2D COMPUTER KINEMATIC FORWARD MODELING OF THE STILLWELL ANTICLINE FOLD SYSTEM, WEST TEXAS: TESTING MODELS OF FOLD EVOLUTION

We performed 2D forward kinematic modeling to constrain the fault system beneath the Stillwell anticline, west Texas. The fold is defined by Cretaceous marine units, with the present-day surface approximately 600 m above Paleozoic basement. The anticline is a NW-trending, NE-vergent, 8 km long, 500 m wide system that displays a left-stepping en echelon pattern with flat-ramp fault-propagation fold geometries, including a gently dipping back-limb, a middle-limb, and a steeply dipping forelimb. However, the 8-12° NE dip of the mid-limb, the 50-100 m vertical offset between hinterland and foreland, and the linear, localized deformation are not consistent with a simple, shallow, flat-ramp fault propagation origin. We hypothesize that reactivation of high-angle faults in the basement were necessary to produce these features. We tested fault-related fold formation using FaultFoldForward (FFF) to constrain relative timing and a possible link between basement and flat-ramp faults.

FFF uses algorithms governed by trishear kinematics that permit folds to develop in a triangular zone of distributed shear that expands ahead of a propagating fault tip. FFF permits manipulation of 5 values: ramp angle, trishear angle, fault slip, propagation-to-slip ratio (P/S), and fault tip location. We first tested the reactivation of a high angle basement fault (Stage 1) that might produce the mid-limb dip, foreland-hinterland offset, and narrow region of deformation as constrained by field data. Our best-fit model supports a reactivated basement fault system initiated approximately 600 meters below the present-day surface, with a vertical offset of approximately 80 m, a 60° ramp angle, 92 m of fault slip, a 35° trishear angle, and a P/S of 1.5.

The best-fit Stage 1 model became the starting point to model a shallow flat-ramp fault-propagation fold (Stage 2) that matches field observations. Systematic testing yielded a range of variable values that successfully produced fold geometries consistent with those documented in the field. Significant discrepancies between Stages 1 and 2 fault slip values and spatial constraints on fault geometries support an asynchronous, 2-stage model of fold formation, with an early-forming monocline acting as a nucleation point for later folding generated by a shallow flat-ramp fault system.