Can slab melting be caused by flat subduction?

Slab melting has been suggested as a likely source of adakitic are magmas ( i.e., andesitic and dacitic magmas strongly depleted in Y and heavy rare ea rth elements). Existing numerical and petrologic models, however, restrict partial melting to very young (less than or equal to 5 Ma) oceanic crust (t ypically at 60-80 km depth). Paradoxically, most of the known Pliocene-Quat ernary adakite occurrences are related to subduction of 10-45 Ma lithospher e, which should not be able to melt under normal subduction-zone thermal gr adients, We propose an unusual mode of subduction known as flat subdution, occurring in similar to 10% of the world's convergent margins, that can pro duce the temperature and pressure conditions necessary for fusion of modera tely old oceanic crust, Of the 10 known flat subduction regions worldwide, eight are linked to present or recent (<6 Ma) occurrences of adakitic magma s. Observations from Chile, Ecuador, and Costa Pica suggest a three-stage e volution: (1) steep subduction produces a narrow calc-alkaline are, typical ly similar to 300 km from the trench, above the asthenospheric wedge; (2) o nce flat subduction begins, the lower plate travels several hundred kilomet ers at nearly the same depth, thus remaining in a pressure-temperature wind ow allowing slab melting over this broad distance; and (3) once flat subduc tion continues for several million years, the asthenospheric wedge disappea rs, and a volcanic gap results, as in modern-day central Chile or Peru. The proposed hypothesis, which reconciles thermal models with geochemical obse rvations, has broad implications for the study of are magmatism and for the thermal evolution of convergent margins.