KINEMATIC DEVELOPMENT OF THE STILLWELL ANTICLINE, WEST TEXAS

We use field data, cross-section construction, and detailed analysis of fault and fracture systems along the length of the northwest-trending Stillwell anticline, west Texas, to demonstrate the relationship between the geometry of fault-propagation folding and the accommodation of strain during the evolution of the anticline system. Geologic map data support a northeast-vergent anticline cored by a reactivated, high-angle, left-stepping en echelon fault system, which divides the anticline into three well-defined segments. We constructed cross-sections across these segments that reveal fault propagation folding with kink-band kinematics, consisting of a planar frontal ramp propagating upward from a flat decollement. At different locations along the fold system, geometries include: 1) an asymmetric, east-vergent anticline with a shallowly-dipping backlimb and a steep forelimb; 2) an asymmetric box fold, with a shallowly-dipping backlimb, a short, sub-horizontal middle limb, and a steeply-dipping forelimb; and 3) an asymmetric box fold, with a shallowly-dipping backlimb, a long sub-horizontal middle limb, and a steeply-dipping forelimb. We consider these geometries to be temporal increments of fold evolution, with the magnitude of slip along the underlying fault system increasing from fold geometries 1 to 3. Within the hinge zone of the anticline at several locations, we documented outcrop-scale (meter-scale displacement) and bed-scale (cm-scale displacement) ramp-flat fault geometries, which accommodate significant shortening and increase the intensity of fracturing at those locations. Detailed analysis of fault formation supports a model where bedding-parallel slip dominated early in fold development, with the formation of ramps within the hinge zone occurring only after significant bed rotation. Most joint and shear fracture sets were initiated prior to or during fold formation, with greater fracture intensity associated with tighter interlimb angle. Based on this relationship, we plan to constrain the most likely locations of maximum fracture formation at each stage of fold evolution utilizing computer kinematic modeling. Initial modeling validates the ramp-flat mechanism, but further testing is needed to constrain geometries of subsurface faulting.