Supradetachment basins that formed during the Devonian extensional collapse of the Scandinavian Caledonides have been explained as hanging-wall basins formed along the listric breakaway zone of a major detachment fault system. This model involves significant rotation of bedding as the detachment flattens to an approximately horizontal orientation, and explains the large stratigraphic thicknesses of east-dipping layers overlying in tectonic contact the top-to-the-west Nordfjord-Sogn detachment zone (NSDZ). However, it fails to explain one of the basins, the Håsteinen basin, where east-dipping Devonian strata rest unconformably on a metamorphic substrate that forms part of the upper plate of the detachment system. Based on detailed field mapping, we present a model where the Håsteinen forms as a ramp basin that develops on the upper plate above a major west-facing ramp in the NSDZ. We test both of these models by forward modeling using 2DMove (two-dimensional kinematic modeling software). The results show how two different types of supradetachment basins can coexist in a subhorizontal detachment zone in an area that underwent many tens of kilometers of lateral crustal extension.

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