Trishear Fault-Propagation Folding

Fault-propagation folds are produced by deformation that takes place just in front of the propagating fault. The fault tip propagates upsection, and the fold develops above the ramp with uniform fold angles.

 Figure: A) Kink-band model, B) Fold above thrust fault,

C) Fold above reverse fault, D) Fold above normal fault.

(Erslev 1991)

 

Fault-propagation fold hinges tighten and converge downward, forming a triangular zone of deformation that is concentrated on the tip of the propagating fault. This downward convergence of deformation is modeled as triangular shear zones. This lends the name to trishear fault-propagation folds.

Figure: Models of homogeneous and heterogeneous

fault-propagation trishear folds. A) Thrust faults,

B) Reverse faults, C) Normal faults. (Erslev 1991)

Understanding the geometry of fault-propagation folding is useful in creating balanced models of fold and thrust belts. Fault-propagation trishear folds are common in the Laramide structures of the Bighorn Basin in Wyoming.

All information from: Eric A. Erslev (1991) Trishear fault-propagation folding

Ramsay Fold Classifications

John Ramsay's Fold Classification

John Ramsay proposed a classification scheme for folds that is used to describe folds in profile based upon curvature of the inner and outer lines of a fold, and the behavior of dip isogons.

dip isogon: a line that connects points of equal inclination or dip on the outer and inner bounding surfaces of a folded layer

Class 1 - Dip isogons converge downward towards axial surface, signifying that the curvature of the outer arc is less than that of the inner arc

Class 1A - Limbs thicker than hinges

Class 1B - Layer thickness constant; parallel fold

Class 1C - Limbs thinner than hinges

Class 2 - Dip isogons are parallel, signifying that the curvature of the outer arc exactly matches the curvature of the inner arc; similar fold

Class 3 - Dip isogons diverge downward towards axial surface, signifying that the curvature of the outer arc is greater than that of the inner arc

Class 1B (parallel) and Class 2 (similar) folds are the most common folds seen in the field. Concentric folds are a special case of Class 1B (parallel) folds where the outer and inner bounding surfaces define arcs with a common center of curvature. These types of folds are common in upper crustal tectonic settings, where most deformation occurs by processes that only permit limited ductile flow of rock. Most of the deformation is accommodated by slip on bedding or layer boundaries (flexural slip folding). Class 2 (similar) folds have relative thinning of the limbs and thicking of the hinges. These types of folds are common in metamorphic terranes, where most deformation occurs by processes that permit extensive ductile flow of rock.


Sources:
Folding and Fracturing of Rocks, John G. Ramsay, 1967
Structural Geology of Rocks, 2nd Edition, George H. Davis & Stephen J. Reynolds, 1996
http://ocw.mit.edu/courses/earth-atmospheric-and-planetary-sciences/12-113-structural-geology-fall-2005/lecture-notes/part6_dctl_fldfb.pdf