Depth of extensional faulting on the Mid-Norway Atlantic passive margin

JON MOSA R NG U-BULL43 7,2000 - PAG E33

Depth of extensional faulting on the Mid-Norway Atlantic passive margin

JON MOSAR

Mosar,J.2000: Depthof exte nsional faulting on theMid-NorwayAtlant icpassive margin.Norgesqeoloqiske under- sekelseBulletin437,33-4 1.

TwocrustaI-scale cross-sectionsof theMid-Norwegia n At lanticpassive marginare discussed.Large W- andE-dip- pingnorm al faultsrelateto extensionof the continentalcrust foll owingtheCaledonianorogenyandthesubse- quentopeningof theAtlantic Ocean.Thepassivemarginexte nds fromthe ocean-conti nentboundarywestofthe VerinqMarginalHighto the innermostextensiona l normalfault s 90 kmwest of theCaledo nian thrustfron t. Based onearthquakedataand publishedresults of geophysicalmod elling andseismicinterpretat ion,the averagedept h to whichthe normal faultsextendinthe offshoredomain is estimated to 20±5 km.Thisdepthcorrespon ds to the brittle-ductiletransitio n inthe crustofthestretched Mid-Norwegiancont inen talmargin.Above thistransitionthe crustexte nded bybritt lefaulting(tiltedblocks);belowthislimit the crustextended inaductilemann er.

JonMosar,Norgesqeoloqiskeundersekelse, N-7491Trondheim,Norway.(e-mail-Jon.Mosar@ngu.no)

Introduction

Know ledge of the present-d ay st ruct ure at crustaI and lithospheric scale and of the depth to the major extensiona l fault shelpsusto understand thelarge-scaletectonicpro- cesses involvedin thedevelopmentof the Mid-Norwegian passivemargin.Two crustaI-scalecross-sections,one in the Lofoten areaand the other along the Storlien-Trondheim- Verinq transectarediscussed (Figs.1and 2).The depth to thedetachmentand/orthedepth towhich the ext ensional faults ext end, hasbeeninvestigated by analysisofeart hq ua- ke depth s (Fig.3) in combination with interpretations of existi ngseismic surveys and geophysicalmodelling .

The Mid-Norwaypassivemargindeveloped in continen- tal crust and reachesfrom theinnermo st exte nsional normal faults, near Are some 90 km westof the Caledonianthrust front close toOstersund(Sweden),to the western terminati- on of the Voring MarginalHigh and the transition to oceanic crust (Figs.1 and 2).Thest ruct ureand geologyin thecent ral part of theMid-Norw ayAtlanticmargin is the result ofapo- lyphasedeformation,including thedevelopmentof an acti- ve margin (Caledonides), followed by multipl e exte nsional event s leadingto thedevelopmentofanewpassivemargin. TheCaledonian structurescompriseasuccession ofstacked nappes,result ingfrom the closure of thelapetu sOcean and the convergence of Baltica and Laurentia during Early Palaeozoictim e,formingtheCaledon ian(Scandi an) orogen.

Asuccession of extension aleventsevent ually culminated wit h theopening of the Nort h AtlanticOcean(Vaq nes etal.

1998,Dore et al. 1999,Gabrielsen etal.1999,Brekke 2000).

TheLatePalaeozoic-Mesozoicsedimentary cover sequences weredeposited in this exte nsionalenviro nme nt. Con t inued exte nsionand deposition sincethelatestCretaceousin this proto-NE Atlantic event ually led to the opening of the Atlantic Ocean.Such sedimentary seq uences are found in

theoffshorerealm wherethe str uct uralst yle isdominated by grabendevelopmentandlocally inverted domest ructu- resof Tertiaryage.For a detaileddiscussionof theregion al geology onshoreand offshorethe reader isreferredto pa- pers by Gee et al. (1985),Roberts &Gee (1985), Stephens&

Gee(1985) and Stephens etal.(1985);and thest ruct uralele- mentsmapsof the Norwegian continentalshelf(Blystad et al. 1995);the metamorphic,st ruct ural and isotope age map, and the bed rock map of Central Fennoscand inavia (Lundqvistet al. 1996, 1997).

