CONTROLS ON FRACTURE DISTRIBUTION AND DEVELOPMENT WITH RELATION TO DETACHMENT FOLDS WITHIN THE LISBURNE GROUP, NORTHEASTERN BROOKS RANGE, AK

The Mississippian and Pennsylvanian Lisburne Group carbonates of the northeastern Brooks Range fold-and-thrust belt have been deformed by detachment folding during Cenozoic time. Two well-exposed folds in the lower Lisburne provide an opportunity to examine the timing and character of fracture development with respect to fold geometry and mechanical stratigraphy. Several stages of deformation have been recorded by fractures that formed before, during, and after detachment fold development. The distribution of calcite filled, syn-fold fractures and other mesoscopic structures suggests that the early and latest stages of shortening were accommodated by flexural slip and flexural flow, and the middle to early-late stages of shortening by homogenous flattening. Mechanical stratigraphy in a multi-layer sequence is commonly defined by stratigraphic bed thickness, lithology, competency contrast, and interface strength. Based on the location of mechanical slip horizons within these folds, stratigraphic bed thickness does not coincide with mechanical bedding. Fractures are not distributed homogeneously throughout the observed stratigraphy, but are strongly influenced by lithology and mechanical bed thickness.

There are two sets of post-fold fractures: (1) an east-west striking set that are sub-perpendicular to bedding, unfilled, and interpreted to be release fractures, and (2) a north-south striking set that are sub-perpendicular to the fold axis, unfilled, and interpreted to be unloading fractures partially influenced by regional in situ stresses. The relative timing of these fracture sets is ambiguous at certain locations, but overall, the north-south set post-dates the east-west set.

This information contributes to: (1) improving our understanding of detachment fold kinematics, and (2) determining the structural and stratigraphic factors that may control reservoir permeability in similar detachment folds in the subsurface, both in northern Alaska and elsewhere.