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1 Tectonophysics 608 (203) Contents lists available at ScienceDirect Tectonophysics journal homepage: Crustal exhumation of the Western Gneiss Region UHP terrane, Norway: 40 Ar/ 39 Ar 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 9306, USA b Geology, Cornell College, Mount Vernon, IA 5234, USA c Geology, Colgate University, Hamilton, NY 3346, USA d University of Oslo, Centre for Earth Evolution and Dynamics (CEED), P.O. Box 048, Blindern, 036 Oslo, Norway article info abstract Article history: Received 29 June 202 Received in revised form 5 June 203 Accepted 27 June 203 Available online 5 July 203 Keywords: Ultrahigh-pressure exhumation Western Gneiss Region 40 Ar/ 39 Ar muscovite thermochronology 40 Ar/ 39 Ar K-feldspar thermochronology Fault-slip analysis New 40 Ar/ 39 Ar 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 (~ 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. 40 Ar/ 39 Ar 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. 203 Elsevier B.V. All rights reserved.. 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, 200). 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., 995; 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 5234, USA. Tel.: ; fax: 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. ) is a window of Baltican Proterozoic gneiss with igneous and metamorphic ages of ~650 Ma, ~200 and ~950 Ma (Austrheim et al., 2003; Skår, 2000; Tucker et al., 99) exposed beneath a stack of allochthons initially emplaced onto the margin of Baltica between ~430 Ma and 45 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., 202). 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, 995) but disconnected /$ see front matter 203 Elsevier B.V. All rights reserved.

2 60 E.O. Walsh et al. / Tectonophysics 608 (203) Carboniferous-Devonian Basins Høybakken Detachment Caledonian Allochthons Autochthon (Baltican basement) 00 km Møre-Trøndelag FC Hitra-Snåsa Fault Trondheim 62 N UHP domains Stadlandet Sørøyane Nordøyane Røragen detachment Hornelen Basin Kvamshesten Basin Nordfjord Nordfjord-Sogn Detachment Jotun Nappe Laerdal-Gjende fault Olestøl fault 0 E Fig.. 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, 998; 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., 200; 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., 995; Krogh Ravna and Terry, 2004; Terry et al., 2000; Wain, 997) over a period of about 20 Myr from ~420 to ~ 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. ), 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., 200). Metamorphic grade increases northwestward across the WGR (Griffin et al., 985; Krogh, 977; Tucker et al., 99) as does the degree of Scandian deformation (Barth et al., 200; Hacker et al., 200; Krabbendam and Wain, 997; Milnes et al., 997; 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, 990; Austrheim, 99), 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., 99; Brueckner and van Roermund, 2004; Duretz et al., 202), or delamination due to an unspecified gravitational instability (e.g., Hacker, 2007; Hurich, 996; 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 ~0 mm/yr or faster (e.g., Carswell et al., 2003; Krabbendam and Dewey, 998; 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, 999) or, more commonly, amphibolite-facies metamorphism at ~ C down to pressures of 0.5 GPa (Labrousse et al., 2004; Root et al., 2005; Spencer et al., 203; 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., 994; Dewey and Strachan, 2003; Fossen, 2000; van Roermund and Drury, 998). 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, 986; Séranne and Séguret, 987), as well as sinistral (rotated normal-sense) shear along the Møre Trøndelag Fault Complex in the north (Braathen et al., 2000; Krabbendam and Dewey, 998; Séranne, 992). 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, 200; Krabbendam and Dewey, 998). Within the Sørøyane UHP domain, this second stage of exhumation began after isothermal (~750 C) decompression to granulite-facies conditions at 5 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.,

