ductile and brittle extension in the southern lofoten archipelago, north norway

21 Pages·2004·1.24 MB·English

[AMERICANJOURNALOFSCIENCE,VOL.299,JANUARY,1999,P.69–89]
DUCTILE AND BRITTLE EXTENSION IN THE SOUTHERN
LOFOTEN ARCHIPELAGO, NORTH NORWAY:
IMPLICATIONS FOR DIFFERENCES IN TECTONIC
STYLE ALONG AN ANCIENT COLLISIONAL MARGIN
ANDRE C. KLEIN*‡, MARK G. STELTENPOHL*, WILLIS E. HAMES*,
and ARILD ANDRESEN**
ABSTRACT. TheLofotenarchipelago,northNorway,occupiesthemostinter-
nal position of the Caledonian belt in northern Scandinavia, and rocks and
structuresexposedtherearecrucialtounderstandingprocessesofhowtheBaltic
basementanditscoverallochthonsrespondedtocontinentallithosphericsubduc-
tion and subsequent continental separation. Relatively little is published about
thestructuralandmetamorphicdevelopmentandespeciallythetimingofthese
events; consequently, it is unknown how features exposed on these spatially
isolated islands relate to those of the adjacent mainland. Rocks in Lofoten were
affected by Caledonian regional metamorphism, and structures record tops-east
contraction and later extension related to late- to post-Caledonian basement
exhumation. Tops-west extension is preferentially developed in meter to km-
scaleductileshearzonescontainingwest-dippingextensionalshearbands,west-
verging rootless folds, and asymmetric feldspar porphyroclasts. West-plunging,
sinistral-oblique elongation lineations in the mylonitic foliation are interpreted
to indicate the line of transport. Mesoscopic backfolds are locally developed
withinthetops-westshearzonesandfurtherdocumentwest-directedtransportof
structurallyhigherrocks.Theductileextensionalshearzoneslocallyarecrosscut
bylow-angle,tops-westcataclasticnormalfaults,reflectingprogressiveunroofing
of the shear zones to shallower crustal levels during late- and post-Caledonian
extensional events. Northeast-striking, high-angle, brittle normal faults truncate
all other fabrics and structures and juxtapose structurally deep undeformed
Precambrian basement with the Caledonian nappe sequence. 40Ar/39Ar data
indicate that ductile extension in Lofoten likely initiated soon after the Caledo-
nian metamorphic peak (at (cid:1)430 Ma) and continued episodically until at least
Permiantime.Rockspassedthroughthebrittle/ductiletransitionrapidlyat(cid:1)275
Ma. Recent paleomagnetic data also point to later phases of brittle faulting in
Lofoten, one in the Jurassic/Cretaceous and one related to Tertiary opening of
the Norwegian Sea. The magnitude and style of late- to post-Caledonian exten-
sionandbasementexhumationinLofotendiffersmarkedlyfromtheformationof
phenomenal Devonian basins and exposures of Caledonian ultra-high pressure
assemblages in the Western Gneiss Region (WGR) of southern Norway. Differ-
ences in the styles of extension between northern and southern Norway are
interpreted to reflect inherited differences in crustal architecture that may have
beenenhancedduringtheCaledonianorogeny.
INTRODUCTION
Extensional structures and fabrics in the Western Gneiss Region (WGR) of south-
western Norway expose deep sections of continental lithosphere exhumed following
continent-continentcollision(Norton,1986;Se´ranneandSe´guret,1987;Andersenand
Jamtveit, 1990; Andersen and others, 1991). Comparable crustal-scale extension has
beenreportedintheCaledonidesofEastGreenland(HartzandAndresen,1995).Over
the past decades there has been little record of syn- to post-Caledonian extension
reported in the basement of northwestern Norway, although it has recently been
recognized in the Lofoten archipelago (Holloman and others, 1995, 1996; Hames and
*DepartmentofGeology,AuburnUniversity,Auburn,Alabama36849
‡Presentaddress:DepartmentofGeologyandGeophysics,RiceUniversity,MS-126,6100MainStreet,
Houston,Texas77005-1892
**DepartmentofGeology,UniversityofOslo,P.O.Box1047,Blindern,0316Oslo,Norway
69
70 AndreC.Kleinandothers—Ductileand
Andresen,1996a;Holloman,ms;Klein,ms;Mooney,ms;Waltman,ms).Theevolution-
ary history of the northern WGR (Lofoten block) differs strikingly from its southern
WGR counterparts, especially in terms of metamorphism, deformation, pressures at-
tained during subduction, and residence time in the footwall of a major fault zone. For
example, the WGR of southern Norway was penetratively-deformed and subjected to
ultra-high pressure (UHP) metamorphic conditions during the Caledonian orogeny
(Bryhni and Sturt, 1985). Many basement exposures in the Lofoten region of northern
Norway, however, appear to have experienced little or no Caledonian deformation
(Hakkinen, ms; Griffin and others, 1978; Tull, 1978; Bartley, 1981, 1982). The differ-
encesbetweentheWGRandLofotenblockareenigmaticgiventhatthetwoareashavea
similarstructuralsetting.Explanationsforthesedifferencesareexaminedhereinbased
ontheresultsofnewstructuralstudiesaimedatcharacterizingtheCaledonianandlater
tectonic evolution of the southern Lofoten islands (Holloman and others, 1995, 1996;
Holloman,ms;Klein,ms;Mooney,ms;KleinandSteltenpohl,1999).Thispaperfocuses
ontheislandsofVestvågøy,Værøy,andRøst(fig.1),becausetheyaretheonlyislandsin
southern Lofoten with exposures of aluminous metasedimentary rocks, making them
idealforanintegratedstructural,geochronologic,andthermobarometricstudy.