The cross-sectionsdiscussed aresimplified regional sec- tions linking theonland wit h the offshore st ruct ures.They are based on existing and available data,combin ed wit h new int erpretati on at depth, and show simpli fied,viable, structura l solutions. The emphasis is on the post-Late Permian extensional structures,and on the location and depthof themajor,normalfault systemsaffectingthe crust.

Adistinctionis made betw eenbasement

sensu lato

(s.l.)and sedimentary cover.Included in the basement s.1. are the Caledonian nappes,the autochthonou s subst rat um (base- ment

se nsu str icto

(s.s.)),and the Devonianand EarlyPermo- Carboniferousgrabenswith their variably metamorpho sed sedimenta ry rocks.The sedimenta ry cover comprises all post-LatePermi andeposits.Thedetailedgeologyandst ruc- turesof the different tectonicunitsare not represented on the sections forreasonsof readability.

Thermo-mechanicaland rheological modelling isbeyond the scope of thispaper,as isthedetailed discussion ofthe possible relation ship of the exte nsional st ruct ures and the Caledoniancompressional(t hrust)/extensionalst ruct ures.

Onshore/offshore cross-section

Seismic invest igat ions, both onshore and offshore, have imaged st ruct ures at depth in different areas betweenthe

NGU-BU LL 43 7,2000 -PAG E34 JON MOSAR

Scandmavtan Caledollldestectoooarattgraphic map:Sveriges geologiskaundersOkningSer.Ba nr 35;compiledbyGeeetal.1985

Compilanon:.kill.Ho..""'._~OO(}

0lft},,,,..map.fromBlysradet,I 1995.Brekkeet at1999;Gabrielsee etal.^J999,Smethurst2000

BoundaryofTertiary1a"'15(innerflows) Oceanicmagnetic anomaly

Subcropof'baseCretaceousbelowQuaternary SubcropoflopBasementbelowQuaternary

300km

} LowerAlIochthon

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Parautochthc nand LowerAIlochthon Tertiary volcani cescarpment

Oceanicfracture zone

OldRed Sandstone Perrnian

UppermostAllochthon UpperAllochthon MiddleAllochthon

Sedimentarycover Precambrian crystallinerod ,s Sedimentarv cc ver Prtcambri~

crystallinerods

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Onshore geclogy

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Fig.1.Simplifiedtecto nostrat igraphic map of the At lant ic Nor w egianpassive margin. Bold blackline s show locati on of crustal-scalecross-section s in Fig.2.ln red,blueand black areshownthepost-Late Permiannormalfault sonsho re,as wellasfaults wit h undeterminedmoveme nt/age. H=Hit ra;

R=Roragen;Trondh.=Trondheim.

ocean ic crust to thewestof the Verinq Marginal High ,and the overt hickened, extended continental crust to theeast (St orlien-Ostersund,Sweden).It appears that the onshore portion is affect edby ext ensional,crusta I-scale faulting and developm ent of small basins(Fig. 2;see also Hurich et al.

1988,Sjostrorn & Bergman 1989,Sjostrornet al. 1991,Wilks &

Cuthbert 1994, Andersen 1996, Hartz & Andresen 1997,

Ande rsen et al. 1999). These stru ctures are similarto those described in west ern and southern Norway (Fossen &

Ry kkelid 1992,Ande rsen1996,Hurich1996,Dunlap&Fossen 1998,Fossen&Dunlap 1998,Osmu nd sen et al.1998,Fossen et al. 1999, Gabrielsenet al.1999,Christiansson etal. 2000).

