Numerical experiments for evolution of backarc basins and melting processes at the Mariana subduction system

Backarc basin basalts can be characterized as fertile, MORB-like end-member mixed with water-rich, previously depleted, arc-like components in various proportions. In addition, an enriched component might also contribute to the magma generation. A number of melting processes have been proposed to account for the chemical systematics of the backarc basin basalts. Previous numerical model experiments for circulation and thermal evolution in the subduction zones generally consider a steady spreading center, corresponding to mature stages. In this study we attempt to better understand the roles of different melting mechanisms and magma sources using a series of thermomechemical models of evolution of backarc basins. In the models the effects of rifting-spreading transition, ridge migration and asymmetric spreading on the thermal field, fluid distribution and melting history are incorporated. We focused on the evolution of the Mariana subduction system because the Mariana subduction zone is one of the best sampled region and its tectonic environment and history are relatively simple. Furthermore, the variations along the strike of the Mariana trough represent the sequence of the evolution of a backarc basin from rifting to mature stages of spreading. The effects of modeling parameters including slab geometry, spreading rate, velocity of subducting slab, lithospheric rheology and crustal thickness have been investigated. How the melting regimes and magma sources may evolve with time in the Mariana arc-basin system will be presented.