Conjugate
Ferrill, David A.1,
Alan P. Morris1, Judith McIntyre2, Iain Sinclair2
(1) Southwest Research Institute®, San Antonio, TX (2) Husky Energy, St.
John's, NF
Crossing conjugate normal faults are
common in hydrocarbon settings. In extension, and in the absence of factors
that might induce a preferred direction of fault dip (preexisting strength
fabric, dipping basal detachment, steep basin margin), normal faults with
opposite dips often develop. This conjugate fault pattern can cause sequential
cross-cutting of faults. Crossing conjugate normal faults commonly occur where
bending strain (hanging wall rollovers, salt-related turtle structures) causes
formation of crestal grabens.
Crossing conjugate normal faults also form between major overlapping parallel
normal faults (displacements of bounding faults are ~10x
greater than the crossing faults), where they accommodate distributed layer
extension. Cross-cutting faults occur in a synthetically dipping panel between
parallel bounding faults or in the damage zone associated with one major fault.
We characterize cross-cutting conjugate normal faults at a range of scales from
less than a meter (subseismic) to greater than tens
of meters displacement (seismic scale) in outcrop (limestone, unlithified clastic sediments,
volcanic tuff) and seismic settings (clastic
sedimentary strata from Terra Nova Field, Newfoundland). Where layer rotation
occurs during and after formation of, and slip on, the conjugate fault system,
faults that are antithetic to the bounding faults rotate to gentler dips while
faults synthetic to the bounding faults progressively rotate to steeper dips.
Crossing faults may remain active throughout this rotation despite their
inappropriate orientation with respect to far-field stresses, because they
continue to accommodate layer-parallel extension imposed by propagation of and
displacement along bounding faults. Crossing normal faults dramatically thin
the section that they displace and are associated with zones of extreme damage.
AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California