The Devonian basins of western Norway: tectonics and kinematics of an extending crust

Summary The Devonian basins of western Norway represent shallow to deep exposures of a synthetic extensional sedimentary basin and provide field evidence for ductile extensional deformation within the basin fill and for the evolution of a brittle low-angle fault and ductile shear zone along the basal contact. The motion along this low-angle (5–25°) detachment is synchronous with both deposition and tilting (25°) of the huge (up to 25-km thick) overlapping coarse detrital Middle Devonian series. Such a geometry requires a minimum dip-slip offset of 50 km. The structural data are consistent with fault-rock associations along the basal contact and with the prograde greenschist metamorphism observed in the southern basin: deeper and deeper levels are observed from N to S. Except along the highly sheared and retrogressed basal shear zone, the footwall remained unaffected by deformation during basin development. We discuss three crustal models for basin development and propose that the displacement along the basal contact of the basins is transformed into pervasive ductile flow within the lower crust both at some distance to the side of the basin and beneath the basin.