Temporal and spatial development of a gravity-driven normal fault array: Middle–Upper Jurassic, South Viking Graben, northern North Sea

Three-dimensional seismic and well data from the South Viking Graben, northern North Sea Basin, is used to investigate the temporal and spatial development of a gravity-driven normal fault array above an evaporite-rich detachment. Two moderate throw (500–900 m), Middle to Upper Jurassic normal faults (the Gudrun and Brynhild Faults) are developed within the study area. Both faults die-out laterally and tip-out upwards at different structural levels within the syn-rift succession. Both faults terminate downwards into Late Permian evaporites (Zechstein Group) and do not offset pre-evaporite basement units. This thin-skinned fault array developed in response to westwards tilting of the hangingwall of the South Viking Graben during Late Jurassic rifting, and consequent westward gliding and extensional break-up of units above the mechanically-weak evaporite horizon. Isochron mapping and well-based correlation of Middle to Upper Jurassic syn-rift units allow constraints to be placed on the temporal evolution of the fault array. Several stages of structural development are observed which document; (i) a period of relatively minor, early (i.e. pre-rift) halokinesis; (ii) variable spatial activity on individual faults within the array; and (iii) the progressive upslope migration of active faulting within the array as a whole. The progressive upslope migration of fault activity is interpreted to reflect progressive “unbuttressing” and extensional faulting of upslope, post-evaporite units. The overall structural style and kinematic evolution identified here shares many characteristics with both ‘rift–raft tectonics’ documented in other rifts developed above an evaporitic sub-stratum and ‘raft tectonics’ described from passive margin basins containing thick mobile salt or shale intervals. This style of fault array evolution differs from that observed in rifts lacking mobile layers at-depth and highlights the importance of these units in the structural development of rifts.