Labrousse, L, Jolivet, L., Andersen, T.B., Agard, P., Maluski, H. and Schärer, U.

30 Pages·2005·4.46 MB·English

Geological Society of America
Special Paper 380
2004
Pressure-temperature-time deformation history of the exhumation of
ultra-high pressure rocks in the Western Gneiss Region, Norway
L. Labrousse*
L. Jolivet
Laboratoire de Tectonique UMR 7072, UPMC T26E1 case 129, 4, place Jussieu 75252 Paris cedex 05, France
T.B. Andersen
Department of Geology, University of Oslo, P.O. Box 1047 Blindern, 0316 Oslo, Norway
P. Agard
Laboratoire de Tectonique UMR 7072, UPMC T26E1 case 129, 4, place Jussieu 75252 Paris cedex 05, France
R. Hébert
Departement des Sciences de la Terre UMR 7072, Université de Cergy-Pontoise, Le Campus Bat I,
95031 Cergy cedex, France
H. Maluski
Laboratoire de Géochronologie UMR 5573, Université Montpellier II, Place Eugène Bataillon,
34095 Montpellier cedex, France
U. Schärer
Géochronologie–Geosciences Azur UMR 6526, Université de Nice—Sophia Antipolis, 06108 Nice cedex 02, France
ABSTRACT
The Nordfjord area, north of the Hornelen Devonian basin in Western Norway,
is the southernmost part of the Ultra-High Pressure (UHP) Province, de ned by the
occurrence of coesite-bearing and diamond-bearing continental rocks. Compilation
of structural, petrological, and chronological data from the area leads to a model for
the formation of dome structures at the crustal scale and the behavior of the conti-
nental crust during its exhumation from mantle depths. The Nordfjord area appears
as a 100 × 50 km dome-shaped boudin affected by at least two deformation stages.
A stage of E-W stretching and top-to-west shearing produced several envelopes of
migmatitic gneisses bounded by narrow high-strain zones over a core preserving the
Precambrian granulite protolith. This dome is affected by the west-vergent Nordfjord
Mylonitic Shear Zone on its southern limb during late exhumation under the Nord-
fjord-Sogn Detachment Zone. The  rst stage of deformation is coeval with reequili-
bration from maximum pressure conditions around 2.8 GPa, 650 °C (THERMO-
CALC multiequilibrium method) in the coesite stability  eld to higher temperature
and lower pressure conditions (1.8 GPa, 780 °C). Subsequent retrogression was
recorded in the amphibolite facies (0.7 GPa, 580 °C) and in the greenschist facies
(0.4 GPa, 420 °C). Dates for these stages yield exhumation velocities higher than 2
mm/yr. 40Ar/39Ar ages in the area, compared to a spectrum of cooling ages along a
*[email protected]
Labrousse, L., Jolivet, L., Andersen, T.B., Agard, P., Hébert, R., Maluski, H., and Schärer, U., 2004, Pressure-temperature-time deformation history of the exhumation
of ultra-high pressure rocks in the Western Gneiss Region, Norway, in Whitney, D.L, Teyssier, C., and Siddoway, C.S., Gneiss domes in orogeny: Boulder, Colorado,
Geological Society of America Special Paper 380, p. 155–183. For permission to copy, contact [email protected]. © 2004 Geological Society of America
155
156 L. Labrousse et al.
500-km-long N-S pro le, show that cooling of the northern part of the Western Gneiss
Complex is at least 20 Ma younger than in the south. The Western Gneiss Complex
is therefore the result of the late juxtaposition of two complexes, the Northwestern
Gneiss Complex, characterized by UHP relics, constrictive stretching, partial melting,
and doming during a multi-stage exhumation from the deep parts of the orogen, and
the Southwestern Gneiss Complex with Devonian basins, a well-developed detach-
ment system, and distinct high pressure to medium pressure units stacked together
during a single and rapid exhumation stage. The two complexes may represent deep
subduction channel dynamics (north) and shallower wedge circulation (south) in the
Caledonian orogen. The Nordfjord Mylonitic Shear Zone appears as a major tectonic
in the Western Gneiss Complex. Partial melting in the Northwestern Gneiss Complex
may have favored the late exhumation of E-W elongated domes such as the Nordfjord
crustal-scale boudin and their juxtaposition to the Southwestern Gneiss Complex
during top-to-west shearing.
