Monocline development by oblique-slip fault-propagation folding: the East Kaibab monocline, Colorado Plateau, Utah

Fault relationships along a 50-km stretch of the East Kaibab monocline in southern Utah suggest that Late Cretaceous/early Tertiary development of the structure involved a significant component of right-lateral strike-slip displacement, accommodated by basement-rooted faulting and fault-propagation folding. Evidence of oblique slip is provided mainly by pervasive map-scale and outcrop-scale faults that define a shear zone occupying the steep east-dipping limb of the monocline for at least its northernmost 50 km. Dominant fault orientations are synthetic and antithetic to the shear zone, and accommodate reverse-right-lateral and reverse-left-lateral slip, respectively. Structural style within the shear zone changes character and increases in intensity with progressively lower structural and stratigraphic levels in the fold, suggesting that the shear zone propagated upward from a basement-rooted fault during monocline formation. We conclude that horizontal, ENE-directed, Laramide compression drove reverse-right-lateral slip on the basement fault zone beneath the developing East Kaibab monocline. The resulting transpressional fault-propagation fold is marked in southern Utah by 1600 m of reverse displacement and possibly 8000 m of right-lateral displacement across the shear zone and associated monoclinal flexure.