Deep electrical structure of the northern Cascadia (British Columbia, Canada) subduction zone: Implications for the distribution of fluids

Soyer, Wolfgang; Unsworth, Martyn

doi:10.1130/g21951.1

Cited by 104 publications

(103 citation statements)

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“…5b) is interpreted as pervasive serpentinization of the forearc mantle wedge (mainly antigorite). These findings are consistent with electrical resistivity studies in the forearc of Cascadia (McGary et al, 2014;Soyer and Unsworth, 2006) and point to…”

Section: Accepted Manuscriptsupporting

confidence: 81%

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Teleseismic constraints on the geological environment of deep episodic slow earthquakes in subduction zone forearcs: A review

2016

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More than a decade after the discovery of deep episodic slow slip and tremor, or slow earthquakes, at subduction zones, much research has been carried out to investigate the structural and seismic properties of the environment in which they occur. Slow earthquakes generally occur on the megathrust fault some distance downdip of the great earthquake seismogenic zone in the vicinity of the mantle wedge corner, where three major structural elements are in contact: the subducting oceanic crust, the overriding forearc crust and the continental mantle. In this region, thermo-petrological models predict significant fluid production from the dehydrating oceanic crust and mantle due to prograde metamorphic reactions, and their consumption by hydrating the mantle wedge. These fluids are expected to affect the dynamic stability of the megathrust fault and enable slow slip by increasing pore-fluid pressure and/or reducing friction in fault gouges.Resolving the fine-scale structure of the deep megathrust fault and the in situ distribution of fluids where slow earthquakes occur is challenging, and most advances have been made using A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPTteleseismic scattering techniques (e.g., receiver functions). In this paper we review the teleseismic structure of six well-studied subduction zones (three hot, i.e., Cascadia, southwest Japan, central Mexico, and three cool, i.e., Costa Rica, Alaska, and Hikurangi) that exhibit slow earthquake processes and discuss the evidence of structural and geological controls on the slow earthquake behavior. We conclude that changes in the mechanical properties of geological materials downdip of the seismogenic zone play a dominant role in controlling slow earthquake behavior, and that near-lithostatic pore-fluid pressures near the megathrust fault may be a necessary but insufficient condition for their occurrence.

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Section: Accepted Manuscriptsupporting

confidence: 81%

“…5a). The forearc of the Cascadia subduction zone has been extensively studied from active and passive source surveys (e.g., Clowes et al, 1987;Green et al, 1986;Soyer and Unsworth, 2006). Bostock (2013) provides a recent review of the structural elements as determined from teleseismic receiver function and other geophysical studies.…”

Section: Cascadiamentioning

confidence: 99%

Teleseismic constraints on the geological environment of deep episodic slow earthquakes in subduction zone forearcs: A review

2016

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“…MT electrical imaging provides additional constraints on fluid content. For instance, the seismogenic portion of the Cascadia subduction interface below Vancouver Island coincides with an inclined tabular zone of high electrical conductivity (Kurtz et al 1990;Soyer and Unsworth 2006). In southwest Japan, likewise, a highly reflective low-velocity layer dips c. 7°NW below Shikoku Island, down-dip from a subducting seamount on the inner wall of the Nankai Trough, defining the subduction interface that ruptured in the 1946 M w 8.1 Nankaido megathrust earthquake.…”

