Exhumation of ultrahigh-pressure rocks beneath the Hornelen segment of the Nordfjord-Sogn Detachment Zone, western Norway Scott Johnston Bradley R. Hacker Department of Earth Science, University of California, Santa Barbara, CA 93106-9630, USA Mihai N. Ducea Department of Geosciences, University of Arizona, Gould-Simpson Building #77, 1040 E 4th St., Tucson, AZ 85721, USA ABSTRACT INTRODUCTION ultrahigh-pressure) provinces, a lack of post- orogenic deformation, and the preservation of The Nordfjord-Sogn Detachment Zone of Ultrahigh-pressure (UHP) terranes—which original tectonostratigraphic contacts between western Norway represents an archetype for range from km-scale nappes to tens-of-thou- the (U)HP provinces with structurally higher crustal-scale normal faults that are typically sands-of-square-kilometer provinces—experi- tectonostratigraphic units, provide a unique cited as one of the primary mechanisms ence rapid and near-isothermal decompression opportunity to reconstruct the history of an UHP responsible for the exhumation of ultrahigh- from metamorphic conditions within the coesite orogen and characterize UHP exhumation. The pressure (UHP) terranes. In this paper, we stability fi eld (>~27 kbar) to the upper crust at Norwegian (U)HP provinces are thought to have investigate the role of normal-sense shear plate-tectonic rates exceeding 10 mm/yr (e.g., been primarily exhumed by the Nordfjord-Sogn zones with respect to UHP exhumation using Baldwin et al., 2004; Glodny et al., 2005; Root Detachment Zone, a major top-W shear zone structural geology, thermobarometry, and et al., 2005; Parrish et al., 2006). To explain that extends >100 km along orogenic strike geochronology of the Hornelen segment of these impressive exhumation rates, a variety (Milnes et al., 1997; Andersen, 1998; Labrousse the Nordfjord-Sogn Detachment Zone. The of kinematic models have been employed that et al., 2004). This quantitative study focuses on Hornelen segment of the zone is a 2–6 km incorporate one or a combination of exhuma- the Hornelen segment of the Nordfjord-Sogn thick, top-W shear zone, primarily devel- tion mechanisms that include: wedge extrusion Detachment Zone to address a specifi c set of oped within amphibolite-grade allochtho- (e.g., Chemenda et al., 2000), channel fl ow (e.g., questions designed to characterize deformation nous rocks, that juxtaposes the UHP rocks of Beaumont et al., 2001), subhorizontal coaxial related to normal-sense displacement above the Western Gneiss Complex in its footwall thinning followed by non-coaxial removal of UHP terranes: (1) using structural geology and with lower-grade allochthons and Carbonif- the upper crust (e.g., Dewey et al., 1993), and electron back-scatter diffraction on quartz- erous-Devonian Basins in its hanging wall. normal-sense reactivation of the suture zone ites, we determine how strain was partitioned New thermobarometry and Sm/Nd garnet (e.g., Hacker et al., 2003). While all of these within the shear zone and across tectonostrati- geochronology show that these top-W fab- models cite normal-sense shear zones along the graphic contacts; (2) using thermobarometry, rics were initiated at lower crustal depths upper contact of the exhuming UHP terrane, we quantify the depth from which different of 30–40 km between 410 Ma and 400 Ma. the amount of offset and the tectonic setting in tectonostratigraphic units were exhumed and Structural geology and quartz petrofabrics which the normal-sense displacement occurred the depth at which normal-sense shear initiated; indicate that top-W shear was initially rela- vary drastically in the different models. These and (3) using Sm/Nd garnet geochronology, we tively evenly distributed across the shear differences in the style of normal-sense shearing constrain the timing of normal-sense displace- zone, and then overprinted by discrete have important geologic implications beyond ment with respect to (U)HP metamorphism. ductile-brittle detachment faults at slower the exhumation of UHP rocks, representing an Ultimately, our results are used to investigate strain rates during progressive deforma- essential step toward a better understanding of models for (U)HP exhumation and quantify the tion and exhumation. These results require fi rst-order plate-tectonic processes as far reach- component of UHP exhumation accomplished a three-stage model for UHP exhumation in ing as the kinematic evolution of continental through normal-sense shear. which normal-sense shear zones exhumed collision and orogeny, the formation and com- UHP rocks from the base of the crust along position of the lower continental crust, melt gen- GEOLOGIC SETTING initially broad ductile shear zones that were eration, the geometry and depositional patterns progressively overprinted by discrete duc- of syn-orogenic basins, and the forces driving The Scandinavian Caledonides formed tile-brittle structures. plate motion. through a series of orogenic events associated We present a case study from western Nor- with the closure of the Iapetus Ocean during the Keywords: ultrahigh-pressure rocks, exhuma- way that places important constraints on the Ordovician-Devonian, and culminated with the tion, Nordfjord-Sogn Detachment Zone, low- style of normal-sense shearing associated with emplacement of the Caledonian nappe stack and angle detachment, western Norway, Hornelen UHP exhumation. The size and excellent expo- the formation of the Norwegian UHP provinces Region. sures of the Norwegian (U)HP (high-pressure/ as Baltica and Laurentia collided (Roberts and GSA Bulletin; September/October 2007; v. 119; no. 9/10; p. 1232–1248; doi: 10.1130/B26172.1; 8 fi gures; 4 tables; Data Repository item 2007209. 