2D AND 3D COMPUTER MODELING OF THE STILLWELL ANTICLINE FOLD SYSTEM, WEST TEXAS: TESTING MODELS OF FOLD FORMATION

The Stillwell anticline is a 500 m wide, 8 km long, well-exposed basement-cored fold system in the Big Bend region containing three NW-trending, NE-vergent, left-stepping en echelon segments with flat-ramp fold geometries. The area is characterized by bedded Cretaceous carbonate rocks, with the present-day surface approximately 600 m above the Paleozoic basement. Like most basement-involved, fault-related anticlines of the Laramide orogeny, the Stillwell anticline is characterized by narrow, steeply-dipping forelimbs expanding into gently-dipping backlimbs. We used 3D Move (Midland Valley) to perform 2D and 3D kinematic modeling of the fault-cored fold system to best constrain formation of the fold system.

3D Move uses geometric algorithms to provide efficient and viable proxies for deformation that follow traditional modeling approaches. Because we had no subsurface information, we used this software to build and test structural models with the goal of developing subsurface fault geometries consistent with the fold system we documented on the surface. We began by building a 3D model of the system through the integration of geologic 2D mapping results, existing 2D geologic cross sections, and digital elevation data. We then imported structural data to construct additional cross sections, and we incorporated hypothetical subsurface fault geometries consistent with surface fold geometries. Finally, we used 3D Move’s Unfolding and Move On Fault modules to restore the area to pre-deformation geometries, thus testing our interpretations of the fault-propagation fold system.

After adjusting our fault interpretations based on model results, we successfully constructed a robust 3D model of fault-cored fold formation. These results improve our understanding of the origin and evolution of Laramide age deformational structures in the Big Bend area and will permit us to evaluate lithologic responses caused by the formation and propagation of fault-cored folds in compressional settings.