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Tectonophysics xx (2005) xxx – xxx
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Eclogite-facies polyphase deformation of the Drbsdal eclogite,
Western Gneiss Complex, Norway, and implications
for exhumation
Ruth Foremana,*, Torgeir B. Andersenb, John Wheelera
a
Department of Earth and Ocean Sciences, University of Liverpool, 4 Brownlow Street, Liverpool, UK
b
Institute for Geology and PGP, University of Oslo, PO Box 1047, Blindern, Oslo 0316, Norway
Received 24 October 2003; accepted 1 October 2004
Abstract
Exhumation of the deep crust during orogenic extension is accepted as a geological phenomenon, but structures formed
during burial and the earliest stages of exhumation are often overprinted by deformation events occurring at shallower crustal
levels. The Western Gneiss Complex (WGC) of Norway comprises variably retrogressed high-pressure (HP) and ultrahighpressure (UHP) eclogite bodies enclosed in predominantly felsic amphibolite-facies rocks. The Drbsdal body, located in the
Sunnfjord area, is one of the largest and best-preserved eclogites in the WGC, and comprises over ca. 3 km2 of exposed
eclogite-facies rocks. It is an excellent example of an eclogite tectonite, and displays a wealth of structures formed during
deformation at a minimum depth of 50 km and peak temperatures of ca. 800 8C. Large volumes of mylonites with the eclogitefacies assemblage garnetFclinopyroxeneFquartzFamphiboleFclinozoisiteFphengiteFkyaniteFrutile are preserved. Structures associated with the eclogite-facies metamorphism include E–W-trending isoclinal folds, boudinage, hinge-parallel
lineations, and meter-scale kyanite-dominated veins. The Drbsdal body was tightly folded on the kilometer scale under eclogitefacies conditions. Subsequently, the shapes of the Drbsdal body and other mafic bodies in the Sunnfjord area were modified
during eclogite-to-amphibolite-facies E–W-directed stretching, to give boudin-like lenses. The timing of formation of a
pervasive eclogite-facies lineation, eclogite-facies folds, and kyanite bearing veins overlapped substantially, and this portion of
the deformation history was dominated by a constrictional strain field. Partitioning of deformation occurred after a substantial
amount of eclogite-facies deformation had already taken place, and resulted in the relative rotation of linear features in specific
zones on the scale of ~400 m. The eclogite-facies lineations and fold hinges within the Drbsdal body are subparallel to
amphibolite and greenschist-facies structures throughout the WGC. Although circumstantial, this suggests that structures
belonging to each of these three metamorphic facies formed during one progressive deformation event corresponding to
extensional exhumation. Our observations are consistent with models involving inhomogeneous deformation of the lower crust
* Corresponding author.
E-mail address: ruth_wings@yahoo.co.uk (R. Foreman).
0040-1951/$ - see front matter D 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.tecto.2004.10.003
TECTO-07305; No of Pages 32
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during early orogenic extension. We envisage that transtensional (noncoaxial) deformation dominated the extensional
deformation of the WGC, but that coaxial deformation must have occurred locally.
D 2004 Elsevier B.V. All rights reserved.
Keywords: Eclogite; Exhumation; Extension tectonics; Folds; Transtension; Western Gneiss Region
1. Introduction
Products of high-pressure (HP) and ultrahighpressure (UHP) metamorphism are exposed in orogenic belts worldwide, and are formed during
episodes of subduction and extreme crustal thickening
(Chopin, 1987; Andersen et al., 1991; Carswell et al.,
2003; Rey et al., 1997; Liou et al., 2004). Unravelling
the dynamics of their formation and subsequent
journey to the Earth’s surface is essential if we are
to understand the processes operative in the deep crust
during orogenesis. Large volumes of UHP and HP
rocks are exposed over an area of approximately
45,000 km2 in the Western Gneiss Complex (WGC),
Norway (Fig. 1). The WGC is largely composed of
Proterozoic quartzo-feldspathic gneisses of granitic to
granodioritic compositions; minor amounts of anorthosites, mafic rocks, ultramafic rocks, and metasediments are also present (Tucker et al., 1990). Extensive
reworking occurred during the Scandian phase of the
Caledonian orogeny (Kullerud et al., 1986; Milnes et
al., 1997), and amphibolite-grade assemblages now
Fig. 1. Simplified geological map of Sunnfjord and the surrounding area, SW Norway (compiled from maps of Engvik and Andersen, 2000;
Krabbendam and Dewey, 1998).
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dominate the WGC. The UHP and HP metamorphism
is Scandian in age (420–400 Ma; Gebauer et al., 1985;
Griffin and Brueckner, 1980), and occurred during the
collision between Baltica and Laurentia. The presence
of such deeply formed rocks at the Earth’s surface
requires exceptional geological circumstances, and
must involve exhumation along a P–T path that
permits metastable behaviour of particular lithologies
over a wide range of metamorphic conditions (Austrheim and Engvik, 1997; Rubie, 1990; Wayte et al.,
1989). Assuming that erosion alone is not sufficient to
expose HP/UHP rocks at the surface, a contribution
from deformation-related processes must be considered. Moreover, the large amount of deformation
involved in the transfer of HP/UHP rocks to the
uppermost crust appears, in many cases, to have
dismembered crustal fabrics and enhanced retrogression to lower-grade assemblages. Accordingly, the
majority of felsic gneisses and some of the mafic
material of the WGC have experienced retrogression
to amphibolite-facies assemblages (Krabbendam and
Wain, 1997). Even so, large and superbly wellpreserved HP, and smaller UHP bodies characterise
the WGC. Exceptionally, some of these bodies have
maintained eclogite-facies structural relationships in
their interiors; these features are rarely seen in UHP
provinces worldwide. Many recent models for exhumation of the WGC involve extensional deformation
in the upper crust, coupled with an overall coaxial
deformation in the lower crust (Andersen and
Jamtveit, 1990; Andersen et al., 1994; Jolivet et al.,
1994). Krabbendam and Dewey (1998) proposed that
exhumation occurred via transtensional (noncoaxial)
deformation. Rey et al. (1997) proposed that plate
divergence caused extensional exhumation of the
WGC, and was triggered by the Variscan collision
between Laurasia and Gondwana.
There are two main issues to resolve concerning the
HP and UHP rocks of the WGC: (1) exhumation of the
HP rocks must be explained, and (2) links between the
HP and UHP and their juxtaposition must be
explained. Possible explanations for the close juxtaposition of preserved HP and UHP assemblages
include those of Wain (1997) and Terry et al.
(2000a,b). Wain (1997) defined a UHP province,
using observations of relic coesite and pseudomorphs
after coesite as evidence for UHP metamorphism of
eclogites along with the surrounding gneisses, or din
3
situ.T In this scenario, the boundary between this UHP
province and the HP portion of the WGC is interpreted
as a folded or imbricated tectonic break, along which
pressure jumps of ~3 kbar are recorded, giving a
bimodal trend in P–T estimates in this dmixed zone,T
and the HP and UHP units were juxtaposed prior to a
late phase of overprinting. The model of Terry et al.
also focuses on explaining the close juxtaposition of
UHP and HP rocks (~40 and ~20 kbar, respectively),
and follows Wain (1997) in separating the UHP and
HP rocks into two distinct tectono-stratigraphic units
with different P–T histories. This model can be
summarised as a two-stage exhumation history in
which the HP and UHP rocks originally reside in
different, distinct crustal blocks separated by a major
structural discontinuity. During the initial stage, the
UHP block is exhumed from a maximum depth of 125
to ~60 km, and top-southeast thrusting and imbrication
brings the UHP rocks into contact with the HP block.
The second stage involves exhumation of the juxtaposed UHP and HP blocks to a depth of ~40 km,
accompanied by reequilibration and top-to-west extensional shearing. Both models have drawn heavily on
studies of the mineral chemistry, structure, and
regional P–T distribution of the UHP and HP rocks.
More recently, evidence of UHP metamorphism has
been found in mafic bodies over a much broader area
than previously anticipated, and UHP indicators have
even been found in the Verpeneset body, hitherto
considered a type example of an HP WGC eclogite
(Carswell et al., 2003). In the light of this new
evidence, it may be necessary to reappraise the models.
Perhaps mixed HP and UHP rocks occur over a larger
area than can be accounted for by models involving the
tectonic juxtaposition of several distinctly different
units; the possibility that there is no major tectonic
break between bodies recording HP and UHP metamorphism in the WGC must be considered. The
observation of a large coherent mixed zone in which
mafic bodies preserve UHP or HP or amphibolitefacies metamorphism depending on their shapes, sizes,
compositions, and distances from major detachment
horizons is perhaps a closer match to the evidence. The
implication is that metastability and nonreaction as
documented in a number of papers on the eclogites of
the Lind3s nappe in the Bergen area as well as in the
WGC (cf. Austrheim and Engvik, 1997) may be more
important than previously suggested, and it is therefore
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difficult to explain the variable maximum pressure
record of a large portion of the WGC by tectonic
juxtaposition as discussed in previous papers (Terry et
al., 2000a,b; Wain, 1997; Wain et al., 2001). Regardless of the details, the history of the UHP rocks of the
WGC cannot be understood without reference to the
HP history.
The Drbsdal mafic body, in Sunnfjord (Fig. 2), is
the focus of this study. It is an exceptional example of
an HP tectonite because it displays several generations
of mesoscale and macroscale structures that were
formed at different times but all within eclogite-facies
conditions. It is among the largest HP occurrences in
the WGC, yet the body had not been mapped in detail
until this study. In fact, it is, to our knowledge, one of
the largest pristine eclogite bodies recorded worldwide. For example, the largest known eclogite body in
the Dabie Shan HP/UHP terrane is also an HP body,
and is 200 m1 km in extent (Castelli et al., 1998; L.
Jiang, personal communication). The Cabo Ortegal
eclogite is the only recorded larger occurrence and is
considerably larger (100 m thick and 17 km long;
Abalos, 1997), but is heavily overprinted. Exceptional
preservation of the V3rdalsneset eclogite body, a
nearby HP tectonite, has been previously reported
by Engvik and Andersen (2000). The V3rdalsneset
eclogite body is similar to the Drbsdal body in many
aspects of its petrography and has some similar
structural characteristics (Engvik and Andersen,
2000); however, only a small area is exposed, and
therefore the size and context of the body are unclear.
In contrast, the whole of the Drbsdal body is more
accessible and can therefore be placed in context.
Structural analysis of the Drbsdal body is applied
alongside petrological evidence, and reveals that this
part of the WGC was both pervasively folded and
extended on the kilometer scale during residence in
the deep crust.
Fig. 2. Simplified geological map of Sognefjord–Dalsfjord region, SW Norway, showing the relationship of the Drbsdal mafic body (D) to
related mafic bodies and regional-scale structures, including the V3rdalsneset eclogite body (V) (modified after Engvik and Andersen, 2000).
