TECTO-125958; No of Pages 21
Tectonophysics xxx (2013) xxx–xxx
Contents lists available at SciVerse ScienceDirect
Tectonophysics
journal homepage: www.elsevier.com/locate/tecto
Crustal exhumation of the Western Gneiss Region UHP terrane,
Norway: 40Ar/39Ar thermochronology and fault-slip analysis
Emily O. Walsh a,b,⁎, Bradley R. Hacker a, Phillip B. Gans a, Martin S. Wong a,c, Torgeir B. Andersen d
a
Earth Science, University of California, Santa Barbara, CA 93106, USA
Geology, Cornell College, Mount Vernon, IA 52314, USA
Geology, Colgate University, Hamilton, NY 13346, USA
d
University of Oslo, Centre for Earth Evolution and Dynamics (CEED), P.O. Box 1048, Blindern, 0316 Oslo, Norway
b
c
a r t i c l e
i n f o
Article history:
Received 29 June 2012
Received in revised form 15 June 2013
Accepted 27 June 2013
Available online xxxx
Keywords:
Ultrahigh-pressure exhumation
Western Gneiss Region
40
Ar/39Ar muscovite thermochronology
40
Ar/39Ar K-feldspar thermochronology
Fault-slip analysis
a b s t r a c t
New 40Ar/39Ar muscovite and K-feldspar thermochronology combined with existing data reveal the timing
and patterns of late-stage exhumation across the Western Gneiss Region (U)HP terrane. Muscovite age contours
show that exhumation into the mid-upper crust progressed westward over a ~20 Myr period (~400–380 Ma).
This exhumation was caused by i) E–W stretching and eastward tilting north of Nordfjord, where muscovite
ages decrease from the foreland allochthons westward into the UHP domains, and ii) differential exhumation
south of Nordfjord, where muscovite ages depict a NE–SW dome-like pattern and the Western Gneiss
Region is bounded by overlying units little affected by the Scandian metamorphism. Exhumation of the UHP
domains into the mid-upper crust by late folding continued through ~374 Ma. The smooth gradient of fairly
flat muscovite age spectra demonstrates minimal influence of excess Ar, which is relatively unusual for a
(U)HP terrane. 40Ar/39Ar spectra and modeled cooling histories from K-feldspar combined with brittle–ductile
and brittle fault data indicate continued exhumation on local structures into the Permian.
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
Over the past three decades, much work has been done to understand the subduction and exhumation of ultrahigh-pressure (UHP)
rocks. Once a controversial concept, subduction of continental crust to
ultrahigh pressures is now known to have occurred repeatedly
throughout the Phanerozoic (Ernst, 2001). UHP exhumation may take
place in two stages at different rates: an initial decompression from
mantle depths to the base of the crust, and a second stage through the
crust (Walsh and Hacker, 2004). Exhumation of continental crust
from mantle depths has often been attributed to changes in buoyancy
or rheology (e.g., Chemenda et al., 1995; Milnes and Koyi, 2000;
Peterman et al., 2009), whereas exhumation of continental crust
through continental crust may be driven by, or be a byproduct of, a
wider range of processes (e.g., Braathen et al., 2004; Dewey and
Strachan, 2003; Johnston et al., 2007). Spatial and temporal variations
in exhumation rate and kinematics across a UHP terrane are critical to
evaluating the processes involved in exhuming UHP rocks through the
crust. Even in the relatively well-studied UHP Western Gneiss Region
(WGR) of Norway, these data remain incompletely known (Kendrick
⁎ Corresponding author at: Geology, Cornell College, Mount Vernon, IA 52314, USA.
Tel.: +1 319 895 4302; fax: +1 319 895 5667.
E-mail address: ewalsh@cornellcollege.edu (E.O. Walsh).
et al., 2004). Here, we present a dense net of low-temperature
thermochronology data and a regionally distributed set of late-stage,
fault-slip data. These data allow us to address the following specific
questions: i) What were the deformation kinematics during exhumation through the crust? Is there an identifiable spatial or temporal variation in the kinematics, and how did exhumation through the crust
differ from earlier, high-temperature exhumation? ii) At what rate did
cooling occur and how did it vary spatially? What does this mean for exhumation rates and their spatial variation? What implications does this
have for the mechanism of crustal exhumation?
2. Geology of the Western Gneiss Region (WGR)
The WGR of Norway (Fig. 1) is a window of Baltican Proterozoic
gneiss with igneous and metamorphic ages of ~ 1650 Ma, ~ 1200 and
~ 950 Ma (Austrheim et al., 2003; Skår, 2000; Tucker et al., 1991) exposed beneath a stack of allochthons initially emplaced onto the margin of Baltica between ~ 430 Ma and 415 Ma (Hacker and Gans, 2005;
Roberts, 2003). The nappe sequence includes part of Laurentia in the
Uppermost Allochthon, ophiolitic rocks from the outboard oceanic
terranes in the Upper Allochthon, and displaced sedimentary and
crystalline rocks of the rifted and hyperextended margin of Baltica
in the Upper, Middle, and Lower Allochthons (Andersen et al.,
2012). These allochthons were originally defined, and are best exposed, east of the WGR, but attenuated equivalents crop out across
the WGR in relatively coherent (Robinson, 1995) but disconnected
0040-1951/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.tecto.2013.06.030
Please cite this article as: Walsh, E.O., et al., Crustal exhumation of the Western Gneiss Region UHP terrane, Norway: 40Ar/39Ar thermochronology
and fault-slip analysis, Tectonophysics (2013), http://dx.doi.org/10.1016/j.tecto.2013.06.030
2
E.O. Walsh et al. / Tectonophysics xxx (2013) xxx–xxx
Høybakken Detachment
Carboniferous-Devonian Basins
Caledonian Allochthons
Autochthon (Baltican basement)
100 km
tra
Hi
ag
el
ønd
Trondheim
FC
Tr
re-
Mø
hm
en
t
Nordøyane
de
ta
c
UHP
domains
t
aul
aF
ås
-Sn
ge
n
Sørøyane
Rø
ra
62°N
Stadlandet
Nordfjord
Hornelen
Basin
Nordfjord-Sogn
Detachment
Jotun
Nappe
Kvamshesten Basin
l-G
rda
e
La
øl
est
lt
au
ef
d
jen
lt
fau
Ol
10°E
Fig. 1. Geologic map of the Western Gneiss Region showing the ultrahigh-pressure domains and the major structures related to exhumation.
fragments (Root et al., 2005; Terry et al., 2000; Tveten, 1998; Walsh
and Hacker, 2004).
The convergence of Baltica and Laurentia resulted in a Himalayatype collision, with NW-directed subduction of the nappes and the
Baltican margin beneath Laurentia (Hacker and Gans, 2005; Labrousse
et al., 2010; Torsvik and Cocks, 2005). This episode, the Scandian
orogeny, resulted in metamorphism of crustal rocks to conditions as
high as 3.6 GPa and 800 °C (Cuthbert et al., 2000; Dobrzhinetskaya
et al., 1995; Krogh Ravna and Terry, 2004; Terry et al., 2000; Wain,
1997) over a period of about 20 Myr from ~420 to ~400 Ma (see summary in Kylander-Clark et al., 2009). UHP rocks are now exposed along
the west coast of the WGR in 3 distinct domains (Fig. 1), which, based
upon the location of UHP rocks beneath HP rocks and younger muscovite ages within the domains, define apparent antiformal culminations
(Hacker et al., 2010). Metamorphic grade increases northwestward
across the WGR (Griffin et al., 1985; Krogh, 1977; Tucker et al., 1991)
as does the degree of Scandian deformation (Barth et al., 2010; Hacker
et al., 2010; Krabbendam and Wain, 1997; Milnes et al., 1997; Young
et al., 2007).
Many different models have been suggested for the exhumation of the
UHP rocks in west Norway; the majority includes two stages: relatively
rapid exhumation from mantle depths to the base of the continental
crust, followed by slower crustal exhumation. The cause of the initial,
mantle stage of exhumation is unknown and variously inferred to have
been caused by removal of a dense lithospheric root (e.g., Andersen and
Jamtveit, 1990; Austrheim, 1991), a change in plate motion (e.g., Dewey
and Strachan, 2003; Fossen, 2000), forced-return flow (e.g., Terry and
Robinson, 2004), slab breakoff and eduction (Andersen et al., 1991;
Brueckner and van Roermund, 2004; Duretz et al., 2012), or delamination
due to an unspecified gravitational instability (e.g., Hacker, 2007; Hurich,
1996; Johnston et al., 2007; Labrousse et al., 2004; Peterman et al., 2009;
Walsh and Hacker, 2004). The calculated rates of exhumation for the
initial stage are often quoted as ~10 mm/yr or faster (e.g., Carswell et
al., 2003; Krabbendam and Dewey, 1998; Kylander-Clark et al., 2008;
Terry et al., 2000; Walsh et al., 2007). Once the rocks reached crustal
depths, they were extensively overprinted by granulite- (Straume and
Austrheim, 1999) or, more commonly, amphibolite-facies metamorphism
at ~650–800 °C down to pressures of 0.5 GPa (Labrousse et al., 2004; Root
et al., 2005; Spencer et al., 2013; Terry and Robinson, 2003; Walsh and
Hacker, 2004).
Extension is commonly called upon during the second stage of exhumation to have moved the rocks from amphibolite-facies conditions at
the base of the crust to greenschist-facies conditions in the upper crust
(alternatives exist, see e.g., Andersen et al., 1994; Dewey and Strachan,
2003; Fossen, 2000; van Roermund and Drury, 1998). Evidence for this
includes vertical shortening combined with strong top-W extension
along the Nordfjord–Sogn Detachment Zone (Johnston et al., 2007;
Marques et al., 2007; Norton, 1986; Séranne and Séguret, 1987), as
well as sinistral (rotated normal-sense) shear along the Møre–Trøndelag
Fault Complex in the north (Braathen et al., 2000; Krabbendam and
Dewey, 1998; Séranne, 1992). During this later stage of exhumation
into the mid-upper crust, Buchan-type amphibolite-facies recrystallization affected local domains in the west, and late-stage folds formed
(Fossen, 2010; Krabbendam and Dewey, 1998). Within the Sørøyane
UHP domain, this second stage of exhumation began after isothermal
(~750 °C) decompression to granulite-facies conditions at 15–20 km
depth (~0.5 GPa), creating an unusually hot geothermal gradient
roughly equivalent to that of the Basin and Range today (Root et al.,
Please cite this article as: Walsh, E.O., et al., Crustal exhumation of the Western Gneiss Region UHP terrane, Norway: 40Ar/39Ar thermochronology
and fault-slip analysis, Tectonophysics (2013), http://dx.doi.org/10.1016/j.tecto.2013.06.030
E.O. Walsh et al. / Tectonophysics xxx (2013) xxx–xxx
the WGR, where the bulk of the eclogites and all the known UHP
rocks crop out, is much less well characterized. This study addresses
that deficiency by analyzing 37 additional samples (Fig. 2; Table 1)
collected mainly from typical quartzofeldspathic gneiss (Fig. 2A–E)
but also from mafic gneiss (Fig. 2F: 8815G12, K5622A5), rocks in
the Møre–Trøndelag Fault Complex (Fig. 2G: J5816A, J5816B,
J5816I), discordant dikes (Fig. 2F: E9814A6, H5622B, P6804A), and
the allochthons (Fig. 2H, I: 8907B5, 929, J5813F, J5814H, J5814N4,
J5814S, J5815K).
