11/30/2015

Geol341 J. Toro Topics

• Fabrics • Foliation, cleavage, lineation Metamorphic Rocks and – Cleavage and Folds –Geometry Cleavage Development – Strain significance • Origin of Cleavage – Pressure solution – Passive rotation – Recrystallization • Shear zones

Many diagrams are from Earth Structure, van der Pluijm and Marshak, 2004 2013

Geothermal Gradient and Metamorphism Naming of Metamorphic Rocks

Slatey cleavage Gneiss Segregation of mafic and felsic components Where does it come from?

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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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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 Metamorphic Fabrics • Foliation : Cleavage, Schistosity • Foliation • Lineation: Mineral Lineation, Intersection Lineation – Cleavage – Schistosity • Lineation

Random fabric

S-tectonite L-tectonite L/S-tectonite Foliation Lineation

S-Tectonites L-Tectonites

Schists

Columbia Pluton, VA Lineated Gneiss

USGS photo U. Western Ontario photo

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L-S Tectonites 3D Strain - Flinn diagram No strain along 3rd dimension

Cigars

S =S >S S1/S2 1 2 3

S1>S2=S3

Lineated and foliated gneiss, Himalayas Prolate Pancakes

A. Tomkins photo S2/S3 Oblate

Two kinds of lineation How dense is the cleavage?

Intersection Lineation Mineral Elongation Lineation (Stretching Penetrative Fabric S0 = bedding lineation) S1 = First cleavage

S0/S1 Intersection S0

Spaced Fabric

S1

Cleavage Spaced cleavage • A kind of foliation • Deformation Fabric • Rock splits along preferred planes

Cambrian Rocks, South Verkhoyansk Lachlan fold belt, Australia foldbelt, Siberia Photo by E.L.Miller

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Axial planar The beginning of Metamorphism: cleavage is the plane Axial Planar of Cleavage flattening

Fossen

Axial Planar cleavage

Keck photo

Other relationships of cleavage to folding Processes of Cleavage Transected folds: Development

In areas with a component of • Pressure solution strike slip • Rotation of minerals • Recrystallization • Crenulation

Fossen

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Cleavage Cleavage formation during thrusting Development

Early Cleavage Vertical compaction- Horizontal cleavage

Tectonic Shortening- Cleavage and folding vertical spaced cleavage

More tectonic shortening- vertical slatey cleavage along axial planes

SEM Image of Clays in Shale Pressure solution in Clay-rich Limestone

Spaced Pressure-Solution Cleavage

Small quartz vein cut by a spaced crenulation/solution cleavage 2 mm 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 Sandstone after compaction

Beginning of foliation 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 Two Foliations! Foliation

S2

S1

Hard Lumps in the Pudding: strain shadows

Calcite

Pyrite New Quartz Evidence for rotation of cleavage during its formation In fibrous mineral growth in the strain Strongly foliated matrix shadows of resistant minerals such as pyrite (From Passchier and Trouw, 1996)

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Strain distribution in a Shear Shear Sense Indicators zone leads to rotations

Evidence for Rotation during non-coaxial deformation 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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