MECHANICS OF CONTRACTIONAL-STEPOVER FORMATION, REGIONAL STRESSES, AND BASEMENT-FAULT REACTIVATION ASSOCIATED WITH THE HUMBOLDT FAULT IN KANSAS

A contractional stepover of upper Pennsylvanian rocks mapped in Pottawatomie County (northeastern Kansas) reveals information on the state of stress associated with the reactivation of basement faults. The contractional stepover is a 2-km² uplifted area mapped at the surface that occurs within the stepover region between two northeast-trending echelon right-lateral faults, one of which is 9-km long. The contractional stepover is bounded at the ends by north-northwest-trending reverse faults. The highest relief on the structure ranges from 5 to 10 meters and occurs near the reverse faults. The vertical displacement along each reverse fault is greatest at a strike-slip fault termination and decreases toward the opposite strike-slip fault. The boundary-element method was used to model the deformation in and adjoining the contractional stepover. The fault geometry used in the models matched that observed in the field and incorporated all known fault segments. The model results indicate that the greatest uplift occurs inside the contractional-stepover region at the strike-slip fault terminations. The uplift remains high along the reverse faults. Although the central portion of the contractional stepover is higher than the adjoining areas outside the contractional stepover, the model indicates that the central area is lower than the areas along the reverse faults, forming an elevated syncline. Varying the trajectory of the remote stresses changes the deformation pattern and the pattern of vertical displacement along the reverse faults. A comparison with the configuration of the mapped contractional stepover indicates that the trajectory for the maximum regional compressive stress was east-northeast during the formation of the contractional stepover. The surface faults overlie a known basement fault that is part of the Humboldt fault zone. The trend of the basement fault is north-northeast, and the strike-slip fault traces have a clockwise rotation with respect to the basement faults. This rotation is consistent with the expected rotation if the strike-slip faults formed as twist segments above a preexisting basement fault in the model-determined regional stress field.