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2. Geologic setting
The Drbsdal mafic body is situated in Sunnfjord,
western Norway, and is surrounded by amphibolitefacies granodioritic gneisses of the WGC (Fig. 1). The
WGC constitutes the lowest exposed tectonic unit in
the hinterland of the Norwegian Caledonides, and is
characterised by the preservation of relatively small
lenses of eclogite-facies material within large tracts of
amphibolite-facies gneiss. The Drbsdal mafic body is
situated in the southerly dipping limb of a regionalscale antiform (Hacker et al., 2003), with the lowgrade Devonian Solund and Kvamshesten Basins
exposed in complimentary synforms ~5 km to the
south and ~15 km to the north, respectively (Fig. 1).
Orogen-scale extension on the Nordfjord–Sogn and
Hornelen Detachment Zones (NSDZ and HDZ,
respectively) is at least partially responsible for
exhumation of the WGC, which now lies in its
footwall (Fig. 3). The NSDZ is well exposed at key
localities beneath the hanging wall. It has carried the
hanging-wall basins down to the west, and in the
Sunnfjord area, the detachment zone comprises ~3–5
km thickness of brittle–ductile mylonites (Andersen
and Jamtveit, 1990). There is an indistinct transition
with increasing depth from the NSDZ mylonites into
the heterogeneous basement gneisses and eclogites of
the WGC (Norton, 1987). The Drbsdal body belongs
to a suite of mafic bodies that have been variably
5
tectonised, and range in character from relatively
undeformed gabbros and coronitic eclogites (Engvik
and Andersen, 2000; Mbrk, 1985), to true eclogites
and eclogite tectonites of variable composition. The
mafic bodies are included as pods and lenses in the
amphibolite-facies gneisses (Figs. 2 and 3). These
mafic bodies also preserve metastable mineral
assemblages related to different stages in the P–T
history of the WGC, including igneous material,
granulite-facies assemblages, and variably deformed
eclogites. In addition to P–T controls, fluid budget is
thought to have had a large control on the initiation
and progress of metamorphic reactions within these
mafic bodies, and in turn on deformation. No
precursors to the eclogite-facies rocks have been
found within the Drbsdal mafic body, but the spatial
distribution and petrographic characteristics of other
mafic bodies in Sunnfjord suggest that these mafic
bodies were intruded into felsic gneisses as a series of
dykes, sills, and larger igneous complexes (Engvik et
al., 2001), and passed through amphibolite- and
granulite-facies before the onset of eclogite-facies
metamorphism (Austrheim and Engvik, 1997; Engvik
et al., 2001). Indeed, Krabbendam et al. (2000) and
Wain et al. (2001) have demonstrated that some WGC
rocks preserve Pre-Caledonian igneous and granulitefacies assemblages. Over 90% of the Drbsdal mafic
body preserves eclogite-facies assemblages and structures, and retrogressive amphibolitisation is largely
Fig. 3. Schematic vertical section showing the structural relationships of the different crustal units in the Hyllestad–Sunnfjord area. Vertical
distances calculated from maps presented herein and those presented by Hacker et al. (2003).
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confined to zones of contact with the surrounding
granodioritic gneisses.
3. Petrography and mineral chemistry
In this section, our aim is to demonstrate that the
rocks at Drbsdal preserve eclogite-facies assemblages;
we will not give a comprehensive metamorphic
history.
3.1. Petrography
The interior of the Drbsdal body comprises rock of
fine to medium grain size, and characteristically
displays eclogite-facies assemblages with little or no
evidence of retrogression. No protolith to the eclogitefacies rock has been observed in the Drbsdal body;
eclogitisation was complete and accompanied by
pervasive deformation. Garnet is always present in
the eclogite-facies assemblage, along with omphacite,
zoisite, quartz, and rutile. Amphibole, phengitic mica,
and kyanite occur as accessory minerals in textural
equilibrium with some or all of the eclogite-facies
minerals. The assemblage varies considerably
between different compositional bands, and can also
vary within and between parts of the mafic body with
different structural trends (Section 4). A gradual
transition to a fine-grained green–black amphibolite
is typically observed at the margins of the mafic body.
This is interpreted to be a retrogressed variety of the
eclogite. Over distances between 5 and 100 m, this
mafic amphibolite passes into a lithology with the
granodioritic amphibolite-facies assemblage quartz,
plagioclase feldspar, potassium-rich feldspar, white
mica, biotite mica, and, in some places, garnet and
sphene.
Brief descriptions of the rock types found within
and around the Drbsdal mafic body and preliminary
P–T estimates follow. Rock types are grouped
7
according to field observations, but descriptions also
include optical microscope observations and chemical
data obtained via microprobe analysis. Mineral
analyses were performed on a Cameca SX100
electron microprobe (wavelength dispersive system)
at the Department of Geology, University of Oslo,
using natural and synthetic mineral standards. An
accelerating voltage of 15 kV was used, with a beam
current of 10 nA and counting time of 10 s for all
analyses. A focused electron beam was used for all
minerals except sheet silicates and feldspars, for
which the beam was defocused to 10 Am diameter.
Amphibole compositions are classified according to
the nomenclature of Leake et al. (1997); clinopyroxene compositions are classified according to the
nomenclature of Morimoto et al. (1988).
Lithologies are grouped as follows: (1) gneissic
eclogites, (2) partially retrogressed eclogites, (3)
amphibolites, and (4) granodioritic amphibolite-facies
rocks. Minor volumes of other rock types are found
within the mafic body, and these are described briefly
for completeness.
3.1.1. Gneissic eclogite
The name gneissic eclogite covers the majority of
rocks in the Drbsdal body, perhaps as much as 80% or
90% of the total volume (Fig. 4A and B). The gneissic
eclogite is characterized by a strong compositional
foliation and a strong pervasive type I lineation
(Section 5.2). Within the gneissic eclogites, garnet,
omphacite, quartz, zoisite, and rutile exist in textural
equilibrium. Kyanite or amphibole may be present
as part of an equilibrium assemblage, although may
also have formed later. Garnet generally occurs as
euhedral grains of up to 1 cm in size. Most garnet
grains larger than 0.5 mm contain numerous inclusions of quartz and zoisite, and occasional pargasitic
amphibole in their cores. Complex compositional
zoning is a conspicuous feature of garnet in the
gneissic eclogites, and is observed in all but the
Fig. 4. Photomicrographs showing typical mineralogy and textures in rocks from the Drbsdal eclogite body. Each pair shows the same area.
Double-headed arrows show lineation direction. Field of view 4 mm width for all photomicrographs: (A) gneissic eclogite (sample P10) viewed
in cross-polarised light (XPL); (B) same view in plane-polarised light (PPL); (C) partially retrogressed eclogite (sample R18a, viewed in XPL);
(D) same view in PPL; (E) amphibolite (sample SK10, viewed in XPL); (F) same view in PPL; (G) granodioritic amphibolite-facies rock
(sample OL6, viewed in XPL); (H) same view in PPL. Abbreviations: amphibole (Amp); biotite (Bt); diopside (Di); epidote (Ep); garnet (Grt);
matrix of plagioclase, quartz, magnesiohornblende, alkali feldspar, and epidote (Mx); omphacite (Omp); phengite (Phg); plagioclase (Pl); quartz
(Qtz); zoisite (Zo).
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smallest of grains (b0.2 mm). Broadly, zoning varies
from core (Alm42–56, Prp14–30, Grs20–27) to rim
(Alm41–48, Prp26–39, Grs10–22) in a fashion commonly
thought to represent prograde growth, but more
detailed analysis reveals a much more complex
pattern than can be explained by a simple model of
prograde growth. Discussion of this problem is
beyond the scope of the present work, and will be
addressed in a separate study, although it is worthwhile to note that since any estimate of P–T assumes
chemical equilibrium between the phases used in the
calculation, P–T estimates for these rocks should be
viewed with considerable caution.
Omphacite grains (Jd29–45, Ae2–25; Na2O content
6.3–7.3 wt.%) generally show a strong shapepreferred orientation, and define a strong lineation,
along with zoisite/clinozoisite and occasional kyanite.
The pervasive centimeter-scale to meter-scale foliation defined by variations in the relative abundances
and grain sizes of the eclogite-facies minerals is
mimicked at the submillimeter scale. Kyanite in the
gneissic eclogite is commonly associated with clinozoisite/zoisite-bearing layers, and it is in these layers
that the shape-preferred orientation is strongest.
Zoisite within these layers is strongly prolate in
shape, and frequent fractures and zones with a high
density of fluid inclusions, interpreted to be dhealedT
fractures, transect zoisite grains at a high angle to the
lineation direction.
Omphacite grains are in many places surrounded by
a rim of symplectite, especially so in quartz-rich
samples. The symplectite consists of delicately intergrown diopside (Jd3–35, Ae1–14; Na2O content 1.48–6.6
wt.%) and plagioclase (Ab60–87, An13–25), and is
interpreted to have grown during static posteclogitefacies conditions.
Barroisitic amphibole often occurs in the matrix of
the eclogite-facies assemblage. It is variably abundant,
and generally shows a similar shape-preferred orientation to clinopyroxenes within the foliation. Commonly, poikiloblastic amphibole grains occur in the
eclogite, with garnet (Alm40–43, Prp34–36, Grs19–20) as
the only included phase. Kyanite occasionally occurs
in these samples as large poikiloblasts, and carries
inclusions of quartz, garnet (Alm41–45, Prp30–34,
Grs17–24), and omphacite (Jd43–47, Ae0–4; Na2O content 6.5–7.0 wt.%) with a shape-preferred orientation
subparallel to lineation.
Grains of phengitic mica are generally present in
small amounts in most samples, and are aligned
within the foliation. Phengite is more abundant in the
margins of the mafic body and is particularly
abundant in samples from the margins of eclogitefacies veins, especially those bearing kyanite and
quartz. It is therefore likely that these phengites are
products of eclogite-facies hydration reactions facilitated by the local availability of a fluid phase. Since
additional potassium is also required, a potassium
source such as a potassium-bearing metasomatic fluid
is also required for new phengite to be produced.
Rutile occurs as individual grains or strings of grains
aligned parallel to lineation.
3.1.2. Partially retrogressed eclogites
These rocks represent partially retrogressed lithologies occurring at the margins of the mafic body,
and contain substantial amounts of minerals typical of
the amphibolite-facies, along with minor amounts of
greenschist-facies material (Fig. 4C and D). Samples
typically contain garnet, plagioclase, diopside-rich
clinopyroxene, amphibole, quartz, and phengite.
Omphacite, zoisite, and rutile are present in small
amounts and are interpreted to represent eclogitefacies remnants. Chlorite, epidote and sphene may be
present as accessory minerals, and are retrogression
products belonging to the amphibolite and possibly
greenschist facies. The widespread presence of
symplectites and reaction rims indicates a high degree
of textural disequilibrium.
Garnet grains (Alm48–58, Prp11–18, Grs20–24) in
these rocks are zoned, indicating disequilibrium
during retrograde metamorphism. They are generally
subhedral to anhedral, and are in many cases
surrounded by a kelyphitic mantle consisting of an
inner rim of hastingsitic amphibole and an outer rim
of chlorite. Grain size of the garnet varies from 0.3
mm to N1 cm, and larger grains have an assortment of
inclusions. Amphibole (taramite to magnesiotaramite)
and carbonate inclusions are common towards the
rims of large grains. Zoisite and quartz inclusions are
present in garnet cores.
