Strain migration during multiphase extension, Stord Basin, northern North Sea rift

In multirifted regions, rift-related strain varies along and across the basin during and between each extensional event, and the location of maximum extension often differs between rift phases. Despite having a general understanding of multiphase rift kinematics, it remains unclear why some parts of the rift are abandoned, with strain accumulating in previously less deformed areas, and how seismic and sub-seismic scale pre-existing structures influence fault and basin geometries. We study the Stord Basin, northern North Sea, a location characterized by strain migration between two rift episodes. To reveal and quantify the kinematics, we interpreted a dense grid of 2D seismic reflection profiles, produced time-structure and isochore maps, collected quantitative fault kinematic data and calculated the amount of extension (β-factor). Our results show that the locations of basin-bounding fault systems were controlled by pre-existing crustal-scale shear zones. Within the basin, rift faults mainly developed at high angles to the Permo-Triassic Rift Phase 1 (RP1) E-W extension. Rift faults control the locus of syn-RP1 deposition, whilst during the inter-rift stage, sedimentary processes (e.g. areas of clastic wedge progradation) are more important in controlling sediment thickness trends. The calculated amount of RP1 extension (β-factor) for the Stord Basin is up to β=1.55 (±10%, 55% extension). During Middle Jurassic-Early Cretaceous (Rift Phase 2, RP2) however, strain localises to the west along the present axis of the South Viking Graben, with the Stord Basin being almost completely abandoned. Migration of rift axis during RP2 is interpreted to be related to the changes in lithospheric strength profile and possible underplating due to the ultraslow extension (<2mm/yr during RP1) and the long period of tectonic quiescence (ca. 70 myr) between RP1 and RP2. Our results highlight the very heterogeneous nature of temporal and lateral strain migration during and between extension phases within a single rift basin.