Watching Dehydration: Seismic Indication for Transient Fluid Pathways in the Oceanic Mantle of the Subducting Nazca Slab

Intermediate depth seismicity in subduction zones often occurs in the form of two slab-parallel bands. We estimated the seismic P to S wave velocity ratio within the shallowest part of the lower seismicity zone (LSZ) in the mantle of the subducting slab of the Central Andean subduction system at 50-km depth, 30 km below the Moho, using local earthquake data. We find an exceptionally high VP∕VS value larger than ∼2.0 that cannot be explained by a realistic solid lithology but requires the presence of fluid-filled porosity. This implies that the incoming Nazca plate must be partially hydrated to this depth below the seafloor. We introduce a state-of-the-art petrophysical model that takes into account the thermodynamic and poroelastic effects of dynamic metamorphic mineral dehydration at 1.8 GPa and consider anisotropic effects. The model shows that a high VP∕VS value generally indicates that the medium is near the percolation threshold, that is, that porosity must be interconnected. This result is consistent with observations from outcrops of paleosubduction zones, laboratory experiments, and numerical simulations. It follows that the shallowest part of the LSZ of the Central Andes must reside at a temperature at which mineral dehydration reactions take place, here between 430 and 500 ∘ C. For the first time, we can confirm that the observations of transient dehydrating fluid-filled vein structures with a pore volume in the order of only 10−3 are reasonable for the LSZ and enough to allow for effective drainage.