3 E.O. Walsh et al. / Tectonophysics 608 (203) ). 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 40 Ar/ 39 Ar 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., 998; Chauvet and Dallmeyer, 992; Fossen and Dunlap, 998; Hacker and Gans, 2005; Lux, 985; Root et al., 2005; Walsh et al., 2007; Warren et al., 202; Young et al., 2007). Most of the data are from the southern half of the WGR; the northern half of 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 ) collected mainly from typical quartzofeldspathic gneiss (Fig. 2A E) but also from mafic gneiss (Fig. 2F: 885G2, K5622A5), rocks in the Møre Trøndelag Fault Complex (Fig. 2G: J586A, J586B, J586I), discordant dikes (Fig. 2F: E984A6, H5622B, P6804A), and the allochthons (Fig. 2H, I: 8907B5, 929, J583F, J584H, J584N4, J584S, J585K). The main amphibolite-facies fabric across the WGR, Scandian coaxial E W stretching (Hacker et al., 200), is overprinted by an A E62C7 WMPA = 387. ± 0.4 Ma H562B WMPA = ± 0.5 Ma E62Q8 WMPA =390.4 ± 0.4 Ma H5622C 397 to 375 Ma Ar/ 40 Ar 36 Ar/ 40 Ar 36 Ar/ 40 Ar 36 Ar/ 40 Ar 40/36=307.0± Ar/ 40 Ar E62C7 Age =.7 ± 7.4 Ma MSWD = 2.99 (<2.26) E62Q8 Age =.7 ± 0.6 Ma MSWD = 2.0 (<2.00) 40/36=386.± Ar/ 40 Ar H562B Age = ±.0 Ma MSWD =.77 (<.85) 40/36=5.9± Ar/ 40 Ar H5622C Age = ± 3.9 Ma MSWD = 0.58 (<2.63) 40/36=5339.4± Ar/ 40 Ar 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. (998); the preferred age is bold and is listed in the Renne column in Table. WMPA = weighted mean plateau age; WMA = weighted mean age. Error shown is ±σ without uncertainty in J.

4 62 E.O. Walsh et al. / Tectonophysics 608 (203) B H5702A WMA = ± 0.4 Ma 0 Cumulative 39 Ar H5702A Age = ± 0.6 Ma MSWD = 0.06 (<2.4) 40/36=36.5± J58D WMPA = 388.2± 0.4 Ma 0 Cumulative 39 Ar J58D Age = ± 0.6 Ma MSWD =.0 (<2.5) 40/36=294.4± J585E WMPA = ± 0.4 Ma 0 Cumulative 39 Ar J585G WMPA = 380. ± 0.4 Ma 0 Cumulative 39 Ar Fig. 2 (continued). J585E Age = ± 0.6 Ma MSWD =.56 (<.85) 40/36=884.±34.9 J585G Age = ± 0.5 Ma MSWD =.6 (<.89) 40/36=363.7± extensive suite of brittle ductile and brittle faults with exposed slip surfaces ranging from 0'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.. 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 ± 0.28 Ma (Renne et al., 998). The uncertainty in the irradiation flux monitor, J, was set conservatively at ±0.2% 2σ. Previously published ages from the study area have been recalculated

5 E.O. Walsh et al. / Tectonophysics 608 (203) C J586J WMPA =.8 ± 0.4 Ma J586J Age =.6 ± 0.5 Ma MSWD =.63 (<.78) 40/36=320.5± J586L WMPA = ± 0.4 Ma P5627E2 WMA = ± 0.4 Ma P5627E2 Age =.7 ± 0.6 Ma MSWD = 2.07 (<2.00) 40/36=355.5±8.0 40/36=052.0±48.0 J586L Age = 397. ± 0.5 Ma MSWD =.24 (<2.5) P5627F3 WMPA = ± 0.4 Ma 40/36=26.±58.8 P5627F3 Age = ± 0.6 Ma MSWD = 3.89 (<.89) Fig. 2 (continued). according to Renne et al. (998), to be consistent with our new data. Ages are reported with σ uncertainty. Uncertainties quoted in the text include uncertainty in J only to facilitate comparison of 40 Ar/ 39 Ar ages of different samples. Full uncertainties including also uncertainties in 40 K decay constant and monitor age are included in Table ; theseare the uncertainties to be used when comparing the 40 Ar/ 39 Ar ages to those determined by other methods (e.g., U Pb). For muscovite, the gas was incrementally released in 4 20 steps per sample, with 5-minute 00 C line blanks run periodically between samples (see Appendix A in the Supplementary Material). For K-feldspar, step experiments were run with isolation times of 5 min to 0 h (see Calvert et al., 999). Many of the K-feldspar samples yielded spectra suitable for full diffusion-domain analysis (Lovera et al., 989, 2002), which we completed using modified 997 versions of Lovera's (992) 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 D o (Lovera et al., 989); 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 40 Ar (Harrison et al., 994) were assigned the age of the youngest step in the group, and ii) steps with low radiogenic yields (b95%) and anomalously old ages were adjusted to provide a smoothly increasing trend. Cooling histories were calculated from initial times Myr older than the oldest step. Only those cooling histories that provide a good fit to the data are shown.

