Lecture Five - Fabrics and folds Flashcards
What scales can folds occur at?
Folds can occur at all scales.
Large scale folds are a major feature of most moutain belts.
Smaller scale folds are also important for accomodating straing and provide cludes to larger scale geometry.
What are foliations and lineations?
Foliations are planar structures associated with folding.
Foliation is the general term used for penetrative planes.
Foliation - penetrative multiple planes penetrating the entire rock mass (fault plane has only one plane of penetration = non-penetrative).
Foliation is known as axial planar foliation.
- Planar surfaces parallel to the fold axial plane/axial surface.
- Tend to develop ~ perpendicular to the principal axis of shortening.
Lineations are linear structures associated with folding.
- Tend to develop ~ parallel to the principal axis of extension and/or where two surfaces intersect (intersection lineation).
These structures have certain geometric relationships to folds.
What happens to rock when it is streched to compressed?
Streching lineation occurs with streching.
Foliation occurs with flattening.
Essentially, the minerals are either compressed or streched, but will keep the same volume (generally).
What is the axial plane of foliation?
Foliation is parallel to the axial plane/axial surface.
What is bedding?
Bedding is the different coloured layers which are parallel to each other in a cross section of a fold.
What are the fabric formation mechanisms?
What are the different specific terms used for foliation regarding grain size (metamorphic grade)?
Cleavage:
- Crenulation cleavage = a fabric formed in metamorphic rocks such as phyllite, schist and some gneiss by two or more stress directions resulting in superimposed foliations.
- Slaty cleavage = alignment of very fine grains and low grade platy minerals.
- Phyllitic cleavage = same as slaty cleavage but larger grain size.
Schistocity = generally higher grade metamorphic rocks. Same as slaty cleavage but larger grain size (larger than phyllitic too).
Gneissic foliation or gneissosity = high temperature fabric mineral growth. Differentiation, melting during deformation.
What is vergence?
Identifies the direction to the nearest antiform.
Vergence from foliation/bedding relationship.
Look down the plunge.
Find acute angle - vergence is the direction from younger to older foliation.
(Using anlge from axial plane of foliatino to bedding).
Explain the relationship between the vergence from a fold and symmetry/asymmetry.
If you find a large scale S fold, you can predict that there will be an M fold next to it, then a Z fold next to that.
