Lower oceanic crust formed by in situ melt crystallization revealed by seismic layering

Abstract

Oceanic crust forms at mid-ocean spreading centres through a combination of magmatic and tectonic processes, with the magmatic processes creating two distinct layers: the upper and the lower crust. While the upper crust is known to form from lava flows and basaltic dykes based on geophysical and drilling results, the formation of the gabbroic lower crust is still debated. Here we perform a full waveform inversion of wide-angle seismic data from relatively young (7–12-Myr-old) crust formed at the slow-spreading Mid-Atlantic Ridge. The seismic velocity model reveals alternating, 400–500 m thick, high- and low-velocity layers with ±200 m s−1 velocity variations, below ~2 km from the oceanic basement. The uppermost low-velocity layer is consistent with hydrothermal alteration, defining the base of extensive hydrothermal circulation near the ridge axis. The underlying layering supports that the lower crust is formed through the intrusion of melt as sills at different depths, which cool and crystallize in situ. The layering extends up to 5–15 km distance along the seismic profile, covering 300,000–800,000 years, suggesting that this form of lower crustal accretion is a stable process.

Seismic evidence for uniform crustal accretion along slow-spreading ridges in the equatorial Atlantic Ocean

“… 80 and the presence of melt sills in the lower crust is determined in ref. 81 . In this model, the melt is not focused to segment centres at the base of the lithosphere 19 , 20 , but migrates upward vertically.…”

A Dual‐Level Magmatic System Beneath the East Pacific Rise, 9°N

“…In the Oman ophiolite and at Hess Deep, the lower gabbroic section exhibits modal layering (e.g., Gillis et al., 2014; Pallister & Hopson, 1981); these centimeter thick layers may have formed via crystallization in thin sills (e.g., Kelemen et al., 1997). Seismic experiments have also detected sill‐like features in the upper part of the lower crust (Carbotte et al., 2021), and in the lower crust at a slow‐spreading ridge (Guo et al., 2022). Sills are not precluded by the tomographic image, which predicts bulk‐average melt fractions over the resolution length of the image, and therefore cannot detect individual sills.…”

Ocean expansion: The role of detachment faults

“…• Poliakov and Buck's model (Poliakov & Buck, 1998) also explains simply why gabbro bodies are always found as sills and never as dykes at shallow levels (Chalot-Prat, 2005;Dick et al, 2003Dick et al, , 2019Garcés & Gee, 2007;Guo et al, 2022;Henstock et al, 1993;Lagabrielle & Cannat, 1990;Maffione et al, 2013;Magott et al, 2021;Whitney et al, 2013). Gabbro bodies, initially vertical, are progressively tilted upward, taking on the appearance of sills at shallow level when the mantle is transferred laterally along detachment faults.…”

“…Mantle rock textures, as those of intrusive gabbro body margins, attest to stretching within a ductile context, stretching which would occur during matter transfer, upward but also laterally on either side of the plate boundary according to the numerical model of Poliakov and Buck (1998). Poliakov and Buck's model (Poliakov & Buck, 1998) also explains simply why gabbro bodies are always found as sills and never as dykes at shallow levels (Chalot‐Prat, 2005; Dick et al, 2003, 2019; Garcés & Gee, 2007; Guo et al, 2022; Henstock et al, 1993; Lagabrielle & Cannat, 1990; Maffione et al, 2013; Magott et al, 2021; Whitney et al, 2013). Gabbro bodies, initially vertical, are progressively tilted upward, taking on the appearance of sills at shallow level when the mantle is transferred laterally along detachment faults. Moreover, mantle cataclasite horizons, capping exhumed mantle rocks at the surface, show evidence of shear deformation along detachment faults during their emplacement at shallow level (Mallows & Searle, 2012).…”