Watching Dehydration: Seismic Indication for Transient Fluid Pathways in the Oceanic Mantle of the Subducting Nazca Slab. (14th September 2018)

• Record Type: Journal Article
• Authors: Bloch, Wasja; John, Timm; Kummerow, Jörn; Salazar, Pablo; Krüger, Oliver S.; Shapiro, Serge A.
• Is Part Of: Geochemistry, geophysics, geosystems. Volume 19:Number 9(2018)
• Page Start: 3189
• Page End: 3207
• Publication Date: 2018-09-14
• Subjects: Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 550.5
• DOI: 10.1029/2018GC007703
• Languages: English
• ISSNs: 1525-2027

Abstract

Abstract: 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 V P / V S 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 V P / V S 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. Plain Language Summary: Beneath the Central Andes, small earthquakes occur along a band 30 km within the subducting Nazca tectonic plate that slowly sinks below the South American plate. Such earthquake bands occur worldwide, and it has been proposed that they happen, because there is water bound in the solid rocks there, which would be released when the subducting plate heats up while it sinks into the Earth. We test this hypothesis by determining a rock property that can indicate whether liquid water is present from the arrival times of earthquake waves. We then theoretically calculate how this property would be for different kinds of rock that can be present at this specific location within the Earth. It results that the rock property that we derived in the earthquake band can best be explained by the presence of liquid water that is stored in thin interconnected veins there. Other researchers saw in laboratory experiments, in computer simulations, and in the field that rocks that release water look exactly like this, so that their and our pieces of the puzzle fit together well. With this knowledge, we can understand better how water sinks into the Earth and how it finds its way back to the surface. Key Points: A high seismic V P / V S ratio is observed in a lower band of seismicity in a subducting slab Active metamorphic mineral dehydration is required to explain this value Poroelastic modeling indicates the presence of a percolating vein network