime.

Numerous intrusions in the Corrovarre area exhibit complex shapes, such aslenses(Fig. 8) and pillow-like shape (Fig. 8A). In addition,the host rockexhibits significant ductile foliation drap-ing around the intrusions. These structurescould casually be in-terpretedaspost-emplacementtectonic boudins,however,chilled marginsobservedatthecontactsoftheigneousbodiesshowthat theyareprimaryemplacementfeatures.Theclose-upphotographs ofFig. 8A and Dshow that the intrusion contacts are highly ir-regular with complex lobes. Note that on Fig. 8A, the hostrock waspartially moltenwhenthedoleritewas emplaced. Such mor-phologystrongly resembles a Saffman-Taylor instability (Saffman andTaylor,1958), whichdevelopsduring the inflow ofa viscous fluid into another fluid of higher viscosity. These structures are alsovery similar to intrusionsemplaced in low-viscosity salt di-apirs (Schofield et al., 2014). These observations show that the emplacement of these intrusions was dominantly controlled by ductiledeformationofthehostrock,whichbehavedasarelatively lowviscosityfluid.

Our observations at Sarek show that magma emplacement

wasdominantlyaccommodated bybrittledeformation andmixed ductile/brittle deformation in the meta-arkose sandstone and the slightly deeper carbonates, respectively (compare Fig. 3 and Fig.6C). Locally,Fig.6C showsthat ductilefoldingaffects carbon-atelayers,whereasbrittlefaultingoffsetcalc-silicatelayers.Finally, our observations at Corrovarre show that magma emplacement wasdominantlyaccommodatedbymixedductile/brittletoentirely ductiledeformation inthe meta-arkoses andthe carbonate

sedi-ments, respectively (compare Fig.7 withFig. 8). Such systematic differencesshow how thehost rock andtheir associated rheolo-gies are critical factors controlling magma emplacement andthe morphologiesoftheintrusivebodies.

4.2. Emplacementofthewholedykeswarm

The Favoritkammen outcrop allows the identiﬁcation of two groupsof dykeswithdistinct orientations,separatedby an acute angleofca.30◦.Theirmutualcross-cuttingrelationships,andthe sudden shift from one direction to another along a single dyke (Fig. 2 and Fig. 3) show that these two dyke groups were con-temporaneous and not the result from two successive intrusive episodes.Thisimpliesthatthestudieddykeswarmisnotasimple stack ofparallel dykes perpendicular to the leastprincipal stress (hereextensionalstressesrelatedtorifting),asexpectedfromthe Anderson’stheory(Anderson,1905).Wethereforeinferthatmany ofthestudieddykeswerenotverticalatthetimeofemplacement buthadadipof70◦ orless.

The restoration showninFig. 5displays that the dyke swarm accommodated for large crustal stretching (94%), as expected in rifts. However, counter-intuitively, the restoration also indicates that the positive dilation associated with the dyke swarm also accommodated for27% of crustal thickening (Fig. 5). This shows that the emplacement of a dyke swarm also can contribute to signiﬁcant crustalthickening,even inariftsetting.Such a mech-anism is only possibleifthe emplacement of thedyke swarm is a forceful process, as corroborated by local-scale structural ob-servations (see section 4.1). Weinfer that the magmainﬂux rate was larger than the tectonic stretching rate, such that the tec-tonicextension was not fastenough to accommodate forthe in-put of magma. This is corroborated by high-precision radiomet-ric (U-Pb zircon) dating of the dykes, which suggests that the whole dyke swarm was emplaced in a periodof

∼

4 Ma or less (Kjøll et al., 2019), which is signiﬁcantly shorter than character-istic time scale of continental rifting (e.g. Courtillot et al., 1999; Menziesetal.,2002).

In both the Sarek and Corrovarre areas, we observed brittle as well as ductiledeformation accommodating the emplacement of the studied intrusions. Based on this study and the previous estimatesofpressure-temperatureconditions forthemagma em-placement(650–700◦Cand3-4kbfromthecontactmetamorphic aureole;Kjølletal.,2019),weinferthatthestudiedintrusive com-plexwasinitiallyemplacedneartheBDT.Inaddition,Fig.6Bshows that an earlythin, sharp-tippeddyke, the emplacementof which was likely controlled by brittle deformation, has been folded in ductile fashion to accommodate theemplacement of later dykes. Thissuggeststhat earlierdykes were emplacedin a brittlecrust, whereaslaterdykesatthesamestratigraphiclevelwereemplaced inamoreductilecrust.ThisstronglysuggeststhattheBDTmoved upward with time during the emplacement of the dyke swarm. Such an upward migrationofthe BDT islikely aresponse tothe heatingofthecrustresultingfromthefastinﬂuxofmaﬁcmagma (Danielsetal.,2014).

