STRUCTURAL OBSERVATIONS ADJACENT TO A LARGE-SCALE EXTENSIONAL DETACHMENT ZONE IN THE HINTERLAND OF THE NORWEGIAN CALEDONIDES

Structural and lithological observations in a N-S traverse across Brem angerlandet, western Norway, demonstrate that the region is separated into two plates by the E-W-striking, S-dipping ductile-brittle Vetvika Shear Zone. The Vetvika Shear Zone places Caledonian high-grade gneis ses, with a Precambrian protolith age of the lower plate, in tectonic contact with heterogeneous gneisses of the upper plate. The Vetvika Sh ear Zone is the uppermost part of the Nordfjord-Sogn Detachment Zone. A gently W-plunging stretching lineation within the S-dipping lower-pl ate augen gneisses is interpreted as having formed within a several ki lometers thick ductile W-dipping extensional shear zone that has under gone later rotation around an E-W axis. A highly attenuated sequence o f nappes occur structurally on top of a heterogeneous basement and cov er quartzite of the upper plate. Pre-Caledonian structures are preserv ed locally in the gneissic upper plate basement. The nappes at Bremang erlandet are correlated with the Caledonian Middle and Upper Allochtho ns, representing Baltic/suspect and exotic terranes, respectively. One large and several medium- to small-scale, brittle, top-to-the-south e xtensional faults are superimposed on ductile contractional structures in the upper plate. Some of the extensional faults merge downward wit h the Vetvika Shear Zone. The contractional structures include E-W-tre nding folds and S-directed thrusts in the cover quartzite, and are sup erimposed on a pronounced W-E stretching lineation. Conjugate sets of fractures and faults related to E-W extension represent the youngest s tructures in the upper plate. The Old Red Sandstones (Devonian) of the Hornelen basin rest unconformably on top of extensionally faulted upp er plate rocks on Bremangerlandet. The unconformity is partly faulted at Bremanger, and truncated by a regionally extensive late, brittle fa ult along the basin margin further east. This suggests a long-lived te ctonic history for the Vetvika Shear Zone and its eastward continuatio n, with movement pre- and post-dating the filling of the collapse basi n, possibly as an eastward migration of the break-away fault.