Abstract A Middle to Late Miocene compression phase has been documented along the Norwegian margin from 62°N to 68°N. Reverse faults and inversion domes are numerous on the 300 to 450 km wide central mid-Norwegian margin, but are only locally observed on the Møre and Lofoten margins. A major regression forced the coastline of the syn-tectonic Kai Formation 50–150 km west of the present coastline and a genetic link between the regression and the smaller-scale contraction structures is observed. Long wavelength (200–300 km) lithospheric bulging is suggested to explain the coastline regression at the onset of the compression phase. The position of the regressed coastline suggests that the continent -shelf transition guides the lithospheric buckling such that the thicker crust segment is uplifted. Long wavelength folding may account for additional crustal shortening along the sharp crustal transitions on the Lofoten and Møre margins. It is suggested that the Norwegian onshore domes were formed during at least two independent stages – a Middle to late Miocene compression-related uplift and later regional glacial isostatic uplift. The syn-tectonic Kai Formation fills in the depressions around the contraction-related structures and is dated as late Middle to Late Miocene. It is suggested that both the Utsira Formation and the assumed northern equivalent, the Molo Formation, were deposited after the compression event and, thus, are mainly early Pliocene in age. The Middle to Late Miocene compression phase is observed throughout the North Atlantic and the sea-level fall at the initiation and rise at the termination corresponds with significant shifts in climate.

Abstract The mild compressional structures of Cenozoic age on the passive margins bordering Norway, the UK, the Faroes and Ireland have been the subject of much discussion in the literature. Nevertheless, their origin remains enigmatic. Candidate mechanisms must be able to explain the generation of sufficient stress to cause deformation, the episodic nature of the structures and why they developed where they did. We examine these mechanisms and conclude that multiple causes are probable, while favouring body force as potentially the most important agent. The geometry and setting of the structures are incompatible with gravitational sliding and toe-thrusting, probably the commonest ‘compressive’ structuring around the Atlantic margins. A passive mode of origin featuring drape or flank sedimentary loading probably emphasized some of the structures, but cannot be invoked as a primary mechanism. Likewise, reactivation of basement structure probably focused deformation but did not initiate it. Far-field orogenic stress from Alpine orogenic phases and from the West Spitsbergen–Eurekan folding and thrusting is also examined. This mechanism is attractive because of its potential to explain episodicity of the compressional structures. However, difficulties exist with stress transmission pathways from these fold belts, and the passive margin structures developed for much of their existence in the absence of any nearby contemporaneous orogeny. Breakup and plate spreading forces such as divergent asthenosheric flow have potential to explain early post-breakup compressional structuring, for example on the UK–Faroes margin, but are unlikely to account for later (Neogene) deformation. Ridge push, generally thought to be the dominant body force acting on passive margins, can in some circumstances generate enough stress to cause mild deformation, but appears to have low potential to explain episodicity. It is proposed here that the primary agent generating the body force was development of the Iceland Insular Margin, the significant bathymetric-topographic high around Iceland. Circumstantially, in Miocene times, this development may also have coincided with the acme of the compressional structures. We show that, dependent on the degree of lithosphere–asthenosphere coupling, the Iceland Plateau may have generated enough horizontal stress to deform adjacent margins, and may explain the arcuate distribution of the compressional structures around Iceland. Assuming transmission of stress through the basement we argue that, through time, the structures will have developed preferentially where the basement is hotter, weaker and therefore more prone to shearing at the relatively low stress levels. This situation is most likely at the stretched and most thermally-blanketed crust under the thickest parts of the young (Cretaceous–Cenozoic) basins. Although several elements of this model remain to be tested, it has the potential to provide a general explanation for passive margin compression at comparatively low stress levels and in the absence of nearby orogeny or gravitational sliding.