Gia Patafio and Finn Chen 2013

EPS116 Chapter Summary

Alyse Briody 2011

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 the geometry 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

10.4 The 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

10.5 Common 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

EPS116 Chapter Summary

Alyse Briody 2011

Gia Patafio and Finn Chen 2013