Synchronous contraction and extension have been documented from several
orogenic belts around the world (the Higher Himalaya, Burchfiel and Royden, 1985;
southwestern Norway, Norton, 1986; the Variscan belt in Europe, Steltenpohl and
others, 1993), and these examples have added greatly to our understanding of the
stressespresentinanevolvingorogen.Modernsubductionzones,liketheonealongthe
western coast of North America, contain a great deal of along-strike variability. Such
variability in ancient subduction zones is commonly masked by later deformation or
unfavorablelevelsoferosion,buttheNorwegianCaledonides,withexcellentexposure
ofsubequalamountsofbasementandcover,provideaparamountopportunitytostudy
an ancient subducted margin. The ancient A-type subduction zone boundary (Hodges
andothers,1982)isexposedalongthewestcoastofNorwayandhasbeenthesubjectof
numerousreports.MoststudieshavefocusedonthesouthernWGR,wherelarge-scale
extensiondrivenbypost-Caledoniangravitationalcollapseoftheorogenapparentlyled
totheformationoftheDevonianmolassebasinsandjuxtaposedthemagainstbasement
eclogitized during Caledonian subduction (Norton, 1986; Se´ranne and Se´guret, 1987;
AndersenandJamtveit,1990;Andersenandothers,1991).
ItmustbenotedherethatthefinalclosureofIapetusduringtheSilurianinvolved
both contractional and extensional stresses at various levels of the Caledonian nappe
stack at various times (Andersen and others, 1991; Northrup, 1996). Therefore, the
distinctionbetweenCaledonianandpost-Caledonian,asusedinthispaper,isnotrelated
to an absolute change in stress regimes. The boundary is drawn, instead, at the end of
prograderegionalmetamorphismoftheBalticbasementanditsoverlyingcoverat(cid:1)430
Ma (Steltenpohl and Bartley, 1988; Hames and Andresen, 1996a; Northrup, 1996;
DunlapandFossen,1998).
RecentdetailedgeologicmappingofthesouthernLofotenarchipelagohasrevealed
that post-Caledonian extension significantly modified the earlier contractional edifice
(Holloman,ms;Klein,ms;Mooney,ms;Waltman,ms;KleinandSteltenpohl,1999).Of
singular interest is how the continental margin at the present-day latitude of Lofoten
(68°N)respondedtothewaningstagesofcontinent-continentcollisionandsubductionto
depths approaching 40 km. In contrast to the spectacular examples of deep crustal
exhumation reported from southern Norway, mentioned above, a master extensional
decollement (Nordfjord-Sogn Detachment Zone of Andersen and Jamtveit, 1990) has
not been reported. In addition, Lofoten does not expose UHP assemblages of Caledo-
nianage,suchasthecoesite-eclogitefaciesbasementoftheWGRofsouthwestNorway
(Griffin and Brueckner, 1980, 1985; Gebauer and others, 1985). Given that the rocks
brittleextensioninthesouthernLofotenarchipelago,northNorway 71
Fig.1. LocationoftheLofotenarchipelagoandstudyarea.LithologiesexposedinLofotenareprimarily
PrecambrianbasementgneissoftheLofotenblock(lightgray)andallochthonousmetasedimentaryrocksofthe
Leknesgroup(black).TheOfotenregionofnorthNorwayisindicated.
exposed in these two terranes are lithologically similar and occupy a comparable
structural setting, why did they behave so differently in response to subduction and
subsequent eduction? This paper addresses this question in light of new structural,
petrologic,andgeochronologicdatafromthesouthernLofotenarchipelago.