The main amphibolite-facies fabric across the WGR, Scandian
coaxial E–W stretching (Hacker et al., 2010), is overprinted by an
2005). Cooling occurred rapidly after this isothermal decompression,
with rates of ~30–90 °C/Myr implied by the difference in U–Pb zircon
and titanite and 40Ar/39Ar muscovite ages (Kylander-Clark et al., 2008;
Root et al., 2005).
K-white mica (henceforth, “muscovite”) thermochronology has
been used effectively over the past several decades to reveal the
timing of exhumation of (U)HP rocks across the WGR (e.g.,
Andersen et al., 1998; Chauvet and Dallmeyer, 1992; Fossen and
Dunlap, 1998; Hacker and Gans, 2005; Lux, 1985; Root et al., 2005;
Walsh et al., 2007; Warren et al., 2012; Young et al., 2007). Most of
the data are from the southern half of the WGR; the northern half of
415
Age = 370.7 ± 7.4 Ma
MSWD = 2.99 (<2.26)
36
0.002
0.001
40/36=3071.0±3614.5
39
Cumulative Ar
0.000
0.000
1
Ar/40Ar
WMPA =390.4 ± 0.4 Ma
0.001
376
370
0
415
Cumulative 39Ar
Ar/40Ar
395
WMPA = 397.7 ± 0.5 Ma
445
Cumulative 39Ar
0.031
Ar/40Ar
0.042
0.052
40/36=511.9±39.8
0.011
0.004
H5622C
425
397 to 375 Ma
0.023
39
0.034
Ar/40Ar
0.045
0.057
H5622C
Atm
0.003
Age = 377.2 ± 3.9 Ma
MSWD = 0.58 (<2.63)
0.002
0.001
405
395
0
39
Age = 393.7 ± 1.0 Ma
MSWD = 1.77 (<1.85)
0.000
0.000
1
435
415
0.021
0.002
0.001
375
365
0
0.010
36
385
0.066
H5621B
Atm
0.003
405
0.052
40/36=386.1±36.1
0.004
H5621B
0.040
Ar/40Ar
Age = 389.7 ± 0.6 Ma
MSWD = 2.01 (<2.00)
0.000
0.000
1
39
0.002
36
382
0.026
E1612Q8
Atm
0.003
394
388
0.013
0.004
E1612Q8
Ar/40Ar
Apparent Age (Ma)
WMPA = 387.1 ± 0.4 Ma
383
400
Apparent Age (Ma)
Ar/40Ar
399
391
E1612C7
Atm
0.003
407
375
0
Apparent Age (Ma)
0.004
E1612C7
36
Apparent Age (Ma)
A
3
40/36=5339.4±543.9
Cumulative 39Ar
1
0.000
0.000
0.011
0.022
39
0.033
Ar/40Ar
0.044
0.055
Fig. 2. Muscovite age spectra and isochron plots for: A–E) quartzofeldspathic gneisses of the Baltican basement; F) mafic rocks and discordant dikes; G) samples from the Møre–
Trøndelag Fault Complex; and H,I) samples from the allochthons. The noted age has been recalculated according to Renne et al. (1998); the preferred age is bold and is listed in the
“Renne” column in Table 1. WMPA = weighted mean plateau age; WMA = weighted mean age. Error shown is ±1σ without uncertainty in J.
Please cite this article as: Walsh, E.O., et al., Crustal exhumation of the Western Gneiss Region UHP terrane, Norway: 40Ar/39Ar thermochronology
and fault-slip analysis, Tectonophysics (2013), http://dx.doi.org/10.1016/j.tecto.2013.06.030
4
E.O. Walsh et al. / Tectonophysics xxx (2013) xxx–xxx
405
0.004
H5702A
389
WMA = 388.8 ± 0.4 Ma
381
Age = 388.7 ± 0.6 Ma
MSWD = 0.06 (<2.41)
0.002
40/36=316.5±38.5
0.001
373
365
H5702A
Atm
0.003
397
36Ar/40Ar
Apparent Age (Ma)
B
0
Cumulative 39Ar
0.000
0.000
1
0.011
0.021
0.032
0.042
0.053
39Ar/40Ar
0.004
J5811D
388
WMPA = 388.2± 0.4 Ma
382
0.002
Age = 388.2 ± 0.6 Ma
MSWD = 1.01 (<2.15)
40/36=294.4±25.0
0.001
376
370
J5811D
Atm
0.003
394
36Ar/40Ar
Apparent Age (Ma)
400
0
Cumulative 39Ar
0.000
0.000
1
0.011
0.021
0.032
0.042
0.053
39Ar/40Ar
0.004
J5815E1
395
WMPA = 378.8 ± 0.4 Ma
385
0
Cumulative 39Ar
Age = 375.6 ± 0.6 Ma
MSWD = 1.56 (<1.85)
0.002
0.001
375
365
J5815E1
Atm
0.003
405
36Ar/40Ar
Apparent Age (Ma)
415
40/36=884.1±34.9
0.000
0.000
1
0.012
0.023
0.035
0.047
0.058
39Ar/40Ar
0.004
J5815G
Atm
0.003
385
36Ar/40Ar
Apparent Age (Ma)
390
380
WMPA = 380.1 ± 0.4 Ma
375
370
365
0
Cumulative 39Ar
1
J5815G
Age = 379.7 ± 0.5 Ma
MSWD = 1.61 (<1.89)
0.002
0.001 40/36=363.7±17.2
0.000
0.000
0.010
0.021
0.031
0.042
0.052
39Ar/40Ar
Fig. 2 (continued).
extensive suite of brittle–ductile and brittle faults with exposed slip
surfaces ranging from 10's of m to cm scale. A few faults appear
from outcrop and thin-section observations to have been active during amphibolite-facies symplectite formation (Hacker, 2007;
Peterman et al., 2009), and their slip surfaces are characterized chiefly
by coarse-grained to fine-grained biotite (Fig. 3A). Most brittle–
ductile faults postdated symplectite formation and are characterized
by recrystallized biotite ± epidote ±feldspar ± quartz (Fig. 3).
Semi-brittle to brittle faults are marked by chlorite ± epidote ±
quartz ± carbonate ± Mn–Fe oxides and, locally, cataclasite, clay or
gouge. At several sites, brittle faults overprint semi-ductile to
semi-brittle faults, but at many outcrops there is a continuum between the brittle–ductile and brittle faults. Because these faults
were active after the major, high-temperature amphibolite-facies
metamorphism, they provide kinematic information about the exhumation of the (U)HP rocks into the upper crust.
3. Analytical techniques
3.1. Thermochronology
Muscovite and K-feldspar samples were irradiated at Oregon State
University for 40 h and analyzed at the University of California, Santa
Barbara, by Staudacher-type resistance-furnace step heating. The irradiation flux monitor was Taylor Creek Rhyolite sanidine, for which we assumed an age of 28.34 ± 0.28 Ma (Renne et al., 1998). The uncertainty
in the irradiation flux monitor, J, was set conservatively at ±0.12% 2σ.
Previously published ages from the study area have been recalculated
Please cite this article as: Walsh, E.O., et al., Crustal exhumation of the Western Gneiss Region UHP terrane, Norway: 40Ar/39Ar thermochronology
and fault-slip analysis, Tectonophysics (2013), http://dx.doi.org/10.1016/j.tecto.2013.06.030
E.O. Walsh et al. / Tectonophysics xxx (2013) xxx–xxx
400
0.004
J5816J
388
WMPA = 389.8 ± 0.4 Ma
382
Age = 389.6 ± 0.5 Ma
MSWD = 1.63 (<1.78)
0.002
40/36=320.5±23.4
0.001
376
370
J5816J
Atm
0.003
394
36Ar/40Ar
Apparent Age (Ma)
C
5
0
Cumulative 39Ar
0.000
0.000
1
0.010
0.021
0.031
0.042
0.052
39Ar/40Ar
0.004
J5816L1
WMPA = 397.3 ± 0.4 Ma
398
392
0
Cumulative 39Ar
Age = 397.1 ± 0.5 Ma
MSWD = 1.24 (<2.15)
0.002
0.001
386
380
J5816L1
Atm
0.003
404
36Ar/40Ar
Apparent Age (Ma)
410
40/36=355.5±18.0
0.000
0.000
1
0.010
0.021
0.031
0.042
0.052
39Ar/40Ar
0.004
P5627E2
Atm
0.003
394
36Ar/40Ar
Apparent Age (Ma)
400
388
WMA = 387.6 ± 0.4 Ma
382
376
370
0
Cumulative 39Ar
0.002
0.001 40/36=1052.0±48.0
0.000
0.000
1
P5627E2
Age = 383.7 ± 0.6 Ma
MSWD = 2.07 (<2.00)
0.010
0.021
0.031
0.042
0.052
39Ar/40Ar
0.004
P5627F3
383
P5627F3
Atm
0.003
389
36Ar/40Ar
Apparent Age (Ma)
395
WMPA = 385.8 ± 0.4 Ma
377
Age = 386.0 ± 0.6 Ma
MSWD = 3.89 (<1.89)
0.002 40/36=261.1±58.8
0.001
371
365
0.000
0.000
0.010
0.021
0.031
0.042
0.052
39Ar/40Ar
Fig. 2 (continued).
according to Renne et al. (1998), to be consistent with our new data.
Ages are reported with 1σ uncertainty. Uncertainties quoted in the text
include uncertainty in J only to facilitate comparison of 40Ar/39Ar ages
of different samples. Full uncertainties—including also uncertainties in
40
K decay constant and monitor age—are included in Table 1; these are
the uncertainties to be used when comparing the 40Ar/39Ar ages to
those determined by other methods (e.g., U–Pb).
For muscovite, the gas was incrementally released in 14–20 steps
per sample, with 15-minute 1100 °C line blanks run periodically
between samples (see Appendix A in the Supplementary Material).
For K-feldspar, 47–104 step experiments were run with isolation
times of 15 min to 10 h (see Calvert et al., 1999). Many of the
K-feldspar samples yielded spectra suitable for full diffusion-domain
analysis (Lovera et al., 1989, 2002), which we completed using modified 1997 versions of Lovera's (1992) modeling routines. A minimum
of four age steps from a spectrum was fit with a line to define the activation energy E and frequency factor Do (Lovera et al., 1989); more
steps were added if the fit improved. The number of domains
was limited to a minimum of three and a maximum of eight. The
diffusion-domain theory predicts constant or monotonically increasing age spectra, and spectra that do not match this ideal were either
not modeled or had the uncertainties of aberrant step ages increased
until the spectrum showed a monotonic age increase: i) multiple isothermal, low-temperature steps designed to identify Cl-correlated excess 40Ar (Harrison et al., 1994) were assigned the age of the youngest
step in the group, and ii) steps with low radiogenic yields (b 95%) and
anomalously old ages were adjusted to provide a smoothly increasing
trend. Cooling histories were calculated from initial times 50–100 Myr
older than the oldest step. Only those cooling histories that provide a
good fit to the data are shown.
