TIMING OF LATE CRETACEOUS FAULT-FOLD INTERACTIONS, EAST KAIBAB MONOCLINE, UTAH: IMPLICATIONS FOR THE ONSET OF LARAMIDE DEFORMATION
Deformation bands in sandstones of the Straight Cliffs and Wahweap formations represent four distinct sets within the steep limb of the monocline. Northeast-striking, northwest-dipping deformation bands parallel the prominent, northeast-striking faults and display right-handed offset. Northwest-striking, northeast-dipping bands with left-handed separation are conjugates related to the northeast-striking fault surfaces. Gently dipping bands formed by thrusting in the synclinal hinge, and bedding-parallel polished surfaces with dip-slip slickenlines indicate flexural slip during monoclinal folding. Mutual cross-cutting relationships among all sets provide strong evidence that the northeast-striking faults in the field area developed during growth of the East Kaibab monocline.
Apparent thickening of the Late Cretaceous Wahweap Formation on the southeastern side of each northeast-striking fault suggests the faults formed as southeast-dipping normal faults, and subsequently rotated into their current positions. Northeast-striking fault surfaces display right-handed separation, but become dip-slip normal faults when unfolded with the steep monoclinal limb. Stress directions inferred from paleo-focal mechanisms yield east-west horizontal compression for gently dipping deformation bands and north-south extension for the prominent faults and related deformation bands; stress directions are consistent with Laramide monoclinal folding. Future work will focus on confirming the presence of syntectonic growth strata in order to provide a tight constraint on the timing of onset of Laramide deformation.