Coupling of supra-to sub-salt structures during gravity gliding: A physical analogue modelling investigation

Abstract

Viscous salt layers can introduce significant structural complexity to the basins in which they are deposited. Along basin margins, salt typically flows basinward due to regional tilting of the margin, and can be influenced by the geometry of the surface that it flows over (e.g. fault scarps on the base-salt surface). This interaction can lead to coupling of sub- and supra-salt structures, with the orientation and distribution of base-salt structures reflected in the structure of the overburden. However, the controls on the degree of strain coupling during salt-detached translation are relatively poorly understood, in particular the roles played by the thickness and mechanical heterogeneity of the salt unit. This is, in part, caused by difficultly in deconvolving the relative contribution of variables such as salt thickness, the magnitude of base-salt relief, sediment supply, and regional tectonic regime. In addition, seismic reflection data provide only the present structure of the basin, from which its evolution must be inferred.To evaluate the influence of salt thickness and heterogeneity on sub-to supra-salt strain coupling during salt-detached translation in the extensional domain, we present a series of physical analogue models with controlled boundary conditions. We use a model apparatus with a simple geometry comprising three oblique basal steps, and vary the thickness and composition of the salt analogue in each experiment to evaluate the proposed variables. X-ray tomography allows us to image the internal structure during model evolution and therefore gain a 4D view of its structural development. Results show that supra-salt structures associated with thicker and more homogeneous salt units are characterised by symmetric extensional structures and large diapirs, with significant vertical and lateral displacements and only weak coupling to the underlying base-salt relief. Conversely, thinner and more heterogeneous salt units are characterised by asymmetric extensional structures and primary welds, with restricted vertical displacement, such that the resultant overburden structure is strongly coupled to the geometry of the base-salt surface. These results document the important role of base-salt relief in the structural evolution of salt basins and provide model analogues that are valuable assets in seismic interpretation efforts on salt-influenced basin margins.