Pattern Formation in 3-D Numerical Models of Down-Built Diapirs Initiated by A Rayleigh–Taylor Instability

fernandez, naiara ; kaus, boris j.p.;

Many salt diapirs are thought to have formed as a result of down-building, which implies that the top of the diapir remained close to the surface during syn-halokinetic sediment deposition. Down-building is largely a 3-D process and in order to better understand what controls the patterns of the diapirs that form by this process, we here perform 3-D numerical models of down-built diapirs initiated by the gravity instability in linear viscous materials and compare the results with analytical models. We vary several parameters of the numerical models such as initial salt thickness, sedimentation rate, salt viscosity, salt-sediment viscosity ratio as well
as the density of sediments. Down-building of 3-D diapirs only occurs for a certain range of parameters and is favoured by lower sediment/salt viscosity contrasts and sedimentation rates in agreement with analytical predictions and findings from previous 2-D models. However, the models show that the sedimentation rate has an additional effect on the formation and evolution of 3-D diapir patterns. At low sedimentation rates, salt ridges that form during early model stages remain preserved at later stages as well. For higher sedimentation rates, the initial salt ridges are covered up and finger-like diapirs form at their junctions, which results in different salt exposure patterns at the surface. Once the initial pattern of diapirs is formed, higher sedimentation rate can also result in covered diapirs if the diapir extrusion velocity is insufficiently large. We quantify the effect of sedimentation rate on the number of diapirs exposed at the surface as well as on their spacing and we explain the observations with analytical predictions using thick-plate analytical models. In some cases, this final pattern is distinctly different from the initial polygonal pattern.