Keywords: ultra-high pressure, exhumation processes, doming, Western Gneiss
Region, Caledonides.
INTRODUCTION present here the results of a multi-method study combining struc-
tural analysis, thermobarometry, and geochronology to propose
During the last two decades, ultra-high pressure (UHP) a model in which syn-collisional upward  ow in the subduction
rocks have been discovered in most of the Alpine, Variscan, and channel and subsequent extensional doming are responsible for
Caledonian orogens (Maruyama et al., 1996; Ernst and Liou, the exhumation of a crustal-scale UHP core, part of the large
2000). Occurrences of UHP index minerals such as coesite and UHP Province of Western Norway.
diamond (Chopin and Sobolev, 1995) have been described in
lithologies of continental af nities such as the pyrope-quartzite GEOLOGICAL SETTING
of the Dora Maira massif (Chopin, 1984), the paragneisses of the
Dabie Shan belt (Okay et al., 1989) and the granodioritic gneisses The Western Gneiss Complex (Milnes et al., 1997) is the
of the Western Gneiss Region (Smith, 1984; Wain, 1998). The deepest exposed unit of the Scandinavian Caledonides (Fig. 1).
burial of low-density material to mantle depths and subsequent Silurian to Devonian metamorphism and deformation in this
exhumation with incomplete retrogression raise questions about segment of Proterozoic continental crust, contemporaneous with
the importance of buoyancy forces in the orogenic wedge. Key southeastward thrusting onto the Baltic shield, is referred to as the
parameters to estimate the force balance are the size, the geom- Scandian phase of the Caledonian orogeny. The latest Caledonian
etry, and the structure of the crustal or lithospheric elements tectonic event recorded in the dominant granitic to granodioritic
involved in the exhumation. gneisses is regional E-W stretching during their equilibration
Structural and petrological analyses in the Dora Maira mas- in the amphibolite facies. This late Scandian extension stage
sif (Henry et al., 1993) and in the Moldefjord area in western resulted in a 35,000 km2 core-complex structure separated from
Norway (Terry et al., 2000b) lead to the conclusion that UHP supracrustal lithologies by the basal Jotunheimen Décollement
units were nappes stacked together in lower grade metamor- Zone in the east, the Bergen Arc Shear Zone in the south, and
phic wedges during exhumation. Thermal considerations for the Nordfjord-Sogn Detachment Zone in the west (Fig. 1). The
preservation of UHP-low temperature (LT) assemblages led to Møre-Trøndelag Fault Zone in the north separates the Western
the idea that they must be rapidly exhumed early in the orogeny Gneiss Complex from the Vestranden Gneiss Complex, recently
(Hacker and Peacock, 1995). Extensional doming, concomitant described as an extensional dome (Braathen et al., 2000).
with thermal reequilibration and partial melting, would therefore Lithological heterogeneities in the dominant amphibolitic
play a minor role in the exhumation process. Nevertheless,  eld gneiss complex recorded the different stages of their Proterozoic
evidence in the Dabie Shan belt indicates that doming and partial and Caledonian tectono-metamorphic history. Kilometer-scale
melting affected the Dabie block and contributed to the exhuma- bodies, preserved from penetrative deformation and refractory to
tion of the UHP paragneisses (Faure et al., 1999; Zhong et al., mineralogical reequilibrations, show mid-Proterozoic HP-granu-
1999). Precise chronological data are clearly necessary to relate litic assemblages in the Flatraket area and even primary igne-
the timing of thermal reequilibration stages and exhumation of ous mineralogy (Krabbendam et al., 2000). Meter-scale ma c
high-grade rocks. lenses, in swarms or isolated in the gneissic matrix, commonly
The Nordfjord area in the Western Gneiss Region is a key preserve eclogitic assemblages in their cores in a 100-km-wide
area for unraveling the structures associated with exhumation of zone along the west coast (Krogh, 1977; Grif n et al., 1985).
UHP rocks and their relationship with shallower structures. We Eclogitic paragenesis described in surrounding felsic gneisses
Pressure-temperature-time deformation history of ultra-high pressure rocks 157
¯
¯
Figure 1. Location map of the different areas
and localities in the Western Gneiss Region.
