FABRICS: FOLIATIONS AND LINEATIONS

Fabric- geometric arrangement of structures within a rock

Random Fabric- no preferred orientation of component elements

Preferred Fabric- fabric elements are aligned or repeated

Continuous Fabric- continuous on millimeter scale.

Continuous Fabric at Ait Ahmane, Bou Azzer, Morocco

Spaced Fabric- visible spacing between fabric elements.

   Domains- deformed planar element

   Microlithons- undeformed rock layers between Domains

        Two main classes:

Foliation- planar fabric

       Lineation- linear fabric

Primary- develop during formation of rock

Tectonic- produced by deformation processes

Tectonites-rocks with penetrative fabrics such that rock character is defined by continuous fabric elements.

        L-Tectonites- Linear fabric elements

        S-Tectonites- Planar fabric elements

PLANAR TECTONIC FABRICS

Cleavage-

    Tectonic planar fabric formed under low temperature conditions

     Imparts a tendency for rocks to split (cleave) along planes.

    Disjunctive Cleavage-

        Spaced foliations

        Form in clay rich (sedimentary) rocks

          Subjected to differential tectonic stress

          Under low (sub-greenschist) metamorphic conditions.   

           Cleavage domains mark stylolitic zones of pressure solution

        Separated by microlithons of relatively undeformed rock.

Spaced cleavage in Silurian limestone from Door County, Wisconsin

        Pencil Cleavage-

        Continuous cleavage

        Marked by the development of elongate pencil-like shards

Forms in weakly deformed shales or mudstone.

May form due to the intersection of two planar elements (two fracture sets, or a fracture set and bedding)

Or may form due to the internal alignment of clay.

�Pencils� are ~5-10 cm long.

Represents an early stage in the development of slaty cleavage.

Pencil Cleavage from Phyllite within the Anti-Atlas Mountains, Morocco

Crenulation Cleavage-

Secondary cleavage imposed upon an earlier cleavage

Axial planes of microfolds defines crenulation cleavage plane

Forms in fine grained rock (slate or phyllite)

Crenulation cleavage may be associated with pressolution.

    Crenulation cleavage in Phyllite, Rt 12, Baraboo Wisconsin

Symmetric Crenulations- accordian-like crenulations develop when the secondary cleavage is parallel to the pre-existing foliation. Common in hinge of folds.

Asymmetric Crenulations- crenulation cleavage is not parallel to the pre-existing foliation, resulting in shear along the crenulation cleavage planes resulting in S- or Z-shaped crenulations. Common on limbs of folds.

Transposition- replacement of an earlier tectonic fabric (S₁) by a more recent tectonic fabric (S₂) by rotation, recrystallization and pressolution.

Slaty Cleavage-

Clay rich rock that has developed continuous cleavage

No uncleavage microlithons remain.

Changes with deformation smectite(H₂O) to illite to mica.

Phyllitic Cleavage-

Forms in strongly aligned micas and clays

Produce silky, sometimes wavy foliation.

Phyllitic cleavage develops in clay rich rocks

Subjected to greenschist facies metamorphism

Smectites, illites transformed to muscovite and chlorite.

Schistosity-

Middle greenschist to higher metamorphic facies

Mineral growth forms visible crystals in strongly foliated fabric.

Mineral assemblages vary based on P, T,  protolith

     Porphyroclasts- large, non-platy pre-existing crystals.

     Augen Schists-   feldspar porphyroclasts (augen= eye)

     Porphyroblasts- large, non-platy, newly formed crystals

Garnet schist with quartz porphyroblast in Jackson County, Wisconsin

Gneissic Layering-

Paragneiss- sedimentary protolith

        Orthogneiss- igneous protolith

         Augen gneiss- visible feldspar �eyes�

Coarse grained, color banded metamorphic fabric

Produced by melting and recrystallization

Common color banding

   light (quartz, feldspar rich) layers

   dark (amphibole, biotite) layers.

The development of a gneissic fabric may involve:

     Inheritance- original compositional banding of parent rock

     Metamorphic Differentiation-

            partial melting (Anatexis) and recrystallization.

    Transposition of earlier (isoclinally) folded layers

            resulting in truncation of folded limbs

    Lit par lit intrusion- sill-like intrusion into parallel layers.

Gneissic banding at Siroua, Morocco

Migmatization-

   Product of an intensely melted rock w

   Exhibits characteristics of metamorphic and igneous fabrics.

   Commonly appears as a semi-chaotic color banding

   �Marble-cake�appearance

   Migmatites may be thought of as an intensely melted gneiss.

Archean Migmatites from the Lake Superior Region

Mylonitic Foliation-

Fine grained, strongly layered fabric

Forms by crystal plastic deformation

Mylonites are strongly foliated and lineated rocks

Lack mesoscopic brittle fabrics

Cleavage and Strain-

   definitely related but usually difficult to assess.

Big Question: how much volume loss?

                Strain Markers:

Reduction spots- small greenish spots in slate produced by reduced iron. If initially spherical these passive markers may be useful as passive strain markers. Reduction spot studies suggest that total shortening strain (e1) associated with slaty cleavage is >60%.

Graptolites- graptolite studies suggest volume loss of up to 50% in slates.

Cleavage, Bedding and Folding- card deck analogy

     Axial Planar Cleavage-

       cleavage is parallel to sub-parallel to the axial plane of the fold.

        Cleavage Refraction-

        Cleavage orientation changes across lithologic boundaries

        Due to different competency (sandstone-shale-sandstone).

Cleavage Refraction between phyllite (left) and quartzite (right)

at Van Hise Rock, Rock Springs, Wisconsin

LINEATIONS --

        linear fabric element.

     Three major types: Form, Surface and Mineral.

Form Lineations-

          Fold hinge lineations

Crenulation lineations

Rods- rootless fold hinges

               Occurs in interbedded phyllite (or schists) and 

               quartzite, or in mylonite.

Mullions-

        Cusp-like corrugation

        Develops in multi-layered rocks of different competencies.

Boudins-

   Sausage-shaped lenses

   More rigid rock layer embedded in more ductile matrix.

Elongated objects- pebbles, vesicles, etc.

Surface Lineations-

        Intersection lineations-

                Form by intersection of two planar fabrics

                (e.g., bedding-cleavage).

        Slip lineations-

                Form on slip surfaces

                Trend parallel to transport direction.

Slickenside Lineations on serpentinized ultramafic rocks within the

Pembine-Wausau volcanic terrane, east of Little Chicago, Wisconsin

        Groove lineations-

                Indented grooves (glacial striations)

        Fiber lineations-

                Vein mineral fibers precipitate along surface

                Veins fill crack via crack-seal processes

   Mineral Lineations-

Elongated mineral grains or grain clusters define linear fabric.

  Form on foliations, shear surfaces or mylonitic planes.

  Mineral alignment may be due to:   

      Growth of a crystal in a preferred orientation

              Rotation of crystals toward a principal strain direction

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