Abstract
In the forearc region, aqueous fluids are released from the subducting slab at a rate depending on its thermal state. Escaping fluids tend to rise vertically unless they meet permeability barriers such as the deformed plate interface or the Moho of the overriding plate. Channeling of fluids along the plate interface and Moho may result in fluid overpressure in the oceanic crust, precipitation of quartz from fluids, and low Poisson ratio areas associated with tremors. Above the subducting plate, the forearc mantle wedge is the place of intense reactions between dehydration fluids from the subducting slab and ultramafic rocks leading to extensive serpentinization. The plate interface is mechanically decoupled, most likely in relation to serpentinization, thereby isolating the forearc mantle wedge from convection as a cold, potentially serpentinized and buoyant, body. Geophysical studies are unique probes to the interactions between fluids and rocks in the forearc mantle, and experimental constrains on rock properties allow inferring fluid migration and fluid-rock reactions from geophysical data. Seismic velocities reveal a high degree of serpentinization of the forearc mantle in hot subduction zones, and little serpentinization in the coldest subduction zones because the warmer the subduction zone, the higher the amount of water released by dehydration of hydrothermally altered oceanic lithosphere. Interpretation of seismic data from petrophysical constrain is limited by complex effects due to anisotropy that needs to be assessed both in the analysis and interpretation of seismic data. Electrical conductivity increases with increasing fluid content and temperature of the subduction. However, the forearc mantle of Northern Cascadia, the hottest subduction zone where extensive serpentinization was first demonstrated, shows only modest electrical conductivity. Electrical conductivity may vary not only with the thermal state of the subduction zone, but also with time for a given thermal state through variations of fluid salinity. High-Cl fluids produced by serpentinization can mix with the source rocks of the volcanic arc and explain geochemical signatures of primitive magma inclusions. Signature of deep high-Cl fluids was also identified in forearc hot springs. These observations suggest the existence of fluid circulations between the forearc mantle and the hot spring hydrothermal system or the volcanic arc. Such circulations are also evidenced by recent magnetotelluric profiles.
Highlights
The forearc region of subduction zones is a location of intense aqueous fluid circulations that influences major geologic phenomena
Geometry of fluid circulation and links with hot springs and the volcanic arc Aqueous fluids are produced from the top of subducting plate by compaction of fractures and porosity, and by thermally induced dehydration reactions of minerals (Iwamori 1998; Peacock 1993; van Keken, et al 2011)
Ultra-low-velocity layers are interpreted as overpressured fluids in portions of the oceanic crust at depths near the seismic to aseismic transition on the subduction megathrust (Audet, et al 2009; Song, et al 2009)
Summary
At greater depths and down to the source of magmatic liquids, connections between fluids rising to the surface and deep fluid sources are tenuous (Kawamoto, et al 2013). Saline fluid conductivity is not very sensitive to temperature and pressure in the range of the forearc mantle (0.5–2 GPa, 573–973 K), and a simple second-order polynomial expression was fitted to experimental (Quist et al 1968) and molecular dynamics data (Sakuma and Ichiki 2016) with a 30 % precision similar to discrepancies between datasets logσf 1⁄4 1:538 þ 0:7386 logm−0:05277ð logmÞ2; ð2Þ where m is the NaCl molality in aqueous fluid. Extrapolation of this empirical fit is uncertain, but minimum values of 8–12 m are obtained at 300 MPa and 200–700 °C, which are close to the highest values estimated for explaining observed electrical conductivities (Fig. 2) These values are above measurements of salinity in fluid inclusions from high-pressure ophiolitic rocks (Philippot, et al 1998; Scambelluri, et al 1997), indicating that aqueous fluids are likely undersaturated in salt at forearc conditions. At the resolution of magnetotelluric studies of a few kilometers or more, the existence of discrete high-porosity fault network zones separated by large impermeable blocks make lower average fractions of connective fluid realistic
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