Metamorphism and Deformation on Subduction Interfaces: 2. Petrological and Tectonic Implications

The compositional range of ∼2,000 marine sediments and ∼19,000 oceanic igneous rocks is encapsulated by a set of 12 sedimentary and 10 mafic rock compositions, allowing computation of phase relationships on P-T paths along subduction interfaces. These are described economically by a partitioning analysis, which connects the mineral assemblages to different parts of the subduction P-T space and facilitates assessment of prograde dehydration, melting, densification, and rheological systematics. Dehydration and densification occur at shallower depths than in studies that neglect shear heating. Lawsonite stability is limited to interfaces where convergence is slower than 20 mm/yr; such rates also favor transport of volatiles beyond the arc. Terrigenous sediments and mafic rocks reach their solidi close to the top of the wedge-slab interface; melt fractions are enhanced by fluid from the dehydrating slab interior. Rheological calculations show that the most abundant sediment types have interface capacities of hundreds of meters to kilometers, and that the strengths of mafic rocks comfortably exceed their buoyancy stresses. Above ∼650°C sediments are weak enough to rise as diapirs into the mantle wedge. Carbonate- and serpentinite-rich lithologies are weaker than other interface rocks, and ascend most rapidly at the cessation of subduction. Ascent rates drop abruptly as rocks enter the plate interface, probably leading to retrograde equilibrium at P ∼ 1–1.5 GPa. The seismic-aseismic transition is expected at about 500°C in mafics, and 400°C in metasediments. Seamounts are weaker than most other interface rocks, and unlikely to form asperities. Slow slip and tremor may be associated with the blueschist-eclogite transition.