--> Abstract: Conjugate Normal Faults and their Role Within Fault Blocks and Fault Damage Zones; #90063 (2007)

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Conjugate Normal Faults and their Role Within Fault Blocks and Fault Damage Zones

 

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