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

Abstract Understanding mantle flow beneath old oceanic plates is essential for linking present-day plate motions with long-term lithospheric evolution, yet direct constraints remain sparse due to limited offshore observations. Here, we investigate upper-mantle seismic anisotropy beneath the Pacific Triangle, one of the oldest preserved fragments of oceanic lithosphere, using shear-wave splitting measurements from the Oldest-1 broadband ocean-bottom seismometer array. We obtained 128 high-quality measurements using teleseismic XKS and direct S phases as well as ScS phases to characterize upper-mantle anisotropy. Fast polarization directions (FPDs) show pronounced regional variability, with predominantly northwest–southeast to north-northwest–south-southeast orientations in the northwest and east-northeast–west-southwest orientations in the southeast, with a mean delay time of ∼1.5 s, higher than the global average of 1.0 s. These FPD variations cannot be explained solely by absolute plate motion (APM) or paleospreading directions, and instead show systematic spatial correspondence with P-wave velocity anomalies imaged at depths of ∼95 to 305 km. In the northern and western parts of the study region, FPDs exhibit more northward orientations relative to APM predictions. In the central-to-eastern region, FPDs exhibit systematic azimuthal variations around a high-velocity lithospheric domain, suggesting coupling between mantle deformation and a mechanically strong lithosphere. In addition, null measurements are observed near the Magellan Seamount Trail and the paleotriple junction, spatially coincident with low P-wave velocity anomalies. Together, these observations document laterally variable and depth-dependent anisotropy beneath the Pacific Triangle and provide new offshore constraints on upper-mantle deformation beneath the oldest Pacific seafloor.