ANALYSIS OF FINITE FOLD GEOMETRY AND VARIATIONS IN STRAIN BASED ON STRUCTURAL POSITION: A CASE STUDY FROM THE STILLWELL ANTICLINE, WEST TEXAS

The analysis of folds and fractures on the surface is critical for our understanding of subsurface fault geometry and evolution. A well-exposed fold in west Texas, the asymmetric Stillwell anticline, is NW-trending, 8 km long, and exhibits a left-stepping en echelon pattern with three prominent segments. The highest fold amplitudes are near the center of each segment, with amplitudes that decrease approaching segment boundaries. The deformation of the well-bedded Cretaceous carbonate rocks that define anticline geometries is especially prominent where the anticline has been dissected by erosion. Previous studies suggest that the anticline is cored by a basement-involved fault system that is best modeled using trishear kinematics.

We used field data, field photography, and photogrammetry to compare fold geometries and strain accommodation at different locations along the anticline system. With photomosaics of the forelimb and hinge zone, we identified zones of maximum shear strain and associated deformation mechanisms. A high-resolution 3D model of a particularly well-exposed cross-sectional view of the anticline was created using Agisoft Photoscan photogrammetric software. This software enabled us to analyze areas of the outcrop that were inaccessible to better constrain variations in fold and fault geometry and fracture intensity.

We documented significant variation in fold geometries along the axis of the anticline system. The northern and southern fold segments exhibited moderately-dipping (20 – 30 degrees) backlimbs, long shallowly-foreland-dipping midlimbs, and steep (>60 degrees) forelimbs. Locations near the central zone of the fold had more gently-dipping (<20 degrees) backlimbs, short to non-existent midlimbs, and steeply-dipping forelimbs. At all locations, the hinge zone and the forelimb exhibited the greatest strain, consistent with previous 2D modeling results. These structural positions exhibited ramp-flat fault geometries, intra-bed faulting, intense fracturing, and zones of brecciation. This study better constrains the distribution of strain at different stages of fault-propagation folding, demonstrates the power of photogrammetry in structural studies, and provides a well-documented model for the interaction of fold segments in complex fault-cored systems.