Discrepencies Different researchers use different cut-off points for what we term low-angle faults. We will go with the text definition of dips <30° Fault workers commonly refer to lenses in fault zones as horses when they are m-km scale structures Triangular features developed by erosion of fault surfaces on fault-bounded mountain fronts are fault facets. Flatirons are morphologically similar structures developed by erosion of tilted sedimentary layers

Finding fault(s) How to evaluate whether a fault is present in the subsurface or an area of poor exposure

Fault….but may be Abrupt lithologic change Abrupt change in orientation of bedding or foliation In both cases, look for evidence of fault-zone deformation with proximity to contact Nonconformity - look for evidence such as basal conglomerate Angular unconformity (see above) or Fold (look for lithologic continuity)

Use of stratigraphy Faults can cut out (omit) or stack (repeat) stratigraphic intervals. Stratigraphic omission or repetition can be evident through mapping or drill core.

Sense of slip in the absence of piercing points Relative ages of hanging wall and footwall materials constrains sense of slip Metamorphic grade of hanging versus footwall constrains kinematics

Heterogeneity and anisotropy Fractures typically nucleate on heterogeneities (e.g., fossil) Low-angle reverse, or thrust, faults localize in mechanically weak layers such as shale and evaporite. They cut across stronger layers in short steps As long as it is favorably oriented, it is apparently energetically easier to reactivate an existing fault than create a new one

Pattern has meaning Large-scale fault patterns reflect tectonic environments (plate movements) Patterns reflect most energetically favorable response to imposed displacement We will look at overall strains and patterns of faulting in a different order than indicated in syllabus: extensional, transcurrent, then contractional regimes