Strong seismic heterogeneity in layer 2A near hydrothermal vents at the Mid-Atlantic Ridge

We present a high-resolution 3D seismic image beneath the Lucky Strike volcano on the Mid-Atlantic Ridge using streamer tomography. To obtain a high-resolution ray coverage in layer 2A, we first downward continue the multichannel seismic (MCS) data close to the seafloor generating a synthetic ocean bottom experiment (SOBE) and then apply 3D travel-time tomography. We find that the upper crust is laterally heterogeneous on 2-3 km scale, with unusually low velocities (1.8-2.2 km. s(-1)) in the upper few hundred meters beneath the Lucky Strike volcanic edifices, but normal layer 2A velocities (2.2-3.0 km. s(-1)) beneath the lava lake. The low velocities could be due to extremely high porosity (25-50%) in recently erupted, highly fractured pillow lavas. The hydrothermal vent fields seem to lie at the boundary between the high-porosity edifices and the lower porosity lava lake. We have also imaged a reflector at the base of the volcanic edifices that is distinct from the deeper high-velocity gradient transition zone from layer 2A to 2B imaged so far. The new technique provides an image of the oceanic crust with resolutions comparable to that of seafloor geology, leading to new insight about volcanic and hydrothermal processes. Citation: Arnulf, A. F., S. C. Singh, A. J. Harding, G. M. Kent, and W. Crawford (2011), Strong seismic heterogeneity in layer 2A near hydrothermal vents at the Mid-Atlantic Ridge, Geophys. Res. Lett., 38, L13320, doi:10.1029/2011GL047753.