TECTONICS,
VOL.
9,
NO. 5, PAGES
1097-1111, OCTOBER
1990
UPLIFF
OF DEEP CRUST DURING
OROGENIC
EXTENSIONAL
COLLAPSE:
A
MODEL
BASED
ON FIELD
STUDIES
IN
THE SOGN-SUNNFJORD
REGION
OF
WESTERN
NORWAY
Torgeir B. Andersenand Bj0m Jamtveit
Departmentof Geology, University of Oslo,
Oslo, Norway
Abstract.The WesternGneissRegion
(WGR) in Norway experiencedhigh-pressure
metamorphismduring Silurian-Devonian
continent-continent
collision.The eclogitebearinglower crest is separatedfrom the
middle and uppercrest by major detachment
zonesformed during extensionalcollapseof the
orogen;formationof the Devonianbasinsis
related to the extension. The footwall
of the
detachmentzonescomprisesthree structuraland
metamorphiczones.The upper zone, zone 1, is
characterizedby penetrativehomogeneous
down-to-the-westsimple sheardevelopedunder
retrogradegreenschist-facies
metamorphism.
Zone 2 sufferedinhomogeneous
simple shearof
the samepolarity. Petrographyand mineral
chemistrydata from the lower zone, zone 3,
show a record of initial eclogitefacies
metamorphismat 600øCand >16 kbar, which
was decompressed
almostisothermallyto
amphibolite-facies
conditionsat 550øCand 1012 kbar. Both the eclogite-and amphibolitefaciesmetamorphism
developedin a regimeof
pure shearwith vertical shortening.The rapid
decompression
recordsan approachof
approximately20 km to the surface,related to
Copyright 1990
by the AmericanGeophysicalUnion.
Paper number 90TC00731.
0278-7407/90/90TC-00731 $10.00
uplift that was probablythe result of the
removal of a thickenedthermal boundarylayer
in the mantle lithosphere.The pure shear
regime, which developedinitially in the lower
crest, was truncatedby zonesof simple shear
as the lower crust was uplifted to middle and
upper crustallevels. Extensionby simple shear
in the upper crest was rooted in the lower crest
where extensionoccurredby pure shear.The
shearzonesin zones 1 and 2 did not penetrate
the pure shearregime of the lower crest. A
considerableamountof tectonicstrippingof the
orogenicwelt predatesdepositionof the
Devonian sedimentarybasins.
INTRODUCTION
Most tectonicmodelsfor the uplift of deeply
buriedmetamorphicterrainsduring extensional
collapseof orogensare basedon field studies
in the middle and upper crest and interpretation
of geophysicaldata from the deep rootsof the
orogen [Dewey et al., 1988; England and
Houseman, 1988]. Few models based on direct
observations
of the deep crest are available.
The Western GneissRegion (WGR) of
southwestNorway experiencedeclogite-facies
metamorphismduring Caledoniancontinentcontinentcollisionin Late Silurian-Early
Devonian times [Griffin and Bmeckner, 1980;
Griffin et al., 1985]. The northwesternpans of
1098
the WGR
Andersen and Jamtveit: Orogenic Extensional Collapse, Western Norway
bear evidence
of extreme crustal
thicknesses.Jamtveit [1987] reported pressures
of around 25 kbar from the Eiksundaleclogite,
and coesite-bearingeclogitesindicating
pressuresas high as 30 kbar have beenreported
from the area north of the Homelen
Basin
sion in the detachment
zones and discusses
their geometry.The main object, however,is to
presenta tectonicmodel for the structural
patterndevelopedbetweenthe deep, eclogitebearingcrust and the middle to uppercrustal
rocks that are juxtaposedacrossthe detachment
(Figure 1) [Smith, 1988; Smith and Lappin,
1989]. The pressure(P) and temperature(T)
increaseprogressivelyin a north westerly
directionthroughthe gneissregion [Krogh,
1977]. Eclogites(P>18 kbar) also occurin the
Bergen arcs, but their structuralrelationshipto
thoseof the WGR is yet unknown [Austrheim
KINEMATIC
and Griffin, 1985; Austrheim, 1987, 1990]. The
detachment zones between the lower, middle,
WGR is stmcturallyoverlain by Caledonian
thrust nappes,parts of which have experienced
only low greenschist-facies
metamorphism,and
the low-gradesedimentarybasinsof Middle
Devonian age [Kildal, 1970]. With the
exceptionof the H•tsteinenBasin, the northern,
eastern,and southernmarginsof the basinsare
and upper crest along the westernmargin of the
WGR from Sognefjordento Bremangerlandet
(Figure 1). The uplifted deep crust comprises
the Fjordaneand Jostedalcomplexesof the
WGR and, probably,also the kyanite-bearing
schistsin Hyllestadand partsof the Askvoll
Group near At10y (Figure 1). Middle crustal
rocks occur mainly in the units belongingto
the Middle Allochthon,whereasthe uppercrust
is representedby the Caledonianoutboard
fault bounded, and their western contacts are
unconformities[Bryhni, 1964]. Several authors
have discussedrecentlythe contactrelationships
among the Devonian basins,the Caledonian
nappes,and the WGR [Hossack,1984; Norton,
1986; 1987; S6ranneand S6guret,1987]. The
hangingwall characteristically
contains
semiductileto brittle fault rocks, although
restricted zones of ductile deformation
of the
Devonian conglomerateshave been described
[Norton, 1987; S6ranneand S6guret,1987]. In
the footwall, mylonite zones,severalkilometers
thick, are present(Figure 2a). Kinematic
indicatorsin the mylonitesshow that they, and
presumablythereforethe Devonianbasins,
formed as a result of extension [Norton, 1986,
1987; S6ranneand S6guret, 1987]. This
interpretationhas been disputedby some
authorsarguingmainly from studiesof
magneticfabric [Torsvik et al., 1987, 1988];
however,the widespreadpresenceand variety
of kinematicindicatorsdescribedby previous
workers and in the presentstudy show that the
basinsformed above major extensional
detachments. As such, the WGR is similar to a
metamorphiccore complex [Davis, 1983]. The
uniqueness
of the area is that it exposesvery
deep sectionsof the continentalcrust,the
former root of a continent-continent
collision
zone. Almost all prior derailedstudiesin the
WGR have focusedon eclogitepetrology;very
few combinedstructuraland metamorphic
studieshave been carried out that may clarify
the mechanismof uplift.
