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.
