Crustal Exhumation of the Western Gneiss Region UHP Terrane, Norway: 40Ar/39Ar Thermochronology and Fault-Slip Analysis

Tectonophysics 608 (2013) 1159–1179

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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 b Geology, Cornell College, Mount Vernon, IA 52314, USA c 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 article info abstract

Article history: New 40Ar/39Ar muscovite and K-feldspar thermochronology combined with existing data reveal the timing Received 29 June 2012 and patterns of late-stage exhumation across the Western Gneiss Region (U)HP terrane. Muscovite age contours Received in revised form 15 June 2013 show that exhumation into the mid-upper crust progressed westward over a ~20 Myr period (~400–380 Ma). Accepted 27 June 2013 This exhumation was caused by i) E–W stretching and eastward tilting north of Nordfjord, where muscovite Available online 5 July 2013 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 Keywords: Ultrahigh-pressure exhumation Region is bounded by overlying units little affected by the Scandian metamorphism. Exhumation of the UHP Western Gneiss Region domains into the mid-upper crust by late folding continued through ~374 Ma. The smooth gradient of fairly 40Ar/39Ar muscovite thermochronology ﬂat muscovite age spectra demonstrates minimal inﬂuence of excess Ar, which is relatively unusual for a 40Ar/39Ar K-feldspar thermochronology (U)HP terrane. 40Ar/39Ar spectra and modeled cooling histories from K-feldspar combined with brittle–ductile Fault-slip analysis and brittle fault data indicate continued exhumation on local structures into the Permian. © 2013 Elsevier B.V. All rights reserved.

1. Introduction et al., 2004). Here, we present a dense net of low-temperature thermochronology data and a regionally distributed set of late-stage, Over the past three decades, much work has been done to under- fault-slip data. These data allow us to address the following specific stand the subduction and exhumation of ultrahigh-pressure (UHP) questions: i) What were the deformation kinematics during exhuma- rocks. Once a controversial concept, subduction of continental crust to tion through the crust? Is there an identifiable spatial or temporal vari- ultrahigh pressures is now known to have occurred repeatedly ation in the kinematics, and how did exhumation through the crust throughout the Phanerozoic (Ernst, 2001). UHP exhumation may take differ from earlier, high-temperature exhumation? ii) At what rate did place in two stages at different rates: an initial decompression from cooling occur and how did it vary spatially? What does this mean for ex- mantle depths to the base of the crust, and a second stage through the humation rates and their spatial variation? What implications does this crust (Walsh and Hacker, 2004). Exhumation of continental crust have for the mechanism of crustal exhumation? from mantle depths has often been attributed to changes in buoyancy or rheology (e.g., Chemenda et al., 1995; Milnes and Koyi, 2000; 2. Geology of the Western Gneiss Region (WGR) Peterman et al., 2009), whereas exhumation of continental crust through continental crust may be driven by, or be a byproduct of, a The WGR of Norway (Fig. 1) is a window of Baltican Proterozoic wider range of processes (e.g., Braathen et al., 2004; Dewey and gneiss with igneous and metamorphic ages of ~1650 Ma, ~1200 and Strachan, 2003; Johnston et al., 2007). Spatial and temporal variations ~950 Ma (Austrheim et al., 2003; Skår, 2000; Tucker et al., 1991) ex- in exhumation rate and kinematics across a UHP terrane are critical to posed beneath a stack of allochthons initially emplaced onto the mar- evaluating the processes involved in exhuming UHP rocks through the gin of Baltica between ~430 Ma and 415 Ma (Hacker and Gans, 2005; crust. Even in the relatively well-studied UHP Western Gneiss Region Roberts, 2003). The nappe sequence includes part of Laurentia in the (WGR) of Norway, these data remain incompletely known (Kendrick 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 ex- ⁎ Corresponding author at: Geology, Cornell College, Mount Vernon, IA 52314, USA. Tel.: +1 319 895 4302; fax: +1 319 895 5667. posed, east of the WGR, but attenuated equivalents crop out across E-mail address: [email protected] (E.O. Walsh). the WGR in relatively coherent (Robinson, 1995) but disconnected

Carboniferous-Devonian Basins Høybakken Detachment Caledonian Allochthons Autochthon (Baltican basement) 100 km Hitra-Snåsa Fault Trondheim

Møre-Trøndelag FC

UHP Nordøyane domains

Sørøyane 62°N Røragen detachment Stadlandet

Nordfjord Hornelen Basin Nordfjord-Sogn Detachment Jotun Nappe Kvamshesten Basin Laerdal-Gjende fault Olestøl fault

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 due to an unspecified gravitational instability (e.g., Hacker, 2007; Hurich, and Hacker, 2004). 1996; Johnston et al., 2007; Labrousse et al., 2004; Peterman et al., 2009; The convergence of Baltica and Laurentia resulted in a Himalaya- Walsh and Hacker, 2004). The calculated rates of exhumation for the type collision, with NW-directed subduction of the nappes and the initial stage are often quoted as ~10 mm/yr or faster (e.g., Carswell et Baltican margin beneath Laurentia (Hacker and Gans, 2005; Labrousse al., 2003; Krabbendam and Dewey, 1998; Kylander-Clark et al., 2008; et al., 2010; Torsvik and Cocks, 2005). This episode, the Scandian Terry et al., 2000; Walsh et al., 2007). Once the rocks reached crustal orogeny, resulted in metamorphism of crustal rocks to conditions as depths, they were extensively overprinted by granulite- (Straume and high as 3.6 GPa and 800 °C (Cuthbert et al., 2000; Dobrzhinetskaya Austrheim, 1999) or, more commonly, amphibolite-facies metamorphism et al., 1995; Krogh Ravna and Terry, 2004; Terry et al., 2000; Wain, at ~650–800 °C down to pressures of 0.5 GPa (Labrousse et al., 2004; Root 1997) over a period of about 20 Myr from ~420 to ~400 Ma (see sum- et al., 2005; Spencer et al., 2013; Terry and Robinson, 2003; Walsh and mary in Kylander-Clark et al., 2009). UHP rocks are now exposed along Hacker, 2004). the west coast of the WGR in 3 distinct domains (Fig. 1), which, based Extension is commonly called upon during the second stage of exhu- upon the location of UHP rocks beneath HP rocks and younger musco- mation to have moved the rocks from amphibolite-facies conditions at vite ages within the domains, define apparent antiformal culminations the base of the crust to greenschist-facies conditions in the upper crust (Hacker et al., 2010). Metamorphic grade increases northwestward (alternatives exist, see e.g., Andersen et al., 1994; Dewey and Strachan, across the WGR (Griffin et al., 1985; Krogh, 1977; Tucker et al., 1991) 2003; Fossen, 2000; van Roermund and Drury, 1998). Evidence for this as does the degree of Scandian deformation (Barth et al., 2010; Hacker includes vertical shortening combined with strong top-W extension et al., 2010; Krabbendam and Wain, 1997; Milnes et al., 1997; Young along the Nordfjord–Sogn Detachment Zone (Johnston et al., 2007; et al., 2007). Marques et al., 2007; Norton, 1986; Séranne and Séguret, 1987), as Many different models have been suggested for the exhumation of the well as sinistral (rotated normal-sense) shear along the Møre–Trøndelag UHP rocks in west Norway; the majority includes two stages: relatively Fault Complex in the north (Braathen et al., 2000; Krabbendam and rapid exhumation from mantle depths to the base of the continental Dewey, 1998; Séranne, 1992). During this later stage of exhumation crust, followed by slower crustal exhumation. The cause of the initial, into the mid-upper crust, Buchan-type amphibolite-facies recrystalliza- mantle stage of exhumation is unknown and variously inferred to have tion affected local domains in the west, and late-stage folds formed been caused by removal of a dense lithospheric root (e.g., Andersen and (Fossen, 2010; Krabbendam and Dewey, 1998). Within the Sørøyane Jamtveit, 1990; Austrheim, 1991), a change in plate motion (e.g., Dewey UHP domain, this second stage of exhumation began after isothermal and Strachan, 2003; Fossen, 2000), forced-return flow (e.g., Terry and (~750 °C) decompression to granulite-facies conditions at 15–20 km Robinson, 2004), slab breakoff and eduction (Andersen et al., 1991; depth (~0.5 GPa), creating an unusually hot geothermal gradient Brueckner and van Roermund, 2004; Duretz et al., 2012), or delamination roughly equivalent to that of the Basin and Range today (Root et al., E.O. Walsh et al. / Tectonophysics 608 (2013) 1159–1179 1161

2005). Cooling occurred rapidly after this isothermal decompression, the WGR, where the bulk of the eclogites and all the known UHP with rates of ~30–90 °C/Myr implied by the difference in U–Pb zircon rocks crop out, is much less well characterized. This study addresses and titanite and 40Ar/39Ar muscovite ages (Kylander-Clark et al., 2008; that deﬁciency by analyzing 37 additional samples (Fig. 2; Table 1) Root et al., 2005). collected mainly from typical quartzofeldspathic gneiss (Fig. 2A–E) K-white mica (henceforth, “muscovite”) thermochronology has but also from maﬁc gneiss (Fig. 2F: 8815G12, K5622A5), rocks in been used effectively over the past several decades to reveal the the Møre–Trøndelag Fault Complex (Fig. 2G: J5816A, J5816B, timing of exhumation of (U)HP rocks across the WGR (e.g., J5816I), discordant dikes (Fig. 2F: E9814A6, H5622B, P6804A), and Andersen et al., 1998; Chauvet and Dallmeyer, 1992; Fossen and the allochthons (Fig. 2H, I: 8907B5, 929, J5813F, J5814H, J5814N4, Dunlap, 1998; Hacker and Gans, 2005; Lux, 1985; Root et al., 2005; J5814S, J5815K). Walsh et al., 2007; Warren et al., 2012; Young et al., 2007). Most of The main amphibolite-facies fabric across the WGR, Scandian the data are from the southern half of the WGR; the northern half of coaxial E–W stretching (Hacker et al., 2010), is overprinted by an