This paper summarizesevidencefor exten-
zones.
INDICATORS
Thick mylonite zonescharacterizethe
terrains and the Devonian basins. In order to
illustrate the evidence for noncoaxial strain and
senseof shearin the mylonitezonesof the
detachmentsadjacentto the lower crest, a
summaryof the kinematicindicatorscommonly
observedat the scaleof outcropwill be
presented.Detailed descriptionsof these
features are discussedelsewhere [Swenssonand
Andersen,1989, also manuscriptin preparation,
1990]. A penetrativestretchinglineationis
developedin the mylonitesin the area, with an
azimuth generallybetweenE-W and WNW-ESE
with shallowplunges,dominantlyto the west.
In the upperparts of the mylonite zones,late
extensionalcrenulationcleavagesare common
in mica-richmylonitesand phyllonites,also
indicating a down-to-the-westmovementof the
hangingwall (Figure 2b). Shearbandsare
ubiquitousin the mylonitesand the asymmetry
betweenS and C planesshowsconsistent
westwardmovementof the hangingwall
relative to the footwall (Figure 2c). Spectacular
examplesof asymmetricalboudinageindicating
the same sense of movement
are common
(Figure 2c). Asymmetrical,west vergentfolds
are common,in many placesassociatedwith
compressional
structuressuchas foliation
duplexes,indicatinga consistentrotationsense.
Fold axes are parallel or subparallelwith the
stretchinglineation, and sheathfolds are
common.In the stmcturallylower part of the
mylonite zones,extensionalveins showing
Andersen and Jamtveit: Orogenic Extensional Collapse, Western Norway
1099
GEOLOGICAL MAP, SOGNEFJORDEN - NORDFJORD AREA. W. NORWAY
lO KM
BREMANGERLANDET
o
o
o
o o o
o
o
o
o o
o
oo o o o o o
o
o
o
o
o
o
o o
o o o o
o oo o
o
o
fz
Ber.en
I•
Oslo
f
ORDE
61ø 25"N
,,,,,•
LEGEND'
KVAMSHESTEN
!o øo 1 Devonian
sediments,
minor
oL_•_.•
j voicanites
andintrusives
in the Solund area
•
•
G•sey
gabbro
(380
_+
26Ma'
Caledonian
outboard
terrane,
Upper AIIochthon, Solund-
StavfjordOphioliteComplex
HYLLESTAD
and associated
rocks
•........
Middle
AIIochthon,
OLUND
continental
basement
&
cover
//SOGNEFJORDEN
••.l•
^^^ I Greenschists,
probably
Caledonian,in
the
Askvoll Group and in Hyllestad
#//////////
Boundary
betweenZone1 & 2
I•,-..,.•
Kyanite-bearing
schists,Hyllestad
area
Boundarybetween Zone 2 & 3
• Fjordane
Complex,
Western
Gneiss
Region
Jostedalen Complex,
"-'•• Western
Gneiss
Region
Detachmentsin Upper and Middlecrust
&
A
DetachmentbetweenUpper/Middle
crust
andeclogite-bearing
Lower
crust
ß ß
Eclogites
i
Fig. 1. Simplifiedgeologicalmap of westernNorway betweenSognefjorden
and
Nordt•ord.The reinterpretation
of the geologyin the northeastcornerof the map
is basedon publishedmaps by Bryhni and coworkers.
1100
Andersen and Jamtveit: Orogenic Extensional Collapse, Western Norway
-'-.-...
:::.
::•::::
..•
,,;•.•%'"':',
,•:,
•!..... .-'
.....
ß'..-
• *• :.'•, ..
Fig. 2. (a) View from $e noah, acrossF•rdefjorden of $e KvamshestenDevonian
Basin •d •d $c underlyingdc•chmcnt zone. Line, dipping to the fight (west),
mekcd by patchesof snow, is $c b•ttlc fault which •ncatc $c detachment
fab•c. Mylonitcs of zones 1 •d 2 fo• $c hi•s between$c •ord •d the late
fault. (b) Extension• crenula•oncleavage•d she• bandsdevelopedin mylonitic
E•nschists of $c Askvoll 6•up in the detachmentzone sous of $c
KvamshcstcnBasin. Displacementis to $c left (west). Wid$ of sectionis =1.20
m. (c) Asymmedc• boudins•om detachmentsouthof the KvamshcstcnBasin.
Boudin necks•d extensionalveins a• filled with qu•zfcldspatic material.
Displacementis to $c left (west). (d) Mylonitic augcngneiss•om zone 1 in the
S•d•cn detachment.Shoe b•ds •d asymmc• of augcn showsdown-to-thewest (left) displacement.
progressiverotationfrom the compressional
to
the extensionalfield of the strainellipsoid
betweenthe S and C planescommonly is very
occur. The sense of shear derived from the
orientation and deformation of the veins is
consistent with the other kinematic indicators.
