Polyphase semi-brittle to brittle deformation of the basement rocks of the Øygarden Complex commenced shortly after Caledonian ESE-directed thrusting and followed ductile W-directed shearing. Two main post-Caledonian fracture sets (I and II) are characterized on basis of their geometric relationships, mineral content and deformation styles. An early set of semi-brittle, low-angle faults with E-W to NE-SW trends (Set Ia) is tentatively ascribed to local strain caused by the formation of major E-W trending folds. A more pronounced and regionally more consistent set of NE-SW trending early fractures (Set Ib) formed in response to NW-SE extension and vertical shortening. Sphene in these fractures is dated at around 396 Ma by the U/Pb method. In agreement with field relations and previous work, this age suggests that this portion of the Caledonian orogen cooled from amphibolite facies conditions into the brittle field during the Early Devonian. The age of set I fractures indicates that large-scale, E-W trending folds in the basement are older than similarly oriented folds associated with the Devonian basins of southwest Norway. Later hydrothermal activity along Set 1b fractures is indicated by Rb/Sr two-point mineral dating of epidote and hydrothermally altered alkali-feldspar in the wall rock (363, 369 and 371 Ma). A later phase of brittle deformation involved reactivation of set I fractures and the formation of steep, mainly NNW-SSE trending set II fractures, comprising calcite-filled veins and breccias with minor striated faults. Kinematic analyses indicate that these fractures reflect a change in the extension direction to E-W, possibly around the Devonian-Carboniferous boundary. Set II fractures acted as preferable sites for later reactivation at progressively shallower crustal levels; first by intrusion of Permian dikes and later by the formation of incohesive fault rocks.
Kinematics and timing of polyphase post-Caledonian deformation in the Bergen area, SW Norway