Low velocities in the oceanic upper mantle and their relation to plumes: insights from SEM-based waveform tomography

The exchange of heat, mass and momentum between tectonic plates and mantle convection controls lithospheric evolution and hotspot volcanism, and must occur at a range of spatial scales. Yet, the detailed morphology of the associated convection patterns continues to elude geophysicists. Because seismic velocities are affected by temperature, seismic tomography can be used to map the patterns of flow in the Earth's mantle. Here, we present a global-scale long-period full-waveform seismic tomographic model SEMum2 constructed using the Spectral Element Method, which can very accurately model wave propagation through highly complex structures, and account for phenomena such as scattering, (de)focusing, and wavefront healing. Notably, SEMum2 achieves more realistic amplitudes of lateral heterogeneity - particularly low velocities in the upper 250km - than previous generations of global models, while still retrieving the long-wavelength structure present in earlier tomographic models. Cluster analysis of profiles of shear velocity in the SEMum2 oceanic upper mantle, confirms the presence of a well marked shear wave low velocity zone (LVZ) beneath the lithosphere, with a velocity minimum which deepens progressively as a function of age of the plate. The LVZ minimum in SEMum2 reaches values that are lower than in previous tomographic global models and in agreement with local estimates where available. Interestingly, reaching below this "classical" low velocity zone, the model reveals a pattern of alternating lower and higher velocities organized into elongated bands in the direction of absolute plate motion (APM), with a quasi-regular spacing of ~2000 km perpendicular to the APM. This fingerlike structure, most prominent around 200-250 km and extending down to 350-400 km, is most prominent beneath the Pacific plate, but also present under the eastern Antarctic plate, in the south Atlantic and in parts of the Indian Ocean Below this depth, the low velocities appear organized into several vertically coherent "conduits", the most prominent under Hawaii and the Pacific superswell, where they appear to be rooted in the lower mantle. These conduits have complex shapes, in particular, the one associated with Hawaii undulates as it "rises", and is deflected towards the ridge as it reaches the bottom of the "fingering" layer. Individual hotspots do not lie immediately above the conduits but in their general vicinity. Nor are the fingers always associated with prominent hotspots. This morphology in the top 400 km of the oceanic mantle suggests the presence of a complex dynamic interplay between plate-driven flow just below the lithosphere, return flow directed toward the ridges, and influx from the deep plume conduits.