The evidence for rotational strains in the
conservativeestimateof displacementbasedon
uncertaindeterminationof shear strain suggests
minimum displacementsof the order of 50-60
km in the detachmentmylonites.
penetratively mylonitizeddetachmentzonesis
ubiquitousand consistentwith a westward
translationof the hangingwall. The zoneshave
structuralthicknessesacrossthe mylonite
foliationof 2-3 km and have evidenceof very
high shearstrains.Althoughthe strainsare
difficult to estimateprecisely,shearstrain>20
representsa minimum estimateas the angle
small and difficult
GEOMETRY
ZONES
to measure. Thus a
OF THE
DETACHMENT
In discussionsof metamorphiccore
complexesa terminologyincludingan upper
Andersen and Jamtveit: Orogenic Extensional Collapse, Western Norway
Norwegian Caledonides-Oslo-Sognefjord-
and a lower plate separatedby a detachmentis
commonlyused [e.g. Davis, 1983; Wernicke,
1985; Hamilton, 1987]; This has been adopted
in models where the Devonian
/•lesund,
excursion
guide,Uppsala
Caledonide
SymposiumA1, pp. 1-151, herein after referred
to as Bryhni et al., 1981). The Caledonian
metamorphicgrade of the Middle and Upper
Allochthonin this area is greenschist/lower
amphibolite-facies.Relics of Precambrian
granuliteshave been describedfrom mangerite
and anorthositein this area (Bryhni et al.,
1981) and from similar rocks in the Daisfjord
Suite below the KvamshestenBasin (Figures 1
and 3). Hence a major part of the middle crust
that preservesCaledoniantectonostratigraphy
has been preservedstructurallybelow the
marginal fault of the Homelen Basin and along
the Standalendetachmentwhere the eclogitebearinglower crust appearsto the south.The
basins in
westernNorway have been describedas a result
of extensional
tectonics. The detachment
faults
have been describedas parts of one continuous
structurevariably termed the MtilCy fault
[Hossack,1984], the Nordfjord-Sogn
Detachment [Norton, 1986, 1987] or the Basal
Contact [S6ranneand S6guret, 1987]. The most
importantcriterionfor identifyinglower plate
rocks in westernNorway is the presenceof
eclogites.These rocks are separatedfrom areas
without eclogitesby shearzones,as shownin
Figure 1; the boundaryof the lower crustal
rocks does not always coincidewith the faults
along the marginsof the Devonian basins.
These relationshipsare particularly well
displayedbetween the Standalenfault and the
southmargin fault of the Hornelen Basin
(Figure 1). This area, describedby Bryhni
[1989, and referencestherein], comprisesrocks
of the Eikefjord and Lykkjeb½groups,
correlated
with the Middle
Allochthon
middle
crustal rocks in this area thus constitute
a middle plate. The rocks in this structural
positionare highly deformed,and severalshear
zonesbranchingfrom major shearzoneslike
the Eikefjorden fault (Bryhni et al., 1981) and
the Standalendetachmenthave been mapped
[Kildal, 1970]. Shear senseindicators from such
zones clearly indicate down-to-the-west
movement,partly by reactivationof earlier east
directedcompressionalstructures.
Mapping by Bryhni and coworkers[Bryhni
and Grimstad, 1970] (Bryhni et al., 1981)
showsthat the middle plate rocks continueeast
and north of the Homelen Basin (Figure 1). In
in the
Caledoniantectonostratigraphy
and parts of the
FjordaneComplex within the WGR. To the
west, rocks of the Upper Allochthonbelongto
the cover sequenceof the Solund-Stavfjord
Ophiolite Complex [-Fumeset al., 1990] and are
unconformablyoverlain by the Htisteinen
Devonian basin (Bryhni et al., The southern
N
1101
this area, however, the shear zones between the
Kvamshesten
Devonian
Daisfjord
Suite
Daisfjord fault
Main detachment
ß \ o % o
Western Gneiss Region
0
• •
0
o
•
0 0
o
--'
0
0
o
•
0 •
0
0
/
ß
1 km
\
Hoyvik
ed.inAskvoll
Gp.
•
--- Greenschists in Askvoll Gp.
DaisfjordSuite, affected
by ductile extensional simple shear
Fig. 3. N-S sectionof the KvamshestenBasin and substrate.Note that the the late
Daisfjord fault truncatesthe detachmentfabric which is subparallelto the
lithological contactsin the substrate.
1102
Andersen and Jamtveit: Orogenic Extensional Collapse, Western Norway
upper, middle and lower crustalrocks gradually
merge and apparentlybranchfrom one major
detachmentnorth of Bremangerlandet(Figure
1). Rocks similar to thoseof the Eikefjord and
Lykkjeb0 groupsalso occurin rocks of the
lower crust where they form a heterogeneous
superstructure,
the Fjordane Complex, on the
migmatiticgneissesof the JostedalenComplex
[Bryhni, 1989].
The configurationof extensionalfaults
RELATIONSHIPS
BETWEEN
THE
DETACHMENT
ZONES AND ROCKS
THE LOWER
CRUST
around the Homelen
Standalen detachment zones are described.
Basin shows that the
OF
As shown above, the detachment zones
comprise2 to 3 km thick mylonite zones
characterizedby very high rotationalstrains.In
the presentstudy,tectonometamorphic
relationshipsfrom the Kvamshestenand
detachmentgeometrywas complex and
probablyconsistedof anastomosing
high-strain
zoneswith variable orientation,cuttingup and
down throughthe Caledoniannappepile. A
similar geometrycan be demonstratedfrom the
Adjacentto the Kvamshesten
Basin [Bryhniand
Skjerlie, 1975], the detachmentis truncated
[Torsvik et al., 1987] by semiductileto brittle
faults of the Daisfjord fault which mark the
presentmargin of the basin(Figure3). To both
Kvamshesten
the north and the south of the basin, continuous
detachment.
On the south side of
the basin (Figure 3), the Daisfjord Suite,
correlatedwith the Jotunnappeof the Middle
Allochthon[Brekke and Solberg, 1987], occurs
in the hangingwall. In this area, the Daisfjord
Suite is unaffectedby ductile strainsrelatedto
the detachment. On the north side, however,
penetrativemylonitizationof meta-anorthosite
and mangeritesyenite(Figure 3) is related to
shearingalong the detachment.These
relationshipssuggestthat the detachments
did
not form low-angle structureseverywhereas
suggestedby S6guretet al. [1989]. A
consequence
of this is that the Devonianbasins
were depositedat variable distancesfrom the
eclogite-bearing
lower crust.