Section: Tensile Overpressure Compartments Along Subduction Interfacesmentioning

confidence: 99%

Tensile overpressure compartments on low-angle thrust faults

Sibson

2017

Earth Planets Space

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Hydrothermal extension veins form by hydraulic fracturing under triaxial stress (principal compressive stresses, σ 1 > σ 2 > σ 3 ) when the pore-fluid pressure, P f , exceeds the least compressive stress by the rock's tensile strength. Such veins form perpendicular to σ 3 , their incremental precipitation from hydrothermal fluid often reflected in 'crackseal' textures, demonstrating that the tensile overpressure state, σ 3 ′ = (σ 3 − P f ) < 0, was repeatedly met. Systematic arrays of extension veins develop locally in both sub-metamorphic and metamorphic assemblages defining tensile overpressure compartments where at some time P f > σ 3 . In compressional regimes (σ v = σ 3 ), subhorizontal extension veins may develop over vertical intervals <1 km or so below low-permeability sealing horizons with tensile strengths 10 < T o < 20 MPa. This is borne out by natural vein arrays. For a low-angle thrust, the vertical interval where the tensile overpressure state obtains may continue down-dip over distances of several kilometres in some instances. The overpressure condition for hydraulic fracturing is comparable to that needed for frictional reshear of a thrust fault lying close to the maximum compression, σ 1 . Under these circumstances, especially where the shear zone material has varying competence (tensile strength), affecting the failure mode, dilatant fault-fracture mesh structures may develop throughout a tabular rock volume. Evidence for the existence of fault-fracture meshes around low-angle thrusts comes from exhumed ancient structures and from active structures. In the case of megathrust ruptures along subduction interfaces, force balance analyses, lack of evidence for shear heating, and evidence of total shear stress release during earthquakes suggest the interfaces are extremely weak (τ < 40 MPa), consistent with weakening by nearlithostatically overpressured fluids. Portions of the subduction interface, especially towards the down-dip termination of the seismogenic megathrust, are prone to episodes of slow-slip, non-volcanic tremor, low-frequency earthquakes, very-low-frequency earthquakes, etc., attributable to the activation of tabular fault-fracture meshes at low σ 3 ′ around the thrust interface. Containment of near-lithostatic overpressures in such settings is precarious, fluid loss curtailing mesh activity.© The Author(s) 2017. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

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“…Since the Vancouver Island and EMSLAB studies from the 1980, new images of the Cascadia subduction zone have been published by several authors (Aprea et al 1998;Vanyan 2002). This has included the first long-period MT data capable of imaging the lower crust and upper mantle of the Cascadia Subduction zone across southern British Columbia (Soyer and Unsworth 2006;see Fig. 13.21).…”

Section: Tectonic Setting and Geophysical Studies Of The Cascadia Submentioning

confidence: 99%

Mapping the Distribution of Fluids in the Crust and Lithospheric Mantle Utilizing Geophysical Methods

Unsworth

Rondenay

2012

Lecture Notes in Earth System Sciences

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Geophysical imaging provides a unique perspective on metasomatism, because it allows the present day fluid distribution in the Earth's crust and upper mantle to be mapped. This is in contrast to geological studies that investigate midcrustal rocks have been exhumed and fluids associated with metasomatism are absent. The primary geophysical methods that can be used are (a) electromagnetic methods that image electrical resistivity and (b) seismic methods that can measure the seismic velocity and related quantities such as Poisson's ratio and seismic anisotropy. For studies of depths in excess of a few kilometres, the most effective electromagnetic method is magnetotellurics (MT) which uses natural electromagnetic signals as an energy source. The electrical resistivity of crustal rocks is sensitive to the quantity, salinity and degree of interconnection of aqueous fluids. Partial melt and hydrogen diffusion can also cause low electrical resistivity. The effects of fluid and/or water on seismic observables are assessed by rock and mineral physics studies. These studies show that the presence of water generally reduces the seismic velocities of rocks and minerals. The water can be present as a fluid, in hydrous minerals, or as hydrogen point defects in nominally anhydrous minerals. Water can further modify seismic properties such as the Poisson's ratio, the quality factor, and anisotropy. A variety of seismic analysis methods are employed to measure these effects in situ in the crust and lithospheric mantle and include seismic tomography, seismic reflection, passive-source converted and scattered wave imaging, and shear-wave splitting analysis. A combination of magnetotelluric and seismic data has proven an effective tool to study the fluid

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Deep electrical structure of the northern Cascadia (British Columbia, Canada) subduction zone: Implications for the distribution of fluids

Cited by 104 publications

References 30 publications

Teleseismic constraints on the geological environment of deep episodic slow earthquakes in subduction zone forearcs: A review

Teleseismic constraints on the geological environment of deep episodic slow earthquakes in subduction zone forearcs: A review

Tensile overpressure compartments on low-angle thrust faults

Mapping the Distribution of Fluids in the Crust and Lithospheric Mantle Utilizing Geophysical Methods

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