Metamorphic fabrics

11/30/2015
Geol341
J. Toro
Topics
• Fabrics
• Foliation, cleavage, lineation
Metamorphic Rocks and
Cleavage Development
– Cleavage and Folds
– Geometry
– Strain significance
• Origin of Cleavage
– Pressure solution
– Passive rotation
– Recrystallization
• Shear zones
Many diagrams are from Earth Structure, van der Pluijm and Marshak, 2004
Geothermal Gradient and
Metamorphism
Slatey cleavage
Where does it come from?
2013
Naming of Metamorphic Rocks
Gneiss
Segregation of mafic and felsic components
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11/30/2015
Looks like bedding, but is it?
Metamorphic layering
Brooks Range, AK
Quartz-mica schist
Isoclinal Fold
Transposition of Layering
Fabric
“Arrangement of component features in a rock” van
der Pluijm & Marshak
•Includes:
•Texture
•Composition
•Microstructure
•Preferred Orientation
Horizontal fabric => Vertical fabric
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11/30/2015
Quartz-mica schist
Fabric Elements
•Bedding (S0)
•Compositional layering
•Crystallographic orientation
•Fold Hinges
•Cleavage planes (S1)
•Mineral elongation lineation
Passchier and Trouw (1996)
Metamorphic Fabrics
• Foliation : Cleavage, Schistosity
• Lineation: Mineral Lineation, Intersection Lineation
Metamorphic Fabrics
• Foliation
– Cleavage
– Schistosity
• Lineation
Random fabric
S-tectonite
Foliation
Lineation
L-Tectonites
L-tectonite
L/S-tectonite
S-Tectonites
Schists
Columbia Pluton, VA
USGS photo
Lineated Gneiss
U. Western Ontario photo
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11/30/2015
L-S Tectonites
3D Strain - Flinn diagram
No strain along 3rd
dimension
Cigars
S1=S2>S3
S1/S2
S1>S2=S3
Lineated and foliated gneiss, Himalayas
Prolate
Pancakes
Two kinds of lineation
Intersection Lineation
S0 = bedding
Oblate
S2/S3
A. Tomkins photo
How dense is the cleavage?
Mineral Elongation
Lineation (Stretching
lineation)
Penetrative Fabric
S1 = First cleavage
S0/S1 Intersection
S0
Spaced Fabric
S1
Spaced cleavage
Cleavage
• A kind of foliation
• Deformation Fabric
• Rock splits along preferred planes
Lachlan fold belt, Australia
Photo by E.L.Miller
Cambrian Rocks,
South Verkhoyansk
foldbelt, Siberia
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11/30/2015
Axial
planar
cleavage
is the plane
of
flattening
The beginning of
Metamorphism:
Axial Planar
Cleavage
Fossen
Axial Planar cleavage
Keck photo
Other relationships of cleavage to
folding
Processes of Cleavage
Development
Transected folds:
In areas with a
component of
strike slip
•
•
•
•
Pressure solution
Rotation of minerals
Recrystallization
Crenulation
Fossen
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11/30/2015
Cleavage
Development
Vertical compactionHorizontal cleavage
Tectonic Shorteningvertical spaced cleavage
Cleavage formation during thrusting
Early Cleavage
Cleavage and folding
More tectonic shorteningvertical slatey cleavage
along axial planes
SEM Image of Clays in Shale
Pressure solution in Clay-rich
Limestone
Spaced Pressure-Solution Cleavage
2 mm
Small quartz vein cut by a spaced crenulation/solution cleavage
Documenting the extent of material removal along these surfaces.
Photo from David Gray, Monash University, Australia
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Shortening by folding vs. shortening by cleavage
Sequential diagram of % shortening by folding (initially by
buckling), cleavage development and dissolution.
Courtesy of David Gray, Monash University, Australia
Bedding/cleavage relationship
Beginning of foliation
Sandstone after compaction
Crenulation Cleavage
Small scale kinks and pressure solution surfaces
So
Shear Bands
Symmetrical
S1
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More Advanced Fabric
Crenulation Cleavage Beginning
Coarse Metasandstone-Strong
Foliation
Two Foliations!
S2
S1
Hard Lumps in the Pudding:
strain shadows
Calcite
Pyrite
New
Quartz
Strongly foliated matrix
Evidence for rotation of cleavage during its formation
In fibrous mineral growth in the strain
shadows of resistant minerals such as pyrite
(From Passchier and Trouw, 1996)
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11/30/2015
Strain distribution in a Shear
zone leads to rotations
Evidence for Rotation during
non-coaxial deformation
Shear Sense Indicators
Sense of Shear Indicators
Garnet in Qtz-mica schist
Take home ideas
• Metamorphic fabric is a product of
deformation
• Foliation is perpendicular to flattening
direction
• Lineation is often parallel to direction of
shear or flow
• Bedding becomes transposed during
foliation development
• Asymmetric fabrics reveal sense of shear.
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