Crustal thinning and nature of extension in the northern North Sea from deep seismic reflection profiling

A regional network of deep seismic reflection profiles recorded in the northern North Sea has been used to map Mesozoic and Cenozoic basin thickness and crustal thickness in the Viking Graben and adjacent platform areas. Depth to the reflection Moho varies from about 20 km in parts of the Viking Graben to about 32 km beneath the Shetlands and the Norwegian margin. The shallowing of the reflection Moho beneath the Viking Graben implies crustal stretching factors for Mesozoic extension greater than 2 in the center of the North Sea basin. The axis of crustal thinning is located directly beneath the Viking Graben, the axis of the sedimentary basin. Steeply dipping basin-bounding faults are imaged in the upper crust and several dipping reflectors are observed in the upper mantle, but no continuous reflective feature extending from the sedimentary basin to beneath the Moho is observed on any of the deep profiles. Thus these data do not support the existence of lithosphere-penetrating low-angle detachments (zones of simple shear) as the cause of extension in the northern North Sea. Some of the mantle reflectors dip to the west and some to the east, suggesting that simple shear, if it occurs in the upper mantle, is not of uniform sense. Rather, these data suggest a complex, depth-dependent pattern of brittle extensional deformation in the upper crust; pervasive, ductile extension (bulk pure shear) in the lower crust (which is decoupled from deformation in the mantle); and extension accommodated by discrete, dipping shear zones in the lithospheric mantle.