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Rock Record of Upper-Crustal Deformation (Brittle-ductile Transition…
Rock Record of
Upper-Crustal Deformation
Deformation Regime
Frictional
domain
Near cold surface
Plate boundary = locked
Seismic events accomodate relative plate motion
Frictional sliding releases accumulated elastic strain in seconds
Viscous
domain
At depth temperatures = high
Slow steady viscous creep of rocks
accomodates relative plate motion
Characterised by a particular dominant deformation mechanism
SAF Transects Entire Lithosphere
Since is a strike slip plate boundary
Three deformation regimes yield distinct fault rocks
Frictional (cataclasites)
Frictional-plastic (cataclasites & mylonites)
Plastic/viscous (mylonites)
Frictional regime
Depth ~ 0 - 10 km
Rate & state-dependent friction
Brittle deformation
N.B. DEPTHS FOR DEF REGIMES ARE ROCK SPECIFIC
Brittle-ductile Transition
Temperature becomes important
a.k.a Frictional-plastic /
-viscous/ transitional region
Depth ~10-15 km
Highly mobile
Line btwn coseismic & posstseismic domains moves through crust during earthquake cycle
∴ Shape of SAF changes across the fault
Plastic-
Viscous Regime
Ductile deformation
Temp activated creep
Frictional
Regime
Fault Rocks
Discrete faults
nr the surface comprising:
( fault rocks listed below with increasing Z)
Incohesive, cataclastic
gouges < 30% visible fragments
breccias > 30% visible fragments
Pseudotachylite
palaeoseismic indicators
deeper - near/in FVT
glass/devitrified glass
Cataclasites
Cohesive
< 30% Angular clasts in fine matrix
Descriptors of maturity = proto vs ultra
Proto vs ultra cataclasite
= Descriptor of cataclasite maturity
Gauged from tectonic grain size reduction
i.e. how much of rock volume is fine grained matrix?
Brittle-Ductile Transition
Semi-brittle mylonites
Pseudotachylites
Phyllonites
Plastic/
Viscous
Mylonites
Foliated
metamorphic rock
Formed by
ductile deformation during intense shearing
Form at depth
where P/T sufficient for plastic deformation
Lockner
Samples
Took drill core sample from interval @ 2.7 km depth
Deformed samples in machine
Measured frictional resistance ∴ inferred friction coefficient
SAF @
2.7 km
Depth
Foliated fault gouge
= clasts floating in muddy/clay matrix
clasts/quartz = host rock remnants
Continually accomodates shear displacement
on SAF
Foliated - reidal, antireidal & p-shears