Fold (geology)

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. 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 very varied conditions of stress, hydrostatic pressure, pore pressure, and temperature, as evidenced by their presence in soft sediments, sedimentary rocks, 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.

Types of folds

• Anticline: linear, strata dip away from axial center, oldest strata in center.
• Syncline: linear, strata dip toward axial center, youngest strata in center.
• 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.
• Monocline: linear, strata dip in one direction between horizontal layers on each side.
• Recumbent: linear, fold axis oriented at low angle resulting in overturned strata below the fold axis.
• Slump: typically monoclinal, result of differential compaction or dissolution during sedimentation and lithification.