A 415 0.004 E1612C7 E1612C7 407 Atm 0.003 Age = 370.7 ± 7.4 Ma MSWD = 2.99 (<2.26) 399 Ar 40 WMPA = 387.1 ± 0.4 Ma 0.002 391 Ar/ 36 383 0.001

Apparent Age (Ma) 40/36=3071.0±3614.5 375 0139 0.000 Cumulative Ar 0.000 0.013 0.026 0.040 0.052 0.066 39Ar/40Ar 400 0.004 E1612Q8 E1612Q8 Atm 394 0.003 Age = 389.7 ± 0.6 Ma MSWD = 2.01 (<2.00) 388 Ar

WMPA =390.4 ± 0.4 Ma 40 0.002

382 Ar/ 36 40/36=386.1±36.1 376 0.001 Apparent Age (Ma) 370 39 0.000 01Cumulative Ar 0.000 0.010 0.021 0.031 0.042 0.052 39Ar/40Ar 0.004 415 H5621B H5621B Atm 405 Age = 393.7 ± 1.0 Ma 0.003 MSWD = 1.77 (<1.85)

395 Ar WMPA = 397.7 ± 0.5 Ma 40 0.002

385 Ar/ 36 0.001 375 40/36=511.9±39.8 Apparent Age (Ma) 365 0.000 01Cumulative 39Ar 0.000 0.011 0.023 0.034 0.045 0.057 39Ar/40Ar 445 0.004 H5622C H5622C Atm 435 Age = 377.2 ± 3.9 Ma 0.003 MSWD = 0.58 (<2.63) 425 Ar 40 397 to 375 Ma 0.002 415 Ar/ 36 405 0.001 Apparent Age (Ma) 40/36=5339.4±543.9 395 0139 0.000 Cumulative Ar 0.000 0.011 0.022 0.033 0.044 0.055 39Ar/40Ar

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. 1162 E.O. Walsh et al. / Tectonophysics 608 (2013) 1159–1179 B 405 0.004 H5702A H5702A 397 Atm Age = 388.7 ± 0.6 Ma 0.003 MSWD = 0.06 (<2.41)

389 Ar WMA = 388.8 ± 0.4 Ma 40 0.002 381 Ar/ 40/36=316.5±38.5 36 373 0.001 Apparent Age (Ma) 365 39 0.000 01Cumulative Ar 0.000 0.011 0.021 0.032 0.042 0.053 39Ar/40Ar

400 0.004 J5811D J5811D 394 Atm Age = 388.2 ± 0.6 Ma 0.003 MSWD = 1.01 (<2.15)

388 Ar 40 0.002 382 WMPA = 388.2± 0.4 Ma 40/36=294.4±25.0 Ar/ 36 376 0.001 Apparent Age (Ma) 370 0.000 01Cumulative 39Ar 0.000 0.011 0.021 0.032 0.042 0.053 39Ar/40Ar

415 0.004 J5815E1 J5815E1 405 Atm Age = 375.6 ± 0.6 Ma 0.003 MSWD = 1.56 (<1.85)

395 Ar 40 0.002 WMPA = 378.8 ± 0.4 Ma 385 Ar/

36 40/36=884.1±34.9 375 0.001 Apparent Age (Ma) 365 0.000 01Cumulative 39Ar 0.000 0.012 0.023 0.035 0.047 0.058 39Ar/40Ar

390 0.004 J5815G J5815G 385 Atm Age = 379.7 ± 0.5 Ma 0.003 MSWD = 1.61 (<1.89)

380 Ar WMPA = 380.1 ± 0.4 Ma 40 0.002

375 Ar/ 36 40/36=363.7±17.2 370 0.001 Apparent Age (Ma) 365 0.000 01Cumulative 39Ar 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 metamorphism, they provide kinematic information about the exhu- surfaces ranging from 10's of m to cm scale. A few faults appear mation of the (U)HP rocks into the upper crust. from outcrop and thin-section observations to have been active during amphibolite-facies symplectite formation (Hacker, 2007; 3. Analytical techniques Peterman et al., 2009), and their slip surfaces are characterized chiefly by coarse-grained to fine-grained biotite (Fig. 3A). Most brittle– 3.1. Thermochronology ductile faults postdated symplectite formation and are characterized by recrystallized biotite ± epidote ±feldspar ± quartz (Fig. 3). Muscovite and K-feldspar samples were irradiated at Oregon State Semi-brittle to brittle faults are marked by chlorite ± epidote ± University for 40 h and analyzed at the University of California, Santa quartz ± carbonate ± Mn–Fe oxides and, locally, cataclasite, clay or Barbara, by Staudacher-type resistance-furnace step heating. The irradi- gouge. At several sites, brittle faults overprint semi-ductile to ation flux monitor was Taylor Creek Rhyolite sanidine, for which we as- semi-brittle faults, but at many outcrops there is a continuum be- sumed an age of 28.34 ± 0.28 Ma (Renne et al., 1998). The uncertainty tween the brittle–ductile and brittle faults. Because these faults in the irradiation flux monitor, J, was set conservatively at ±0.12% 2σ. were active after the major, high-temperature amphibolite-facies Previously published ages from the study area have been recalculated E.O. Walsh et al. / Tectonophysics 608 (2013) 1159–1179 1163 C 400 0.004 J5816J J5816J Atm 394 Age = 389.6 ± 0.5 Ma 0.003 MSWD = 1.63 (<1.78)

388 Ar WMPA = 389.8 ± 0.4 Ma 40 0.002

382 Ar/ 40/36=320.5±23.4 36 0.001 376 Apparent Age (Ma) 370 0.000 0 Cumulative 39Ar 1 0.000 0.010 0.021 0.031 0.042 0.052 39Ar/40Ar

410 0.004 J5816L1 J5816L1 Atm 404 Age = 397.1 ± 0.5 Ma 0.003 WMPA = 397.3 ± 0.4 Ma MSWD = 1.24 (<2.15)

398 Ar 40 0.002

392 Ar/

36 40/36=355.5±18.0 0.001 386 Apparent Age (Ma) 380 0.000 0 Cumulative 39Ar 1 0.000 0.010 0.021 0.031 0.042 0.052 39Ar/40Ar 400 0.004 P5627E2 P5627E2 Atm 394 Age = 383.7 ± 0.6 Ma 0.003 MSWD = 2.07 (<2.00)

388 Ar WMA = 387.6 ± 0.4 Ma 40 0.002

382 Ar/ 36 0.001 40/36=1052.0±48.0 376 Apparent Age (Ma) 370 0.000 0 Cumulative 39Ar 1 0.000 0.010 0.021 0.031 0.042 0.052 39Ar/40Ar

395 0.004 P5627F3 P5627F3 Atm 389 0.003 Age = 386.0 ± 0.6 Ma MSWD = 3.89 (<1.89)

383 Ar WMPA = 385.8 ± 0.4 Ma 40 0.002 40/36=261.1±58.8

377 Ar/ 36 0.001 371 Apparent Age (Ma) 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. of four age steps from a spectrum was ﬁt with a line to deﬁne the ac-

Ages are reported with 1σ uncertainty. Uncertainties quoted in the text tivation energy E and frequency factor Do (Lovera et al., 1989); more include uncertainty in J only to facilitate comparison of 40Ar/39Ar ages steps were added if the fit improved. The number of domains of different samples. Full uncertainties—including also uncertainties in was limited to a minimum of three and a maximum of eight. The 40K decay constant and monitor age—are included in Table 1;theseare diffusion-domain theory predicts constant or monotonically increas- the uncertainties to be used when comparing the 40Ar/39Ar ages to ing age spectra, and spectra that do not match this ideal were either those determined by other methods (e.g., U–Pb). not modeled or had the uncertainties of aberrant step ages increased For muscovite, the gas was incrementally released in 14–20 steps until the spectrum showed a monotonic age increase: i) multiple iso- per sample, with 15-minute 1100 °C line blanks run periodically thermal, low-temperature steps designed to identify Cl-correlated ex- between samples (see Appendix A in the Supplementary Material). cess 40Ar (Harrison et al., 1994) were assigned the age of the youngest For K-feldspar, 47–104 step experiments were run with isolation step in the group, and ii) steps with low radiogenic yields (b95%) and times of 15 min to 10 h (see Calvert et al., 1999). Many of the anomalously old ages were adjusted to provide a smoothly increasing K-feldspar samples yielded spectra suitable for full diffusion-domain trend. Cooling histories were calculated from initial times 50–100 Myr analysis (Lovera et al., 1989, 2002), which we completed using modi- older than the oldest step. Only those cooling histories that provide a fied 1997 versions of Lovera's (1992) modeling routines. A minimum good fit to the data are shown. 1164 E.O. Walsh et al. / Tectonophysics 608 (2013) 1159–1179 D 395 0.004 P5627K P5627K 389 Atm Age = 385.3 ± 0.8 Ma 0.003 MSWD = 0.47 (<1.94)

383 Ar

WMPA = 386.5 ± 0.4 Ma 40 0.002 40/36=486.8±121.4

377 Ar/ 36 371 0.001 Apparent Age (Ma) 365 0.000 0 Cumulative 39Ar 1 0.000 0.0110.022 0.034 0.045 0.056 39Ar/40Ar

400 0.004 P5629O P5629O Atm 394 Age = 388.0 ± 0.6 Ma WMPA = 387.4 ± 0.4 Ma 0.003 MSWD = 1.98 (<2.00)

388 Ar 40 0.002 40/36=239.1±30.4

382 Ar/ 36 0.001 376 Apparent Age (Ma) 370 0.000 0 Cumulative 39Ar 1 0.000 0.0100.021 0.031 0.042 0.052 39Ar/40Ar

395 0.004 P6805H2 P6805H2 Atm 389 Age = 388.2 ± 0.4 Ma 0.003 MSWD = 1.21 (<1.85)

383 WMPA = 388.2 ± 0.4 Ma Ar 40 0.002

377 Ar/ 40/36=317.4±7.4 36 0.001 371 Apparent Age (Ma) 365 0.000 0 Cumulative 39Ar 1 0.000 0.0110.022 0.034 0.045 0.056 39Ar/40Ar

0.004 395 P6806J P6806J Atm 389 Age = 379.8 ± 0.5 Ma 0.003 MSWD = 1.07 (<2.15)

383 Ar 40/36=458.1±14.0 40 0.002

377 WMA = 382.2 ± 0.4 Ma Ar/ 36 0.001 371 Apparent Age (Ma) 365 0.000 0 Cumulative 39Ar 1 0.000 0.011 0.021 0.032 0.042 0.053 39Ar/40Ar

Fig. 2 (continued).