Omphacite (Jd34–42, Ae10–12; Na2O content 6.1–
7.5 wt.%) in the partially retrogressed eclogites is
largely replaced by symplectic intergrowths of plagioclase (Ab87–93, An8–13) and diopside (Jd17–19,
Ae9–11; Na2O content 3.7–4.2 wt.%). The extent of
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symplectisation varies within and between samples on
the millimeter scale. The widespread presence of
delicate mimetic symplectite in these samples indicates static retrogression.
Phengitic mica is more abundant than in the
gneissic eclogites, and makes up as much as 15% of
the rock in some samples. It is present as aggregates
N2 mm in size, often oriented subparallel to the
sample lineation, and wraps around large garnet
grains, sometimes forming pressure shadows. Locally
abundant phengite may signify the availability of a
fluid phase and a potassium source to facilitate
eclogite-facies hydration reactions (as hypothesized
in Section 3.1.1), but does not strictly require the
presence of a fluid phase since hydrous minerals can
also exist when water activity is b1 (i.e., no free
water). The abundant eclogite-facies phengite in these
rocks may indicate that they were probably particularly hydrous under eclogite-facies conditions. Our
interpretation is that the present assemblages and
textures are the result of locally enhanced retrogression of marginal rocks with particularly hydrous
mineral assemblages.
3.1.3. Amphibolites
Amphibolites comprise the dark-coloured iron and
magnesium-rich rocks associated with the Drbsdal
mafic body (Fig. 4E and F). They occur at the contact
of the eclogite-facies mafic rocks and the granodioritic
gneisses surrounding them, and also as pods of b5 m2
found dfloatingT in the granodioritic material close to
the Drbsdal mafic body. These rocks are often finegrained and in many cases contain plagioclase,
amphibole, alkali feldspar, biotite, quartz, epidote,
white mica, and sphene as part of the amphibolitefacies assemblage. Chlorite is occasionally present.
The fabrics within these amphibolites vary from
statically retrogressed symplectic and granoblastic
fabrics after eclogite-facies textures to strongly
lineated and mylonitic textures. Very similar assemblages and textures also characterise the fine-grained
amphibolites developed at amphibolite-facies vein
margins (symplectic and granoblastic) and in shear
zones (lineated to protomylonitic) within the body
(see Section 4.7 for structural description of shear
zones). Garnet (Alm42–47, Prp22–37, Grs16–30) is
largely replaced by chlorite, alkali feldspar, and
magnesiohornblende. Relict grains are transected by
9
chloritised cracks and mantled by chlorite, alkali
feldspar, and biotite.
Retrogressive metamorphism has produced a
medium- to coarse-grained matrix of plagioclase
(Ab64–84, An15–37), quartz, magnesiohornblende,
occasional alkali feldspar, and epidote, with occasional biotite aggregates. Rutile is commonly present,
and grains are often mantled by a fine-grained
aggregate of sphene.
3.1.4. Granodioritic amphibolite-facies rocks
The granodiorites surrounding the mafic body (Fig.
4G and H) are made of plagioclase, quartz, alkali
feldspar, biotite, white mica, and epidote. Chlorite is
also commonly present, and garnet occurs in small
amounts in the granodiorites closest to the eclogite
body (approximatelyb100 m away). A felsic matrix of
quartz, plagioclase (Ab85–91, An8–15), and alkali
feldspar (Ab3–9, An0) constitutes between 65% and
85% of these samples, with the remainder made up of
sheet silicates, garnet, and epidote.
Where present, garnet (Alm51–64, Prp2–14, Grs27–41)
grains appear to be in textural equilibrium with the
amphibolite-facies matrix, and larger grains (N3 mm)
contain inclusions of carbonates, sphene, zircons, and
quartz. Biotite occurs in mimetic association with
phengite, partially or completely replacing the phengite grains. Garnet porphyroclasts are often accompanied by quartz-filled pressure shadows and wrapped
by sheet silicates to give a diffuse banding.
3.1.5. Other lithologies
3.1.5.1. Garnet–quartz layers. Occasional bands of
garnet-rich rock occur within the eclogite body. Finegrained garnet of b0.5 mm grain size constitutes 70–
90% of the rock. Abundant inclusions of allanite, iron
oxides, and rutile occur within garnet grains. The
garnet forms a matrix with interstitial quartz, omphacite, phengite, rutile, and iron oxides. Rutile grains of
up to 3 mm in diameter constitute 3–5% of the matrix,
appearing as elongate grains with long axes parallel to
the lineation of the gneissic eclogite.
3.1.5.2. Garnet–amphibole rocks. Rocks consisting
almost entirely of coarse-grained amphibole and
garnet occur locally at the margins of the mafic body.
A key locality is on the eastern edge of Svardalsvatnet
10
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Fig. 5. Structural map of the Drbsdal eclogite body. Positions of detailed maps of Tinghaugen (see Fig. 10) and Teiges3ta (see Fig. 12), and cross-section line A–B (see Fig. 8) are
indicated.
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(Fig. 5), where the southern boundary of the mafic
body is in contact with granodioritic gneiss.
3.1.5.3. Felsic bodies. Rocks of felsic composition
make up approximately 20% of the mafic body, and
consist largely of quartz and phengite with small
amounts of garnet, rutile, and zoisite, and occasional
omphacite. These felsic bodies are highly variable
with regard to size and morphology, with dimensions
in the range of 10 cm to N100 m length, and 2 cm to
15 m width. They are interpreted to have originated
under eclogite-facies conditions, and their structural
features are described in more detail in Section 4.5.
3.2. P–T estimates
Temperature and pressure estimates have been
calculated for the formation of the gneissic eclogite,
according to the clinopyroxene–garnet thermometers
of Ellis and Green (1979) and Powell (1985b), the
garnet–amphibole thermometers of Powell (1985a)
and Graham and Powell (1984), the garnet–clinopyroxene–phengite barometer of Waters and Martin
(1993), and using the THERMOCALC program.
Temperature calculations based on cpx–grt pairs
for the two thermometers yielded either extremely
large ranges (e.g., 420–715 8C for T max (Fet) for a
single mineral pair), or no temperature at all. Large
uncertainties are involved in the calculation of ferric
iron contents for clinopyroxenes, especially in the
specific compositional range of the omphacites
studied here. These uncertainties arise because: (1)
the calibrations of the calculations used for cpx–grt
exchange thermometry are highly sensitive to variations in Fe2+ of omphacite, and (2) Fe3+ was estimated
by stoichiometry and is therefore subject to further
uncertainties related to analytical errors and nonstoichiometry. The temperature estimates obtained via
cpx–grt thermometry for all methods are therefore
rejected (Koons, 1984).
Grt–amp thermometry using the methods of
Graham and Powell (1984) and Powell (1985a,b)
gives similar estimates of T max (Fet); 537–633 and
531–631 8C, respectively, identical within error.
Pressures of 17F2 kbar were calculated using the
calibration of Waters and Martin (1993) for grt–cpx–
pheng grains in close proximity, assuming a temperature of 600 8C.
11
Using the THERMOCALC program, estimates of
T=720–830 8C and P=19–21 kbar were obtained for
gneissic eclogite samples. The following four assemblages were used in the THERMOCALC calculations:
garnet+omphacite+zoisite+amphibole, garnet+omphacite+zoisite+amphibole+kyanite, garnet+omphacite+amphibole, and garnet+omphacite+zoisite+
phengite. THERMOCALC could not calculate P or
T estimates for assemblages not containing amphibole
or phengite due to a dlack of independent reactionsT.
On a more cautionary note, uncertainties related to
Fe3+ in omphacite and poorly understood complex
zonation patterns in garnet from Drbsdal indicate that
these P–T estimates may deviate substantially from
the peak P–T conditions of these rocks.
Relevant previous P–T work on eclogites in the
Sunnfjord region (Fig. 2) includes estimates from the
V3rdalsneset eclogite body (Engvik and Andersen,
2000), from localised eclogite-facies layers within
the B3rdsholmen banded granulite-facies complex
(Engvik and Andersen, 2000), and from smaller
eclogite pods in the Lavik–Hyllestad area (Hacker et
al., 2003). The V3rdalsneset rocks yielded estimates
of T=677F21 8C and P=16F2 kbar using the cpx–
grt geothermometer of Powell (1985b) and the
geobarometer of Waters and Martin (1993), respectively; and the B3rdsholmen rocks gave estimates of
T=455–490 8C after Powell (1985b) and P min=12
kbar using the method of Holland (1980), assuming
T=500 8C. Labrousse et al. (2002, in preparation)
recalculated these data using THERMOCALC, to
give T=615F22 8C and a pressure of 22.7 kbar for
V3rdalsneset, and T=525F46 8C and a pressure of
23.4 kbar for B3rdsholmen. The Lavik–Hyllestad
samples gave estimates of ~700 8C and 23 kbar,
using THERMOCALC (Hacker et al., 2003). All
published pressure estimates obtained using THERMOCALC are higher than those obtained using
exchange reactions. A possible reason for this is
that THERMOCALC uses larger assemblages than
the exchange reactions.
4. Structural relations
The Drbsdal eclogite body is enveloped by
granodioritic amphibolite gneisses (Fig. 5), and its
E–W-trending long axis in map view is subparallel to
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the linear trend of regional-scale late extensional
features in this area (Andersen et al., 1994; Krabbendam and Dewey, 1998; Milnes et al., 1997). Structural
mapping and analysis have allowed us to link grainand outcrop-scale structures with map-scale features,
and thus infer the shape of the body in three
dimensions.
4.1. Foliation and folding
Throughout most of the Drbsdal mafic body, a
strong compositional foliation is defined by variations
in the modal abundances of the eclogite-facies
minerals garnet, omphacite, quartz, zoisite, amphibole, and kyanite, or their amphibolite-facies equivalents in zones of retrogression. Compositional bands
are typically b1 cm to ~1 m in thickness, and steeply
dipping. Variations in orientation are associated with
pervasive folding of the body. As shown in Figs. 5
and 6, consistent structural trends are observed within
several map-scale domains (Section 4.3).
In the eastern part of the body, the foliation trends
ENE–WSW and is folded into tight to isoclinal
upright structures with wavelengths of 20 cm to 20
m (Figs. 5 and 6). These folds typically have ENE–
WSW-trending subcylindrical hinges with plunges of
15–408 to the west, and can be traced laterally for
distances of 3–10 m. All eclogite-facies folds lack
axial planar cleavage or fabric. The folds affect the
eclogites, and the amphibolitic and granodioritic
gneisses in their immediate vicinity. The granodioritic
gneisses surrounding the eastern end of the body
contain a relatively weak ENE–WSW-oriented foliation, which is also subparallel to the lithological
contact to the mafic body.
Towards the western end of the body, the foliation
is less steep. Its typical orientation is NNE–SSW in
the interior, and close to E–W at the margins. Foliation
within the granodioritic gneiss surrounding this part of
the body also trends E–W, although structural features
are somewhat weaker in the granodioritic lithologies.
