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(Variscan Orogeny )]] 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 . DESCRIBING FOLDS Folds are classified by their size, fold shape, tightness, dip of the axial plane. 2D Fold Terms Looking at a fold surface in profile the fold can be divided in to a hinge portion and the '''limbs'''. The limbs are the flanks of the fold and the hinge is where the flanks join together. The hinge point is the point of minimum radius of curvature for a fold. The '''crest''' of the fold is the highest point of the fold surface, and the '''trough''' is the lowest point. The '''inflection point''' of a fold is the point on a limb at which the concavity reverses, on regular folds this is the mid-point of the limb. 3D Fold Terms The hinge points along an entire folded surface form a hinge line. The Trend And Plunge of a linear hinge line gives you information about the orientation of the fold. To completely describe the orientation of a fold, one must use the '''axial surface'''. The axial surface is the surface defined by connecting all the hinge lines of stacked folding surfaces. If the axial surface is a planar surface then it is called the '''axial plane''' and can be described by the Strike And Dip of the plane. The '''axial trace''' is the line of intersection of the axial surface with any other surface (ground, side of mountain, geological cross-section). Finally, folds can have, but don’t necessarily have a '''fold axis'''. A fold axis, “is the closest approximation to a straight line that when moved parallel to itself, generates the form of the fold.” (Davis and Reynolds, 1996 after Donath and Parker, 1964; Ramsay 1967). 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. Fold shape It is necessary to convey a sense of the shape of the fold. A fold can be shaped as 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. Fold tightness Fold tightness is defined by the angle between the fold's limbs, called the ''interlimb angle''. Gentle folds have an interlimb angle of between 170° and 180° , '''open''' folds range from 170° to 90°, '''tight''' folds from 90° to 10°, and '''isoclinal''' folds have an interlimb angle of between 10° and zero, with parallel limbs. Fold symmetry 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 '''asymmetrical'''. Asymmetrical folds will generally have an axis which is at an angle to the original, unfolded surface which they formed upon. Deformation style classes Folds which maintain uniform layer thickness are classed as concentric folds; those which 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 which results from deformation of the layers, whereas similar folds usually form by some form of dislocation between the layers (sliding), with extension and contraction of the thickness of rock layers differently in the limb ad hinge zones FOLD TYPES
FOLDING MECHANISMS Folding of rocks must balance the deformation of layers with the conservation of volume in a rock mass. This occurs by several mechanisms. Flexural slip Flexural slip allows folding by creating layer-parallel slip between the layers of the folded strata which, altogether, result in deformation. The best analog is bending a phone book, where volume preservation is accommodated by slip between the pages of the book. Buckling Typically, folding is thought to occur by simple buckling of a planar surface and its confining volume. The volume change is accommodated by ''shortening'' the volume, which grows in ''thickness''. Folding under this mechanism is typically of the similar fold style, as thinned limbs are shortened horizontally and thickened hinges do so vertically. Mass displacement But what happpens if the folding deformation cannot be accommodated by flexural slip or volume-change shortening (buckling)? Generally the rocks respond by removing themselves from the path of the stress! This is achieved by Pressure Dissolution , a form of metamorphic reaction, in which rocks shorten by dissolving constituents which move to areas of lower strain. Folds created in this way include examples in Migmatite s, and areas with a strong axial planar cleavage. REFERENCES
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