Our inability to adequately image or accurately estimate fault dimensions in the subsurface represents a substantial knowledge gap with wide-ranging implications, from predicting natural or induced seismicity to quantifying fault penetration probabilities critical to subsurface containment or movement of fluid at sites for waste disposal, CO2 sequestration, gas storage, and resource (hydrocarbon, geothermal) extraction. Here we leverage outcrop exposures of normal faults in sedimentary successions to directly observe and quantify heights and displacement patterns of naturally occurring faults in mechanically layered rocks. Field-based outcrop characterization, high-resolution photogrammetry, digital fault and horizon mapping, and X-ray diffraction (XRD) mineralogy analysis were used to characterize interactions between mechanical stratigraphy and fault geometry at study sites encompassing a wide range of fault displacements (centimeter to decameter), a variety of sedimentary rock types (e.g., sandstone, siltstone, mudstone, carbonate, and volcanic ash units), and varied depositional environments, structural settings, and geologic ages of exposed strata. Digital fault mapping and fault displacement measurements were used to quantify fault displacement gradient for exposed faults, and companion XRD mineralogy data allowed relationships between fault displacement gradient (change in displacement over distance along fault) and bulk rock XRD mineralogy to be quantified.
Fault displacement gradient vs. XRD mineralogy regression analyses generally demonstrate (i) positive correlations for displacement gradient vs. total clay, (ii) negative correlations for displacement gradient vs. total carbonate, and (iii) negative correlations for displacement gradient vs. combined quartz and feldspars. Generally negative correlations between fault displacement gradient and calcite, dolomite, quartz, and feldspars are interpreted to reflect dominantly brittle deformation in rock layers with higher percentages of these strong minerals. In contrast, generally positive correlations between displacement gradient and total clay reflect a trend for increased ductile deformation within rock layers containing greater proportions of weak minerals. Deviations from the general patterns described above are interpreted to be caused by (i) structural complexities such as abutting or overlapping faults influencing displacement gradients, (ii) bed-scale variations in mineralogy that are not captured by collected samples and associated XRD analyses, and (iii) other mechanical stratigraphic factors (e.g., weak bed interfaces and variations in bed thicknesses) that may result in more ductile deformation behavior of the overall rock mass than is predicted by XRD mineralogy data alone.