The southern termination of the Nordland Ridge, mid-Norway exhibits variations in extensional style which can be attributed to differing degrees of coupling between pre-salt 'basement' and postsalt 'cover' strata. South of the Nordland Ridge, Jurassic and Cretaceous extension is distributed relatively uniformly over the 80 km wide Halten Terrace. Further to the north the zone of extension narrows to less than 40 km and the majority of the extension is taken up on the Revfallet Fault, which separates the Donna Terrace from the Nordland Ridge. Displacement on the Revfallet Fault, at basement level, increases progressively from its zero displacement tip to a maximum of 7 km, 60 km to the north. At low displacements, extension in the cover is decoupled from the basement extension and vertical level changes are accommodated by monoclinal flexure. As displacement on the Revfallet Fault increases, the degree of basement-cover coupling increases and a much closer spatial link between deformation in the basement and in the cover is evident. This progressive increase in degree of coupling is characterized by increased footwall uplift and decreased extension in the cover away from the master fault. The degree of basementcover coupling can also be shown to vary not only with displacement but also with time, in response to variable strain rates. Initial extension during the Jurassic was predominantly decoupled and occurred over 50 Ma period. After 40-50 Ma of relative quiescence, the major extensional episode occurred within a comparatively short 10 Ma period in the mid-Cretaceous. This later period of rapid extension requires strain rates at least an order of magnitude greater than those of the Jurassic. The results of this study have led to significant changes in the regional tectono-stratigraphic model.

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