Please cite this article as: Walsh, E.O., et al., Crustal exhumation of the Western Gneiss Region UHP terrane, Norway: 40Ar/39Ar thermochronology
and fault-slip analysis, Tectonophysics (2013), http://dx.doi.org/10.1016/j.tecto.2013.06.030
6
E.O. Walsh et al. / Tectonophysics xxx (2013) xxx–xxx
395
0.004
P5627K
383
WMPA = 386.5 ± 0.4 Ma
377
0.002 40/36=486.8±121.4
0.001
371
365
P5627K
Age = 385.3 ± 0.8 Ma
MSWD = 0.47 (<1.94)
Atm
0.003
389
36Ar/40Ar
Apparent Age (Ma)
D
0
0.000
0.000
1
Cumulative 39Ar
0.011
0.022
0.034
0.045
0.056
39Ar/40Ar
394
36Ar/40Ar
WMPA = 387.4 ± 0.4 Ma
382
0
40/36=239.1±30.4
0.000
0.000
1
Cumulative 39Ar
0.004
P6805H2
0.021
0.031
0.042
0.052
P6805H2
Age = 388.2 ± 0.4 Ma
MSWD = 1.21 (<1.85)
Atm
0.003
389
WMPA = 388.2 ± 0.4 Ma
383
0.010
39Ar/40Ar
36Ar/40Ar
Apparent Age (Ma)
395
377
0.002
40/36=317.4±7.4
0.001
371
0
0.000
0.000
1
Cumulative 39Ar
0.011
P6806J
Atm
0.003
389
383
WMA = 382.2 ± 0.4 Ma
377
0.022
0.034
0.045
0.056
39Ar/40Ar
0.004
395
0.002
P6806J
Age = 379.8 ± 0.5 Ma
MSWD = 1.07 (<2.15)
40/36=458.1±14.0
0.001
371
365
0.002
0.001
376
365
P5629O
Age = 388.0 ± 0.6 Ma
MSWD = 1.98 (<2.00)
Atm
0.003
388
370
Apparent Age (Ma)
0.004
P5629O
36Ar/40Ar
Apparent Age (Ma)
400
0
Cumulative 39Ar
1
0.000
0.000
0.011
0.021
0.032
0.042
0.053
39Ar/40Ar
Fig. 2 (continued).
3.2. Electron-probe microanalysis
Muscovite grains from each of the 37 rock samples were analyzed
on a Cameca SX-50 electron microprobe with 5 wavelength spectrometers at the University of California, Santa Barbara, using an accelerating voltage of 15 kV, a current of 15 nA, a spot size of 2 μm,
and natural and synthetic mineral standards. Core and rim analyses
reported are averages of 2 spot analyses.
3.3. Fault-slip analysis
We measured fault slip surfaces throughout the study area, recording the orientation of slip planes, the orientation of lineations/
striae, sense of slip (Petit, 1987), and a measure of our confidence in
the latter based on the type of indicator and degree of preservation
(using a scale of 1 to 4, where 1 is certain, 2 is reliable, 3 is inferred, and
4 is unknown, Angelier, 1984). We recorded the type of mineralization
along the faults, mineralization within associated mode-I veins, and apparent overprinting relationships to attempt assessment of the relative
ages of different fault sets in each outcrop (Table 3).
Like Braathen (1999) and Braathen and Bergh (1995), we separated
different fault sets at individual outcrops (or sets of nearby outcrops)
manually, rather than relying on computationally based separation;
this was done principally because the gneissic anisotropy likely affected
the orientation and abundance of fault sets. In general, each outcrop
is dominated by a particular kind of fault with a characteristic
Please cite this article as: Walsh, E.O., et al., Crustal exhumation of the Western Gneiss Region UHP terrane, Norway: 40Ar/39Ar thermochronology
and fault-slip analysis, Tectonophysics (2013), http://dx.doi.org/10.1016/j.tecto.2013.06.030
E.O. Walsh et al. / Tectonophysics xxx (2013) xxx–xxx
395
36Ar/40Ar
383
WMPA = 384.7 ± 0.4 Ma
377
395
Cumulative 39Ar
0.000 0.011
0.004
383
WMPA = 384.5 ± 0.4 Ma
0.022 0.034
39Ar/40Ar
0.045
0.056
P6818C2
Atm
0.003
389
Age = 383.7 ± 0.6 Ma
MSWD = 1.19 (<2.00)
0.002
40/36=349.8±22.8
0.001
371
365
40/36=349.6±17.1
0.000
1
P6818C2
377
Age = 384.2 ± 0.5 Ma
MSWD = 1.50 (<1.82)
0.002
0.001
371
0
P6816B1
Atm
0.003
389
365
Apparent Age (Ma)
0.004
P6816B1
36Ar/40Ar
Apparent Age (Ma)
E
7
0
Cumulative 39Ar
1
0.000
0.000
0.011
0.022
0.034
0.045
0.056
39Ar/40Ar
900
840
780
720
931 to 450 Ma
660
0
425
Apparent Age (Ma)
405
H5621A
Apparent Age (Ma)
Apparent Age (Ma)
960
Cumulative 39Ar
1
H5622E
399
WMA = 397.2 ± 0.4 Ma
393
387
381
375
0
Cumulative 39Ar
1
J5816K
419
413
WMA = 402.4 ± 0.5 Ma
407
401
395
0
Cumulative 39Ar
1
Fig. 2 (continued).
mineralization and a relatively limited variation in planar or linear elements. Faults with a large range in measured planar or linear elements
were not placed in a single group unless there was a continuum of measurements. Strike-slip and dip-slip faults were considered separately
unless they exhibited the same calculated principal directions. Each
fault-slip dataset was analyzed following Sperner et al. (1993)
using the NDA program (Spang, 1972) written by Sperner and
Ratschbacher (1994) and an angle of 45° between σ1 and each fault
plane for the brittle–ductile faults and 30° for the brittle faults (Table 3).
Our objective was to better understand the regional, postamphibolite-facies deformation of the WGR and to characterize
large-scale spatial and temporal variations in the deformation. Because
we observed no systematic differences in brittle–ductile and brittle faults
at most outcrops, we treat the kinematic data from each locality as a single data set. We do not attempt to resolve principal directions at high
precision, largely because most of the rocks are anisotropic and many
of the brittle–ductile faults reactivate foliation planes. We follow
Lacombe (2012) in interpreting the calculated principal directions as
indicators of paleostress.
4. Results
4.1. Muscovite thermochronology
Most of the analyzed muscovite samples yielded slightly U-shaped
age spectra; nine samples produced spectra for which weighted mean
Please cite this article as: Walsh, E.O., et al., Crustal exhumation of the Western Gneiss Region UHP terrane, Norway: 40Ar/39Ar thermochronology
and fault-slip analysis, Tectonophysics (2013), http://dx.doi.org/10.1016/j.tecto.2013.06.030
8
E.O. Walsh et al. / Tectonophysics xxx (2013) xxx–xxx
395
0.004
8815G12
383
377
WMPA = 382.1 ± 0.4 Ma
Cumulative 39Ar
Age = 381.7 ± 0.5 Ma
MSWD = 0.66 (<2.07)
0.002
0.001
371
365
0
8815G12
Atm
0.003
389
36Ar/40Ar
Apparent Age (Ma)
F
40/36=359.6±14.3
0.000
0.000
1
0.010
0.021
0.031
0.042
0.052
39Ar/40Ar
WMPA = 389.3 ± 0.4 Ma
Cumulative 39Ar
0.004
H5622B
36Ar/40Ar
WMPA = 397.9 ± 0.4 Ma
0.031
0.042
0.052
Age = 397.5 ± 0.6 Ma
MSWD = 1.33 (<1.82)
0.002
0.001 40/36=404.8±13.8
Cumulative 39Ar
1
567
K5622A5
405
395
385
414 to 403 Ma
Cumulative
39Ar
0.000
0.000
0.010
0.021
0.031
0.042
0.052
39Ar/40Ar
415
375
0
0.021
H5622B
Atm
0.003
383
425
0.010
39Ar/40Ar
399
375
0
0.002 40/36=554.3±118.0
0.000
0.000
1
407
391
Age = 386.4 ± 1.3 Ma
MSWD = 2.73 (<2.26)
0.001
391
415
Apparent Age (Ma)
36Ar/40Ar
411
401
E9814A6
Atm
0.003
421
381
0
Apparent Age (Ma)
0.004
E9814A6
Apparent Age (Ma)
Apparent Age (Ma)
431
1
P6804A
547
527
507
487
558 to 471 Ma
467
0
Cumulative 39Ar
1
Fig. 2 (continued).
plateau ages (WMPA) (N 50% of 39Ar released) could be calculated, 25
produced relatively flat spectra for which we computed weighted
mean ages (WMA), and 3 samples yielded spectra with a range of
step ages. Twenty-five of the 37 samples yielded well-fit inverse
isochrons; the isochron ages are preferred because they account for deviation of the trapped 40Ar/36Ar ratio from atmospheric and, in some
samples, use a greater percentage of the data than the WMPA.
The new muscovite 40Ar/39Ar ages from basement gneisses increase
eastward across the WGR toward the foreland (Fig. 4). Ages at the
eastern limit of the WGR are generally ~400 Ma (e.g., 398.5 ± 0.5 Ma
at J5814H, 402.4 ± 1.0 Ma at J5816K); they increase gradually into
the foreland allochthon stack to the east and increase more dramatically
into the Jotun Nappe (558–471 Ma at P6804A, 931–450 Ma at H5621A)
to the southeast. The samples from the Møre–Trøndelag Fault Complex
range from ~387–383 Ma, younger than previously dated muscovite
from the Central Norway basement window and the Høybakken detachment zones (Dallmeyer et al., 1992; Eide et al., 2005) and similar
to the hornblende and biotite 40Ar/39Ar ages from detachment
mylonites (Kendrick et al., 2004). The youngest ages (e.g., 370.2 ±
0.9 Ma from strain shadows in a mylonite at Terry929 on Fjørtoft,
Nordøyane) are from samples within UHP domains.
The Si content of the muscovites ranges from 3.01 to 3.31 atoms per
formula unit (apfu) (Table 2). There is up to 0.10 Si apfu difference between the cores and rims of individual grains, but the difference is not
systematically positive or negative within samples; more-detailed transects by Warren et al. (2012) on similar samples show that some WGR
muscovites have Si-rich cores and Si-poor rims. There is no relationship
apparent between the Si content of the muscovite cores or rims and age
Please cite this article as: Walsh, E.O., et al., Crustal exhumation of the Western Gneiss Region UHP terrane, Norway: 40Ar/39Ar thermochronology
and fault-slip analysis, Tectonophysics (2013), http://dx.doi.org/10.1016/j.tecto.2013.06.030
E.O. Walsh et al. / Tectonophysics xxx (2013) xxx–xxx
0.004
J5816A
388
WMA = 389.3 ± 0.4 Ma
382
Age = 386.6 ± 1.2 Ma
MSWD = 0.42 (<1.89)
0.002
40/36=827.9±207.8
0.001
376
370
J5816A
Atm
0.003
394
36Ar/40Ar
Apparent Age (Ma)
G 400
9
0
Cumulative 39Ar
0.000
0.000
1
0.010
0.021
0.031
0.042
0.052
39Ar/40Ar
0.004
J5816B
385
WMPA = 383.0 ± 0.4 Ma
375
0
Cumulative 39Ar
Age = 382.7 ± 0.9 Ma
MSWD = 2.19 (<2.15)
0.002
0.001
365
355
J5816B
Atm
0.003
395
36Ar/40Ar
Apparent Age (Ma)
405
40/36=353.0±102.2
0.000
0.000
1
0.010
0.021
0.031
0.042
0.052
39Ar/40Ar
0.004
J5816I
388
WMPA = 387.5 ± 0.4 Ma
382
Age = 384.8 ± 1.1 Ma
MSWD = 1.68 (<1.94)
0.002
40/36=674.9±186.7
0.001
376
370
J5816I
Atm
0.003
394
36Ar/40Ar
Apparent Age (Ma)
400
0
Cumulative 39Ar
1
0.000
0.000
0.010
0.021
0.031
0.042
0.052
39Ar/40Ar
Fig. 2 (continued).