Nor m al faulting alsooccurredinthe contin en talcrustinthe Ofot en-Lofot en area further north (Rykkelid & Andresen

JON MOSAR

1994,Coker etal. 1995,Hames &Andresen 1996,Klein et al.

1999).Simi larfau ltsarealso presentin theoffshore substra- tum of the Trondelag Platform,in the Voring Basin, aswellas benea th theLofoten and Utrest Rid ges(Blyst ad etal. 1995, Doreetal.1999, Gabriel senetal. 1999).

Tw oE-W cross-sectionsarediscussed (Fig s. 1 and 2):one fro m east of Are (Sw eden), through Trondh eim and the Fosen Penin sula, andacross the Trondelag Platform into the Vor ingBasin;anda second acro ssthe Lofotenarea, through the Utrest High int o ocean ic crust. Int er p reta t ions of the offshore set t ing of bothsect io ns are based on:1) published geo seism icprofiles aft erseismic refl ection data (Blyst ad et al.1995), 2)seism ic refraction dat a (Plan keet al. 1991,Mj eld e etal. 1993,1996,1997 ,1998,Planke &Eldholm 1994),and 3) geophysi cal modelli ng (Go ldsc h mi dt-Roki t a et al. 1988, Sko gseid et al. 1992,Skogs eid 1994, Skogseid & Eldholm 1995,Olesen et al. 1997,Digraneset al. 1998).

Interpret ations of thedifferentpublished data, whi lelar- gely in ag ree me nt, are not necessarily identical.The cross- sect io ns discussedhereintry tosat isfy the most relevant ob- ser vat ions and attem pt to highlight the most important changes in basin st ruct u re, fault geometry, and base- ment/Moh odepth.

In the so ut he rn Voring-Ar e secti on, four main domains are recognised (Fig. 2):[I] the Are-Trondheim-Fosen area,[11]

theTrondelag Platform, [Ill] the Verinq Basin,[IV] and the Verinq Marginal High that forms the transition to oceanic lithosphere to the west.Thesefour domains are underlain by continentalcrust. Along the Voring-Ar esection,the passive margin israther wide (500 km offshore

+

220 km onshore) comparedwith the Lofoten margin (250 km)or many other seg m ents ofthe AtlanticOcean.However, equally wide mar- gins exist, for exampl e, off Newfoundland (Tankard &

Welsink1989,Welsinket al. 1989,Driscoll et al. 1995 ) or along the northeastmargin of Greenland.

In the Are-Trondelag portion of thesect io n,it is possible to utilisedeep seism ic ref lection data (Hu rich et al.1988, 1989, Gee 1991, Palm 1991,Palmet al. 1991,Hurich 1996, Hurich&Roberts 1997 and in prep.)to constrainthest ruct u- resat depth.This profile shows a 10-15 km-thick st ack of Caledonian nappes(Fig.2),which resultedfrom thrustingof the exoticterranesfrom Laurentia/lapetus and the imbrica- tio n of theW-subducting marg in of Baltica (Gee etal.1985, Stephens et al. 1985,Stephens & Gee 1989,Rey et al. 1997).

Post-dating the Caledonian st ruct u res,aseriesof at least four, main ly west-d ip p ing, normal faultscut the fold-and- thru st belt (No rt o n 1986, 1987, Gee 1988, Sj6str6m &

Bergm an 1989,Sj6str6m etal. 199 1,Geeet al. 1994,Wilks &

Cut h b er t 1994, And ersen 1996, Hurich & Roberts 1997, Andersen et al. 1999) (Figs. 1 and 2).

The innermost no rm al faults, incl ud ing the Rorag en detachment (Sj ost ro rn&Bergman1989,Gee etal. 1994), are locat ed west of the frontal thru stof the Caledonianoro g enic wed ge. The fault with the largest norm aloff set islocate d wit hin the Meraker Nappe, and has been termed the

NGU-BULL 43 7 , 200 0 - PAGE 35

Kopperafault (Hur ich & Roberts, in prep.,D.Roberts, pers.

comm.2000).Farther east is the Roragen detachment, lo ca- ted closeto the Norwegian-Swedishborder and there over- printing amaj o r thrust;and the in nermos t no rmal fault - here calledprovision allytheArefault- located 1 km west of Are (Swe d en)(Figs. 1and 2).

