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fabric foliation cleavage cross-


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									Lecture 17: fabrics, foliations, cleavages                                       fabric
                                                                                          texture; arrangements of grains and “elements” in a rock

  websites from which images are drawn:


                                                                                           any planar fabric in a rock
                                                                                               cleavage; phyllitic fabric; schistosity; gneissosity

 cleavage                                                                            cross-cutting relationship with bedding
    ubiquitous secondary fabric element
       • tendency to split into planes (planes of weakness) -- closely spaced,
                aligned, planar to curviplanar, penetrative surfaces
       • low temperature metamorphic to non-metamorphic conditions
                (below greenschist facies, i.e. < 350°C)
        • cross-cutting relationship with bedding

    associated with folded rocks

     cleaved rocks often are folded
     not all folded rocks are cleaved

  do not confuse cleavage with fracture
fracture and cleavage may be penetrative
…but cleavage has no loss of cohesion…

cross-cutting relationship with bedding (thin section)                           cross-cutting relationship with bedding

cross-cutting relationship with bedding                cross-cutting relationship with bedding

                   where is the bedding?                                  where is the cleavage?

cross-cutting relationship with bedding                cleavage typically is parallel to axial surface
                                                                i.e. forms perpendicular to shortening
                                                                        …z of strain ellipse…

         what is kinematic significance of cleavage?                      identify the folds…
                  associated with folded rocks

cleavage typically is parallel to axial surface        cleavage typically is parallel to axial surface
         i.e. forms perpendicular to shortening                 i.e. forms perpendicular to shortening
                 …z of strain ellipse…                                  …z of strain ellipse…

                   find the cleavage…                                     now, identify the axial surfaces…

cleavage typically is parallel to axial surface                                                 cleavage typically is parallel to axial surface
         i.e. forms perpendicular to shortening                                                  if this is the case, we can use bedding/cleavage angles to determine
                 …z of strain ellipse…                                                                where we are in larger structure -- cleavage vergence
                                                                                                                 think in terms of bedding or parasitic folds

categories of cleavage (based on appearance/morphology)                                         disjunctive cleavage: domainal structure can be seen with naked eye
                                                                                                                                                  why domains?
                                                                                                                                            distribution of clay in rock
                                                                                                                                                    not uniform
                                                                                                                                          evolution of cleavage domains
                                                                                                                                             where clay concentration high;
                                                                                                                                          form perpendicular to shortening;
                                                       from: van der Pluijm and Marshak, 1997
                                                                                                                                             • dissolution on faces normal to
“domainal structure”                                                                                                                                shortening;
  -- cleavage domains (foliae) vs. intervening rock (microlithons)                                                                          • diffusion of material away
               cleavage, C: thin, mica/clay rich, strong preferred orientation                                                                    from site of dissolution;
               microlithons, M: original rock, equigranular minerals                                                                        • precipitation where shortening
                                                                                                                                                    is less;
 • continuous cleavage--domainal structure not visible to naked eye                                                                         • collapse of clay flakes
        (slaty cleavage, phyllitic structure, schistosity)
                                                                                                                                           field observations indicate rock
 • disjunctive cleavage--domainal structure visible to naked eye                                                                              must have > 10% clay for
         (spaced, pencil and crenulation cleavage)                                                                                                 cleavage to develop
                                                                                                 from: van der Pluijm and Marshak, 1997

spaced cleavage:                                                                                spaced cleavage
 spacing of cleavage domains determines nature of spaced cleavage                                  array of parallel to anastamosing, stylolitic to smooth,
        -- depends on initial clay content and strain --                                                   partings occupied by clayey and carbonaceous matter
              high clay content: domains more closely spaced
              high strain: domains more closely spaced

                                              sketches of morphological
                                           characteristics of spaced cleavage

                                                     d is considered
                                                 anastamosing cleavage
                                                    (wavy domains)
                                                                                                                                                             from: Twiss and Moores

                                                                                                     …spacing of partings (cleavage, C, above) ~1 to 10 cm
                                                                                                     …thickness of partings (cleavage, C, above) ~0.02 to 1 mm
                                                                                                                   C (cleavage) separates M, microlithons
                                    from: van der Pluijm and Marshak, 1997

can have different levels of preferred orientation in M, microlithons                                  characteristic of spaced cleavage is offset of markers along cleavage

