PROCESSES OF FAULT DEVELOPMENT IN A LAYERED SANDSTONE AND SHALE SEQUENCE: AN EXAMPLE FROM THE MOAB FAULT, UTAH

Large basin-scale faults grow over long periods of time through changing deformation conditions and offset multiple rock types. Thus, the heterogeneity observed in many fault zones should be expected, but is not directly addressed by simple theories such as Mohr-Coulomb or simple statistical descriptions. To address the source of such heterogeneity, we examined the variation of fault rock and damage zone structures in sandstone and shale along the Moab fault, a basin-scale normal fault system with nearly one kilometer of throw. We found that fault rock and surrounding damage zone structures vary along strike and dip related to changes in the mechanism of faulting that coincide with changes in fault geometry and the lithology faulted.

In detail, the Moab normal fault is composed of structures resulting from three distinct mechanisms. In sandstone we differentiate two structural assemblages: (1) deformation bands, zones of deformation bands, and slip surfaces, and (2) joints, sheared joints, and breccia and related fine grained fault rock. These assemblages are the products of the deformation band-based mechanism and the shearedjoint-based mechanism respectively. Deformation band-based structures occur in sandstones exposed along the entire fault length. However, sheared joint-based structures preferentially occur in relays and intersections between fault segments and in folded sandstone. Where both assemblages are present, the sheared joint-based system is always younger. Where the fault crosses shale, shale is folded and attenuated along the fault zone rather than faulted, and gouge occurs along the fault slip surfaces. This assemblage is the product of the third faulting mechanism, ductile deformation of shale (smearing).

These faulting mechanisms comprise a conceptual model for fault growth and development consistent with spatial and temporal variation in fault components and faulting mechanisms. Furthermore, these mechanisms, together with knowledge of fault geometry and stratigraphy, provide a basis for predicting fault zone properties that ensures physically plausible fault zone architecture and is more robust than simple empirical or statistical descriptions of faults. We applied this methodology to the Moab fault as a 3D case study.