The role of hydraulic fractures and intermediate-depth earthquakes in subduction-zone magmatism

The presence of magmatism and intermediate-depth (70–300 km deep) seismicity at subduction zones is at first sight surprising. Magmatism is unexpected because the subduction of cool oceanic lithosphere makes these regions the coldest in the mantle. The current model for subduction-zone magmatism is that water released from the subducting slab enters the relatively warm mantle wedge, leading to a reduction in melting temperature and magmatism1, 2, 3, 4. But there is a problem with this scheme because it is thought that water cannot leave the slab by porous flow to enter the wedge. The occurrence of intermediate-depth earthquakes is surprising because of the inhibitory effect of the very high frictional stress on faults expected from the high pressure at these depths. One proposal put forward to explain intermediate-depth seismicity is that high pore-pressure might facilitate faulting by decreasing the friction5, 6, 7. The hypothesis presented here is that non-percolating water provides the high pore-pressure, that the consequent faulting temporarily interconnects the water pores and, when a sufficient vertical height of water is interconnected, a hydrofracture is produced which transports the water out into the mantle wedge, thereby generating subduction-zone magmatism