3.2. Electron-probe microanalysis 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 Muscovite grains from each of the 37 rock samples were analyzed (using a scale of 1 to 4, where 1 is certain, 2 is reliable, 3 is inferred, and on a Cameca SX-50 electron microprobe with 5 wavelength spec- 4isunknown,Angelier, 1984). We recorded the type of mineralization trometers at the University of California, Santa Barbara, using an ac- along the faults, mineralization within associated mode-I veins, and ap- celerating voltage of 15 kV, a current of 15 nA, a spot size of 2 μm, parent overprinting relationships to attempt assessment of the relative and natural and synthetic mineral standards. Core and rim analyses ages of different fault sets in each outcrop (Table 3). reported are averages of 2 spot analyses. Like Braathen (1999) and Braathen and Bergh (1995), we separated different fault sets at individual outcrops (or sets of nearby outcrops) 3.3. Fault-slip analysis manually, rather than relying on computationally based separation; this was done principally because the gneissic anisotropy likely affected We measured fault slip surfaces throughout the study area, re- the orientation and abundance of fault sets. In general, each outcrop cording the orientation of slip planes, the orientation of lineations/ is dominated by a particular kind of fault with a characteristic E.O. Walsh et al. / Tectonophysics 608 (2013) 1159–1179 1165 E 395 0.004 P6816B1 P6816B1 Atm 389 Age = 384.2 ± 0.5 Ma 0.003 MSWD = 1.50 (<1.82)

383 Ar WMPA = 384.7 ± 0.4 Ma 40 0.002

377 Ar/ 36 40/36=349.6±17.1 0.001 371 Apparent Age (Ma) 365 0.000 0 Cumulative 39Ar 1 0.000 0.0110.022 0.034 0.045 0.056 39Ar/40Ar

395 0.004 P6818C2 P6818C2 Atm 389 Age = 383.7 ± 0.6 Ma 0.003 MSWD = 1.19 (<2.00)

383 Ar WMPA = 384.5 ± 0.4 Ma 40 0.002 377 Ar/

36 40/36=349.8±22.8 0.001 371 Apparent Age (Ma) 365 0.000 0 Cumulative 39Ar 1 0.000 0.0110.022 0.034 0.045 0.056 39Ar/40Ar

960 405 H5621A H5622E 900 399

840 393 WMA = 397.2 ± 0.4 Ma

780 387

720 381 Apparent Age (Ma) 931 to 450 Ma Apparent Age (Ma) 660 375 0 Cumulative 39Ar 1 0 Cumulative 39Ar 1

425 J5816K 419

413

407 WMA = 402.4 ± 0.5 Ma

401 Apparent Age (Ma)

395 0 Cumulative 39Ar 1

Fig. 2 (continued).

mineralization and a relatively limited variation in planar or linear ele- at most outcrops, we treat the kinematic data from each locality as a sin- ments. Faults with a large range in measured planar or linear elements gle data set. We do not attempt to resolve principal directions at high were not placed in a single group unless there was a continuum of mea- precision, largely because most of the rocks are anisotropic and many surements. Strike-slip and dip-slip faults were considered separately of the brittle–ductile faults reactivate foliation planes. We follow unless they exhibited the same calculated principal directions. Each Lacombe (2012) in interpreting the calculated principal directions as fault-slip dataset was analyzed following Sperner et al. (1993) indicators of paleostress. using the NDA program (Spang, 1972)writtenbySperner and

Ratschbacher (1994) and an angle of 45° between σ1 and each fault 4. Results planeforthebrittle–ductile faults and 30° for the brittle faults (Table 3). Our objective was to better understand the regional,post- 4.1. Muscovite thermochronology amphibolite-facies deformation of the WGR and to characterize large-scale spatial and temporal variations in the deformation. Because Most of the analyzed muscovite samples yielded slightly U-shaped we observed no systematic differences in brittle–ductile and brittle faults age spectra; nine samples produced spectra for which weighted mean 1166 E.O. Walsh et al. / Tectonophysics 608 (2013) 1159–1179 F 395 0.004 8815G12 8815G12 Atm 389 Age = 381.7 ± 0.5 Ma 0.003 MSWD = 0.66 (<2.07)

383 Ar 40 0.002

377 WMPA = 382.1 ± 0.4 Ma Ar/ 36 40/36=359.6±14.3 0.001 371 Apparent Age (Ma) 365 0.000 0 Cumulative 39Ar 1 0.000 0.0100.021 0.031 0.042 0.052 39Ar/40Ar 431 0.004 E9814A6 E9814A6 Atm 421 Age = 386.4 ± 1.3 Ma 0.003 MSWD = 2.73 (<2.26)

411 Ar 40 0.002 40/36=554.3±118.0

401 Ar/ WMPA = 389.3 ± 0.4 Ma 36 0.001 391 Apparent Age (Ma) 381 0.000 0 Cumulative 39Ar 1 0.000 0.0100.021 0.031 0.042 0.052 39Ar/40Ar 415 0.004 H5622B H5622B Atm 407 Age = 397.5 ± 0.6 Ma 0.003 MSWD = 1.33 (<1.82)

399 Ar 40 0.002

391 WMPA = 397.9 ± 0.4 Ma Ar/ 36 0.001 40/36=404.8±13.8 383 Apparent Age (Ma) 375 0.000 0 Cumulative 39Ar 1 0.000 0.0100.021 0.031 0.042 0.052 39Ar/40Ar 425 567 K5622A5 P6804A 415 547

405 527

395 507

385 487 Apparent Age (Ma) Apparent Age (Ma) 414 to 403 Ma 558 to 471 Ma 375 467 0 Cumulative 39Ar 1 0 Cumulative 39Ar 1

Fig. 2 (continued).

plateau ages (WMPA) (N50% of 39Ar released) could be calculated, 25 range from ~387–383 Ma, younger than previously dated muscovite produced relatively flat spectra for which we computed weighted from the Central Norway basement window and the Høybakken de- mean ages (WMA), and 3 samples yielded spectra with a range of tachment zones (Dallmeyer et al., 1992; Eide et al., 2005) and similar step ages. Twenty-five of the 37 samples yielded well-fit inverse to the hornblende and biotite 40Ar/39Ar ages from detachment isochrons; the isochron ages are preferred because they account for de- mylonites (Kendrick et al., 2004). The youngest ages (e.g., 370.2 ± viation of the trapped 40Ar/36Ar ratio from atmospheric and, in some 0.9 Ma from strain shadows in a mylonite at Terry929 on Fjørtoft, samples, use a greater percentage of the data than the WMPA. Nordøyane) are from samples within UHP domains. The new muscovite 40Ar/39Ar ages from basement gneisses increase The Si content of the muscovites ranges from 3.01 to 3.31 atoms per eastward across the WGR toward the foreland (Fig. 4). Ages at the formula unit (apfu) (Table 2). There is up to 0.10 Si apfu difference be- eastern limit of the WGR are generally ~400 Ma (e.g., 398.5 ± 0.5 Ma tween the cores and rims of individual grains, but the difference is not at J5814H, 402.4 ± 1.0 Ma at J5816K); they increase gradually into systematically positive or negative within samples; more-detailed tran- the foreland allochthon stack to the east and increase more dramatically sects by Warren et al. (2012) on similar samples show that some WGR into the Jotun Nappe (558–471 Ma at P6804A, 931–450 Ma at H5621A) muscovites have Si-rich cores and Si-poor rims. There is no relationship to the southeast. The samples from the Møre–Trøndelag Fault Complex apparent between the Si content of the muscovite cores or rims and age E.O. Walsh et al. / Tectonophysics 608 (2013) 1159–1179 1167

G 400 0.004 J5816A J5816A Atm 394 0.003 Age = 386.6 ± 1.2 Ma MSWD = 0.42 (<1.89)

388 Ar WMA = 389.3 ± 0.4 Ma 40 0.002

382 Ar/

36 40/36=827.9±207.8 376 0.001 Apparent Age (Ma) 370 0.000 0 Cumulative 39Ar 1 0.000 0.0100.021 0.031 0.042 0.052 39Ar/40Ar 405 0.004 J5816B J5816B Atm Age = 382.7 ± 0.9 Ma 395 0.003 MSWD = 2.19 (<2.15)

385 Ar 40 WMPA = 383.0 ± 0.4 Ma 0.002 375 Ar/ 36 40/36=353.0±102.2 0.001 365 Apparent Age (Ma) 355 0.000 0 Cumulative 39Ar 1 0.000 0.0100.021 0.031 0.042 0.052 39Ar/40Ar 400 0.004 J5816I J5816I Atm 394 Age = 384.8 ± 1.1 Ma 0.003 MSWD = 1.68 (<1.94)

