THE ROLE OF PREEXISTING STRUCTURES DURING EXTENSION OF CALEDONIAN CRUST AND FORMATION OF THE NORTHERN NORTH SEA RIFT

The transformation of the Lower Paleozoic Caledonian orogen into the North Sea Mesozoic rift basin involved a prolonged history of extension that involved both reuse of preexisting structures and the formation of new extensional faults. Initially (~400 Ma), extension was taken up by reactivation of Caledonian thrusts as low-angle extensional detachments, with the basal Caledonian thrust acting as a major top-to-the hinterland décollement that related to eduction of the Baltican margin. After some extension, new and steeper extensional shear zones developed that transected the reactivated Caledonian thrusts and the underlying crust. The transition from thrust reactivation to the formation of new and steeper hinterland-dipping shear zones is linked to uplift and rotation of the hinterland (Western Gneiss Region) and the basal thrust zone during the extension.

Some of these Devonian extensional shear zones show evidence of significant Permian and later onshore brittle fault development, suggesting that these ductile extensional shear zones were reactivated during extension phases that can be correlated with North Sea Rifting. Brittle reactivation of low-angle (< 30° dip) extensional shear zones may seem surprising from a mechanical point of view, suggesting that they were mechanically very weak structures. However, most onshore Permo-Triassic faults and fractures are steep structures, and most major Permo-Triassic rift-related fault complexes in the northern North Sea appear to be new-cut fault structures, but preexisting structures have guided their location and influenced their geometries and segmentation patterns. During a second (late Jurassic) phase of North Sea rifting, many (but not all) early-stage rift faults were reactivated, and some new ones formed, depending on differences in strain localization during the two rift phases and probably also fault strength and fault orientation relative to the stress field. Clearly the location of the northern North Sea rift is controlled by extensional structures formed during the collapse of the Caledonian orogeny, revealing a complicated history of tectonic inheritance that can be traced from the late Jurassic phase of rifting through several phases of extensional deformation to the late-Caledonian structural framework of this orogen.