Geometry and kinematics of thrust-truncated and/or flattened asymmetrical folds in the eastern Brooks Range, Alaska

Abstract

Exceptional exposures of thrust-related folds within carbonate rocks of the Lisburne Group of the Brooks Range of northern Alaska are classified into four groups: unbroken/parallel folds, thrust-truncated/parallel folds, unbroken/flattened folds and thrust-truncated/flattened folds. The geometry of these folds varies greatly along strike, suggesting that serial sections do not represent successive stages in fold evolution. Geometric and kinematic modeling of individual folds reveals that no single fault-related fold model can reproduce the geometry of a given fold. Instead, successful forward modeling of the folds requires some combination of detachment folding with fault-bend folding or fault-propagation folding. All folds followed one of two kinematic paths: the initially parallel folds either broke through without flattening or flattened first and then broke through. Preferred hypothesis for why folds would deform by one path verses the other include: I) Original asymmetry will promote breakthrough over flattening, so that originally asymmetric folds will break through without flattening while initially symmetric folds will flatten before breaking through; and 2) A thin detachment unit may limit fold growth resulting in breakthrough without flattening, while a thick detachment units will not limit fold growth, allowing folds to shorten via flattening before breaking through.