Announcements Draft #1 of fault paper due next Wed.! Late papers: lose 5% of grade per day.
Today: Cleavages and Passive Folding (Davis and Reynolds, pp )
Cleavage: closely spaced, aligned, planar surfaces; associated with folds- impart a splitting property to the rock- often cuts bedding
Cleavages: commonly parallel to axial planes of folds. They cut bedding! "Axial planar cleavage"
Cleavages: may fan slightly, but still close approximation for axial surface
Sometimes cleavages interact with fissility (tendency for rock to split along bedding plane) to form "pencil" structures
Cleavage can be very closely spaced- this is called a continuous cleavage (vs. a spaced cleavage)
Cleavage is an expression of variations in mineralogy and fabric (grain shape, size, and configuration)- these variations formed during deformation (secondary) "domainal" nature of cleavage
continuous cleavage under a microscope, showing domainal nature
Types of continuous cleavage: Purely based on grain size (which increases with higher T) (1) Slaty cleavage (fine-grained) (2) Phyllitic cleavage (med. grained- shiny) (3) Schistosity (coarse grained- can see micas)
Slaty cleavage
microfolds in mica layers "sheen" appearance phyllitic structure
Schistosity: coarser grained fabric
older layering can be folded + cut by younger cleavage to form Crenulation Cleavage
more... Crenulation Cleavage
Crenulation Cleavage at the microscopic scale
Strain Significance of Cleavage Relationship with folding: suggests cleavages form as a response to shortening and flattening Form perpendicular to 1 and S3 Confirmed in field and laboratory
How does cleavage form?? (1) Mechanical rotation of minerals (contributes a bit) (3) Pressure solution (dominant!!) 2) Directional Recrystallization (contributes a bit)
pressure solution "dimples" in conglomerate clasts
Formation of crenulation cleavage
stylolites- results of pressure solution in limestones
Passive Folds: wavelength shortened by pressure dissolution along cleavage planes
Passive Folds: generally similar folds- thickening in hinge zone. Intersection line between cleavage and bedding yields trend and plunge of hinge line!
Origin of Passive folds
TRANSPOSITION Lithologic layering is often mistaken for coherent bedding- it's not! It's layering due to major deformation cleavage/foliation is parallel to lithologic layering; lensoidal layers
How bedding gets transposed:
the beginning of limb destruction
destroy those limbs!
A lost limb due to transposition
transposition to the max!
Cleavage is a "flattening" fabric Pure shear (coaxial) vs. Simple shear (noncoaxial)? What about loss of volume?
What is it? What is the closely spaced fabric? Are these similar or parallel folds? Free folds, forced folds, or passive folds? What is direction of S1 (maximum stretch)? axial planar cleavage similar folds passive folds S1 is horizontal
Next Lecture: Foliations, lineations, and tectonites Please read (D&R, pp ; )
Important terminology/concepts cleavage axial planar cleavage continuous cleavage slaty cleavage phyllitic cleavage schistosity crenulation cleavage strain significance of cleavage pressure solution stylolites passive folds transposition flattening (pure shear) fabric volume loss (dilational strain)