5. Discussion

5.1. Regionalconstraints

Inthisstudy,weintegrateﬁeldobservationsfromtwolocalities, SarekandCorrovarre,whichareseparatedby

∼

300kmintheSeve andKalaknappecomplexesoftheCaledonidesandhasbeen trans-portedoverlargedistances(e.g.Jakobetal.,2019).Thesimilarities in exposed crustal depth, in intrusion age andhost rock litholo-giesstronglysuggest that theseareascanbe correlated,andthat the studied intrusions represent the same magmatic event. The

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Fig. 9. Conﬁgurationofdykes(DA andDB),bedding(S0)anddirectionofmaximalstretching(Xmax).Threeindistinguishablescenariosareproposed.Notethatallthree

scenariosresultinatleastonedykeorientationbeinginclined.A)Beddingwashorizontalattimeofemplacement.B)Themaximumstretchingdirectionwashorizontalat thetimeofemplacement.C)Thebisectorlineoftheobtuseanglebetweenthetwodykesetsmarksthehorizontal.

CaledoniandeformationrotatedthecrustalblockexposedatSarek, such thatit ischallenging toreconstructthepalaeo-horizontal at the time of emplacement of the dykes. Our observations, how-ever, allow us to propose three scenarios to discuss the palaeo-horizontalatthetimeofdykeemplacement(Fig.9).Inscenario1, thehostrocklayeringS0 issub-horizontalandthedykeswith

ori-entationDaare sub-vertical(Fig.9A).Inthiscase,thedykeswith

orientation Db andthe ﬁnite maximal stretching direction

calcu-latedinFig.5areinclined.We wouldinterprettheinclinedﬁnite maximal stretching direction, as a result of a syn-intrusion tec-tonicshear. In scenario2, themain stretching directionof Fig. 5

is horizontal, andthe dykes with orientation Db are sub-vertical

(Fig. 9B). In thiscase,the hostrocklayering isalreadytilted and thedykes withorientation Da are inclined.Finally, inscenario 3,

the two dyke orientations are a conjugatefracture set relatedto syn-magmaemplacement,andthebisectorlineoftheobtuse an-gle marks the palaeo-horizontal (Fig. 9C). In this case, the host rocklayering istilted andboth dyke orientations Da andDb are

inclined.Ourobservationsdonotallowustodistinguishbetween thesethreescenarios.Nevertheless,inall threecases,atleastone dyke orientation is inclined, and we interpret that the opening ofthesedykesaccommodatedfortheobservedcrustalthickening. Hencethesethreescenariosarecompatiblewithcrustalthickening inducedbytheemplacementofthedykeswarm.

5.2. Tectonicandgeodynamicimplicationsformagma-richmargins

The geodynamic interpretation of crustal thickening resulting fromthe emplacement ofthe dyke swarm strongly relies onthe robustness of the kinematic restoration of Fig. 5. This kinematic restoration was computed from a 2D cliff section, whereas the restoredobjects are 3D. The dykes are, however,almost perpen-dicular to the cliff, and so the exposed outcrop is very close to displaythesectionperpendiculartothedykesegmentsand there-fore we infer that the kinematic restoration in the plane of the cliff is reliable. We cannot rule out some out-of-plane displace-ments, which are not reachable with our method. However, the geodynamicsettingatthetimeofdykeemplacementwasinferred tohavealimited,ifnotnegligible,trans-tensionalcomponent im-plyingthat out-of-planekinematicsislikelylimited(Svenningsen,

1995).

The emplacement of the studied dyke swarm accommodates

foralmost 100% stretching (

β

-factor

=

1.94; Fig.5), as expected in a rift setting. However, more surprisingly the restoration in-dicatesthat the emplacement ofthe dyke swarm accommodated

27% of crustal thickening. Such thickening was possible because the dyke swarm consisted of at least one dyke population that was not verticalandnot orientedperpendicular tothe rifting di-rection (Fig. 9). Furthermore,we show that some dyke segments areemplacedasmixedmodeIandII,whichincreasestheamount of thickening relative to an inclined dyke opening as a mode I fracture (Fig. 3A). Their opening vector therefore had a vertical component resultingincrustal thickening. We inferthat the tec-tonic stretching ratedid not balance the rateof magmainjected in the crust, hence the magma input dominated over the tec-tonicextension.Thisissupportedbythehigh-precision radiomet-ric dating, which showsthat the peak magmaticevent occurred within a short period of time (