388 Ar 40 0.002

382 WMPA = 387.5 ± 0.4 Ma Ar/ 40/36=674.9±186.7 36 0.001 376 Apparent Age (Ma) 370 0.000 0 Cumulative 39Ar 1 0.000 0.0100.021 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 378–374 Ma from nearby samples (Root et al., 2005) suggest that the observed correlation between muscovite age and composition is with modeled cooling history is reasonable. Sample R9828C12, from a Mg# [Mg/(Fe + Mg)], which ranges from 0.00 to 0.73 apfu and shows syndeformational pegmatite on Sandsøya south of the Sørøyane an increase with increasing age (Fig. 5B). Such a relationship was noted antiform, yielded a well-defined spectrum similar to that from by Scaillet et al. (1992) in high-pressure rocks from Dora Maira and Nerlandsøya, but without the unusually young intermediate step ages. was attributed partly to increased Ar retentivity in Mg-rich muscovite. 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) 4.2. K-feldspar thermochronology and thermal-history modeling until ~320 Ma, at which time the cooling rate decreased even further to b1 °C/Myr. The shape of the spectrum, however, suggests a We measured 40Ar/39Ar spectra and modeled the cooling histories of more-complicated cooling history from 383 Ma to 320 Ma that is not fifteen K-feldspar single crystals from an area spanning ~350 km north resolved by the modeling. The remaining three samples from this area to south and ~400 km east to west (Fig. 6). Five of the samples were col- (Gurskøya, Gødøya and Runde) have spectra affected by excess Ar and lected from within and near the Sørøyane UHP domain. Root et al. are difficult to model. The sample from Gurskøy (8815G5) is from (2005) inferred that the UHP rocks in this area occupy the core of an orthogneiss at the southern edge of the Sørøyane UHP domain. A antiform surrounded by high-pressure rocks. Muscovites within the ‘hump’ in the age spectrum followed by a drop in r/ro, suggests prema- core of the antiform are 374 and 378 Ma, whereas muscovites in the ture in vacuo melting of the K-feldspar; a good model fit was not high-pressure synform to the south are 384–380 Ma (Root et al., obtained, but the spectrum (combined with nearby muscovite ages of 2005); this difference has been interpreted to indicate that the folding 384–383 Ma (Root et al., 2005)) suggests slow cooling (~1 °C/Myr) is younger than 374 Ma. The K-feldspar sample from Nerlandsøya until ~280(?) Ma, followed by a more moderate cooling rate until (8905A3) is a granitic segregation at the margin of an eclogite boudin ~250 Ma. The Gødøya sample, 8822A5, is from a K-feldspar–biotite– from the center of the Sørøyane UHP domain. The spectrum includes a quartz pegmatite in amphibolite north of the Sørøyane UHP domain. series of intermediate steps that have unusually young ages but isotopic The spectrum complexity suggests variable release of excess 40Ar; be- ratios otherwise similar to the other steps (i.e., no elevated 38Ar or 36Ar, cause of this modeling was not attempted. The spectrum—plus the see Appendix B in the Supplementary Material); it was modeled fact that the sample site lies between a 374 Ma muscovite sample and to show moderate cooling from ~320 Ma to ~300 Ma (6 °C/Myr), a 390 Ma biotite sample (Root et al., 2005)—can be interpreted to re- followed by very slow cooling (b1 °C/Myr). Muscovite ages of flect slow cooling from ~374 to ~245 Ma, with rapid cooling around 1168 E.O. Walsh et al. / Tectonophysics 608 (2013) 1159–1179 H 395 0.004 8907B5 8907B5 389 Atm 0.003 Age = 382.4 ± 0.5 Ma MSWD = 1.11 (<2.00)

383 Ar WMPA = 382.9 ± 0.4 Ma 40 0.002

377 Ar/

36 40/36=370.0±24.4 0.001 371 Apparent Age (Ma) 365 0.000 0 Cumulative 39Ar 1 0.000 0.0110.022 0.032 0.043 0.054 39Ar/40Ar

485 0.004 Terry #929 Terry #929 Atm 459 0.003 Age = 367.3 ± 0.9 Ma MSWD = 1.86 (<3.83)

433 Ar 40 0.002 40/36=605.1±27.7

407 Ar/ 36 0.001 381 Apparent Age (Ma) WMA = 370.2 ± 0.4 Ma 355 0.000 0 Cumulative 39Ar 1 0.000 0.0110.022 0.032 0.043 0.054 39Ar/40Ar

400 0.004 J5813F J5813F 394 Atm 0.003 Age = 389.3 ± 0.5 Ma MSWD = 1.88 (<1.89)

388 Ar WMPA = 389.5 ± 0.4 Ma 40 0.002

382 Ar/

36 40/36=360.4±26.1 0.001 376 Apparent Age (Ma) 370 0.000 0 Cumulative 39Ar 1 0.000 0.0110.022 0.032 0.043 0.054 39Ar/40Ar

415 0.004 J5814H J5814H 405 Atm 0.003 Age = 398.5 ± 0.6 Ma MSWD = 1.92 (<2.00)

395 Ar WMPA = 398.7 ± 0.4 Ma 40 0.002

385 Ar/ 40/36=313.6±34.0 36 0.001 375 Apparent Age (Ma) 365 0.000 0 Cumulative 39Ar 1 0.000 0.0110.022 0.032 0.043 0.054 39Ar/40Ar

Fig. 2 (continued).

330–320 Ma. Sample 8829A1 from Runde is an undeformed dike cutting a pyroxene granulite in the Jotun nappe, gave a relatively K-feldspar–biotite–quartz vein from the north edge of the Sørøyane simple spectrum; modeling indicates moderate cooling from 280 to UHP domain; the spectrum suggests a slow rate of cooling that 260 Ma (7 °C/Myr) followed by very slow cooling (b0.5 °C/Myr). The may have increased around 315 Ma and decreased again shortly Laerdal sample, 8818C, from Baltica basement granitic orthogneiss 5 m thereafter. beneath the Laerdal–Gjende fault could not be ﬁt with reasonable MDD We analyzed four samples from the basement and overlying models, but the spectrum is compatible with extremely slow cooling allochthons along the southeastern edge of the WGR to extend the until ~270 Ma, when the cooling rate increased to ~2 °C/Myr. Sample work of Dunlap and Fossen (1998). Sample 8817A, from granitic 8818J, from a K-feldspar–quartz vein in the Valdres sedimentary cover orthogneiss of the easternmost Western Gneiss Complex, has consider- of the Jotun nappe, yielded a spectrum with excess 40Ar in early steps able excess Ar in the middle of the spectrum. Modeling, combined with that was removed with temperature cycling and excess 40Ar in later a nearby muscovite age of 400 Ma (Fossen and Dallmeyer, 1998), sug- steps associated with melting; modeling gives a relatively complete gests a moderate cooling rate from ~330–280Ma(4°C/Myr)andvery cooling history of extremely slow cooling (0.3 °C/Myr) until ~260 Ma, slow cooling (~0.6 °C/Myr) thereafter. Sample 8818A, from a granitic when the cooling rate increased to ~2 °C/Myr. E.O. Walsh et al. / Tectonophysics 608 (2013) 1159–1179 1169 I 400 0.004 J5814N4 J5814N4 Atm 394 Age = 388.1 ± 0.5 Ma 0.003 MSWD = 1.60 (<2.41) 388 Ar WMPA = 388.9 ± 0.4 Ma 40 0.002 382 Ar/ 40/36=313.5±19.1 36 0.001 376 Apparent Age (Ma) 370 0.000 39 0 Cumulative Ar 1 0.000 0.0100.021 0.031 0.042 0.052 39Ar/40Ar

405 0.004 J5814S J5814S Atm 397 Age = 388.6 ± 0.5 Ma 0.003 MSWD = 1.62 (<1.94)

389 Ar WMPA = 389.0 ± 0.4 Ma 40 0.002

381 Ar/ 36 40/36=348.5±19.3 0.001 373 Apparent Age (Ma) 365 0.000 0 Cumulative 39Ar 1 0.000 0.0110.022 0.034 0.045 0.056 39Ar/40Ar

395 0.004 J5815K J5815K Atm 389 Age = 384.2 ± 0.6 Ma 0.003 MSWD = 2.00 (<2.00)

383 Ar WMPA = 384.9 ± 0.4 Ma 40 0.002

377 Ar/ 36 0.001 40/36=403.5±33.2 371 Apparent Age (Ma) 365 0.000 0 Cumulative 39Ar 1 0.000 0.0100.021 0.031 0.042 0.052 39Ar/40Ar

Fig. 2 (continued).

