Timing
of Late Cretaceous fault-fold interactions, East Kaibab
Monocline, Utah: implications for the onset of Laramide
deformation
Bernard,
Jonathan, Jenesky, Timothy, Tester, Edward, Orsulak, Megan,
Tindall, Sarah E.,
and Simpson,
Edward
L., 2006, Timing
of
Late
Cretaceous
fault-fold
interactions, East Kaibab Monocline, Utah: implications for the
onset of Laramide deformation [abs]: Rocky Mountain
Section–58th Annual Meeting (17–19 May 2006), Geological Society of
America Abstracts with Programs, Vol. 38, No.6, p. 38..
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.