Regional structural evolution of the North Sea: oblique slip and the reactivation of basement lineaments

Abstract

Basement lineaments pre-dating the dominant Mesozoic basin formation of the North Sea are fundamental controlling features within the polyphase tectonic evolution of NW Europe. These inherent zones of weakness were repeatedly reactivated during the evolution of the North Sea rift system. The regional and geometrical constraints imposed by the presence of these pre-existing tectonic grains during the Mesozoic extensional deformation suggest that oblique-slip movement must have occurred along many of the basin-bounding faults in a non-preferential orientation for dip-slip reactivation. While the North Sea can be considered in broad terms as a series of linked, elongated grabens developed in response to dominantly E–W extensional intra-plate stresses, the orientation, distribution and character of pre-existing shear zones ultimately controlled the geometry of many Mesozoic basins and their faulted margins. Four types of inherent basement features are recognized as having influenced subsequent basin development: (a) wide, diffuse zones of NW–SE and NE–SW basement shear, e.g. in the Central Graben; (b) narrow shear zones oblique to primary Mesozoic extensional faults, e.g. in the East Shetland Basin; (c) E–W-trending discrete fault systems, e.g. on the Western Platform; and (d) massive palaeohighs, e.g. the Halibut Horst. The distribution of these distinct structural features throughout the North Sea was controlled by Early Paleozoic plate tectonic evolution. Repeated Mesozoic and Cenozoic reactivation along fundamental zones of weakness, including oblique-slip displacements and basinal inversion, has led to areas of highly complex structuration. Hence, there is the potential for numerous structural and stratigraphic hydrocarbon trapping mechanisms associated with these underlying basement lineaments. Although the total amount of horizontal crustal movement may be limited, understanding of the role of oblique-slip along pre-existing zones of weakness and the effect of long-lived palaeohighs within the basin is crucial to place Mesozoic and Tertiary hydrocarbon accumulations within a realistic North Sea tectono-stratigraphic framework.