Response of salt structures to loading and unloading by ice-sheets – insights from numerical modelling

Salt rocks are mechanically weak and behave like viscous fluids when deforming at geological time scales and strain rates. The weight of an ice sheet advancing into a salt-bearing basin may cause sufficient differential load to induce salt flow. Ice loading has been postulated as a trigger for Pleistocene deformation at a number of salt structures in the Central European Basin System. We conducted 2D-finite-element models (ABAQUS) with a setup representing a simplified salt diapir to test existing conceptual models and evaluate the controlling factors. Different parameter sets for the rheology of salt and overburden rocks, including linear versus non-linear viscosity of the salt, were tested. Model results show lateral salt flow into the diapir and diapiric rise during the ice advance, while a transgression of the diapir by the ice sheet leads to overall downwards displacement. During unloading, displacements are largely restored due to the dominance of the elastic response. Displacements never exceed few metres and are always larger in models with linear viscosity than in those with non-linear viscosity. Linear viscous salt behaviour seems reasonable, considering the low differential stresses caused by the load of a few hundred-metres-thick ice sheet and the time-scale of several thousand years. The elastic parameters also have a strong impact, with lower Young's moduli leading to larger displacements. Our findings demonstrate that both the viscosity and the elasticity exert a fundamental control on ice-load driven salt movement during glacial-interglacial cycles and highlight the importance of a careful parameter choice in numerical modelling.