Simpli ed geological contours after Roberts
and Gee (1985), coesite occurrences after
Wain(1998) and Cuthbert et al. (2000), and
diamond occurrence after Dobrzhinetskaya
et al. (1995). BASZ—Bergen Arc Shear
Zone; L—Lavik; LGFZ—Laerdal-Gjende
Fault Zone; NSDZ—Nordfjord-Sogn De-
tachment Zone.
demonstrate that the whole region has experienced the high basins (Hossack, 1984) in its hanging wall. Top-to-the-west shear
pressure stage recorded by those “external” eclogites (Krogh, and E-W folding of the Nordfjord-Sogn Detachment Zone during
1980a; Krabbendam and Wain, 1997; Wain, 1998). The “inter- the deposition of the Devonian basins (Osmundsen and Ander-
nal” eclogites are also associated with kilometer-scale ultrama c sen, 2001) resulted in the juxtaposition of the deep gneisses in
bodies incorporated into the continental material before or dur- the core of antiforms with the hanging wall material in synforms.
ing the Caledonian subduction (Carswell et al., 1983; Brueckner The evolution of structures from the cores of anticlines toward
and Medaris, 2000; Brueckner et al., 2002). North of Nordfjord, the Nordfjord-Sogn Detachment Zone is therefore representative
more than twenty localities with preserved coesite or coesite of the successive deformations experienced by the gneisses dur-
pseudomorphs (Smith, 1984; Cuthbert et al., 2000; Wain et al., ing their exhumation (Andersen et al., 1994). Early coaxial E-W
2000), occurrences of diamond in gneisses (Dobrzhinetskaya et stretching is progressively overprinted by top-to-west shearing
al., 1995) and in majorite-bearing orogenic peridotite lenses (van when approaching the Nordfjord-Sogn Detachment Zone
Roermund and Drury, 1998; van Roermund et al., 2002) led to (Andersen and Jamtveit, 1990). On the Nordfjord-Sogn Detach-
the de nition of an “UHP Province” from Nordfjord to Moldef- ment itself, brittle faults and mylonitic textures show the latest
jord (Krabbendam and Wain, 1997; Wain, 1998; Wain et al, 2000; localization of deformation (Andersen et al., 1994).
Cuthbert et al., 2000).
The main extensional event structured the whole crust from RECENT SCENARIOS FOR THE EXHUMATION OF
the Western Gneiss Complex to higher structural levels of the THE WESTERN GNEISS COMPLEX AND THE UHP
nappe stack (Andersen, 1998). The main extensional structure in PROVINCE
the Western Gneiss Region is the Nordfjord-Sogn Detachment
Zone (Norton, 1986), in which the top-to-west movement was Recent structural, petrological, and radiochronological
contemporaneous with the deposition of the continental Devonian data lead to different interpretations of depth-time paths for the
158 L. Labrousse et al.
exhumation of UHP rocks in Western Norway. The  rst stages of The dominant forces driving rocks upward depend on the
exhumation are systematically fast and syn-collisional (Fig. 2). inferred geometry of the Caledonian orogen at its climax. The
This stage is responsible for 30 km (Wilks and Cuthbert, 1994) widespread extensional structures recorded by the Western
to 60 km (Terry et al., 2000a) of vertical motion, and brings UHP Gneiss Complex during retrogression into amphibolites were
to HP rocks to a depth of ~60 km in all the models (Fig. 2). Post- explained by models of gravitational collapse during thickening
collisional exhumation, due to changing boundary conditions of the lithosphere (Andersen and Jamtveit, 1990). The northwest-
from convergence to divergence, is correlated in all models with ward polarity both in the peak metamorphic conditions recorded
decreasing exhumation velocity below 3 mm/yr. Estimates of the by eclogites (Krogh, 1977; Grif n et al., 1985) and in the Caledo-
timing of the boundary conditions inversion vary from 425 Ma nian imprint lead to asymmetrical models with continental sub-
for the earliest (Wilks and Cuthbert, 1994) to 395 Ma for the duction plunging to the northwest and subsequent eduction of the
latest (Milnes et al., 1997). Dates on zircons and monazites from Western Gneiss Complex (Andersen et al., 1991) as a coherent
UHP rocks in Western Norway (Terry et al., 2000a; Root et al., portion of continental crust. Eduction may have been triggered
2001) show that burial of continental material was active at least by a change of buoyancy of the subducting lithosphere due to
until 400 Ma, thus con rming the latest estimations of the begin- delamination during convergence (Andersen et al., 1991) or by a
ning of divergence (Milnes et al., 1997). creation of free space by plate divergence (Fossen, 1992, 2000).
