Dipping Seismic Reflectors, Electrically Conductive Zones, and Trapped Water in the Crust Over a Subducting Plate

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Abstract
The LITHOPROBE program across the northern Cascadia subduction zone at Vancouver Island has obtained geophysical data that suggest a dipping zone of trapped free pore water in the overlying continental crust. Land and marine multichannel seismic reflection profiles show several bands of reflectors. The primary one at a depth of about 30 km is about 5 km thick, dips inland, and appears to coincide with an electrically conductive layer defined by magnetotelluric data. The downgoing oceanic crust is imaged as a thin reflector dipping at about 15° and 10 km deeper than the primary reflector, a location that is consistent with seismicity and seismic refraction data. Both the reflectors and the conductor have been modeled by near‐horizontal lamellae with 1–4% saline water porosity. Detailed geothermal measurements defining heat flow that decreases inland indicate that the dipping bands are nearly isothermal at a temperature of 400°–500°C. It is suggested that free water generated by dehydration of the downgoing oceanic plate migrates upward until reduced temperature results in hydration reactions and mineral precipitation that forms an impermeable barrier. Low vertical permeability is required to keep the water from separating to the top of the layer. This can be reconciled with the low electrical resistivity if pore interconnection is through thin grain boundary tubes as predicted by theoretical equilibrium pore geometry, although very small effective grain size is necessary. An abrupt transition from high permeability to very low permeability is predicted at a several percent porosity, in agreement with the modeled porosity. Such pore geometry with low permeability may be restricted to temperatures above about 400°C, i.e., in greenschist to amphibolite facies conditions, where grains will deform to equilibrium geometry and cracks are annealed. The region above the reflective and conductive band is estimated to be within blueschist conditions, consistent with the inferred high seismic velocity from refraction data and high density from gravity data.