Deformation, evolution and controls of halokinetic megaflaps driven by vertically-stacked and laterally-shifting depocenters

Megaflaps comprise steeply dipping to overturned panels of the oldest suprasalt strata flanking steep diapirs, and represent the roofs of early inflated salt. These large-scale structures result from salt-sediment interaction at minibasin scales and entail multiple kilometres of folding and vertical relief. They are divided into two end-member types (halokinetic and contractional) and form by some combination of limb rotation and kink-band migration. They can be difficult to image and interpret adjacent to flaring diapirs and beneath allochthonous salt due to steep bedding dips and suboptimal illumination. Using physical models, we investigate halokinetic megaflaps driven by differential loading. Models with vertically-stacked vs. laterally-shifting loading above a prekinematic layer have been run to determine the main processes and mechanisms controlling the growth and kinematic evolution of megaflaps. Parameters such as the thickness of the prekinematic cover, the width of the proto-salt wall, the synkinematic sedimentation rate, and variations in the mechanical properties of the prekinematic cover have been tested to evaluate their role in megaflap generation. The experimental results demonstrate that in absence of tectonic forces, halokinetic megaflaps are generated by a combination of 1) an early increase of pressure-head gradient between two adjacent minibasins with different rates of sedimentation and subsidence, and 2) the disappearance of this gradient that occurs when welding occurs beneath the more quickly subsiding minibasin. The geometry, kinematic evolution, and degree of small-scale deformation of the megaflaps in our analogue models are consistent with both exposed (e.g., Paradox Basin) and seismically imaged halokinetic megaflaps (e.g., deepwater northern Gulf of Mexico). •We investigate halokinetic megaflaps driven by differential loading using analogue models.•Modelled megaflaps stretch and thin upward when they are made by sand, and have a constant thickness when are more cohesive.•They formed by rapid subsidence of a minibasin once the adjacent one was welded and pressure-head gradient disappears.•Halokinetic megaflap folding was dominantly by bed rotation early, but mostly by kinkband migration late in the history.•Megaflap’s development ended when allochthonous salt broke out and overrode them.