Fractures and other mesoscopic structures formed at different times during the evolution of individual detachment folds in Lisburne Group carbonates of the northeastern Brooks Range. These structures provide clues to the mechanism of folding, the conditions under which folds evolved and the paragenesis of fractures in the fold-and-thrust belt as a whole. The earliest fractures strike NNW and probably represent orogen-normal extension fractures that developed in the foreland basin in advance of the fold-and-thrust belt. These rocks and fractures were later incorporated into the thrust belt, where they were thrust-faulted and folded. Later fractures, strained markers and dissolution cleavage developed during detachment folding as a result of flexural slip and homogeneous flattening. Fracturing associated with flexural slip occurred early in the development of folds. These early fractures were commonly overprinted or destroyed by ductile strain as later homogeneous flattening accommodated additional shortening. This penetrative strain was in turn overprinted by late extension fractures that formed during flexural slip in the waning phases of folding or after folding due to unroofing of the orogenic wedge.
Early fracturing, overprinting by ductile structures and subsequent later fracturing in detachment-folded Lisburne Group emphasizes the importance of understanding the unique character and history of each fold-and-thrust belt in a successful hydrocarbon exploration effort. In particular, the mechanical stratigraphy and conditions of deformation play an important role in the type of fold that develops, the fold mechanisms that are active and the subsequent distribution and character of fractures and other mesoscopic structures.