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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

•Includes:

•Texture

•Composition

•Microstructure

•Preferred Orientation

Fabric

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Quartz-mica schistFabric Elements

•Bedding (S0)

•Compositional layering

•Crystallographic orientation

•Fold Hinges

•Cleavage planes (S1)

•Mineral elongation lineation

Passchier and Trouw (1996)

Random fabric

Foliation Lineation

Metamorphic Fabrics• Foliation : Cleavage, Schistosity

• 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

Shear Bands

Symmetrical

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Crenulation Cleavage BeginningMore Advanced Fabric

Coarse Metasandstone-Strong Foliation

Two Foliations!

S2

S1

Hard Lumps in the Pudding: strain shadows

Pyrite

New Quartz

Strongly foliated matrix

Calcite

Evidence for rotation of cleavage during its formationIn fibrous mineral growth in the strain

shadows of resistant minerals such as pyrite (From Passchier and Trouw, 1996)

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Strain distribution in a Shear zone leads to rotations

Shear Sense Indicators

Evidence for Rotation during non-coaxial deformation

Garnet in Qtz-mica schist

Sense of Shear Indicators

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.