The team, led by postdoctoral researcher Jingchuan Wang, used advanced seismic imaging techniques to analyze Earth's mantle, the layer between its crust and core. They found a thick area in the mantle transition zone, located between 410 and 660 kilometers beneath the surface. The discovery may also explain a previously misunderstood structure in the Pacific Large Low Shear Velocity Province (LLSVP), as the slab appears to split this large mantle feature.

"This thickened area is like a fossilized fingerprint of an ancient piece of seafloor that subducted into the Earth approximately 250 million years ago," Wang said. "It's giving us a glimpse into Earth's past that we've never had before."

Subduction, where one tectonic plate slides under another and is pushed into the mantle, usually leaves surface evidence such as volcanoes and earthquakes. However, Wang and his colleagues, Geology Professors Vedran Lekic and Nicholas Schmerr, used seismic waves to probe the ocean floor. The method allowed them to map hidden mantle structures by studying the way seismic waves moved through Earth's layers.

"You can think of seismic imaging as something similar to a CT scan. It's basically allowed us to have a cross-sectional view of our planet's insides," Wang explained. "Usually, oceanic slabs of material are consumed by the Earth completely, leaving no discernible traces on the surface. But seeing the ancient subduction slab through this perspective gave us new insights into the relationship between very deep Earth structures and surface geology, which were not obvious before."

Their analysis revealed that material was moving through the Earth's interior more slowly than previously expected. Wang believes this slow movement points to colder material within the mantle transition zone, where oceanic slabs may get stuck as they sink.

"We found that in this region, the material was sinking at about half the speed we expected, which suggests that the mantle transition zone can act like a barrier and slow down the movement of material through the Earth," Wang added. "Our discovery opens up new questions about how the deep Earth influences what we see on the surface across vast distances and timescales."

The team plans to extend their research to other parts of the Pacific Ocean and beyond. Wang aims to create a more detailed map of ancient subduction and upwelling zones, which may offer further insights into how tectonic plates have shifted throughout Earth's history.

"This is just the beginning," Wang said. "We believe that there are many more ancient structures waiting to be discovered in Earth's deep interior. Each one has the potential to reveal many new insights about our planet's complex past - and even lead to a better understanding of other planets beyond ours."

Research Report:Mesozoic intraoceanic subduction shaped the lower mantle beneath the East Pacific Rise

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