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Serpentine Mineral Behaviour (Serpentine Affects Strength (Rock lacking…
Serpentine Mineral Behaviour
Serpentine strength
Serpentine minerals = very weak / low strength
Rock
strength
Dunite
= olivine + pyroxene
strong mantle rock
sustains high diff stress before failing
Serpentine
bearing rocks
~ Weathered dunites
∴ Varying serpentine content (9 - >90% lizardite/chrysotile)
Much weaker - irrespective of serpentine %
Serpentine Affects Strength
At confining P ~ 200MPa
Rock lacking serpentine
strength ~ 1250 MPa
Rock w/10% serpentine
strength decreased signif < 500 MPa
10% serpentine = sufficient to weaken peridotite to strength of pure serpentinite!
10% serpentine
= sufficient to weaken peridotite to strength of pure serpentinite!
∴ Suggests deformation = accommodated
entirely by serpentine minerals
Whilst
olivine remains relatively undeformed
Gouge
= rock crushed by deformation processes
Friction coefficient
μ = shear stress / effective normal stress
= ratio of shear to normal force
required to initiate sliding (μs) / maintain sliding (μd)
~ Frictional strength
Serpentine gouge strength
Friction coefficient differs
for different serpentine minerals
Serpentine gouge of varying serpentine phase composition = placed in triaxial press
Examine strength
μ varies
Different serpentine phases have different μ
Lizardite / antigorite μ ~ 0.5
Chrysotile μ ~ 0.2
Chrysotile - especially low μ ∴ easier to deform
Chrysotile μ
Absorbed water weakens @ low P/T
Low P/T - μ especially low ~ 0.1
Stronger with dehydration @ high P/T
High P/T - Higher μ ~ 0.5 (similar to lizardite & antigorite)
Water in structure makes mineral malleable
μ = pressure & temperature dependent
Experimental Design
Moore et al 2009
Room temp ∆
due to air conditioning switching on / off
-->
caused transient drops in μ
Why ?
Affects length of sample column
btwn fault & displacement msmt