Late-Caledonian structures, differential retrogression and structural position of (ultra)high-pressure rocks in the Nordfjord-Stadlandet area, Western Gneiss Region

Abstract

Late-Caledonian, amphibolite-facies structures in the Nordfjord-Stadlandet area comprise an 8 km-wide zone of non-coaxial shear below the Nordfjord-Sogn Detachment (NSD). Below this zone, coaxial structures dominate. The gneissic layering within the coaxial zone, in its present form, resulted mainly from pervasive late-Caledonian east-west extension. East-West trending fold hinges and prolate feldspar augen indicate substantial north-south shortening during east-west extension. This pervasive deformation played a significant role in the differential retrogression of eclogite-facies assemblages, so that most felsic rocks now have amphibolite-facies assemblages. Intergration of petrological data with the structural data shows that attenuation of the crust is unevenly distributed and that locally ultrahigh-pressure (UHP) eclogites occur structurally above HP (p-20-24kbar) eclogites. Some 15 km of crust has been exhumed from above the NSD. 57 km of crust is now cut out by the NSD, but only about half of that has been exhumed by non-coaxial strain. Below the NSD attenuation to 72-86% occurred. Locally, however, attenuation to 93-96% has occurred as suggested by the occurrence of HP and UHP eclogites less than 1 km apart. Even so, no distinct high-strain zones occur in the vicinity of UHP eclogite pods.