Stagnant slab, wet plumes and Cenozoic volcanism in East Asia

Data from seismology and mineral physics indicate the existence of low-velocity zones in the deep part of the upper mantle wedge and the mantle transition zone under East Asia (Zhao and Ohtani, 2009; Zhao et al., 2009). It suggests a link between the deep subduction (and stagnation) and deep dehydration of the Pacific slab below this area and the Cenozoic volcanism observed at the surface. Here we present a numerical model describing the dynamics of a hydrated layer covering a slab that is stagnating in the Earth⿨tm)s mantle transition zone. Specifically, our model includes the effects of water on density and viscosity of the mantle minerals. We conclude that the presence of water is likely to induce Rayleigh⿢Taylor type instabilities (wet plumes) that are able to transport water up to the base of the surficial lithosphere in a relatively short time scale (a few million years). Under initial conditions representative of the subducting Pacific slab under East Asia, melting is induced where wet plumes reach the base of the lithosphere. According to our results and analysis of the known composition of Cenozoic basalts from Southwest Japan (Iwamori, 1991 and Iwamori, 1992), wet plumes and their melting induced by water related to the stagnant slab provides an appealing hypothesis to explain Cenozoic volcanism in East Asia.