6 64 E.O. Walsh et al. / Tectonophysics 608 (203) D P5627K WMPA = ± 0.4 Ma P5629O WMPA = ± 0.4 Ma P6805H2 WMPA = ± 0.4 Ma P6806J WMA =.2 ± 0.4 Ma P5627K Age = ± 0.8 Ma MSWD = 0.47 (<.94) 40/36=486.8±2.4 P5629O Age = ± 0.6 Ma MSWD =.98 (<2.00) 40/36=239.±30.4 P6805H2 Age = ± 0.4 Ma MSWD =.2 (<.85) 40/36=37.4±7.4 P6806J Age = ± 0.5 Ma MSWD =.07 (<2.5) 40/36=458.± Fig. 2 (continued) 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 5 kv, a current of 5 na, a spot size of 2 μm, and natural and synthetic mineral standards. Core and rim analyses reported are averages of 2 spot analyses 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, 987), and a measure of our confidence in the latter based on the type of indicator and degree of preservation (using a scale of to 4, where is certain, 2 is reliable, 3 is inferred, and 4 is unknown, Angelier, 984). 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 (999) and Braathen and Bergh (995), 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

7 E.O. Walsh et al. / Tectonophysics 608 (203) E P686B WMPA = ± 0.4 Ma P686B Age = ± 0.5 Ma MSWD =.50 (<.82) 40/36=349.6± P688C2 WMPA = ± 0.4 Ma P688C2 Age =.7 ± 0.6 Ma MSWD =.9 (<2.00) 40/36=349.8± H562A H5622E WMA = ± 0.4 Ma to 450 Ma J586K WMA = ± 0.5 Ma 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. (993) using the NDA program (Spang, 972) written by Sperner and Ratschbacher (994) and an angle of 45 between σ and each fault planeforthebrittle 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 (202) in interpreting the calculated principal directions as indicators of paleostress. 4. Results 4.. Muscovite thermochronology Most of the analyzed muscovite samples yielded slightly U-shaped age spectra; nine samples produced spectra for which weighted mean

8 66 E.O. Walsh et al. / Tectonophysics 608 (203) F G2 WMPA =. ± 0.4 Ma E984A6 WMPA =.3 ± 0.4 Ma G2 Age = 38.7 ± 0.5 Ma MSWD = 0.66 (<2.07) 40/36=359.6±4.3 E984A6 Age = ±.3 Ma MSWD = 2.73 (<2.26) 40/36=554.3± H5622B WMPA = ± 0.4 Ma K5622A5 H5622B Age = ± 0.6 Ma MSWD =.33 (<.82) 40/36=404.8± P6804A to 403 Ma 558 to 47 Ma Fig. 2 (continued). plateau ages (WMPA) (N50% of 39 Ar 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 40 Ar/ 36 Ar ratio from atmospheric and, in some samples, use a greater percentage of the data than the WMPA. The new muscovite 40 Ar/ 39 Ar ages from basement gneisses increase eastward across the WGR toward the foreland (Fig. 4). Ages at the eastern limit of the WGR are generally ~ Ma (e.g., ± 0.5 Ma at J584H, ±.0 Ma at J586K); they increase gradually into the foreland allochthon stack to the east and increase more dramatically into the Jotun Nappe ( Ma at P6804A, Ma at H562A) to the southeast. The samples from the Møre Trøndelag Fault Complex range from ~387 Ma, younger than previously dated muscovite from the Central Norway basement window and the Høybakken detachment zones (Dallmeyer et al., 992; Eide et al., 2005) and similar to the hornblende and biotite 40 Ar/ 39 Ar ages from detachment mylonites (Kendrick et al., 2004). The youngest ages (e.g.,.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 3.0 to 3.3 atoms per formula unit (apfu) (Table 2). There is up to 0.0 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. (202) 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

