1Metamorphic Rocks and Cleavage DevelopmentGeol341J. ToroMany diagrams are from Earth Structure, van der Pluijm and Marshak, 2004Chapter 12- FossenTopics• Fabrics• Foliation, cleavage, lineation– Cleavage and Folds–Geometry– Strain significance• Origin of Cleavage– Pressure solution– Passive rotation– Recrystallization• Shear zonesGeothermal Gradient and MetamorphismNaming of Metamorphic RocksSlatey cleavageGneiss2Looks Like Bedding, but is it? Brooks Range, AKMetamorphic layeringIsoclinal FoldQuartz-mica schist“Arrangement of component features in a rock” van der Pluijm & Marshak•Includes: •Texture•Structure•Microstructure•Preferred OrientationFabricFabric Elements•Bedding•Compositional layering•Crystallographic orientation•Fold Hinges•Cleavage planes•Mineral elongation lineationPasschier and Trouw (1996)3Random fabricFoliation LineationMetamorphic Fabrics• Foliation : Cleavage, Schistosity• Lineation: Mineral Lineation, Intersection LineationS0 = beddingS1 = First cleavageIntersection LineationS0S0/S1 IntersectionS1Mineral Elongation Lineation (Stretching lineation)Metamorphic Fabrics• Foliation – Cleavage– Schistosity• LineationS-tectoniteL-tectonite L/S-tectonitePenetrative FabricSpaced FabricCleavage• A kind of foliation• Deformation Fabric• Rock splits along preferred planesLachlan fold belt, AustraliaPhoto by E.L.MillerPaleozoic Quartzite, Brooks Range, AK4Spaced cleavage Cambrian Rocks, South Verkhoyansk foldbelt, SiberiaThe beginning of Metamorphism: Axial Planar CleavageRelationship of cleavage to foldingAxial-planar cleavageKeck photoAxial Planar cleavage Processes of Cleavage Development• Rotation of minerals • Recrystallization• Pressure solution • Crenulation5Cleavage DevelopmentVertical compaction-Horizontal cleavageTectonic Shortening-vertical spaced cleavageMore tectonic shortening-vertical slatey cleavage along axial planesCleavage formation during thrustingSEM Image of Clays in ShalePressure solution in Clay-rich LimestoneSpaced Pressure-Solution CleavageSmall quartz vein cut by a spaced crenulation/solution cleavageDocumenting the extent of material removal along these surfaces.Photo from David Gray, Monash University, Australia6Sequential diagram of % shortening by folding (initially by buckling), cleavage development and dissolution. Courtesy of David Gray, Monash University, AustraliaShortening by folding vs. shortening by cleavageBedding/cleavage relationshipSandstone after compactionBeginning of foliationCrenulation CleavageSmall scale kinks and pressure solution surfaces7Crenulation Cleavage BeginningMore Advanced FabricCoarse Sandstone-Strong FoliationTwo Foliations!S2S1Hard Lumps in the Pudding: strain shadowsPyriteQuartzImpressive 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 zoneShear Sense IndicatorsEvidence for Rotation during non-coaxial deformationGarnet in Qtz-mica schistSense of Shear IndicatorsTake 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