Thermomechanical models for the dynamics and melting processes in the Mariana subduction system

Abstract

Melt generation in the arc-basin system of the subduction zones depends not only on fluid distribution and detailed temperature and pressure conditions, but also on melting mechanism and melting history. These factors are closely linked to the lithospheric extension, the proximity of the back-arc basin to the arc, and the migration of the spreading center. In this study the evolution of the back-arc basin is incorporated to explore the first-order features of the dynamics and melting processes in the Mariana subduction zone, using coupled thermomechanical numerical experiments. In the models continuous stretching thins the lithosphere, ultimately resulting in breakup of the lithosphere. The spreading center develops and migrates subsequently to form the new ocean basin in the back-arc region. Potential flux melting and decompression melting regimes are delineated based on the resultant thermal models, experimentally determined solidi, and phase diagrams. The results predict an overlap between the two regimes for several million years, suggesting that both melting mechanisms contribute to arc magma and back-arc basin basalts during the rifting phases and the earlier stages of spreading. The decompression melting regime progressively separates from the flux melting regime as the ridge migrates along. The mantle flow field driven by subduction and spreading, together with the ridge migration, generate intricate particle paths. This could lead to multiphase melt extraction for both the arc magma and the back-arc basin basalts and thus cause complex geochemical signatures. These processes may explain the first-order geochemical characteristics observed in most of the Mariana arc-basin system. Copyright 2010 by the American Geophysical Union.