Detachment fold

A detachment fold, in geology, occurs as layer parallel thrusting along a decollement (or detachment) develops without upward propagation of a fault; the accommodation of the strain produced by continued displacement along the underlying thrust results in the folding of the overlying rock units. As a visual aid, picture a rug on the floor. By placing your foot on one end and pushing toward the other, the rug slides across the floor (decollement) and folds upward (detachment fold). Figure 1, is a generalized representation of the geometry assumed by a detachment fault.

Detachment folding occurs as strain imposed on a mechanically weak layer or incompetent unit, such as shale or salt, or at the boundary between an incompetent and more competent unit, induces resistance from the units resulting in folding typically observed in the competent unit. Once the resistance of these units is overcome with strain or the variation in strain between the units becomes great enough, a shearing motion known as a detachment fault may occur. Defined, a detachment fault may be located within an incompetent unit or at the boundary of an incompetent and a competent unit, which accommodates for strain differences between the units and allows displacement to occur in a planar field. Detachment folding occurs in regions of thick-skinned deformation, where the basement is involved in deformation and thin-skinned deformation, where deformation occurs at relatively shallow depth in the crust.

One of the principal ideas that should be recognized in each model is the law of conservation of volume, as conservation is a fundamental law in physics; it should also apply to geology. Two ways to maintain volume conservation are thickening of units and synclinal deflection of incompetent material; it is likely that both may occur.

J. Contreras (2010) developed a model for low amplitude detachments using the conservation of mass equation. The results suggest the occurrence of layer thickening as an initial response to shortening and volume conservation. Hayes and Hanks (2008) confirm layer thickening during the onset of folding, specifically their field data places the thickening at the hinges of folds rather than the limbs. When defining the geometry of detachment folding it may be necessary to define layer thickening as it has been recorded to affect the overall geometry. Though variable limb thickness is assumed; over time, limb rotation and limb length become the dominant mechanisms for deformation, leading to an increase in fold amplitude.

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