Lower Crustal Folding and the Miracle of Exhumation- Insights and Oversights from Norwegian Eclogites

Ruth Foreman*, Torgeir B. Andersen# & John Wheeler*

*Department of Earth Sciences, University of Liverpool, 4 Brownlow Street, Liverpool, UK

# Institutt for Geologi, University of Oslo, P.O. Box 1047, Blindern, 0316 Oslo, Norway

r.foreman@liv.ac.uk

Exhumation of the deep crust during orogenic collapse is an accepted geological phenomenon, but structures formed during the earliest stages of exhumation are often overprinted by shallower deformation events. The Middle-Silurian to Early-Devonian collision of Baltica with Laurentia resulted in ultrahigh-pressure (UHP) and high-pressure (HP) metamorphism of the Western Gneiss Region of Norway (WGR). The WGR now comprises variably retrograded mafic eclogite bodies enclosed in more felsic amphibolite facies rocks. Gradual partitioning of strain into the more felsic rocks, and finally into the mylonites and schists of nearby extensional detachment zones has allowed some of the mafic bodies to be exhumed through the amphibolite and greenschist facies with minimal structural and mineralogical overprint. Large volumes of eclogite-bearing lower crustal rocks have thus been juxtaposed with allochthonous mid- to upper-crustal units, including several Devonian basins across the main extensional Nordfjord-Sogn Detachment Zone. The Drøsdal body, in Sunnfjord, southwestern Norway, is one of the largest and best-preserved mafic eclogites in the WGR.

At Drøsdal, some of the most spectacular eclogite facies rocks in the world are exposed over c. 3km2, displaying a wealth of structures which were formed during deformation at a minimum depth of 50km and at peak temperatures of c. 700°C. Although the Drøsdal body does not preserve UHP assemblages, it is much bigger than any of the UHP bodies, and is an excellent example of an eclogite tectonite. Large volumes of mylonites with the eclogite facies assemblage garnet + clinopyroxene ± quartz ± amphibole ± zoisite/clinozoisite ± phengite ± kyanite ± rutile are preserved. E-W trending isoclinal folds, boudinage, hinge-parallel lineations and meter scale kyanite veins are among the eclogite facies structures represented in this body. Through structural analysis of field data, along with petrological evidence, this study reveals that the Drøsdal body was pervasively folded on the kilometer scale during the early eclogite facies phase of exhumation and that map scale boudinage of the body followed, during exhumation through the eclogite to amphibolite transition.