Tectonophysics 608 (2013) 1159–1179 Contents lists available at ScienceDirect Tectonophysics journal homepage: www.elsevier.com/locate/tecto Crustal exhumation of the Western Gneiss Region UHP terrane, Norway: 40Ar/39Ar thermochronology and fault-slip analysis Emily O. Walsh a,b,⁎, Bradley R. Hacker a, Phillip B. Gans a, Martin S. Wong a,c, Torgeir B. Andersen d a Earth Science, University of California, Santa Barbara, CA 93106, USA 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 0040-1951/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tecto.2013.06.030 1160 E.O. Walsh et al. / Tectonophysics 608 (2013) 1159–1179 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.