Along the coast,the polyphase Mer e-Tre nd elaq Faul t Complex forms a major high-strain shear zone cut t ing thr o ug h the Central Norwegian Caledonide s (Gab rielsen 1989,Grenlie &Roberts1989,Serann e1992,Robert s1998).

Multiple react ivation recorded alo ng this fault complex ranges from sinist ral ductile movement in the Devoni an period (Gre n lie & Roberts 1989) to brittle offset s in Lat e Cretaceous-Early Tertiary time (Gronlie et al. 1990, 1991).

From thedistrib utionofeart hq uake s(Fig. 3) it ap pears that the present -day seism ic activityalong this faultis verylow or non-existent (NFR/NORSAR 1998).The Mere-Trend elaq Fau lt Complexap pears to be an upper crusta I featurethat term i- nates against a major extensional fault at depth (Fig.2),as hasbeen interpreted from deep seismic profiling (Hurich&

Roberts 1997).A similar conclusionhasbeenreach edfrom recent investig ation s on the Great Glen Fault (M cBride 1995 ),which bearsanalogieswith the Mer e-Trend elaqFau lt Complex (Cow ard 1993,Chauvet&Seranne 1994).

Offshore,the southern cross-sect ion shows a strongly variablethicknessof the crust.Thisisrelated torifting,part ly in Permo-Juras sictime, but mainly during the Cretaceous period (Dare et al. 1997,1999,Swiecicki etal. 1998,Brekke et al. 1999,Brekke 2000).The Trondel agPlatform isaffect ed by few,major, normal fault syst em s and the crust appears to have been only moderately stretched.Mesozoi cbasin s and st ret ching were possibly superim posed on impo rt ant Palaeozoic (Devono-Carboniferou s?) basins. Normal faults with large throws were active mainly in theVer inq Basin (Brekkeet al. 1999, Brekke 2000).In the centralVor inq Basin, the crustis very thin (Fig.2)and lithosph er icmantl eispre- sent quite close to the floor of the sed ime nta ry basin (Skogseid 1994, Skogseid &Eldholm 1995 ).Conversely,the sedimentary basinsare very deep with up to 8-12 km of sedi m ent s.

The cross-section in the Lofoten area shows a typical, tilted block, margin geometry with major,west- and east- dipping, normal faults.The section show s four different structural highs: [I] onshore area, [11] Lofoten Ridge, [Ill]

Marmzele Spur, and [IV] Rest High. The most import ant thinning of the crust is observed to the west of the Vestfjorden Basin,beneath the Lofoten Ridge(Mj eld e 1992, Mokhtari & Pegrum 1992,Mjelde et al. 1993,Gold schm idt- Rokita et al. 1994,l.eseth &Tveten 1996).Unlike the Are- Trondheim -Voring section,the Lofoten cross-section shows a width of onlyso m e 250 km for the passivemargin realm.

The strong thinning of the lower crust beneath the Lofoten Ridge -from 11.5 to 2 km - ap pears to be duemainly to ductilestretching in thelowercrust.

Underplatedmagmatic materi aloccur sin both sect io ns

I)

Loro/ ell area

z

Clc

(JJ

cc- c-

.j:>

w

"

IV

oo o Okm

1

^{150 km}

1

fo/il/

1

Lofcteu Ridge Utrost Ridge

--- b

100km

RustHigh Marma-lespur

RibbanBasin Vestfjorden Basin

Okm N\\'

