UNRAVELING GEOMETRY: TAKING THE CONES OUT OF PERICLINES

Investigation and interpretation of folded rocks utilizes geometrical analytical techniques to describe their three-dimensional geometry. Recent technologic and software developments allow for robust evaluation of folds from large pseudo-3D (e.g. 2.5D) to fully 3D datasets. However, given that perfectly exposed folds are rare, collecting high spatial resolution datasets is difficult at best. This warrants the traditional approach of fold characterization, of lower spatial resolution field datasets, utilizing pseudo-3D methods of domain analyses with stereographic projection and tangent diagrams. While these methods show merit for many analyses (e.g. determining cylindricity, finding preferred orientations, etc.), for many fold geometries they yield equivocal interpretations. For example, a periclinal fold can be misrepresented as a conical fold if the entirety of the structure is not given adequate analysis.

This study utilizes high accuracy, high spatial resolution, “strike and dip” measurements extracted from a virtual periclinal fold. This virtual field data is analyzed by various analysis techniques (stereographic projection, tangent diagrams, SCAT analysis, and three-dimensional curvature) via Midland Valley’s Move™ software, along with a traditional interpretation of actual field data. The results highlight the highly equivocal nature of traditional fold analysis utilizing low spatial resolution field data. For example, using the traditional approach, periclinal folds are shown to form similar patterns on stereonets and tangent diagrams as conical folds in certain exposure scenarios. To circumvent ambiguity in fold characterization we propose adaptation of a combination of analysis techniques is critical, especially for non-cylindrical folds, which is the common reality of geologic structures. Stereographic analysis and tangent diagrams augmented by SCAT analysis, and/or three-dimensional curvature, can yield a more rigorous interpretation and thus, a better description of the actual 3D geometry of the fold. Adaptation of this approach should result in more representative geometry of folded surfaces, which in turn will enable a more thorough analysis of the underlying deformation mechanism.