Date of Award

4-17-1995

Document Type

Dissertation

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.

Handle

http://hdl.handle.net/11122/9430

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