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3D analogue models of variable displacement extensional faults: applications to the Revfallet Fault system, offshore mid-Norway

Abstract The Revfallet Fault system, Heidrun area, offshore mid-Norway, illustrates considerable variation in structural style along strike which is attributed to variation in coupling between basement and cover (separated by Triassic salt), due to the variable displacement along the fault system and to distinct phases of shallow level as well as basement-involved extension. Deformation in the southern part of the fault is partitioned over a number of cover-graben and associated with the development of a monoclinal flexure above the basement fault system. With an increase in displacement northwards, cover-graben become grounded on the pre-salt basement resulting in the rotation and breaching of the extensional drape fold and, eventually, partial and full coupling between basement and cover fault systems. Analogue models were designed to simulate the 3D evolution of cover deformation above a variable displacement extensional fault separated from the cover by a ductile basal unit and to validate the structural interpretations and evolutionary models for the Revfallet Fault system. Analogue model results reveal the changing geometry along strike from a gentle extensional drape fold with minimal brittle deformation at low extension, to a breached drape fold system with significant rotation and collapse of the cover across the basement fault as displacement increases. Models conducted with a two-phase history — shallow extension (orthogonal and oblique) followed by basement-involved extension illustrate variable reactivation of cover-graben along strike during the basement-involved event, and collapse and rotation of the extensional footwall to the basement fault as displacement increases to a maximum. Coupling between cover faults and the basement fault only occurred where an oblique, precursor cover-graben cuts across the basement fault from hanging wall to footwall. Closer examination of the structural and stratigraphic relationships reveals major discrepancies between throw on cover and basement fault segments. The analogue model results compare favourably to seismic data from the Revfallet Fault system and validate the structural model for this fault system.