19Cleavage (Estud)

1Metamorphic Rocks and Cleavage Development

Geol341J. Toro

Many diagrams are from Earth Structure, van der Pluijm and Marshak, 2004Chapter 12- Fossen

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 Gneiss

2Looks Like Bedding, but is it?

Brooks Range, AK

Metamorphic layering

Isoclinal FoldQuartz-mica schist

Arrangement of component features in a rock van der Pluijm & Marshak

Includes:

Texture

Structure

Microstructure

Preferred Orientation

FabricFabric Elements

Bedding

Compositional layering

Crystallographic orientation

Fold Hinges

Cleavage planes

Mineral elongation lineation

Passchier and Trouw (1996)

3Random fabric

Foliation Lineation

Metamorphic Fabrics Foliation : Cleavage, Schistosity Lineation: Mineral Lineation, Intersection Lineation S0 = bedding

S1 = First cleavage

Intersection Lineation

S0S0/S1 Intersection

S1

Mineral Elongation Lineation (Stretching

lineation)

Metamorphic Fabrics Foliation

Cleavage Schistosity

Lineation

S-tectonite L-tectonite L/S-tectonite

Penetrative Fabric

Spaced Fabric

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

Lachlan fold belt, AustraliaPhoto by E.L.Miller Paleozoic Quartzite, Brooks Range, AK

4Spaced cleavage

Cambrian Rocks, South Verkhoyansk foldbelt, Siberia

The beginning of Metamorphism:

Axial Planar Cleavage

Relationship of cleavage to folding

Axial-planar cleavage

Keck photo

Axial Planar cleavage Processes of Cleavage Development

Rotation of minerals Recrystallization Pressure solution Crenulation

5Cleavage Development

Vertical compaction-Horizontal cleavage

Tectonic Shortening-vertical spaced cleavage

More tectonic shortening-vertical slatey cleavage along axial planes

Cleavage formation during thrusting

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 cleavageDocumenting the extent of material removal along these surfaces.Photo from David Gray, Monash University, Australia

6Sequential 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 Crenulation Cleavage

Small scale kinks and pressure solution surfaces

7Crenulation Cleavage BeginningMore Advanced Fabric

Coarse Sandstone-Strong Foliation Two Foliations!

S2

S1

Hard Lumps in the Pudding: strain shadows

PyriteQuartz

Impressive evidence for rotation of cleavage during its formationcan sometimes be read from fibrous mineral growth in the strain

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

8Strain distribution in a Shear zone 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 parallel to direction of shear or

flow Bedding becomes transposed during

foliation development Asymmetric fabrics reveal sense of shear.