Structure and ^sup 40^Ar/^sup 39^Ar thermochronology of an ultrahigh-pressure transition in western Norway

• Published in: Journal of the Geological Society Vol. 168; p. 887
• Main Authors: Young, D J, Hacker, B R, Andersen, T B, Gans, P B
• Format: Journal Article
• Language: English
• Published: Oxford Geological Society Publishing House 01.07.2011
• Subjects: Basins; Geology; Plate tectonics; Rocks
• ISSN: 0016-7649; 2041-479X

The Nordfjord region of western Norway hosts an archetypal subducted crustal section, underpinned by ultrahigh-pressure (UHP) eclogite, overlain by Devonian sediments, and cored by a crustal-scale extensional shear zone. Structural mapping reveals two distinct displacement zones that played different roles during the formation and exhumation of this section: (1) the Sandane Shear Zone is a NW-dipping, amphibolite-facies, high-strain zone near the base of the eclogite-bearing crust that separates allochthonous units from underlying crystalline basement; it may have originated during early thrusting, but was overprinted by top-to-the-west extensional fabrics at lower crustal depths; (2) structurally above this, the Nordfjord-Sogn Detachment Zone is a top-to-the-west, amphibolite- to greenschist-facies detachment shear zone within allochthonous units that defines the upper boundary of the eclogitized crust and was responsible for exhumation through at least mid-crustal depths. Muscovite ^sup 40^Ar/^sup 39^Ar ages suggest that amphibolite-facies deformation below the Nordfjord-Sogn Detachment was mostly finished by c. 397 Ma, whereas muscovite ages from the deeper parts of the UHP domain indicate that it cooled after 390 Ma. During exhumation through the middle crust, west-directed stretching was accompanied by north-south folding. Late sinistral transpressional faulting in the middle to upper crust truncated the earlier folds and shear zones. [PUBLICATION ABSTRACT]