11/30/2015 1 Metamorphic Rocks and Cleavage Development Geol341 J. Toro 2013 Many diagrams are from Earth Structure, van der Pluijm and Marshak, 2004 Topics • Fabrics • Foliation, cleavage, lineation – Cleavage and Folds – Geometry – Strain significance • Origin of Cleavage – Pressure solution – Passive rotation – Recrystallization • Shear zones Geothermal Gradient and Metamorphism Naming of Metamorphic Rocks Slatey cleavage Where does it come from? Gneiss Segregation of mafic and felsic components
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Metamorphism Naming of Metamorphic Rockspages.geo.wvu.edu/~jtoro/Structure/ppt/19Cleavage.pdf11/30/2015 1 Metamorphic Rocks and Cleavage Development Geol341 J. Toro Many diagrams are
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11/30/2015
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Metamorphic Rocks and Cleavage Development
Geol341
J. Toro
2013Many diagrams are from Earth Structure, van der Pluijm and Marshak, 2004
Topics
• Fabrics• Foliation, cleavage, lineation
– Cleavage and Folds– Geometry– Strain significance
• Origin of Cleavage– Pressure solution– Passive rotation– Recrystallization
• Shear zones
Geothermal Gradient and Metamorphism Naming of Metamorphic Rocks
Slatey cleavageWhere does it come from?
Gneiss Segregation of mafic and felsic components
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Looks like bedding, but is it?
Brooks Range, AK
Metamorphic layering
Quartz-mica schistIsoclinal Fold
Transposition of Layering
Horizontal fabric => Vertical fabric
“Arrangement of component features in a rock” van der Pluijm & Marshak
• Lineation: Mineral Lineation, Intersection Lineation
Metamorphic Fabrics• Foliation
– Cleavage
– Schistosity
• Lineation
S-tectonite L-tectonite L/S-tectonite
L-Tectonites
Columbia Pluton, VA Lineated Gneiss
USGS photo U. Western Ontario photo
S-Tectonites
Schists
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L-S Tectonites
Lineated and foliated gneiss, Himalayas
A. Tomkins photo
3D Strain - Flinn diagram
Cigars
Pancakes
S2/S3
S1/S2
S1>S2=S3
S1=S2>S3
No strain along 3rd
dimension
Oblate
Prolate
S0 = bedding
S1 = First cleavage
Intersection Lineation
S0
S0/S1 Intersection
S1
Mineral Elongation Lineation (Stretching
lineation)
Two kinds of lineation
Penetrative Fabric
Spaced Fabric
How dense is the cleavage?
Cleavage• A kind of foliation
• Deformation Fabric
• Rock splits along preferred planes
Lachlan fold belt, AustraliaPhoto by E.L.Miller
Spaced cleavage
Cambrian Rocks, South Verkhoyansk foldbelt, Siberia
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Axial planar
cleavage is the plane
of flattening
Fossen
The beginning of Metamorphism:
Axial Planar Cleavage
Keck photo
Axial Planar cleavage
Other relationships of cleavage to folding
Transected folds:
In areas with a component of strike slip
Fossen
Processes of Cleavage Development
• Pressure solution
• Rotation of minerals
• Recrystallization
• Crenulation
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Cleavage Development
Vertical compaction-Horizontal cleavage
Tectonic Shortening-vertical spaced cleavage
More tectonic shortening-vertical slatey cleavage along axial planes
Cleavage formation during thrusting
Early Cleavage
Cleavage and folding
SEM Image of Clays in Shale Pressure solution in Clay-rich Limestone
Spaced Pressure-Solution Cleavage
2 mmSmall quartz vein cut by a spaced crenulation/solution cleavageDocumenting the extent of material removal along these surfaces. Photo from David Gray, Monash University, Australia
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Sequential diagram of % shortening by folding (initially by buckling), cleavage development and dissolution. Courtesy of David Gray, Monash University, Australia
Shortening by folding vs. shortening by cleavage
Bedding/cleavage relationship Sandstone after compaction
Beginning of foliation
So
S1
Crenulation CleavageSmall scale kinks and pressure solution surfaces