72 AndreC.Kleinandothers—Ductileand
GEOLOGIC SETTING
TheLofotenarchipelago(68°N)isabasementridgeinthenorthNorwegianshelf,
associated with gravity, magnetic, and topographic highs. Exposures in Lofoten repre-
sent the most distal component of the Caledonides in the north Norwegian margin
(fig. 1). The Caledonides are composed of a variety of vertically stacked nappes, thrust
from west to east on to the Baltoscandian platform during partial subduction beneath
Laurentia in early to middle Paleozoic time (Gee, 1975; Roberts and Gee, 1985). The
relationship of the Lofoten region to the Caledonides on the Norwegian mainland has
beendebatedsincedetailedmappingoftheareawasbegunintheearly1970’s(Griffin
andTaylor,1978;Tull,msand1978),althoughitisnowgenerallyacceptedthatBaltic
basement exposed on the mainland is continuous with Lofoten basement (Olesen and
others,1996).TheLofotenblockiscomposedofprimarilyPrecambrianbasementgneiss
(Foslie, 1941; Vogt, 1942; Andresen and Tull, 1986). Griffin and others (1978) report
areally extensive mangeritic and charnokitic intrusions in Lofoten dated at (cid:1)1700 to
1800Ma.Theso-called‘‘mangeritesuite’’wasemplacedundergranulite-facies(clinopy-
roxene-zone)conditions.TheLofotenbasementwaslocallyretrogradedtoamphibolite-
facies in shear zones accompanying allochthonous Caledonian rocks, although, as a
whole, the basement was little-affected by Caledonian reheating (Griffin and others,
1978).
In central Lofoten, allochthonous cover rocks, informally known as the ‘‘Leknes
group’’ (Tull, ms and 1978; Sigmond and others, 1984), lie in thrust contact with
PrecambriangneissesandgraniticplutonstypicalofthenorthernWGRexposedinthe
Lofotenislands(Griffinandothers,1978).Basedonpoorly-constrainedRb-SrandK-Ar
datesfromtheLeknesgroup,Tull(msand1978)andGriffinandothers(1978)suggested
that the Leknes group was emplaced onto the basement during an amphibolite-facies
metamorphiceventthatoccurredbetween1200to1100Ma.Tull(ms)indicatedthatthe
metamorphism (his M ) of the Leknes group is unrelated to the Caledonian metamor-
2
phism described by Bartley (ms) on Hinnøy ((cid:1)100 km northeast of Vestvågøy). New
isotopic data, however, indicate development of amphibolite-facies porphyroblasts in
schists and amphibolites of the Leknes group occurred during Caledonian metamor-
phism(HamesandAndresen,1996a,b).Recordsofthiseventarevariablyoverprinted
bysubsequentretrogressiveevents(KleinandSteltenpohl,1999).
The type locality of the Leknes group on Vestvågøy is composed of aluminous
schist,calc-silicategneiss,quartzite,marble,amphibolite,andvoluminousquartzofelds-
pathic mica schist (fig. 2). Two lithologically distinct sequences are found within the
Leknes group on Vestvågøy (Klein, 1997). The ‘‘southern units’’ (fig. 3) are in thrust
contact with the mangeritic basement and contain the bulk of the quartzofeldspathic
mica schist and amphibolite, as well as thin ((cid:1)5 m) layers of quartzite, marble, and
calc-silicategneiss.The‘‘northernunits’’havebeenfaultedoverthesouthernunitsand
containpredominantlyamphibole-richschist,two-micaschist,andquartz-richamphibo-
lite.
STRUCTURAL DEVELOPMENT
ThestructuraldevelopmentofthesouthernLofotenarchipelagoisoutlinedintable
1 and covers mainly the Caledonian and later history of the region. Due to the local
intensity of Caledonian deformation and metamorphism, evidence relating to the
pre-Caledonian geologic history has been largely obscured near Caledonian shear
zones. With depth below Caledonian thrusts or across post-Caledonian normal faults
withlargedisplacements,Caledoniandeformationgenerallyisabsent,andProterozoic
structuresremainintact.Ingeneral,thepre-Caledoniangeologichistoryisdominatedby
development of the Baltic Shield and the northern WGR by a series of orogenic and
brittleextensioninthesouthernLofotenarchipelago,northNorway 73
Fig.2. LithotectonicmapofthestudyareaonVestvågøyanddiagrammaticcrosssectionA-A(cid:1)(same
horizontalscaleasmap).Ingeneral,theLeknesgroupisacomplexlyinterlayeredandanastomosingsequence
ofvariablyfelsicandmaficschistsandgneisseswithminoramountsofquartzite,marble,andamphibolite.See
figure3forameasuredsectionofpartoftheLeknesgroup.