∼

4 Ma; Kjøll et al., 2019) and is in good agreementwith estimatedshort durationsof magma-tism relatedtoLargeIgneousProvinces(e.g.Svensenetal.,2012; Tegner etal., 2019).The observationspresentedheresuggestthat voluminousandfastemplacementoflargedykeswarmsatdepths of 10 to 14 km can contribute to significant crustal thickening, even in rifting settings, if the magma influx rate is larger than thetectonicstretchingrate.Ourobservationsdifferfromcommon observations in“normal” volcanic rifts andrift zonesofhot spot volcanism,wheredykeemplacementisoftenaccompaniedby nor-mal faulting (e.g. Rubin, 1992). In these settings, magma influx ratesaresignificantlylowerthanthoseinLIPs,andwesuggestthat themagmainfluxratedidnotfullybalancethetectonicstretching rate.

Thethickeningaccommodatedbytheemplacementofthedyke swarm isestimatedto

∼

27% (Fig. 5).Ifthisthickeningis applied to anormal 15-km thick brittlecrust, thiscan leadto

∼

4 kmof crustalthickening.Unlessaccommodatedforbydykeinduced nor-malfaultsintheuppercrust,suchthickeningwouldresultin sub-stantialsurfaceuplift(Fig.10).InLargeIgneousProvinces,upliftis observedandsystematicallyinterpretedasdynamictopography re-sultingfromthe interactionsbetweena risingviscous plumeand the overlying lithosphere (e.g. Pik etal., 2008). Our observations suggest instead that at least parts of this uplift may be caused by theemplacement ofadyke swarm inthecrust.Such thicken-ing canexplain thepresence ofthick distalcrustal segments (so-called outer highs) along magma-richmargins (e.g.Mjelde etal.,

2001). This is supported by recent geophysical observations that an outer highinthe mid-Norwegianmargin likely hosts adense dykeswarm(Abdelmalaketal.,2015).

The ﬁeld observations from Sarek suggest that the BDT may have moved upward during the

∼

4 Ma-long period of

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emplace-H.J. Kjøll et al. / Earth and Planetary Science Letters 518 (2019) 223–235 233

Fig. 10. ConceptualmodelshowingevolutionoftheBrittle-DuctileTransition(BDT)duringriftingandwhattypeofdykegeometriescanbeexpectedatdifferentlevelsinthe rift.AtTime1,theBDTisslightlyelevatedduetoriseoftheunderlyingasthenosphere.BrittlefractureswillgovernthedykeemplacementabovetheBDTandvisco-elastic ﬁngeringgoverntheemplacementatandbelowtheBDT.ThisisalsothecaseforTime2,buttheriseoftheBDTcausesductileconditionswherethedykeemplacement ﬁrstwascontrolledbybrittlemechanisms.Wherethehostrockisweak,likewherecarbonatesdominate,thedykemorphologycanbecomecomplex.

mentofthedyke swarm (Fig.3andFig.6). Wepropose thatthe fastemplacement andcooling of large volumes of maﬁc magma advectedheat intothecrust (e.g.Danielsetal.,2014;Kjøll etal.,

2019), whichlead toshallowing oftheBDT, thinning ofthe brit-tle crust and a viscosity decrease of the ductile crust (Fig. 10). This process has the potential to considerably weaken the crust (Fig.6D).Suchaninterpretationisinagreementwiththemodelof BastowandKeir (2011) whichsuggestthattheintrusion ofdykes intheEastAfricaRiftsystemandtheDanakildepressioninduced weakeningof the crust. Such mechanismcan promote stretching andthereby thinningoftheductilecrust andlithosphere, aswell asbrittle failure ofthe uppercrust, leading to enhanced decom-pressionmeltingandeventually theﬁnal break-up(Bastow etal.,

2018; Keir etal., 2013). Ourinterpretation impliesthat fast, vo-luminousinjectionof magmacan deeply andquicklymodify the rheologicalstructure ofthe lithosphere, whichis proven to be a ﬁrst-order parameter on tectonic deformation style (e.g. Clercet al., 2015; Labrousse et al., 2016). However, models of continen-talrifting usuallyonlyaccountforthetectonictime-scalethermal evolutionofthe lithosphere, butnot forthe thermalimpactof a short, voluminous magmatic event (e.g. Keller et al., 2013). Our studyshowsthat implementing magma input is essential for re-vealingthetectonicevolutionofmagma-richriftedmargins.