                                                                                                                                                         arrows show “offset”

                                                                                                          looks fault-like, but is not…cleavage surfaces are NOT faults
                              reflects increasing strain

            when microlithons have strong preferred orientation                                            truncation of fossils/markers at boundaries of microlithons
                      similar to cleavage domains                                                            cannot be restored by fault-slip motions       material lost
                 …approaching continuous cleavage…
                                                                                                                     …stay tuned -- more on this later…

pencil cleavage (type of spaced cleavage)                                                                                                      formation of pencil cleavage
   • breaks into pencil-like pieces 5-10 cm long and 0.5 cm in diameter                                                                           • clay attains orientation parallel
   • found in weakly deformed shale                                                                                                                         to bedding during
                                                                                                                                                      deposition and compaction
                                                                                                                                                          (strain ellipse, a)
                                                                                                                                                  • shale undergoes layer-parallel

                                                                                                                                                  • mica/clay grains rotate to
                                                                                                                                                    perpendicular to shortening
                                                                                                                                                       (strain ellipse, b)
                                                                                                                                                   • incipient foliation forms, which
                                                                                                                                                            produces pencils

                                                                                                                                                      continued shortening produces
                                               from: http://www.rci.rutgers.edu/~geolweb/slides.html
                                                                                                                                                               slaty cleavage
                                                                                                                                                        at high angles to bedding
   reflects tendency to part along bedding plane equal to that of
                                                                                                                                                             (strain ellipse, c)
     parting along incipient cleavage (i.e. cleavage not well formed)
                                                                                                                                              from: van der Pluijm and Marshak, 1997

 crenulation cleavage                                                                                   crenulation cleavage                        from: http://earth.leeds.ac.uk/learnstructure/index.htm

      • lithology containing closely and evenly spaced foliation
      • crenulation is re-working of a pre-existing foliation
                --“crinkles”, or folds, original foliation
                 -- axial plane of microfolds define crenulation foliation


                                                                                                        two thin-sections reflect
 from: van der Pluijm and Marshak, 1997
                                                                                                          different stages of
                                          symmetric                 asymmetric
                                                                                                             crenulation cleavage
         crenulation cleavage may be symmetric or asymmetric
                  …remember, it is a second foliation….
                                                                                                                          from: http://www.rci.rutgers.edu/~geolweb/slides.html          asymmetric

two types of crenulation cleavage:
  discrete: narrow cleavage domains cut continuous cleavage                                                                                                        discrete
               of microlithons (forms in slate)
                   … looks like small faults
  zonal: wide cleavage domains coincide with limbs of microfolds
          in pre-existing continuous cleavage (forms in phyllite/schist)
                            …pre-existing cleavage preserved in microlithons

           discrete (Sc2)                                            zonal
from: http://slate.ias.hiroshima-u.ac.jp/photo.htm    from: http://www.rci.rutgers.edu/~geolweb/slides.html

                                                                                                              differentiation during formation of crenulation cleavage

                                                                                                               from: van der Pluijm and Marshak, 1997

                                                                                                                    • as crenulations form, quartz removed from limbs of microfolds
                                                                                                                              where applied stress is greatest…
                                                                                                                       and is re-precipitated in hinges where differential stress is less;
                                                                                                                        phyllosilicates (do not dissolve) concentrate on limbs and quartz in hinge

                                                                                                                    • as process continues, phylloscilicates rotate and may recrystallize
                                                                                                                       into parallelism with new foliation
                                              zonal                                             discrete
                                                                                                                     note: in right image, can no longer see microfolds---
                                                                                                                          --difficult to tell that S2 is a second foliation--no remnants of S1

                                                                                                              continuous cleavage: slaty cleavage
                                                                                                                   closely spaced domains such that no uncleaved microlithons exist;
                                                                                                                      …strong dimensional-preferred orientation of clays/micas
                                                                                                                   smooth and planar; penetrative
                                                                                                                     …resulting rock is a slate…can be split into thin sheets
                                                                                                                             a property that made slate a popular roofing material

