Competent unit thickness variation in detachment folds in the Northeastern Brooks Range, Alaska: geometric analysis and a conceptual model

Abstract

Detailed measurements of six map-scale detachment folds in the northeastern Brooks Range, Alaska, document significant variations in structural thickness of the competent Lisburne Limestone. Thickness variations occur mainly by parasitic folding and penetrative strain, and may be controlled by differences in mechanical stratigraphy, relative thicknesses of the competent and incompetent units, and structural relief of the underlying basement. The geometry of these detachment folds is not consistent with key assumptions of existing geometric and kinematic models, such as constant competent unit thickness or constant detachment depth. We propose a new model that allows both competent and incompetent unit thicknesses to vary throughout the fold. This model allows a more realistic geometric description of some natural detachment folds than previous models, but the number of variables makes unique reconstruction of specific fold geometries or kinematics difficult. Comparison of models with natural folds demonstrates that significant error can result if shortening estimates are based on models that incorrectly assume constant competent unit thickness or constant detachment depth. Use of surveying techniques to quantify map-scale fold geometry can provide better reconstructions of fold geometry, better shortening estimates, and information to constrain kinematic reconstructions.