5.3.Mechanicalimplications

Akeyobservationfromthestudiedoutcropisthetwoinclined dykesegmentorientations.Severalfactorshavebeenidentiﬁedto control inclined dyke emplacement. First, the two dyke segment orientationsareclearly contemporaneousandthereforeconjugate, but with acute angles of about 30◦ rather than 60◦, as would be expected under the Mohr-Coulomb brittle regime. Given that the dyke swarm was emplaced in a tectonic rift setting,the in-clineddykescouldbe interpretedtobecontrolledby pre-existing conjugateextensionalshearfractures.However,thishypothesis im-pliesthat we wouldexpect an acuteangle of

∼

60◦ betweenthe dyke orientations, in disagreement with the observed 30◦ acute angle. Second, topographic loading can deviate dyke trajectories

(Maccaferrietal.,2014).However, thedeviationisexpectedtobe gently gradual over the whole length of the dykes, which is in-compatible with the observed abrupt orientation changes of the studied dykes (Fig. 3). Third, the interference betweendykes in-trudingsimultaneouslycanalsoleadtodeviationoftheirtrajectory (KühnandDahm,2008).Thismechanism,however,cannotexplain thesystematicgroupingofthemeasureddykeorientationsintwo sets.

Other key observations necessaryto explain the inclined con-jugate orientationsofthe studied dykesare that (1)the opening ofthedykesexhibitashearingcomponent(Fig. 3andFig.4),and (2) theemplacement of the dyke swarm lead to crustal thicken-ing (Fig. 5). These observations are in good agreement with the 2D experiments of Abdelmalak et al. (2012) and Bertelsenet al. (2018), which show that the forceful propagation of a dyke tip intoacohesivebrittlecrust isaccommodatedbyalocalconjugate setofsmall-scaleshearstructuresthatcancontrolthesubsequent oblique propagation ofthe tip. In these experiments,there were no tectonic extension. The resulting dykes consisted of steeply-dipping segments of alternating dip-directions. In addition, the emplacementofthedykesintheseexperimentsalsotriggered sur-face uplift and thickening ofthe models. The similarity between these experiments and the observations presented here suggests that theoblique segments ofthe dykesare markersof aforceful dykeemplacementmechanism,i.e.theemplacingdykesgenerated their ownstress ﬁeldthat overcameboth thevertical loadofthe overburdenandthetectonicstresses.

This mechanism is supported by seismological data, which show that the propagation of dykes in active volcanoes triggers signiﬁcantshearfailureofthehost,wherethefaultplanesexhibit smallacuteanglebetweengroupsoffaultplanes(Ágústsdóttir et al., 2016; Whiteet al., 2011). Another mechanismhasbeen pro-posed by Weinberg and Regenauer-Lieb (2010), who argue that conjugatemicro-scalesheardamageistriggeredbyductile fractur-ing,whichsubsequentlycontrolstheorientation oftheemplacing magma.TheseelementsstronglysuggestthathybridmodeI-II fail-ureofthehostrockplaysamajorroleintheemplacementofthe studied dyke swarm. This is in contradiction to the established

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theory assuming pure mode I dyke propagation along a tensile fractureperpendicularto

σ

3.

Ourﬁeld observationsshowdistinctmodesofemplacementof theintrusionshighlightedbycontrastingdeformationmechanisms ofthehostrock.Rubin (1993) andGallandetal. (2014) showthat magmaemplacementiscontrollednotonlybythemechanical be-haviour of thehost rock, butalso by the magma properties and emplacement rate. In our study area, all intrusions are of very similar maﬁc composition (Tegneret al., 2019) and have a sim-ilar magma crystallization temperature (

∼

1150◦C) over the 900 kmlongexposureofthedykecomplex(Kjølletal.,2019).We in-ferthatthemagmapropertiesalongthewholedykecomplexwere relativelyhomogeneous,whichstronglysuggeststhattheobserved emplacementmechanismsdominantlyresultedfromdistinct brit-tle/ductilerheologyofthehostrock(Fig.10).

