Schematic cartoons showing the differences in dynamic regime and geothermal gradient between the different stages of plate convergence (IMAGE)
Caption
(a) Low-angle subduction, compressional regime prevails in the overlying plate. Because the plate interface is at low geothermal gradients, the subducting crust experiences ultrahigh pressure eclogite facies metamorphic dehydration at subarc depths, where partial melting of the hydrated small mantle wedge does not immediately happen and thus lack of arc magmatism. (b) High-angle subduction, rollback of the subducting slab results in its decoupling with the mantle wedge at subarc depths, leading to elevation of the geothermal gradient in the subduction zone and switch to extensional regime forming lithospheric rifting in the overlying plate. This would cause not only dehydration melting of the subducting crust but also partial melting of the metasomatites in the small mantle wedge.
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