Pervasive folding of both the mafic body and the
country rock is observed, and tight upright folds with
subcylindrical hinges and wavelengths of 20 cm to 20
m are again typical. Fold hinges have a spread of
orientations, varying from E–W-trending fold hinges
Fig. 6. Structural data for Drbsdal eclogite, subdivided into three (A–C) distinct structural zones as indicated by boxed domains. Stereonets
plotted as lower hemisphere equal area projections.
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13
Fig. 7. Field photographs of typical eclogite-facies folds from Drbsdal. (A) Isoclinal m-fold in gneissic eclogite. Note more felsic layers at top of
photograph are folded more chaotically than mafic layers near pencil. Length of pencil, 15 cm. Profile view, looking west. (B) Large tight fold in
gneissic eclogite. Length of hammer, ca. 80 cm. Profile view, looking west; outcrop dips gently towards viewer.
associated with plunges of 10–208 to the west, to N–
S-trending fold hinges associated with plunges of 10–
208 to the south.
The centimeter- to meter-scale folds of the Drbsdal
body are commonly asymmetric, but local vergence is
not always obvious. In the eastern end of the body,
these folds are characterized by steep southerly
dipping axial planes and tight to isoclinal shapes
(Fig. 7A and B). Cross-sections through the body (Fig.
8) show that meter- and kilometer-scale folds are also
asymmetric, and are also characterised by steep
southerly dipping axial planes, northward vergence,
and tight fold shapes.
4.2. Lineations
Two types of lineation can be distinguished at
Drbsdal. Since these are somewhat similar in mineralogy and appearance, the two types will be referred
to as types I and II.
Type I lineations (Fig. 9) are strongly developed
throughout the majority of the Drbsdal body, and are
Fig. 8. Vertical cross-section through eastern end of Drbsdal eclogite body (location shown on Fig. 5). Data shown as dip indicators.
Vertical=horizontal scale. Height measured in meters above sea level.
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R. Foreman et al. / Tectonophysics xx (2005) xxx–xxx
Fig. 9. Schematic block diagram showing the relationship between
folds, eclogite-facies type I lineations, and boudinage in a typical
block of the Drbsdal eclogite.
commonly defined by a grain shape fabric of the
eclogite-facies minerals omphacite, zoisite, quartz,
and/or kyanite. A polycrystalline lineation is also
common, defined by elongate clusters or aggregates
of omphacite, zoisite, quartz, or kyanite grains.
Distinction between the grain shape and polycrystalline lineations is not always possible in the field, but
observations at the thin section scale indicate that
grain shape lineations defined by zoisite are stretching lineations (Section 5.2). In general, the eclogite
has an LNS fabric; layering is compositional, and
grains are only slightly elongate in sections perpendicular to lineation. Lineations measured in the
amphibolite-facies granodioritic gneisses outside the
mafic body are subparallel to those inside the mafic
body, and are defined by polycrystalline alignments of
elongated quartz or feldspar grains. As with the
compositional foliation, lineations have a wide variety
of orientations throughout the whole of the mafic
body, but trends characteristic of specific structural
domains can be distinguished at the map scale (Figs. 5
and 6). These lineations are interpreted as stretching
lineations, and their modal orientation (in area C;
shown in Fig. 6) coincides strongly with the regional
linear E–W trend. Previously, the linear E–W trend
has been reported in amphibolite-facies rocks (Andersen et al., 1994; Krabbendam and Dewey, 1998), but
at Drbsdal, minerals defining the E–W-trending
lineation clearly grew under eclogite-facies conditions, suggesting that the trend may have been
initiated before the late amphibolite-facies stage (see
discussion in Section 5.4).
The lineations generally define a penetrative
crystal fabric, and are aligned within the plane of
the compositional foliation. In the western part of
the body, at Teiges3ta and Tinghaugen (areas A and
B), the orientation of lineations in the eclogite is
highly variable. In area A, for example, a progressive change in the orientation of lineations is
observed from ENE–WSW-trending lineations with
westward plunges to ESE–WNW lineations with
eastward plunges (Figs. 5 and 6). However, the
presence of centimeter-scale to kilometer-scale shear
zones within and at the margins of the body
complicates the structural trends further in this area
(see Section 4.7). In the eastern part of the body, at
Ramsgro, the observed lineations follow an ENE–
WSW trend, and typically plunge 25–408 to the west
or southwest.
The second type of lineation (type II), described as
a surface lineation, is observed locally, particularly a
few tens of meters north of Svanetjorna (Fig. 10). It is
defined by finely dcorrugatedT aggregates of micas,
amphiboles, and zoisite crystals, and looks similar in
many respects to the lineations described above.
However, this type of lineation is not penetrative
but confined to the surfaces of isolated foliation
planes within the rock, and micas are conspicuously
abundant on these planes. Linear mineral orientations
of this type trend E–W, and plunge 5–258 to the east
or west, and are therefore generally much shallower
than the plunges of the dominant penetrative lineation
(Fig. 11). Perhaps, the type II lineation is locally
superimposed on the earlier fabric defined by type I
lineations.
4.3. Map-scale variations in structure
Map-scale observations indicate local disturbances
to the east–west trend of mesoscale fold hinges and
other linear features (Sections 4.1. and 4.2). Planar
features such as foliation planes are also affected,
locally becoming less steep.
In the Teiges3ta area (Fig. 12), lineations, foliation, and fold hinges measured in the granodioritic
rocks surrounding the Drbsdal mafic body exhibit
subparallelism with the regional E–W trend of
gneissic fold hinges, foliation, and lineations. The
same is true of measurements from very marginal
parts of the mafic body, but towards its interior, these
are locally highly oblique to the regional E–W trend
(Figs. 6 and 12). This gradual change in obliquity of
linear structures towards the interior of the body
indicates a rotation of the margins of the mafic body
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15
Fig. 10. Structural map of the Tinghaugen area (see Fig. 5 for location).
relative to its interior in this area. Within the
unrotated block, fold morphologies are less tight
than those throughout the rest of the body; but
otherwise, structures and mineralogies are typical for
the Drbsdal eclogite. The observed lineation pattern
can be explained by partitioning of strain into the rest
of the body, and a resulting clockwise rotation
relative to the Teiges3ta dblock.T The open folds at
Tieges3ta would be preserved remnants within a low-
Fig. 11. Conceptual sketch of the relationships between surface
lineation (depicted in black) and penetrative lineation (depicted in
grey) in the Drbsdal eclogite.
strain lozenge. This shearing is compatible with the
transtensional strain regime proposed by Krabbendam
and Dewey (1998), who presented evidence that
exhumation through amphibolite-facies occurred this
way (see Section 5.7).
A similar obliquity of structural features with
respect to the regional E–W trend is observed in the
Tinghaugen area (Fig. 10). Fold style at Tinghaugen is
more chaotic than at Teiges3ta, at times being
disharmonic, with poorly defined layering. Lithologies at Tinghaugen are much more retrogressed than
at Teiges3ta, but thin section analysis shows that the
zones of retrogression are dominated by symplectic
material after eclogite-facies minerals such as omphacite and garnet. We conclude that retrogression of the
rocks in the Tinghaugen area was largely static, so the
obliquity was established at eclogite facies (as at
Teiges3ta). Differences in deformation style between
Tinghaugen and Teiges3ta may simply be the result of
differences in bulk composition or distance from the
upper or lower margins of the body (out of the map
plane). The relationship between N–S-trending linear
features at Tinghaugen and those at Teiges3ta is
difficult to assess due to these fundamental differences
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R. Foreman et al. / Tectonophysics xx (2005) xxx–xxx
Fig. 12. Detailed structural map of Teiges3ta area (see Fig. 5 for location).
in deformation style, and as a result of this, the timing
of formation of structures in the two areas remains
unresolved.
4.4. Boudinage
Boudinage is most common in the eastern
portion of the Drbsdal body and structures range
from pinch-and-swell to completely detached boudins. The extension direction implied by the
orientation of boudins trends SW–NE to E–W, and
therefore coincides strongly with the trends of
stretching lineation and fold hinges within the
Drbsdal mafic body. Ductile deformation of less
competent felsic layers is observed to have accommodated the boudinage of more competent mafic
layers. Asymmetric truncation of the ends of layers
within boudins is common, but both sinistral and
dextral shear senses are observed, implying that no
dominant shear sense is present (Goscombe and
Passchier, 2003). Boudin necks are commonly sites
of vein development (see Section 4.6). Both boudins
and veins in boudin necks comprise eclogite-facies
mineralogies.
4.5. Felsic bodies
Rocks of felsic composition make up approximately 20% of the mafic body (see Section 3.1.5 for
petrographic description). The felsic bodies themselves are often foliated, and some may also be traced
around eclogite-facies folds.
Some of the largest foliated felsic eclogite bodies,
however, have a discordant cross-cutting relationship
with the foliation in the surrounding mafic eclogite,
and contain pods of mafic eclogite. Their discordant
relationship with the surrounding eclogite indicates
that they postdate the majority of eclogite-facies
deformation, while their mineralogy indicates that they
crystallized under eclogite-facies conditions. Little or
no retrogression is observed either at the margins of
these bodies or within the included pods (Fig. 13A and
B). These features lead to the interpretation that most
of these dyke-like felsic bodies were introduced under
eclogite-facies conditions, possibly as a fluid phase or
melt, and that pods and lenses of mafic eclogite were
entrained during dintrusionT of these felsic bodies, but
it is also possible that some are older than the eclogitefacies event. Many of these large felsic bodies are also
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17
Fig. 13. Field photographs showing key structural features within the Drbsdal eclogite body. Locations are numbered and shown on Fig. 5. (A)
Lens of unretrogressed eclogite enclosed within large foliated felsic dyke; notebook is 20 cm in length (location 1). Map view; looking east. (B)
Protrusion at margin of felsic dyke; white markers indicate contact and black marker indicates foliation in surrounding unretrogressed eclogite.
Note that the foliation runs broadly from the left to the right of the photograph, and is clearly cut by the protrusion, seen in the centre of the
lower part of the photograph (location 1). Map view; looking northeast. (C) Folded kyanite-bearing vein in mylonitic eclogite (type I vein as
described in text; foliation of eclogite runs from the left to the right of the photograph); black markers indicate opposite ends of an individual
kyanite lath, which is oriented at a high angle to foliation. Length of pencil, 15 cm (location 2). Profile view; looking north. (D) Cross-cutting
kyanite-bearing vein (type II vein) in folded mylonitic eclogite; individual laths of kyanite up to ~10 cm are visible in this view. Note that laths
are oriented subparallel to foliation. Length of hammer, ca. 80 cm (location 3). Profile view; looking northwest. (E) Close-up view of kyanitebearing vein in boudin neck (type III vein). Note clustering of kyanite laths towards vein neck (location 4). Map view; looking north. (F)
Amphibole–quartz vein in mylonitic eclogite. Note highly irregular vein margins with tapering protrusions into the wallrock. Pencil (15 cm in
length) indicates position of ca. 2-cm amphibole crystal at vein margin (location 5). Looking north; outcrop dips gently towards viewer. (G)
Network of E–W-trending plagioclase–quartz–amphibole veins near margin of eclogite body. Clinometer, 10 cm long (location 6). Looking
north; map view. (H) Submeter-scale ductile amphibolite-facies shear zone within eclogite in the Teiges3ta area. Shear sense is sinistral. Pencil
(15 cm in length) indicates orientation of eclogite-facies foliation outside the zone of influence of the shear zone (location 7). Map view; looking
north. (I) Meter-scale brittle amphibolite-facies shear zone (vein with small shear displacement). Markers indicate limits of related retrogression
in eclogite. Length of clinometer is 10 cm (location 8). Map view; looking east.
associated with the occurrence of large quantities of
smaller felsitic bodies, occurring as net veins or
disordered arrays of dykelets. This suggests that much
of the felsitic material may have been derived locally,
by partial melting of the eclogite itself, or sourced from
the surrounding granodioritic gneisses.
