Catherine L. Hanks¹, J. Ryan Shackleton¹, John Lorenz², Wesley K. Wallace¹

(1) University of Alaska, Fairbanks, AK
(2) Sandia National Laboratories, Albuquerque, NM

ABSTRACT: Origin of multiple generations of extension fractures in detachment folds of the northeastern Brooks Range, Alaska

Most published literature on the origin of regional extension fractures focuses on extension fractures in either flat-lying rocks or folded rocks. However, it is likely that fracturing occurs in a continuum with other semi-ductile structures as flat-lying rocks are incorporated into a fold-and-thrust belt. Correctly interpreting the multiple generations of fractures and other penetrative structures can yield important clues to the sequence and conditions of deformation.

The complex history of deformation and related fracturing in detachment-folded carbonate rocks of the northeastern Brooks Range illustrates this point. Regional extension fractures probably formed parallel to maximum horizontal in situ stress in relatively undeformed rocks ahead of and orthogonal to the fold-and-thrust belt. These early strike-normal fractures formed as a brittle response to layer-parallel shortening under relatively low temperature, low differential stress, and high pore fluid pressure. Subsequent deeper burial of these fractured strata and advance of the deformation front increased differential stress, temperature and strain rate. Layer-parallel shortening under these conditions probably was accommodated by semi-ductile processes. Subsequent detachment folding of the layered rocks was initially dominated by flexural slip, with associated strike-parallel and/or strike-perpendicular shear and extension fractures. Ductile processes became increasingly important as folds tightened. With waning folding and/or uplift, temperature, strain rate and differential stress decreased and the rocks once more deformed brittlely with formation of late strike-normal extension fractures.

This general deformational sequence suggests that extension fractures with similar orientations but due to different stress regimes may form at different times during the evolution of a fold-and thrust belt. This may explain apparent conflicting relative ages of extension fractures in these settings.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado