Seafloor acoustic imagery surrounding the Rainbow massif, Mid-Atlantic Ridge 36°N

The MARINER (Mid-Atlantic Ridge INtegrated Experiments at Rainbow) seismic and geophysical mapping experiment was carried out in April-May 2013 and was designed to examine the relationship between tectonic rifting, heat/melt supply, and oceanic core complex formation at a non-transform offset (NTO) of the Mid-Atlantic Ridge (36°N) the site of the ultramafic-hosted Rainbow hydrothermal system. One component of this experiment was dense acoustic multibeam backscatter and bathymetry data collection. We present acoustic imagery of the seafloor extending across two segments of the Mid-Atlantic Ridge separated by the Rainbow NTO massif. The acoustic imagery provides a broad view of the character of the ridge system, emphasizing the strong variability of seafloor morphology, tectonics, and lava emplacement and reveals the general tectono-magmatic setting of the Rainbow massif. The amplitude data were collected via a hull-mounted multi-beam sonar system (Kongsberg EM-122) aboard the R/V Marcus G. Langseth. The multi-fold, multi-directional coverage of the amplitude data allows for a compilation of all data into a common grid, as is usually done for depth data. Corrections for amplitude loss and grazing angle, in combination with multiple data coverage averages out noisy data, local slope dependence, and removes along-track artifacts that tend to be endemic to sonar images. The benefit is a complete sonar image for the area that can be examined with little distortion due to artifacts, and whose features can be interpreted as being principally derived from the intrinsic reflectivity of the seafloor rather than from look-direction and local seafloor slope. The main features of the image include: (1) newer seafloor within the axial valleys with some discernible individual lava flows; (2) large and small fault scarps and possible fissure systems; (3) sediment-filled basins; (4) terrains composed nearly entirely of small volcanic cones; (5) volcanic ridges; (6) regions of shallow topography exhibiting low sedimentation. The Rainbow massif itself consists of a mixture of high and low backscatter amplitudes, reflective of a complex tectonic and sedimentation history.