ORDOVICIAN SILURIAN DEVONIAN
Llandovery We. Lu. Lower Middle Upper
Pridoli
450 440 430 420 410 400 390 380 370 360
0
Syncollisionnal Ringerike Hitra W.Basins
Age (Ma)
exhumation
} SVWC94
Inversion of plate\
20
relative motion
Pexohsutcmolaltiisoionnnal AWC94AKD98 AAJ90 } H P & U H P wed ge SVT2000
40 Mode II
AM97 AT2000
EAJ90
) 60 Mode I
km (AJ90) EM97 ET2000
( EWC94
h (F92 in M97)
pt
De 80
d 10 mm/a
100 UHPWC94 UUHHPPAK9D198 nit isolate 5 m2m m/am1/ amm/a
u
UHP UHPF2000 M97
120
UHPT2000
(A91)
F2000
WC94
Figure 2. Comparative depth-time paths recently proposed for the Western Gneiss Region. Time-scale after Tucker and McKerrow (1995).
A—amphibolitic stage; E—HP eclogitic stage; GS—greenschist stage; UHP—UHP stage. Index: AJ90—(Andersen and Jamtveit, 1990); A91—
(Andersen et al., 1991); WC94—(Wilks and Cuthbert, 1994); KD98—(Krabbendam and Dewey, 1998); T2000—(Terry et al., 2000a, 2000b);
F2000—(Fossen, 2000); M97—(Milnes et al., 1997). Deposition timing for the Ringerike Group from Bjørlykke (1983), for the Hitra Basin from
Bockelie and Nystuen (1985), for the western basins from Wilks and Cuthbert (1994).
Pressure-temperature-time deformation history of ultra-high pressure rocks 159
Syncollisional gravitational collapse would be compatible with tant for determining whether the Western Gneiss Complex must
the  rst hypothesis, whereas late overall extension would feed be considered as a coherent unit during the Scandian phase.
the second scenario. It is then crucial to constrain the relative
timing of extension in the upper levels of the crustal wedge and FIELD EVIDENCE FOR CRUSTAL-SCALE
subduction of continental material. The change from south- BOUDINAGE AND MIGMATIZATION DURING
eastward thrusting to northwestward extension in the southern EXHUMATION IN THE NORDFJORD AREA
part of the Western Gneiss Region and the overlying nappes
is constrained by 40Ar/39Ar crystallization ages on syntectonic The regional E-W stretching direction observed in the West-
micas between 402 and 408 Ma (Fossen, 2000). The deposition ern Gneiss Region (Andersen, 1998; Fossen, 1992) is the rule
of the western syn-extensional detrital basins may have begun in the central part of the studied area with only local rotation to
as soon as the Praguian-Emsian boundary (ca. 409 Ma; Tucker N040 on the Nordfjord shores and to N140 on Gurskøy Island
and McKerrow, 1995), and the recent ages on UHP rocks (Terry (Fig. 1). We have mapped foliation and stretching lineation tra-
et al., 2000a; Root et al., 2001) indicate that burial was active jectories as well as kinematic indicators in the whole Nordfjord
until at least 400 Ma. There would therefore be an overlap of area (Fig. 3). Foliation trajectories (Fig. 3A) indicate a dome
~10 m.y. between the extension period in shallower levels and structure formed by several units (data from Krabbendam et al.,
active subduction at depth. 2000, and this work):
Sinistral strike-slip between Baltica and Laurentia (Ziegler, 1. A core region (south Stadlandet and Flatraket) where
1985; Torsvik et al., 1996) has been recorded in the Western foliation shows an intense folding pattern around kilometric pods
Gneiss Region both by ductile deformation in the gneisses (Krab- of preserved granulites (Krabbendam et al., 2000);
bendam and Dewey, 1998) and the geometry of Devonian basins 2. Layered metatexites in the Stadlandet, Vanylven, and
(Osmundsen and Andersen, 2001). This implies a non-cylindrical Volda areas, structurally above the Flatraket core and showing
three-dimensional geometry for the exhumation and extension systematic intense E-W stretching and folding of foliation;
processes. The sinistral activation of the Møre Trøndelag Fault 3. A kilometer-scale mylonitic shear band limiting the
Zone in the Devonian (Roberts, 1983) would be responsible for a region to the south, interpreted as the ductile expression of the
component of constriction (Krabbendam and Dewey, 1998; Terry Nordfjord-Sogn Detachment Zone in the gneisses (Krabbendam
et al., 2000b) and for the progressive counter-clockwise rotation and Wain, 1997).
