Abstract
Subduction of hydrated oceanic lithosphere can carry water deep into the Earth, with important consequences for a range of tectonic and magmatic processes. Most fluid is released at relatively shallow depths in the forearc where it is thought to play a critical role in controlling mechanical properties and seismic behavior of the subduction megathrust. Here we present results from three-dimensional inversion of nearly 400 long-period magnetotelluric sites, including 64 offshore, to provide new insights into the distribution of fluids in the forearc of the Cascadia subduction zone. Our amphibious dataset provides new constraints on the geometry of the electrically resistive Siletzia terrane, a thickened section of oceanic crust accreted to North America in the Eocene, and the conductive accretionary complex, which is being underthrust all along the margin. Fluids accumulate, over time-scales likely exceeding 1 My, above the plate interface in metasedimentary units, while the mafic rocks of Siletzia remain dry. Fluids in metasediments tend to peak at fixed slab-depths of 17.5 and 30 km, suggesting control by metamorphic processes, but also concentrate around the edges of Siletzia, suggesting that this mafic block is impermeable, with dehydration fluids escaping up-dip along the megathrust. Our results demonstrate that lithology of the overriding crust can play a critical role in controlling fluid transport and sequestration in a subduction zone, with potentially important implications for mechanical properties.
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