A geometric model of fault zone and fault rock thickness variations

Abstract

The thicknesses of fault rock and fault zones and the fault normal separations for breached and intact relay zones each show a positive correlation with fault displacement. The displacement to thickness ratio, or average shear strain, varies for the different structures increasing from intact relay zones (median value = 0.27) to fault rocks (median value = 50). The correlation for fault rocks is widely interpreted as a growth trend controlled by fault rock rheology, but the progression of displacement to thickness ratios for the different structures suggests an alternative model. In this alternative model a fault initiates as an array of irregular fault segments. 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 large scale range on which fault segmentation and irregularities occur provides the basis for application of this model over a scale range of at least 7 orders of magnitude.