Folding

A geologic fold occurs when a flat surface is bent or contorted. For a rock or sediment to show its folds, it must have a set of parallel surfaces or different layers inside it. A very homogeneous rock mass with no internal variation would not show that any force had been applied to it to make it fold. Folds form in sediments and rocks generally from near the surface to as much as 45 km (kilometers) deep in the Earth, where elevated temperatures and the high rock overburden and fluid pressures cause the rocks to be ductile and change shape as they are subjected to tectonic or other forces from glaciers and surface processes. This allows their development in sediments and sedimentary rocks, in a full spectrum of metamorphic [Page 1141]rocks, and even as primary flow structures in some igneous rocks. Folds can even occur in soft sediments, where varied conditions of confining stress, hydrostatic pressure, and pore pressure allow their development. Folds in rocks occur at scales ranging from the microscopic to vast regions best observed by satellites in space. Folds distributed on a regional scale constitute a fold belt, which is a common feature of orogeny or mountain-building zones. Folds can also range from aesthetically pleasing, simple waveforms all the way to vital traps for hydrocarbon accumulation; their great variety has excited the curiosity of lay people as well as the geologists and geo-physicists who study them intensively. Three main approaches have been used to study folds:

• 1.
  The study of the geometry and internal structure of natural folds
• 2.
  Theoretical analysis of folding processes
• 3.
  The formation of experimental folds in the laboratory

In the course of the study of folds, an extensive and sometimes confusing descriptive terminology of form has been developed to classify them by their size and shape, or geometry and morphology, the tightness of the fold, and the dip of the axial plane, as well as by their mechanics of growth, kinematics, and tectonic frameworks. The first consideration is fold description and classification. The region of maximum curvature of a fold is the hinge zone or fold nose, where the limbs of the fold close. The axial surface of a fold connects the hinge zones of the various folded layers, and an axial line connects the points of maximum curvature in any one bedding surface of a fold. Folds that close upward are antiforms, and those closing downward are synforms. Sedimentary rocks have a variety of structures that show superposition or the right way up (younging direction). If an antiform has its younging direction upward, it can be considered an anticline, whereas a synform with the same superposition direction is a syncline. Folds face in the direction of the stratigraphically younger rocks; vergence is the direction in which structures face. If sedimentary rocks are overturned from their original superposition, established when they were deposited, it is then possible to have synformal anticlines and antiformal synclines. The plunge inclination is the attitude of the axial line relative to a horizontal plane.

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