Effects of Oceanic Crustal Thickness on Intermediate Depth Seismicity

Abstract

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. These faults have been shown to extend to depths of over 30 km and can channel water to depths of 20 km or more beneath the seafloor. However, the amount of water introduced into the oceanic mantle lithosphere, and the role of that water in the formation of intermediate depth seismicity, has been the topic of ongoing research.Here we compile evidence from areas where the subducted oceanic crust is likely thicker than the penetration depth of water into the downgoing plate. These regions comprise aseismic plateaus and ridges (hot spot tracks) that can be compared directly to adjacent segments of the oceanic plate where oceanic crust of normal thickness is subducted. Regions with thick oceanic crust show little to no seismicity at intermediate 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 that intermediate depth earthquakes observed in regions with thinner oceanic crust are caused by mantle dehydration reactions that are not possible in regions where the oceanic mantle was never hydrated because the thickness of the oceanic crust exceeded the penetration depth of water into the plate. We compare our observations to phase diagrams of hydrous basalt and hydrated depleted peridotite to determine pressures and temperatures that would be consistent with our observations. These can provide valuable constraints, not only on the degree of hydration and dehydration in the downgoing plate, but also as ground-truth for thermal models of these regions, all of which have complex, three-dimensional, time-variant subduction geometries and thermal histories.

Slab Transport of Fluids to Deep Focus Earthquake Depths—Thermal Modeling Constraints and Evidence From Diamonds

“…Plate‐bending at the outer rise produces lithospheric fractures that allow seawater to circulate deeply (see the summary in Faccenda, 2014) and serpentinize mantle lithosphere and low seismic velocities in the mantle portion of the slab (e.g., Ranero & Sallarès, 2004). Geophysical modeling (e.g., Faccenda, 2014), as well as a growing number of seismic studies (Bloch et al., 2018; Boneh et al., 2019; Cai et al., 2018; Faccenda, 2014; Fromm et al., 2006; Grevemeyer et al., 2018; Hatakeyama et al., 2017; Ranero et al., 2003; Wagner et al., 2020) indicate that fracture systems can result in hydration some 20–30 km into the slab. This is significant, not only for the total volume of water subducted with the slab, but also because plate interiors remain cold to greater depths, delaying the release of water from these parts of the slab.…”

“…As described by Zhan (2017), the size of faults associated with deep earthquakes is much larger than any proposed metastable olivine wedge or, for that matter, any plausible region of slab hydration. While slabs are now believed to be hydrated to depths of tens of kilometers (Bloch et al., 2018; Boneh et al., 2019; Cai et al., 2018; Faccenda, 2014; Fromm et al., 2006; Grevemeyer et al., 2018; Ranero et al., 2003; Wagner et al., 2020), the very largest seismic events likely require a secondary mechanism such as shear heating to propagate into surrounding “stable” areas. However, smaller seismicity should be able to remain localized within the region of phase transformation, and so may provide information on the size of that region.…”

“…In most subduction zones, however, only the crust is expected to dehydrate by those depths (Komabayashi et al., 2004; Maruyama & Okamoto, 2007; Ohtani, 2015; Ohtani et al., 2004; Omori et al., 2004; Thompson, 1992). The amount of water present in the mantle lithosphere of subducting slabs is a topic of ongoing research, but a number of recent studies highlight potentially tens of kilometers of hydration below the oceanic Moho made possible by extensional outer‐rise faulting as the plate bends to enter the trench (Bloch et al., 2018; Boneh et al., 2019; Cai et al., 2018; Faccenda, 2014; Fromm et al., 2006; Grevemeyer et al., 2018; Ranero et al., 2003; Wagner et al., 2020). The fate of this deeper reservoir of water in descending plates has been the topic of significant research (Barcheck et al., 2012; Hacker, 2008; Syracuse et al., 2010; van Keken et al., 2011) though often primarily at depths <300 km.…”

Full Waveform Inversion Beneath the Central Andes: Insight Into the Dehydration of the Nazca Slab and Delamination of the Back‐Arc Lithosphere

“…depth seismicity shuts off quickly downdip (Ferrand et al, 2017;Peacock, 2001;Sippl et al, 2018;Wagner et al, 2020).…”

Impact of the Juan Fernandez ridge on the Pampean flat subduction inferred from full waveform inversion

“…The northeast extension in the northern slab limb parallel to the JFR is superimposed on dominant slab pull (downdip extension), which is also reflected in the velocity field (Hu & Liu, 2016) and azimuthal anisotropy (Hu et al, 2017;Lynner et al, 2017). The south branch is coincident with the track of the JFR and attributed to the reactivation of the preexisting normal 10.1029/2021GL095509 7 of 13 faults, causing vigorous intra-slab seismicity (Ammirati et al, 2015;Anderson et al, 2007;Gans et al, 2011;Ranero et al, 2005;Wagner et al, 2020). Heit et al (2008) detected a strong oceanic LAB signal west of 𝐴𝐴 69 • W that suddenly disappears and even changes polarity further east below the slab tearing zone (Figure 3b).…”