Jun 6 2014
Fold is a geological bend or flexure in stratified rocks, resulting from the permanent deformation of sedimentary strata. The width of the folds varies from a few inches to several miles.
The rocks experience fractures when the stresses in the Earth exceed the strength of the rocks. A typical fold includes two limbs, which rise or fall in the direction opposite to a central axial plane.
Folds are generally classified based on the dip of the axial plane, tightness, fold shape, and size. They can be either isolated, or occur in extensive fold trains.
Folding may be caused by mass displacement, buckling, or flexural slip under the conditions of temperature, pore pressure, hydrostatic pressure, and stress.
Types of Folds
There are several types of folds which include the following:
- Recumbent fold – The limbs and axial plane of the fold lie in a horizontal plane. It is caused by excess pressure
- Isoclinal fold – It is formed by the continuous lateral compression on an overfold
- Anticlinal fold – It is formed when the beds of the rock are bent upwards
- Synclinal fold – This type of fold is caused by the downward bending of the beds or strata
- Asymmetrical fold – In this fold, one limb of the fold structure is greater than the other
- Antiform fold – The strata dip in the direction away from the center of the axial line
- Synform fold – The strata dip in the direction towards the center of the axial line
Causes of Fold Formation
The key factors associated with the formation of folds are listed below:
- Igneous intrusion – The displacement of igneous intrusions leads to the deformation and, in turn, folding of the surrounding country rock
- Folding in sediments – Sediments are generally weak mechanically, and easily subjected to remobilization before lithification, causing folding
- Fault-related folding – Most of the folds are directly related to the displacement and propagation of faults, and the presence of strains between the faults.
- Folds in shear zones – This type of folds is caused by the instability of the shear flow, or the orientation of pre-shearing layering
- Layer-parallel shortening - Homogeneous shortening, folding or faulting may be caused by the shortening of a layered rock sequence parallel to its layering