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Development of Crenulation and Crenulation Cleavage

Structural Geology

Andy Migliazzo

on 5 May 2014

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Transcript of Development of Crenulation and Crenulation Cleavage

What is Crenulation?
What can crenulation tell us?
This secondary cleavage fabric is restricted to rocks with a pre-existing, well developed foliation defined by anisotropic phylosillicate minerals.
Progression of crenulation cleavage
Andrew Migliazzo
Development of Crenulation and Crenulation Cleavage
Crenulated Peach Bottom Slate
Crenulated Amphibolite
Slaty cleavage
S/C fabric
-The progression of re-folding these fabrics into a series of micro-folds, leads to creating new cleavage planes. Cleavage planes tend to form perpindicular to maximum shortening direction (X)
-Secondary microfold fabrics developed in low grade metamorphic environment, by the combination of
1.) micro folding
2.) buckling instibilities
3.) pressure solution
as a result of two or more stress factors.
Stress mechanisms
-Under the conditions during crenulation, the minerals in the rock are also undergoing meramorphic differentiation. This involves three processes
-Pressure solution (also known as wet diffusion)
-Mineral migration
-Crenulation will either form symmetrical or Asymmetrical depending on the angle ratio between the existing foliation and secondary stress field.
-Buckling stages of micaceous fabric producing spaced planes of weakness due to the alignment of mica flakes on the limbs
-Evidence of crenulation fabrics can indicate the locality and direction of a larger ductile shear zone boundary.
Crenulation cleavage
is a series of these microfolds at the centimeter scale or less with parallel axial surfaces.
-mm to cm scale
-First classified in the mid 1980's, since then
has had 19 different names due to the variability
of its structure, but they are all developed by the
same processes.
-As differential stress is applied, the strain response/rate vary between mineral domains.
Metamorphic Differentiation
and pressure solution
This being said, as the minerals are being pushed into
new orientations as they group into "domains" of new mineral precentiles.
If they form Asymmetrical..
-mica-rich domains in the long limbs
-quartz-feldspar-rich domains in the short limbs

-mica-rich domains in the limbs of the folds
-quartz-feldspar-rich domains in the hinges
Mica-Schist w/chlorite
section a-43.5% Muscovite, 14.6% biotite, 28.3% quartz, 13.6% plagioclase
section b-47.6% muscovite, 17.9% biotite, 23.1% quartz, 11.4% plagioclase
section c-41.5% muscovite, 11%biotite, 34.3% quartz, 13.6% plagioclase
Mica-Schist during different stages of crenulation
-Mineral migration and reassemble through stages
-Stages in these crenulation development can indicate
location boundaries in the larger shear zone.
-figure A & B above showing
different locations and structures along a larger fold
crenulation in
Peach Bottom Slate
-maximum principal stress
is applied parallel to the coordinate
direction (mineral fabric)
-the material deforms by plain strain
-the material is anisotropic
-Paired folds with paired axial surfaces
symmetrical to each other
formation model of
1.)conjugate crenulation
cleavage (Muff (in Peach et el 1909)
2.) modified model
Crenulated bed sheet
-Goes for $145
-comes in two kinds
The end
Garnet-Mica Schist
Slight crenulation in
Distinct crenulation cleavage
Full transcript