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. This is because studies of upper crustal accretion have been hampered by the inability to directly image lava flow structure, resulting in the use of proxies such as velocity gradients to distinguish between lava flows and sheeted dykes. Using 3D seismic reflection data, here we report for the first time imaging of a sequence of lava flows beneath Axial volcano. These lava flows dip inward towards the caldera and rift zones, resulting from subsidence caused by the emptying of magma reservoirs and rift zone extension. Furthermore, our images reveal that the sheeted dyke complex is completely absent. Instead, lava flows are rotated and brought into direct contact with the magma reservoir, leading to assimilation of these hydrous units. 3D images also show that melt is injected along lava flow bedding planes, suggesting that the upper crust at Axial volcano is formed by complex interactions between injected melt sills and lava flows.