Geoscientists Discover Mysterious Subduction Zone beneath Pacific Ocean

During the Mesozoic era, between 250 and 120 million years ago, an ancient seafloor sank deep into Earth in the East Pacific Rise, a tectonic plate boundary on the floor of the southeastern Pacific Ocean, says a team of geoscientists from the University of Maryland and the University of Alberta.

University of Maryland researcher Jingchuan Wang and colleagues used innovative seismic imaging techniques to peer deep into Earth’s mantle, the layer between our planet’s crust and core.

They found an unusually thick area in the mantle transition zone, a region located between about 410 and 660 km below the Earth’s surface.

The zone separates the upper and lower mantles, expanding or contracting based on temperature.

The newly-discovered seafloor may also explain the anomalous structure of the Pacific Large Low Shear Velocity Province (LLSVP) — a massive region in Earth’s lower mantle — as the LLSVP appears to be split by the slab.

“This thickened area is like a fossilized fingerprint of an ancient piece of seafloor that subducted into the Earth approximately 250 million years ago,” Dr. Wang said.

“It’s giving us a glimpse into Earth’s past that we’ve never had before.”

Subduction occurs when one tectonic plate slides beneath another, recycling surface material back into Earth’s mantle.

The process often leaves visible evidence of movement, including volcanoes, earthquakes and deep marine trenches.

While geologists typically study subduction by examining rock samples and sediments found on Earth’s surface.

By examining how seismic waves traveled through different layers of Earth, the researchers were able to create detailed mappings of the structures hiding deep within the mantle.

“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,” Dr. Wang said.

“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.”

What the authors found surprised them: material was moving through Earth’s interior much more slowly than previously thought.

The unusual thickness of the area they discovered suggests the presence of colder material in this part of the mantle transition zone, hinting that some oceanic slabs get stuck halfway down as they sink through the mantle.

“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,” Dr. Wang said.

“Our discovery opens up new questions about how the deep Earth influences what we see on the surface across vast distances and timescales.”

The results were published in the journal Science Advances.

_____

Jingchuan Wang et al. 2024. Mesozoic intraoceanic subduction shaped the lower mantle beneath the East Pacific Rise. Science Advances 10 (39); doi: 10.1126/sciadv.ado1219