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Strain Softening Mechanisms (Reaction softening (Weakens rocks by…
Strain Softening Mechanisms
1. Deformation Mechanism ∆
Change to more efficient mechanism
i.e. mechanism best suited to boundary conditions
e.g. Grain size reduction usually favours:
grain boundary sliding
pressure solution
Due to increased surface area
DOMINANT MECHANISM STRAIN SOFTENING
3. Continual recrystallisation
= Dynamic reorganisation of crystal
-->
reduces dislocation density
∴ Providing more efficient arrangement
Ensures there are always some new strain free grains
7. 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
5. Shear Heating
Heat released by deformation causes positive feedback
Heat increases deformation efficiency
Conducive to temp dependent processes
More def ∴ heat transfer occurs
Temp increase results from high stresses/fast strain rate
Shear zones
= zones v high ductile defomation formed by deformation localisation
6. Chemical Softening
Results from ∆ trace element content
e.g. water input - hydrolytic effects
--> retrogression weakens quartz & olivine
Reaction softening
e.g. Metamorphic reactions
Weakens rocks
by producing:
Smaller grain size
favours grain size sensitive creep
New grains -
strain free lattice easier to deform
Different phases/minerals
with different deformation behaviour (weaker?)
Syn-kinematic reactions = during deformation
Lower ρ
dislocation
= lower level of disorder in lattice
∴ easier to accommodate deformation
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
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
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