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

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 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.

Earthquake Depth Phase Extraction With P Wave Autocorrelation Provides Insight Into Mechanisms of Intermediate‐Depth Earthquakes

“…Neither in Japan nor Chile do we find anomalous sP to pP differential times for events in the lower layers of the double seismic zones, which argues against deep penetration of water into the upper mantle and, thus, dehydration embrittlement as the mechanism for the lower of the double seismic zone. This is somewhat inconsistent with Bloch et al (2018), who observed very high Vp/Vs in the lower layer. But the depth where they found anomalous Vp/Vs (about 55 km) is shallower than what we observed with sP to pP differential time.…”

“…But the depth where they found anomalous Vp/Vs (about 55 km) is shallower than what we observed with sP to pP differential time. Using a similar approach to Lin and Shearer (2007) and Bloch et al (2018), we do not find high Vp/Vs in the lower layer in the Japan subduction zone ( Figure S7). It could be interesting to explore deeper regions in the lower layer of the double seismic zone in northern Chile using the approach of Bloch et al (2018) to see if both measurements are consistent with each other and confirm if dehydration extends deeper in the lower layer in warm slabs.…”

“…Recently findings of correlation of seismicity rate and off-trench faults distribution (Boneh et al, 2019), deep hydration in the Mariana subduction zone (Cai et al, 2018), and high Vp/Vs in the lower layer in northern Chile (Bloch et al, 2018) point to a dehydration embrittlement mechanism for intermediate earthquakes.…”

“…Recently findings of correlation of seismicity rate and off‐trench faults distribution (Boneh et al, ), deep hydration in the Mariana subduction zone (Cai et al, ), and high Vp/Vs in the lower layer in northern Chile (Bloch et al, ) point to a dehydration embrittlement mechanism for intermediate earthquakes. However, the high value in the upper layer (Florez & Prieto, ) may lead to the dominance of upper layer seismicity in estimating the seismicity rate in the intermediate‐depth range, thus suppressing the contribution of lower layer seismicity which may operate with other mechanisms.…”

Modification of the Seismic Properties of Subducting Continental Crust by Eclogitization and Deformation Processes

“…Another important characteristic used to study rocks at depth is the V P / V S ratio, which is of specific interest in receiver function imaging, because this method is based on the differences in arrival times of converted waves at boundaries with contrasting seismic velocities (Kind et al, ). Additionally, the V P / V S ratio is often used to extract lithological information of the rocks at depth or to detect the presence of fluids (e.g., Bloch et al, ; Zhang et al, ).…”