THE INFLUENCE OF PRE-EXISTING BASEMENT STRUCTURE ON LARAMIDE-AGE CONTRACTIONAL DEFORMATION: EVOLUTION OF THE STILLWELL ANTICLINE, WEST TEXAS

The influence of pre-existing structures upon subsequent deformation has been well-documented at many localities along the length of the Laramide orogeny. In west Texas, workers have demonstrated that earlier rifting and orogenic events have likely controlled Laramide-age and younger fold and fault systems within the northwest-trending Texas lineament. On the eastern margin of the lineament, the geometry of the Laramide-age Stillwell anticline suggests an origin associated with basement reactivation of an en echelon, high-angle normal fault system.

The 8 km long fold system consists of 3 northwest-trending, left-stepping en echelon segments, with fold amplitudes that decrease near the boundaries between segments. The geometry of the northeast-vergent, asymmetric anticline most commonly consists of a shallow, southwest-dipping backlimb and a steep northeast-dipping to vertical forelimb. Cross-sections constructed based on field and remotely-sensed data reveal two important features of the system: 1) subhorizontal stratigraphy to the southwest of the anticline is 50 – 100 m higher than stratigraphy to the northeast of the anticline; and 2) fold geometries along significant portions of the anticline are consistent with formation by fault propagation along a subhorizontal decollement and a shallow, northeast-vergent ramp.

These features permit inference that the fold system is cored at depth by a high-angle, southwest-dipping, en echelon reverse fault system, with 50 – 100 meters vertical throw. Importantly, this fault system was likely a reactivated normal fault system in the Paleozoic basement, approximately 500 m below the present-day surface. As reverse displacement was initiated by east-northeast directed compression, upward propagation of each fault segment must have been strongly influenced by the mechanical stratigraphy of younger, well-bedded Mesozoic marine units. These variations in mechanical strength controlled refraction of the upward – propagating faults, resulting in the flat-ramp geometries present along significant portions of the overall system. Thus, the orientation and complex geometries of the Stillwell anticline fold system appear to have been controlled by a combination of pre-existing crustal weaknesses and stratigraphic variations in mechanical strength.