Subduction is a process in which oceanic crust and upper mantle are consumed beneath the margins of island arcs, such as Japan and Sumatra, or the margins of continents, as along the northwest coast of the U.S. Subducton gives rise to violent volcanic eruptions as well as great ?megathrust? earthquakes and tsunamis. Dramatic demonstrations of the destructive effects of subduction have occurred recently in Indonesia, Japan, and Chile. Water released from the subducted plate is a significant, but poorly understood, contributor to these events; this seismic survey of the Juan de Fuca Plate has the specific aim of characterizing the plate from its formation at the ridge, through alteration and hydration of the plate as it ages, to subduction of the plate along the Cascadia margin. Owing to the very high risk of large earthquakes in Cascadia, improved understanding of the role of water in the subduction process is a primary broader impact of the survey. The project is also an important adjunct to the Cascadia Initiative, a multi-year deployment of instruments both onshore and offshore to study the structure and seismicity of the Cascadia region.
Understanding geological processes occurring at subduction zones is of great importance because of their association with many geohazards (great megathrust earthquakes, tsunamis, and arc volcanoes). A key parameter that influences many subduction processes is water entering the subduction zone. The water stored and transported in the down-going tectonic plate is released at depth through mechanical and metamorphic dehydration. As the water is released at depth, it affects a number of processes such as arc magmatism and the mechanical characteristics of the megathrust fault. At the Cascadia subduction zone, where volcanic eruptions and megathrust and intraslab earthquakes pose significant hazards in the heavily populated northwestern US, the state of hydration of the down-going Juan de Fuca plate is unknown. The main goal of this project was to quantify the state of hydration of the Juan de Fuca plate before is subducted beneath Washington and Oregon states. In particular, we want to determine: (3) What processes lead to plate hydration? (2) When after formation at the mid-ocean ridge is water incorporated into the plate? (3) How is water distributed at depth within the plate? We found that in the interior of the plate hydration occurs locally at propagator wakes, which are deformed zones formed at the mid-ocean ridge by propagation of spreading segments. Near the subduction zone, down-bending of the plate promotes faulting, which allows water to enter the 6-km-thick crust and reach the uppermost mantle. Water is incorporated into the crust as both, free fluid and chemically bound to alteration minerals. In contrast, the elevated temperatures of the Juan de Fuca mantle prevent formation of significant amounts of hydrated minerals in the mantle, thus at depths greater than ~8 km below the seafloor water is stored is small amounts along faults and within fractures. We find that the state of hydration of the plate varies along the subduction margin. Offshore Washington the plate is minimally affected by bending-related faulting, which results in a largely dry plate at depths greater than ~7 km below the seafloor. As the effect of faulting increases to the south toward offshore Oregon, the plate is able to incorporate more of water at deeper levels, albeit in modest amounts. This north-south variation in hydration state is likely to be linked to the documented along-margin changes in seismogenic properties of the Cascadia subduction zone.
