Continental basement inheritance and multiscale rifted ocean margin architectures: case studies from the N Atlantic-Greenland region

It is widely believed that pre-existing basement structures significantly influence the development of rifts in both intracontinental and margin settings, but this hypothesis remains to be tested in many areas.We first investigate how fault reactivation controls the distribution and growth of individual faults in the post-breakup cover sequence of the Norwegian margin. We then use a combined onshore and offshore approach to address the wider problem of how intraplate basement structures control subsequent regional-scale fault patterns and kinematics. The influence of pre-existing, rift-related normal faults upon the early stages of fault growth in the post-breakup cover sequence is well illustrated offshore in the Vring basin. Here Maastrichtian to Palaeocene age rift-related normal faults on the Nyk High were blanketed by Plio-Pleistocene sediments, which are cut locally by small (maximum throw < 30 ms) postglacial normal faults. These occur directly above points of maximum throw or offsets along underlying rift-related faults, which therefore clearly control the location and architecture of later structures. Both upward and downward fault propagation from basement to cover and vice versa are recognised during this reactivation. Elastic interactions between en-echelon fault segments situated above basement heterogeneities are likely to promote the rapid growth of reactivated fault systems. Basement structures are often oriented significantly oblique to later rifting directions and can lead to transtensional deformation patterns. The Davis Strait of West Greenland contains the Ungava transform fault zone, which separates the failed spreading centres of the Labrador Sea and Baffin Bay. Detailed onshore studies of fault patterns and kinematics, at regional to outcrop scales, reveal the key roles played by two main transtensional fault systems. An older system of N-S trending en echelon normal faults bound a series of deep sedimentary basins of mid- upper-Cretaceous age that are linked by ESE-trending normal faults which reactivate basement fabrics. This system is then reactivated and overprinted by a younger Eocene system of polymodal strike-slip faults. The transition from extension- to wrench-dominated transtension marks a switch in instantaneous shortening from vertical to horizontal and coincides with the onset of sea-floor spreading in the Labrador Sea. Our observations suggest that the basement structures have influenced the location and geometry of this oblique/ transform margin segment, but that direct reactivation is limited to localised shear zones. Our onshore and offshore approach demonstrates that pre-existing structures can influence deformation patterns (fault locations, geometries, kinematics) at every stage in the evolution of ocean margins. However, the scales of faulting and displacement magnitudes during reactivation events are commonly modest compared to the regional-scale deformation of the margin. Nevertheless, the resulting seismic to sub-seismic scale, structures (faults, fractures, folds) will impact very significantly on features such as trap geometry, integrity and reservoir performance.