1

Okm

100km

1

Oceanic crust

Tertiary andPost-riftsedi ments Syn-riftsediments

E.Devono-Pennianbasin (Basements.l.) Continentalcrust(Basements.s.) Lithospheri cmantle Underplating

a

Brittle-ductile transition

b

Lithosphericreflector ..uuattributed

C Basementss.-covercontact(...--known._...possible)

d

Base Upper Crust (fromMjelde et al.1996)

• •

• ^o

Lithos phe ric-scale cross-sec tions

of the Mid-N orwegian Atlantic passi ve margin

2)

Vor;llg - S/orliell area

Okm

1

^100km

1 1

SI:

Ire

synform

A D

KF RD

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Stjordalen Storlicn Trondheirns

fjord l'osen

SF MTFC SSZ C TrnndelagPlatform

VoringBasin

AD-AreDetachment

"I'-KopperaFault

MTFC-Mcre-TrondelagFaultComplex RI>-RoragenDetachment

SSZ-Stedle ndshearzone VoringMarginalHigh

100km

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Okm

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100km

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NGU

Fig.2. Crustal-scalecross-sectionsof theMid-Norwegi anAtlanticpassivemargin. Cross-sect ionsareco mpiled and simplified fromexist ing data(seereferencesin text ).Fault sin the offshore domain havebeen extended to depth intothe zoneof thebrittle-ductiletransit ion.Caledo nian basem ent-cover st ructures in the offshoreareaare infer- red,byanalogywit hinfor mat io nfrom the onshorestructures.Onshore st ruct ures in theFosen-Storlienarea are from deep seismicpro filing(Hurich&Roberts1997),The Swedishportionofthe sectio nisfromPalmet al.(1991),

JON MO SAR NGU-B UL L437 , 20 0 0 - PAG E37

Northern BalticShield Scandinavia -Svalba rd-Mohns/KolbenseyRidges

110tension compression

2000

_ ...._r-.

~-

.

.,^,

1500

...

o, o•

-

^.,

1000

• • • • • • Brittle-Ductiletransition

Onshore/Offshore earth qui-"a"'k""es: - ---,--_ _--,

Oilcontinentalcrust EarthquakesinNorthernEuropein1965 ~1 997

Earthquakefrequency 500

Earth qu akesillVoring Basin 0

0 10-

•

^ill

30 40- 50

~.c

cSE.

100

• •

wet . dry D Strength envelope

-1000 0 1000 3000

Strength (M Pa)

(modifiedfromCloetingh&Banda1992;EGT)

90

(Data from Instituteof Seismolog y.University of Helsinki )

Fig.3.Depthofeart hquakesfrom NorwayandtheNorwegianSea and thest rengt henvelopefor the Nort hernBaltic Shield. Analysesofeart hq uake depthsshow thatmost of the seismicityoccursin themiddle-uppercrust above25km depth.The st rength-de pt hrelat ionshipofthelit hosph ere is expressed by rheo logic alprofil es orstrengthenvelopes.The baseof maximu m occurrences ofearthquakescorrespondswellwit h thebase of the mechanicallyst rong crust (MSC) deduc edfrom the strength profile andcoincideswit hthebritt le-ductiletransition (rheologiesare forquartzite, diabase andolivi ne/d unitelayering;Cloet ingh&Banda1992).

along the ocean-contin ent (passive margin) bou nd ary, at the crust-mantletransition(Skogseid et al. 1992,Mjeldeet al.

1997,1998).This underplated material hasbeen interpreted from high-velocitybodiesbelowthecrust, and isthought to have resulted from adiabatic decompressional melting of the mantle, follo wed by preferent ial melt migratio n along the crust-mant le boundary (Skogseid et al. 1992).There appears to be a smaller volume of underp lated material alongtheLofot en sect ion thanalongthe Voring-Areprofile.

Ingeneral,theamou nt of unde rplatedmaterial(high-velo- city intrusionsin thelowercrust)decreasesland wards.Some magm aunderplating maybeof Cretaceousage,in view of the stro ngerextension that affected theVoring Basin during thisperiod.Similarmagmaticunderplati ng hasbeen recor- ded along manyprofiles along the European Atlanti cmargin (Faeroe, Rockall,Greenland,No rt hSea,More).