(Fig. 5A), in accord with the findings of Warren et al. (2012). The only
observed correlation between muscovite age and composition is with
Mg# [Mg/(Fe + Mg)], which ranges from 0.00 to 0.73 apfu and shows
an increase with increasing age (Fig. 5B). Such a relationship was noted
by Scaillet et al. (1992) in high-pressure rocks from Dora Maira and
was attributed partly to increased Ar retentivity in Mg-rich muscovite.
4.2. K-feldspar thermochronology and thermal-history modeling
We measured 40Ar/39Ar spectra and modeled the cooling histories of
fifteen K-feldspar single crystals from an area spanning ~350 km north
to south and ~400 km east to west (Fig. 6). Five of the samples were collected from within and near the Sørøyane UHP domain. Root et al.
(2005) inferred that the UHP rocks in this area occupy the core of an
antiform surrounded by high-pressure rocks. Muscovites within the
core of the antiform are 374 and 378 Ma, whereas muscovites in the
high-pressure synform to the south are 384–380 Ma (Root et al.,
2005); this difference has been interpreted to indicate that the folding
is younger than 374 Ma. The K-feldspar sample from Nerlandsøya
(8905A3) is a granitic segregation at the margin of an eclogite boudin
from the center of the Sørøyane UHP domain. The spectrum includes a
series of intermediate steps that have unusually young ages but isotopic
ratios otherwise similar to the other steps (i.e., no elevated 38Ar or 36Ar,
see Appendix B in the Supplementary Material); it was modeled
to show moderate cooling from ~320 Ma to ~300 Ma (6 °C/Myr),
followed by very slow cooling (b 1 °C/Myr). Muscovite ages of
378–374 Ma from nearby samples (Root et al., 2005) suggest that the
modeled cooling history is reasonable. Sample R9828C12, from a
syndeformational pegmatite on Sandsøya south of the Sørøyane
antiform, yielded a well-defined spectrum similar to that from
Nerlandsøya, but without the unusually young intermediate step ages.
The model for the Sandsøya sample (combined with nearby muscovite
ages of 384–383 Ma (Root et al., 2005)) suggests slow cooling (~2 °C/Myr)
until ~320 Ma, at which time the cooling rate decreased even further
to b1 °C/Myr. The shape of the spectrum, however, suggests a
more-complicated cooling history from 383 Ma to 320 Ma that is not
resolved by the modeling. The remaining three samples from this area
(Gurskøya, Gødøya and Runde) have spectra affected by excess Ar and
are difficult to model. The sample from Gurskøy (8815G5) is from
orthogneiss at the southern edge of the Sørøyane UHP domain. A
‘hump’ in the age spectrum followed by a drop in r/ro, suggests premature in vacuo melting of the K-feldspar; a good model fit was not
obtained, but the spectrum (combined with nearby muscovite ages of
384–383 Ma (Root et al., 2005)) suggests slow cooling (~1 °C/Myr)
until ~280(?) Ma, followed by a more moderate cooling rate until
~250 Ma. The Gødøya sample, 8822A5, is from a K-feldspar–biotite–
quartz pegmatite in amphibolite north of the Sørøyane UHP domain.
The spectrum complexity suggests variable release of excess 40Ar; because of this modeling was not attempted. The spectrum—plus the
fact that the sample site lies between a 374 Ma muscovite sample and
a 390 Ma biotite sample (Root et al., 2005)—can be interpreted to reflect slow cooling from ~374 to ~245 Ma, with rapid cooling around
Please cite this article as: Walsh, E.O., et al., Crustal exhumation of the Western Gneiss Region UHP terrane, Norway: 40Ar/39Ar thermochronology
and fault-slip analysis, Tectonophysics (2013), http://dx.doi.org/10.1016/j.tecto.2013.06.030
10
E.O. Walsh et al. / Tectonophysics xxx (2013) xxx–xxx
WMPA = 382.9 ± 0.4 Ma
0
0.004
Terry #929
36Ar/40Ar
407
0.032
0.043
0.054
0.002
Age = 367.3 ± 0.9 Ma
MSWD = 1.86 (<3.83)
40/36=605.1±27.7
0.001
381
0
400
WMA = 370.2 ± 0.4 Ma
Cumulative 39Ar
0.000
0.000
1
0.004
J5813F
388
WMPA = 389.5 ± 0.4 Ma
415
0.043
0.054
Age = 389.3 ± 0.5 Ma
MSWD = 1.88 (<1.89)
40/36=360.4±26.1
0.000
0.000
1
0.011
0.022
0.032
0.043
0.054
39Ar/40Ar
0.004
J5814H
J5814H
Atm
0.003
405
395
0.032
0.002
0.001
376
Cumulative 39Ar
0.022
J5813F
Atm
0.003
382
0
0.011
39Ar/40Ar
394
WMPA = 398.7 ± 0.4 Ma
385
Age = 398.5 ± 0.6 Ma
MSWD = 1.92 (<2.00)
0.002
40/36=313.6±34.0
0.001
375
365
0.022
Terry #929
Atm
0.003
433
370
0.011
39Ar/40Ar
459
355
40/36=370.0±24.4
0.000
0.000
1
36Ar/40Ar
Apparent Age (Ma)
Cumulative 39Ar
Age = 382.4 ± 0.5 Ma
MSWD = 1.11 (<2.00)
0.002
0.001
371
485
Apparent Age (Ma)
36Ar/40Ar
383
377
8907B5
Atm
0.003
389
365
Apparent Age (Ma)
0.004
8907B5
36Ar/40Ar
Apparent Age (Ma)
H 395
0
Cumulative 39Ar
1
0.000
0.000
0.011
0.022
0.032
0.043
0.054
39Ar/40Ar
Fig. 2 (continued).
330–320 Ma. Sample 8829A1 from Runde is an undeformed
K-feldspar–biotite–quartz vein from the north edge of the Sørøyane
UHP domain; the spectrum suggests a slow rate of cooling that
may have increased around 315 Ma and decreased again shortly
thereafter.
We analyzed four samples from the basement and overlying
allochthons along the southeastern edge of the WGR to extend the
work of Dunlap and Fossen (1998). Sample 8817A, from granitic
orthogneiss of the easternmost Western Gneiss Complex, has considerable excess Ar in the middle of the spectrum. Modeling, combined with
a nearby muscovite age of 400 Ma (Fossen and Dallmeyer, 1998), suggests a moderate cooling rate from ~330–280 Ma (4 °C/Myr) and very
slow cooling (~0.6 °C/Myr) thereafter. Sample 8818A, from a granitic
dike cutting a pyroxene granulite in the Jotun nappe, gave a relatively
simple spectrum; modeling indicates moderate cooling from 280 to
260 Ma (7 °C/Myr) followed by very slow cooling (b0.5 °C/Myr). The
Laerdal sample, 8818C, from Baltica basement granitic orthogneiss 5 m
beneath the Laerdal–Gjende fault could not be fit with reasonable MDD
models, but the spectrum is compatible with extremely slow cooling
until ~270 Ma, when the cooling rate increased to ~2 °C/Myr. Sample
8818J, from a K-feldspar–quartz vein in the Valdres sedimentary cover
of the Jotun nappe, yielded a spectrum with excess 40Ar in early steps
that was removed with temperature cycling and excess 40Ar in later
steps associated with melting; modeling gives a relatively complete
cooling history of extremely slow cooling (0.3 °C/Myr) until ~260 Ma,
when the cooling rate increased to ~2 °C/Myr.
Please cite this article as: Walsh, E.O., et al., Crustal exhumation of the Western Gneiss Region UHP terrane, Norway: 40Ar/39Ar thermochronology
and fault-slip analysis, Tectonophysics (2013), http://dx.doi.org/10.1016/j.tecto.2013.06.030
E.O. Walsh et al. / Tectonophysics xxx (2013) xxx–xxx
400
WMPA = 388.9 ± 0.4 Ma
405
Cumulative 39Ar
395
0.052
36Ar/40Ar
40/36=348.5±19.3
0.011
0.022
0.034
0.045
0.056
39Ar/40Ar
0.004
J5815K
Atm
0.003
WMPA = 384.9 ± 0.4 Ma
377
371
Cumulative 39Ar
0.042
Age = 388.6 ± 0.5 Ma
MSWD = 1.62 (<1.94)
0.000
0.000
1
383
0
0.031
0.002
0.001
389
365
0.021
J5814S
Atm
0.003
WMPA = 389.0 ± 0.4 Ma
Cumulative 39Ar
0.010
0.004
J5814S
373
0
40/36=313.5±19.1
39Ar/40Ar
389
365
0.002
0.000
0.000
1
397
381
Age = 388.1 ± 0.5 Ma
MSWD = 1.60 (<2.41)
0.001
376
0
Apparent Age (Ma)
36Ar/40Ar
388
382
J5814N4
Atm
0.003
394
370
Apparent Age (Ma)
0.004
J5814N4
36Ar/40Ar
Apparent Age (Ma)
I
11
1
J5815K
Age = 384.2 ± 0.6 Ma
MSWD = 2.00 (<2.00)
0.002
0.001 40/36=403.5±33.2
0.000
0.000
0.010
0.021
0.031
0.042
0.052
39Ar/40Ar
Fig. 2 (continued).
Two samples were analyzed from the northern WGR. Sample 8814A
collected near Stokken from a granitic pegmatite in an eclogite boudin
strain shadow gave a relatively simple spectrum with some resolvable
excess 40Ar in early steps. Modeling implies slow cooling from 370 to
310 Ma (3 °C/Myr) and then very slow cooling (0.3 °C/Myr) until
≤260 Ma. The spectrum from Bergsøya sample 8814B, a granitic
pegmatite cutting an eclogite boudin, is not as simple as the previous
sample, but the MDD model suggests a similar cooling history. These
two localities gave hornblende ages of 403 and 402 Ma and biotite
ages of 397 Ma (Root et al., 2005).
We analyzed two samples from the Tømmerås Window where
WGR-type basement is exposed in a window beneath the overlying
allochthons. A quartzite near Stiklestad, 8813A1, from the Leksdalsvatn/
Offerdal nappe, yielded an age spectrum with an unusually complete
200 Myr age range. Excess 40Ar appears to have been released episodically during in vacuo heating, but the monotonic increase in age
implies that it was mostly removed by temperature cycling. The cooling
models show very slow cooling (0.4 °C/Myr) from N510 Ma through
300 Ma, at which time slow cooling (N 1 °C/Myr) ensued. A Lund
orthogneiss, 8813D, from the core of the Tømmerås Window gave an
uninterpretable spectrum contaminated by excess Ar.
