Folding thanksgiving paper napkins
On this Wikipedia the language links are at the top of the page across from the article title. Folds of alternate layers of limestone and chert occur in Greece. The limestone and chert were originally deposited as flat layers on the floor of a deep sea basin. These folds were folding thanksgiving paper napkins by Alpine deformation.
In structural geology, a fold is a stack of originally planar surfaces, such as sedimentary strata, that are bent or curved during permanent deformation. Folds in rocks vary in size from microscopic crinkles to mountain-sized folds. The fold hinge is the line joining points of maximum curvature on a folded surface. This line may be either straight or curved. The term hinge line has also been used for this feature. The axial surface is defined as a plane connecting all the hinge lines of stacked folded surfaces. If the axial surface is planar, it is called an axial plane and can be described in terms of strike and dip.
Folds can have a fold axis. A fold that can be generated by a fold axis is called a cylindrical fold. This term has been broadened to include near-cylindrical folds. Often, the fold axis is the same as the hinge line. Minor folds can, however, often provide the key to the major folds they are related to. A fold can be shaped like a chevron, with planar limbs meeting at an angular axis, as cuspate with curved limbs, as circular with a curved axis, or as elliptical with unequal wavelength.
Not all folds are equal on both sides of the axis of the fold. Those with limbs of relatively equal length are termed symmetrical, and those with highly unequal limbs are asymmetrical. Asymmetrical folds generally have an axis at an angle to the original unfolded surface they formed on. Vergence is calculated in a direction perpendicular to the fold axis.
Folds that maintain uniform layer thickness are classed as concentric folds. Those that do not are called similar folds. Similar folds tend to display thinning of the limbs and thickening of the hinge zone. Concentric folds are caused by warping from active buckling of the layers, whereas similar folds usually form by some form of shear flow where the layers are not mechanically active. Anticline: linear, strata normally dip away from the axial center, oldest strata in center irrespective of orientation.
Syncline: linear, strata normally dip toward the axial center, youngest strata in center irrespective of orientation. Antiform: linear, strata dip away from the axial center, age unknown, or inverted. Synform: linear, strata dip toward the axial center, age unknown, or inverted. Monocline: linear, strata dip in one direction between horizontal layers on each side. Recumbent: linear, fold axial plane oriented at a low angle resulting in overturned strata in one limb of the fold. Dome: nonlinear, strata dip away from center in all directions, oldest strata in center. Basin: nonlinear, strata dip toward center in all directions, youngest strata in center.
