The middle Devonian basins of western Norway: sedimentary response to large-scale transtensional tectonics?

The Devonian basins of western Norway were formed during late- to post-orog enic extension of overthickened Caledonian crust. The basins are situated i n the hanging wall of the extensional Nordfjord-Sogn Detachment Zone (NSDZ) and display extensional half-graben geometries in sections parallel to the local direction of principal extension. Based on overall facies configurat ions, paleocurrent patterns and intrabasinal structures, we infer an anticl ockwise rotation of the syndepositional extension direction from NW-SE in t he south (Solund basin) to WSW-ENE in the north (Hornelen basin). The axes of folds that are roughly parallel to the local extension direction are rot ated correspondingly. The Kvamshesten basin is located between the Solund a nd Hornelen basins. Sedimentological and structural data show evidence of a n early, southeastwards tilt direction followed by a more eastwards tilt an d associated E-W flowing paleodrainage. Correspondingly, NW-SE trending fol ds and reverse faults are superposed by E-W trending ones at low to interme diate stratigraphic levels. The variations in apparent tilt direction for t he basins together with variations in intrabasinal structure is interpreted to reflect an anticlockwise rotation of the regional syndepositional strai n field. The above observations and inferences indicate that the Devonian b asins in western Norway formed in a strain field dominated by regional tran stension, accommodated by extension along the NSDZ and sinistral strike-sli p along orogen-parallel shear zones and faults to the north of the basins; alternatively, NW-directed extension preceded the introduction of a sinistr al strike-slip component. The models are in accordance with recent work car ried out in the footwall of the NSDZ and illustrates the tectono-sedimentar y response to a complex interplay between extension and strike-slip that ap pears to have been fundamental in the late-stage disintegration of the Cale donian orogen. (C) 2001 Elsevier Science B.V. All rights reserved.