One sample was measured from the easternmost Trondheim nappes.
H1602M is a megacrystic K-feldspar augen gneiss of the Risberget
nappe; modeling indicates extremely slow cooling (~0.1 °C/Myr) until
~340 Ma, at which point the cooling rate increased to ~7 °C/Myr.
4.3. Fault-slip analysis
We observed no systematic and clear differences between brittle–
ductile fault sets and brittle fault sets at most outcrops; although
more-detailed studies might find otherwise, we thus consider all of
the data from each outcrop together (one exception is noted in
Fig. 7). The pre-existing gneissic foliation strongly influenced the
orientations of faults at any given outcrop, such that the bulk of the
measured brittle–ductile faults formed by slip along existing foliation
planes (Fig. 7, green and purple). Because the existing foliation planes
have variable orientations (e.g., due to outcrop-scale and larger
folds), the calculated principal directions for any given fault set are
more variable than if the faults had developed in an isotropic medium, and this limits their interpretative value. In cases where the
pre-existing gneissic foliation was poorly oriented for slip, new faults
formed, cutting the foliation (Fig. 7, blue and yellow). Specific examples of this include outcrops E9806A, E9806F, E9808K, E9810A, and
Y1611AL. Note that the calculated principal directions for the two
different types of faults (i.e., reactivated foliation and offset foliation)
are similar (Fig. 7).
When the fault-slip data are considered in a broad way—as is
appropriate given the limitations imposed by the anisotropy of the
medium—nearly all of the data fit a simple pattern of E–W stretching.
At any given outcrop this E–W stretching occurred i) along
NNW-trending dextral faults and ENE-trending sinistral faults (Fig. 7,
Please cite this article as: Walsh, E.O., et al., Crustal exhumation of the Western Gneiss Region UHP terrane, Norway: 40Ar/39Ar thermochronology
and fault-slip analysis, Tectonophysics (2013), http://dx.doi.org/10.1016/j.tecto.2013.06.030
12
E.O. Walsh et al. / Tectonophysics xxx (2013) xxx–xxx
Table 1
40
Ar/39Ar muscovite age data.
Sample
Geologic context
UTM
WMPAa
8815G12
8907B5
#929 of Terry
E1612C7
E1612Q8
E9814A6
H5621A
H5621B
H5622B
H5622C
H5622E
H5702A
Layered mafic block
Bio-kfs gneiss
Strain shadows in ky-gar mylonite
Tonalitic-granodioritic gneiss
2-Mica granodioritic gneiss
qtz-pl-kfs sweats in granulite
Basement gneiss
Basement gneiss
Basement gneiss leucosome
Dioritic gneiss
Muscovite gneiss
Mus-qtz segregation in
quartzofeldspathic gneiss
Tonalitic-dioritic gneiss
Blåhø quartzite
Allochthon gneiss
Blåhø schist
Concordant pegmatite in Blåhø
kfs-gneiss
Granodioritic gneiss
kfs augen gneiss
Blåhø gneiss
MTFZ mylonite zone
MTFZ mylonitic tonalite
MTFZ mylonitic tonalite
Augen gneiss
Muscovite schist with quartzite
Muscovite schist
Eclogite
Granodioritic gneiss
Bio-dioritic gneiss
Granitic gneiss
kfs-augen gneiss
Pegmatite cutting kfs-augen gneiss
Bio-plg symplectitic gneiss
kfs-augen gneiss
Bio-kfs gneiss
Sanidine gneiss
0319906 6907793
0321490 6911190
Not listed
0342680 6899388
0386325 6884503
0366201 6909234
0467229 6782883
0321952 6731670
0325254 6785784
0349757 6784619
0348278 6863184
0347533 6931555
376.9
377.7
365.2
381.9
385.1
384.0
902.5
392.3
392.5
394.7
391.8
383.5
0400791
0451392
0476914
0324548
0420254
0387893
0398970
0399601
0463998
0475666
0484213
0499845
0502823
0534950
0506601
0308467
0352994
0349529
0361434
0325474
0435220
0316216
0299773
0328243
0380936
383.0 ± 0.4
384.2 ± 0.4
393.3 ± 0.4
383.6 ± 0.4
383.7 ± 0.4
372.4 ± 0.4
373.7 ± 0.4
375.0 ± 0.4
379.7 ± 0.4
384.0 ± 0.4
377.8 ± 0.4
382.3 ± 0.4
384.5 ± 0.4
396.9 ± 0.5
391.9 ± 0.4
na
382.4 ± 0.4
380.6 ± 0.4
381.3 ± 0.4
382.2 ± 0.4
na
383.0 ± 0.4
377.0 ± 0.4
379.5 ± 0.4
379.34 ± 0.4
J5811D
J5813F
J5814H
J5814N4
J5814S
J5815B1
J5815E1
J5815G
J5815K
J5816A
J5816B
J5816I
J5816J
J5816K
J5816L1
K5622A5
P5627E2
P5627F3
P5627K
P5629O
P6804A
P6805H2
P6806J
P6816B1
P6818C2
6865298
6948000
6931581
6970208
6951810
6966797
6970946
6981198
7013119
7004251
7024582
7023429
7003520
7027640
6978402
6805345
6906801
6911676
6918751
6882843
6834377
6874705
6894405
6911792
6951415
±
±
±
±
±
±
±
±
±
±
±
±
0.4
0.4
0.4
0.4
0.4
0.4
0.9
0.5
0.4
0.5
0.4
0.4
∑39%
Isochrona
∑39%
MSWD
40/36i
TFA
Renneb
±2σ
75
80
27
76
71
73
63
80
94
22
47
26
376.5
377.2
362.3
365.7
384.5
381.2
na
388.4
392.1
372.1
na
383.4
±
±
±
±
±
±
96
99
32
82
94
73
na
96
94
40
na
26
0.66
1.11
1.86
2.99
2.01
2.73
na
1.77
1.33
0.58
na
0.06
360 ± 14
370 ± 24
605 ± 28
3071 ± 3614
386 ± 36
554 ± 118
na
512 ± 40
343 ± 46
5339 ± 543
na
317 ± 39
376.9
377.7
384.2
383.5
385.7
387.1
878.0
392.9
392.7
401.3
392.1
385.9
381.7
382.4
370.2
387.1
389.7
389.3
931 to 450
393.7
397.5
397 to 375
397.2
388.8
0.9
1.0
0.9
0.9
1.1
0.9
na
2.0
1.1
na
0.9
0.9
94
53
95
90
86
68
72
54
65
46
63
94
95
26
76
na
67
77
73
93
na
77
55
78
86
383.0
384.1
393.1
382.9
383.3
372.7
370.5
374.6
379.0
381.4
377.5
379.6
384.3
na
391.7
na
378.5
380.8
380.1
382.8
na
382.9
374.7
379.0
378.5
±
±
±
±
±
±
±
±
±
±
±
±
±
94
91
95
63
94
68
97
96
90
82
86
94
99
na
72
na
93
77
73
93
na
86
66
100
94
1.01
1.88
1.92
1.60
1.62
2.67
1.56
1.61
2.00
0.42
2.19
1.68
1.63
na
1.24
na
2.07
3.89
0.47
1.98
na
1.21
1.07
1.50
1.19
294 ± 25
360 ± 26
314 ± 34
313 ± 19
348 ± 19
276 ± 40
884 ± 35
364 ± 17
403 ± 33
828 ± 208
353 ± 102
675 ± 187
320 ± 23
na
355 ± 18
na
1052 ± 48
261 ± 59
487 ± 121
239 ± 30
na
317 ± 7
458 ± 14
350 ± 17
350 ± 23
383.3
384.7
393.6
383.7
384.6
372.8
376.1
375.3
379.9
383.6
377.9
382.3
384.5
398.9
392.2
404.1
384.8
380.7
381.5
382.1
510.8
383.1
377.5
379.7
379.1
388.2
389.3
398.5
388.1
388.6
377.5
375.6
379.7
384.2
386.6
382.7
384.8
389.6
402.4
397.1
na
383.7
385.8
385.3
387.4
558 to 471
388.2
379.8
384.2
383.7
0.9
0.9
1.1
1.0
0.9
0.8
1.2
1.0
1.1
2.3
1.8
2.2
0.9
1.0
0.9
na
1.2
0.8
1.7
0.9
na
0.9
1.0
1.0
1.2
WMPA, weighted mean plateau age (N50% cumulative 39Ar); WMA, weighted mean age, for cumulative
MSWD, mean standard weighted deviation for isochron; TFA, total fusion age.
Preferred age in boldface.
a
Error reported as ±1sigma with error in J.
b
Renne: preferred age recalculated following Renne et al. (1998).
green and blue), suggesting coincident N–S shortening, or ii) along
E- and W-dipping normal faults (Fig. 7, purple and yellow), indicating
coincident vertical thinning. These strike-slip dominated and dip-slip
dominated fault sets have similar mineralization, such that the simplest
interpretation is that they formed simultaneously. If so, this is a
constrictional strain field. We find no significant record of vertical
stretching or N–S stretching in the fault-slip data.
39
0.5
0.5
0.9
7.4
0.6
1.3
± 1.0
± 0.6
± 3.9
± 0.6
0.6
0.5
0.6
0.5
0.5
0.6
0.6
0.5
0.6
1.2
0.9
1.1
0.5
± 0.5
±
±
±
±
0.6
0.6
0.8
0.6
±
±
±
±
0.4
0.5
0.5
0.6
Ar b50%.
5. Discussion
5.1. Geochronology
Our new muscovite ages are combined with previously published
work (see references in Hacker, 2007; Warren et al., 2012) in Fig. 4 to
reveal several striking features. Generally, muscovite across the WGR
Fig. 3. A) Biotite-bearing brittle–ductile fault discordant to foliation. B) Chlorite-bearing brittle–ductile fault parallel to foliation.
Please cite this article as: Walsh, E.O., et al., Crustal exhumation of the Western Gneiss Region UHP terrane, Norway: 40Ar/39Ar thermochronology
and fault-slip analysis, Tectonophysics (2013), http://dx.doi.org/10.1016/j.tecto.2013.06.030
E.O. Walsh et al. / Tectonophysics xxx (2013) xxx–xxx
closed to Ar loss over ~20 Myr, from about 400 Ma at the eastern edge
through about 380 Ma in the west.
North of Nordfjord, basement gneisses and rocks interpreted to
represent allochthons folded into the basement record muscovite
40
Ar/39Ar ages that increase eastward toward the foreland from
375.6 ± 0.6 Ma (J5815E1) to 398.5 ± 0.5 Ma (J5814H); such ages
continue to increase within the nappes farther east (Hacker and Gans,
2005) as well as structurally upward in the hanging wall to the WGC
in Sunnfjord and Nordfjord. This entire domain thus represents a progressively unroofed or eastward-tilted domain. The UHP domains in
the west have been interpreted as E-plunging antiforms (Root et al.,
2005) formed after ~378–374 Ma during long-term N–S shortening
(Braathen, 1999; Osmundsen and Andersen, 2001; Torsvik et al., 1988).
