SHEAR ZONES

BrittleMechanisms:

   Frictional sliding

    Cataclastic flow

   Slip by growth of fault parallel veins.

Ductile Mechanisms-

   Cataclastic flow

   Crystal-plastic processes

        (dislocations, twinning)

   Diffusional flow

        Pressure Solution

   Recrystallization

Brittle-Ductile Transition- 

   Occurs 10-15 km depth (T~300⁰C)

   Separates upper frictional regime and the lower plastic regime.

Ductile Shear Zones-

zones of displacement

Rocks subjected to ductile deformation    

High strain rates.

Ductile indicators:

Mylonites

Isoclinal folds

Disrupted layers

Transposed bedding

Rotated clasts

Foliations/lineations

Mylonite-

   Fine grained, foliated metamorphic rock

   Experienced ductile deformation

   Recrystallization of grains

   Develop at Temperatures > 300⁰C.

Fault Zone/Shear Zone Rocks-

     Fault Breccia- coarse grained, brittle fault rock

      Cataclastite- cohesive, fine grained fault rock  

             Produced by grain scale brittle processes

      Gouge- Incohesive, fine grained fault rock

            Formed by brittle processes

     Mylonite- Cohesive, foliated fault rock

            Formed by crystal-plastic processes

      Pseudotachylite- glassy fault rock along fractures   

               Formed by frictional heating/melting of rock

SHEAR SENSE INDICATORS

       Grain Tail Complexes:

              Tails �point� in the direction of shear

             Tails form by:

                  Highly attenuated preexisting grains    

                        (commonly feldspars)

                  Dynamic recrystallization at rim of grain

                  Neocrystallization in pressure shadows

          s-type (sigma)-

                wedge shaped tails

                tails do not cross the reference plane of shear

               May represent slow grain rotation

                    relative to tail growth  

Sigma Grain Tail Complex in mylonite thin section; Sinistral Shear

Sigma Grain Tail Complex in mylonite thin section; Sinistral Shear

d-type (delta)-

                curved tails cross the reference plane of shear

               Thought to represent fast grain rotation

                        relative to tail growth

          Fractured Grains-

                Occurs with feldspars in mylonites

        Synthetic Fractures-

            fractures at low angles to mylonitic foliation

            displacement consistent with sense of shear

        Antithetic Fractures-

        fractures oriented at angles >45⁰  to foliation

      opposite movement sense to overall sense of shear.

Sigmoidal Antithetic Fractures in a Dextral Shear Zone,

Tiddiline Conglomerate, Bou Azzer inlier, Morocco

En Echelon Antithetic Veins in a Sinistral Shear Zone in the Baraboo Quartzite

Block with the keychain moved down and to the left.

Mica Fish-

            lenticular (fish-shaped) porphyroclasts        

            muscovite or biotite indicate shear sense.

           observed in thin section or hand specimen

           Common in mylonitic rocks

Fish-Flash-

            maximum sun  reflectivity on rock surface        

        looking down the lineation direction

      S-C Foliations- common in mylonites

           S= schistosity (foliation)

            C= cisaillement (shear)-

                    shear direction lies in the C plane

    C-C� Foliations-

              C= cisaillement (shear)-

                    shear direction lies in the C plane

              C�= shear bands or extensional crenulations

                    that offset mylonitic foliation

      Rotated Grains-

            rotated porphyroclasts or pyphyroblasts

            (e.g. Snowball Garnet)

            may record internal vorticity of rotational strain

    Rotated Garnet Animation: