feart-08-00244 July 10, 2020 Time: 11:11 # 1 HYPOTHESIS AND THEORY published: 10 July 2020 doi: 10.3389/feart.2020.00244 Effects of Oceanic Crustal Thickness on Intermediate Depth Seismicity Lara S. Wagner1*, Mark J. Caddick2, Abhash Kumar3, Susan L. Beck4 and Maureen D. Long5 1 Department of Terrestrial Magnetism, Carnegie Institution for Science, Washington, DC, United States, 2 Department of Geosciences, Virginia Polytechnic Institute and State University, Blacksburg, VA, United States, 3 Department of Geology and Environmental Science, University of Pittsburgh, Pittsburgh, PA, United States, 4 Department of Geosciences, University of Arizona, Tucson, AZ, United States, 5 Department of Geology and Geophysics, Yale University, New Haven, CT, United States The occurrence of intermediate depth seismicity (70–300 km) is commonly attributed to the dehydration of hydrous phases within the downgoing oceanic plate. While some water is incorporated into the oceanic crust at formation, a significant amount of water is introduced into the plate immediately before subduction along outer-rise faults. Edited by: Vlad Constantin Manea, These faults have been shown to extend to depths of over 30 km and can channel National Autonomous University water to depths of 20 km or more beneath the seafloor. However, the amount of of Mexico, Mexico water introduced into the oceanic mantle lithosphere, and the role of that water in the Reviewed by: Fenglin Niu, formation of intermediate depth seismicity, has been the topic of ongoing research. Rice University, United States Here we compile evidence from areas where the subducted oceanic crust is likely Wasja Bloch, thicker than the penetration depth of water into the downgoing plate. These regions German Research Centre for Geosciences, Helmholtz Centre comprise aseismic plateaus and ridges (hot spot tracks) that can be compared directly Potsdam, Germany to adjacent segments of the oceanic plate where oceanic crust of normal thickness is *Correspondence: subducted. Regions with thick oceanic crust show little to no seismicity at intermediate Lara S. Wagner [email protected] depths, whereas adjacent regions with normal oceanic crust (∼6–8 km thick) have significant seismicity at similar depths and distances from the trench. We hypothesize Specialty section: that intermediate depth earthquakes observed in regions with thinner oceanic crust This article was submitted to Structural Geology and Tectonics, are caused by mantle dehydration reactions that are not possible in regions where a section of the journal the oceanic mantle was never hydrated because the thickness of the oceanic crust Frontiers in Earth Science exceeded the penetration depth of water into the plate. We compare our observations Received: 28 December 2019 to phase diagrams of hydrous basalt and hydrated depleted peridotite to determine Accepted: 04 June 2020 Published: 10 July 2020 pressures and temperatures that would be consistent with our observations. These can Citation: provide valuable constraints, not only on the degree of hydration and dehydration in Wagner LS, Caddick MJ, the downgoing plate, but also as ground-truth for thermal models of these regions, Kumar A, Beck SL and Long MD (2020) Effects of Oceanic Crustal all of which have complex, three-dimensional, time-variant subduction geometries and Thickness on Intermediate Depth thermal histories. Seismicity. Front. Earth Sci. 8:244. doi: 10.3389/feart.2020.00244 Keywords: flat-slab, seismicity, hydration, intermediate-depth, aseismic-ridges, oceanic crust, oceanic ridges Frontiers in Earth Science| www.frontiersin.org 1 July 2020| Volume 8| Article 244 feart-08-00244 July 10, 2020 Time: 11:11 # 2 Wagner et al. Oceanic Crust and Deep Seismicity INTRODUCTION Ferrand et al.(2017) at 3.5 GPa, relatively little antigorite ( ∼5%) is needed to induce seismicity, consistent with small amounts of Intermediate depth earthquakes (70–300 km depth) occur at hydrated material that might be present deep within the mantle pressures and temperatures (P/T) that should produce ductile lithosphere of subducted slabs. deformation rather than brittle failure (e.g., Green and Houston, There has also been increased interest in the relationship 1995). There has long been significant debate about the role of between observations of heterogeneities within the oceanic dehydration reactions in the formation of intermediate depth plate as it enters the trench, and seismicity patterns in the seismicity. The P/T conditions at which intermediate depth subducting slab at intermediate depths (e.g., Shillington et al., earthquakes occur have repeatedly been shown to be consistent 2015). Recent studies looking at the seismic velocity structure of with dehydration phase boundaries for both crustal and mantle oceanic plates as they approach the trench show that outer-rise lithospheric hydrous compositions (e.g., Kirby, 1995; Kirby et al., faulting consistently results in extensive hydration of the oceanic 1996; Peacock, 2001; Hacker et al., 2003; Yamasaki and Seno, plate (see Grevemeyer et al., 2018 and references therein). In 2003; Abers et al., 2013). The most commonly cited link between studies with sufficiently deep resolution, hydration is observed dehydration and intermediate depth seismicity is dehydration to extend ∼10–12 km below the oceanic Moho (e.g., Ranero embrittlement. As originally conceived, the term “dehydration et al., 2003). Fromm et al.(2006) used aftershock locations from embrittlement” means the hydraulic fracturing of rock caused by a Mw 6.7 outer-rise earthquake to show faulting to depths of the production of free fluids and resultant overpressure in pore ∼ 25 km. In rare cases (e.g., Bloch et al., 2018; Cai et al., spaces that in turn could overcome the ambient pressure (e.g., 2018), evidence exists for even deeper hydration. Recent work Raleigh and Paterson, 1965; Meade and Jeanloz, 1991). However, by Boneh et al.(2019) shows a correlation between the observed more recent experimentation has shown that (a) above some fault offsets on outer-rise faults from bathymetry data and the pressure, volume change associated with dehydration reactions rate of intermediate depth seismicity observed down-dip, which is likely negative, complicating the overpressure argument (e.g., suggests the extent of hydration in the oceanic plate due to outer Dobson et al., 2002; Jung et al., 2004; Gasc et al., 2011), and (b) rise faulting may control the amount of seismicity that is possible some experiments on antigorite and other hydrous phases have at intermediate depths. produced no acoustic emissions (AEs) upon dehydration and/or While links between hydration and seismicity are consistent show evidence solely of stable sliding or distributed deformation, with dehydration embrittlement or related mechanisms, bringing the concept of “dehydration embrittlement” into they do not address whether it is the dehydration of the question (e.g., Chernak and Hirth, 2010; Chernak and Hirth, oceanic crust or mantle that is responsible for the generation 2011; Gasc et al., 2011; Proctor and Hirth, 2015; Gasc et al., 2017). of intermediate depth earthquakes. Our hypothesis is that One possible contribution from dehydration reactions unrelated regions with thinner oceanic crust produce intermediate to pore pressure is the formation of nanocrystalline reaction depth seismicity due to mantle dehydration reactions, and products whose small grain size might contribute to vein growth, that slabs with very thick oceanic crust do not produce channel flow, and/or localized weakening (e.g., Incel et al., 2017; intermediate depth events because the previously hydrated Plümper et al., 2017). Other theories have focused more directly overthickened crust has already experienced its most on localized weakening due to slip, i.e., thermal run-away/shear important dehydration reactions by this point, and because heating mechanisms that also incorporate reduced grain size the mantle below was never hydrated to begin with and along discrete planes of weakness (e.g., Kelemen and Hirth, 2007; therefore cannot produce the dehydration reactions needed to John et al., 2009; Prieto et al., 2013; Poli et al., 2016; Proctor and generate seismicity. Hirth, 2016; Ohuchi et al., 2017). Thermal run-away mechanisms Our hypothesis is based on the clearest and simplest may be self-initiating or may serve as a propagation mechanism observation of this pattern: along the Nazca Ridge in Peru for events triggered by another mechanism (e.g., Zhan, 2017). (∼18 km thick oceanic crust) (Hampel et al., 2004; Kumar et al., A recent theory that matches many of these observations is the 2016). We present our hypothesis in the context of this location, “dehydration destabilization stress transfer” mechanism (DDST; and investigate other possible factors such as plate age, evidence Ferrand et al., 2017; Ferrand, 2019). In this model, hydrated for outer rise faulting, oceanic crustal structure, and any available peridotite produces AEs at specific conditions that depend on direct evidence for oceanic mantle hydration. We also investigate the amount of hydrated material present within the peridotite the plausibility of our hypothesis in terms of the metamorphic matrix and the pressure at which the dehydration reaction occurs. reactions and pressure-temperature conditions required for this Rather than increasing pore pressure to hydraulically fracture to be true. We then assess the hypothesis at a number of other the surrounding rock matrix, the dehydration of antigorite