The basal unconformityof the Homelen
Basin is exposedalong the west and northwest
parts of the basin (Figure 1). In this area, the
unconformityoverstepsfrom little-deformed
low-grade regional metamorphicrocks of the
Upper Allochthon in the Kalv•g Melange
[Bryhni and Lyse, 1985; Fumes et al., 1990]
near Sm0rhavn,onto highly deformedgneisses
and garnet amphibolitesof unknown
tectonostratigraphic
positionson the islands
northwestof FlorO. On Bremangerlandet,
the
basal unconformityof the Homelen Basin
overstepsamphibolite-faciesrocks of the Middle
Allochthon and greenschist-facies
rocks of the
Upper Allochthon.Between these
tectonostratigraphic
units, steeplydipping
semiductile to ductile extensional fabrics
indicatingdown-to-the-westmovementare
superimposed
on east directedshortening
structures.These relationshipssuggestthat the
Devonian sedimentswere depositedon a
basementof Caledonianrocks which already
had undergoneconsiderableextensionaltectonic
strippingand uplift.
sectionsfrom the faulted top of the detachment
to the eclogite-bearing
lower crustare exposed
as the entire region is folded in large-scaleEW trendingfolds. The detachmentmylonitesare
more tightly folded than the brittle faults along
the basins(Figure 3). Torsvik et al. [1987,
1988] proposedthat the folding was a resultof
late PaleozoicN-S compression.
The
mechanismand timing for this folding will not
be discussedhere. Its presence,however,
providesa deep structuralsectionthroughthe
moderatelyto steeplydipping detachmentzones
into rocks of the lower crust.
A cross section from the Kvamshesten
detachmentinto the deepestpartsof the lower
crust that are exposedalong FOrdefjordenis
described as a basis for the discussion of
deformationmechanismduring the extension
(Figure 4). The sectionhas a slructural
thicknessof approximately4-5 km acrossthe
foliation which dips about 45ø north and south
away from the axial trace of the F0rdefjorden
antiform. The detachmentmylonitesare
truncatedby semibrittleto brittle fault rocks at
the contactwith the hanging wall, and these
rocks will
not be discussed further here.
Within the mylonitesof the detachment,
three structural zones with transitional contacts
can be identified on the basis of variation in
deformationintensity,strainpatternand degree
of metamorphicretrogression.
Zone 1 on the north side of the
Kvamshesten basin has a structural thickness of
approximately1.5 km (Figure 4). The entire
zone is characterizedby very high strains
developedduring rotationaldeformation.The
rocks comprisegreen and grey phyllonites,
ultramylonites,mylonites,and mylonitic
gneisses.The phyllonitescommonlyhave large
Andersenand Jamtveit:OrogenicExtensionalCollapse,WesternNorway
1103
E - W section along the Kvamshestendetachment zone
Daisfjord
SuiteHeyvik
Gp.
Brittle
fault
cutting
main •I Melange
detachmentfabricI •
/
2 km
Kvamshesten
basin
I .[ ,L•,,,/
Ophiolite
sea level
- •-.+2
km
=4
km
--
ß.•=_-_ _ _ •
-
•
_,
- _ _-- -- ----- •_-_-•:-z-:-•-I
•.one
1
- In•homogenous
•:_ ,----_•'--•-•-J..-
!nhomogenous
pure
shear
•-%--.-••--'runcared
byan•,-,--' -?-•'
'--••_....-•--•.
ß
+6
km
5 km
10 km
15 km
Fig. 4. E-W section
of theEvamsh½stcn
basinarmsubstrate.
In thehanging
wall,
theCaledonian
compressional
t½ctonostratigraphy
hasbeenpreserved,
although
previousthrustshavebeenre-activatedas normalfaults.In the footwall,the zonal
subdivision of the detachment is shown. Contacts between the zones are
transitional.
In zone3, therocksarecharacterized
by earlyinhomogeneous
irrotationalstrainfabricsdeveloped
at eclogiteandamphibolite-facies
metamorphism.
Thisis cut by lower-grade
zonesof simpleshear,whichdisplaces
down-to-the-west. The 4- to 6-km structural thicknesscan be studied because of
the foldingaroundE-W fold axes,seeFig 6.
diaphtoretic
whitemicasandhavebeenmapped
cleavage.Garnetiferousrocks have not been
previouslyas mica schists[Kildal, 1970].
Locally, characteristic
lithologies,suchas white
weatheringmeta-anorthosites
and gabbroswith
observed in zone 1 of the Kvamshesten
relict igneoustexturesmay be recognizedas
protolithsin the mylonites.Narrow zonesof
grey quartz-feldspar-muscovite
schistshave been
interpretedas originalpsammites[Kildal, 1970].
Kinematicindicators(seepreviousdiscussion
and Figure2) occurthroughout
this zoneand
showconsistentsimpleshear-typedeformation
with down-to-the-west
movement
of the hanging
wall. Quartz-poorlithologieswith grain-size
reductionand retrogradesyntectonic
crystallization of epidote, biotite, chlorite, muscovite,
and albite are characteristic
of the mylonitesin
this zone.Quartzveinsand segregations
with
dynamicallyrecrystallized
quartzare common.
The chlorite-richphyllonites,particularly
abundantin the structurallyhigherpartsof zone
1, developspectacularextensionalcrenulation
detachment,and the syntectonicmineral
assemblages
indicate that the deformation
occurredduringprogressive
retrogression
at
greenschist-facies
conditions.
