Abstract
We have shown the presence of Wadati-Benioff " double " zones in most subduction zones down to ~ 200 km depth, and described the distinct characteristics of earthquakes on each of the planes, one in the crust of the plunging plate, the other, deeper, in the mantle. We introduced the notion of " thermal parameter ", the product of the plate's vertical velocity and its age, and showed how the maximum depth of earthquakes in a given subduction zone is linked to it. We introduced " serpentinite ", the hydrated phase of olivine, antigorite, its high-temperature-stable polymorphic form, and its dehydration diagram. This led us to present the petrological models proposed for the plunging plate, and the seismic observations supporting the presence of hydrated crust.
The second part of this session was devoted to showing how the lithospheric plate can become deeply hydrated as it folds at the level of the ocean trench and descends into the surrounding mantle. Deep faults open in extension, allowing water to penetrate and be incorporated into the rock as the plate sinks. We have given a few examples of observations and modelling using seismological and, more recently, electro-magnetic imaging to support this phenomenon.