South of Nordfjord, the pattern of muscovite 40Ar/39Ar ages is quite
different: the youngest ages are in the center of the WGR, and the
ages increase both west and east. The ages increase smoothly eastward
into the allochthons as they do farther north, but the age contours are
closer together within the Jotun Nappe. To the south and west, ages
jump abruptly upward within allochthonous units—the Bergen Arcs,
the Høyvik Group, and the Dalsfjord Suite—that did not undergo
Scandian subduction with the WGR (Andersen et al., 1998; Eide et al.,
1997). Thus, the mica contours describe a dome-like structure about a
NE–SW axis that is mirrored by the outcrop pattern of the WGR. This
differential exhumation may represent Middle Devonian footwall uplift
13
to the Nordfjord–Sogn Detachment Zone, possibly in combination with
rotation of the WGR due to shear on the Hardangerfjord Shear Zone
(Fossen and Hurich, 2005).
Determining precisely what each age represents is not straightforward. The muscovite age at any given location might reflect thermally
mediated Ar redistribution, deformation-driven Ar redistribution, or
fluid-driven Ar redistribution. A thermal control on Ar loss seems
credible for many samples given that i) the main amphibolite-facies
deformation was 600–800 °C (Spencer et al., 2013), substantially
hotter than the 400–450 °C closure temperature of white mica to Ar
volume diffusion inferred from experiments (Harrison et al., 2009);
ii) the eastern half of the WGR was weakly deformed during the
Scandian but still has Scandian muscovite ages (Hacker et al., 2010);
iii) in the southern half of the study area, the perimeter of the WGC
is more deformed than the interior and yet has older muscovite
ages; and iv) most of the youngest ages come from gneiss with only an
amphibolite-facies fabric (Table 1). Recrystallization- or fluid-driven Ar
loss also seems possible because i) some samples have heterogeneous
laser spot ages within and among grains (Warren et al., 2012); ii) other
grains have uniform laser spot ages from rim to rim (Warren et al.,
2012); iii) some strongly deformed parts of the WGR have muscovite,
biotite and amphibole ages that are similar [e.g., Møre–Trøndelag Fault
Complex local detachment mylonites (Kendrick et al., 2004)]; iv) muscovite 40Ar/39Ar ages locally overlap with the youngest titanite U–Pb ages
Carboniferous-Devonian Basins
428-420
451-417
Caledonian allochthons
418
422
autochthon (Baltica basement)
410 Ma
400 Ma 410
J5816B
383
J5816I
J5816K
397
J5815K 384
J5816A J5816J 402
387
385 Ma
390
100 km
390 Ma
396
394
418
396
402 409 393
407
397 397
382 406
404
419 402 397
406-395
395 Ma
401
401
Ma
440 Ma 430 423- 401 412400398
454 443 402 444
451-402
401 394
452-402 406-395
427
452-402
K5622A5 H5622E
430
397 4001
414-403
397
408-405
413-403
H5622B
401
H5622C
397
394
420 Ma
420
419
408
404
J5815G 380
J5814N4
J5816L1
J5815E1 376
388
397
J5815B1 378
J5814S
395
Ma
P6816B
Terry929
P6818C2 388
J5813F
8907B 384 P5627E2 370
384
382
389
384
J5814H
H5702A P5627K
399
391
389
383
P6806A
385
380
359 8815G
385
E9814A6
391
62°N
386
382 389
P5627F3
387
385
387
399
386
380
394
390
E1612C
380 Ma 374
E1612Q8
387 P5629O 390
385 Ma
390
395
392387 P6805H2
390
388388406-382
389
406-382
J5811D 393
412 388
424
407
398
395
406 404-414
398
388
398
386-409
396 399
426
421
410
406-403
P6804A
558-471
400 Ma
404
413-408
420-405
421-413
402
931–450 H5621A
397-375
406
410 Ma
450 Ma
394 H5621B
10°E
Fig. 4. Muscovite ages and age contours (blue) for the Western Gneiss Region. Muscovite ages from this study in bold. Additional muscovite data in small type from: Andersen et al.
(1998), Chauvet et al. (1992), Fossen and Dunlap (1998), Hacker and Gans (2005), Lux (1985), Root et al. (2005), Walsh et al. (2007), Warren et al. (2012) and Young et al. (2007,
2011).
Please cite this article as: Walsh, E.O., et al., Crustal exhumation of the Western Gneiss Region UHP terrane, Norway: 40Ar/39Ar thermochronology
and fault-slip analysis, Tectonophysics (2013), http://dx.doi.org/10.1016/j.tecto.2013.06.030
14
E.O. Walsh et al. / Tectonophysics xxx (2013) xxx–xxx
Table 2
EMP muscovite analyses.
8815G12r
8815G12c
8907B5r
8907B5c
929r of Terry
929c of Terry
E1612C7r
E1612C7c
E1612Q8r
E1612Q8c
E9814A6r
E9814A6c
H5621Ar
H5621Ac
H5621Br
H5621Bc
H5622Br
H5622Bc
H5622Cr
H5622Cc
H5622Er
H5622Ec
H5702Ar
H5702Ac
J5811Dr
J5811Dc
J5813Fr
J5813Fc
J5814Hr
J5814Hc
J5814N4r
J5814N4c
J5814Sr
J5814Sc
J5815B1r
J5815B1c
J5815E1r
J5815E1c
J5815Gr
J5815Gc
J5815Kr
J5815Kc
J5816Ar
J5816Ac
J5816Br
J5816Bc
J5816Ir
J5816Ic
J5816Jr
J5816Jc
J5816Kr
J5816Kc
J5816L1r
J5816L1c
K5622A5r
K5622A5c
P5627E2r
P5627E2c
P5627F3r
P5627F3c
P5627Kr
P5627Kc
P5629Dr
P5629Dc
P6804Ar
P6804Ac
P6805H2r
P6805H2c
P6806Ar
P6806Ac
P6806Jr
P6806Jc
P6816B1r
P6816B1c
P6818C2r
SiO2
Al2O3
TiO2
FeO
MgO
K2O
Total
Si apfu
Mg# apfu
47.46
47.47
48.12
47.26
48.38
49.34
46.36
48.33
49.73
48.08
48.59
48.57
49.31
49.32
48.02
48.50
46.96
47.18
48.05
47.59
48.32
48.52
48.36
48.15
46.87
47.69
49.00
47.67
48.16
49.07
48.33
49.52
50.30
50.33
48.26
48.40
47.66
47.95
47.78
47.39
48.32
48.11
48.05
47.59
48.35
47.77
47.27
47.87
48.61
49.16
49.96
50.73
49.31
49.28
49.93
50.80
47.91
47.49
47.18
46.93
47.56
47.16
47.00
47.06
48.52
47.01
49.45
49.34
47.50
47.40
48.25
47.85
45.91
46.42
46.45
33.86
31.86
31.96
32.41
31.34
32.15
32.50
31.76
32.30
34.34
32.09
32.08
32.26
31.38
37.67
36.87
36.80
36.71
32.56
34.87
29.43
29.42
32.25
32.34
31.24
30.77
34.51
35.69
30.74
30.18
29.49
28.88
32.30
31.56
31.30
35.50
35.16
34.76
35.26
34.82
35.43
35.39
32.56
34.87
31.48
30.95
33.70
33.78
33.73
32.87
34.74
33.12
27.59
27.44
30.20
28.25
32.55
31.19
33.90
34.46
35.23
34.13
35.28
35.33
32.96
34.19
31.33
31.36
33.39
33.55
31.78
31.62
33.91
34.20
34.84
0.20
0.81
0.82
0.82
0.00
0.00
0.61
0.20
0.83
0.62
0.00
0.00
0.00
0.20
0.63
0.42
0.00
0.00
0.00
0.00
0.60
0.61
0.82
1.03
0.40
0.41
1.25
1.45
0.81
0.61
1.22
1.02
0.83
0.62
0.82
0.42
0.62
0.62
0.62
1.03
1.04
1.25
0.00
0.00
0.61
0.61
1.43
1.44
0.21
0.62
0.63
0.84
1.02
1.22
0.41
0.61
0.82
0.61
0.82
1.02
0.21
0.61
0.62
0.82
0.00
0.00
0.62
0.62
1.02
1.02
1.02
0.82
1.22
0.41
0.82
4.79
5.66
5.33
5.50
6.20
5.18
6.18
5.69
3.72
4.27
5.52
5.70
4.99
5.53
1.51
1.69
3.92
3.92
5.87
4.80
6.52
6.91
4.62
4.98
7.10
7.29
2.44
2.61
6.03
6.23
6.02
5.50
2.62
3.18
6.06
3.73
3.90
4.45
3.91
4.26
2.62
2.61
5.87
4.80
5.89
6.20
3.68
3.33
3.90
3.72
1.70
1.89
7.12
6.57
5.90
6.24
5.16
6.04
4.96
4.78
4.45
5.16
4.25
4.07
6.43
6.94
4.06
4.44
4.78
4.78
5.71
6.05
5.11
5.30
4.60
1.96
2.05
2.37
2.26
1.74
1.87
2.24
2.37
2.09
1.25
1.96
1.96
1.76
2.07
0.53
1.16
1.99
1.78
1.96
1.24
2.64
2.76
2.38
2.07
2.86
2.76
1.79
0.94
2.67
2.47
2.86
3.50
2.63
3.15
2.58
0.94
1.25
1.35
1.15
1.25
1.05
1.15
1.96
1.24
2.79
2.76
1.55
1.56
1.98
2.30
2.33
2.76
3.89
4.30
1.96
2.57
2.38
3.08
1.24
1.03
1.14
1.24
1.24
1.04
0.52
0.00
2.59
2.59
1.44
1.45
2.48
2.67
1.33
1.13
1.03
11.72
12.20
11.45
11.67
12.32
11.40
12.15
11.67
11.45
11.43
11.81
11.69
11.63
11.48
11.60
11.36
10.40
10.41
11.55
11.51
12.59
11.80
11.51
11.37
11.45
11.11
10.96
11.72
11.63
11.42
12.07
11.66
11.30
11.15
10.96
11.01
11.44
10.95
11.22
11.29
11.52
11.51
11.55
11.51
10.86
11.72
12.43
12.02
11.56
11.34
10.66
10.65
11.01
11.24
11.48
11.55
11.23
11.29
11.93
11.82
11.42
11.60
11.63
11.65
11.56
11.85
11.99
11.64
11.93
11.82
10.75
11.06
12.55
12.57
12.18
99.99
100.05
100.05
99.92
99.98
99.94
100.04
100.02
100.12
99.99
99.97
100.00
99.95
99.98
99.96
100.00
100.07
100.00
99.99
100.01
100.10
100.02
99.94
99.94
99.92
100.03
99.95
100.08
100.04
99.98
99.99
100.08
99.98
99.99
99.98
100.00
100.03
100.08
99.94
100.04
99.98
100.02
99.99
100.01
99.98
100.01
100.06
100.00
99.99
100.01
100.02
99.99
99.94
100.05
99.88
100.02
100.05
99.70
100.03
100.04
100.01
99.90
100.02
99.97
99.99
99.99
100.04
99.99
100.06
100.02
99.99
100.07
100.03
100.03
99.92
3.08
3.11
3.13
3.08
3.17
3.19
3.05
3.15
3.20
3.10
3.16
3.16
3.19
3.20
3.05
3.08
3.01
3.02
3.13
3.08
3.19
3.19
3.13
3.12
3.08
3.13
3.12
3.06
3.15
3.20
3.17
3.23
3.21
3.22
3.14
3.10
3.07
3.09
3.07
3.06
3.09
3.08
3.13
3.08
3.14
3.13
3.07
3.09
3.13
3.16
3.16
3.21
3.23
3.23
3.24
3.31
3.11
3.12
3.07
3.05
3.07
3.06
3.04
3.04
3.16
3.08
3.20
3.19
3.09
3.08
3.13
3.12
3.01
3.04
3.02
0.42
0.39
0.43
0.43
0.34
0.39
0.39
0.43
0.50
0.35
0.39
0.38
0.39
0.40
0.37
0.53
0.48
0.45
0.37
0.31
0.42
0.42
0.48
0.43
0.42
0.40
0.58
0.40
0.44
0.42
0.46
0.53
0.64
0.64
0.43
0.32
0.36
0.35
0.36
0.33
0.43
0.44
0.37
0.31
0.46
0.45
0.42
0.45
0.47
0.52
0.71
0.73
0.49
0.54
0.37
0.42
0.45
0.47
0.31
0.28
0.31
0.30
0.34
0.31
0.13
0.00
0.53
0.51
0.35
0.35
0.44
0.44
0.32
0.27
0.29
Please cite this article as: Walsh, E.O., et al., Crustal exhumation of the Western Gneiss Region UHP terrane, Norway: 40Ar/39Ar thermochronology
and fault-slip analysis, Tectonophysics (2013), http://dx.doi.org/10.1016/j.tecto.2013.06.030
E.O. Walsh et al. / Tectonophysics xxx (2013) xxx–xxx
15
Table 2 (continued)
P6818C2c
P6824B1r
P6824B1c
SiO2
Al2O3
TiO2
FeO
MgO
K2O
Total
Si apfu
Mg# apfu
47.38
49.49
49.87
35.34
29.38
28.29
0.62
0.82
0.41
4.07
6.05
6.77
1.14
2.78
3.08
11.53
11.53
11.51
100.08
100.05
99.93
3.06
3.23
3.26
0.32
0.45
0.45
Oxide weight percent back-calculated from the formula. Each analysis is the average of two spots: r, rim; c, core.
