1
EPS116 Chapter Summary
Alyse Briody 2011
Gia Patafio and Finn Chen 2013
10.1 Geometric Parts of Folds
Inflection Line: a point at which sense of curvature changes (i.e. from convex up to convex down)
Fold train: A series of folds having alternating senses of curvature
Antiforms: Folds that are convex upward
Synforms: Folds that are concave upward
Fold system: A set of regional folds with similar geometry presumed to be of common origin
Curvature: A measure of the change of orientation per unit distance along a surface
Hinge: The line in the folded surface along which the curvature is the greatest
Hinge Zone: most highly curved portion of a fold near the hinge line.
Limbs/flanks: the regions with the lowest curvature (includes inflection lines)
Crest line: Line of highest elevation on a folded surface
Trough line: Line of lowest elevation on a folded surface
Fold Profile: The trace of the folded surface seen when it is viewed parallel to its hinge.
Fold axis:the closest approximation to a straight line that when moved parallel to itself, generates the form of the fold
Anticline: Fold in which the older layers on the concave side of a bedding surface and the younger layers are on the convex side
Syncline: Fold in which the younger layers are on the concave side of the bedding surface and the older layers are on the convex side
Culminations: where crest lines go through maximum elevation
Depression: where trough lines go through minimum elevation
Cylindrical fold: fold axis can be moved along the folded surface without losing contact with it at any point. All the poles to its bedding should lie on a great circle.
Conical fold: whose surface is everywhere at a constant nonzero angle to a line of fixed orientation.All the poles to its bedding should lie on a small circle
Axial surface trace: the intersection of the axial surface with a surface of exposure is a linear feature called the axial surface trace
Hinge surface/axial surface/axial plan: the surface joining all hinge lines in a particular nested set of folds
10.2 Fold Scale and Attitude
Enveloping Surfaces: The two surfaces that bound the fold train developed in a single folded surface
Median Surface: The surface that includes all inflection lines of a fold train in a single surface (See Twiss & Moore figure 10.11)
Amplitude: the distance from the median surface to either of the enveloping surfaces, measured parallel to the fold axis
Wavelength: the distance measured parallel to the median surface, between one point on a fold and the geometrically similar point on a neighboring fold
Upright: term used to describe a fold in which the dip of the axial surface is close to vertical
Recumbent: term used to describe a fold in which the dip of the axial surface is close to horizontal
Reclined fold: A fold whose hinge plunges down the dip of the axial surface
Overturned fold: An inclined or recumbent fold in which one limb is rotated 90 degrees from its original horizontal position
Monocline: a fold pair that has two long horizontal limbs connected by a relatively short inclined limb
Structural terrace: fold pair with two long planar inclined limbs connected by a relatively short horizontal limb.
10.3 The Elements of Fold Style
The style of a folded surface is defined by certain features.
Folding angle: the angle between the normal to the folded surface constructed at the two inflection points of a fold
Interlimb angle: angle between the tangents to the two fold limbs constructed at the inflection point.
Cylindricity: The degree to which a fold approximates thegeometry of a cylindrical fold. This is determined with a stereographic projection.
Symmetry: Whether or not a folded surface creates a mirror-image on both sides of the fold hinge
Aspect Ratio (P): The ratio of the amplitude (measured along the axial surface) to the distance (M) measured between adjacent inflection points that bound the fold
Tightness: Feature defined by the folding angle or the interlimb angle. Folding angle increases and interlimb angle decreases with increased degree of folding.
Bluntness: The radius of the curvature of the fold at its closure relative to a reference circle tangent to the limbs tangent to the limbs of the fold at inflection points
b = rc / r0; for rc≤ r0
b = 2 - (r0/rc ); for rc ≥ r0
rc= radius of curvature
r0= radius of reference circle
Ramsay’s Classification to determine the style of a folded layer is defined by three geometric parameters.
Dip Isogon: the line across the layer connecting two points of equal dip on opposite surfaces of the layer
Orthogonal Thickness: the perpendicular distance between the two parallel tangents
Axial Trace Thickness: the distance between the two tangents measured parallel to the axial surface trace
Ramsay’s classification of folded layers:
Harmonic fold: A fold that is continuous along its axial trace for many multiples of the half-wavelength
10.4The Order of Folds
Fold order: Characterizes scales of related folds, with the largest-scale folds being first-order, and the smaller-scale (also known as parasitic) folds being higher-order numbers
Pumpelly’s Rule: the style and attitude of higher-order folds are generally similar to that of lower-order folds
10.5Common Styles and Structural Associations of Folding
Chevron/Kink Folds: cylindrical, harmonic folds with sharp hinges. Chevrons are symmetrical and kinks are asymmetrical
Ptygmatic folds: disharmonic folds that typically develop in individual layers of sedimentary or metamorphic rock
References & Resources
Robert J. Twiss, Eldridge M. Moores, Structural Geology 2nd edition, (W. H. Freeman), p. 273-295, 2006
1
EPS116 Chapter Summary
Alyse Briody 2011
Gia Patafio and Finn Chen 2013