74 AndreC.Kleinandothers
Fig.3. DiagrammaticillustrationofthetectonostratigraphicarchitectureoftheLeknesarea(noscale).
SmallarrowsareCaledonianthrustfaults.Excellentexposureinthesouthernunitsallowedforthedevelop-
mentofadetailedtectonostratigraphiccolumn.LithologiesaresummarizedfromKlein(ms).
intrusive events that are difficult to evaluate in rocks of the study area due to the high
metamorphicgradeandlocalizedmigmatizationofmanyoftherocks.
Caledonian orogeny.—The onset of Caledonian orogeny in the Lofoten region is
characterized by east-directed thrusting of Leknes group assemblages onto the Baltic
craton. Thrusting was accompanied by local retrogression of basement rocks from
granulite-faciestoamphibolite-faciesmetamorphicassemblages.Metamorphicgradeof
allochthonouscoverrocksincreasedfromeasttowest,withschistsexposedintheRøst
districthavingexperiencedkyanite-orthoclasezoneconditionsandlocalmigmatization
(Holloman, ms; Mooney, ms; Waltman, ms). Thermobarometric data reflect this west-
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76 AndreC.Kleinandothers—Ductileand
ward increase, with conditions on Vestvågøy averaging 600°C and 6 kb, increasing to
750°C and 8.2 kb on Røst (Mooney, ms; Waltman, ms). Similarly, 40Ar/39Ar ages of
muscovite porphyroblasts indicate that rocks of the Røst district cooled through their
respective blocking temperatures (cid:1)100 my later than rocks exposed on Vestvågøy
(HamesandAndresen,1996a).Thesedataarenottoosurprisingconsideringthelocation
of these rocks in the footwall of a decollement. Rocks exposed on Vestvågøy and Røst
were most likely farther apart during the onset of Caledonian orogeny than they are
today.
The Caledonian orogeny was a major contractional event (D of Klein, ms) at the
2
latitude of Lofoten, producing numerous thrust faults, large-scale basement-cored folds
(F ),andpervasivemetamorphicand/ormyloniticfoliations(S )(Bartley,1982;Stelten-
2 2
pohl, 1987; Holloman, ms; Klein, ms; Mooney, ms). The Caledonian foliation disap-
pears with depths of less than 0.5 km into the basement below the basal Caledonian
thrust.ThisphenomenonwasrecognizedbyearlyworkersthroughoutLofoten-Ofoten
(Tull, ms; Griffin and others, 1978) and initially was attributed to the Lofoten terrane
havingoccupiedahighcrustallevelduringCaledonianorogenesis(e.g.Hakkinen,ms;
Griffinandothers,1978).Itisnowgenerallyaccepted,however,thattheLofotenblock
wastectonicallyburiedbythecompositeCaledonianallochthon,andexistingpressure-
temperaturedatasuggestthatLofotenwassubductedtodepthsof25to35km(Mooney,
ms;Waltman,ms).
Late-topost-Caledonian.—FollowingthepeakofCaledonianmetamorphism(at(cid:1)430
Ma), rocks of the Lofoten archipelago underwent two kinematically distinct folding
events (F and F of Klein, ms; F a and F b of Waltman, ms) related to regionally
3 4 3 3
diachronous deformation (D and D on Røst, Holloman, ms; D /D on Vestvågøy,
3 4 3 4
Klein,ms;D ofWaltman,ms).F foldsonVestvågøyhaveadominantwest-northwest
3 3
trend, parallel to the Caledonian (and post-Caledonian, see below) transport direction
(fig.4A).Thesefoldsaretermedcrossfoldsandappeartorepresentshorteningdirected
atasmallangletothetrendoftheorogen(SteltenpohlandBartley,1988).F foldaxes,
4
ontheotherhand,paralleltheorogenictrend(north-northeast)butvergetothewestin
the opposite sense from that of nappe transport (fig. 4B) and thus are considered
backfolds(SteltenpohlandBartley,1988).Interestingly,thecrossfoldsareconcentrated
atthehighestexposedstructurallevelsonVestvågøy,whereasthebackfoldsonlyoccur
proximal to the basement-allochthon contact. As a result, fold interference patterns
betweenF andF wereneverobserved.
