Crustal-scale extensional denudation and exhumation of crustal roots in mountain belts have a complex evolution, as can be documented for the Engebøfjellet Eclogite of the Western Gneiss Region, Norway. This eclogite is situated in a large antiform below the major extensional Nordfjord– Sogn Detachment, with an unroofing history attesting to six events; (Phases D1–2) complete HP eclogitisation of a gabbroic complex during vertical shortening and horizontal E–W stretching. (D3–4) Retrogression along amphibolite- and greenschist-facies structures signifying highly ductile top-W shear. (D5–6) Following folding, earlier structures were cut during top-W shear on the Nordfjord–Sogn Detachment, prior to renewed regional E–W folding. The P–T path points to rapid near-isothermal decompression from c. 17 to 7 kbar (D1 to D4 ), followed by a period of cooling from 525°C to ?300°C (brittle conditions; D5 and D6 ) during a pressure decrease of 3–4 kbar. During extension, exhumation of the crust within a growing major antiform parallel to tectonic transport was caused by either (i) repeated locking and unlocking of extensional detachment(s) that alternated with folding from transpression-transtension events on the major Møre–Trøndelag Fault Complex, or (ii) progressive folding interacting with extensional shear zones along crustal boundary layers.
Alvar Braathen, University Centre in Svalbard, Box 156, N–9171 Longyearbyen, Norway. Department of Geosciences, University of Oslo, P.O.Box 1047 Blindern N–0316, Oslo , Norway. Muriel Erambert, Department of Geosciences, University of Oslo, P.O.Box 1047 Blindern N–0316, Oslo , Norway.