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

Abstract

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.