The growth and evolution of polygonal fault tiers

Polygonal faults are layer-bound arrays of normal faults confined to specific stratigraphic intervals called tiers. Typically hosted in fine-grained sediments, polygonal faults are thought to have the potential for fluid leakage and represent a potential seal bypass mechanism. Intergral to understanding the impact of polygonal faults on regional top seal is timing and evolution of polygonal fault tiers. Whilst there are numerous studies imaging and describing polygonal faults in numerous basins around the world, very few specifically consider the growth of polygonal faults. Additionally, very few polygonal fault studies examine the evolution of fault hierarchies and how these hierarchies accommodate strain and deformation within the tier. This study examines two wedge-shaped polygonal fault tiers on different passive margins. The first polygonal fault tier studied is from offshore Angola and is hosted in sediment thickness of less 500 m, in Plio-Pleistocene claystones. The second tier examined is from the Modgunn Arch, Norwegian Margin and is hosted in a sequence of siliceous oozes and claystones of Eocene to Pliocene age, up to 1200 m in thickness. This study uses the differences in sediment ages to compare and contrast polygonal fault tiers at different stages of evolution and aims to examine common features between young and mature tiers in order to constrain tier evolution. This study considers two aspects of polygonal fault growth. Firstly it considers the utility of perturbations in polygonal fault tiers around features of the base of the tier. These perturbations have the potential to be a useful proxy for elucidating the nucleation position of polygonal faults in the tier. The second aspect of polygonal fault tier evolution reviews the mechanisms that control organisation hierarchies within a tier and examines the role of linkage in creating large master polygonal faults. The results of this thesis show that the polygonal faults can initiate at low temperatures and low pressures, in sediment thickness of 100 m or less. Moreover, it can be demonstrated that polygonal fault tips propagate preferentially upwards with basal tips pinned by a mechanical boundary at the base of the tier and lateral tips pinned by early forming branchline intersections. The transition from shallow to deep burial shows that polygonal fault hierarchies are naturally forming and may relate to variation in the propagation rate of polygonal faults within the tier. With increasing burial, there 3 is an increased likelihood of branchline interaction. Branchline interaction can occur laterally with lateral tips abutting against other faults in the network. Vertical abutments occur where upwardly propagating tips interact to form triangular abutments and are a key mechanism forcing in the organisation of polygonal faults and growth of Master faults. Both abutment styles also impact the spatial distribution of displacement maxima. Polygonal fault tiers also show subtle spatial variablilty in orientation and linkage mechanisms that can be attributed to the distribution and magnitude of far field stresses as well as the early fault dimensions. This thesis also presents a primie facie case for diachronaity in polygonal fault growth driven by variations in sediment loading and branchline linkage complexity. Overall, this thesis describes and accounts for some key behaviours of incipient polygonal faults tiers