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Shear Zone Deformation Mechanisms (Wire analogy (Dislocation Density…
Shear Zone Deformation Mechanisms
Mechanism
∆ at SZs
Rock weakens where strain localises
i.e. becomes easier to deform rock in SZ than surrounding rock
Weaker rocks = more effective at dissipating strain energy i.e. accomodating imposed deformation
SZ Narrowing
More lateral parts ' switch off ' &
deformation concentrated across smaller width
Change to more efficient mechanism
i.e. mechanism best suited to boundary conditions
Shear zone = narrow, subparallel-sided zone where deformation localises
Strain
softening
Rock adjusts to become more efficient at accomodating deformation
By 'weakening' as displacement increases
2. Geometric Softening
Particles reoriented in order to minimise reistance to slip
Slip plane aligned with shear stress
i.e. foliation development
Analogue - ice crystals aligned to facilitate basal slip
4. Pore
Fluid
Effects
Pore fluid presence
significantly decreases rock strength
Twofold effect:
Lowers fracture strength
Water resists confining pressure thus lowering effective stress
Encourages cataclasis
Ductility increased
(wet rocks weaker vs dry)
Fluid flow localises in pores produced/altered by deformation
3. Chemical Softening
Results from ∆ trace element content
e.g. water input - hydrolytic effects
-->weakens quartz & olivine
Dominant strain softening mechanism
--> Change to more efficient mechanism
Change in deformation mechanism
i.e. mechanism best suited to boundary conditions
Transition to grain-size sensitive def
Fine grained mineral matrix forms
in mid/lower crustal shear zones
Interconnected mechanical framework
starts to carry load
Deforms by diffusion creep mechanisms
Conditions conducive to reactions
Water influx along precursory fractures
Retrograde greenschist facies
Reactions occur producing small grain sizes
Reaction softening
Lower ρ
dislocation
∴ easier to accommodate deformation
= lower level of disorder in lattice
Syn-kinematic reactions = during deformation
Weakens rocks
by producing:
Different minerals
with different deformation behaviour (weaker?)
New grains -
strain free lattice easier to deform
Smaller grain size
favours grain size sensitive creep
e.g. Metamorphic reactions
Small grain size = more efficient def
Larger surface area
∴ Surface processes become more efficient
grain boundary sliding
diffusion along grain bounaries
Favours grain-size sensitive creep
Wire analogy
Bend metal wire - forces it to deform
produces lattice defects
(dislocations - extra half plane added to lattice)
Internal strain energy increased
Eventually wire breaks (brittle def)
when strain exceeds yield point
Dislocation Density
Influences capacity of mineral to deform
Critical dislocation exists
at which deformation mechanism changes
--> due to ∆ mechanical properties
Higher dislocation density - harder to deform
Consequence: adjacent qtz grains with different dislocation density --> different deformation capacities