The Impact of Mélange Formation on the Rheology and H2O Budget of the Subduction Zone Interface, With Implications for Diapir Nucleation

Abstract

At the subduction zone interface, a combination of mechanical deformation and (fluid-mediated) chemical mixing of oceanic crust, sediment and mantle lithologies yields heterogeneous mélange lithologies. These mélanges have compositions and metamorphic mineral assemblages dissimilar to their endmember source lithologies, resulting in a subduction zone interface with physical properties (e.g., H₂O capacity, density, and viscosity) that are distinct from both the source lithologies and simple mechanical mixtures of these lithologies. Using phase equilibria modeling, we find that a large range of mélange compositions carry more H₂O to sub-arc depths and are less dense compared to equivalent mechanical mixtures. We further show that mélange is much weaker than the overlying mantle but remains stronger than quartzite deforming by dislocation creep. Lastly, we synthesize these results and find that at all subduction zones, a range of mélange compositions are likely to form diapirs at sub-arc depths and sub-solidus temperatures.