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4.6. Veins
A highly characteristic feature of the Drbsdal body
is the spectacular development of abundant eclogitefacies veins with a variety of morphologies and
compositions. Based on their mineral assemblages,
these veins are divided into four main types. The types
are ordered according to relative abundance: (1)
kyanite–quartz–phengite, (2) quartz, (3) amphibole–
quartz, and (4) feldspar–quartz–amphibole.
4.6.1. Kyanite–quartz–phengite veins
Veins dominated by kyanite are present throughout
the mafic body, and field observations indicate that
they contain the mineral assemblage kyanite+quartz+white micaFomphaciteFgarnetFamphiboleFrutile.
Individual kyanite laths may be up to 15 cm in
length, and laths of 5–10 cm length are common. No
alteration of the eclogite wall rock is observed at the
margins of kyanite-dominated veins; therefore, we
conclude that the veins formed at eclogite facies.
Kyanite veins are divided into three types (I–III), on
the basis of structural setting with respect to the
surrounding eclogite.
Type I veins are oriented subparallel to the
compositional foliation, and can often be traced
around eclogite-facies folds. Most type I veins contain
a strong lineation, defined by the shape-preferred
orientation of kyanite laths, which plunge 15–408 to
the west or southwest (Figs. 13C and 14A). Where
kyanite veins are folded, the kyanite laths may be
concentrated into bundles at the hinge region, and are
generally oriented subparallel to the fold hinge.
Kyanite lineations in the dfold limbT sections of such
veins may also be aligned along the fold limbs, at a
high angle to the fold hinge and lineation in the
eclogite. Type I veins are spectacular, varying from 5
cm to N3 m in total traceable length, and vary in
thickness from 1 cm to 1 m.
Type II veins cut the eclogite-facies foliation at
various angles. They are generally relatively straightsided and taper towards each end (Figs. 13D and
14B). Lineations within these veins are again defined
by a grain shape fabric of kyanite, and plunge 15–408
to the WSW. Although the veins are often oriented at
a high angle to the eclogite-facies foliation, there may
be a continuity of the eclogite-facies foliation through
the kyanite laths from one side of the vein to the other.
Fig. 14. Schematic sketches showing characteristic relationships of
kyanite-bearing veins with surrounding eclogite. (A) dType IT vein;
characteristically subparallel to eclogite-facies foliation and can be
traced around folds. (B) dType IIT vein; clearly cross-cutting
eclogite-facies foliation and with kyanite laths oriented subparallel
to compositional foliation in the surrounding rock. (C) dType IIIT
vein; characteristically located in the neck regions of boudins, with
kyanite laths subparallel to the local foliation.
These veins range from 2 to 50 cm in width, and from
10 cm to N4 m in length.
Type III kyanite veins are developed in the necks
of eclogite-facies boudins, with kyanite fibres up to 10
cm in length mimicking the lineation and foliation in
the boudinaged eclogite itself, forming a fan-shaped
aggregate on each side of the boudin neck. These
aggregates are 2–15 cm in width and 5–30 cm in total
length, and lineations within them have plunges of
20–408 to WNW, W, or WSW (Figs. 13E and 14C).
4.6.2. Quartz veins
Quartz veins of 10 cm to 1 m in length and 5 mm
to 40 cm in width are common throughout the mafic
body. Occasional fine-grained (b1 mm) zoisite,
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omphacite, and garnet grains are included in the vein
fill, but are only observable in thin section. Vein
margins are often enriched in white mica and sometimes in rutile. Most of the quartz veins are steeply
dipping and oriented E–W to ENE–WSW, thus lying
subparallel to the pervasive compositional foliation
within the body, but occasionally they cut the foliation
at a high angle. Some contain a weak lineation defined
by grain shape fabric of quartz, plunging moderately
or steeply to the NW or W. Veins that are oriented
subparallel to the compositional foliation are commonly deformed within eclogite-facies structures such
as folds or boudins (Sections 4.1 and 4.4). Alteration
of the eclogite at the margins of quartz veins is rare,
but occasionally there is a change in colour of the
wallrock from green to grey or black towards the vein.
Eclogite-facies mineralogy and structural relationships
(i.e., folding) of the majority of the veins clearly
indicate that they originated under eclogite facies.
4.6.3. Amphibole–quartz veins
Amphibole-bearing veins are less common than
kyanite-dominated veins, and are composed of barroisitic amphibole+quartzFgarnetFwhite micaF
kyanite. These veins vary from 5 cm toN1 m in
length and from 1 to 30 cm in width. In most cases,
amphibole-bearing veins crosscut the eclogite-facies
foliation, dipping steeply to the west or northwest.
Lineations within these veins are defined by amphibole grain shape fabric, and plunge 15–408 to the
WNW, W, or WSW. Vein margins are often highly
irregular, with tapering protrusions into the wallrock
(Fig. 13F), but field observations suggest that no
alteration of the eclogite wallrock occurs at the vein
margins.
4.6.4. Feldspar–quartz–amphibole veins
Feldspar-bearing veins are divided into two groups,
based on their orientation and mineral content. N–Strending alkali feldspar-bearing veins are observed
throughout the eclogite body, and constitute one endmember of a continuum of structures from brittle
veins with no apparent offset to mineralised ductile
shear zones. These are described in more detail in
Section 4.7.
Plagioclase-bearing E–W-trending veins are common at the margins of the mafic body, and generally
have moderate to steep dips. They run subparallel to
19
the lithological contact between the eclogite and the
granodioritic gneiss, and are associated with retrogression of the eclogite to a fine-grained grey to black
amphibolite. They vary in thickness from b1 to N20
cm, and commonly form a pervasive network of
anastomosing veins, giving the rock a stripy appearance (Fig. 13G).
4.7. Shear zones
Measurable shear zones at the outcrop scale are
rare in the Drbsdal mafic body as a whole. However, a
range of shear zone styles is observed (Fig. 13H),
ranging discontinuously from brittle to ductile. In the
brittle-style shear zones, the shear plane itself consists
of a vein filled with an amphibolite-facies mineral
assemblage of feldspars (these vary within and
between different veins, and can be plagioclase or
alkali feldspar), quartz, and, in some places, amphibole (Fig. 13I). The eclogite-facies compositional
foliation is transected at a high angle and may be
offset by distances of up to 50 cm. All types of
kyanite veins are also transected by these brittle
structures. This type of shear zone is generally
accompanied by a diffuse zone of alteration, 10 cm
to 1 m in width, in which the eclogite is replaced by a
fine-grained grey rock towards the shear zone (Fig.
15A). Brittle shear zones occur in the interior of the
mafic body, and are less common at its margins.
Ductile shear zones are characterized by intensification and reorientation of the compositional foliation,
and are also often accompanied by a change in colour
and grain size of the eclogite. This deformation is
most intense in the shear plane (Fig. 15B). Ductile
shear zones occur at or near the margins of the mafic
body, particularly where the eclogite has a relatively
narrow outcrop width, or tapers laterally; they are not
common within the interior of the eclogite body. For
both ductile and brittle types of shear zones, where a
displacement is observed, the dextral shear zones
typically dip steeply or moderately to the SW, whereas
sinistral shear zones generally dip steeply to the SE or
SW. It is likely that these orientations represent
conjugate sets of structures (Fig. 16) and that shear
senses are as they appear (i.e., a mixture of top-towest and top-to-east). However, it is possible that the
real shear sense of some of these shear zones is
different from the apparent shear sense, since shear
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R. Foreman et al. / Tectonophysics xx (2005) xxx–xxx
Fig. 15. Characteristics of amphibolite-facies shear zone type features: (A and B) Cartoons showing end-members of a continuum of
amphibolite-facies shear zone features found within the Drbsdal eclogite (A, ductile end-member; B, brittle end-member).
zones are commonly viewed in section, and exposure
of shear planes or lineations is rare.
5. Structural and petrographic interpretation
In this section, each type of structure will be
discussed and interpreted, and then a large-scale
model for the structural evolution of the Drbsdal
eclogite body will be presented and explained.
Fig. 16. Orientation measurements of shear zones plotted as poles to
shear planes, equal area projection, and upper hemisphere.
5.1. Eclogite-facies folding
The Drbsdal mafic body is a coherent sheet of
compositionally banded eclogite that is tightly folded
on the kilometer scale. Folds at the kilometer scale are
not seen clearly in map view; however, the vergence
of outcrop-scale folds indicates that the body is folded
on a larger scale. Investigation of the symmetry of
meter- and centimeter-scale folds via the construction
of cross-sections provides further evidence that kilometer-scale folds have affected the whole of the
eastern part of the body. The larger-scale folds are
interpreted to have morphologies resembling those of
the mesoscale parasitic folds, which are generally
asymmetric, tight to isoclinal structures. The apparent
lack of large-scale fold axes in map view is a result of
the steepness and bulk asymmetry of structures in the
area; fold axes are not necessarily associated with
obvious switches in dip direction of the foliation. The
mesoscale folding affects layers with well-preserved
eclogite-facies mineralogy, indicating that folding was
active during metamorphism at eclogite facies. There
is a possibility that folding began before the onset of
eclogite-facies conditions, continuing into the eclogite-facies phase of metamorphism, but the lack of
preserved protolith to the eclogite precludes further
investigation.
At the map scale, the pinching out of the mafic
body at its extreme west and east, and the spatial
relationships between the Drbsdal eclogite and other
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mafic bodies in its immediate vicinity, indicate that
boudinage of mafic layers and lenses has contributed
to the present distribution of these mafic bodies, and
has also modified their shapes (see Figs. 1 and 2).
Zones of retrogressive amphibolite-facies material are
strongly associated with local shearing of the Drbsdal
body. These shear zones are concentrated in the
Teiges3ta area and in the termination of the Drbsdal
body to the NW of Trausdalsvatnet. This indicates that
eclogite-to-amphibolite facies E–W-directed stretching and boudinage were partially accommodated by
movement on submeter-scale shear zones, and that
this deformation further altered the original shape of
the tightly folded eclogite layer to give the observed
lensoid shape. Additional evidence of this boudinage
is found further to the west, in the eclogite around
Svanetjorna.
Folding is relatively well constrained for the
eastern end of the body. Notably, there is a finger of
granodioritic amphibolite-facies rock protruding into
the mafic body to the south of Butjorna, which is
interpreted as the core of an antiform (Figs. 5 and 8).
