Evidence and implications of crustal magmatism on the flanks of the East Pacific Rise

Formation of oceanic crust along the East Pacific Rise is thought to be complete within a few kilometers of the rise axis. Here, however, we present evidence for magmatism 20 km from the spreading center in 300-ka-old crust. Seismic data reveal an intrusive complex ~ 2 km beneath the seafloor, which is limited in lateral extent (< 5 km) and comprises a melt lens underlain by low-velocity, high-attenuation crust. The crustal magma body overlies a region of off-axis delivery of mantle melt that results from a long-lived skew between the axes of mantle upwelling and plate spreading. Magmatism on the flanks of the EPR may be commonplace, thus affecting the architecture of oceanic crust and providing the necessary conditions to drive off-axis hydrothermal activity.