This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: 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 deformation around features such as a sunken remnant of the lithosphere (the crust and the uppermost brittle mantle) 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 into 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 1,000 kilometers (620 miles) 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 orientations aligned with 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 on 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.
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