TIMING OF LATE CRETACEOUS FAULT-FOLD INTERACTIONS, EAST KAIBAB MONOCLINE, UTAH: IMPLICATIONS FOR THE ONSET OF LARAMIDE DEFORMATION

Examination of timing and structural compatibility of deformation bands and slickenlined fault surfaces in Late Cretaceous strata near the northern end of the East Kaibab monocline reveals that two enigmatic northeast-striking faults developed as normal faults simultaneously with Laramide monoclinal folding.

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.