Mantle dynamics beneath the North Atlantic region from integrated seismic imaging using new regional seafloor data and global datasets

Mantle plumes are enigmatic hot upwelling rising from instabilities at Earth's core-mantle boundary. When approaching the surface, they are thought to cause the emplacement of Large Igneous Provinces (LIPs), whose catastrophic volcanism led to mass extinctions through Earth history. Continued ascent of hot material within the plumes' tails is believed to create volcanic hotspots, with continual eruptions over tens of m.y. or more, such as in Iceland. Mantle plumes have been difficult to image seismically, and LIP magmatism is often varied chemically and scattered over thousands of km. This prompted alternative, no-plume explanations for LIP and hotspot volcanism. The properties of mantle plumes (starting with their existence, according to sections of the community) and whether and how they cause LIPs are outstanding, first-order questions of Earth science.

The Iceland Plume is a type example of the phenomenon and a subject of long-standing debate. It is thought to cause Iceland's present volcanic activity and the NE Atlantic Ocean's anomalously shallow (per seafloor age) bathymetry. It may also have created the N. Atlantic Igneous Province (NAIP), its ~60 m.y. old volcanic areas dispersed from Britain to western Greenland. Geophysical and petrological data indicate anomalously hot sub-lithospheric mantle below Iceland, and the Iceland and NAIP basalts show isotopic ratios with deep-mantle signatures, consistent with a plume origin.

The Iceland Plume was a target of many seismic imaging studies, but the gap in station coverage around Iceland (no stations on the seafloor) translated into major gaps in the data sampling of the mantle. Available seismic models show frustratingly low mutual consistency. The vertical and lateral mantle flow below NE Atlantic is largely unknown.

In 2018-20, the project SEA-SEIS, led by the PI, operated Ocean-Bottom Seismometers (OBS) across a large part of NE Atlantic, filling much of the sampling gap. SEA-SEIS' own focus was on structure and seismicity of Ireland's offshore, but the network was designed to also image the Iceland Plume. The proposed project capitalises on the exceptional opportunity of using unique new data (scheduled for public release at SEA-SEIS' end) at no data-collection cost.

This project's goal is a breakthrough in our understanding of the structure and dynamics of the Iceland Plume and of how it could cause the NAIP magmatism. This will be achieved by combining the new data with all other relevant seismic data available and applying a suite of complementary imaging methods that will resolve plume structure and plume-induced flow.

Seismic tomography with new and all pre-existing data will yield a detailed 3D image of the region's upper and lower mantle. We will combine waveform inversion of surface and regional S waves (sampling the upper mantle) with multi-frequency, teleseismic travel-time tomography (also sampling the lower mantle).

The thickness and, by inference, temperature of the mantle transition zone (TZ, ~410-660 km depths) will be mapped using receiver functions (RF). Recent maps show intriguing small-scale variations beneath NE Atlantic but a large gap in the key area between Iceland and Britain. We will fill this gap and expect to learn where and how the hot plume rises through the TZ.

Waveform tomography and interstation surface-wave measurements will constrain a lithospheric-thickness map and show whether or not thin-lithosphere channels connect Iceland with NAIP sites to the east.

Seismic anisotropy indicates fabric created by flow of the rock at depth. It will be mapped with complementary shear-wave-splitting and surface-wave methods and show current directions of convective flow in the NE Atlantic upper mantle.

The combined, integrated evidence will illuminate the Iceland Plume and convective currents it creates in unprecedented detail. It will bring important new insights into the mechanisms of intraplate volcanism globally.