3 4
D was a major extensional episode throughout the entire Lofoten archipelago
4
(Løseth and Tveten, 1996; Holloman, ms; Mooney, ms; Klein, ms). Late-phase, tops-
west, ductile shear zones that affect the basement and cover units on Vestvågøy are
mainlylocalizedwithin1kmofthebasement-allochthoncontact(fig.5).Raretops-west
shear zones occur higher in the nappe stack, where they were observed to truncate F
3
folds;hencethelate-phaseshearzonesareinterpretedasD structures.Theeast-northeast-
4
trending, northwest-dipping shear zones are defined by an S extensional crenulation
4
cleavage(PlattandVissers,1980)thatissubparalleltoS butgenerallyhasasteeperdip
2
(fig. 6A). These relationships lead to the development of local S-C composite fabrics
(Lister and Snoke, 1984) in the basement and S-NSC (Dennis and Secor, 1987)
composite fabrics in the cover units (fig. 6B). Local high-strain mylonite zones are
defined by stretched quartz and feldspar and may represent competence contrasts
between different lithologies. A mineral elongation lineation (L ) in the S mylonitic
4 4
foliationplungeswestnorthwestandisinterpretedtoindicatethelineoftransport(Klein,
ms;KleinandSteltenpohl,1999).TheserelationssuggestthattheD sliplinewaslargely
4
coaxial with D , and that post-Caledonian transport of structurally-higher rocks was in
2
theoppositedirectionfromthatofCaledoniannappeemplacement.Thewest-northwest
plungeofL inthenorth-dippingD shearzonesindicatesacomponentofleft-slipand
4 4
brittleextensioninthesouthernLofotenarchipelago,northNorway 77
Fig.4(A)FieldphotographofanF crossfoldwithinmaficgneissoftheLeknesgroup,(cid:1)2kmwestof
Leknes.SurfacebeingfoldedisS gneis3sositydevelopedduringCaledonianthrusting.Hammeris(cid:1)30cm
2
long.(B)OutcropphotoofamesoscopicF backfoldinbasementmangeriteorthogneiss,lookingnorthand
downplunge,(cid:1)4kmsouthwestofLeknes.N4otewestvergenceofaxialsurfaceandlackofaxialplanarfabric.
Lenscapis(cid:1)5cmindiameter.
78 AndreC.Kleinandothers—Ductileand
Fig. 5. Map of D ductile shear zones within the Leknes group and subjacent basement mangerite
4
(outlinedinboldlines,gradientpattern).Thetwoshearzonesmaybeconnectedbutarenotshowntodosodue
tolackofexposure.Lower-hemisphereequal-areaprojectionsareofL mineralelongationlineationdatafrom
4
eachshearzone.Contoursare2,6,and8percent.
characterizes the D extensional event. Local S-NSC relations also indicate a strong
4
sinistralcomponenttoD (KleinandSteltenpohl,1999).
4
ArgonisotopicanalysisofmetamorphicmineralsrecrystallizedduringD provides
4
cluesastotheabsolutetimingofthislate-phaseshearingevent.Metamorphicconditions
during D were in the greenschist facies, with chlorite commonly overgrowing biotite
4
within D shear zones. The shear zones also commonly contain fine-grained, recrystal-
4
lizedmuscovitethatclearlyisyoungerthanthecoarsemicaflakesformedduringD (that
2
is, the coarse-grained muscovite is often preserved in boudins within S , whereas the
4
fine-grainedmicadefinesS ).RecrystallizedmicasinS fabricsonVestvågøyyieldages
4 4
of (cid:1)365 Ma (Hames and Andresen, 1996a). Similar fabrics along basement-cover
contactsonVærøyandRøstyieldlepidoblasticmuscovitewithagesof(cid:1)275Ma.These
relationships suggest a diachronous or otherwise time-transgressive evolution of D
4
structuresfromnortheasttosouthwestLofoten.
D brittlefaulting.—Brittlefaultstruncateearlierductilestructuresandfabricsandcan
5
generallybedividedintotwomaintypes:fabricformingandnon-fabricformingfaults.
Fabric forming faults range in orientation from low-angle to high-angle. The low-angle
faultsoccuronlyinthecoverrocks,andtheirorientationmayhavebeeninfluencedby