Two samples were analyzed from the northern WGR. Sample 8814A 4.3. Fault-slip analysis collected near Stokken from a granitic pegmatite in an eclogite boudin strain shadow gave a relatively simple spectrum with some resolvable We observed no systematic and clear differences between brittle– excess 40Ar in early steps. Modeling implies slow cooling from 370 to ductile fault sets and brittle fault sets at most outcrops; although 310 Ma (3 °C/Myr) and then very slow cooling (0.3 °C/Myr) until more-detailed studies might find otherwise, we thus consider all of ≤260 Ma. The spectrum from Bergsøya sample 8814B, a granitic the data from each outcrop together (one exception is noted in pegmatite cutting an eclogite boudin, is not as simple as the previous Fig. 7). The pre-existing gneissic foliation strongly influenced the sample, but the MDD model suggests a similar cooling history. These orientations of faults at any given outcrop, such that the bulk of the two localities gave hornblende ages of 403 and 402 Ma and biotite measured brittle–ductile faults formed by slip along existing foliation ages of 397 Ma (Root et al., 2005). planes (Fig. 7, green and purple). Because the existing foliation planes We analyzed two samples from the Tømmerås Window where have variable orientations (e.g., due to outcrop-scale and larger WGR-type basement is exposed in a window beneath the overlying folds), the calculated principal directions for any given fault set are allochthons. A quartzite near Stiklestad, 8813A1, from the Leksdalsvatn/ more variable than if the faults had developed in an isotropic medi- Offerdal nappe, yielded an age spectrum with an unusually complete um, and this limits their interpretative value. In cases where the 200 Myr age range. Excess 40Ar appears to have been released episodi- pre-existing gneissic foliation was poorly oriented for slip, new faults cally during in vacuo heating, but the monotonic increase in age formed, cutting the foliation (Fig. 7, blue and yellow). Specific exam- implies that it was mostly removed by temperature cycling. The cooling ples of this include outcrops E9806A, E9806F, E9808K, E9810A, and models show very slow cooling (0.4 °C/Myr) from N510 Ma through Y1611AL. Note that the calculated principal directions for the two 300 Ma, at which time slow cooling (N1 °C/Myr) ensued. A Lund different types of faults (i.e., reactivated foliation and offset foliation) orthogneiss, 8813D, from the core of the Tømmerås Window gave an are similar (Fig. 7). uninterpretable spectrum contaminated by excess Ar. When the fault-slip data are considered in a broad way—as is One sample was measured from the easternmost Trondheim nappes. appropriate given the limitations imposed by the anisotropy of the H1602M is a megacrystic K-feldspar augen gneiss of the Risberget medium—nearly all of the data fit a simple pattern of E–Wstretching. nappe; modeling indicates extremely slow cooling (~0.1 °C/Myr) until At any given outcrop this E–W stretching occurred i) along ~340 Ma, at which point the cooling rate increased to ~7 °C/Myr. NNW-trending dextral faults and ENE-trending sinistral faults (Fig. 7, 1170 E.O. Walsh et al. / Tectonophysics 608 (2013) 1159–1179

Table 1 40Ar/39Ar muscovite age data.

Sample Geologic context UTM WMPAa ∑39% Isochrona ∑39% MSWD 40/36i TFA Renneb ±2σ

8815G12 Layered maﬁc block 0319906 6907793 376.9 ± 0.4 75 376.5 ± 0.5 96 0.66 360 ± 14 376.9 381.7 0.9 8907B5 Bio-kfs gneiss 0321490 6911190 377.7 ± 0.4 80 377.2 ± 0.5 99 1.11 370 ± 24 377.7 382.4 1.0 #929 of Terry Strain shadows in ky-gar mylonite Not listed 365.2 ± 0.4 27 362.3 ± 0.9 32 1.86 605 ± 28 384.2 370.2 0.9 E1612C7 Tonalitic-granodioritic gneiss 0342680 6899388 381.9 ± 0.4 76 365.7 ± 7.4 82 2.99 3071 ± 3614 383.5 387.1 0.9 E1612Q8 2-Mica granodioritic gneiss 0386325 6884503 385.1 ± 0.4 71 384.5 ± 0.6 94 2.01 386 ± 36 385.7 389.7 1.1 E9814A6 qtz-pl-kfs sweats in granulite 0366201 6909234 384.0 ± 0.4 73 381.2 ± 1.3 73 2.73 554 ± 118 387.1 389.3 0.9 H5621A Basement gneiss 0467229 6782883 902.5 ± 0.9 63 na na na na 878.0 931 to 450 na H5621B Basement gneiss 0321952 6731670 392.3 ± 0.5 80 388.4 ± 1.0 96 1.77 512 ± 40 392.9 393.7 2.0 H5622B Basement gneiss leucosome 0325254 6785784 392.5 ± 0.4 94 392.1 ± 0.6 94 1.33 343 ± 46 392.7 397.5 1.1 H5622C Dioritic gneiss 0349757 6784619 394.7 ± 0.5 22 372.1 ± 3.9 40 0.58 5339 ± 543 401.3 397 to 375 na H5622E Muscovite gneiss 0348278 6863184 391.8 ± 0.4 47 na na na na 392.1 397.2 0.9 H5702A Mus-qtz segregation in 0347533 6931555 383.5 ± 0.4 26 383.4 ± 0.6 26 0.06 317 ± 39 385.9 388.8 0.9 quartzofeldspathic gneiss J5811D Tonalitic-dioritic gneiss 0400791 6865298 383.0 ± 0.4 94 383.0 ± 0.6 94 1.01 294 ± 25 383.3 388.2 0.9 J5813F Blåhø quartzite 0451392 6948000 384.2 ± 0.4 53 384.1 ± 0.5 91 1.88 360 ± 26 384.7 389.3 0.9 J5814H Allochthon gneiss 0476914 6931581 393.3 ± 0.4 95 393.1 ± 0.6 95 1.92 314 ± 34 393.6 398.5 1.1 J5814N4 Blåhø schist 0324548 6970208 383.6 ± 0.4 90 382.9 ± 0.5 63 1.60 313 ± 19 383.7 388.1 1.0 J5814S Concordant pegmatite in Blåhø 0420254 6951810 383.7 ± 0.4 86 383.3 ± 0.5 94 1.62 348 ± 19 384.6 388.6 0.9 J5815B1 kfs-gneiss 0387893 6966797 372.4 ± 0.4 68 372.7 ± 0.6 68 2.67 276 ± 40 372.8 377.5 0.8 J5815E1 Granodioritic gneiss 0398970 6970946 373.7 ± 0.4 72 370.5 ± 0.6 97 1.56 884 ± 35 376.1 375.6 1.2 J5815G kfs augen gneiss 0399601 6981198 375.0 ± 0.4 54 374.6 ± 0.5 96 1.61 364 ± 17 375.3 379.7 1.0 J5815K Blåhø gneiss 0463998 7013119 379.7 ± 0.4 65 379.0 ± 0.6 90 2.00 403 ± 33 379.9 384.2 1.1 J5816A MTFZ mylonite zone 0475666 7004251 384.0 ± 0.4 46 381.4 ± 1.2 82 0.42 828 ± 208 383.6 386.6 2.3 J5816B MTFZ mylonitic tonalite 0484213 7024582 377.8 ± 0.4 63 377.5 ± 0.9 86 2.19 353 ± 102 377.9 382.7 1.8 J5816I MTFZ mylonitic tonalite 0499845 7023429 382.3 ± 0.4 94 379.6 ± 1.1 94 1.68 675 ± 187 382.3 384.8 2.2 J5816J Augen gneiss 0502823 7003520 384.5 ± 0.4 95 384.3 ± 0.5 99 1.63 320 ± 23 384.5 389.6 0.9 J5816K Muscovite schist with quartzite 0534950 7027640 396.9 ± 0.5 26 na na na na 398.9 402.4 1.0 J5816L1 Muscovite schist 0506601 6978402 391.9 ± 0.4 76 391.7 ± 0.5 72 1.24 355 ± 18 392.2 397.1 0.9 K5622A5 Eclogite 0308467 6805345 na na na na na na 404.1 na na P5627E2 Granodioritic gneiss 0352994 6906801 382.4 ± 0.4 67 378.5 ± 0.6 93 2.07 1052 ± 48 384.8 383.7 1.2 P5627F3 Bio-dioritic gneiss 0349529 6911676 380.6 ± 0.4 77 380.8 ± 0.6 77 3.89 261 ± 59 380.7 385.8 0.8 P5627K Granitic gneiss 0361434 6918751 381.3 ± 0.4 73 380.1 ± 0.8 73 0.47 487 ± 121 381.5 385.3 1.7 P5629O kfs-augen gneiss 0325474 6882843 382.2 ± 0.4 93 382.8 ± 0.6 93 1.98 239 ± 30 382.1 387.4 0.9 P6804A Pegmatite cutting kfs-augen gneiss 0435220 6834377 na na na na na na 510.8 558 to 471 na P6805H2 Bio-plg symplectitic gneiss 0316216 6874705 383.0 ± 0.4 77 382.9 ± 0.4 86 1.21 317 ± 7 383.1 388.2 0.9 P6806J kfs-augen gneiss 0299773 6894405 377.0 ± 0.4 55 374.7 ± 0.5 66 1.07 458 ± 14 377.5 379.8 1.0 P6816B1 Bio-kfs gneiss 0328243 6911792 379.5 ± 0.4 78 379.0 ± 0.5 100 1.50 350 ± 17 379.7 384.2 1.0 P6818C2 Sanidine gneiss 0380936 6951415 379.34 ± 0.4 86 378.5 ± 0.6 94 1.19 350 ± 23 379.1 383.7 1.2

