KINEMATIC EVOLUTION THE STILLWELL ANTICLINE SYSTEM, WEST TEXAS: IMPLICATIONS FOR FLUID FLOW WITHIN SUBSURFACE SYSTEMS

The kinematic and dynamic links between fold and fracture formation are critical for evaluation of subsurface fluid flow rates and pathways. Extensive research has focused on the correlation of fractures with final fold geometry, but other factors such as initial and transitional fold geometries and pre-existing fracture systems can also influence fracture formation. The well-exposed asymmetric Stillwell anticline, on the eastern margin of Laramide deformation in west Texas, provides an opportunity to develop a model of anticline formation that will serve as a framework for studies of fold-related fracture formation.

We use field data, cross-section construction, and computer kinematic modeling to demonstrate the relationship between documented fold geometries and the evolution of fault-propagation folding along the Stillwell anticline system. The fold system consists of three northwest-trending, left-stepping en echelon segments, with fold amplitudes that decrease near the boundaries between segments. Thus, cross-section geometries from different locations along the Stillwell anticline system capture different temporal snapshots of fold evolution, likely related to different magnitudes of displacement along the fault system that cores the anticline. These snapshots reveal a fold system that evolved according to classic kink-band kinematics, likely cored by three blind thrust faults with planar frontal ramps propagating upward from flat decollements. This model implies both displacement-parallel and strike-parallel propagation of a subsurface fault system, with linkage between these faults taking place at the left steps in the anticline system; fluid pathways would likely be dependent on the stage of thrust fault propagation and linkage. At locations with greater fold amplitudes, we documented significant meter-scale faulting and fracturing focused adjacent to the steep front limb of the asymmetric system, consistent with the position of greatest strain predicted by computer kinematic modeling of fault-propagation folding. This investigation constrains likely fluid pathways within the underlying fault system, suggests enhanced localized fracturing, and provides context for future studies of fracture formation in a well-constrained anticline system.