Polyphase extensional faulting and tilted block margin

Both orogenic and post-orogenic (post-Late Permian), polyph ase,exte nsional deform ati on dissected the Caledo- nides.Extensionalfault ing was active at diff erent periods andat different dept hs,and hasbeen documentedfrom iso- topeages,mineral assemblagesandsediment at ion history (Boundy etal. 1996,Torsvik etal.1997,Andersen1998, Klein etal. 1999).

Ext ension isknown to have started in Early Devoni an (c.405 Ma) at depth sof c.10 km,follo w ing the Scandian collisional climax.Simultaneouslyand continu ing into the Midd le Devoni an, sedime nts were deposited in actively exte nding half-grabens in thehangin gwallsto theCaledo-

nian nappe s (Stee l 1976, Bee et al. 1989, Osmundsen &

Andersen 1994, Osmundsen et al. 1998,Andersen et al.

1999).These sedimentswere foldedand metam orph osed at sub-greenschist tolowergreenschist facies(Bee et al.1989) and subsequentlybrought to the surface,mostlikelydueto normalfaultingand exten sion (Fossen 1992, 2000).

Impor tant detach ment and high-ang le faults affected both the present-day onshore secti o n and the concealed offshore partsof the Caledonid es.These exte nsional faults arelinkedto rift ing associatedwit h thebreakup ofPangea whichstart edin Permo -Triassictime and continued into the Mesozoic (Eide etal. 1997,1999,Torsvik etal. 1997,Andersen et al.1999,Braathen 1999).Thus, by wayofexample, during Jurassic-Cretaceoustime,a small spoon-shaped sedimen- tary half-grabendeveloped in Beitstadfjorden,in theinner- most Trondh eimsfjord, along theVerranFaultbranch of the More-Trondelag Fault Complex(Bee&Bjerkli1989).Further inlandto the east,the Roragen detachm ent appears to be genetic ally linked with the l.eerdal-Gjend e Fault fart her sout h.Recentinvestigations have shownthatmovement on this latter fault system occurred in the Permian and the Jurassic(Eide et al.1997, Andersenet al.1999).By analo gyit is suggested that movements on the Are-Roragen detach- ment syste m were also occurring during Permo-Jurassic time.

Durin g Permo -Triassic times, basin develop ment was most likely concent rated in the Trondelag Platform region (but also along the present-day Greenland coast).while during the Mesozoic, the major deeper basinsdevelop ed fart her west (Brekke et al. 1999, Brekke 2000, Reemst &

Cloet ingh 2000, Skog seid et al.2000).Associated wit h the

NGU-BULL 437,20 00 -PAGE 38

general extensionof the passive margin,smaller basins also develop ed farther east and west (w est of the Nordland Ridg e). Basin developm ent continu ed throu gh out the Cretaceous, particularly in the Verinq Basin,and finally cul- minated in theopen ing of the Nort h Atl antic Ocean-eastof the Verinq MarginalHigh - andthe separation of Greenland and Norway(Blystad et al.1995,Dore et al.1997,Lundin &

Dore1997).The Norwegian Atlant ic margin developedfrom there on asapassivemargin.

NW-SEdirecte dcom pressional(thrust ing)and exte nsio- nal(normal)faulting and strike-slipfaulting isstil l activeat the present-dayas seen from earthquake data(Bungum et al.1991forlocationand detaileddiscussion).The mostacti- ve regions lie along the western and eastern bordersof the Trend elaqPlatform, as wellas in the Lofote narea.

Depth of the faults/detachment- rheology/earthquake data

Geological,geophysical and laboratory exper im ents sug- gest thatthemechanicallith osphereisrheological lystratifi- ed. Thislayering reflectschanges in the mechanicalbehavi- ourand flowprocesses oflithospheric rocks,as determined by dept h-dependent physical (P,T)and chemical (m ineral composition,%water)environments.

