Detachment folds of the northeastern Brooks Range, Alaska: A basis for geometric and kinematic models of detachment folds

Abstract

Detachment anticlines are defined by mechanically competent rock layers and form both by internal deformation of an adjacent weak layer and detachment above a lower competent unit. This study is important for: (1) Other fold-and-thrust belts. Detachment folds are probably very common in fold-and-thrust belts worldwide, but they are rarely recognized as such and are commonly mistaken for other fold-types. This is partly because a rigorous general model for detachment folds that allows for changes in detachment depth and for fixed-arc length kinematics is lacking in the geologic literature. A general detachment fold model is presented here that: (a) is based on observations of natural folds in the northeastern Brooks Range of Alaska; (b) does not assume constant detachment depth or hinge-migration kinematics and; (c) allows quantification of non-plane strain. The folds observed can be modeled kinematically as fixed-hinge buckle folds, whereas the fold geometry and distribution of strain indicators in each fold precludes the migrating-hinge kinematic interpretation that is common in published models. Layer-parallel shortening, initial fold asymmetry, initial stratigraphic thickness of the incompetent unit, and the nature of rheological gradations each predictably influence fold evolution. This study suggests a general scenario for the evolution of a typical detachment fold. The area defined by a detachment anticline increases rapidly during early stages of folding and this is accompanied by a decrease in depth to detachment beneath synclines and the formation of fixed-hinge parasitic and disharmonic folds. This trend continues until the interlimb angle of the primary fold reaches 90$\sp\circ$. Increased shortening requires volume-loss in the core and/or an increase in detachment depth beneath the fold. Finally, depending on the rheology of the system, the fold may lock and/or be truncated by a thrust fault. (2) Regional tectonics. The western part of the northeastern Brooks Range is mostly a passive-roof duplex, but this study shows that forward-propagating deformation occurred at various structural positions. (3) Economics. Detachment folds may form petroleum traps that require a treatment different than that for fault-bend or fault-propagation folds. Detachment fold traps may exist beneath the coastal plain of the Arctic National Wildlife Refuge.