In situ lower crustal accretion in the equatorial Atlantic Ocean by melt sill injection revealed by seismic layering

Oceanic crust is formed at mid-ocean spreading centres by a combination of magmatic, tectonic and hydrothermal processes. The crust formed mainly by magmatic process consists of an upper crust generally composed of basaltic dikes and lava flows and a lower crust presumed to mainly contain slightly to moderately altered gabbro whereas that dominated by tectonic process can be very heterogeneous and may even contain mantle rocks. Although the formation and evolution of the upper crust are well known from geophysical and drilling results, those for the lower crust remain a matter of debate. Here, we present results from the application of an elastic full waveform inversion method to the crustal turning waves (Pg) recorded by 8 ocean bottom seismometers spaced at 10-20 km covering 7 to 12 Ma old oceanic crust formed at the slow spreading Mid-Atlantic Ridge in the equatorial Atlantic Ocean. Our results show the presence of alternate 400-500 m thick layers with ±100-200 m/s P-wave velocity variations extended over 5-10 km in the lower crust. We suggest that these layers are formed in situ by melt sill injection, cooling and crystallisation at different depths in the lower crust. The vertical velocity gradient in the upper crust is higher than that obtained from conventional tomographic studies and the thickness of the upper crust is ~400 m thinner. The transition from the upper crust to the lower crust is marked by a 400 m thick low velocity layer, which may represent the base of hydrothermal circulation at the ridge axis. Based on the geochemical study of gabbro at ODP Hole 735B, we suggest that the lower part of the layering is formed by a stack of repeated recharge of primitive thin melt sills whereas the upper part consists of a homogeneous evolved magma mush formed by upward reactive porous flow, progressive differentiation and accumulation. This process would lead to layering structures, containing olivine-rich gabbro and troctolites at the base with a boundary separating more evolved olivine and olivine-bearing gabbro. Taken together, these suggest that the magmatism plays more important roles in the crustal accretion process at slow spreading ridges than tomographic study indicated, and that in-situ lower crustal accretion might be the main process for the formation of lower oceanic crust.