Salt tectonics above complex basement extensional fault systems: results from analogue modelling

Abstract

Physical model studies of salt tectonics related to thick-skinned extension have been largely confined to 2Dstructures. An experimental programme, utilizing silicone polymer and silica sand/ceramic beads as ductile and brittle analogues respectively, was designed to investigate the 3D relationships between salt diapirs and intersecting basement fault systems seen in the Central Graben, UK North Sea.During extension, intersection points in the basement fault system generate complex single, two-way or three-way, flap structures in the overburden, localizing deformation and footwall diapiric activity on the rift margins. Flap structures and associated diapirs are located adjacent to, but diagonally inboard of, the basement intersection points and consist of single, or multiple, convex-to-the-hanging-wall fault segments that are gradually breached with increasing extension. Major diapirs that attain passive status accommodate much of the continued basement extension through down building processes. Intra-basin horst systems develop inwardly dipping graben, cored by a major salt wall, during initial extension. Axial flow along this structure feeds growing diapirs. Grounding of the brittle overburden results in source cut-off and deflation of the salt wall due to continued expansion by extensional faulting.During subsequent inversion major diapirs are reactivated, exhibiting rapid active rise through the overburden, suggesting that buoyancy forces associated with diapirs play a major role in their reaction to subsequent tectonic stresses. Basin margins with major corner point systems develop highly segmented inversion fronts related to the localized nature of brittle structuring during extension. Diapir crests and entrained salt bodies are nucleation sites for the development of brittle reverse faults. Basin margins with minor corner systems commonly demonstrate reactivation of the basin margin fault system, producing a linked inversion front. The models display close similarities to structures seen in the UK Central North Sea and other salt basins, and have important implications for basement control on diapir locations during post-salt basin extension.