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

Abstract

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-s!ip 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.A key problem for large-scale extensional tectonics is to determine the relationships between brittle faulting near to the surface and ductile stretching at depth, which controls the structure and evolution of extensional sedimentary basins.Using a theoretical approach, McKenzie (1978) proposed a model for lithospheric stretching.

Extensional tectonics in the Caledonides of southern Norway, an overview

“…For details and documentation refer to the original papers (cf. Norton, 1986;Séranne and Séguret, 1987;Chauvet and Séranne, 1989), and papers by the present author and co-workers (see references). Fig.…”

“…Bryhni, 1964;Nilsen, 1968;Steel and Gloppen, 1980) were focused on the basin fill. After the recognition of the large-scale extensional detachment, models for formation of the basins have principally emphasised the importance of the detachment as the major control on Devonian sedimentation (Hossack, 1984;Norton, 1986;Séranne and Séguret, 1987;Séguret et al, 1989). Clasts from the lower plate of the detachment have, however, not been identified within any of the basins (Cuthbert, 1991;Osmundsen, 1996;Osmundsen et al, 1998).…”

“…Structures formed at various upper to lower crustal levels related to the extensional collapse have been identified in several on-shore cross-sectional traverses along-strike of the orogen (e.g., Norton, 1986;Séranne and Séguret, 1987;Andersen and Jamtveit, 1990;Fossen, 1992;Rykkelid and Andresen, 1994). The late-to post-orogenic extension in southern Norway resulted in penetrative reworking and decompression of Caledonian high-pressure metamorphic rocks, formation of large extensional detachments and the Devonian supra-detachment basins Osmundsen, 1996;Osmundsen et al, 1998).…”

Ductile and brittle extension in the southern Lofoten Archipelago, North Norway; implications for differences in tectonic style along an ancient collisional margin

“…Extensional structures and fabrics in the Western Gneiss Region (WGR) of southwestern Norway expose deep sections of continental lithosphere exhumed following continent-continent collision (Norton, 1986;Sé ranne and Sé guret, 1987;Andersen and Jamtveit, 1990;Andersen and others, 1991). Comparable crustal-scale extension has been reported in the Caledonides of East Greenland (Hartz and Andresen, 1995).…”

“…The ancient A-type subduction zone boundary (Hodges and others, 1982) is exposed along the west coast of Norway and has been the subject of numerous reports. Most studies have focused on the southern WGR, where large-scale extension driven by post-Caledonian gravitational collapse of the orogen apparently led to the formation of the Devonian molasse basins and juxtaposed them against basement eclogitized during Caledonian subduction (Norton, 1986;Sé ranne and Sé guret, 1987;Andersen and Jamtveit, 1990;Andersen and others, 1991).…”