WMPA, weighted mean plateau age (N50% cumulative 39Ar); WMA, weighted mean age, for cumulative 39Ar b50%. 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 5. Discussion E- and W-dipping normal faults (Fig. 7, purple and yellow), indicating coincident vertical thinning. These strike-slip dominated and dip-slip 5.1. Geochronology dominated fault sets have similar mineralization, such that the simplest interpretation is that they formed simultaneously. If so, this is a Our new muscovite ages are combined with previously published constrictional strain field. We find no significant record of vertical work (see references in Hacker, 2007; Warren et al., 2012)inFig. 4 to stretching or N–S stretching in the fault-slip data. 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. E.O. Walsh et al. / Tectonophysics 608 (2013) 1159–1179 1171 closed to Ar loss over ~20 Myr, from about 400 Ma at the eastern edge to the Nordfjord–Sogn Detachment Zone, possibly in combination with through about 380 Ma in the west. rotation of the WGR due to shear on the Hardangerfjord Shear Zone North of Nordfjord, basement gneisses and rocks interpreted to (Fossen and Hurich, 2005). represent allochthons folded into the basement record muscovite Determining precisely what each age represents is not straightfor- 40Ar/39Ar ages that increase eastward toward the foreland from ward. The muscovite age at any given location might reflect thermally 375.6 ± 0.6 Ma (J5815E1) to 398.5 ± 0.5 Ma (J5814H); such ages mediated Ar redistribution, deformation-driven Ar redistribution, or continue to increase within the nappes farther east (Hacker and Gans, fluid-driven Ar redistribution. A thermal control on Ar loss seems 2005) as well as structurally upward in the hanging wall to the WGC credible for many samples given that i) the main amphibolite-facies in Sunnfjord and Nordfjord. This entire domain thus represents a pro- deformation was 600–800 °C (Spencer et al., 2013), substantially gressively unroofed or eastward-tilted domain. The UHP domains in hotter than the 400–450 °C closure temperature of white mica to Ar the west have been interpreted as E-plunging antiforms (Root et al., volume diffusion inferred from experiments (Harrison et al., 2009); 2005) formed after ~378–374 Ma during long-term N–S shortening ii) the eastern half of the WGR was weakly deformed during the (Braathen, 1999; Osmundsen and Andersen, 2001; Torsvik et al., 1988). Scandian but still has Scandian muscovite ages (Hacker et al., 2010); South of Nordfjord, the pattern of muscovite 40Ar/39Ar ages is quite iii) in the southern half of the study area, the perimeter of the WGC different: the youngest ages are in the center of the WGR, and the is more deformed than the interior and yet has older muscovite ages increase both west and east. The ages increase smoothly eastward ages; and iv) most of the youngest ages come from gneiss with only an into the allochthons as they do farther north, but the age contours are amphibolite-facies fabric (Table 1). Recrystallization- or fluid-driven Ar closer together within the Jotun Nappe. To the south and west, ages loss also seems possible because i) some samples have heterogeneous jump abruptly upward within allochthonous units—the Bergen Arcs, laser spot ages within and among grains (Warren et al., 2012); ii) other the Høyvik Group, and the Dalsfjord Suite—that did not undergo grains have uniform laser spot ages from rim to rim (Warren et al., Scandian subduction with the WGR (Andersen et al., 1998; Eide et al., 2012); iii) some strongly deformed parts of the WGR have muscovite, 1997). Thus, the mica contours describe a dome-like structure about a biotite and amphibole ages that are similar [e.g., Møre–Trøndelag Fault NE–SW axis that is mirrored by the outcrop pattern of the WGR. This Complex local detachment mylonites (Kendrick et al., 2004)]; iv) musco- differential exhumation may represent Middle Devonian footwall uplift vite 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 426 J5816B 400 Ma 410 383 J5816I 420 Ma 397 J5816K J5815K 384 402 J5816A 420 J5816J 419 385 Ma 387 390 100 km 390 Ma 404 408 J5815G 380 J5814N4 J5815E1 376 J5816L1 421 J5815B1 388 397 378 J5814S 395 Ma P6816B Terry929 384 P6818C2 388 J5813F 8907B P5627E2 370 384 382 384 389 J5814H H5702A P5627K 399 P6806A 383 389 385 391 380 359 8815G 385 E9814A6 391 62°N 382 389 386 410 387 P5627F3 385 387 399 380 E1612C 386 390 394 380 Ma 374 E1612Q8 387 P5629O 385 Ma 390 390 395 392 388388 387 P6805H2 390 406-382 406-382 389 407 412 388 398 J5811D 393 424 406 404-414 395 398 388 398 386-409 396 399 396 406-403 418 394 402 409 393 396 413-408 407 397 397 382 406 404 P6804A 420-405 406-395 419 402 397 395 Ma 401 401 558-471 401 400 Ma 440 Ma 430 423- 412 421-413 454 402 398 400 Ma 443 451-402 444 452-402 406-395 401 394 452-402 427 430 K5622A5 H5622E 397 1 404 400 414-403 397 402 408-405 413-403 H5622B 931–450 H5621A 401 397 H5622C 394 397-375

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). 1172 E.O. Walsh et al. / Tectonophysics 608 (2013) 1159–1179

Table 2 EMP muscovite analyses.

SiO2 Al2O3 TiO2 FeO MgO K2O Total Si apfu Mg# apfu 8815G12r 47.46 33.86 0.20 4.79 1.96 11.72 99.99 3.08 0.42 8815G12c 47.47 31.86 0.81 5.66 2.05 12.20 100.05 3.11 0.39 8907B5r 48.12 31.96 0.82 5.33 2.37 11.45 100.05 3.13 0.43 8907B5c 47.26 32.41 0.82 5.50 2.26 11.67 99.92 3.08 0.43 929r of Terry 48.38 31.34 0.00 6.20 1.74 12.32 99.98 3.17 0.34 929c of Terry 49.34 32.15 0.00 5.18 1.87 11.40 99.94 3.19 0.39 E1612C7r 46.36 32.50 0.61 6.18 2.24 12.15 100.04 3.05 0.39 E1612C7c 48.33 31.76 0.20 5.69 2.37 11.67 100.02 3.15 0.43 E1612Q8r 49.73 32.30 0.83 3.72 2.09 11.45 100.12 3.20 0.50 E1612Q8c 48.08 34.34 0.62 4.27 1.25 11.43 99.99 3.10 0.35 E9814A6r 48.59 32.09 0.00 5.52 1.96 11.81 99.97 3.16 0.39 E9814A6c 48.57 32.08 0.00 5.70 1.96 11.69 100.00 3.16 0.38 H5621Ar 49.31 32.26 0.00 4.99 1.76 11.63 99.95 3.19 0.39 H5621Ac 49.32 31.38 0.20 5.53 2.07 11.48 99.98 3.20 0.40 H5621Br 48.02 37.67 0.63 1.51 0.53 11.60 99.96 3.05 0.37 H5621Bc 48.50 36.87 0.42 1.69 1.16 11.36 100.00 3.08 0.53 H5622Br 46.96 36.80 0.00 3.92 1.99 10.40 100.07 3.01 0.48 H5622Bc 47.18 36.71 0.00 3.92 1.78 10.41 100.00 3.02 0.45 H5622Cr 48.05 32.56 0.00 5.87 1.96 11.55 99.99 3.13 0.37 H5622Cc 47.59 34.87 0.00 4.80 1.24 11.51 100.01 3.08 0.31 H5622Er 48.32 29.43 0.60 6.52 2.64 12.59 100.10 3.19 0.42 H5622Ec 48.52 29.42 0.61 6.91 2.76 11.80 100.02 3.19 0.42 H5702Ar 48.36 32.25 0.82 4.62 2.38 11.51 99.94 3.13 0.48 H5702Ac 48.15 32.34 1.03 4.98 2.07 11.37 99.94 3.12 0.43 J5811Dr 46.87 31.24 0.40 7.10 2.86 11.45 99.92 3.08 0.42 J5811Dc 47.69 30.77 0.41 7.29 2.76 11.11 100.03 3.13 0.40 J5813Fr 49.00 34.51 1.25 2.44 1.79 10.96 99.95 3.12 0.58 J5813Fc 47.67 35.69 1.45 2.61 0.94 11.72 100.08 3.06 0.40 J5814Hr 48.16 30.74 0.81 6.03 2.67 11.63 100.04 3.15 0.44 J5814Hc 49.07 30.18 0.61 6.23 2.47 11.42 99.98 3.20 0.42 J5814N4r 48.33 29.49 1.22 6.02 2.86 12.07 99.99 3.17 0.46 J5814N4c 49.52 28.88 1.02 5.50 3.50 11.66 100.08 3.23 0.53 J5814Sr 50.30 32.30 0.83 2.62 2.63 11.30 99.98 3.21 0.64 J5814Sc 50.33 31.56 0.62 3.18 3.15 11.15 99.99 3.22 0.64 J5815B1r 48.26 31.30 0.82 6.06 2.58 10.96 99.98 3.14 0.43 J5815B1c 48.40 35.50 0.42 3.73 0.94 11.01 100.00 3.10 0.32 J5815E1r 47.66 35.16 0.62 3.90 1.25 11.44 100.03 3.07 0.36 J5815E1c 47.95 34.76 0.62 4.45 1.35 10.95 100.08 3.09 0.35 J5815Gr 47.78 35.26 0.62 3.91 1.15 11.22 99.94 3.07 0.36 J5815Gc 47.39 34.82 1.03 4.26 1.25 11.29 100.04 3.06 0.33 J5815Kr 48.32 35.43 1.04 2.62 1.05 11.52 99.98 3.09 0.43 J5815Kc 48.11 35.39 1.25 2.61 1.15 11.51 100.02 3.08 0.44 J5816Ar 48.05 32.56 0.00 5.87 1.96 11.55 99.99 3.13 0.37 J5816Ac 47.59 34.87 0.00 4.80 1.24 11.51 100.01 3.08 0.31 J5816Br 48.35 31.48 0.61 5.89 2.79 10.86 99.98 3.14 0.46 J5816Bc 47.77 30.95 0.61 6.20 2.76 11.72 100.01 3.13 0.45 J5816Ir 47.27 33.70 1.43 3.68 1.55 12.43 100.06 3.07 0.42 J5816Ic 47.87 33.78 1.44 3.33 1.56 12.02 100.00 3.09 0.45 J5816Jr 48.61 33.73 0.21 3.90 1.98 11.56 99.99 3.13 0.47 J5816Jc 49.16 32.87 0.62 3.72 2.30 11.34 100.01 3.16 0.52 J5816Kr 49.96 34.74 0.63 1.70 2.33 10.66 100.02 3.16 0.71 J5816Kc 50.73 33.12 0.84 1.89 2.76 10.65 99.99 3.21 0.73 J5816L1r 49.31 27.59 1.02 7.12 3.89 11.01 99.94 3.23 0.49 J5816L1c 49.28 27.44 1.22 6.57 4.30 11.24 100.05 3.23 0.54 K5622A5r 49.93 30.20 0.41 5.90 1.96 11.48 99.88 3.24 0.37 K5622A5c 50.80 28.25 0.61 6.24 2.57 11.55 100.02 3.31 0.42 P5627E2r 47.91 32.55 0.82 5.16 2.38 11.23 100.05 3.11 0.45 P5627E2c 47.49 31.19 0.61 6.04 3.08 11.29 99.70 3.12 0.47 P5627F3r 47.18 33.90 0.82 4.96 1.24 11.93 100.03 3.07 0.31 P5627F3c 46.93 34.46 1.02 4.78 1.03 11.82 100.04 3.05 0.28 P5627Kr 47.56 35.23 0.21 4.45 1.14 11.42 100.01 3.07 0.31 P5627Kc 47.16 34.13 0.61 5.16 1.24 11.60 99.90 3.06 0.30 P5629Dr 47.00 35.28 0.62 4.25 1.24 11.63 100.02 3.04 0.34 P5629Dc 47.06 35.33 0.82 4.07 1.04 11.65 99.97 3.04 0.31 P6804Ar 48.52 32.96 0.00 6.43 0.52 11.56 99.99 3.16 0.13 P6804Ac 47.01 34.19 0.00 6.94 0.00 11.85 99.99 3.08 0.00 P6805H2r 49.45 31.33 0.62 4.06 2.59 11.99 100.04 3.20 0.53 P6805H2c 49.34 31.36 0.62 4.44 2.59 11.64 99.99 3.19 0.51 P6806Ar 47.50 33.39 1.02 4.78 1.44 11.93 100.06 3.09 0.35 P6806Ac 47.40 33.55 1.02 4.78 1.45 11.82 100.02 3.08 0.35 P6806Jr 48.25 31.78 1.02 5.71 2.48 10.75 99.99 3.13 0.44 P6806Jc 47.85 31.62 0.82 6.05 2.67 11.06 100.07 3.12 0.44 P6816B1r 45.91 33.91 1.22 5.11 1.33 12.55 100.03 3.01 0.32 P6816B1c 46.42 34.20 0.41 5.30 1.13 12.57 100.03 3.04 0.27 P6818C2r 46.45 34.84 0.82 4.60 1.03 12.18 99.92 3.02 0.29 E.O. Walsh et al. / Tectonophysics 608 (2013) 1159–1179 1173

