On the extent of mantle hydration caused by plate bending

When bent at subduction zones, oceanic plates are damaged by normal faulting, and this bending-related faulting is widely believed to cause deep mantle hydration, down to ∼20-30 km deep. The buoyancy of water (or equivalently, confining pressure), however, makes it difficult to bring water down even if faulting is deep. Extension associated with plate bending generates negative dynamic pressure, but the magnitude of such dynamic pressure is shown to be insufficient to overcome confining pressure. Seismic velocity anomalies that have been used to infer the extent of mantle hydration are reviewed, and it is suggested that small crack-like porosities, which can be produced by thermal cracking and further enhanced by bending-related faulting, is sufficient to explain such velocity anomalies. The presence of such porosities, however, does not necessarily lead to the substantial hydration of oceanic plates because of confining pressure. Whereas the depth extent of bending-generated porosities is uncertain, the theory of thermal cracking can be used to place a lower bound on the amount of water contained in the slab mantle (0.03-0.07 wt% H2O), and this lower bound is suggested to be more than sufficient to explain the lower-plane earthquakes of the double seismic zone by dehydration embrittlement.