FAULT REACTIVATION MECHANICS AND MID-LARAMIDE EVOLUTION OF THE EASTERN COLORADO PLATEAU: NEW INSIGHTS FROM THE BOWIE AREA, WESTERN COLORADO

A shift in fault style between Late Cretaceous NW-trending normal faults and Eocene ENE-trending normal faults, and face cleat in coal, may indicate a major shift in paleostress axes with the onset of the Laramide orogeny in the eastern Colorado Plateau. In addition, these Eocene faults and face cleats record left-lateral oblique-slip structures, implying a counterclockwise rotation of the major far-field horizontal stress axis during the Laramide.

A reverse-reactivated, penecontemporaneous normal fault in the Bowie No. 2 and No. 3 coal mines exhibits a unique style of faulting with two fault planes, separated by up to 15 m, bounding a fault block with beds rotated between 35° and 45°. Fault throw on either plane is variable, ranging from 15 m reverse to 10 m normal on either plane; however, the cumulative throw (calculated by extrapolating hanging wall and footwall coal seams across the fault zone) is everywhere normal with an average of 9 m. Fault reactivation was selective and appears to have alternately favored both fault planes.

The estimation of fault offset is complicated by the presence of a footwall fold within 300 m of the fault plane with a backlimb dip of 12° into the fault. Where the fold is pronounced, reverse fault throw approaches 15 m. However, extrapolation of unrotated beds on both sides of the fault yields a cumulative normal throw of ~9 m, consistent with that measured across the fault. Observations of this and other reactivated faults suggest that footwall folds may be a relatively common phenomenon in fault reactivation, yielding insights into fault reactivation mechanics and a potential tool for detecting reactivated faults in the field. Preliminary numeric modeling suggests that a controlling factor in the formation of a footwall fold is the direction of fault propagation. Stress shadows on early normal faults or the presence of overpressure, among other things, may cause reactivated faults to propagate downward, creating features like those observed at Bowie.