Abstract - The thicknesses of fault rock and fault zones and the fault normal separations for intact and breached relay zones each show a positive correlation with fault displacement. The displacement to thickness ratio for these different structures increases from intact relay zones (median value = 0.28) to fault rocks (median value = 50). The frequently recorded positive correlation between fault displacement and fault rock thickness is often interpreted as a growth trend controlled primarily by fault rock rheology. However recognition of similar correlations for the other fault components suggests a geometrical model may be appropriate. In this model a fault initiates as a segmented array of irregular fault surfaces. As displacement increases, relay zones separating fault segments are breached and fault surface irregularities are sheared off, to form fault zones containing lenses of fault bounded rock. With further displacement these lenses are progressively comminuted, and ultimately converted to zones of thickened fault rock. The final fault rock thickness is therefore influenced strongly by fault structure inherited from the geometry of the initial fault array. The model is one of progressive strain concentration within a zone within which the active fault surface progressively approaches, albeit along a potentially complex path, a more planar geometry. The large scale range on which fault segmentation and irregularities occur provides the basis for application of this model over a scale range of 8 orders of magnitude. The model is consistent with outcrop observations of the internal structure of fault zones, such as the lack of a clear variation in fault rock thickness with lithology and large variations in fault rock thickness observed for a given displacement, and with recently developed discrete element models of fault zone evolution.
Abstract of talk given to:
Fault Zones Structure, Mechanics and Fluid Flow, Geological Society of London, September 2008.