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
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)
Characterised by a particular dominant deformation mechanism
Frictional regime
Depth ~ 0 - 10 km
N.B. DEPTHS FOR DEF REGIMES ARE ROCK SPECIFIC
Rate & state-dependent friction
Brittle deformation
Brittle-ductile Transition
Temperature becomes important
a.k.a Frictional-plastic /
-viscous/ transitional region
Depth ~10-15 km
Plastic-
Viscous Regime
Ductile deformation
Highly mobile
Line btwn coseismic & posstseismic domains moves through crust during earthquake cycle
∴ Shape of SAF changes across the fault
Frictional
Regime
Fault Rocks
Proto vs ultra cataclasite
= Descriptor of cataclasite maturity
Gauged from tectonic grain size reduction
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
Cataclasites
Cohesive
< 30% Angular clasts in fine matrix
Descriptors of maturity = proto vs ultra
deeper - near/in FVT
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
glass/devitrified glass
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
Temp activated creep
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