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Metamorphic Fabrics

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