(Spencer et al., 2013); and v) the youngest age is from a mylonitic rock
(Table 1).
Unlike the broad-scale, relatively simple patterns in muscovite
ages, the K-feldspar spectra from the WGR have considerably more
heterogeneity. Within the WGR there is as much variation in the
calculated cooling histories for K-feldspar within the different areas
studied—e.g., northern WGR, Jotun nappe (including one sample
from the WGR), and Sørøyane UHP domain (including one sample
from north of the UHP domain)—as there is in the cooling histories
for the region as a whole. Moreover, our sample 8817A and Dunlap
and Fossen's (1998) sample N28, which are both from the WGR and
are separated by only ~ 10 km, yield considerably different spectra.
These observations imply that the variations in the K-feldspar cooling
histories should not be interpreted too literally. Until a more-detailed
investigation shows otherwise, we conservatively interpret the WGR
K-feldspar data as indicating cooling through ~ 400 °C in the broad
interval of 390–330 Ma and through 200 °C around 310–230 Ma.
The cooling through ~ 400 °C that we infer occurred around
390–330 Ma is substantially younger than the ~ 400–390 Ma cooling
inferred by Dunlap and Fossen (1998). This difference in calculated
cooling histories reflects substantial differences in the step ages of
the K-feldspars measured in the two studies: the spectra measured
by Dunlap and Fossen are generally older than those we measured.
This age difference (older ages to the east) matches the diachronous
cooling recorded in muscovite age data and ties the early cooling history
of the K-feldspar to the progressive westward unroofing of the WGR.
The cooling through ~ 200 °C that we infer occurred around
310–230 Ma is consistent with the ~ 320–260 Ma cooling inferred
by Dunlap and Fossen (1998) for samples along a transect in or near
the SW corner of Fig. 6; Dunlap and Fossen attributed this episode
of accelerated cooling to the onset of rifting in the Oslo Graben and
the North Sea.
Eide et al. (1999) inferred that two of their K-feldspar samples from
the Nordfjord–Sogn detachment zone (south of our UHP samples)
cooled through 200 °C by ~350 Ma, substantially earlier than any of
our samples. They used samples from the Nordfjord–Sogn detachment
zone to infer that late-stage E–W folding in the WGR continued until
~340 Ma. Three new K-feldspar samples from Sørøyane may also record the effects of this Carboniferous E–W folding: Nerlandsøya, located
at the center of the antiform, cooled the fastest (~6 °C/Myr until
slowing at ~300 Ma); Sandsøya, located at the southern edge of the
antiform, cooled more slowly (~2 °C/Myr until ~320 Ma, followed by
slower cooling); and Gurskøya, even farther south, cooled the slowest
(1 °C/Myr until ~280 Ma).
5.2. Late-stage structures
A
Muscovite age vs. Si apfu
3.25
Si apfu
3.20
3.15
3.10
3.05
3.00
2.95
350
360
370
380
390
400
390
400
Age (Ma)
B
Muscovite age vs. Mg #
0.8
0.7
Mg #
0.6
0.5
0.4
0.3
0.2
0.1
350
360
370
380
Age (Ma)
Fig. 5. A) Lack of apparent correlation between Si apfu and age in new muscovite data.
B) Weak positive correlation between Mg# [Mg/(Mg + Fetotal)] and age.
The faulting that we report has been studied to the north and south of
the WGR and is attributed to deformation during exhumation at upper
crustal levels (Andersen et al., 1999; Bering, 1992; Braathen, 1999;
Fossen, 1998, 2000; Larsen et al., 2002, 2003; Redfield et al., 2005). As
noted above, the relationship of the faults to amphibolite-facies
symplectite and the mineralization along the faults suggest that slip
mainly post-dated amphibolite-facies conditions; therefore the faults
are chiefly younger than the amphibolite-facies ductile, E–W stretching
summarized by Hacker et al. (2010). They likely also postdate the
dominantly amphibolite-facies quartz fabrics interpreted by Barth et al.
(2010) to indicate a mix between plane strain and constriction. Overall,
the dominant E–W stretching inferred from the faults intimates a continuation of this constrictional regime, first identified by Krabbendam and
Dewey (1998).
The brittle–ductile to brittle faults characterized by (biotite) ±
chlorite ± epidote ± quartz ± carbonate ± Mn–Fe oxides and the
brittle faults characterized by clay or gouge are similar to the set I
and set II faults, respectively, of Larsen et al. (2003) in the Bergen
area. Those authors interpreted a 396 Ma titanite date from their set
I faults to indicate the start of post-Caledonian brittle deformation
in the Bergen area; ~ 363–371 Ma Rb/Sr isochron ages from set I faults
are interpreted to date hydrothermal alteration and syn- or post-set I
deformation (Larsen et al., 2003). The set II faults, then, were likely
active from shortly after the Late Devonian set I faults until Permian
dike intrusion at ~ 260 Ma (Larsen et al., 2003).
Fossen and Dallmeyer (1998) and Fossen and Dunlap (1998) used
40
Ar/39Ar muscovite ages and an inferred closure temperature of
350 °C to constrain the initiation of lower amphibolite-facies
WNW-directed extension (their “Mode I”) in southwestern Norway
to ~400 Ma. Semi-ductile to brittle normal-sense faults associated
Please cite this article as: Walsh, E.O., et al., Crustal exhumation of the Western Gneiss Region UHP terrane, Norway: 40Ar/39Ar thermochronology
and fault-slip analysis, Tectonophysics (2013), http://dx.doi.org/10.1016/j.tecto.2013.06.030
16
E.O. Walsh et al. / Tectonophysics xxx (2013) xxx–xxx
local muscovite ages
350
Runde
200
0 cumulative 39Ar released 1
Bergsøya
300
Lund
200
0 cumulative 39Ar released 1
Tømmerås Window
250
200
local muscovite ages
350
300
Stokken
350
0 cumulative 39Ar released 1
Nerlandsøy
Stokken
Bergsøya
300
Godøy
Runde
Nerlandsøy
Sandsøya Gurskøy
350
Sandsøya
Stokk
M
Gurskøy
200
0 cumulative 39Ar released 1
100
200
8817A
apparent age (Ma)
400
local muscovite ages
Laerdal
350
300
700
H1602M
500
300
0 cumulative 39Ar released 1
400
500
Jotun Nappe
8818A
8818J
Laerdal
8817A
250
200
900
Time (Ma)
300
0 cumulative 39Ar released 1
400
apparent age (Ma)
160
1100
UHP
Jotun
northern WGR
Tømmerås Window
Trondheim Nappes
en
200
300
250
1602M
Stiklest
modeled cooling
local muscovite ages
Sandsøya
Trondheim Nappes
ad
H1602
apparent age (Ma)
400
300
/m
C
1°
apparent age (Ma)
0 cumulative Ar released 1
Bergsøya
7A
39
881
160
.y.
Nerlandsøy
10°C/m.y.
øy
rsk
Gu
8818A
400
200
8J
250
88
1
apparent age (Ma)
400
apparent age (Ma)
250
Lund
Stiklestad
Stiklestad
400
400 local hornblende & biotite ages
Godøy
300
160
northern WGR
Temperature (°C)
apparent age (Ma)
400
apparent age (Ma)
500
UHP
350
local muscovite ages
8818J
300
250
200
8818A
0 cumulative 39Ar released 1
Fig. 6. K-feldspar age spectra and cooling models for locations across the Western Gneiss Region. Measured age spectra shown with black fill, modeled age spectra shown by gray
dashed lines. Local muscovite, biotite or hornblende ages shown where available. Modeled cooling paths in center subfigure are color-coded by sample locality.
with deposition of the Devonian–Carboniferous basins and slip along
the Nordfjord–Sogn Detachment Zone (NSDZ) also happened at this
time (Andersen, 1998; Braathen, 1999; Eide et al., 2005). Chauvet and
Séranne (1994) specifically noted that a transition from E–W extension
and vertical thinning to E–W constriction occurred during the deposition of these basins. Fossen and Dallmeyer (1998) and Fossen and
Dunlap (1998) further noted that later, steeper semi-brittle to brittle
faults accommodating NW–SE extension (“Mode II”) predate Permian
dikes and may be associated with K-feldspar cooling at 320–260 Ma.
Thus, the brittle–ductile faults we observed most likely formed when
muscovite was closing from 400 Ma to 370 Ma, with the brittle faults
forming from latest Devonian into the Permian.
5.3. Absence of excess argon
Most (U)HP terranes are plagued by excess 40Ar (El-Shazly et al.,
2001; Kelley, 2002; Warren et al., 2012). It is particularly common in
K-poor minerals in K-poor rocks encased in K-rich rocks (e.g., amphibole
in eclogite in granitic gneiss), but also widespread in high-pressure white
mica (Kokchetav, Hacker et al., 2003; Dabie, Hacker et al., 2000). In
Please cite this article as: Walsh, E.O., et al., Crustal exhumation of the Western Gneiss Region UHP terrane, Norway: 40Ar/39Ar thermochronology
and fault-slip analysis, Tectonophysics (2013), http://dx.doi.org/10.1016/j.tecto.2013.06.030
E.O. Walsh et al. / Tectonophysics xxx (2013) xxx–xxx
17
Table 3
Fault-slip data.
