Extensional tectonics and crustal structure: deep seismic reflection data from the northern North Sea Viking graben

Abstract

Summary A preliminary interpretation of a deep seismic reflection profile across the northern North Sea provides an insight into the crustal structure beneath the major extensional rift, the Viking graben. The Viking graben was initiated during the early Mesozoic and evolved during the Jurassic-Cretaceous into an asymmetrical half-graben, fault-bounded on its western side. Deep seismic reflections enable an interpretation of the position of the Moho to be made, and an estimate of the amount of crustal extension, within the plane of the section, of 50% has been calculated. The pre-Mesozoic crust has been thinned to a minimum of 14 km, but generally varies from 15–20 km beneath the rift. Interpretation of extensional faults within the Jurassic generally gives less than 50% extension, and much of the early Mesozoic faulting is probably not imaged. Calculation of Mesozoic subsidence in the graben leads to an estimated stretching factor of 3 or greater. A larger scale of linked tectonics involving major strike-slip movements along the length of the graben is invoked to explain the discrepancy between subsidence and crustal extension. Fault-related subsidence occurred in three distinct phases of finite duration—during the Triassic (40 My), the late Jurassic (30 My), and the early Tertiary (30 My). During the late stages of pervasive crustal thinning (late Jurassic-Early Cretaceous) a series of major fault and shear zones developed, dipping E and passing through the crust, offsetting the Moho. The change in tectonic style corresponds to a change from a dominantly extensional to a dominantly strike-slip mode of rift evolution. The seismic profile also images the Magnus basin related to the Great Glen fault system, and the Horda Platform near the coast of Norway.