Table 2 (continued)

SiO2 Al2O3 TiO2 FeO MgO K2O Total Si apfu Mg# apfu P6818C2c 47.38 35.34 0.62 4.07 1.14 11.53 100.08 3.06 0.32 P6824B1r 49.49 29.38 0.82 6.05 2.78 11.53 100.05 3.23 0.45 P6824B1c 49.87 28.29 0.41 6.77 3.08 11.51 99.93 3.26 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 by Dunlap and Fossen are generally older than those we measured. (Table 1). This age difference (older ages to the east) matches the diachronous Unlike the broad-scale, relatively simple patterns in muscovite cooling recorded in muscovite age data and ties the early cooling history ages, the K-feldspar spectra from the WGR have considerably more of the K-feldspar to the progressive westward unrooﬁng of the WGR. heterogeneity. Within the WGR there is as much variation in the The cooling through ~200 °C that we infer occurred around calculated cooling histories for K-feldspar within the different areas 310–230 Ma is consistent with the ~320–260 Ma cooling inferred studied—e.g., northern WGR, Jotun nappe (including one sample by Dunlap and Fossen (1998) for samples along a transect in or near from the WGR), and Sørøyane UHP domain (including one sample the SW corner of Fig. 6; Dunlap and Fossen attributed this episode from north of the UHP domain)—as there is in the cooling histories of accelerated cooling to the onset of rifting in the Oslo Graben and for the region as a whole. Moreover, our sample 8817A and Dunlap the North Sea. and Fossen's (1998) sample N28, which are both from the WGR and Eide et al. (1999) inferred that two of their K-feldspar samples from are separated by only ~10 km, yield considerably different spectra. the Nordfjord–Sogn detachment zone (south of our UHP samples) These observations imply that the variations in the K-feldspar cooling cooled through 200 °C by ~350 Ma, substantially earlier than any of histories should not be interpreted too literally. Until a more-detailed our samples. They used samples from the Nordfjord–Sogn detachment investigation shows otherwise, we conservatively interpret the WGR zone to infer that late-stage E–W folding in the WGR continued until K-feldspar data as indicating cooling through ~400 °C in the broad ~340 Ma. Three new K-feldspar samples from Sørøyane may also re- interval of 390–330 Ma and through 200 °C around 310–230 Ma. cord the effects of this Carboniferous E–W folding: Nerlandsøya, located The cooling through ~400 °C that we infer occurred around at the center of the antiform, cooled the fastest (~6 °C/Myr until 390–330 Ma is substantially younger than the ~400–390 Ma cooling slowing at ~300 Ma); Sandsøya, located at the southern edge of the inferred by Dunlap and Fossen (1998). This difference in calculated antiform, cooled more slowly (~2 °C/Myr until ~320 Ma, followed by cooling histories reﬂects substantial differences in the step ages of slower cooling); and Gurskøya, even farther south, cooled the slowest the K-feldspars measured in the two studies: the spectra measured (1 °C/Myr until ~280 Ma).

5.2. Late-stage structures

A Muscovite age vs. Si apfu The faulting that we report has been studied to the north and south of 3.25 the WGR and is attributed to deformation during exhumation at upper 3.20 crustal levels (Andersen et al., 1999; Bering, 1992; Braathen, 1999; Fossen, 1998, 2000; Larsen et al., 2002, 2003; Redfield et al., 2005). As 3.15 noted above, the relationship of the faults to amphibolite-facies 3.10 symplectite and the mineralization along the faults suggest that slip mainly post-dated amphibolite-facies conditions; therefore the faults Si apfu 3.05 are chiefly younger than the amphibolite-facies ductile, E–Wstretching summarized by Hacker et al. (2010). They likely also postdate the 3.00 dominantly amphibolite-facies quartz fabrics interpreted by Barth et al. 2.95 (2010) to indicate a mix between plane strain and constriction. Overall, 350 360 370 380 390 400 the dominant E–W stretching inferred from the faults intimates a contin- Age (Ma) uation of this constrictional regime, first identified by Krabbendam and Dewey (1998). – B Muscovite age vs. Mg # The brittle ductile to brittle faults characterized by (biotite) ± 0.8 chlorite ± epidote ± quartz ± carbonate ± Mn–Fe oxides and the 0.7 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 0.6 area. Those authors interpreted a 396 Ma titanite date from their set 0.5 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

Mg # 0.4 are interpreted to date hydrothermal alteration and syn- or post-set I 0.3 deformation (Larsen et al., 2003). The set II faults, then, were likely 0.2 active from shortly after the Late Devonian set I faults until Permian dike intrusion at ~260 Ma (Larsen et al., 2003). 0.1 Fossen and Dallmeyer (1998) and Fossen and Dunlap (1998) used 350 360 370 380 390 400 40Ar/39Ar muscovite ages and an inferred closure temperature of Age (Ma) 350 °C to constrain the initiation of lower amphibolite-facies “ ” Fig. 5. A) Lack of apparent correlation between Si apfu and age in new muscovite data. WNW-directed extension (their Mode I ) in southwestern Norway

B) Weak positive correlation between Mg# [Mg/(Mg + Fetotal)] and age. to ~400 Ma. Semi-ductile to brittle normal-sense faults associated 1174 E.O. Walsh et al. / Tectonophysics 608 (2013) 1159–1179

500 UHP Stiklestad Lund 400 400 local muscovite ages northern WGR Stiklestad 350 Godøy 400 local hornblende & biotite ages 300 300 Stokken 350

apparent age (Ma) Lund 250 Bergsøya 200 0139 Runde 300 cumulative Ar released 200 apparent age (Ma) Tømmerås Window 160 250 0139 cumulative Ar released apparent age (Ma) 400 200 Stokken local muscovite ages 01cumulative 39Ar released 350 Nerlandsøy Bergsøya 300

250 400 Nerlandsøy 200 10°C/m.y. apparent age (Ma) Bergsøya 1°C/m.y. Trondheim Nappes 160 0139 Gurskøy 1602M cumulative Ar released 8818A Godøy Stiklestad 1100 Runde 300 8817A Sandsøya Nerlandsøy 900 Temperature (°C) Temperature Sandsøya Gurskøy 8818J H1602M 400 700 local muscovite ages modeled cooling 350 UHP 500 Sandsøya 200 Jotun Stokken apparent age (Ma) 300 northern WGR 300 Gurskøy Tømmerås Window 01cumulative 39Ar released 250 Trondheim Nappes H1602M 200

apparent age (Ma) Time (Ma) 100 160 200 300 400 500 01cumulative 39Ar released

Jotun Nappe 8817A 400 local muscovite ages 8818A 400 local muscovite ages 8818J Laerdal 8818J 350 350 Laerdal 300 8817A 300

250 250 8818A apparent age (Ma) apparent age (Ma) 200 39 200 01cumulative Ar released 01cumulative 39Ar released

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 muscovite was closing from 400 Ma to 370 Ma, with the brittle faults the Nordfjord–Sogn Detachment Zone (NSDZ) also happened at this forming from latest Devonian into the Permian. time (Andersen, 1998; Braathen, 1999; Eide et al., 2005). Chauvet and Séranne (1994) speciﬁcally noted that a transition from E–Wextension 5.3. Absence of excess argon and vertical thinning to E–W constriction occurred during the deposition of these basins. Fossen and Dallmeyer (1998) and Fossen and Most (U)HP terranes are plagued by excess 40Ar (El-Shazly et al., Dunlap (1998) further noted that later, steeper semi-brittle to brittle 2001; Kelley, 2002; Warren et al., 2012). It is particularly common in faults accommodating NW–SE extension (“Mode II”)predatePermian K-poor minerals in K-poor rocks encased in K-rich rocks (e.g., amphibole dikes and may be associated with K-feldspar cooling at 320–260 Ma. in eclogite in granitic gneiss), but also widespread in high-pressure white Thus, the brittle–ductile faults we observed most likely formed when mica (Kokchetav, Hacker et al., 2003;Dabie,Hacker et al., 2000). In E.O. Walsh et al. / Tectonophysics 608 (2013) 1159–1179 1175

Table 3 Fault-slip data.