Plot name
Sites included
n
σ1
E9803E
E9803G
E9803P
E9804A
E9804I
E9805C
E9805L
E9805P
E9806A
E9806F
E9806I
E9807A
E9807B
E9808D
E9808F
E9808K
E9809G
E9809H
E9810A
E9816F-1
E9816F-2
E9817A
E9820F
E9820M
E1606A
E1606H
E1607A
E1612C
E1612P
J5810A
J5813F
J5815C
J5814B
P6809A
R9821B
R9821C
R9822C
R9823G
R9826E
Y0816C
Y0816M
Y0817A
Y1611A-biotite
Y1611A-chlorite
E9801O, E9803E, c E1617A
E9803F, E9803G, E9803K, E1616D
E9803P, E9803S
E9804A, E9804B, E9804C, E9804D
E9804I, E9804J, E1615A, E1615B
E9805C, E9805G, E1614C, E1614D
E9805L, E9805M
E9805P, E9805T, E9808E
E9806A, E9806B, E9806E
E9806F, E9806H
E9806I
E9807A
E9807B, E9813B
E9808D
E9805P, E9808F, E9808G, E1613M
E9808K, E9815B, E9815D, E1613I
E9809G, E9809I
E9809H
E9809K, E9809O, E9810A
E9816F, E9818F
E9816F
E9817A, E9817B, E9818F
E9820F
E9820M
E1606A, E1606B
E1606C, E1606E, E1606H, E1606J, E1606L
E1607A
E1612C, E1612D
E1612P
E9811F, J5810A-C
J5813F
J5815C
J5814B
P6809A
R9821B
R9821C
R9822C, R9822D
R9823G, R9823H
R9826E
Y0816C, Y0816D, Y0816E
Y0816M, Y0816J
Y0817A
Y1611A, B, I, K, L
Y1611A, B, I, K, L
214
50
5
35
28
17
6
40
16
26
6
13
34
10
18
10
9
6
17
14
6
6
3
10
6
14
10
14
4
27
12
4
6
9
5
10
38
14
14
21
41
8
19
41
218
197
001
168
048
005
312
053
345
169
180
055
109
169
190
347
253
023
182
022
194
135
264
197
205
215
217
009
051
204
047
222
170
052
305
157
060
331
135
034
175
176
029
172
σ2
73
82
03
27
72
08
29
60
14
01
78
61
77
75
04
23
67
14
18
18
66
73
64
23
06
20
51
12
60
13
00
05
53
48
47
02
55
55
57
34
27
10
24
60
357
024
111
017
208
256
175
233
145
271
281
180
229
330
031
160
003
143
344
158
041
339
016
034
333
018
011
196
163
343
139
358
335
189
164
063
200
177
351
173
025
043
176
004
13
08
82
60
12
68
53
30
75
84
02
18
06
14
86
67
08
64
71
65
22
15
11
66
80
69
36
78
12
73
85
83
36
33
36
63
28
32
28
48
59
70
61
29
σ3
R
F (°)
nev
Mineralization
090 11
294 01
271 08
265 13
300 6
098 21
055 21
323 00
253 05
079 06
011 12
277 23
321 11
061 05
280 01
256 02
096 21
287 22
090 05
287 16
307 10
248 07
111 23
289 06
114 08
123 06
110 13
099 02
259 27
111 11
317 05
131 05
071 07
295 23
058 21
248 27
300 19
079 13
252 17
289 22
272 13
270 14
293 14
271 05
0.53
0.17
0.51
0.84
0.69
0.58
0.52
0.52
0.84
0.50
0.35
0.50
0.58
0.48
0.82
0.63
0.48
0.43
0.72
0.60
0.51
0.59
?
0.58
0.60
0.37
0.48
0.75
0.43
0.51
0.54
0.49
0.52
0.57
0.50
0.63
0.44
0.66
0.74
0.80
0.65
0.61
0.75
0.66
17
18
5
19
21
11
5
24
14
20
15
9
21
14
21
17
6
35
14
16
17
27
5
17
20
13
17
16
10
12
12
3
14
10
4
9
18
14
19
10
12
16
23
16
0
1
0
1
0
2
0
4
0
0
1
0
3
1
1
1
0
0
0
1
0
1
0
0
0
1
0
0
0
0
2
0
0
0
0
1
1
0
0
0
0
0
1
2
Chlorite biotite
Chlorite epidote biotite
Chlorite biotite
Chlorite epidote biotite
Chlorite epidote biotite
Quartz epidote
Biotite chlorite quartz
Quartz chlorite epidote
Quartz oxides clay
Clay epidote cataclasite
Chlorite quartz
Biotite chlorite clay quartz
Biotite chlorite quartz carbonate
Chlorite quartz
Quartz chlorite epidote
Quartz epidote
Quartz chlorite epidote
Chlorite quartz
Chlorite quartz
Epidote chlorite quartz
Chlorite quartz oxides
Biotite chlorite epidote
Biotite chlorite quartz
Biotite chlorite quartz
Chlorite quartz
Chlorite quartz
Chlorite quartz
Chlorite quartz
Biotite
Biotite
Biotite chlorite quartz
Biotite feldspar quartz
Biotite chlorite quartz
Biotite chlorite quartz
Chlorite quartz
Chlorite quartz
Biotite chlorite quartz epidote
Biotite quartz
Quartz
Chlorite quartz
Chlorite quartz epidote carbonate
Chlorite
Biotite
Quartz chlorite epidote
All analyses performed by numerical dynamic analysis (after Spang, 1972) using the program NDA (Sperner and Ratschbacher, 1994).
n, number of faults; R, (σ2 − σ3) / (σ1 − σ3); F, average angle of misfit; nev, number of faults with slip sense opposite of expected.
contrast, the well-behaved muscovite age spectra, the smooth gradient
in ages across the study area, the inter-sample precision (see discussion
in Walsh et al., 2007), and the ‘fit’ of the muscovite ages within the context of other geochronological studies, all demonstrate a general absence
of excess Ar within most of the white mica in the WGR. Why the WGR
should be “exempt” from this widespread problem of excess Ar is
unclear. One, perhaps remote, possibility is that none of the micas
recrystallized at pressures higher than 1 GPa, Barrovian metamorphism
(Peterman et al., 2009; Walsh and Hacker, 2004).
constrictional regime, and b) a NE–SW dome-like structure south of
Nordfjord that experienced differential exhumation, possibly due to
footwall uplift along the Nordfjord–Sogn Detachment Zone plus rotation due to shearing on the Hardangerfjord Shear Zone. E–W extension
continued along brittle–ductile faults through the Late Devonian, after
which exhumation occurred along local structures (as recorded by
K-feldspar ages and brittle faults) as late as ~260 Ma.
6. Conclusions
Funded by NSF EAR-9814889, EAR-0510453, EAR-0911485, and
NFR Centre of Excellence grant to PGP. We thank Blanka Sperner for
help with data handling and Haakon Fossen and Loic Labrousse for
detailed reviews that resulted in considerable improvement to the
manuscript.
New 40Ar/39Ar thermochronology and fault-slip data have notable
implications for the late-stage exhumation of the WGR (U)HP terrane
(Fig. 8). Muscovite ages from the northwestern WGR record the exhumation of the (U)HP rocks into the mid-upper crust approximately
20 Myr after the UHP metamorphism ended. Strong E–W stretching
along with westward propagation of the Nordfjord–Sogn Detachment
Zone created: a) a block north of Nordfjord that experienced eastward
tilting and the development of local E–W trending folds in a
Acknowledgments
Appendix A. Supplementary data
Supplementary data to this article can be found online at http://
dx.doi.org/10.1016/j.tecto.2013.06.030.
Please cite this article as: Walsh, E.O., et al., Crustal exhumation of the Western Gneiss Region UHP terrane, Norway: 40Ar/39Ar thermochronology
and fault-slip analysis, Tectonophysics (2013), http://dx.doi.org/10.1016/j.tecto.2013.06.030
18
E.O. Walsh et al. / Tectonophysics xxx (2013) xxx–xxx
A
E9806F
fault striae
E9803P
E9806A
E9805L
J5813F
E9808F
certain
reliable
inferred
unknown
σ1, σ2, σ3
foliation
extension vein
chiefly strike slip
R9823G
E9804A
chiefly dip slip
E9805C
E1612C
E9816F-1
Y1611ALbio
Y0816M
J5810A
Nordfjord-Sogn
Detachment Zone
E1606H
E9809H
E9808K
Y0816C
E9810A
E9820M
B
fault striae
E9804I
R9821C
certain
reliable
inferred
unknown
J5814B
σ1, σ2, σ3
R9821B
E9805P
foliation
extension vein
E9807B
chiefly strike slip
E9803G
chiefly dip slip
E9803E
R9822C
E9817A
Nordfjord-Sogn
Detachment Zone
R9826E
E9807A
E9820F
E1607A
E1606A
Y0817A
E9809G
P6809A
Y1611ALchl
E9808D
E9816F-2
E1612P
Fig. 7. Ductile–brittle faults and brittle faults show dominantly E–W stretching at amphibolite facies to greenschist facies. Each stereoplot is a lower-hemisphere, equal-area
projection; symbol explanation in upper-right corner. A) Fault sets are grouped into i) strike-slip along reactivated steep foliation planes (green), and ii) strike-slip on faults
that cut foliation (blue). B) Fault sets are grouped into i) normal-slip reactivation of moderately inclined foliation planes (purple), and ii) dip-slip on steep faults (yellow). Fault
sets composed of relatively few data are uncolored.
Please cite this article as: Walsh, E.O., et al., Crustal exhumation of the Western Gneiss Region UHP terrane, Norway: 40Ar/39Ar thermochronology
and fault-slip analysis, Tectonophysics (2013), http://dx.doi.org/10.1016/j.tecto.2013.06.030
E.O. Walsh et al. / Tectonophysics xxx (2013) xxx–xxx
19
A) ~425–400 Ma: (U)HP metamorphism in subducted Baltica basement and allochthons
Caledonian allochthons
Laurentia
100 km
Baltica
UHP metamorphism
by ~410 Ma: allochthons in east cooled through muscovite closure at ~10 km depth
B) North of Nordfjord
C) South of Nordfjord
~400–380 Ma: E–W extension, chiefly along NSDZ, exhumed increasingly
deep rocks to west
Nordfjord–Sogn
380 385
detachment zone
390
400 410 420
395
~374 Ma: N–S shortening further exhumed UHP rocks in west
sedimentary basins NSDZ
formed above NSDZ 380 385
390
395
400 410 420
~300 Ma: E–W extension continued on NSDZ into Permian
NSDZ
sedimentary basins 380 385
390
395
400 410 420
~400 Ma: parautochthonous basement cooled through muscovite closure
across southern WGR
400
Dalsfjord Suite
NSDZ
400
405
Jotun Nappe
~395 Ma: continued E–W extension caused differential exhumation in
south-central WGR
Hardangerfjord
NSDZ
400 395 395 400
405
Shear Zone
~260 Ma: southeastern WGR cooled moderately; followed by Late
Permian slip on Laerdal–Gjende fault
sedimentary basins NSDZ 400 395 395 400
405 Laerdal–Gjende fault
Fig. 8. Tectonic scenario for late-stage exhumation of UHP rocks north and south of Nordfjord, after Hacker et al. (2010). Muscovite age contours shown in blue. Note that exact
timing of WGR exposure remains uncertain. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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