Salt tectonics driven by differential sediment loading: stability analysis and finite-element experiments

At many continental margins, diierential sediment loading on an underlying salt layer drives salt deformation and has a signi¢cant impact on the structural evolution of the basin.We use 2-D ¢niteelement modelling to investigate systems in which a linear viscous salt layer underlies a frictionalplastic overburden of laterally varying thickness. In these systems, diierential pressure induces the £ow of viscous salt, and the overburden experiences updip deviatoric tension and downdip compression. A thin- sheet analytical stability criterion for the system is derived and is used to predict conditions under which the sedimentary overburden will be unstable and fail, and to estimate the initial velocities of the system.The analytical predictions are in acceptable agreement with initial velocity patterns of the numerical models. In addition to initial stability analyses, the numerical model is used to investigate the subsequent ¢nite deformation. As the systems evolve, overburden extension and salt diapirism occur in the landward section and contractional structures develop in the seaward section.The system evolution depends on the relative widths of the salt basin and the length scale of the overburden thickness variation. In narrow salt basins, overburden deformation is localised and characterised by high strain rates, which cause the system to reach a gravitational equilibrium and salt movement to cease earlier than for wide salt basins. Sedimentation enhances salt evacuation by maintaining a diierential pressure in the salt. Continued sedimentary ¢lling of landward extensional basins suppresses landward salt diapirism. Sediment progradation leads to seaward propagation of the landward extensional structures and depocentres. At slow sediment progradation rates, the viscous £ow can be faster than the sediment progradation, leading to e⁄cient salt evacuation and salt weld formation beneath the landward section. Fast sediment progradation suppresses the viscous £ow, leaving salt pillows beneath the prograding wedge.