EXTENSION-PARALLEL FOLDING IN THE SCANDINAVIAN CALEDONIDES - IMPLICATIONS FOR LATE-OROGENIC PROCESSES

Structural analyses of the Devonian basins and basement rocks in the Scandinavian Caledonides show the existence of large-scale E-W-oriented folds which affect the para-autochthonous Western Gneiss Region of the Baltic Craton. These folds mostly occur in areas of widespread and intense late-orogenic extension. Chronology, based on structural relationships both within the Devonian sediments and Caledonian rocks, shows that folding has been active from the first stages of the ductile extension and ended after Devonian basin sedimentation. Folding evolved through time towards crustal-scare transcurrent left-lateral faulting in the zones of maximum deformation. Such an evolution is explained by progressive unloading of the Western Gneiss Region due to extension, which involved a permutation of the maximum stress from a vertical position at the onset of extension to a horizontal position in the subsequent stages. Folding parallel to late-orogenic extension, as well as left-lateral translation on east- to northeast-trending crustal-scale faults suggest the occurrence of a regional direction of compression perpendicular to the direction of extension. This in turn argues for an external force triggering the late-orogenic extensional process in the Scandinavian Caledonides.