Metamorphic Rocks Foliations, and Lineations

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

Looks Like Bedding, but is it?

Metamorphic layering

Broosk Range, AK

Isoclinal Fold

Quartz-mica schist

Fabric Elements•Bedding

•Compositional layering

•Mineral grains

•Fold Hinges

•Axial planes

•Planes of parting or fracture

•etc

Metamorphic Fabrics• Foliation

– Cleavage– Schistosity

• Lineation

S-tectonite L-tectonite L/S-tectonite

Random fabric

Foliation

Lineation

Penetrative Fabric

Spaced Fabric

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

The beginning of Metamorphism:

Axial Planar Cleavage

Relationship o cleavage to folding

Axial-planar cleavage

Axial Planar cleavage

Processes of Cleavage Development

• Rotation of minerals

• Crenulation

• Pressure solution

• Recrystallization

SEM Image of Clays in Shale

Pressure solution in Clay-rich Limestone

Early pressure-solution cleavage in limestone

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

Sequential diagram of % shortening by folding (initially by buckling), cleavage development and dissolution. Courtesy of David Gray, Monash University, Australia

Sandstone after compaction

Beginning of foliation

Crenulation Cleavage

Small scale kinks and pressure solution surfaces

Crenulation Cleavage Beginning

More Advanced Fabric

Coarse Sandstone-Strong Foliation

Two Foliations!

S2

S1

Spaced Pressure-Solution Cleavage

Shortening by folding vs. shortening by cleavage