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Thermal and Metamorphic
Environment of Subduction-Zone
Episodic Tremor and Slip
Simon M. Peacock
Dept. of Earth and Ocean Sciences
University of British Columbia
Southern Vancouver Island
Slab age at deformation front = 7.5 Ma (Wilson, 1993)
Orthogonal convergence rate = 40 mm/yr (NUVEL-1A)
Plate geometry of Rogers (1998)
Southern Vancouver Island
Predicted interface T at 30 km depth = 510 °C
Forearc heat flow ~ 60 mW/m2
ETS within subducting oceanic crust is not linked to a
specific temperature or metamorphic reaction
Hacker et al.
(2003)
Vancouver Island:
Shikoku:
Kii Peninsula:
amphibolite facies
greenschist to epidote-blueschist facies
blueschist facies
Is H2O present in subduced oceanic crust
at ETS depths?
• Seismological observations of high Vp/Vs and high
Poisson’s ratio
• Pore pressures are likely high (~lithostatic), at least
where dehydration reactions are taking place.
• H2O production rates are relatively small, 100 mL
per m2 column per yr
• Very low permeabilities are required for significant
volumes of H2O to accumulate beneath the slab
interface
Subducted oceanic crust
beneath Vancouver Isl.
Poisson’s ratio ~ 0.4,
requires pervasive fluid
at high pore pressures
Audet et al.
(2009, Nature)
At ETS depths in warm
subduction zones, H2O is
liberated from the slab
by metamorphic
dehydration reactions
and possibly by the
collapse of porosity in
the upper crust.
The amount of H2O
released is predicted to
be small:
0.1 x 10-3 m3 / (m2 yr) =
100 milliliters of H2O per
m2 column per year
Hyndman and Peacock (1999)
Very low permeability of slab interface
• Foliated cataclasites and mylonites (deformation induced
grain-size reduction )
• Precipitation of minerals from migrating fluids
• Audet et al. (2009, Nature)
estimates permeabilities to be
~5 x 10-25 to ~5 x 10-22 m2
• Permeability may vary during
seismic (or ETS) cycle, but
anomalous seismic properties
require >>500 yrs of fluid
production
Tectonic melange
© Milling, AGI
The End
SW Japan (Shikoku and Kii Peninsula)
Slab age at Nankai trough =
15 Ma (Shikoku) and
20 Ma (Kii Peninsula)
(Hibbard and Karig, 1990)
Fossil Shikoku Ridge requires
transient model (last 15 Myr):
Shikoku 0 to 15 Ma
Kii Peninsula 5 to 20 Ma
Orthogonal convergence rate = 58 and 52 mm/yr (REVEL)
Plate geometry of Hirose et al. (2008)
SW Japan (Shikoku transect)
Predicted interface T at 30 km depth = 390 °C
Forearc heat flow ~ 50 mW/m2
SW Japan (Kii Peninsula transect)
Predicted interface T at 30 km depth = 280 °C
Forearc heat flow ~ 40 mW/m2
Southwest Japan Subduction Zone
Obara (2002)
Tremor at 35-45 km depth
Pronounced gap beneath Kii channel
Southwest Japan Subduction Zone
Low-frequency earthquakes associated with deep
tremor occur at ~30 km depth (Hirose et al., 2008)
Northern Cascadia Subduction Zone
Kao et al. (2005)
Observed tremor extends from ~30-40 km depth (within
subducted slab) upward into the overlying forearc crust
Shikoku
Southwest Japan
ETS: High Vp/Vs (1.9-1.95)
Shelley et al. [2006]
Kii Peninsula
ETS: High Poisson’s ratio
Wang et al. [2006]
Absence of ETS
NE Japan due to
lack of fluid
production (slab
dehydration
reactions) at
shallow depths or
higher interface
permeability?
Kii channel (SW Japan) due to lack of hydrous
minerals in incoming crust (tonalite, Seno and
Yamasaki, 2003) or slab contortion increase
permeability?
Is H2O present in Cascadia forearc crust?
Kao et al. (2005)
• H2O entering forearc crust will encounter cool rocks
capable of absorbing H2O through hydration rxns:
ultramafic rocks  serpentine minerals
mafic rocks  greenschist minerals (chlorite, amphibole, …)
felsic rocks  not very hydratable
• Free H2O may exist locally in faults zones and fractures
lined by hydrous minerals, but is unlikely to be uniformly
present throughout the forearc
Conclusions
• ETS within subducted oceanic crust is not linked
to a specific temperature or metamorphic rxn
• Seismic observations suggest H2O present at
high pore pressures
 requires very low permeability interface to permit
fluids produced by metamorphic dehydration
reactions to accumulate
• Free H2O is unlikely to be present throughout
the forearc crust, but may be present locally
along fault zones
Northern Cascadia
Subduction Zone
Kao et al. (2007)