1. Subduction Fluids Channelled to Forearc Mantle Corner:
ETS and Vp/Vs-defined Silica Deposition in Cascadia
R.D. Hyndman1,2, P.A. McCrory3 A.G. Wech4, H. Kao1,2, and J. Ague5
1,2Pacific Geoscience Centre, Geol. Surv. Canada and Univ. Victoria
3U.S. Geological Survey, Menlo Park, Ca
4U.S. Geol. Survey, Alaska Volcano Obs., Anchorage, AK 5Yale University, New Haven, Connecticut
R.D. Hyndman
Pacific Geoscience Centre
Geological Survey of Canada

2. ETS zone is controlled by fluids
expelled into crust at forearc mantle corner
Cascadia 3 1 2

3. Episodic Tremor and Slip (ETS)
Seismic tremor with no clear onset
Slow slip events on the subd. thrust
downdip of seismogenic zone
Tremor at 35 km, 500C, where
thrust should be aseismic!
What generates ETS and what controls controls downdip location?
1. Not continuous with seismogenic zone; ~50 km gap
2. ETS associated with upward fluid expulsion at forearc mantle corner
3. Silica deposition above corner from upward fluid expulsion E
Time
After Rogers and Dragert (2003)

4. Cascadia Rupture Area Review Hyndman, JGR 2013 Thermal Constraint
Geodetic Data
"Locked/transition
Zone"
1. Geodetic locked/transition
GPS, levelling, tide gauge, etc
2. Thermal constraint, C
Heat flow etc.
3. Coseismic subsidence
Coastal marsh data
4. Thrust reflection character
Thin seismic to thick ductile
5. Forearc basins
6. Landward limit small eqs.

5. Locked/transition zone vs slow slip and tremor
(Wang et al., Kao et al., Wech et al.)

6. Cascadia forearc mantle
corner and ETS tremor
Summary
McCrory et al., GGG 2014
Corner
Tremor
Wech et al. (2010)

7. Cascadia forearc mantle corner and ETS tremor
Locked zones: Holtkamp
and Brudzinski (2010)
Corner: McCrory et al. (2012)
transition
locked

8. Dehydration fluids of downgoing plate
Channelled to corner by anisotropic sheared serpentinite
(based on Kawano et al., 2011)

9. Evidence for fluids in deposited silica
from rising silica saturated fluids
SILICA AND SERPENTINITE IN SUBDUCTION FOREARC

10. Quartz veins in Otago Schists accretionary prism
from rising subduction slab-derived water
Breeding and Ague (2002)

11. Cascadia shear tomography
Low Poisson's Ratio (and Vp/Vs) over forearc mantle corner <0.22, only silica is that low
Poisson's Ratio related to Vp/Vs
Ramachandran and Hyndman (2010)

12. Low Poisson’s
Ratio corresponds
To tremor location

13. Other Hot Subduction Zones
For both SW Japan and Mexico, ETS lies approx. over forearc mantle corner
SW Japan has older subducting crust and is cooler than Cascadia. Its thermally limited seismogenic zone extends nearly to forearc mantle corner so a smaller gap
Mexico subduction zone has variable age subducting crust; there is a gap between seismogenic zone and ETS (and mantle corner) for hotter northern part

14. Summary
1. Gap between Cascadia seismic rupture zone
and ETS slow slip
2. ETS not T controlled
3. ETS Overlies mantle corner
4. Low permeability and anisotropic serpentinite
channel fluids from dehydrating ocean crust to the corner
5. Low Poisson's Ratio evidence for silica saturated
fluids in crust overlying corner. Quartz veins formed with fluid cooling above corner
Thank you Roy Hyndman, PGC-GSC