From seismicprof iles onsho re,as well as offshore,itcan be seen that the faults which limit themajo r half-grabens extend intothe upper crust(Fig.2).Depend ingon their age of formation (from Devonian through to Cret aceous) and location,onshorevs.offshore,these grabens havebeenfilled wit hsediments.Their dept h in thebrittl eport ion of the crust can be estimated from geophysicalmethods and by conside- ring thest rength profile ofthe conti nenta lcrust and the depth of the present-day earthquakes.The present-day strength profile in the Central Baltic Shield (Cloet ingh &

Banda 1992) indica tes that the mechanically strong crust extendsdowntoa depth of21km(Fig.3 - MSC

=

baseofthe mechanically stro ngcrust).Thebrittle-du ct ile transitionzone forms thelower boundary to the seismicacti vit y.Therefor e, the earthquakedepth is controlled by the thicknessof the brittle part of thecru st.In extensionaldomains,aswell asin compressiona lorogens,faultsfrequentlyroot inthis zone.

The depth dist ribut io n of eart hq uakes in Norway,the Norweg ian shelf (including Svalbard) and the oceanic domain east of the mid-oceani cridge has beenanalysed to determinethe depth to the baseofthe present-day brittle crust (Fig. 3).These data include documented NW-SEexten- sional faulting and local NW-SE compressive events (Bungum et al.1991,Atakanet al.1994,NFR/NORSAR 1998).

In this oceanic-co nt inental domain,the majority of the earthquakes are locatedbetween0and25km depth(Fig.3).

A mean depth of14.2kmwascalculatedfor the whole data set (oceanandcontine nt;num berofearthquakes = 5308, standard deviat ion

=

11km).The depth distr ibution of the eart hq uakes shows a stro ngly skewed distr ibut ion wit h a majorityof earthquakesabovethe mean value anda rapidly

JON MOSAR

dim inishing number of eart hq uakesbelowthemean value.

Domains underlain exclusivelyby continen tal crust(Verinq Basin,Trendel aqPlatform and onshore)showameandepth of 12.9 km(number of earthqu akesanalysed

=

3006;stan- dard deviation

=

^10.4km). Theearthquake frequency dimi- nishesmarkedl y below thisdept h and becom esvery weak below25-30km.The dataanalysessuggestthatthe brittle- ductiletransit ionislocatedat aroun d15- 25km depth.Thus, the dept h to a possib le major detachment,or the dept h where the exte nsional faultsroot,rangesfrom 15to25km.

This is consistent with more detailed work from the Norwe gi an Atlanticmargin repo rted byGab rielsen (1989), Gregersen & Basham (1989), Bung um et al. (1991) and NFR/NORSAR(1998).

The overthickened crust ofthe Baltic Shield is furth er stretche d beneath thebase level of thesefault s,mainly in the Verinq Basin (where inte rpretation of refract ion data suggeststhat thelowercrust almostcom p letelydisapp ears) and the Lofoten Ridge areas.This extension occurs below theductile-brittletransiti on zoneinthelowercrust,where the crust deforms in a ductile fashion and may sustain substant ial stre tc hing, whil e the upper crust is extended largely by brittl e normal faulting. Permo-Triassic,Jurassic and Cretaceous(Eideet al.1997,Andersen et al.1999)to Tertiary extens ionacted upon the continental lithosphere createdaftertheCaledonianorogeny(followingthe orogenic collapse, the subducting slab-b reakoff/remo val ofthe gravi- tati on ally unsta blecrustaIroot;Andersen & Jam tveit 1990, Koyi et al. 1999).Thisisin accor dance with the results of analogue modelling, where the up per and lower crust decouple and develop str ong boud inage(necking) in the low er ductilecrust,eventuallyjuxtaposingupper crustwith asthe nospheric mantle(Brun &Beslier1996,Gart rell 1997).

