Detachment fold

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. ==Modes of detachment folding==

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. 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. Withdrawal from the regional position is dependent on thickness and viscosity differences between the competent and incompetent units as well as the ductile nature of the incompetent unit, like Contreras, recognized a transition from unit deflection and material migration, to limb rotation and limb lengthening. ==Detachment fold evolution==

Though many models have been developed to help explain the kinematic evolution of single layer detachment faulting; many models do not account for multiple layers, complex fold geometries These models may not be good indicators of detachment folding on a large scale and are better suited to assist in interpreting fold geometries of detachment folds as their kinematic evolution is generally associated with single fold, single unit deformations. The definition of disharmonic folds (below) however, incorporates many types of symmetric folds over a larger area encompassing many geometries and attributes of the basic models and may be better suited to the application of these models. By incorporating elementary fold geometries Present day examples detachment folding can be found in the Jura Mountains of Central Europe. This region complements the idea of detachment fold evolution put forth by Mitra Further compression dominated by hinge migration, yields tightening of folds and space accommodation issues within the anticlinal core; leading to the formation of disharmonic folds . Epard and Groshong, (1994) recognize a similar pattern to disharmonic folding they label it second-order shortening. Basic models and experiments as well as concentric fold models fail to recognize disharmonic folds as they focus on single layer detachment folding, lack the resolution in experimental methods or, though the assumption of multiple units is made, restrict unit parameters which may cause disharmony through deformation. Continued shortening and excess material within the anticlinal core not only results in increased amplitude and disharmonic folds, but may lead to the onset of thrusts out of the folded synclinal or anticlinal regions. Through further deformation by limb rotation and through hinge migration, isoclinal folds eventually assume lift-off geometries. Thrust faults in the synclinal fold, if any formed, may also be rotated to assist in the formation of detached lift-off folds upon further tightening and rotation (figure 4). ==Detachment faulting==

It is documented in many cases that faulting may develop from detachment folding or through the kinematic mechanisms involved with folding. In general, faulting may occur during fault slip and detachment folding in two ways. Firstly, faulting may be induced when progressive folding or tightening of a folded limb reaches its maximum fold geometry resulting in a transition from folding to shearing. being triangular in shape; while a parallel deformation zone transmits shear across the units of the fold and typically takes on the form of a parallelogram or is rectangular in geometry. These two deformation patterns may exist in a single fold and at some time during continued deformation may reconnect with the detachment. It is also the case that a backthrust may occur in an asymmetric fold geometry as shear across the forelimb due to rotation and migration of beds. Symmetric faults were essentially covered previously under the name ‘lift-off’ folds, see Figure 4. Progressive limb rotation and lock-up in a symmetric fold induces shear at both the forelimb and backlimb of the fold which may then result in faults on both limbs causing lift-off. Like the asymmetric fold faulting, as progressive slip along the basal detachment occurs, either the forelimb or backlimb (the limb closest to the source of thrust) thrust may reconnect with the basal detachment. For a more robust definition of faulting reference Mitra 2002. ==References==