REVIEW

HOW DOES FLUID PRESSURE CHANGE THE MOHR-COULOMB FRACTURE ENVELOPE?

[[sigma]]s = [[sigma]]0 + tan [[phi]] ([[sigma]]N-[[sigma]]FP)

DEFINITIONS

anticline - fold that is convex in the direction of the youngest beds

 antiform- "a fold that is a fold that points up": convex upward

 syncline- fold that is concave in the direction of the youngest beds

 synform - "a fold that points down": concave upward\

 overturnde folds: when one limb is overturned, i.e. younger rocks are underneath

 fold limb: flandk of a fold on either side of a

fold hinge: zone of maximum curvature

 crest: highest point of a folded surface

 crest line: collection of crests along on a common folded surface

 inflection points: Location on a fold where curved limb segments of opposing convexity meet.

 hingeline: collection of hinge points along a singel folded surface

 axial surface: surface that passes through the hingelines of various folded layers in a sequence (axial trace is the intersectin of the axial surface with any other surface, e.g. outcrop)

 fold axis is a line that when moved parrallel t itself orchestrates afold

 cylindricalfolds are folds that possess fold axes; every part of the folded surface is oriented such that it contains a line whose orientation is identical to that of the hingeline
 
 

READINGS

 Ch. 11 Folds

LECTURE

SLIDE

Folding in central Australia is a spectacular example of a Proterozoic collisional boundary.

 Folding can be recognized fromthe air.

 Recognition of folds in maps is more complicated and we'll be examining those cases in the labs.

 In the field folds are not always immediately obvious. Erosion often removes the hinge area and all we have is a set of parallel beds.

* Age relationships can reveal folds

 * small-scale structures (minor strutures) reveal the alrger scale geometry
 
 

Folds are improtant intheoil industry because the zone of closure can serve as a structuraltrap.
 
 

Show age relationships
 
 

FOLD CLASSIFICATION

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

 SIZE

 SYMMETRY: asymmetricalfolds 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