Mechanical controls on the spatial and temporal variability of faulting mechanisms in sandstone along the Moab normal fault, Utah

Segmentation is a fundamental characteristic of faults. However, the effect of segmentation on the process of fault development, the architecture of the fault zone, and the properties of faults are poorly understood. Along the Moab fault, a basin scale normal fault with ~1 km of throw in SE Utah, segmentation is associated with localized changes in the density and types of structures associated with faulting in sandstone. Changes in the types of structural elements are associated with fault development by two different mechanisms in sandstone: (1) cataclastic shear failure that produces deformation bands and (2) the repeated formation and subsequent shearing of joints that leads to the formation of a brecciated fault zone. Deformation bands are prevalent along the entire length of the fault system and band density is greatest within relays between normal fault segments that are subjected to a component of strike-parallel contraction. The joints and sheared joints only occur at intersections between normal fault segments and relays that are subjected to strike-parallel extension where they overprint deformation bands. We contend that spatial variation of the faulting mechanisms in sandstone is associated with the stress perturbation around the fault. We used the geometry and kinematics of the fault segments and an estimated burial depth of 2 km to simulate the mechanical behavior of the fault system in linear elastic boundary element models using Poly3D. We looked specifically for changes in the stress state that would cause a transition from deformation band formation to joint formation because joints are the youngest structural elements wherever they occur. Joints form normal to the least compressive principal stress when this stress exceeds the tensile strength of the rock. We also note that cataclasis in deformation bands represent a loss of volume, whereas jointing and breccia formation are dilatant processes. Consequently the mean stress can act as an indicator to distinguish locations favored for deformation bands versus those that might favor jointing. These simulations predict less compressive mean stress and least compressive principal stress localized at extending relays and intersections where joints are observed in the field. Furthermore, the orientations of joints predicted from the mechanical models correspond to the orientation of joints measured in the field. Conversely, locally more compressive mean stress is predicted in contractional relays where the highest deformation band density is observed in the field. We therefore propose that mechanical interaction between fault segments can cause a change in faulting mechanism, and thus control the distribution of structural elements along the fault. These mechanical interactions probably change as a fault grows or is exhumed leading to temporal evolution of the fault system such as the localized transition from deformation banding to jointing. The distribution of structural elements strongly controls a fault's permeability structure. A fault's permeability structure will likely develop differently in areas where the tips of fault segments interact in contrast to portions of fault segments that are isolated from the fault tips and other segments. The overprinting of deformation band-related by joint-related structural elements indicates temporal evolution of the fault system which should be associated with changing fault properties such as permeability. We therefore suggest that fault architecture, and thus permeability, will vary systematically as fault segments grow and interact.