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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, Australia Photo 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 cleavage Documenting 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
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