Controls on the development and termination of failed continental rifts: Insights from the crustal structure and rifting style of the North Sea via ambient noise tomography

The mid to lower crust plays an important role in rift initiation and evolution, particularly when large scale sutures and/or terrane boundaries are present. These inherited features can focus strain or act as inhibitors to extensional deformation. Ancient tectonic features are known to exist beneath the iconic failed rift system of the North Sea making it the ideal location to investigate the complex interplay between pre-existing regional heterogeneity and rifting. To this end, we produce a 3D shear-wave velocity model from transdimensional ambient seismic noise tomography to constrain crustal properties to ~30 km depth beneath the North Sea and its surrounding landmasses. Major North Sea sedimentary basins appear as low shear-wave velocity zones that are a good match to published sediment thickness maps. We constrain relatively thin crust (13-18 km) beneath the Central Graben depocentres that contrasts with crust elsewhere that is at least 25-30 km thick. Significant variations in rift style and structure are identified along the failed rift system, varying between symmetric and strongly asymmetric extension, that is related to the location of Laurentia-Avalonia-Baltica paleo-plate boundaries. We identify clear variations between paleo-plates with strong lateral gradients in crustal velocity related to Laurentia-Avalonia-Baltica plate juxtaposition and reduced lower crustal velocities in the vicinity of the Thor suture, possibly representing the remnants of a Caledonian accretionary complex. Our results provide fresh insight into the pivotal role that ancient terranes can play in the formation and failure of continental rifts, and may help explain the failure of other similar continental rifts such as Bass Strait in Australia.