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