| Faults and fractures are ubiquitous structures in the Earth's crust. The evolution of fault systems and their impact on fluid flow may be evaluated through analysis of the petrophysical properties of the structural elements that comprise a fault and how they vary spatially and temporally. In this study, I use the structural components of faults to distinguish faults that formed by distinct deformation mechanisms. This study investigates fault zone evolution by contrasting faults formed from a single mechanism in sandstone, faults formed by two mechanisms in sandstone, and faults formed by three mechanisms in faulted sandstone-shale stratigraphy.; In sandstone, two principal faulting mechanisms localize deformation: (1) cataclastic deformation band faulting, which involves pore collapse and grain-scale fracturing, and (2) joint-based faulting, which involves cyclic jointing and subsequent shearing along joints that produces multiple generations of splay fractures. In sandstone-shale sequences faulting in the shale proceeds by penetrative shear that causes rotation and attenuation of the shale unit and might also result in larger scale folding. Shale gouge develops along the contact between the shale smear and fault plane. Each of these processes produces a distinct set or assemblage of structural components providing the basis to infer the mechanism of fault formation in the field. In addition, these deformation mechanisms have important mechanical requirements and implications. Thus, basic concepts of fracture mechanics can be used to infer what mechanism is likely in the subsurface, enabling predictions of subsurface fault architecture.; Faults in clastic rocks develop by one or more of these mechanisms. This is consistent with the growth of faults and propensity for established faults to be reactivated so that a fault may experience a variety of deformation conditions or fault multiple rock types. In detail, the type, distribution and density of elemental structures along a fault is related to the fault's geometry, slip distribution, rock type and the state of stress based on the physical processes of fault formation. |
