Exhumation of high‐pressure rocks beneath the Solund Basin, Western Gneiss Region of Norway

Abstract

The Solund-Hyllestad-Lavik area affords an excellent opportunity to understand the ultrahigh-pressure Scandian orogeny because it contains a near-complete record of ophiolite emplacement, high-pressure metamorphism and large-scale extension. In this area, the Upper Allochthon was intruded by the c. 434 Ma Sogneskollen granodiorite and thrust eastward over the Middle ⁄ Lower Allochthon, probably in the Wenlockian. The Middle ⁄ Lower Allochthon was subducted to c. 50 km depth and the structurally lower Western Gneiss Complex was subducted to eclogite facies conditions at c. 80 km depth by c. 410-400 Ma. Within < 5-10 Myr, all these units were exhumed by the Nordfjord-Sogn detachment zone, producing shear strains > 100. Exhumation to upper crustal levels was complete by c. 403 Ma. The Solund fault produced the last few km of tectonic exhumation, bringing the nearultrahigh-pressure rocks to within c. 3 km vertical distance from the low-grade Solund Conglomerate.

Diffusion versus recrystallization processes in Rb–Sr geochronology: Isotopic relics in eclogite facies rocks, Western Gneiss Region, Norway

“…Our P,T estimates of 800-900°C for the granulite facies and of 19-25 kbar, 650-750°C for the eclogite facies metamorphism are well within the reported range of P,T estimates from similar rocks in the Dalsfjord region Hacker et al, 2003;Foreman et al, 2005), as summarized above. Specifically, all available evidence implies that during Caledonian subduction, conditions of >650°C and ~20 kbar have been reached at a regional scale in the WGR.…”

“…Available evidence suggests that this part of the WGR experienced near-UHP eclogite facies metamorphism. P,T work in the region resulted in estimates of 700°C at ~23 kbar for eclogites in the Solund-Hyllestad-Lavik area (Hacker et al, 2003), some 20 km south of Bårdsholmen (Fig.1). For the Drøsdal eclogite, ~ 8 km south of Bårdsholmen, Foreman et al (2005) arrived at T=720-830°C and P=19-21 kbar using the THERMOCALC multiequilibrium approach.…”

Applications of inclusion behaviour models to a major shear zone system: The Nordfjord-Sogn Detachment Zone in western Norway

“…2). The NSDZ constitutes a system of large-scale ductile extensional shear zones with cumulative top to the west displacements in the order of 50e100 km (Hacker et al, 2003;Johnston et al, in press;Wain et al, 2000). The NSDZ brings lower pressure Caledonian nappes and overlying sediments of Devonian supra-detachment basins in the hanging wall on top of late Caledonian high-(HP) and ultra-high-pressure (UHP) rocks of the Western Gneiss Complex (WGC) in the footwall (Osmundsen and Andersen, 2001).…”

Ultra High Pressure Metamorphism☆

“…of such mafic rocks did transform to eclogite, with the rest remaining unreacted (Straume and Austrheim, 1999;Krabbendam et al, 2000;Walsh and Hacker, 2004). A Barrovian to Buchan amphibolite-facies overprint-with local partial meltingoccurred at 650-800°C during post-UHP decompression from >1.5 GPa to~0.5 GPa, almost completely obliterating the record of (U)HP metamorphism (Krogh, 1980;Chauvet et al, 1992;Dransfield, 1994;Straume and Austrheim, 1999;Hacker et al, 2003b;Terry and Robinson, 2003;Labrousse et al, 2004;Walsh and Hacker, 2004;Root et al, 2005;Engvik et al, 2007). Importantly, this metamorphic history documents that the UHP rocks were exhumed nearly isothermally to depths of 15-20 km.…”

“…4). They consist mostly of gently plunging ENE-WSW to ESE-WNW lineations, isoclinal lineationparallel folds, and generally symmetrical fabrics implying coaxial strain histories with a constrictional component (Andersen et al, 1994;Dransfield, 1994;Krabbendam and Wain, 1997;Krabbendam and Dewey, 1998;Labrousse et al, 2002;Hacker et al, 2003a;Terry and Robinson, 2003;Engvik et al, 2007;Barth et al, 2010). Along the western edge of the WGR, these fabrics are overprinted by or merge into the Nordfjord-Sogn Detachment Zone (NSDZ), an amphibolite-to greenschist-facies, W-dipping, top-W shear zone that formed toward the end of the Scandian orogeny (Norton, 1987;Andersen and Jamtveit, 1990; see summary in Johnston et al, 2007a).…”