9 E.O. Walsh et al. / Tectonophysics 608 (203) G J586B WMPA =.0 ± 0.4 Ma J586A WMA =.3 ± 0.4 Ma J586I WMPA = ± 0.4 Ma J586A Age = ±.2 Ma MSWD = 0.42 (<.89) 40/36=827.9± J586B Age =.7 ± 0.9 Ma MSWD = 2.9 (<2.5) 40/36=353.0± J586I Age = ±. Ma MSWD =.68 (<.94) 40/36=674.9± Fig. 2 (continued). (Fig. 5A), in accord with the findings of Warren et al. (202). 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. (992) in high-pressure rocks from Dora Maira and was attributed partly to increased Ar retentivity in Mg-rich muscovite K-feldspar thermochronology and thermal-history modeling We measured 40 Ar/ 39 Ar spectra and modeled the cooling histories of fifteen K-feldspar single crystals from an area spanning ~350 km north to south and ~ 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 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 38 Ar or 36 Ar, 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 C/Myr). Muscovite ages of Ma from nearby samples (Root et al., 2005) suggest that the modeled cooling history is reasonable. Sample R9828C2, 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 Ma (Root et al., 2005)) suggests slow cooling (~2 C/Myr) until ~320 Ma, at which time the cooling rate decreased even further to b C/Myr. The shape of the spectrum, however, suggests a more-complicated cooling history from 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 (885G5) is from orthogneiss at the southern edge of the Sørøyane UHP domain. A hump in the age spectrum followed by a drop in r/r o, 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 Ma (Root et al., 2005)) suggests slow cooling (~ 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 40 Ar; 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

10 68 E.O. Walsh et al. / Tectonophysics 608 (203) H B5 WMPA =.9 ± 0.4 Ma Terry #929 WMA =.2 ± 0.4 Ma 8907B5 Age =.4 ± 0.5 Ma MSWD =. (<2.00) 40/36=.0± Terry #929 Age = ± 0.9 Ma MSWD =.86 (<3.83) 40/36=605.± J583F WMPA =.5 ± 0.4 Ma J583F Age =.3 ± 0.5 Ma MSWD =.88 (<.89) 40/36=360.4± J584H WMPA = ± 0.4 Ma J584H Age = ± 0.6 Ma MSWD =.92 (<2.00) 40/36=33.6± Fig. 2 (continued) Ma. Sample 8829A 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 35 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 (998). Sample 887A, 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 Ma (Fossen and Dallmeyer, 998), suggests a moderate cooling rate from ~ Ma(4 C/Myr)and very slow cooling (~0.6 C/Myr) thereafter. Sample 888A, 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, 888C, 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 888J, from a K-feldspar quartz vein in the Valdres sedimentary cover of the Jotun nappe, yielded a spectrum with excess 40 Ar in early steps that was removed with temperature cycling and excess 40 Ar 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.

