Polyphase brittle faulting in the Sunnfjord Region, Western Norway: kinematics and timing

Abstract

Structural studies in the Sunnfjord region of western Norway, focused on faulting episodes that post-date the major period of Devonian extension, yield data that are relevant to the style and evolution of brittle faulting. Major brittle structures in the region generally divide into (i) E-W faults and (ii) N-S lineaments with subordinate NE-SW and NW-SE structures. In close association with these large-scale features, four distinct groups\/ populations of mesofaults can be distinguished. Two reference areas, the eastern parts of the Kvamshesten and Hornelen Devonian basins, feature clear cross-cutting relationships of major faults and folds. Such cross-cutting relations and kinematic patterns of the distinct populations of meso-faults, and their relationship to dated fault rocks and dykes, form the basis for a four-stage history of post-Late Devonian brittle faulting. Stage 1 is characterised by N-S shortening by thrusting and folding, related to either Late Devonian - Carboniferous transpression or compression. Stage 2 is characterised by N-S shortening and E-W extension on conjugate strike-slip faults and bisecting normal faults. This fracture pattern could relate to the regional E-W folding (as ac- and hkO-fractures) during progressive shortening evolving from stage 1 to stage 2; Stage 3 is charac- terised by approximate E-W extension and is manifested as silicified fault breccias of possible Permian age that mainly bound the eastern margin of the Devonian basins. Stage 4 features N-S extension on E-W striking normal faults that possibly relates to a regional Late Jurassic-Early Cretaceous extensional phase. In addition, neotectonic reactivation may have occurred along some of the major structures. Significant movements on some of the faults emphasise the importance of Late Devonian and younger faulting and leave the impression that these deformation episodes had a re