INTERLIMB ANGLE

 BISECTING SURFACE

 [[pi]]-diagram

 [[beta]]-diagram

REVIEW

FOLD CLASSIFICATION

 TIGHTNESS: gentle, open, tight, isoclinal//based on interlimb angles

 SIZE

 SYMMETRY: asymmetric folds have limbs of different lengths

 LAYER THICKNESS: concentric ore parallel folds: individual layers maintain a constant thickness similar folds display significant thickening in the hinge and significant thinning on the limbs CLASS 1B and class 2 folds

DEFINITIONS

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

READINGS

 Ch. 11 Folds

LECTURE

Preparation for Alabama Field trip

 1. Stereographic determination of folds: interlimb angles, [[pi]] and [[beta]] diagrams

 2. Stereographic determination of plots

 Because folds are seldomly perfectly cylindrical or similar you will have to take many readings of structures in the field to statistically find a good representative group of measurements that will faithfully describe the folds. Normally a [[pi]]-diagram is not clearly defined and requires a best fit to the data by eye. A more objective way of determining the distribution of poles is to determine the scatter or pole density by counting out the number of poles per square subdivision of the stereonet. Because you are using an equal area net any given scatter of poles in one point of the net can be compared directly with another pole scatter elsewhere on the net.
 
 

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

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 thelayers can take place too. The strain may deformt he rock but not break it, but it can also create joints.

 Flexural folding has a variant where there are alternating competent and incompetent layesrs. 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 inthis environment whenfolding 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