Zone 1 of the Standalen detachment has
been studiedin sectionsat Steindalen(UTM
125 266) near the H•tsteinenBasin and at
Standal(Figure 1). The zone is approximately1
km wide in theseareas,and the mylonites
generallyhave steepdips between40ø and 70ø
to the north. At Standal,a dark ultramylonite
with an indistinguishable
protolithoccursalong
the contactto the brittly deformedhanging
wall. Structurallybelow the ultramylonite,
protolithsof meta-anorthosite,mica schist,
quartzite,and a megacrysticaugengneiss
with
large porphyroclasts
of K-feldsparcan be
identifiedin the mylonite sequence.The mica
schistin the lower part of the zone contains
1104
Andersen and Jamtveit: Orogenic Extensional Collapse, Western Norway
partly to completelyretrogradedgarnetsup to a
centimeterin diameter,which in outcropcan be
seento contain inclusionfabrics indicating
syntectonicgrowth prior to the retrogression.
Zone 2 in the Kvamshesten
detachment
has
a structuralthicknessof approximately1.5 km.
Becauseof variable dips of the foliation, which
are a result of the E-W folding, and its
transitionalcontacts,it is difficult to give a
preciseestimateof the structuralthickness.The
dominantlithologieswithin zone 2 comprise
bandedgrey-greenquartz-poor,biotiteplagioclasegneissespossiblyof a supracrustal
origin, amphiboliticrocks, and granodioritic
augengneisses.The structuralstyle of zone 2 is
characterizedby anastomosing
mylonitic and
phyllonitic fabrics of variable intensity.Shear
sense indicators characteristic of rotational
strainsare developedin thesezonesand show
consistent
down-to-the-west
movement
of the
structurallyhigher parts of the zone. Banded
garnet amphibolites,locally, have a bright green
color, characteristicof retrogradedeclogites.
Relict eclogitesare found mainly in the lower
part of zone 2 in both the Standalenand
Kvamshesten
sections. Near the Kvamshesten
Basin, however, an eclogite body, not yet
studiedby us, occursas close as 1.5 km from
the upperplate, possiblywithin zone 1 (Figure
4), of the Kvamshestendetachment(W.L.
Griffin and M.B.E. M0rk, Eclogitesand basal
gneissesin westernNorway, excursionguide,
UppsalaCaledonideSymposiumB1, pp. 1-88,
1981).
The heterogeneityof the mylonitic
lithologiesin zone 2 suggeststhat it is best
correlatedwith similar rocksassignedto the
FjordaneComplex elsewherein the WGR. The
deformationpatternis characterized
by variable
strains,developedin an anastamosing
pattern.
Within the high-strainzones,however,
unambiguousshear senseindicatorsshow a
consistent,simple shear-typedeformation,which
is syntheticwith the senseof shearfound
within zone 1. Hence a structuralcontinuity,
regardingdeformationin both a simpleshear
regime and shear sense,betweenzones 1 and 2,
can be suggested.
Zone 3 constitutesthe deepestpart of the
WGR that is exposedin the area.It comprises
rocks assignedto the FjordaneComplexand the
dominantlyquartz-feldspathic
gneissesof the
JostedalenComplex [Bryhni, 1989]. Both
complexesare eclogite bearingbut in the
studiedarea aroundF0rdefjorden(Figure 1), the
eclogitesare mainly confinedto rocks of the
FjordaneComplex.Becausethe eclogite-facies
metamorphismis Caledonian[Griffin and
Brueckner, 1980], the structural and
metamorphicmodificationsof theserocks are
taken to be of Late Silurian-EarlyDevonian
age. A comprehensive
studyof zone 3 has not
yet been carried out. However, a number of
observations from the coast and road section
along F0rdefjordenclearly distinguishthe
structuralpatternin this zone from that of
zones 1 and 2.
The graniticand migmatiticbandedgneisses
of the Jostedalen
Complex,comprisingmajor
parts of zone 3 in the studiedarea, are
heterogeneously
deformed.Protomylonitic
granitoidgneissesgradeinto fine-grained
mylonites.The high-strainzonesanastomose
between areas in which older structures and
textures,presumablyof Precambrianage, are
preserved.The structuralintensitywithin the
complex apparentlyincreasestoward the illdefinedcontactwith the FjordaneComplexin
which eclogitesare common.A numberof the
high-strainzones have been examinedto
determine their shear sense; no consistent
indicators of noncoaxial deformation have been
found in the zones(Figure 4). Within a single
high-strainzone, kinematic indicators,which are
ubiquitousand consistentwithin zones 1 and 2,
may be absentor show inconsistentsenseof
shear(Figure 5c). Later shearzones,which
usually are narrow and have a more phyllonitic
character,may show a down-to-the-westsense
of sheardevelopedin a simpleshearregime.
Both generations
of high-strainzonesgenerally
strike E-W.
Structuraland metamorphicaspectsof highstrain zonesin and adjacentto the eclogites
south of Naustdal (UTM 236 232), near the
previously mappedcontactbetweenthe
Fjordane and Jostedalencomplexes(Figure 1),
have been studied in some detail. The
mineralogicaland petrologicalcharacteristics
of
theseeclogitesare consistentwith Krogh's
[1980] descriptionof the Sunnfjordeclogites.
Electronmicroprobeanalysesin the present
study show that the preservedeclogite-facies
mineral assemblage(Figure 5a and 5b) consists
of progradezoned(increasingMg/Fe ratio
rimward) garnets,omphacite(Jd 50), phengite
(X celadonite= 0.30-0.35), barroisitic
amphibole,clinozoisite,quartz, and ruffle.
Andersen and Jamtveit: Orogenic Extensional Collapse, Western Norway
1105
Fig. 5. (a) Cut and polishedspecimenof eclogiteboudinin amphibolite-facies
matrix from zone 3 on the north side of FOrdefjorden.Width of photographis 15
cm. Note that conjugateshearbandsin both the eclogiteand the matrix displaces
both to the left (west) and fight (east),indicatingan overall irrotational
deformation.(b) Micrographof eclogitefacies shearbanddefinedby omphacite
(with rim of dark symplectite),clinozoisite,phengite,and lenticularquartz. Shear
bands,displacingin both directions(east and west), anastomose
in the thin section.
