Seismic evidence for the upper crustal accretion by long-distance lateral dyke injection at the Lucky Strike segment of Mid-Atlantic Ridge

Oceanic crust is formed along mid-ocean ridges by a combination of magmatic accretion and tectonic extension. Conventional view of oceanic crustal accretion is that magma migrates from the mantle to a crustal magma reservoir, which erupts to the surface forming lava flow through narrow vertical dykes, together making up the upper crust that is subsequently modified and thinned by faulting. This simple one-dimensional model magmatic accretion and two-dimensional tectonic extension stems from the observation of nearly continuous axial melt lenses (AMLs) beneath fast- to intermediate-spreading ridges and limited variation in upper crustal thickness along ridge axis. However, the available observations at slow-spreading ridges suggest that the AML is concentrated to the central portion of ridge segments, challenging the conventional model of oceanic crustal accretion. By applying full waveform inversion to wide-angle ocean bottom seismometer data from the ~70 km-long slow-spreading Lucky Strike segment at 37oN on the Mid-Atlantic Ridge, we show that although the crustal thickness varies from ~8.4 km at the segment centre to ~3.9 km at segment ends, the upper crustal thickness remains nearly constant (~3.0 km). The large variation in crustal thickness is dominantly due to the thinning of lower crust, which accounts for ~2/3 of crustal thickness at the segment centre but only 10% at segment ends. We suggest that most of the upper crust on this ridge segment is formed by lateral dyke propagation from the melt-rich segment centre to melt-poor segment ends. The petrology of basalts from the Lucky Strike segment indicates that melt is more primitive at the segment centre and gets more evolved towards segment ends, supporting our interpretation. This interpretation is consistent with the large-distance dyke propagation during the 2005 eruption of Afar rift, Ethiopian, the 1998 and 2015 eruptions of Axial Seamount, several eruptions in Iceland and the 2022 earthquake swarm on the 53.3o–54.3oN Mid-Atlantic Ridge. Taken together, these observations suggest that the magmatic accretion and tectonic extension along slow-spreading centres and rift zones are three-dimensional.