THE LOCALIZATION OF SHEAR STRAIN WITHIN A CONTRACTIONAL FAULT-PROPAGATION FOLD SYSTEM: A CASE STUDY FROM THE STILLWELL ANTICLINE, WEST TEXAS

We use the Stillwell anticline in west Texas (USA) to develop a strain-predictive model of contractional fault-propagation fold formation in layered carbonate rocks. The Laramide-age anticline, located within the Texas lineament, is a NW-trending, NE-vergent, asymmetric fold. We integrate field observations, geologic and structural data, 2D kinematic modeling, and 3D fracture network models to establish how strain is accommodated during fold formation.

Previous researchers established a two-stage model of fold formation, including: 1) reverse reactivation of a pre-existing, normal fault system in Paleozoic basement rocks to generate monoclonal flexures in overlying Cretaceous carbonates; and 2) the formation of a subsequent flat-ramp fault system that propagated horizontally along a mechanically-weak unit before ramping up at the hinge of the pre-existing monocline. We use a combination of 2D kinematic modeling and outcrop observations to show that shear strain is focused within the forelimb of the system, in front of the propagating fault tip, and is accommodated by a combination of interlayer slip, flat-ramp faulting at both the cm- and the m-scale, and fracturing proximal to planes of slip.

To evaluate bed-scale deformation, we collected oriented samples from structural positions across the anticline, from the same stratigraphic unit and from beds of similar thickness. We used a new “slice-and-scan” technique and medical imaging software (ImageJ) to build 3D models of the fracture networks within each sample. Fractures in the forelimb sample follow a low angle path relative to bedding, with fractures that both follow intrabed layering and cut across intrabed layers at a low (~30 degree) angle, similar to fault systems at the outcrop scale. In contrast, fracture network models built from other structural positions displayed fractures formed almost exclusively at a high angle to bedding, similar to outcrop-scale fractures documented by previous researchers at those structural positions. These mm-scale findings are geometrically consistent with outcrop-scale observations at the cm- to m-scale, suggesting a fractal nature to the accommodation of strain across the fold. These results can be used to predict strain accommodation across scales in less well-exposed fault-propagation folds elsewhere.