of stretching direction from Sunnfjord to Moldefjord (Krabben- Lithological heterogeneities in gneiss led to boudinage
dam and Dewey, 1998). in the E-W direction from the centimeter (more silicic layers
Most of these models consider the Western Gneiss Com- in the mylonites) to the kilometer scale (pods of granulites in
plex as a coherent body at least during its retrograde history Flatraket area). L-tectonites and folding of foliation along E-
(Andersen et al., 1991; Wilks and Cuthbert, 1994; Milnes et al., W axes (Fig. 4) testify for constriction during extension at all
1997; Fossen, 2000), with continuous gradients from SE to NW scales (Krabbendam and Dewey, 1998). Shear sense indicators
in the equilibrium conditions of eclogites (Krogh, 1977; Grif n in gneisses such as shear bands, asymmetric boudinage, and drag
et al., 1985; Cuthbert et al., 2000), in the intensity of Caledonian folds indicate an overall dextral shear in subvertical layers and
reworking, and in the constrictional component of stretching top-to-west shearing in subhorizontal gneisses. Segregation of
(Krabbendam and Dewey, 1998). partial melt and retrogression of eclogite lenses (Wain, 1997) are
Only scenarios making the distinction of a UHP province coeval with stretching and shearing in the surrounding gneisses.
within the Western Gneiss Complex (Terry et al., 2000b; Wain The construction of the E-W elongated 100 × 50 km dome (i.e.,
et al., 2000) consider it as a composite body. The southern part boudin at the crustal scale) with core-preserving protolith bodies
of the UHP Province in Stadlandet (Fig. 3) is separated from and migmatized rims thus occurred during decompression from
the HP rocks by a large HP-UHP cryptic transition zone (Krab- eclogite to amphibolite facies conditions and is correlated to
bendam and Wain, 1997). Terry et al.’s (2000b) model considers exhumation.
the UHP rocks of Moldefjord as a nappe incorporated late into
a HP wedge. Finite Structure of the Nordfjord Area
The recent description of a large eclogite-bearing gneiss
province in the Laurentian basement exposed in the East Green- The average strike of foliation in gneisses is E-W in the
land Caledonides (Gilotti, 1993) and the discovery of UHP central part of the area with local perturbations and turns to N160
eclogites in these terranes (Gilotti and Ravna, 2002) are new in Stadlandet, N-S in Vanylven, and N140 in Gurskøy, with local
arguments for a wide eclogitic root within a thickened litho- perturbations of the foliation trajectories mainly due to lithologi-
sphere (Ryan, 2001) or for several continental subductions with cal heterogeneities in gneisses. Foliations dip systematically to
different senses (Gilotti and Ravna, 2002). the east in the peninsulas of Stadlandet, Volda, and Vanylven, as
In this context, the study of structures inside the Western well as in the island of Gurskøy, making the Flatraket region the
Gneiss Complex and the precise relationships between UHP deepest unit of the area. There, the granulitic bodies of Ulvesund
eclogite-bearing gneisses and their surrounding rocks is impor- and Flatraket preserved from distributed ductile deformation,
