Seafloor seismometers find a mixed record of mantle flow under the Pacific's oldest plate

The paper's first author is Seung-Heon Choi, and YoungHee Kim of Seoul National University described the results ^[2]. The method depends on how flowing rock lines up. Mantle flow pulls the crystals of upper-mantle minerals into a lattice-like orientation, and a shear wave crossing that rock splits into fast and slow components ^[3]. "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 said ^[4].

The Pacific Triangle lies about 1,000 kilometres east of the Mariana Trench and is 160 to 180 million years old ^[6]. Three plates once met there at a triple junction of spreading ridges, pulling apart as new ocean crust welled up between them ^[7]. "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," Kim said ^[10].

The picture they recovered is complicated in the way she anticipated ^[1]. Fossil anisotropy in the lithosphere "could therefore coexist" with anisotropy produced by present-day flow of the upper mantle, Kim said ^[11]. The two local anomalies rest on different kinds of evidence. The deflection near the sunken lithosphere fragment is a pattern in the fast directions, and it agrees with a structure that tomography had already hinted at ^[12]. The possible hotspot upwelling near the Magellan Seamount Trail is inferred from the absence of a clear signal ^[13]. Kim attached a caveat to both. "Neither interpretation is unique, but both offer clues to how local mantle structure may influence deformation," she said ^[14].

The evidence supports revising plate-motion-only readings of this seafloor ^[5]. It does not yet show that old oceanic mantle in general carries the same mixed record, because the data come from a single array, Oldest-1 ^[8]. The phys.org account does not give the number of stations or splitting measurements, so the coverage behind each anomaly cannot be judged from it. The thing this doesn't tell you is whether the Pacific Triangle is typical of old seafloor, or unusual because three plates met there ^[7].

The next data come from closer to the trench. The team is analysing the Oldest-2 array, deployed west of Oldest-1 and nearer the Mariana Trench, to see whether the trends continue or change ^[15]. "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 said ^[16].