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 s outhern Utah suggest that Late Cretaceous/early Tertiary development of the structure involved a significant component of right-lateral strike-slip di splacement, 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. Dominan t fault orientations are synthetic and antithetic to the shear zone, and ac commodate 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-d irected, Laramide compression drove reverse-right-lateral slip on the basem ent 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. (C) 1999 Elsevier Science Ltd. All rights reserved.