11 E.O. Walsh et al. / Tectonophysics 608 (203) I J584N4 WMPA = ± 0.4 Ma J584N4 Age = 388. ± 0.5 Ma MSWD =.60 (<2.4) 40/36=33.5± J584S WMPA =.0 ± 0.4 Ma J584S Age = ± 0.5 Ma MSWD =.62 (<.94) 40/36=348.5± J585K WMPA = ± 0.4 Ma J585K Age = ± 0.6 Ma MSWD = 2.00 (<2.00) 40/36=403.5± Fig. 2 (continued). Two samples were analyzed from the northern WGR. Sample 884A collected near Stokken from a granitic pegmatite in an eclogite boudin strain shadow gave a relatively simple spectrum with some resolvable excess 40 Ar in early steps. Modeling implies slow cooling from to 30 Ma (3 C/Myr) and then very slow cooling (0.3 C/Myr) until 260 Ma. The spectrum from Bergsøya sample 884B, 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, 883A, from the Leksdalsvatn/ Offerdal nappe, yielded an age spectrum with an unusually complete 200 Myr age range. Excess 40 Ar 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 N50 Ma through 300 Ma, at which time slow cooling (N C/Myr) ensued. A Lund orthogneiss, 883D, from the core of the Tømmerås Window gave an uninterpretable spectrum contaminated by excess Ar. One sample was measured from the easternmost Trondheim nappes. H602M is a megacrystic K-feldspar augen gneiss of the Risberget nappe; modeling indicates extremely slow cooling (~0. C/Myr) until ~340 Ma, at which point the cooling rate increased to ~7 C/Myr 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, E980A, and Y6AL. 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,

12 70 E.O. Walsh et al. / Tectonophysics 608 (203) Table 40 Ar/ 39 Ar muscovite age data. Sample Geologic context UTM WMPA a 39% Isochron a 39% MSWD 40/36i TFA Renne b ±2σ 885G2 Layered mafic block ± ± ± B5 Bio-kfs gneiss ± ± ± #929 of Terry Strain shadows in ky-gar mylonite Not listed.2 ± ± ± E62C7 Tonalitic-granodioritic gneiss ± ± ± E62Q8 2-Mica granodioritic gneiss ± ± ± E984A6 qtz-pl-kfs sweats in granulite ± ± ± H562A Basement gneiss ± na na na na to 450 na H562B Basement gneiss ± ± ± H5622B Basement gneiss leucosome ± ± ± H5622C Dioritic gneiss ± ± ± to 375 na H5622E Muscovite gneiss ± na na na na H5702A Mus-qtz segregation in ± ± ± quartzofeldspathic gneiss J58D Tonalitic-dioritic gneiss ± ± ± J583F Blåhø quartzite ± ± ± J584H Allochthon gneiss ± ± ± J584N4 Blåhø schist ± ± ± J584S Concordant pegmatite in Blåhø ± ± ± J585B kfs-gneiss ± ± ± J585E Granodioritic gneiss ± ± ± J585G kfs augen gneiss ± ± ± J585K Blåhø gneiss ± ± ± J586A MTFZ mylonite zone ± ± ± J586B MTFZ mylonitic tonalite ± ± ± J586I MTFZ mylonitic tonalite ± ± ± J586J Augen gneiss ± ± ± J586K Muscovite schist with quartzite ± na na na na J586L Muscovite schist ± ± ± K5622A5 Eclogite na na na na na na 404. na na P5627E2 Granodioritic gneiss ± ± ± P5627F3 Bio-dioritic gneiss ± ± ± P5627K Granitic gneiss ± ± ± P5629O kfs-augen gneiss ± ± ± P6804A Pegmatite cutting kfs-augen gneiss na na na na na na to 47 na P6805H2 Bio-plg symplectitic gneiss ± ± ± P6806J kfs-augen gneiss ± ± ± P686B Bio-kfs gneiss ± ± ± P688C2 Sanidine gneiss ± ± ± WMPA, weighted mean plateau age (N50% cumulative 39 Ar); WMA, weighted mean age, for cumulative 39 Ar b50%. MSWD, mean standard weighted deviation for isochron; TFA, total fusion age. Preferred age in boldface. a Error reported as ±sigma with error in J. b Renne: preferred age recalculated following Renne et al. (998). 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. 5. Discussion 5.. Geochronology Our new muscovite ages are combined with previously published work (see references in Hacker, 2007; Warren et al., 202) infig. 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.