Plot name Sites included n σ1 σ2 σ3 R F (°) nev Mineralization

E9803E E9801O, E9803E, c E1617A 214 218 73 357 13 090 11 0.53 17 0 Chlorite biotite E9803G E9803F, E9803G, E9803K, E1616D 50 197 82 024 08 294 01 0.17 18 1 Chlorite epidote biotite E9803P E9803P, E9803S 5 001 03 111 82 271 08 0.51 5 0 Chlorite biotite E9804A E9804A, E9804B, E9804C, E9804D 35 168 27 017 60 265 13 0.84 19 1 Chlorite epidote biotite E9804I E9804I, E9804J, E1615A, E1615B 28 048 72 208 12 300 6 0.69 21 0 Chlorite epidote biotite E9805C E9805C, E9805G, E1614C, E1614D 17 005 08 256 68 098 21 0.58 11 2 Quartz epidote E9805L E9805L, E9805M 6 312 29 175 53 055 21 0.52 5 0 Biotite chlorite quartz E9805P E9805P, E9805T, E9808E 40 053 60 233 30 323 00 0.52 24 4 Quartz chlorite epidote E9806A E9806A, E9806B, E9806E 16 345 14 145 75 253 05 0.84 14 0 Quartz oxides clay E9806F E9806F, E9806H 26 169 01 271 84 079 06 0.50 20 0 Clay epidote cataclasite E9806I E9806I 6 180 78 281 02 011 12 0.35 15 1 Chlorite quartz E9807A E9807A 13 055 61 180 18 277 23 0.50 9 0 Biotite chlorite clay quartz E9807B E9807B, E9813B 34 109 77 229 06 321 11 0.58 21 3 Biotite chlorite quartz carbonate E9808D E9808D 10 169 75 330 14 061 05 0.48 14 1 Chlorite quartz E9808F E9805P, E9808F, E9808G, E1613M 18 190 04 031 86 280 01 0.82 21 1 Quartz chlorite epidote E9808K E9808K, E9815B, E9815D, E1613I 10 347 23 160 67 256 02 0.63 17 1 Quartz epidote E9809G E9809G, E9809I 9 253 67 003 08 096 21 0.48 6 0 Quartz chlorite epidote E9809H E9809H 6 023 14 143 64 287 22 0.43 35 0 Chlorite quartz E9810A E9809K, E9809O, E9810A 17 182 18 344 71 090 05 0.72 14 0 Chlorite quartz E9816F-1 E9816F, E9818F 14 022 18 158 65 287 16 0.60 16 1 Epidote chlorite quartz E9816F-2 E9816F 6 194 66 041 22 307 10 0.51 17 0 Chlorite quartz oxides E9817A E9817A, E9817B, E9818F 6 135 73 339 15 248 07 0.59 27 1 Biotite chlorite epidote E9820F E9820F 3 264 64 016 11 111 23 ? 5 0 Biotite chlorite quartz E9820M E9820M 10 197 23 034 66 289 06 0.58 17 0 Biotite chlorite quartz E1606A E1606A, E1606B 6 205 06 333 80 114 08 0.60 20 0 Chlorite quartz E1606H E1606C, E1606E, E1606H, E1606J, E1606L 14 215 20 018 69 123 06 0.37 13 1 Chlorite quartz E1607A E1607A 10 217 51 011 36 110 13 0.48 17 0 Chlorite quartz E1612C E1612C, E1612D 14 009 12 196 78 099 02 0.75 16 0 Chlorite quartz E1612P E1612P 4 051 60 163 12 259 27 0.43 10 0 Biotite J5810A E9811F, J5810A-C 27 204 13 343 73 111 11 0.51 12 0 Biotite J5813F J5813F 12 047 00 139 85 317 05 0.54 12 2 Biotite chlorite quartz J5815C J5815C 4 222 05 358 83 131 05 0.49 3 0 Biotite feldspar quartz J5814B J5814B 6 170 53 335 36 071 07 0.52 14 0 Biotite chlorite quartz P6809A P6809A 9 052 48 189 33 295 23 0.57 10 0 Biotite chlorite quartz R9821B R9821B 5 305 47 164 36 058 21 0.50 4 0 Chlorite quartz R9821C R9821C 10 157 02 063 63 248 27 0.63 9 1 Chlorite quartz R9822C R9822C, R9822D 38 060 55 200 28 300 19 0.44 18 1 Biotite chlorite quartz epidote R9823G R9823G, R9823H 14 331 55 177 32 079 13 0.66 14 0 Biotite quartz R9826E R9826E 14 135 57 351 28 252 17 0.74 19 0 Quartz Y0816C Y0816C, Y0816D, Y0816E 21 034 34 173 48 289 22 0.80 10 0 Chlorite quartz Y0816M Y0816M, Y0816J 41 175 27 025 59 272 13 0.65 12 0 Chlorite quartz epidote carbonate Y0817A Y0817A 8 176 10 043 70 270 14 0.61 16 0 Chlorite Y1611A-biotite Y1611A, B, I, K, L 19 029 24 176 61 293 14 0.75 23 1 Biotite Y1611A-chlorite Y1611A, B, I, K, L 41 172 60 004 29 271 05 0.66 16 2 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 constrictional regime, and b) a NE–SW dome-like structure south of in ages across the study area, the inter-sample precision (see discussion Nordfjord that experienced differential exhumation, possibly due to in Walsh et al., 2007), and the ‘ﬁt’ of the muscovite ages within the con- footwall uplift along the Nordfjord–Sogn Detachment Zone plus rota- text of other geochronological studies, all demonstrate a general absence tion due to shearing on the Hardangerfjord Shear Zone. E–W extension of excess Ar within most of the white mica in the WGR. Why the WGR continued along brittle–ductile faults through the Late Devonian, after should be “exempt” from this widespread problem of excess Ar is which exhumation occurred along local structures (as recorded by unclear. One, perhaps remote, possibility is that none of the micas K-feldspar ages and brittle faults) as late as ~260 Ma. recrystallized at pressures higher than 1 GPa, Barrovian metamorphism (Peterman et al., 2009; Walsh and Hacker, 2004). Acknowledgments

6. Conclusions Funded by NSF EAR-9814889, EAR-0510453, EAR-0911485, and NFR Centre of Excellence grant to PGP. We thank Blanka Sperner for New 40Ar/39Ar thermochronology and fault-slip data have notable help with data handling and Haakon Fossen and Loic Labrousse for implications for the late-stage exhumation of the WGR (U)HP terrane detailed reviews that resulted in considerable improvement to the (Fig. 8). Muscovite ages from the northwestern WGR record the exhu- manuscript. mation of the (U)HP rocks into the mid-upper crust approximately 20 Myr after the UHP metamorphism ended. Strong E–W stretching Appendix A. Supplementary data along with westward propagation of the Nordfjord–Sogn Detachment Zone created: a) a block north of Nordfjord that experienced eastward Supplementary data to this article can be found online at http:// tilting and the development of local E–W trending folds in a dx.doi.org/10.1016/j.tecto.2013.06.030. 1176 E.O. Walsh et al. / Tectonophysics 608 (2013) 1159–1179

A E9806F E9803P fault striae certain E9805L reliable E9806A J5813F inferred E9808F unknown

σ1, σ2, σ3 foliation extension vein chiefly strike slip R9823G chiefly dip slip E9804A

E9805C E1612C

E9816F-1

Nordfjord-Sogn Y1611ALbio Detachment Zone J5810A Y0816M E1606H E9809H E9808K

Y0816C E9820M E9810A

B fault striae certain E9804I J5814B reliable R9821C inferred unknown

σ1, σ2, σ3 R9821B E9805P foliation extension vein E9807B chiefly strike slip E9803G chiefly dip slip E9803E

R9822C

E9817A

Nordfjord-Sogn Detachment Zone R9826E E1606A E9820F E1607A E9807A 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. E.O. Walsh et al. / Tectonophysics 608 (2013) 1159–1179 1177

A) ~425–400 Ma: (U)HP metamorphism in subducted Baltica basement and allochthons Laurentia Caledonian allochthons

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 ~400 Ma: parautochthonous basement cooled through muscovite closure deep rocks to west Nordfjord–Sogn across southern WGR detachment zone NSDZ 380 385 390 395 400 410 420 400400 405 Dalsfjord Suite Jotun Nappe

~374 Ma: N–S shortening further exhumed UHP rocks in west ~395 Ma: continued E–W extension caused differential exhumation in sedimentary basins NSDZ south-central WGR formed above NSDZ 380 385 390 395 400 410 420 NSDZ 400 395 395 400 405 Hardangerfjord Shear Zone

~300 Ma: E–W extension continued on NSDZ into Permian ~260 Ma: southeastern WGR cooled moderately; followed by Late Permian slip on Laerdal–Gjende fault NSDZ sedimentary basins 380 385 390 395 400 410 420 sedimentary basins NSDZ 400395 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.

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