C
n n 10 km

n n
n

n

m n km
m m 10

mm ntavladnni nr o Hn

m n dfjord
Nordin
m m mmm nµ µ n n 3D sketch of the crustal-scale bou
6°00' E ørkedale n B
m Bj m
m m m m m n n n n 10 k

5°00' E5°30' E A mm mGURSKØY mmmmmmmmmVANYLVENSTADLANDETVOLDAm mmBreidteigelva mmµmµDrageµ µµµmµ µµmµ µµAlmklovdalenµµmµ µFlatraketµµµµµµµµµ mµµ µµmµµUlvesund NORDFJORD n n n n n n nn Bortnepollenn nn n nn n n nn n Structural map of the Nordfjord regionFoliation patterns, structural and petrological LegendGraniteboundaries Devonian BasinsQuartzite(Old Red Sandstones) ! Ultrabasite VANYLVEN !Hornelen detachment ! STADLANDETVOLDAEclogiteIntermediate units Amphibolite Nordfjord-Sogn detachment! ! ! UmObserved partial meltingHP MangeritePROVEclogitic lensesINAnorthositeCEUHP eclogitic lenses (1)µLayered gneissLineation direction and UHP/HP TRANSITION ZONEsense of shearGranodioritic gneissFoliation planeMylonitic gneiss Foliation patternAugen gneissHP PROVINCENMSZVertical foliationTNEMMuscovite gneissHCATED NGBoundaries of structural units (this study)OS-Granitic gneissDROJFDRONBoundaries of UHP, HP and UPPER UNITStransitional zones (Carswell et al., 2000)
n
N N N
62°10' 62°00' 61°50'
Figure 3. A. Structural map of
the Nordfjord area. Litholo-
gies modi ed from Norges A
Geologiske Undersøking
maps (Kildal, 1970; Lutro et
al., 1998; Lutro and Tveten, ෴
(cid:56)
1998; Tveten et al., 1998);
structural data compiled from ᝓ(cid:1)ᙶ֊(cid:24)֊(cid:1)(cid:25)֊ᙶ
this study and Bryhni (1966)
(cid:58)ᥴ
and Krabbendam and Wain
(1997). Coesite occurrences (cid:58)(cid:59) (cid:66)
from Wain (1997), Smith
(cid:66)
(1984), and Cuthbert et al.
(2000). UHP, UHP-HP and
ᇹϻ๛(cid:70)⃪(cid:69)⑊⋿(cid:66)␦(cid:1)
HP zones limits after Cuth- C ⅋(cid:70)ỹ(cid:66)᥏⃪(cid:66)⑊⓿⑊
bert et al. (2000). B. Foliation
trajectories for the differ- ״֊(cid:1)(cid:68)⅋
ent units. C. Interpretative ᝓ(cid:56) ፼(cid:1)ᙶ֊(cid:24)(cid:25)(cid:1)ݏ֊(cid:56)
diagram of the crustal-scale
B
boudin structure of the Nor- ᥴ(cid:59)
dfjord area. A larger size ver-
sion of this  gure is included
on the CD-ROM accompany-
ing this volume.
෴(cid:66)␦⃪➾(cid:1)ᙶϻᝓ(cid:1)▟(cid:70)Ↄ⑊(cid:1)ỹ(cid:66)⓿῾(cid:70)␦(cid:70)(cid:69)
፼(cid:66)⓿(cid:70)(cid:1)ᙶϻᝓ(cid:1)▟(cid:70)Ↄ⑊
෴(cid:66)␦⃪➾(cid:1)෴ϻ(cid:56)(cid:1)▟(cid:70)Ↄ⑊(cid:1)⑊⓿␦(cid:70)⓿(cid:68)῾(cid:70)(cid:69)(cid:1)
D E
(cid:56)
(cid:69)
״֊(cid:1)(cid:68)⅋
(cid:71)
F
௺⅋⋿῾᥏⇿⃪⓿(cid:70)(cid:1)⃪(cid:70)Ↄ⑊(cid:70)⑊ (cid:56)
G (cid:70)(cid:66)␦⃪➾(cid:1)⏐╿(cid:66)␦⓿⣿(cid:1)▟(cid:70)Ↄ ፼ᙶ֊(cid:26)ۿ
ỹ(cid:66)⓿῾(cid:70)␦(cid:70)(cid:69)(cid:1)
෴
(cid:70)
ໝↃ(cid:70)⑊⑊(cid:68)(cid:1)⃪(cid:70)Ↄ⑊
״(cid:1)⅋
Ᏼ(cid:68)(cid:66)⑊(cid:68)῾⑊⓿⑊
Figure 4. Field evidence of stretching and constriction. A, B. Detailed photograph and outcrop sketch in Bortnepollen. Augen gneiss shows a
penetrative E-W stretching lineation and a discrete foliation. C. Layered gneiss outcrop photograph in Flatraket showing constrictive folding
parallel to E-W stretching direction. D, E, F, G. Photographs and outcrop sketches in the Nordfjordeid region showing folding of early veins (C)
and N-S late veins (D) perpendicular to stretching direction.
