Deep mantle deformation is linked to subducted slabs, global map shows

Deep within Earth's interior, slow convection currents drive the movement of tectonic plates while simultaneously deforming the mantle material itself. A groundbreaking new study published in The Seismic Record has confirmed that much of this deformation in Earth's lowest mantle layer correlates with regions where scientists believe deeply subducted tectonic slabs exist.

While researchers had long hypothesized this connection, the study represents the first comprehensive global analysis of this phenomenon, examining nearly 75% of the lowest mantle layer positioned just above the core-mantle boundary, approximately 2,900 kilometers beneath Earth's surface.

Jonathan Wolf at the University of California, Berkeley, and his research team developed this unprecedented global map by collecting and analyzing an extraordinary database containing over 16 million seismograms from 24 data centers worldwide.

The research relies on seismic anisotropy, a phenomenon where earthquake-generated shear waves travel at varying speeds depending on their direction through different materials. This directional variation in wave speed, determined by the material's structure and composition, enables scientists to identify areas experiencing mantle deformation.

These deformation patterns provide crucial insights into mantle convection processes. "We know that deformation in the upper mantle is dominated by the drag of the plates that move across it," Wolf explained. "That extremely well approximates what we know from seismic anisotropy about the deformation of the upper mantle. But we don't have any of this kind of large-scale understanding for flow in the lowermost mantle. That's really what we want to get at."

The researchers analyzed specific phases of seismic waves that traveled through the mantle, into the core, and back through the mantle using what Wolf believes may be "the largest-ever assemblage of earthquake seismic data." These wave types prove particularly effective for mapping seismic anisotropy at lateral scales of hundreds of kilometers, delivering detailed information about anisotropy distribution in the lowest mantle.

The analysis revealed anisotropy in approximately two-thirds of the sampled lower mantle region. While the anisotropy pattern displays complexity, most occurrences align with locations where scientists suspect the presence of deeply subducted slabs.

"This isn't that surprising in a sense, because that is predicted by geodynamic simulations," Wolf noted. "But at the scale that we're looking at, it's not really been shown using the methods that we're using."

The scientific community continues to debate the origins of seismic anisotropy within these subducted slabs. While the slabs may retain "fossil" anisotropy traces from their time nearer to the surface, seismic anisotropy more likely results from severe deformation these slabs experience upon reaching the core-mantle boundary, combined with deformation of the material they displace.

The extreme pressure and temperature conditions within the mantle can also alter the slab's mineral phases during descent, creating a different type of anisotropic "fabric."

Wolf emphasized that the absence of anisotropic signals in certain areas of their lowest mantle map doesn't necessarily indicate the absence of anisotropy. In some locations, the signal may be too weak for their analytical methods to detect.

The massive dataset utilized in this research represents a "treasure trove" that Wolf and his colleagues continue to explore for additional deep mantle insights. "If I can dream, we will someday have enough information to really say much more about global flow directions of the lowermost mantle, knowing the seismic anisotropy across different lateral scales in the mantle, illuminating it from many directions," he said.