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11/30/2015 Geol341 J. Toro Topics • Fabrics • Foliation, cleavage, lineation Metamorphic Rocks and Cleavage Development – Cleavage and Folds – Geometry – Strain significance • Origin of Cleavage – Pressure solution – Passive rotation – Recrystallization • Shear zones Many diagrams are from Earth Structure, van der Pluijm and Marshak, 2004 Geothermal Gradient and Metamorphism Slatey cleavage Where does it come from? 2013 Naming of Metamorphic Rocks Gneiss Segregation of mafic and felsic components 1 11/30/2015 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 2 11/30/2015 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 • Foliation : Cleavage, Schistosity • Lineation: Mineral Lineation, Intersection Lineation Metamorphic Fabrics • Foliation – Cleavage – Schistosity • Lineation Random fabric S-tectonite Foliation Lineation L-Tectonites L-tectonite L/S-tectonite S-Tectonites Schists Columbia Pluton, VA USGS photo Lineated Gneiss U. Western Ontario photo 3 11/30/2015 L-S Tectonites 3D Strain - Flinn diagram No strain along 3rd dimension Cigars S1=S2>S3 S1/S2 S1>S2=S3 Lineated and foliated gneiss, Himalayas Prolate Pancakes Two kinds of lineation Intersection Lineation S0 = bedding Oblate S2/S3 A. Tomkins photo How dense is the cleavage? Mineral Elongation Lineation (Stretching lineation) Penetrative Fabric S1 = First cleavage S0/S1 Intersection S0 Spaced Fabric S1 Spaced cleavage Cleavage • A kind of foliation • Deformation Fabric • Rock splits along preferred planes Lachlan fold belt, Australia Photo by E.L.Miller Cambrian Rocks, South Verkhoyansk foldbelt, Siberia 4 11/30/2015 Axial planar cleavage is the plane of flattening The beginning of Metamorphism: Axial Planar Cleavage Fossen Axial Planar cleavage Keck photo Other relationships of cleavage to folding Processes of Cleavage Development Transected folds: In areas with a component of strike slip • • • • Pressure solution Rotation of minerals Recrystallization Crenulation Fossen 5 11/30/2015 Cleavage Development Vertical compactionHorizontal cleavage Tectonic Shorteningvertical spaced cleavage Cleavage formation during thrusting Early Cleavage Cleavage and folding More tectonic shorteningvertical slatey cleavage along axial planes SEM Image of Clays in Shale Pressure solution in Clay-rich Limestone Spaced Pressure-Solution Cleavage 2 mm 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 6 11/30/2015 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 Beginning of foliation Sandstone after compaction Crenulation Cleavage Small scale kinks and pressure solution surfaces So Shear Bands Symmetrical S1 7 11/30/2015 More Advanced Fabric Crenulation Cleavage Beginning Coarse Metasandstone-Strong Foliation Two Foliations! S2 S1 Hard Lumps in the Pudding: strain shadows Calcite Pyrite New Quartz Strongly foliated matrix Evidence for rotation of cleavage during its formation In fibrous mineral growth in the strain shadows of resistant minerals such as pyrite (From Passchier and Trouw, 1996) 8 11/30/2015 Strain distribution in a Shear zone leads to rotations Evidence for Rotation during non-coaxial deformation Shear Sense Indicators 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. 9
