From metamorphic core complex to crustal scale rollover: Post-Caledonian tectonic development of the Utsira High, North Sea

Abstract

The Utsira High (North Sea) records rift faulting that culminates with Jurassic-Cretaceous crustal-scale rollover towards the Viking Graben. This deformation is superimposed on Paleozoic fault-bounded basins on a substrate of Caledonian nappes. The latter contains multilevel crustal-scale shear systems that domed under the Utsira High during the Devonian, as indicated by mapping and interpretation of a large long-offset 2D seismic reflection dataset. Restorations show that isostatically driven doming from excision of overthickened crust caused uplift and erosion of basins and the underlying nappe stack. Doming took place above a crustal rollback system as a symmetrical metamorphic core complex formed. The dome discloses vertical flattening kinematics below bi-directional shear systems, evident by opposite shear-fabric kinematics on opposite dome flanks. A major detachment marks the upper boundary of a transition between upper and lower plate strain regimes, coinciding with low-reflective granitic units above strongly reflective Caledonian nappes. This detachment hosts transport-parallel corrugations that strike E-W to NE-SW. A series of deeply eroded half-grabens on top of the upper plate are bounded by pre-Permian faults exhibiting top-NE extensional kinematics. Faults sole out in the fundamental detachment level, locating the brittle-ductile transition, with all structures subsequently rotated during doming. A new localized detachment formed at a shallower level above the region of maximum crustal uplift, seen as upwards detachment migration driven by heating. Later faulting locates to the dome flanks, either rejuvenating rotated shear zones or cutting to deeper crustal levels while recording predominantly down-eastward transport. During Jurassic-Cretaceous rifting, deformation localized to the Viking Graben Boundary Fault, giving room for thick growth wedges in the Viking Graben and establishing the Utsira High as a crustal-scale rollover structure.