Lecture 5 - Fabric 1: Foliations, Lineations, and Sheer Sense in Mylonites Flashcards
What is a foliation?
e use the term foliations for a planar feature that occurs penetratively in rocks. As such rythmic bedding may also be called a foliation in addition to cleavage, schistosity or compositional layering
Name five features of primary cleavage
- sedimentary structures present
- composition and grain size vary
- thickness shows variation
- rarely symmetric to layering
- not parallel to axial plane of folds
List six features of secondary cleavage
- no sedimentary structures present
- approximately constant thickness
- bimodal composition
- lensoid or anastomosing (interlapping)
- symmetric parallel to layering
- parallel to axial plane of folds
What are the 3 common stages of foliation formation?
Diagenetic foliation (pressure from above), Pencil foliation (squeezed from the sides) then slaty foliation (further squeezing from the side)
What are the 4 mechanisms of foliation development?
Roation, shape change, dissolution and recrystallization
How do styolites form?
Stylolites develop during localized dissolution of a rock.
What do styoites look like?
They appear as dark and rough seams. The stylolite plane is oriented perpendicular to 1. Stylolite teeth are large vertical columns on the stylolite, they grow parallel to 1. Stylolite roughness develops as small particles that do not dissolve “pin” the surface.
How does a secondary foliation form?
First the old foliation is lightly folded, then a crenulation cleavage develops and is folded progressively and finally the old foliation disappears completely and a new one is formed
Describe the relationship between folds and foliation
Foliations that are associated with folds form parallel to the axial surface. Competent layers have foliations with a wider spacing than incompetent layers. Foliations in competent layers are “refracted”, they tend to be oriented perpendicular to bedding. If the competence contrast is very high, foliations in the incompetent beds may become parallel to bedding.
What can happen to foliations during polyphase deformation?
During poyphase deformation younger foliations crosscut the folds, they are not parallel to the axial plane. The large fold in the example is the older fold and the young foliation runs across it and is associated with smaller young folds.
What are strain lineations used to help identify?
The long axis of an ellipsoid, which is very important for mylonites
What is a catalclasite?
Cataclasite is a type of cataclastic rock that is formed by fracturing and comminution during faulting. It is normally cohesive and non-foliated, consisting of angular clasts in a finer-grained matrix.
What is a mylonite?
Mylonite is a fine-grained, compact rock produced by dynamic recrystallization of the constituent minerals resulting in a reduction of the grain size of the rock.
How are mylonites formed?
Mylonites are ductilely deformed rocks formed by the accumulation of large shear strain, in ductile fault zones
How do you find the shear sense?
First find the foliation. The main foliation is parallel to the shear zone boundary. Second find a stretching lineation. This can be stretched crystals, stretched veins, pegmatites etc. The stretching lineation will lie in the foliation plane. Then look at sections perpendicular to the foliation and parallel to the stretching lineation. These will show the right shear sense.
