Normal fault growth: mechanical controls, basin architecture and relay zone kinematics
Creators: name: Redpath, David
Publisher: name: Imperial College London
Year: 2022

description: The growth and evolution of normal faults is key to our understanding of rift evolution, the presence and distribution of earth resources and our ability to mitigate earthquake hazards. As such, their growth, geometry, and activity have implications for how we understand earth systems and its resources. Observations from 3D seismic data in sedimentary basins where synkinematic growth strata are present have inspired the propagating, constant-length, and hybrid fault growth models. In developing these models, it is often assumed that the host rock is isotropic and the basin ‘filled to spill’- simplifying assessments of fault throw. There has also commonly been an assumption that where two faults link, slip rates are enhanced time but this process is poorly understood and constrained. In this thesis, I tackle these outstanding questions by conducting seismic investigations across the Kwanza Basin, offshore Angola and the Exmouth Plateau, NW Shelf, offshore Australia, to improve our understanding of normal fault growth, by addressing three key research topics to: (i) evaluate the conditions under which fault scaling relationships may breakdown, (ii) develop a novel technique to measure fault growth in sediment starved settings and, (iii) analyse the behaviour of throw minima on breaching faults over geological timescales to better understand fault segment linkage. A high-quality 3D seismic reflection dataset on the Angolan segment of the West African passive margin was analysed. Evidence for mechanical stratigraphy controlling the dimensions of seismic-scale faults was identified, showing that an array of normal faults was vertically and laterally restricted by mechanical stratigraphy. In addition, their aspect ratios and throw distributions did not adhere to fault scaling relationships, despite their geometric data sitting within the parameters of a global D-L dataset. Thus, providing one of a few examples of mechanical stratigraphy’s control on seismic-scale fault dimensions using 3D seismic data. Extensive 3D seismic reflection datasets across the Exmouth Plateau were analysed to study degradation-fan complexes on basin-bounding normal faults. A new method for constraining fault growth in sediment-starved settings was proposed using volume-balancing of the degradation and fan volumetrics. This technique used the spatiotemporal distribution of footwall-derived fans to determine the evolution of fault tip propagation and mode of fault growth. Thus, developing an additional tool to evaluate fault growth where substantial growth strata are not present. In addition, 3D seismic reflection datasets were analysed to evaluate the throw rate variability on relay-breaching faults over geological timescales. The results suggest that maximum throw rates on relay-breaching faults exceed those of their adjacent segments but that they persist for a brief period of the faults life. Throw minima were not removed but reduced by throw rate variability on relay-breaching faults that had a strike oblique to their concomitant faults. This study is the first to reconcile observations on active, modern faults that show breaching rates that could remove throw minima in as little as &lt; 1 Myr, and those made on ancient, tectonic faults that show the presence of throw minima over geological timescales by suggesting that high throw rates on relay-breaching faults do not persist over geological timescales. The additional knowledge provided by this thesis: (i) helps to improve the application and understanding of fault scaling relationships by highlighting the need to use such data in context of their host rock properties; (ii) increases the ability to constrain fault growth in sediment starved settings and (iii) advances understanding of relay zone kinematics and informs our understanding of throw rate variability during segment linkage.