Shear Zone Structures
Mylonite Formation
- Ductile deformation due to intense shearing
during folding / faulting
- Original minerals almost completely broken down
- Recrystallise as smaller grains
--> tightly intergrown ∴ form hard, dense rock
- 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
- Shear bands (slip surfaces) develop (C)
parallel to shear zone walls
- 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
- Fracture formed via brittle fracture
- Fracture allowed fluid ingress
- Facilitates retrogression in feldspar
∴ Temp activated deformation possible i.e. ductile shear