162 L. Labrousse et al.
as well as from Caledonian prograde and retrograde metamor- The stretching lineation is penetrative in most gneiss litholo-
phism, induce large folding of the wrapping gneiss (Krabbendam gies (Fig. 4A, B). Constrictive strain ellipsoids can be locally
and Wain, 1997) and rotation of the fold axis from E-W to N-S deduced from L-tectonites in augen gneiss. The direction of
locally (Fig. 3B). This region contrasts with the regularly layered lineation turns from N140 in Gurskøy to N090-080 in the major
Stadlandet unit, which is directly above the Flatraket core. They part of the studied area, following the sigmoidal shape of the
are separated by a layer of  nely layered gneiss with abundant foliation pattern in the Nordfjord-Sogn Detachment Zone. The
sheath folds (Fig. 5C, D), mapped as a mylonitic shear zone average plunge of the lineation is 10° E and rarely exceeds 30°
(Lutro et al., 1998). Sheath folds are markers of intense shear E. The stretching direction is also indicated by the geometry of
(Cobbold and Quinquis, 1980) along the boundary between Fla- sheared quartz veins (Fig. 4D, F). When initially parallel to the
traket and Stadlandet units. A second strip of mylonites (Lutro et stretching direction (e.g., E-W), the earliest veins are thinned and
al., 1998) corresponds to a break in the foliation trajectories in truncated, but are tightly folded when perpendicular to stretching
the Åheim region between the units of Stadlandet and Vanylven. (e.g., N-S).
Further east, the Volda peninsula and Gurskøy island unit is the
uppermost structural unit with foliation strike turning to N140. Rotational Deformation and Instantaneous Strain
On the Nordfjord shores, foliation shows a rotation in strike
from N080 in the north to N040 and then back to N090 along the The rotational component of deformation is expressed in
Nordfjord-Sogn Detachment Zone proper, drawing a 10 km thick gneisses by C and C! shear bands (Fig. 5A, B), asymmetric pres-
dextral shear-band interpreted as the ductile expression of the sure shadows around garnets, mica  shes, and "- and #-rotated
Nordfjord-Sogn Detachment Zone in its footwall (Wain, 1998; objects. Asymmetric boudinage of more viscous horizons in the
Krabbendam and Dewey, 1998). gneiss (Fig. 6) is used as a criterion for sense of shear when the
A ໝ(cid:66)␦Ↄ(cid:70)⓿ϻ᥏(cid:70)(cid:66)␦Ↄỹ(cid:1)ỹↃ(cid:70)⑊⑊
ᝓ(cid:1)ᙶ״ޱ(cid:25)(cid:1)ޱ֊෴
(cid:56) ෴
(cid:66)
B ״֊(cid:1)(cid:68)⅋
C ෴ϻ(cid:56)(cid:1)(cid:66)⚡⑊ Figure 5. Field evidence of shear cri-
ᙶ ⑊῾(cid:70)(cid:70)⓿῾(cid:1)(cid:71)⇿⃪(cid:69)⑊ ᝓ teria. A, B. Photograph and outcrop
sketch of top-to-west shear-bands in a
garnet-bearing gneiss in Breidteigelva.
C, D. Photograph and outcrop sketch of
(cid:68)
E-W trending sheath folds in migmatitic
gneiss in the Stadlandet-Flatraket high
shear zone. E. outcrop sketch of sinistral
shear criteria in Nordfjordeid.
ޱ֊(cid:1)(cid:1)(cid:68)⅋ D
෴ ⑊Ↄ⑊⓿␦(cid:66)⃪(cid:1)⑊῾(cid:70)(cid:66)␦(cid:1)(cid:68)␦⓿(cid:70)␦(cid:66) (cid:56)
E
״(cid:1)⅋
s. nt
෴ỹↃ(cid:70)⑊⑊(cid:1)⃪(cid:66)➾(cid:70)␦⑊ ⅋⋿῾᥏⇿⃪⓿(cid:68)(cid:1)ỹↃ(cid:70)⑊⑊ ᜔╿(cid:66)␦⓿⣿ϻ␦(cid:68)῾(cid:1)ỹↃ(cid:70)⑊⑊(cid:1)⃪(cid:66)➾(cid:70)␦⑊ yers preserving eclogitic paragenesic block-faulting of a more compete
ϻ␦(cid:68)῾(cid:1) ௺ sic lantheti
╿(cid:66)␦⓿⣿ etabaD. Sy
᜔ d mar.
ee
uncatof sh
y trnse
allse
metricextral
md
ya
of ascate
h di
pn
Photogramarkers i
௺⅋⋿῾᥏⇿⃪⓿(cid:68)(cid:1)ỹↃ(cid:70)⑊⑊ ޱ(cid:24)֊ᙶ ᙶ״״(cid:21) ᙶ״(cid:21)(cid:25) (cid:68)(cid:69)֊ݏ֊ᙶ ෴(cid:68)⃪⇿ỹ⓿(cid:70)(cid:1)⃪(cid:66)➾(cid:70)␦⑊C ௺⅋⋿῾᥏⇿⃪⓿(cid:68)(cid:1)ỹↃ(cid:70)⑊⑊D age at any scale in Verpeneset. A. Outcrop sketch. B. z-rich layer. Asymmetry of boudins and de ection of
boudind quart
᥏ ޱ⅋ 6. Field evidence of ograph of a truncateolitic gneiss layer.
(cid:56) A B Figure C. Photamphib
164 L. Labrousse et al.
de ection of markers in the boudins is synthetic to the sense of sinistral in the regions of subvertical foliation. The asymmetric
rotation of the blocks (Grasemann and Stüwe, 2000). The asym- amphibolitized rims of metabasic lenses included in the gneisses
metry of meter-scale inclusions and particularly metabasic lenses are concordant with local senses of shear, as observed at the
(Fig. 7B) is systematically consistent with other shear sense Drage site in Stadlandet (Fig. 7A, B).
criteria at the outcrop scale and can therefore be used as a kine- Top-to-west shearing is thus contemporaneous with retro-
matic indicator. In the northern part of the area, rotational criteria gression of eclogites to amphibolites. The three eastern units
are mostly to the west in shallow-dipping, foliated gneisses or have been sheared toward the west over the Flatraket deeper unit
B A
(cid:70)(cid:68)⃪⇿ỹ⓿(cid:68)(cid:1)(cid:68)⇿␦(cid:70)⑊
(cid:56) ෴
(cid:66)⅋⋿῾᥏⇿⃪⣿(cid:70)(cid:69)(cid:1)␦⅋⑊ (cid:66)⑊➾⅋(cid:70)⓿␦(cid:68)(cid:1)⃪(cid:70)Ↄ⑊(cid:70)⑊
᥏
(cid:56) ෴ (cid:68) (cid:69)
ۿ(cid:1)⅋
C
״(cid:1)⅋
(cid:56) ෴
⋿␦(cid:70)⑊(cid:70)␦▟(cid:70)(cid:69)(cid:1)
⃪(cid:70)╿(cid:68)⇿⑊⇿⅋(cid:70)(cid:1)(cid:68)⇿⃪⃪(cid:70)(cid:68)⓿(cid:70)(cid:69) (cid:70)(cid:68)⃪⇿ỹ⓿(cid:68)(cid:1)(cid:68)⇿␦(cid:70)⑊
Ↄ(cid:1)⋿␦(cid:70)⑊⑊╿␦(cid:70)ϻ⑊῾(cid:66)(cid:69)⇿◿⑊
(cid:21)֊(cid:1)(cid:68)⅋ (cid:56) ␦(cid:70)⓿␦⇿ỹ␦(cid:70)⑊⑊(cid:70)(cid:69)(cid:1)␦⅋⑊ ෴
D (cid:70)
E
״(cid:1)⅋
⑊╿␦␦⇿╿Ↄ(cid:69)Ↄỹ
⅋ỹ⅋(cid:66)⓿⓿(cid:70)⑊
ᙶ״ۿ֊ (cid:26)֊ F
֊(cid:24)(cid:1)֊
ᙶ ֊(cid:26)֊
ᙶ
Figure 7. Field evidence for recording of the late Caledonian deformation by eclogite lenses in the Nordfjord area. A. Outcrop overview in Drage
(Stadlandet). B. Photograph of asymmetric metabasic lenses. C. Detail sketch of deviated eclogitic foliation around the boudin rim. D. Detail
sketch of asymmetrical boudin. E. Photograph of pegmatitic pressure shadows around metabasic lenses. F. Outcrop sketch showing asymmetric
metabasic lenses in a sheared migmatite (Gurskøy).