Imaging of the Mantle Transition Zone with SS Precursors in the Iceland-Mid-Atlantic Ridge Region

The nature of Iceland hotspot, located on the Mid-Atlantic Ridge, has been subject of contentious debate. Earlier seismic tomographic studies have suggested the presence of a deep mantle plume or confined to the upper mantle without a clear plume affinity. In this study, we utilize SS precursors to image the Mantle Transition Zone (MTZ) beneath Iceland. We collected a large SS precursor dataset that contains teleseismic recordings from all available global broadband stations between year 1976 and 2023. The resulting dataset enables a dense sampling of the Iceland and the Mid-Atlantic Ridge (i.e., 40°-80° N and -70°W to 10° E) with more than 3000 high-quality SS precursor waveforms. We adopted a recently proposed Robust Damped Rank-Reduction method to process the SS precursor data, which enables exploiting the signal coherency in multi-dimensional (4D) data and significantly improving the quality of the weak precursory arrivals. Seismic imaging based on the processed SS precursors effectively captures regional-scale topographic variations of mantle discontinuities, eliminates contaminating noises that produce small-scale artifacts, and enhances the lateral coherency of the MTZ structure. The resulting MTZ images reveal that the 410 km discontinuities are depressed by 5 km compared to regional average, whereas the 660 km discontinuities are uplifted by 6 km, leading to a MTZ of 230 km thick beneath Iceland. The region of depressed 410 extends southwards from the eastern edge of Greenland along the Mid-Atlantic Ridge for a distance about 20 degrees. In comparison, the elevation of 660 is more wide-spread, reaching as far as the northwestern end of Greenland. These observations suggest the interaction of a deep mantle plume with the 660 km discontinuity in a broad area covering the north Mid-Atlantic Ridge. The thick lithosphere beneath Greenland may have channeled the mantle flow towards the ridge, thereby causing the thinning of the MTZ centering on Iceland. In the southeast of the study area, our model also reveals a thinning area, isolated from the Iceland anomaly. The observed complex MTZ topography, in conjunction with multiple low-velocity centers in global tomographic models, may suggest the presence of a bifurcated, deep mantle plume originated from the lower mantle that may feed the shallow hotspots near the Iceland-Mid-Atlantic Ridge.