Caledonian compressional and lateorogenic extensional deformation in the Staveneset area

Abstract--The studied area in Western Norway constitutes part of the hangingwall of the extensional Kvamshesten Detachment Zone (KDZ). The KDZ separates eclogite-bearing lower crust from a hangingwall consisting of a Precambrian basement-cover pair, Silurian continental margin sediments, a Silurian obduction melange, a Late-Ordovician ophiolite and the D vonian Kvamshesten Basin, deposited unconformably onthe Pre-Devonian rocks. Contractional deformation related to ophiolite obduction and to the Caledonian Scandian Orogeny produced a suite of SE-verging structures d veloped under greenschist facies conditions. As orogenic collapse commenced, extensional shear zones were formed in the upper and middle crust, re-activating contractional shear zones and fabrics. The direction of transport on the inherited fabrics was reversed from top-to-southeast to top-to-northwest, and structures related to the regional extension were superimposed on the contractional structures. Back-folding of the main contractional foliation by asymmetric W-vergent folds, together with NW and W-directed shearing along weak lithologies and semi-ductile faulting in the m61ange, are the main structural expressions of early stages of the orogenic ollapse in the Staveneset area. There is apparently no metamorphic break between the contractional f brics and the arliest extensional structures. The extensional structures developed, however, under progressively more brittle conditions. The Devonian sediments were deposited upon a sequence of rocks in the upper plate of the Kvamshesten Detachment that had alread