Oceanic upper crustal accretion by melt sill and lava flow interaction at Axial volcano

Magmatically-accreted upper crust along mid-ocean ridges is traditionally considered to consist of lava flows overlying a sheeted-dyke complex. However, how the upper crust is formed at hotspot-influenced ridge segments remains unknown. Using three-dimensional seismic reflection data, here we report images of lava flow layering beneath Axial volcano on the Juan de Fuca Ridge. These lava flow layers dip inward towards the caldera and rift zones, resulting from subsidence caused by magma withdrawal beneath the summit and rift zone extension. Furthermore, our images reveal that the classic, laterally continuous sheeted-dyke complex is absent. Instead, lava flow layers are rotated and brought into direct contact with the magma domain, leading to partial assimilation of these units. Three-dimensional images also show that melt is injected outward along lava flow layers, suggesting that the upper crust at Axial volcano is formed by complex interactions between injected melt sills and lava flow layers.