Seismic data captured by ocean bottom instruments have revealed "fossilized" traces of mantle flow beneath the Pacific Ocean's oldest preserved piece of crust.
The patterns of ancient flow traced in the uppermost mantle are complicated, tracking changes in tectonic plate motion over time as well as mantle deforming around features such as a sunken lithospheric (the crust and the uppermost brittle mantle) remnant and upwelling hotspots.
The study published in Seismological Research Letters offers a rare glimpse at past mantle flow beneath old oceanic plates, how mantle flow drives plate tectonics and the evolution of Earth's crust, according to YoungHee Kim of Seoul National University and colleagues.
Kim and her colleagues study seismic anisotropy to find traces of past mantle movement. The mantle's flow tugs the crystalline structure of its upper mantle minerals into a lattice-like orientation. These orientations can be detected by the way seismic shear waves split in fast and slow components—anisotropy—as they pass through the aligned minerals.
"The central question is how the directional pattern we observe in the mantle reflects both deformation associated with present-day plate motion and structures inherited from the plate's long history," Kim explained.
"Our results show that the anisotropy beneath the Pacific Triangle is spatially variable and cannot be explained by simple plate-motion–driven deformation alone," she added.
The Pacific Triangle, located about 1000 kilometers east of the Mariana Trench, is 160 to 180 million years old. It represents a site where three tectonic plates once met at a triple junction of spreading ridges, where the plates pulled apart to make way for upwelling new ocean crust.
The shear wave data analyzed in the study were collected by broadband ocean bottom seismometers that are part of Oldest-1, a Korean-Japanese collaborative experiment conducted from 2018 to 2019. "Seismic anisotropy was one of its scientific targets, along with other aspects of mantle structure and dynamics," Kim explained.
The researchers' analysis uncovered a complicated picture of mantle flow in the region, with some anisotropic orientation aligned to ancient and more recent plate motions and other orientations differing from those expected from plate motion predictions.
"We expected some complexity because the region's long history, including changes in plate motion and hotspot interactions, may have preserved or modified earlier deformation fabrics," said Kim. Fossil anisotropy in the lithosphere "could therefore coexist" with anisotropy produced by present-day flow of the upper mantle, she said.
The researchers also got a closer look at some anomalies in the mantle below the Pacific Plate that had been spotted earlier using techniques such as seismic tomography, which produces a sort of CT scan of subsurface structures in the Earth.
The new analysis found a systematic change in wave direction that suggested mantle flow was being redirected around a sunken piece of old lithosphere that had been hinted at through tomographic imaging. The researchers also found spots with no clear anisotropic signal around the Magellan Seamount Trail and the ancient triple plate junction, where there may be localized hotspot upwelling of the mantle.
"Neither interpretation is unique, but both offer clues to how local mantle structure may influence deformation," Kim said.
The research team is now analyzing seismic anisotropy from the Oldest-2 ocean bottom seismometer array, which was deployed west of the Oldest-1 experiment and closer to the Mariana Trench. They want to see if some of the trends they observed in their first study continue or change closer to the trench.
"Combining the two arrays will allow us to trace this pattern over a wider area and better assess possible contributions from regional mantle flow, inherited lithospheric deformation, and the nearby subduction system," Kim explained. "This should help place the Oldest-1 observations within a more comprehensive picture of mantle deformation beneath the oldest Pacific seafloor."
