Waveform tomography of the upper mantle beneath the North Atlantic region using regional seafloor and global data.

The Iceland hotspot and its interaction with the Mid-Atlantic Ridge have strongly influenced the structure and evolution of the North Atlantic region. The Iceland hotspot has been linked to the presence of the underlying Iceland plume, rising from the deep mantle below Iceland. Due to the sparsity of the seismic data coverage in the North Atlantic Ocean, the structure and dynamics of the underlying plume, as well as its influence on the surrounding North Atlantic region, remain a topic of debate. As part of the project SEA-SEIS (Structure, Evolution And Seismicity of the Irish offshore), a network of ocean bottom seismometers was deployed across a large part of the North Atlantic Oceanfrom Ireland and Britain to near Icelandin September 2018. Over a period of 19 months, they recorded seismic data on the seafloor. 14 OBSs were retrieved in May 2020, of which 12 successfully recorded 19 months of seismic data. The data was thoroughly preprocessed, including the reduction of compliance and tilt noise on the vertical-component waveforms of teleseismic events. These waveforms were individually inverted for surface, S- and multiple S-waves using the Automated Multimode Inversion (AMI), extracting structural information on the lithosphere and underlying mantle. Here, we combine this new seafloor seismic data with a massive global dataset of waveform fits for almost 1.5 million seismograms, with the coverage maximised in the hemisphere around the North Atlantic, and compute a new seismic waveform tomography model of the North Atlantic upper mantle: NA24. The tomography capitalises on the improved data coverage and reveals both the S-wave velocity and the azimuthal anisotropy structure below the Iceland hotspot, the adjacent mid-ocean ridge, and the entire North Atlantic region at a new level of detail. It shows low seismic velocities below Iceland and the adjacent Reykjanes Ridge and Kolbeinsey Ridge down to ~260 km depth, below which they merge into one low-velocity body located west of Iceland at around 330 km depth. Around 410 km depth, the low-velocity body is present below eastern Greenland where it remains throughout the transition zone. This observation implies that the Iceland plume is located below eastern Greenland in the transition zone, moving eastward as it rises in the upper mantle. In the shallow upper mantle, a strong ridge-parallel azimuthal anisotropy structure is revealed along the full length of the Reykjanes Ridge. This implies a strong channelled horizontal flow from the Iceland plume towards the south below the ridge axis, while any sign of a radial outward flow of the plume is absent.