This feature appears again as a dwindowT to the west,
where a steep westward-facing hillside has cut
through the eclogite exposing the granodioritic core
of the fold (Fig. 5). This antiform is also seen as the
westward protrusion of eclogite to the north of
Trausdalsvatnet (Fig. 5). The curve in the eclogite
boundary slightly further north, at Svanetjorna, is due
to the presence of a westward-plunging synform. The
elongate, bifurcated shape of the body is therefore the
result of folding on the kilometer scale, modified by
boudinage (see Section 5.7; Figs. 17 and 18).
The Drbsdal body is currently situated in the
southerly dipping limb of a large-scale antiformal
culmination between the Kvamshesten and Solund
Devonian basins (Fig. 1). Folds in this system
typically have wavelengths in the 10–15 km range.
The relationship of the eclogite-facies folding to this
regional-scale structure is discussed in more detail
below.
5.2. Eclogite-facies type I lineation
The parallelism of eclogite-facies fold hinges with
lineations defined by eclogite-facies minerals indicates a link between these structure types. This
relationship between lineations and fold hinges is
21
found in structural domains dominated by a N–S
linear trend and also in those dominated by and E–W
linear trend (Figs. 5 and 6). Grain shape lineations
defined by omphacite, zoisite, and kyanite are
interpreted as stretching lineations for a number of
reasons. Firstly, the opening direction of fractures and
healed fractures within zoisite grains always coincides
with lineation direction. Textural evidence suggests
that the zoisite and omphacite are in equilibrium at the
thin section scale and therefore represent the same
phase of eclogite-facies deformation. Secondly, this
lineation coincides with the orientation of kyanite
laths and amphibole fibres precipitated as vein fillings
during eclogite-facies metamorphism. These grains
can often be traced through veins with opening
directions that cut foliation and lineation of the
surrounding eclogite at a high angle. These veins
therefore opened with an E–W extension direction (in
the present frame of reference) during eclogite-facies
metamorphism. Thirdly, hinge parallel boudinage of
the eclogite and associated development of kyanite
veins in boudin necks are further evidence for large
amounts of eclogite facies E–W stretching in the
eastern end of the Drbsdal body, since no change in
metamorphic conditions appears to accompany the
boudinage.
A mechanism for developing fold axes parallel to a
linear stretching feature must therefore be inferred.
Folds with hinges parallel to a lineation may develop
in the following ways: (1) an array of folds with either
random or common orientation may be subjected to a
strain large enough to rotate the folds towards
parallelism with a subsequent stretching direction
(Escher and Watterson, 1974; Sanderson, 1972;
Skjernaa, 1980); (2) in situations where the intermediate strain orientation Y is constrained to be
perpendicular to the original layering, and deformation takes place under plane strain conditions, buckle
folds may form with axes parallel to the regional or
local stretching direction and therefore have similar
orientations to simultaneously formed linear stretching features (Grujic and Mancktelow, 1995; Watkinson, 1975); (3) a rock mass containing a preexisting
linear fabric may be folded; for hinge lines to form
parallel to the fabric, or dbending anisotropy,T the
fabric must be sufficiently well defined to exert a
mechanical control on the orientation of new fold axes
(Cobbold and Watkinson, 1981); (4) folds may initiate
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R. Foreman et al. / Tectonophysics xx (2005) xxx–xxx
Fig. 17. Schematic 3D diagram showing stage 3 of the structural model for formation and deformation of the Drbsdal eclogite body (see Section
5.7 of text). c=shear strain.
with hinges parallel, oblique or perpendicular to the
transport direction, as a result of differential movement on a shear plane, and subsequently oblique or
perpendicular hinges experience passive rotation
towards the transport direction with an increase in
strain intensity (Coward and Potts, 1983); (5) stretching lineations and fold hinges may form simultaneously with a common orientation, specifically
within a constrictional strain field (Krabbendam and
Dewey, 1998).
The high degree of compositional heterogeneity
between layers and the widespread presence of class 3
folds are indicative of high competence contrasts. The
morphologies of linear and planar eclogite-facies
structures suggest that high strains were achieved
and that ductile behaviour dominated the folding and
boudinage. Although omphacite–zoisite-rich layers of
the rock contain a strong lineation that may possibly
have formed before nucleation of the earliest folds, the
bending anisotropy due to this lineation is unlikely to
have been marked enough to affect the orientations of
new folds (Cobbold and Watkinson, 1981). The
production of folds in this way is only likely in
situations where the linear feature causing the bending
anisotropy is composed of deformation-resistant rods
or fibres that are embedded in a much dweakerT matrix
and do not deform. It is likely that the large variations
in mineralogy observed between layers led to significant competence contrasts during deformation, and
that layer competence therefore had a much larger
control on the mechanism and morphology of the
folding than the linear features themselves. We
therefore conclude that mechanism (3) (Cobbold and
Watkinson, 1981) is unlikely to have caused the
parallelism of lineations and fold hinges in the
Drbsdal eclogite.
If the fold hinges developed parallel to the extensional lineation according to the model of Coward and
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Potts (1983), the fold hinges would not necessarily
have the same orientations as the lineations. The
model dictates that folds may form with hinges
oblique, perpendicular or parallel to the transport
direction, and that oblique or perpendicular folds may
subsequently rotate into parallelism with the transport
direction as the shear zone advances. If fold hinges in
the Drbsdal eclogite had been reorientated postfolding
but synchronously with the lineation according to this
model, we would expect to see a statistical deviation
around the modal orientation for fold hinges, but a
smaller deviation or no deviation from the modal
orientation for lineations. Stereonets show tight
clusters for orientations of both fold hinges and
lineations for the eastern portion of the Drbsdal body,
and for the western portion of the body, we see a large
range in orientations of lineations and a comparatively
smaller range in orientations of fold hinges. The
model also requires large shear strains (c=20 or more)
to rotate folds initiated at high angles to the transport
direction into subparallelism with the shear direction
(Alsop and Holdsworth, 1999; Skjernaa, 1980). Given
the large amounts of deformation intrinsic to any
tectonic model for exhumation, it is possible that the
Drbsdal body has undergone shear strain of this order
of magnitude. However, the large spread in lineation
orientations is not easy to reconcile with this model.
It is also unlikely that the fold hinges and lineations
formed coevally and parallel to a regional or local
stretching direction, under plane strain conditions with
Y constrained to be perpendicular to the original
layering as described by Grujic and Mancktelow
(1995) and Watkinson (1975) (mechanism (1)). Folds
produced in such a manner are observed to have low
amplitude–wavelength ratios, even at high strains.
This is not consistent with our observations.
The strength of the L-fabric and its coincidence
with stretching direction of fold hinges and the long
axes of boudins suggest that formation of folds and
lineations was simultaneous. The mechanism involving a constrictional strain field (Krabbendam and
Dewey, 1998) may be responsible for the parallelism
of fold hinges, lineations, and long axes of boudins.
Populations of fold axes initiated under purely
constrictional strain conditions would perhaps be
expected to have randomly oriented axial planes.
However, it is likely that the orientation of layering
(D1) present in the Drbsdal rocks before folding began
23
would also have a large effect on the formation of the
fold population. Consider the orientations of the three
principle strain axes for the folding event (D2): X is
oriented parallel to present fold hinges (broadly E–W
on map), Y is oriented perpendicular to X but within
the axial surface (in and out of map plane), and Z is
perpendicular to fold hinges and axial surfaces (N–S
on map). If X/YJY/Z, and YNZ, then folds forming in
a dhorizontalT multilayer (i.e., layer subparallel to X–Z
plane) with large competence contracts between layers
would develop with hinges parallel to the X direction
and amplify in the Y direction whilst shortening in the
Z direction. Fold axial planes produced in these
conditions would be consistently steep to upright in
orientation, as shown for the eclogite-facies folds at
Drbsdal. It is also necessary to consider that an
existing population of upright folds could be progressively rotated into parallelism under constrictional
conditions, and thus also be parallel to simultaneously
formed stretching features such as lineations and long
axes of boudins.
The preferred interpretation for the initial stage of
fold formation is the constrictional model of Krabbendam and Dewey (1998), which is compatible with
our data for cases where X/YJY/Z. Perturbations to
this strain field, perhaps due to evolving rheological
contrasts at the margins of the body, or volume
changes due to metamorphic reactions taking place
after initial formation of folds and stretching lineations could explain the obliquity of linear structures
in the Teiges3ta and Tinghaugen areas. However, a
more likely explanation of the observed obliquities is
the onset of top-west–directed overshear (Fig. 17).
Evidence of top-west shear is seen throughout the
WGC in amphibolite- and greenschist-facies rocks
(Andersen and Jamtveit, 1990; Fossen, 1992; Eide et
al., 1999; Hacker et al., 2003). We propose that the
onset of a precursor to this amphibolite- and
greenschist-facies shear began at eclogite facies. This
top-west-directed overshear (Fig. 17) tightened folds
in area C, and rotated fold hinges in areas A and B
clockwise in the present-day map view (area C is the
area above plane B in Fig. 17, and areas A and B are
below plane B in Fig. 17; these also correspond with
areas A, B, and C in Fig. 6). The fold rotation models
of Sanderson (1972), Escher and Watterson (1974),
and Skjernaa (1980) may also apply, but are considered to be less important than the constrictional
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R. Foreman et al. / Tectonophysics xx (2005) xxx–xxx
transtension model of Krabbendam and Dewey (1998)
in the present case.
In summary, we propose that a strong preexisting
planar foliation was folded to produce a population of
folds with similarly oriented axial planes. These folds
were subsequently subjected to sufficient constrictional strain to ensure parallelism of their axes with
each other and with lineation, and eventually result in
hinge-parallel boudinage. The eclogite-facies folds,
stretching lineations, and boudins were formed at
different but perhaps overlapping stages of one
progressive event. It follows that the folds trending
N–S in area A (Fig. 6) record an early phase of
eclogite-facies deformation. Subsequent deformation
was partitioned, and only involved those parts of the
Drbsdal body outside area A. The early folds have
subsequently been tightened, particularly in the eastern end of the body and its margins. As a result of this
partitioning, the majority of the eclogite body and its
surroundings has undergone a clockwise rotation
relative to the dTeiges3ta block.T In map view, the
structures in area A are oriented N–S to NE–SW, and
the structures in the rest of the body are oriented
broadly E–W. The E–W- to ENE–WSW-trending
eclogite-facies structures within the rest of the Drbsdal
body are oriented subparallel to the amphibolite-facies
structures in the surrounding granodioritic rocks. This
indicates that although ductile deformation of the
Drbsdal body must have ceased by the onset of
amphibolite-facies conditions, no major rotation of the
Drbsdal body relative to its surroundings occurred
after this time. Although structures within the whole
of the WGC probably rotated due to transtensional
deformation during exhumation, the fact that structures within the body are subparallel to those outside it
indicates that rotation of the whole Drbsdal body
relative to its surroundings was limited. This lack of
rotation may be a result of the large size and elongate
shape of the Drbsdal body, features that are undoubtedly little changed since eclogite facies. Smaller
eclogite bodies would perhaps have experienced more
substantial passive rotation since eclogite facies.
