According to one study, recumbent folds can form through the process of ductile horizontal extension affecting rocks with an initially steep layering or anisotropy.
Understanding Recumbent Fold Formation
Recumbent folds are a specific type of fold where the axial plane is essentially horizontal. Their formation is a result of intense deformation of rock layers under specific conditions.
The Role of Ductile Horizontal Extension
One mechanism suggested for their formation involves ductile horizontal extension. In geological terms:
- Ductile: This means the rocks deform plastically under stress without fracturing, much like bending clay.
- Horizontal Extension: This refers to the rock mass being stretched horizontally in one direction while shortening occurs in another perpendicular direction.
Initial Conditions
For this mechanism to produce recumbent folds, the rocks must initially have steep layering or anisotropy.
- Layering: Refers to distinct strata or beds within the rock.
- Anisotropy: Describes a material whose properties (like strength or how it deforms) vary with direction. This could be pre-existing fabrics or structures within the rock.
- Initially Steep: The layers or anisotropic fabrics must be oriented at a high angle, close to vertical, before the deformation begins.
The Process: Shortening and Folding
During ductile horizontal extension applied to these steeply oriented rocks, the reference states that the layering or anisotropy is shortened and folded. As the horizontal stress acts and the rock extends horizontally, the initially steep layers are compressed and buckle downwards or upwards, eventually lying nearly flat as the folding intensifies.
Type of Deformation
The study also notes that the overall (bulk) deformation involved in this process can be coaxial or noncoaxial.
- Coaxial Deformation: Principal strain axes maintain a fixed orientation relative to the material during deformation.
- Noncoaxial Deformation: Principal strain axes rotate relative to the material during deformation.
Both types of deformation can potentially lead to the necessary shortening and folding of the steeply oriented layers to form a recumbent fold structure.
In summary, this mechanism highlights that recumbent folds can develop when rocks with steep internal layering are subjected to ductile horizontal stretching, causing the steep layers to be compressed and folded into a horizontal orientation.
