Journal
GEOLOGY
Volume 39, Issue 5, Pages 471-474Publisher
GEOLOGICAL SOC AMER, INC
DOI: 10.1130/G31649.1
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Funding
- U.S. government through U.S. National Science Foundation [EAR-0409343, OCE-0752322]
- Canadian government through U.S. National Science Foundation [EAR-0409343, OCE-0752322]
- Natural Sciences and Engineering Research Council of Canada [RGPIN 138004-04]
- Miller Institute for Basic Research in Science (University of California-Berkeley
- Division Of Ocean Sciences
- Directorate For Geosciences [0752322] Funding Source: National Science Foundation
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In the Cascadia subduction zone, beneath southern Vancouver Island at 25-45 km depth, converted teleseismic waves reveal an similar to 5-km-thick landward-dipping layer with anomalously high Vp/Vs averaging 2.35 +/- 0.10 (2 sigma), interpreted as subducted oceanic crust of the Juan de Fuca plate. This layer is observed downdip of the inferred locked seismogenic zone, in the region of episodic tremor and slip. Laboratory velocity measurements of crystalline rock samples made at 200 MPa confining pressure and elevated pore pressures demonstrate that Vp/Vs increases with increasing fluid-filled porosity. The observed high Vp/Vs values are best explained by pore fluids under near lithostatic pressure in a layer with a high porosity of 2.7%-4.0%. Such large volumes of fluid take similar to 1 m.y. to accumulate based on reasonable rates of metamorphic fluid production of similar to 10(-4) m(3)/(m(2) yr) in subducting Juan de Fuca crust and mantle. Accordingly, the permeability of the plate interface at these depths must be very low, similar to 10(-24) to similar to 10(-21) m(2), or the porous layer must have a permeability < 3 x 10(-20) m(2).
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