The effects of subsalt relief on gravity-driven salt tectonics: Results from analogue modelling

Abstract

Gravity-driven salt-related deformation on passive margins is commonly interpreted as kinematically-linked domains of updip extension and downdip contraction with an intermediate, undeformed zone of translation. Our study uses analogue models to show how salt flow is affected by the subsalt geometry resulting in complex deformation and the alternation of extensional and contractional domains in space and time in translational salt provinces. In our work, the analogue models incorporate subsalt relief associated with half-graben and graben that are realistic geometries along passive margins and combine the effects of gravity spreading in which deformation is driven by differential sedimentary loading, and an early-stage of gravity gliding induced by basin tilt. This approach allows for analysis of how salt flow and overburden deformation are affected by: (1) smoothly dipping subsalt relief; (2) subsalt relief characterized by half-graben with a gently landward dipping extensional fault; (3) subsalt half-graben connected laterally with a distal symmetric graben. For each of these models, we evaluate: 1) the interplay between slipping deformation corresponding to gravity gliding, gravity spreading mechanisms, and subsalt geometry; 2) the spatial and temporal distribution of salt-related structural domains; and 3) the contrasting styles of salt tectonics for different subsalt geometries. In addition, some of the experiments have been repeated considering thicker salt and pre-kinematic overburden and different sedimentation rates. We show that the structures of the subsalt relief variably resist salt flow producing flow mismatch. This variable resistance controls the orientation of the extension and contraction supra-salt structures. Salt surplus and contraction occur over landward dipping base-salt ramps, and a zone of salt depletion and subsidence limited by extensional and contractional hinges occurs over the basinward dipping base-salt ramp. Variations in width and the dip of these base-salt ramps result in stronger flow mismatches and deformation complexity. We also discuss variations in salt flow profile in response to the gravitational forces that drive deformation, allowing a good first-order approximation of viscous salt flow at a regional scale that can be directly compared to examples from various salt-bearing continental margins, particularly the relatively less understood but extremely economically important distal domains of Brazil's Campos and Santos basins.