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/m 2

Vancouver Island: amphibolite facies Shikoku: greenschist to epidote-blueschist facies Kii Peninsula: blueschist facies ETS within subducting oceanic crust is not linked to a specific temperature or metamorphic reaction Hacker et al. ( 2003 )

Is H 2 O 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. H 2 O production rates are relatively small, 100 mL per m 2 column per yr Very low permeabilities are required for significant volumes of H 2 O 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)

Hyndman and Peacock (1999) At ETS depths in warm subduction zones, H 2 O is liberated from the slab by metamorphic dehydration reactions and possibly by the collapse of porosity in the upper crust. The amount of H 2 O released is predicted to be small: 0.1 x m 3 / (m 2 yr) = 100 milliliters of H 2 O per m 2 column per year

Very low permeability of slab interface Foliated cataclasites and mylonites (deformation induced grain-size reduction ) Precipitation of minerals from migrating fluids Tectonic melange © Milling, AGI Audet et al. (2009, Nature) estimates permeabilities to be ~5 x to ~5 x m 2 Permeability may vary during seismic (or ETS) cycle, but anomalous seismic properties require >>500 yrs of fluid production

The End

Orthogonal convergence rate = 58 and 52 mm/yr (REVEL) Plate geometry of Hirose et al. (2008) 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

SW Japan (Shikoku transect) Predicted interface T at 30 km depth = 390 °C Forearc heat flow ~ 50 mW/m 2

Predicted interface T at 30 km depth = 280 °C Forearc heat flow ~ 40 mW/m 2 SW Japan (Kii Peninsula transect)

Southwest Japan Subduction Zone Tremor at km depth Pronounced gap beneath Kii channel Obara (2002)

Low-frequency earthquakes associated with deep tremor occur at ~30 km depth (Hirose et al., 2008) Southwest Japan Subduction Zone

Kao et al. (2005) Observed tremor extends from ~30-40 km depth (within subducted slab) upward into the overlying forearc crust Northern Cascadia Subduction Zone

Southwest Japan Wang et al. [2006]Shelley et al. [2006] ETS: High Vp/Vs ( )ETS: High Poisson’s ratio ShikokuKii Peninsula

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 H 2 O present in Cascadia forearc crust? H 2 O entering forearc crust will encounter cool rocks capable of absorbing H 2 O through hydration rxns: ultramafic rocks  serpentine minerals mafic rocks  greenschist minerals (chlorite, amphibole, …) felsic rocks  not very hydratable Free H 2 O may exist locally in faults zones and fractures lined by hydrous minerals, but is unlikely to be uniformly present throughout the forearc Kao et al. (2005)

Conclusions ETS within subducted oceanic crust is not linked to a specific temperature or metamorphic rxn Seismic observations suggest H 2 O present at high pore pressures  requires very low permeability interface to permit fluids produced by metamorphic dehydration reactions to accumulate Free H 2 O is unlikely to be present throughout the forearc crust, but may be present locally along fault zones

Kao et al. (2007) Northern Cascadia Subduction Zone