Shear Zone Structures

Mylonite Formation

  1. Ductile deformation due to intense shearing
    during folding / faulting
  1. Original minerals almost completely broken down
  1. Recrystallise as smaller grains
    --> tightly intergrown ∴ form hard, dense rock
  1. Preferential growth along foliation planes
    --> parallel to direction of shear

Porphyroblasts may also be flattened/smeared/rotated

S-C'
Fabric

Continued deformation backrotates S to become near parallel with C

∴ New, oblique shear bands (C') form oblique to CS

S-C
Fabric

1. Foliation (S) develops in shear zone parallel to XY plane of strain ellipsoid

  1. Shear bands (slip surfaces) develop (C)
    parallel to shear zone walls
  1. Foliation curves in & out of shear surfaces (C)

As strain accumulates

--> sense of foliation deflection ∴ SZ shear sense

C vs S
Surfaces

Shear Bands (C)

= small scale shear zones which affect foliation

Foliation (S)

S for schistosity

obliquely transect foliation within main SZ

may back-rotate foliation

C'

New, oblique shear bands

Back-rotate CS foliation (then termed S)

CS now termed S

Occurs when strain is especially high

Angle
btwn S+C

Typically 25 - 45 ° (variable)

Reliable shear indicator --> if angular relation = consistent

Mylonite

= Fault rock formed by ductile deformation

Cohesive

Exhibits well developed schisosity

due to tectonic grain size reduction

Shear Zone Nucleation

Nucleate along weakest planes of rock

i.e. micaceous layers, veins, fracture, dykes etc

∴ Brittle fracture = precursor to ductile shear zones

i.e. ductile shear overprints brittle features

Example SZ Nucleation

  1. Fracture formed via brittle fracture
  1. Fracture allowed fluid ingress
  1. Facilitates retrogression in feldspar

∴ Temp activated deformation possible i.e. ductile shear