                                                                                                                                                        slaty cleavage

high magnification shows slaty cleavage has                                        strain significance of slaty cleavage
    …lenticular aggregates of quartz and feldspar (QF) that are                     …debated for over 100 years…many things need explaining:
                 enveloped by anastamosing mica-rich laminae (M)
                                                                                            • geometric coordination of folds and cleavage
       M-domains: (do not confuse with M for microlithon)
                mica-rich domains--cleavage domains; < 5µm thick
                                                                                            • mechanical role of cleavage in folding process
                       …original rock completely altered and changed
       QF-domains: quartz-feldspar domains--microlithons                                    • kinematic development of preferentially oriented minerals
                        …individual grains equigranular, represent original rock

       “flattened” nature of grains in M-domains accentuated by
                                                                                            • division into domainal structure
  …overgrowths of chlorite and quartz…beards on quartz, feldspar, pyrite
                                                                                            • occurrence of clay-filled partings in rocks with
                                                                                                           spaced cleavages

                                                                                            • development of bearded overgrowths of chlorite/quartz
                                                                                            • whereabouts of missing parts of truncated fossils

                                                                                            • overall strain significance of cleavage

                                                                                   slaty cleavage and strain
strain significance of slaty cleavage
                                                                                      cleavage generally forms perpendicular to Z (shortening)
  mechanisms proposed:
                                                                                          …thus cleavage plane is XY plane of strain
    1) rigid body formation of individual platy minerals                            fossils, reduction spots, other structures in slates
    2) prefered directional recrystallization of individual minerals                         typically are flattened in the plane of cleavage,
                                                                                                     providing evidence for shortening
    3) fault-like translation along closely spaced fault surfaces                   …first noted in mid 1800s by Phillips, Sharpe, Sorby in Britain

                                                                                          shortening on average of 65%-75% for slaty cleavage
  all assumed constant volume
      …now realize that dominant process likely is pressure solution
            with removal of original rock material                                 generally accepted that cleavage forms perpendicular to shortening--
                                                                                   what is mechanism to permit rocks to shorten and develop cleavage?

slaty cleavage and strain                                                          mechanical (rigid body) rotation is not sufficient on its own
                                                                                         …does not explain why micas/clays are densely
generally accepted that cleavage forms perpendicular to shortening--                             concentrated in M-domains….
what is mechanism to permit rocks to shorten and develop cleavage?
                                                                                   let us examine recrystallization a little bit more…
  early focus on platy minerals: mica and clays
     …how to bring them into parallel alignment?                                     directional crystallization--new minerals grow in cleavage plane
                                                                                           …enhances flattened, elongate appearance of minerals…
  …rigid-body rotation--micas/clays rotate certain amount                            new growth in pressure shadows next to large rigid grains…
                              depending upon initial orientation                            …shadows grow as microscopic beards from
                                                                                                   pyrite, quartz, feldspar, garnet grains
  …recrystallization--micas/clays grow in common direction
                               in response to stress

                                                                                    direction of crystal growth is direction of incremental extension

                                                                                                  recrystallization is also not sufficient to explain slaty cleavage
                                                                                                         …does not explain domainal character and
                                                                                                                  thinness of quartz and feldspar in M-domains

                                                                                                   examine pressure solution…
                                                                                                         …can accommodate significant distortion in
                                                                                                                 sedimentary and metamorphic rocks…

                                                                                                    the pressing issue was whether low-grade cleavage formation
                                                                                                            is volume-constant or volume-loss

                                                                                                     when it became evident that constant volume deformation for
                                                                                                                 cleavage formation doesn’t work…
                                                                                                          …role of pressure solution became more clear…

pressure solution is important                                                                    deformed quartzite-pebble conglomerates
 •   provides mechanism for removal of material;                                                       lent insight into process
 •   indicates volume loss of 50% in some cases; others none;                                     …when quartzite pebbles in conglomerate
 •   depends on open vs. closed system;
 •   implies deformation important in rock-water interactions and fluid/rock ratios
                                                                                                       are forced into contact during
                                                                                                         deformation, quartzite in
                      loss                             no-loss                                         one or both pebbles dissolves
                                                                                                  …dissolution accommodates shortening…

                                                  preferential volume loss

                                                         from: van der Pluijm and Marshak, 1997

what causes pressure solution?                                                                     slaty cleavage shows all signs of pressure solution:
                                                                                                           …lenslike and trapezoidal grains of quartz/feldspar
        • high stress concentration at grain contacts makes material                                              are remnants of more equant grains
                there more soluble                                                                         cleavage parallel sides of grains are loci of pressure solution
        • material there migrates along grain boundary to places on
               sides of grains where stress is lower and deposits

                                                           solution of material
                                                             at grain-to-grain
                                                                        (purple)                         some missing material (dissolved grains) may be accounted for in
                                                                                                                overgrowths in pressure shadows…but much cannot…
                                                                                                               …and must have been transported out of system….
                      movement of dissolved ions along chemical gradient
                                                                                                    densely packed concentrations of micas and carbonaceous matter
                        redeposition at “sheltered” grain margins (gray)                            in cleavage seams are accumulations of insoluble residue

                                                                        stylolites: seams of insoluble residue oriented normal to σ1
                                                                                     σ1 teeth are concentrations of clays, carbonaceous matter

                                                                                                      sometimes, volume is preserved…
                                                                                                        …veins form perpendicular to stylolites;
                                                                                                      more often, volume loss occurs;
                                                                                                        …as much as 50% has been documented

 pressure solution and grain alignment                                   estimates of volume loss
    examination of chlorite in Martinsburg Slate (Appalachians)
                                                                              can evaluate volume loss quantitatively if you have
                                                                                     objects of known original shape and size
                                                                              …done in Martinsburg Formation with graptolites
          selective corrosion
               of grains
                                                                          deformed graptolites
                                                                            were narrower and shorter
        yields physical alignment,                                               than undeformed
         but not crystallographic
             (no new growth)
                                                                           50% shortened perpendicular
                                                                                 to cleavage
                                                                           …represents 50% volume loss

                                                                                            from: Davis and Reynolds, 1996

                  from: Davis and Reynolds, 1996
                                                                        slaty cleavage represents significant shortening and dilatational strain

constraints on pressure solution                                        crenulation cleavage development also linked to pressure solution
       temperature: most common between 50° and 400°C
                                                                           • cleavage domains of S2 emerge along limbs (or former limbs)
                       (between diageneseis and greenschist facies)
                                                                                   of the microfolds of the original S1 continuous cleavage
       grain size: faster at smaller grain sizes for constant stress;
                       (surface area increases with decreasing size)       • spacing of cleavage is function of wavelength of microfolds
       impurities: enhanced pressure solution                                      and amount of solution-induced shortening across limbs
   …upper temperature limit for pressure solution:                         • dissolution of soluble material takes place along fold limbs
            100 µm grain: quartz is 450°C; calcite is 300°C               • transport and concentration of insoluble residue occurs in hinges
           1000 µm grain: quartz is 300°C; calcite is 200°C
 pressure solution provides for
   diffusional mass transfer
   at shallow crustal depths

              not limited to cleavage              apparent
                fossil                                                                                                       from: Davis and Reynolds, 1996

            (e.g. crinoid)                                                 if enough shortening occurs, fold limbs are completely removed

continuous cleavage: phyllitic cleavage and schistosity
     • if metamorphic conditions reach greenschist facies…                                 porphyroblast
              …clay and illite react to form white mica and chlorite;
     • if deformation is occurring, a strong preferred orientation results;
           phyllite with phyllitic cleavage
     • if metamorphic conditions are higher than greenschist,
              coarser-grained mica and other minerals form
           form schist with schistosity--not as easily split as slaty cleavage
       porphyroclasts (relict large, non-platy crystals) may occur
       porphyroblasts (newly grown large, non-platy crystals) may occur
         micas curve around porphyroblasts and porphyroclasts
                                                        schist                                                                        porphyroclast

                                           with micas
from: http://www.leeds.ac.uk/learnstructure/index.htm                                                                                  from: microstructures photo-CD by Carol Simpson

                                                                                 gneissic layering: defined by compositional banding in gneiss
                                                                                    (metamorphic rock)
                                                                                                              how does it form?

                                                                                                                                  from: http://www.rci.rutgers.edu/~geolweb/slides.html

                                                                                                                           • inheritance from original lithology, a

                                                                                                                           • transposition to create new layering, b

                                                                                                                           • metamorphic differentiation, c

                                                                                  from: van der Pluijm and Marshak, 1997


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