Presentation on theme: "Chapter 17 Cleavage and Foliations. Cleavage ► Cleavage – A prominent planar structure that may differ in orientation from the bedding and indicate subsequent."— Presentation transcript:
Cleavage ► Cleavage – A prominent planar structure that may differ in orientation from the bedding and indicate subsequent deformation and metamorphism
Cleavage ► Continuous Cleavage – Pervades the rock mass ► Spaced Cleavage – Domains of uncleaved rocks separated by cleavage planes ► Microliths – Uncleaved zones between cleavage surfaces
Continuous Cleavage ► Gradation of continuous cleavage intensity Slaty Cleavage – In fine grained rocks and associated with low grade metamorphism Phyllitic Structure - Phyllitic texture with satiny luster Schistosity – Forms in medium grained rocks under medium to high grades of metamorphism
S-Surfaces ► S-Surface – All planar and some curved structures in deformed rocks i.e. Cleavage, foliations, and bedding S 0 – Oldest surface, S 1 – next oldest, etc. Fold Sets – F 1, F 2, etc. Linear Structures – L 1, L 1, etc. Deformation Structures – D 1, D 2, etc.
Discrete Crenulations Scale 1mm; Discrete crenulated truncating an earlier crenulated fabric. Ordovician rock New South Wales; Seyfert 1987 Scale 125 micrometer; Discrete crenulations in slate fabric. Thin films of muscovite. Seyfert 1987.
Progressive Cleavage Development in Fine- Grained Sediment 1. Undeformed condition 2. Earliest deformation stage – Volume loss from reorientation and expulsion of water 3. Pencil structure – Formed from intersection of bedding and cleavage 4. Embryonic cleavage stage – Parallel to fold axis resulting from pressure solution and recrystalization 5. Cleavage stage – Strong planar fabric from reorientation and recrystallization of clays 6. Strong cleavage with mineral lineation – More of above with mineral-elongation parallel to the X direction of strain
Zonal Crenulations Scale 500 micrometers; Zonal crenulations in schist fabric from South Australia. Dissolution of quartz and albite in microlithons and increase in biotite. Seyfert 1987. Scale 2mm; Zonal crenulations of carbonaceous and micaceous domains with dissolution of quartz in the initial fabric from New South Wales; Seyfert 1987.
Stages of Cleavage Development Pencil structure Embryonic Cleavage Stage Cleavage stage Well-developed Cleavage stage with mineral lineations
Cleavage Formation ► Cleavage is related to folding and it frequently parallels the axial planes of folds. ► The maximum principal shortening direction is perpendicular to cleavage planes. ► The XY plane in the strain ellipse parallels cleavage planes. ► As much as 75% shortening has been observed
Cleavage in Slate Mudstone devoid of cleavage from Wales; Clay flakes mainly chlorite with random arrangement Ill-cleaved mudstone from Wales; Planar structure developing Highly cleaved slate from Wales; Closely spaced layers with thin lenticels of randomly arranged flakes Borradaile et al. 1982; 20 micrometer scale SEM