Discovery of a crustal lithosphere-asthenosphere boundary (LAB) topping a large magma reservoir at Axial volcano in the Eastern Pacific

The lithosphere-asthenosphere boundary (LAB) is fundamental to plate tectonics, with the overriding, brittle lithosphere floating on a ductile, convecting asthenosphere, allowing for plate motion at the surface of the Earth, especially plate divergence at mid-ocean spreading centers. The nature of this boundary in the vicinity of spreading centres has hitherto remained largely unknown, except for the presence of a narrow, 1-2 km wide mid-crustal axial magma lens (AML) observed beneath the axis at intermediate- to fast-spreading centers, separating the brittle upper crust from the lower-crustal mush zone. Using three-dimensional (3D) seismic reflection data, the team has imaged this boundary within the newly-created oceanic crust from about 1.2 km down to ~6 km below seafloor continuously beneath a large mid-ocean ridge volcano.

The 3D seismic reflection data were acquired in 2019 on board the United States marine seismic vessel, R/V Marcus Langseth (Fig. 1), at the Axial volcano in the eastern Pacific Ocean. Axial volcano, which sits at the intersection of the intermediate-spreading Juan de Fuca Ridge and Cobb-Eickelberg hotspot, has a flat-topped summit with a horseshoe-shaped 8 km x 3 km caldera lying at about 1.4 km water depth. It hosts several hydrothermal fields, and has been the site of three eruptions in the last decades, in 1998, 2011 and 2015. During the 2019 marine survey, the Langseth was equipped with four 6-km long streamers spaced at 150 m and two sound sources spaced at 75 m, acquiring a 300-m wide swath at each vessel pass. The total area covered is 40 km by 16 km surface area, and the images provide information down to ~6 km depth below seafloor.