Fault Void Fills: Internal architectures of near-surface faults and insights into basement cover relationships during rifting

Natural fault systems are typically complex, with networks of cavities, which are filled with a variety of geological materials. During early rift development, major basin-defining faults may be connected to surface ruptures, where they are subject to near-surface processes, such as sedimentation and surficial fluid circulation. How faults systems are influenced by these processes can have major implications for the hydrodynamic properties of the faults and basement, and therefore the long-term development of economic resources.Recent numerical and analogue modelling studies have shown how fracture cavities and their fills collapse, grow, and deform as faults develop. By examining natural examples of fault voids fills from 5 selected locations, and comparing them to examples from sub-surface core, we aim to test the predictions of these models. This enables a better understanding of the geological controls, processes, and relationships surrounding the internal architectures and fills of faults which are active within a km of the surface.The fault voids studied here vary in scale from micro-cracks, to decametre wide fissures and in-fills. Despite the variety of different aspect ratios and apertures, fault voids consistently demonstrate the pervasive nature of infilling materials. For example, sedimentary material that is passively deposited into open cavities, has been routinely shown to occupy up to 45% of the bulk volumes of host rock, altering the expected mechanical and hydrodynamic properties of the unit.Fault void fills preserve valuable information on the regional structural evolution, in particular preserved fault-rock textures can give vital information regarding fluid migration. Traditionally fracture-vein associations assume fault-valving as a predominant formational mechanism, however observations of cockade and cavity filling mineralisation textures demonstrate that in the near-surface environment, open cracks remain permeable to fluid in the long-term, and the relationship between seismicity and fluid flow in the uppermost crust is highly variable.