|
|
|
Fold Related Microstructures |
|
|
|
 |
|
|
During folding, the bending of bedding surfaces (So) is accommodated by the internal deformation of rocks. This deformation may produce the rotation and the alignement of mineral grains and biogenic clasts parallel to the fold axial surface. This produces a secondary anisotropy plane called a cleavage plane (S1). Deformation can also change the shape of grains through a mechanism called pressure-solution. According to this mechanism of deformation, the part of the grain enjoying the larger stress is dissolved and the material removed migrates toward areas of lower stress. Through pressure-solution a grain of spherical shape is transformed into an ellipsoid whose flattening plane (defined by its long and intermediate axes) tends to be parallel to the fold axial surface hence participating in the development of a cleavage plane. Both mechanisms are presented in slides 16 to 18 in Geol 1002. Two others mechanisms participate to the development of axial planar cleavage. Fracture cleavage involves the development of shear fractures in competent (hard) layers. These shear fractures tend to be parallel to the fold axial surface. Finally, crenulation clevage develops because of the preferential alignement of limbs in micro-folds. |
|
|
|
|
|
|
|
|
|
|
