See also:
folding The term fold is used in
geology when one or a stack of originally flat and planar surfaces, such as
sedimentary strata, are bent or curved as a result of plastic (i.e. permanent)
deformation. Synsedimentary folds are those due to slumping of material before deformation. Folds in rocks vary in size from microscopic crinkles to mountain-sized folds. They occur singly as isolated folds and in extensive fold trains of different sizes, on a variety of scales. Folds form under varied conditions of
stress,
hydrostatic pressure,
pore pressure, and
temperature, as evidenced by their presence in soft
sediments, the full spectrum of
metamorphic rocks, and even as primary flow structures in some
igneous rocks. A set of folds distributed on a regional scale constitutes a fold belt, a common feature of
orogenic zones.
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