This ductile extensio n isconsidered to be related to the stretching ofthelit hosp here associated with the prog ressi- vedevelop ment oftheNort hAtlant ic oceanicrealm .

Though it s thermo-mechanical structure may have chang ed throughou t the evolution of the margin,it has been show nthatthebrittle-d uctilestructure ofthe crust is a perm anentfeatu re.The positionof the brittle-du ctile transi- tion,toget her with the zonesof weaknesscreated by the developm ent of successivenormalfaults,havedeterm ined theevolut ionof the deformatio n in the crust. It has,how- ever, been shown that for successive rift episode s the necking levelremai ns atarather constantdepthwit ha best- fit solutio nat around20km forthe V0 ring Basin(Reemst &

Cloetingh2000).The neckinglevel represent s a zone of con- cent rated brit tledeform at ion associated with a detachment zone,which giveswayto a moredist ribut ed deforma tion in the low ercrust. This levellargelycont rolsthe kinematicsof extension in passivemarg ins(Kooi&Cloetingh1992,Kooi et al.1992,vanderBeek etal.1994,Reemst&Cloet ing h2000).

Inasimp lifiedfirst- orderapproachit is suggested herethat itis therefor erealisti c to admit thatsince the Early Mesozoic, the position of the ductile-br ittle transitio n has remained

JON MOSA R

withinthe samedepth range,even thou ghthe geometryof the crust has changedsubsta nt ially in localised zones.

Conclusions

Crustal-scalecross-sect ion s of the Mid-Norwegian Atlant ic passive margin illustratethe overall geom et ry of themargin and the main st ruct uralfeature s.The Mid-Norway passive marginreaches from the innermostnormal faultsnear Are (Swede n),some90 kmwestof the Caledoni anthrustfront nearbstersund (Swede n), tothewestern term ination of the Verinq Marginal High and the transition to oceanic crust.

Polyphaseextensionalevents between theLate Permia n and thePalaeoceneledtobreak-upand development of the NorthAtlanticOcean in theEarlyEocene(magnet ic anoma ly 24).Caledonian structures and nappes were cut by Late Palaeozoic to Mesozoic nor mal faults,and the continen tal crustwasst retc hed repeate d ly,leadin gtothedevelopm ent of deep sedimentary basins.The possible reactivation, as normal faults,of Caledonian thrust s or Palaeozoic extensio- nal faultsin theTrond elag Platform -Vorlnq Basin areacan- not be excluded,butis difficu lt to demo nst ratewit h thedata availableto date.Thesamestruct ures observedonlandare expected to occur offshore, beneat h the Mesozoic and Cenozoicsedimentarysuccessio ns.Thenorm alfaultsdefin e a series oftilt edblocksform ing importan t half-g rabens with associated structures such as roll-overs, hangingwa ll gra- bens andantit het ic fault s (mainlyobservedin the sedime n- tary coverseque nces).Thenorm alfault sboundingthe diff e- rent tilted blocksdo not necessarilymerge intoonesingle decollernenthorizon.

During the successive extens io nal events, the upper- middlecrustbehaved inabrittl emanner and is characteri - sed bynormalfault s.The rootsofthese faultsare interpreted to coincide with the brittle-ductile transiti on zone.From earth qu akedepth sthistransiti on zone hasbeen determi- nedto lie ataround 15-25kmdept h.

Ackno wledgemen ts

Iwouldliketothank E.Eide, 0.OlesenandD.Robertsfor many st imula- tingdiscussio ns,S.Sherlock for help wit h theEngli sh,aswellasJ.Dehls forhelpin gme wit h the earthq uakedata.The reviewers,E.Lundin andC.

Hurich, great lyhelped to improvethemanuscriptwith their comme nts andsuggest io ns.This work has been part of theBATresearchprogram- meat Norgesgeologi skeundersek else,Tron dh eim .

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