REVIEW

INTERLIMB ANGLE : For a given folded surface, this angle is the angle between the flanks

 BISECTING SURFACE: Surface which splits the interlimb angle in half

 [[pi]]-diagram: stereographic plot of the poles of folded surfaces

 [[beta]]-diagram: stereographic plot of the great circles folded surfaces.

 The intersection of the great circles approximates the fold axis orientation
 
 

DEFINITIONS

READINGS

 Ch. 11 Folds

LECTURE

Preparation for Alabama Field trip
 
 

ORIGIN OF FOLDS

Folds imply non-brittle deformation in most cases. Folds imply plastic deformation because the strains permanent.

 Various factors control the deformation of rocks as we have previously discussed ....

What controls fold shape?

Strength (competency ) of the layering and contrast

Insert figure

 When the layers are strong but actively slipping past each other (contacts) are of low cohesive strength) we can produce concentric folds.

In flexural folding buckling is accommodated by

(1) layer parallel slip between layers -- flexural slip folding. Minor structures expected: striae, slickenside.
 
 

However, some deformation within the layers can take place too. The strain may deform he rock but not break it, but it can also create joints.
 
 

Flexural folding has a variant where there are alternating competent and incompetent layers. The less competent layers flow between the more competent.
 
 

At higher degrees of flattening, we develop foliation: or a preferred orientation of minerals, parallel to the axial surface (known as axial surface foliation). The origin of this foliation is debatable but similar folds are produced in this environment when folding is very intense. Shortening can be accomplished by dissolution and recrystallization. It is udner these conditions that thicknesses parallel to the shortening direction are diminished but thicknesses at right angles are not. In this type of folding the layers are not truly buckled and the layering does not have much mechanical influence on the fold geoometry.
 
 

SLIDES