1232 For permission to copy, contact [email protected] © 2007 Geological Society of America Hornelen segment of the Nordfjord-Sogn Detachment Zone Sturt, 1980; Cuthbert et al., 1983; Gee et al., metamorphic conditions in these basement Following continental subduction and UHP 1985; Hacker and Gans, 2005). The Caledonian gneisses range from upper-amphibolite facies in metamorphism, the Caledonides were reshaped nappe stack, best preserved in the foreland of the east near the foreland (Walsh and Hacker, by a major extensional event that rapidly eastern Norway and Sweden, consists of a series 2004), through UHP coesite-eclogite facies in exhumed rocks from lower crustal and mantle of tectonostratigraphic units (Fig. 1): fragments the west (Smith, 1984; Wain, 1997). The fel- depths into the upper crust. In the foreland, of Laurentia in the Uppermost Allochthon; ophi- sic gneisses of the Western Gneiss Complex this extension was accommodated through top- olitic mélanges, ocean-margin sediments, and include outcrop- to km-scale eclogite boudins W reactivation of older top-SE contractional outboard Baltica terranes in the Upper Alloch- that record northwestward increasing P-T con- detachments, and the nappe stack was exhumed thon; and imbricated basement-cover sequences ditions (Krogh, 1977; Carswell and Cuthbert, through muscovite closure to Ar by ca. 400 Ma representing distal regions of the Baltica margin 2003), suggesting subduction of Baltica beneath (Fossen and Dunlap, 1998). In the hinterland, in the Middle and Lower Allochthons (Gee et Laurentia up to UHP depths by 415–400 Ma muscovite cooling ages become progressively al., 1985; Roberts and Gee, 1985). These nappes (Krogh and Carswell, 1995; Carswell and Cuth- younger westward and down section from were thrust southeastward >200 km over the bert, 2003; Root et al., 2004; Kylander-Clark 400 Ma at higher structural levels in the east Proterozoic granodioritic-granitic gneisses et al., 2007). The Western Gneiss Complex is to 380 Ma in the westernmost UHP provinces of the Western Gneiss Complex, correlative overlain by complexly infolded orthogneisses (Root et al., 2005; Walsh et al., 2007). Most to the (par)-autochthonous Baltica basement, and paragneisses correlated with the structurally of this exhumation is thought to have occurred in a series of events that initiated as early as higher allochthons (Robinson, 1995). Eclogite through top-W, normal-sense displacement the Wenlockian (ca. 425 Ma, Andersen et al., boudins within these allochthons suggest that along a series of detachments that crop out 1990), and continued through 415–408 Ma in the allochthons were also involved in the Late- along the west coast of Norway, combined with the Upper and Middle Allochthons (Fossen and Caledonian UHP event (Terry et al., 2000; Root non-coaxial normal-sense shear and vertical Dunlap, 1998; Hacker and Gans, 2005). Peak et al., 2005; Young, 2005). thinning in the detachment footwalls ( Andersen extensional structures 6°E 8°E ductile–brittle detachments UHP provinces top-W mylonites Western tectonostratigraphy Devonian–Carboniferous basins Hornelen Upper Allochthon Region Middle Allochthon study area e 62°N Lower Allochthon n o Z Western Gneiss Complex t Hornelen n e & m h c Håsteinen Basins ta Gneiss e D n Kvamshesten Basin og –S rd fjo rd Caledonian N o Solund Basin N Complex 61°N Nappes 50 km Bergen 6°E 8°E Figure 1. Regional map of the Norwegian Caledonides showing the relative location of the Caledonian nappe stack, the Western Gneiss Complex, the Devonian basins, and the Nordfjord-Sogn Detachment Zone. Geological Society of America Bulletin, September/October 2007 1233 Johnston et al and Jamtveit, 1990; Milnes et al., 1997; Ander- and Cuthbert, 1994; Krabbendam and Dewey, cross-cutting quartz dioritic-granitic dikes and sen, 1998; Fossen and Dunlap, 1998). Of 1998). This previous body of work lays the con- lenses, and local amphibolite bodies up to 200 m these extensional detachments, the Nordfjord- ceptual foundation for the present quantitative in length. These cross-cutting dikes are variably Sogn Detachment Zone is the largest and best study of the strain portioning, metamorphic con- transposed into the foliation, many forming exposed, stretching ~100 km from Sognefjord ditions, and timing within the Hornelen segment asymmetric boudins; top-W shear-sense indi- to Nordfjord (Fig. 1, Norton, 1987). of the Nordfjord-Sogn Detachment Zone. cators are pervasively developed throughout The Nordfjord-Sogn Detachment Zone is a the Svartekari Group.