Seismic analysis reveals “fossilized” traces of ancient mantle flow beneath the Pacific Triangle, the Pacific Ocean’s oldest preserved oceanic crust (about 160- 180 million years old).

The findings come from a study led by YoungHee Kim of Seoul National University and colleagues, published in Seismological Research Letters (announced by the Seismological Society of America on 7 October 2026).

Researchers analyzed seismic anisotropy, directional variations in wave speeds caused by the alignment of minerals (mainly olivine) in the upper mantle under the influence of flow, using data from broadband ocean-bottom seismometers in the Korean- Japanese Oldest- 1 experiment (2018- 2019).

Key results

• The Pacific Triangle lies ~1,000 km east of the Mariana Trench and marks the site of an ancient triple junction of spreading ridges.
• Anisotropy patterns in the uppermost mantle are spatially variable and complex. They partly align with both ancient (fossil) and more recent plate motions but cannot be explained by simple present- day plate- motion- driven deformation alone.
• Some orientations reflect inherited structures from the plate’s long tectonic history (changes in plate motion), while others show mantle deforming around features such as a sunken lithospheric remnant (redirecting flow, consistent with earlier tomographic images) and possible localized hotspot upwelling near the Magellan Seamount Trail and the old triple junction (areas with weak or absent clear anisotropic signals).

This provides a rare window into past mantle flow beneath old oceanic lithosphere, the interaction between plate tectonics and mantle dynamics, and the evolution of Earth’s crust. Fossil anisotropy in the lithosphere can coexist with signals from ongoing upper- mantle flow.

The team is now analyzing data from the nearby Oldest- 2 array (closer to the Mariana Trench) to extend the picture and better separate contributions from regional flow, inherited deformation, and the subduction system.

Source: Seismological Society of America news release summarizing the Seismological Research Letters paper by Kim et al. (2026).

_____________________________________________________________________________________

The oldest large- scale oceanic crust still preserved on the seafloor is in the western Pacific Ocean, specifically in the region known as the Pacific Triangle (or Pigafetta Basin area, east of the Mariana Trench). 

It is generally dated to about 160-180 million years old (Middle to Late Jurassic).⁠

• This crust formed at the birth of the Pacific Plate around an ancient triple junction of spreading ridges within the Panthalassa Ocean.
• Direct sampling (e.g., Ocean Drilling Program Site 801) and magnetic anomaly data support ages around 167- 170 Ma for the main mid- ocean ridge basalts, with some studies indicating the plate began accreting as early as ~190- 192 Ma.⁠
• Oceanic crust is continuously created at mid- ocean ridges and destroyed by subduction, so almost none survives longer than ~200 million years. The western Pacific holds the oldest extensive, relatively undisturbed examples.

Possible older remnant

A 2016 study using magnetic data proposed that crust in the eastern Mediterranean Sea (Herodotus Basin) could be ~315- 365 million years old (Carboniferous–Permian, potentially a remnant of the ancient Tethys Ocean). This would make it the oldest known oceanic crust still in place under the seafloor.

However, this interpretation is more debated: the area is covered by very thick sediments, direct sampling is limited, and many standard references still cite the Jurassic western Pacific crust as the oldest well- documented large- scale oceanic lithosphere.

In summary: For the vast majority of scientific contexts and global seafloor age maps, the oldest oceanic crust is the Jurassic crust of the western Pacific (~160- 180 and more Ma). Older fragments exist as ophiolites emplaced on land (sometimes billions of years old), but those are no longer part of the intact ocean floor.

_____________________________________________________________________________________

Complex Upper‐Mantle Anisotropy Beneath the Pacific Triangle Revealed by Shear‐Wave Splitting 

The full text appears to be behind a GeoScienceWorld, SSA paywall or institutional access (attempts to retrieve the complete page content were blocked). The publicly available summary and related coverage provide the following key points:

Summary of the study

Seismic data from broadband ocean-bottom seismometers in the Korean- Japanese Oldest- 1 experiment (2018–2019) reveal complex, spatially variable seismic anisotropy in the uppermost mantle beneath the Pacific Triangle, a ~160- 180-million- year- old region of oceanic crust ~1,000 km east of the Mariana Trench. This area marks an ancient triple junction of spreading ridges.

• Method: Analysis of shear- wave splitting (anisotropy), which records the lattice preferred orientation of minerals (primarily olivine) aligned by past and present mantle flow.
• Main finding: The directional patterns of anisotropy are complicated. Some orientations align with both fossil (ancient plate- motion) and more recent plate motions, while others deviate from simple predictions based on present- day plate motion alone. Fossil anisotropy preserved in the lithosphere coexists with signals from ongoing upper-mantle flow.
• Additional observations:
  • Systematic changes in wave direction indicate mantle flow redirected around a sunken lithospheric remnant (previously hinted at by tomography).
  • Weak or absent clear anisotropic signals near the Magellan Seamount Trail and the ancient triple junction, consistent with possible localized hotspot upwelling.
• Significance: Provides a rare view of how mantle flow evolves beneath the oldest preserved oceanic plate, reflecting the interplay of plate-motion changes, inherited structures, and local features over geological time.

The research team is extending the work with data from the nearby Oldest- 2 array (closer to the Mariana Trench) to map regional trends and better separate contributions from inherited deformation, regional flow, and subduction- related processes.

For the complete abstract, figures, methods, and discussion, institutional or personal access to GeoScienceWorld / Seismological Research Letters is required. The SSA news release remains the most detailed open summary currently available.