Detachment folds with fixed hinges and variable detachment depth, northeastern Brooks Range, Alaska

Abstract

Detachment anticlines in the northeastern Brooks Range accommodated displacement above a detachment by buckling of a competent unit over an incompetent unit. Meso- and microstructures in hinges and a lack of relict hinge structures in limbs suggest that these folds grew with fixed hinges. The structural thickness of the incompetent unit beneath the folds (detachment depth) varies from less than to greater than the stratigraphic thickness. A model in which incompetent unit thickness varies with fold area better approximates the geometry of the folds than does a more conventional constant-depth model. Additional discrepancies between modelled and observed incompetent unit thickness and field observations suggest non-plane strain and/or transport of material through the boundaries of the fold in the plane of the cross-section. The results of this study suggest a typical evolutionary sequence for detachment folds in the northeastern Brooks Range, which may be applicable elsewhere. Anticlines initiate as fixed-hinge buckle folds. Rapid initial increase in anticlinal cross-sectional area results in a decrease in incompetent unit thickness. Fold area begins to decrease with tightening beyond an interlimb angle of 90 dg. Decreasing fold area is accommodated through some combination of structural thickening of the incompetent unit, transport of solid or dissolved material out of the plane of section, transport of material through the boundaries of the fold in the plane of the cross-section, and/or truncation by thrust faults.