5.3. Eclogite-facies type II lineation (surface
lineation)
The confinement of the dtype IIT or surface
lineation to foliation planes may indicate that it
formed by slip on the foliation surfaces. Where
present, the surface lineation is oriented at a high
angle to the pervasive, penetrative lineation, and this
strongly suggests that the two types of structure
formed at a different times. If the surface lineation
formed as suggested, via slip on foliation planes, it
cannot be an earlier feature than the penetrative
lineation, as formation of the pervasive, penetrative
lineation should have overprinted most small-scale
linear features, especially those defined by hydrous
phases.
5.4. Relationship of L at Drbsdal to regional linear
dexhumationT features
The orientations of fold axes, boudin long axes,
and lineations in the eastern end of the Drbsdal body
correspond closely to those of regional linear features
formed during extension, such as the crenulation
cleavage and amphibolite- to greenschist-facies shear
sense indicators associated with movement along the
NSDZ (Andersen et al., 1994; Krabbendam, 1998;
Krabbendam and Dewey, 1998), and the eclogite- to
amphibolite-facies stretching lineations described by
Engvik et al. (2000). They are also parallel to the
extensional fabric observed in the nappe units
(Fossen, 1993).
Although structures within the Drbsdal body and
those further north within the WGC and overlying
mid- to upper-crustal units and detachment zones
were formed in different spatial settings, they may all
have formed within a short time frame and corresponding to a stage of relatively fast burial followed
by rapid extensional exhumation. Recent U–Pb ages
from zircons and 40Ar/39Ar closure data from WGC
and Hyllestad Complex localities ~3 km south of
Drbsdal indicate that a mere N5–10 Ma may have
passed between maximum burial of the WGC (ca.
410–400 Ma) and exhumation to upper crustal levels
(ca. 403 Ma) (Chauvet and Dallmeyer, 1992; Hacker
et al., 2003). If this is the case, then the Drbsdal
structures could have formed either in several separate
regional events with short total duration or during one
short-lived phase of progressive deformation.
However, evidence that links the formation of
eclogite-facies stretching lineations in the lower crust
to the amphibolite- and greenschist-facies extensional
structures of the mid- and upper crust is largely
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circumstantial. The presence of several structurally
different domains within the Drbsdal body itself
implies that deformation within the lower crust was
gradually partitioned during passage through eclogitefacies P–T space, and was influenced by different
factors such as local composition of the eclogite,
thickness of the mafic body, and distance from the
margins of the body. The domains containing E–Wtrending structures therefore represent zones of
reworking through which the early-formed folds have
been tightened, stretched, and rotated by subsequent
deformation, perhaps during early exhumation. Rey et
al. (1997) postulated extension with top-west-directed
shear operating at eclogite facies as well as lower
grades, and our data support this possibility.
The parallelism of structures formed in vastly
different parts of P–T space may be explained if no
major changes in deformation regime occurred during
the exhumation from eclogite-facies to amphibolitefacies conditions. This parallelism of structures formed
over a range of metamorphic facies is not typical of the
WGC rocks. For instance, Krabbendam et al. (2000)
show that granulite-facies and eclogite-facies lineations are almost perpendicular in the Nordfjord area,
and Engvik and Andersen (2000) show that dearlyformedT eclogite-facies lineations are consistently at
high angles to dlater-formedT eclogite-facies lineations.
Where lineations are consistently oblique, this indicates that an earlier part of the eclogite-facies history is
seen locally, and where lineations within small
granulite or eclogite bodies have variable lineations,
this is likely to indicate subsequent rotation of these
small bodies relative to their surroundings. The
Drbsdal body is large, and the majority of its smallscale structures are subparallel to those in the
surrounding rocks, as are its boundaries. The oblique
fabrics at Tiegesata and Tinghaugen represent parts of
the body that ceased to deform at an earlier stage.
Partitioning of strain into the rest of the eclogite body
resulted in a clockwise rotation of structures in area C
relative to areas A and B. This obliquity of fabrics does
not necessarily reflect a different strain regime at the
crustal scale. Moreover, although the whole of the
Drbsdal body could have rotated since it was present at
eclogite-facies depth, this is unlikely due to the volume
of rock involved and its elongate nature. Indeed, the
idea of a constant strain regime during the exhumation
from eclogite-facies conditions to amphibolite-facies
25
conditions fits well with the short-lived exhumation
event proposed by Hacker et al. (2003) for the WGC.
A sequential partitioning of deformation into domains
of different rheologies would explain the preservation
of the deepest-formed structures in the largest mafic
bodies and the preservation of shallowly formed
structures in granodioritic or lithologically variable
units such as the felsic gneisses or finely interleaved
mafic and felsic rocks.
There is a conspicuous lack of systematically
oriented outcrop-scale eclogite-facies shear sense
indicators preserved in the Drbsdal mafic body. This
indicates that coaxial flow may have dominated the
deformation regime(s) active within the Drbsdal body
during the latter stages of residence at eclogite facies,
and possibly the whole of the eclogite-facies history.
This interpretation agrees with those of Andersen and
Jamtveit (1990), Andersen et al. (1994), and Jolivet et
al. (1994), who argued for coaxial deformation of the
lower crust during early extensional collapse. The
model of Krabbendam and Dewey (1998) involves
bulk noncoaxial deformation of the lower crust, with
exhumation controlled largely by sinistral transtension. Lattice-preferred orientation (LPO) of omphacite
grains in gneissic eclogite samples from the Drbsdal
body (Foreman and Wheeler, in prep.) shows no
asymmetry—an observation that is consistent with
models involving coaxial deformation of the lower
crust. However, it is also possible that deformation of
the WGC as a whole was noncoaxial, but that
deformation was inhomogeneous, and coaxial deformation occurred locally. It follows that the Drbsdal
body may have rotated anticlockwise in a large-scale
shear affecting the WGC, while deforming internally
in a coaxial manner. However, the subparallelism of
the latest eclogite-facies structures with amphibolitefacies structures outside the body indicates that
posteclogite-facies rotation of the body relative to its
immediate surroundings was limited (see Section 5.2).
The evidence presented in the present contribution
shows that a number of stages of pervasive ductile
deformation affected the Drbsdal mafic body during
residence at eclogite-facies conditions. It is therefore
likely that the E–W-oriented eclogite-facies fabrics
preserved at Drbsdal were developed during the early
stages of exhumation, since there is no evidence for a
major change in orientation of the strain field between
eclogite facies and amphibolite facies.
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5.5. Amphibolite-facies deformation features
The presence of mafic amphibolite-facies mineral
assemblages as vein fillings in brittle shear zones,
and the replacement of eclogite-facies minerals by
amphibolite-facies material in both brittle and ductile
shear zones indicate that the deformation they
represent occurred under amphibolite faces conditions, during exhumation. There are two possible
interpretations to explain the relative timing of
ductile and brittle shear zones. The first is that the
ductile shear zones predate the brittle ones and
therefore formed earlier in the exhumation history.
If this is the case, brittle deformation processes
would have become dominant as exhumation progressed and veins would form with progressively
more brittle features. The alternative is contemporaneous formation of brittle and ductile shear zones,
but in different parts of the mafic body. The presence
of a fine-grained, weakly foliated, amphibolite-facies
carapace around the mafic body indicates that
metamorphic reactions and deformation were possible in mafic material at the margins of the body
during the amphibolite-facies stage, but that no
deformation and very little retrogression occurred at
this stage within the interior of the body. It is
therefore likely that amphibolite-facies shear zones
formed in the felsic material outside the mafic body
during exhumation and in the mafic material at the
very margins of the body, where deformation was
enhanced by reaction softening and possibly also (in
the mafic margins) by an influx of fluid from the
surrounding felsic rocks, and brittle veins formed
simultaneously in the interior of the body where
ductile deformation of the eclogite had already
ceased. Top-west shear fabrics are observed in the
granodioritic amphibolite-facies rocks less than 5 m
from the margin, indicating that rotational deformation began to occur during the amphibolite-facies
exhumation stage.
The presence of numerous amphibolite-facies
shear zones at the two western terminations of the
body and the coincidence of the long axes of
eclogite-facies mafic bodies in the area with both
eclogite-facies stretching features and regional
amphibolite to greenschist-facies stretching features
indicate that the regional-scale boudinage of mafic
material occurred before or during the early amphib-
olite-facies stage of exhumation. Following this
boudinage event, deformation was largely accommodated in the surrounding granodioritic lithologies.
During the amphibolite-facies stage of exhumation,
the mafic bodies behaved as rigid rafts of material
within a more deformable granodioritic matrix, and
therefore retained much of their eclogite-facies
mineralogy and structure.
5.6. Relationship of late regional-scale folding to
structures at DrØsdal
Regional-scale folds with E–W-trending hinges
dominate the present outcrop pattern of the WGC.
Modelling based on Ar spectra from K-feldspar (Eide
et al., 1999) and muscovite (Andersen et al., 1998)
imply that although decompression-related cooling of
UHP and HP rocks from ~700 to ~350 8C had
already occurred by 390F10 Ma (Andersen et al.,
1998), there was a marked increase in cooling rate
during Late Devonian–Early Carboniferous time
(360–340 Ma; Eide et al., 1999). This phase of rapid
cooling was in turn followed by slower cooling
during the Permian and Late Jurassic–Early Cretaceous (300–140 Ma). The Late Devonian–Early
Carboniferous cooling event correlates well with the
final stages of N–S shortening affecting the entire
existing crustal sequence (Eide et al., 1999) (see also
Torsvik et al., 1986; Krabbendam and Dewey, 1998;
Osmundsen and Andersen, 2001). It is important to
note, however, that the Middle Devonian rocks were
deposited in separate topographically constrained
basins, and not as a continuous sheet that has been
subsequently folded (Andersen et al., 1998), so some
N–S shortening must have occurred prior to their
deposition.
As part of the WGC, the Drbsdal mafic body and
its surrounding gneisses were undoubtedly reoriented
as a result of the dlateT E–W folding discussed
immediately above. However, there is no evidence
to suggest that structures within either the granodioritic rocks of the WGC or the mafic rocks they
envelope were in any way reworked during this event
(see also Section 5.7 and Fig. 18). The Drbsdal mafic
body is currently situated in the southerly dipping
limb of a regional-scale antiform, close to the NSDZ.
It records predominantly WSW-plunging eclogitefacies lineations, and lineations preserved within the
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R. Foreman et al. / Tectonophysics xx (2005) xxx–xxx
27
Fig. 18. Schematic 3D diagram showing geometry of the Drbsdal eclogite body after stage 4 of the structural model (see Section 5.7 of text).
surrounding gneisses are similarly oriented (Fig. 5).
According to its position on the map, the V3rdalsneset
eclogite body is situated in the northerly dipping
limb of the same antiform, and is also close to the
NSDZ (Figs. 1 and 2). Engvik and Andersen (2000)
report subhorizontal N–W-trending lineations for
V3rdalsneset. Unfolding of the regional antiform
between the Solund and Kvamshesten basins reveals
that the eclogite-facies lineations preserved within the
Drbsdal and V3rdalsneset bodies match closely,
provided that the lineations originally plunged shallowly to the SW or WSW. This not only implies that
the two bodies record simultaneous eclogite-facies
events, but that minimal rotation of the mafic bodies
as individual dpodsT has occurred since formation of
the eclogite-facies structures. Although circumstantial, this observation adds weight to the suggestion
that the Drbsdal eclogite body records a sequence of
structures formed at a range of times within its
eclogite-facies history. In particular, structures formed
during early residence at eclogite facies are partially
overprinted or reoriented by structures developed
during passage through the eclogite-facies portion of
the exhumation path.