Thetimescaleofemplacementofmaﬁcsheetintrusions(days to years) is orders of magnitude shorter than the rates of duc-tile tectonicdeformation (10−13 to 10−15 s−1; e.g. Sassier etal.,

2009).Thishasbeenusedtoarguethatatsuchshorttimescales, the ductile crust can rupture in a brittle fashion to accommo-datethe emplacement ofdykes.However, ourobservations show that ductiledeformation also plays a major role in accommodat-ing the emplacement of magma in the studyareas. This implies thattheemplacementofdykesintheductilecrustisnot necessar-ilycontrolledbybrittlefracturing.Thisiscorroboratedbyourfield observationsatCorrovarreandSarek,whichshow thatsignificant volumesof magma can be transportedalong sheets that are not purely brittle structures. The laboratory experimentsof Bertelsen etal. (2018) showthatsheet-likeintrusionscanformby viscoelas-ticfingering.Intheseexperiments,thevisco-elasticfingersexhibit straight, parallel walls and blunt tips that push their host rock ahead, exactly like theintrusions shown inFig. 7. We infer that dykes in the ductile crust can be emplaced by viscoelastic fin-gering,sothattheir systematicinterpretationasbrittlestructures shouldbedonewithcaution.Theobservationspresentedherealso implythatductiledeformationoftheEarth’scrust cantakeplace atratesthat are severalorders ofmagnitudefaster thantectonic strainrates.

6. Conclusions

This paperpresents detailed ﬁeld observations of a spectacu-larly exposed dyke swarm emplaced at10 to 15 km depth at a magma-richriftedmarginrelatedtothebreakupofthe palaeocon-tinentsBalticaandLaurentiaat

∼

606Ma.Suchlevelofa magma-rich rifted margin is rarelyexposed at thesurface andthese ob-servations are relatively unique in thissetting. This study,based on ﬁeld examples from northern Sweden andNorway, allows us to reveal fundamental features related to (1) dyke emplacement mechanismsnear thebrittle-ductile transition, and(2)to discuss the geodynamic implications of the fast emplacement of a dyke swarminamagma-richriftedcontinentalmargin.

Ourconclusionsondykeemplacementmechanismsare:

•

The emplacementof numerousdykes was accommodated by

brittle failure, whereas the emplacement of some dykes was accommodated by signiﬁcantductile deformation ofthehost rock.

•

Thebrittle dykesformaconjugateset,separatedby an aver-ageacuteangleof30◦ Hybridmode I-IIfracture propagation accommodated the emplacement of thisconjugate dyke sys-tem, indisagreementwiththe commonlyestablishedmodels assumingapuremodeIpropagation.

•

The dyke-tip shapes andthe associated brittle-ductile struc-turessuggestthatdykescanalsobeemplacedas‘visco-elastic ﬁngers’nearthebrittle-ductiletransition.

•

In ductile host rock, magma conduits may exhibit strongly lobateshapessuggestingthatmagmatransportcanbe accom-modated dominantly by ductile ﬂow of the host rock along ﬁnger-likechannels.

Thegeodynamicimplicationsofthisstudyarethefollowing:

•

Dykes emplaced contemporaneously may exhibit two main

orientations anddemonstrate that dykesare not always sys-tematicallyperpendiculartotheriftingdirection.

•

The studied dyke swarm accommodated for both crustal

stretching (94%) and crustal thickening (

∼

27%), indicating that when magmaemplacement rateisgreater than tectonic stretching rate, the magma pressure can lift the overburden andthickenthecrust.

•

Even inan active rift setting,the dykeswere emplaced in a forceful manner and did not accommodate passively for the tectonicstretching.

•

The emplacementofthe dyke swarmlead toa shallowingof thebrittle-ductiletransition.

Ourstudyshowsthatthethermalimpactofadykeswarmcan considerably weaken the crust and potentially affect the geody-namic evolutions of magma-rich rifted margins and be a major factorcontrollingcontinentalbreak-up.

Acknowledgements

HJK, L.L. and TBA acknowledges support from the Research CouncilofNorway(NFR)throughitsCentresofexcellencefunding scheme,toCEED,ProjectNumber223272.Theﬁeldworkand anal-yses conductedforthispaperwas funded byNFR Fri-Nat project number: 250327. We also acknowledge NationalPark authorities in Norway and Sweden for authorizingthe use of helicopters to access the remote areas described in this study. The manuscript greatly beneﬁted fromtwo thorough andconstructive reviews by Janine Kavanagh and Craig Magee.Tamsin Mather isthanked for commentsandeditorialhandlingofthemanuscript.

Appendix A. Supplementarymaterial

Supplementarymaterialrelatedtothisarticlecanbefound on-lineathttps://doi.org/10.1016/j.epsl.2019.04.016.

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