Width of the micrographis approximately3 mm. (c) Amphibolitefacies, highstrainzone in quartz-feldspar
augengneissof the JostedalComplexwithin zone 3
in the FOrdefjordenarea. Note that no systematicsenseof shearcan be determined
in this lithology. (d) Micrographshowingthe amphibolitefacies foliation defined
by quartz,plagioclase,epidote,muscovite,and hornblendein a retrogradedeclogite
from zone 3. Dark mineral aggregatesare symplectiteswith quartz, plagioclase,
and hornblendeformedby retrogrademetamorphism
of the eclogite.Width of
micrographis approximately3 mm.
Additional phasespresentas inclusionsin the
garnetsare A1- and Fe-rich sodic-calcic
amphibole,paragonite,and hematite.These
phases,however,are not observedamongthe
matrix minerals.The eclogite-faciesmineralogy
is partly to completelyreplacedby an
amphibolite-facies
mineralogycharacterizedby
the assemblage
plagioclase,calcic amphibole,
epidote,phlogopite,and sphene.
The metamorphicconditionsduring eclogite
formation,basedon the Fe-Mg exchange
reactionbetweencoexistinggarnet and
omphacite,calibratedby Ellis and Green
[1979], and on the jadeite-contentof omphacite
coexistingwith quartz [Holland, 1980], are T =
600 + 25øC, P=• = 16 kbar. The estimated
pressurefor the eclogitesin Sunnfjord
corresponds
to a crustaldepthof approximately
1106
Andersen and Jamtveit: Orogenic Extensional Collapse, Western Norway
50 km. North of the Homelen Basin, however,
the pressureestimatesindicatea crustaldepth
of near 100 km [Smith and Lappin, 1989] and
this suggeststhat areasthat have experienced
major differentialuplifts exist within the WGR.
The tectonometamorphic
relationshipsof the
LiabO eclogitesshow that the high-P
assemblagecrystallizedsyntectonically(Figures
5a and 5b). A L > S fabric definedby
omphacite,clinozoisite,phengite,barroisite,and
lenticularquartz-segregations
was developed.
Textural analysisshowsthat the high-P fabric
comprisesshearbands(C planes)as well as a
foliation defined by crystal shapeand crystal
lattice fabrics (S planes)(Figure 5b). At the
scaleof a thin section,the compositefoliation
showsan inconsistentrelationshipof the acute
angle betweenthe S and C planes.Similarly,
shearbandsobservedat outcropand in hand
specimen(Figure 5a) show shearsensein
oppositedirections.This implies that the
deformationassociatedwith the high-P metamorphismwas characterizedby inhomogeneous
coaxial strainsin a pure shearregime.
The superimposed
amphibolite-facies
metamorphism
characterized
aboveapparently
crystallizedas a resultof nearly isothermal
uplift of the eclogites.Krogh [1980] suggested
T = 550øC and P = 10 to 12 kbar for the
amphibolite-facies
assemblages
superimposed
on
the eclogite-faciesparageneses
in the area.
The amphibolite-faciesfoliation, in which
preservedrelics of eclogitesare found as
boudins,has a pure sheardeformationpattern
similar to the pattern describedabove for the
eclogitefacies deformation(Figure 5d). This
suggeststhat the lower crest in this area
underwentcoaxialdeformationduringuplift
from P > 16 kbar to P -- 10 kbar,
corresponding
to a crustalthinningof at least
15-20
km.
L > S fabricswith E-W lineafionsuggest
that the strains were constrictional
with a
subverticalprincipalaxis of shorteningand an
intermediatestrainaxis producingN-S
compression.In some localities,however, no
obviousstretchinglineafion has been observed,
and this suggestspure shearflatteningstrains;
presentlywe have inadequatedata to evaluate
the role of flatteningversusconstrictional
strains.
A general observation,however, is that the
eclogite-faciesand the early amphibolite-facies
anastomosinghigh-strainzoneswithin zone 3
(Figure 4) developed,apparently,during
nonrotationaldeformationin a pure shear
regime becauseshearbandsin theserocks
define a conjugatepattern.The strainsare
particularlyhigh in quartz-richsegregations
where the rheology of quartz has controlledthe
deformation(Figure 5a). Later greenschist
facies
shear zones in zone 3 have evidence of
rotationaldeformationsimilarto that developed
in zones 1 and 2 of the detachments.
DISCUSSION
AND
CONCLUSIONS
Modeling of the strengthof continentalcrest
suggests
that extensionalcollapseof
overthickened
continentalcrestin orogens
occursby lateral spreadingas a resultof body
forcesin the thickenedorogenicwelt [England
and Houseman,1988; Dewey, 1988]. The array
of extensionalmylonite zonesin west Norway,
which partly truncateearlier compressional
structures and underlie the Devonian basins,
representsevidenceof tectonicthinningof the
Caledoniannappepile. The very deep sections
of the continentalcrest exposedin the WGR in
the footwall of the detachments
providea
uniqueopportunityto studythe geological
processesthat have occurredduring crustal
extensionand uplift. The configurationof
Devonian basins, detachment zones, strain
pattern,and E-W folding is shownin Figure 6.
A popular model for post compressional
extensionof orogensimpliesuniform simple
shearof the continentallithospherefollowing
Wemicke's [1985] model from the Basin and
Range Provincein the westernUnited States.
