Interaction of polygonal fault systems with salt diapirs

Salt diapirs are some of the most dynamic geological structures in sedimentary basins and vertical rise through their overburdens leads to the development of complex fault systems. Polygonal fault systems constitute a major structural element of fine-grained sediments in sedimentary basins, forming without the requirement of tectonic extension during the early burial phase of sediment compaction and dewatering. These same fine-grained sedimentary overburdens are also deformed by salt diapirs and their associated fault systems in basins such as the Gulf of Mexico, the South Atlantic margins and the North Sea. The aim of this research was to investigate the interactions between salt diapirism and faulting in overburdens deformed by polygonal fault systems using 3D seismic data sets from the Central North Sea and the extensional domain of the Espírito Santo Basin on the SE \ncontinental margin of Brazil. \nIn both case studies the regional isotropic planform arrangements of polygonal faults mapped in the post-salt overburden have preferred (radial) orientations around salt \nstocks, orthogonal alignments with tectonic faults and salt walls, and concentric arrangements in withdrawal basins. Radial faults around salt stocks are invariably layer-bound, conforming to the same discrete layer of stratigraphy as laterally equivalent to layer-bound polygonal fault systems. In the Espírito Santo Basin, the \nlateral distribution and stacking of polygonal faults is heavily influenced by the distribution of proximal and distal seismic facies. In the Central North Sea spacing, \nthrow and fault length vary as a function of tier thickness. \nPolygonal and radial faults in the same tier have a similar range of maximum throws and spacing but differ in length and aspect ratios. Radial faults are classified as perturbed members of the basin-wide polygonal fault system which propagate primarily under the influence of compaction and contraction but in an anisotropic stress field. \nStacked arrays of layer-bound radial and polygonal faults formed sequentially where upper tier boundaries date the cessation of fault activity. The radial fault zone is between 2-4 stock radii wide. The radial fault zone expanded as salt growth intensified or widened, and contracted as they slowed or became narrower. However, \nthe width of the radial fault zone is not related to the hinge in the domed overburden discrediting arching as mechanism forming the radial faults. Instead, the transition \nboundary separating zones of radial and polygonal faults in a tier is interpreted to reflect the lateral-extent of hoop stress around salt stocks during faulting. \nAn upward change in the regional polygonal planform and dip polarity of polygonal faults in the Espírito Santo Basin is attributed to a change in the regional stress field during the cessation of thin-skinned extension and gravity gliding and the onset of inversion. The results of this thesis highlight the sensitivity of polygonal fault system to local stress anisotropy and provide a potential route for reconstructing the palaeostate of stress around salt diapirs.