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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)