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High pore pressures and porosity at 35 km depth in the Cascadia subduction zone

Simon M. Peacock, Nikolas I. Christensen, Michael G. Bostock and Pascal Audet
High pore pressures and porosity at 35 km depth in the Cascadia subduction zone
Geology (Boulder) (May 2011) 39 (5): 471-474

Abstract

In the Cascadia subduction zone, beneath southern Vancouver Island at 25-45 km depth, converted teleseismic waves reveal an approximately 5-km-thick landward-dipping layer with anomalously high Vp/Vs averaging 2.35+ or -0.10 (2sigma ), 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 approximately 1 m.y. to accumulate based on reasonable rates of metamorphic fluid production of approximately 10 (super -4) m (super 3) /(m (super 2) yr) in subducting Juan de Fuca crust and mantle. Accordingly, the permeability of the plate interface at these depths must be very low, approximately 10 (super -24) to approximately 10 (super -21) m (super 2) , or the porous layer must have a permeability <3X10 (super -20) m (super 2) .


ISSN: 0091-7613
EISSN: 1943-2682
Coden: GLGYBA
Serial Title: Geology (Boulder)
Serial Volume: 39
Serial Issue: 5
Title: High pore pressures and porosity at 35 km depth in the Cascadia subduction zone
Affiliation: University of British Columbia, Department of Earth and Ocean Sciences, Vancouver, BC, Canada
Pages: 471-474
Published: 201105
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
References: 27
Accession Number: 2011-046136
Categories: Solid-earth geophysicsApplied geophysics
Document Type: Serial
Bibliographic Level: Analytic
Annotation: With GSA Data Repository Item 2011151
Illustration Description: illus. incl. sects., sketch map
N48°30'00" - N49°30'00", W125°30'00" - W123°30'00"
Secondary Affiliation: University of California, Berkeley, USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Reference includes data from GeoScienceWorld, Alexandria, VA, United States. Reference includes data supplied by the Geological Society of America, Boulder, CO, United States
Update Code: 201126
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