13 E.O. Walsh et al. / Tectonophysics 608 (203) closed to Ar loss over ~20 Myr, from about 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/ 39 Ar ages that increase eastward toward the foreland from ± 0.6 Ma (J585E) to ± 0.5 Ma (J584H); 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 ~ Ma during long-term N S shortening (Braathen, 999; Osmundsen and Andersen, 200; Torsvik et al., 988). South of Nordfjord, the pattern of muscovite 40 Ar/ 39 Ar 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., 998; Eide et al., 997). 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 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 C (Spencer et al., 203), substantially hotter than the 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., 200); 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 ). 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., 202); ii) other grains have uniform laser spot ages from rim to rim (Warren et al., 202); 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 40 Ar/ 39 Ar ages locally overlap with the youngest titanite U Pb ages 40 Ma J586B Ma 40 J586I J586K J585K J586A 402 J586J 385 Ma km 390 Ma 404 J585G 380 J584N4 J585E 376 J586L J585B P686B J584S Ma Terry B 384 P688C2 388 P5627E2 J583F J584H H5702A P5627K 399 P6806A G 385 E984A P5627F Ma E62C E62Q8 385 Ma 387 P5629O P6805H J58D N Ma Carboniferous-Devonian Basins Caledonian allochthons autochthon (Baltica basement) Ma Ma K5622A5 H5622E H5622B 397 H5622C P6804A Ma H562A Ma Ma Ma H562B 0 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. (998), Chauvet et al. (992), Fossen and Dunlap (998), Hacker and Gans (2005), Lux (985), Root et al. (2005), Walsh et al. (2007), Warren et al. (202) and Young et al. (2007, 20).

14 72 E.O. Walsh et al. / Tectonophysics 608 (203) Table 2 EMP muscovite analyses. SiO 2 Al 2 O 3 TiO 2 FeO MgO K 2 O Total Si apfu Mg# apfu 885G2r G2c B5r B5c r of Terry c of Terry E62C7r E62C7c E62Q8r E62Q8c E984A6r E984A6c H562Ar H562Ac H562Br H562Bc H5622Br H5622Bc H5622Cr H5622Cc H5622Er H5622Ec H5702Ar H5702Ac J58Dr J58Dc J583Fr J583Fc J584Hr J584Hc J584N4r J584N4c J584Sr J584Sc J585Br J585Bc J585Er J585Ec J585Gr J585Gc J585Kr J585Kc J586Ar J586Ac J586Br J586Bc J586Ir J586Ic J586Jr J586Jc J586Kr J586Kc J586Lr J586Lc K5622A5r K5622A5c P5627E2r P5627E2c P5627F3r P5627F3c P5627Kr P5627Kc P5629Dr P5629Dc P6804Ar P6804Ac P6805H2r P6805H2c P6806Ar P6806Ac P6806Jr P6806Jc P686Br P686Bc P688C2r

15 E.O. Walsh et al. / Tectonophysics 608 (203) Table 2 (continued) SiO 2 Al 2 O 3 TiO 2 FeO MgO K 2 O Total Si apfu Mg# apfu P688C2c P6824Br P6824Bc Oxide weight percent back-calculated from the formula. Each analysis is the average of two spots: r, rim; c, core. (Spencer et al., 203); and v) the youngest age is from a mylonitic rock (Table ). 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 887A and Dunlap and Fossen's (998) sample N28, which are both from the WGR and are separated by only ~0 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 ~ C in the broad interval of Ma and through 200 C around Ma. The cooling through ~ C that we infer occurred around Ma is substantially younger than the ~ 390 Ma cooling inferred by Dunlap and Fossen (998). 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 Ma is consistent with the ~ Ma cooling inferred by Dunlap and Fossen (998) 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. (999) 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 ( C/Myr until ~280 Ma). A Si apfu B Mg # Muscovite age vs. Si apfu Age (Ma) 0.8 Muscovite age vs. Mg # 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 + Fe total )] and age Late-stage structures 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., 999; Bering, 992; Braathen, 999; Fossen, 998, 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. (200). They likely also postdate the dominantly amphibolite-facies quartz fabrics interpreted by Barth et al. (200) 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 (998). 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; ~ 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 (998) and Fossen and Dunlap (998) used 40 Ar/ 39 Ar 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 ~ Ma. Semi-ductile to brittle normal-sense faults associated