This model has been adoptedwith few modificationsto explain the extensionand Old Red
basin formation in westernNorway [Norton,
1986, 1987; McClay et al., 1986; S6ranneand
S6guret, 1987; S6guretet al., 1989]. From our
work, mainly adjacentto the Kvamshestenand
Standalendetachmentsit is clear that they
preserveevidenceof a simple shearregime
during their evolutionand that displacements
in
excess of 50 km were accommodated
in these
zones. In the Sunnfjord area of western
Norway, the Middle Devonian basinsare
juxtaposedacrossthe detachmentswith deep
crustcontainingeclogitesthat crystallizedat
pressurescorresponding
to depthsof >50 km.
The presentdistancebetweenthe Devonian
supracrustal
rocks and the eclogite-bearing
complexesis less than 3 km in the area north
Andersen and Jamtveit: Orogenic Extensional Collapse, Western Norway
Pure shear fabric
Kvamshesten
basin
'.%.'o;,.,,,
•.
•...•.• •.
•',',',,;,•.'--.-•
\, • ,
%-- • •-.-.•.
•--,•-• ...• •
'.•:,,•;,_,.';,j\'-,' \
1107
,\
•
-
.":•."-'.,.-."•
•'•
..
•
•
Middle
Upper AIIochthon
Standalen
-,;:--.',,. -'.'-•.....•.- ,'.•., '• • ß'/ ,,•;,-,-.-'_',.•
.....
_....:,•.-'-','-c
X
detachment
Fordefjorden antiform
•-
-
•
•
• >'
//'
FjordaneComplex
'U-, ,-.-.•-,_- '•,.,.,•r.:
c,- %.,•,
-'.:,,,,,-•.-.-,.-'."..•.•,.-:,
.•,'.-•",
Jostedal Complex
Fig. 6. Block diagramwith N-S and E-W sectionsof the centralSunnfjordarea
showingthe relationships
betweenthe lower, middle,and uppercrustalplatesof
the area. Note that the simplesheardomainis developedin the upperpart of the
lower crustandthatpurethe shearregimeoccursin the deepestpartsof the area.
The E-W, nearlyverticalfoldsprovidethe deepsectioninto the lower crustalplate
of the WGR.
of the Kvamshesten Basin. This shows that a
minimum of 40-50 km of crustal thickness has
been excisedalong the detachments.If this
occurredon a uniformly inclined (30ø)
extensionalshearzone penetratingthe entire
continentallithosphere,it would imply a
displacementof the order of 105 km on the
detachment. However, if the extension were
accompanied
by uplift, which is suggested
by
the progressiveretrogrademetamorphismand
which is inherent in the Wernicke model, the
detachmentsurfacewould graduallyattain very
low dips and become subhorizontal.In this
case,the crustalthinningby uniform simple
shearwould be inefficient. Consequently,the
displacementwould have to be increasedby a
factor of 10, dependingon the orientationof
the shear zone in the framework
of time and
amountof extension.This is incompatiblewith
the regionalgeologyin southernNorway and
unacceptablein the presentcase.It seemsclear
that the uplift of the WGR cannotbe explained
fully by Wernicke's [1985] model. However,
the presenceof the major detachments
characterizedby a simple shearregime in zones
1 and 2 of the detachmentclearly showsthat
uniform simple sheardeformationwas
important.A commonfeature of the rotational
deformation
fabric is that it records deformation
mainly at greenschist-facies
metamorphic
conditions.Hence the simple shearmodel
appearsto explain the structuraland metamorphicrelationshipsrecordedin zones 1 and 2
in the upper and middle crust.
As suggestedabove,the eclogite-bearing
rocks in zone 3 underwentinhomogeneous
deformationin a pure shearregime during
eclogite-faciesconditions(P > 16 kbar) and in
the time interval during which they were
isothermallyuplifted to amphibolite-facies
conditions(P = 10-12 kbar). Radiometricage
determinations
from WGR
are numerous and
have been summarizedby Kullerud et al.
[1986]. These data indicate that rocks in the
WGR experiencedeclogite-faciesmetamorphism
at around420 Ma, althoughother workershave
suggestedthat the maximum crustalthickness
was reached at around 450 Ma [Cuthbert et al.,
1983]. From studiesin the hangingwall of the
detachment,it is clear that crustalthickening
went on in Middle Silurian times (= 425 Ma),
as the Solund-StavfjordOphiolite Complexof
Middle-Late Ordovicianage [Dunningand
Pedersen, 1988] was obducted at this time
[Andersenet al., 1990]. The widespread
amphibolite-facies
metamorphismin the WGR
1108
Andersen and Jamtveit: Orogenic Extensional Collapse, Western Norway
occurred at around 400 Ma [Bmeckner, 1972;
Kullemd et al., 1986]. This suggeststhat the
nearly isothermaldecompression
occurredover
a period of approximately20 m.y. Basedon PT estimatesfrom the WGR of the $unnfjord
area, this suggestsa denudationrate of
approximately
1 mm yr'•. This estimateis very
close to the highestrates suggestedfrom the
Himalayaswithin the periodof the last 5 m.y.
[Zeitler, 1985]. Carefully designedcombined
smactural,metamorphic,and geochronological
studiesare necessaryto milne estimatesof
uplift in WGR.