5.7. The model
5.7.1. Stage 1
Formation of an eclogite sheet, probably southerly
dipping, occurs. The precursor to this eclogite sheet
was probably an igneous sheet of basic to intermediate
composition that had already passed through amphibolite and granulite facies, but may or may not have
chemically equilibrated at these metamorphic facies
(see also Section 2). The eclogite sheet has developed
a foliation defined by compositional differences, and
also has both shape fabric and lineation. Although this
initial stage of sheet formation and/or deformation at
eclogite facies is compatible with a constrictional
strain regime, direct evidence for transtension this
early in the strain history is lacking.
5.7.2. Stage 2
The eclogite sheet becomes folded on all scales
ranging from b1 m to N1 km, with smaller folds
parasitic to larger ones. The folds form with variable
hinge orientations, generally plunging W to SW, and
with axial surfaces generally dipping gently to the
south. Lineations form coevally, broadly parallel to
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28
R. Foreman et al. / Tectonophysics xx (2005) xxx–xxx
fold hinges, implying a constrictional strain field with
the maximum stretching direction oriented roughly
east–west, and shortening in the north–south and
vertical directions, in the present reference frame.
These structures may have formed under transtensional strain conditions, as described by Krabbendam
and Dewey (1998) with reference to amphibolite- and
greenschist-facies structures. It is interesting to note
that eclogite-facies constrictional fabrics related to
transtension have not been observed elsewhere in the
WGC, which implies either (1) that the Drbsdal area
has a different local strain history to other parts of the
WGC, or that (2) the other eclogite bodies in the
WGC ceased to deform internally at an earlier stage
than the Drbsdal body.
5.7.3. Stage 3
Boudinage of the eclogite sheet occurred under
continuing constrictional strain, presumably in the
same transtensional regime. Long axes of the kilometer-scale boudins were broadly E–W-trending (Fig.
17). Orientation of boudin necks is difficult to
ascertain, but they are arbitrarily shown perpendicular to lineation (i.e., stretching direction) in Fig. 17.
The onset of top-west-directed overshear then caused
rotation of the W-plunging antiform–synform pair,
which made up the upper part of the Drbsdal sheet at
this time (Fig. 17). The rotation would appear
clockwise in present-day map view. This rotation
tightened the existing folds throughout area C, and
but caused rotation only at the edges of the eclogite
body in areas A and B (area C is the area above
plane B in Fig. 17, and areas A and B are below
plane B in Fig. 17; these also correspond with areas
A, B, and C in Fig. 6). This top-west-directed
overshear probably occurred after boudinage but
could have begun before the sheet was boudinaged,
and is a precursor to the top-west shear evidenced
throughout the WGC in amphibolite- and greenschist-facies rocks (Fossen, 1992; Andersen et al.,
1998; Krabbendam and Dewey, 1998). Folds mainly
predate the onset of top-west-directed shear. It should
be noted that since folds were already quite tight with
many hinges not far from their final orientation, as
shown by fold style in area A (the NW part of the
Drbsdal body), large additional shear strains would
not have been necessary to reach the present-day
structural geometries.
5.7.4. Stage 4
Reorientation of the Drbsdal eclogite body into its
present-day position (Fig. 18) occurred by folding
around the Solund synform. The Solund synform
developed during regional N–S shortening, which
affected the whole crustal sequence during Late
Devonian–Early Carboniferous times, and which can
also be related to transtension (Krabbendam and
Dewey, 1998; Osmundsen and Andersen, 2001).
5.8. Regional implications
Two main unresolved questions concerning the HP
and UHP rocks of the WGC were put forward in the
introduction to this contribution: (1) What was the
mechanism of exhumation of the HP rocks? (2) What
is the relationship between the HP and UHP rocks,
and how did they become juxtaposed?
Based on the compilation of thermochronological
data, it can be demonstrated that the UHP rocks
constitute the lowermost structural units in western
Norway (Hacker et al., 2004). The UHP rocks cooled
below Ar blocking in muscovite after 380 Ma,
whereas higher structural levels in the WGC also
containing eclogites cooled between 400 and 380 Ma.
The large-scale EW folds also fold the muscovite Ar
isochrons in the WGC (Hacker et al., 2004).
The model presented above suggests a structural
development of an HP eclogite body under conditions
of constrictional transtension, with an additional
component of top-west-directed overshear at a relatively late stage of the eclogite-facies history. Krabbendam and Dewey (1998) presented a transtensional
model for the exhumation of the WGC, in which
relatively homogeneous constriction produced linear
fabrics in the deeper part of the orogen, including
lineation-parallel folding. However, the model was
based on observations of late-orogenic amphibolitefacies structures; indeed, the authors state that a gap of
about 5–10 Ma exists in the structural record
(Krabbendam and Dewey, 1998). This dgapT relates
to the time interval between the formation of eclogitefacies structures (Andersen et al., 1991, 1994) and the
late-orogenic amphibolite-facies structures on which
the transtensional model is based (Krabbendam and
Dewey, 1998). Since the linear (constrictional) structures preserved within the Drbsdal eclogite body are
subparallel to the regional principle stretching direc-
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R. Foreman et al. / Tectonophysics xx (2005) xxx–xxx
tion of the transtensional strain field of Krabbendam
and Dewey (1998), we propose that these exceptional
eclogite-facies structures fill the dgapT in the structural
record by providing a record of constrictional
deformation in transtension at conditions of T=720–
830 8C and P=19–21 kbar. Additionally, we have
shown that at least some of the eclogite bodies in the
WGC continued to deform significantly after the
onset of exhumation. Previously, data related to
eclogite-facies deformation in western Norway were
very limited, and could not be easily linked to the
amphibolite- and greenschist-facies portions of the
exhumation path. This contribution allows this link to
be made at least in one area.
The relationship between HP and UHP units cannot
be directly elucidated from the data presented in this
contribution. Even so, a few useful discussion points
can still be raised regarding possible links between the
HP rocks and UHP rocks. We have presented a model
involving exhumation of an HP eclogite body under
conditions of constrictional transtension; however,
horizontal stretching of the orogenic pile (including a
mantle wedge) via transtension would not give us the
observed structural sequence. This stretching alone
would not remove the mantle wedge from above the
WGC; it would just become thinner. Perhaps extensional shearing at an early stage in the exhumation
history facilitated the juxtaposition of UHP and HP
units in a thickened crustal welt, in a fashion similar to
that proposed for the UHP and HP rocks of the
Piemonte zone by Reddy et al. (1999) and Wheeler et
al. (2001), and the WGC by Andersen et al. (1991), Rey
et al. (1997), and Terry et al. (2000a,b). Since the
Drbsdal mafic body does not appear to preserve UHP
assemblages or structures, it cannot provide answers to
questions about the UHP history. Nevertheless, it
provides a much-needed link between the eclogiteand amphibolite-facies portions of the exhumation path
of the WGC, since the structures it preserves are
broadly coaxial with the later amphibolite-facies
structures as outlined above.
6. Conclusions
!
The Drbsdal mafic body is a coherent sheet of
compositionally banded eclogite. It is tightly
folded on the scale of hundreds of meters; folds
29
are asymmetric with upright axial planes, and their
hinges plunge moderately to the WSW. These
folds formed under conditions of constrictional
transtension at eclogite facies.
! Eclogite- to amphibolite-facies E–W-directed
stretching followed by top-west-directed overshear
after the onset of exhumation further modified the
shape of the Drbsdal body and other mafic bodies in
its immediate vicinity. This phase of deformation was
partially accommodated by movement on submeterscale shear zones, giving rise to the present boudinlike distribution and elongate shapes of eclogite
lenses in the area. Long axes of the lenses, and the
orientations of lineations within them are consistent
with stretching features in the surrounding amphibolite-facies gneisses, indicating continuity of deformation regime during the transition between
eclogite- and amphibolite-facies exhumation.
! Pervasive eclogite-facies stretching lineations
within the Drbsdal eclogite lie subparallel to
eclogite-facies fold hinges, and to eclogite-facies
lineations within veins (defined by kyanite and
omphacite). Structural and metamorphic data
indicate that the timing of formation of the
pervasive lineation, the eclogite-facies folds, and
the kyanite-bearing veins overlapped substantially
during the eventful eclogite-facies history dominated by a constrictional strain field, with X/YNY/Z
and YNZ, and Yb1. The X-axis of the strain
ellipsoid was oriented broadly E–W, the Z-axis
was oriented N–S, and the Y-axis was subvertical
in the present reference frame.
! Eclogite-facies fold hinges and lineations in the
Tinghaugen and Teiges3ta areas are oblique to the
present E–W-dominated structures observed
throughout the rest of the Drbsdal body. These
oblique fabrics represent parts of the body that
ceased to deform at an earlier stage. Partitioning of
strain into the rest of the eclogite body resulted in a
relative clockwise rotation of the majority of the
eclogite body. This observed obliquity was achieved
by syneclogite-facies top-west-directed overshear.
! Fold axes, boudin long axes, and lineations in the
majority of the Drbsdal body are subparallel to the
regional linear features formed during extension.
These include the crenulation cleavage and shear
sense indicators associated with movement along
the NSDZ (Andersen et al., 1994; Krabbendam,
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R. Foreman et al. / Tectonophysics xx (2005) xxx–xxx
1998; Krabbendam and Dewey, 1998), the eclogite- to amphibolite-facies stretching lineations
described by (Engvik and Andersen, 2000), and
the extensional fabric observed in the nappe units
(Fossen, 1992, 1993). Circumstantial evidence
from Drbsdal suggests that the subparallel eclogite-, amphibolite-, and greenschist-facies structures of the Sunnfjord area formed during one
progressive deformation event corresponding to
extensional exhumation. Our observations fit well
with a model involving N–S shortening coupled
with E–W (broadly horizontal) extension in the
lower crust during early extensional exhumation.
Local coaxiality of deformation during eclogite
facies is demonstrated by coaxial structures at
Drbsdal, but noncoaxial transtensional deformation probably characterized the large-scale behaviour of the WGC. Sequential partitioning of
deformation into domains of different rheologies
during exhumation may explain the preservation
of varied mineralogies and structures in this part of
the WGC, despite the shared P–T history. Broad
antiforms have obviously rotated the rocks within
the WGC. However, the close match in orientation
of lineations from the Drbsdal and V3rdalsneset
eclogites implies that minimal rotation of mafic
bodies relative to each other has occurred since
formation of the eclogite-facies structures.
Acknowledgements
This work has benefited considerably from the
constructive reviews of P. Rey and an anonymous
reviewer. G. Potts and A. McCaig are also thanked for
constructive comments, which improved the manuscript. H. Austrheim, M. Erambert, and M.G. Lund
are thanked for their help with chemical analyses, and
numerous useful discussions. NERC funding for R.F.
is acknowledged.
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