Englandand Houseman[1988] suggestthat
catastrophic
removal of a major part of a
thickenedthermal boundarylayer (TBC) in the
lithosphericmantle will causerapid uplift of the
orogenicwelt [Dewey, 1988; Plan and Vissers,
1989]. The increasedpotentionalenergyof the
welt duringuplift providesbody forcesresulting
in deviatoric
extensional
stresses which result in
spreadingof the welt [Dewey, 1988; Dewey et
al., 1988]. This model is very appealingin the
presentcaseas it explainssatisfatorily
decompression
from eclogite-faciesto
amphibolite-facies,
which occurredisothermally
and rapidly. Dewey [1988] arguesthat the
replacementof the TBL by hot mantle material
will causea higher geothermalgradient
accompaniedby late to posttectonicmagmatism
in the extendinglithosphere.In western
Norway, the presenceof late Caledonian
magmaticrocks has been documented.They
have, however, not yet been systematically
studied and have received
little attention in
geotectonicmodeling. Devonian intermediateto
acid alkaline lavas [Fumesand Lippard, 1983]
occur in the Solundarea and a suite of gabbros
and granitic rocks from the same area is
probablyalso of Devonian age. Late dolerites
of continentaltholeitic affinity occur at several
localities,particularlyin the FlorOarea (Figure
1) [Skjerlie and Tysseland,1981]. A major
calc-alkalinegabbrodated at 380 + 26 Ma by
the SmoNdmethod occursnear Fr0ya. [Fumes
et al., 1989). Also a number of late Caledonian
granodiorificdykes and bodiesoccurin the area
betweenHyllestad and Flor0. Most of the
intmsivesoccurin rocksin middle and upper
crustalposition.Some of the continental
tholeiitedykes,however,intruderocksof both
the uppermiddle and lower crest in the Floro
area (Figure 1). Farther to the north in the
WGR, a numberof minor graniticrocks of
Devonianage have been recognized[Kullemd
et al., 1986, and referencestherein]. It is
suggestedthat the formation of the Devonian
magmaticrocks may be relatedto introduction
of hot mantlematerialto the lower part of the
lithosphere,as proposedby Dewey [1988].
Their very heterogeneous
geochemical
signatures
may be relatedto partial meltingof
a highly variable source,includingcrest and
mantlelithosphereand possiblyalso sublithosphericmantle.The apparentscarcityof
Devonianmagmaticrocks in the lower crustal
rocksof the WGR is enigmaticwith respectto
this model.
A schematicmodel for crustaluplift and
extensionis shownin Figure 7. In stage1, the
compressional
consreaction
of the orogenicwelt
is completed,and the orogencontainsa
smacmralanisotropydefinedby the
compressional
thrust.Stage2 of the model
(Figure 7) representsthe main uplift stage,
causedby TBC removal; this is where vertical
• •:":"::"::•••••
.........................
• Removal
of
thickened
TBL
:: ,-,..-...
-,__e_e__e•-'•...i
__','.',;
,; •',...
.'.•
.........................
.:•:
•...
.:•
:•i!:
..?TBL
thinnea
andmain
..............................
,.
..............................
.,•.........:....._..,-,.,......_.,r,•
ß
....................
:..•::::_.'_:.'..:.:..'.::.r..•r-.•:'
"'base of uplift initiated
'•
3
•
T
'•
•
T
Vertical
pure
shear
in
lower crust at eclogite
facies metamorphism
......'•;;t'i'•';i""l•'•';;'
sh;arin
uplifted lower crust at
4
amphibolite
facies
Reworkingby simpleshear
Penetrative simple shear
fabric at greenschistfacies
Fig. 7. Model, discussedin the text, for
Devonianuplift and extensionin the Norwegian
Caledonides.
Andersen and Jamtveit: Orogenic Extensional Collapse, Western Norway
(•) compressional
stresses
causepure shear
eclogite-faciesdeformationin the lower crest.
The isothermaldecompression
resultsin
amphibolite-facies
overprintingof the eclogitefacies fabric. The observed structures described
above suggestthat a deformationregime of
inhomogeneous
pure shearcharacterizesthe
deepestparts of the crest during the uplift. The
northwestpart of the WGR, in which the
deepestpart of the region is exposed,has not
yet been studied.Observationsfrom Molde area
by Kullemd [1987] show that the gneisseswere
thoroughlyreworkedin amphibolite-facies
shear
zones(T = 650øC,P = 10 kbar) which post
dated the eclogite-faciesmetamorphism.A
characteristic feature of these zones, which are
steeplydipping and have subhorizontal
stretchinglineations,is that they lack consistent
shear senseindicators [Kullemd, 1987], and this
is consistentwith our observations.
Apparently,
they constitutea systemof anastomosing
highstrain zonesup to 1 km wide, which formed
under inhomogeneous
pure shearconstrictional
strains.Stage 3 of the model representsthe
main spreadingevent and showsreworking of
structuresformed in the pure shearregime by
simple shear,also in accordancewith the
observations
described above.
The model that we suggestfor the
extensionalcollapseof the Caledonianorogenic
welt in southernNorway is a combinedpure
and simpleshearmodel. It corresponds
closely
with the model suggestedfor the Tibetan
plateauwhich is basedon lithosphericmodeling
[Englandand Houseman,1988], observationsof
seismicactivity, and surfacegeologyin upper
and middle crustalrocks [Dewey et al., 1988,
and referencestherein].The simple shear
domain in the upper and middle crest was
apparentlyrooted and progressivelycut into the
pure sheardomain of the lower crest as the
uplifted deep crest was cooled from an
amphibolite-facies
to a greenschist-facies
metamorphiccondition.The rapid uplift
recordedby the presenceof eclogitesformed at
extremedepthsin the continentallithosphere
was probablya result of removal by the TBC
layer of the lithosphericmantle.Introductionof
juvenile hot mantle material to the base of the
lithospheremay explain the Devonianmagmatic
rocksin the area. Extensioncontinuedby
deformation
in localized
extensional
shear zones
in an alreadythinnedcrest and resultedin
formationof the Middle Devoniansedimentary
basins.
1109
Acknowledgments.The manuscripthas
benefited from discussionswith Arild Andresen,
Harald Fumes, Eivind Swensson, Hlikon
Austrheim,Henry Berry, Olav Eldholm, and
JohnDewey, who also correctedthe English.
Financial supporthas been receivedfrom
NAVF. Norwegian ILP contribution95.
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Bevegelsesindikatorer
langsKvamshesten
of
standalforkastningene-indikasjoner
p•t senkaledonsknormalog sidelengsbevegelseog
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acceptedMarch 14, 1990.)
(ReceivedSeptember9, 1989;