Multi‐Scale Spatial Variations in Pacific Mantle Seismic Anisotropy: Constraints on Plate Evolution and Asthenospheric Flow

Abstract

We present array‐scale (∼500 × 500 km) anisotropic shear‐velocity models from two Pacific ocean‐bottom seismometer (OBS) arrays of the OBS Research in the Convecting Asthenosphere (ORCA) experiment: Young ORCA (∼43 Ma) and Old ORCA (∼90 Ma). Rayleigh‐wave phase velocities spanning 5–150 s are combined with Love‐wave phase velocities from 5 to 10 s to constrain and its azimuthal variation to 300‐km depth, and radial anisotropy azimuthal parameter to ∼50 km depth. Both sites show a lithospheric lid over a low‐velocity zone, with lid thickness correlating with plate age, and positive through the crust and upper lithosphere. Young ORCA has moderate lithospheric azimuthal anisotropy ( ∼ 3%) consistent with spreading‐parallel corner flow to ∼30–50 km, while Old ORCA displays weak (∼1%–2%), laterally variable lithospheric anisotropy that is rotated ∼30 from fossil spreading, suggesting perturbed corner‐flow processes at the ridge. A sharp fabric change at ∼70‐km depth in Old ORCA marks the lithosphere‐asthenosphere boundary (LAB), consistent with dehydration‐controlled plate thickness. Beneath the lithosphere, both regions exhibit strong asthenospheric anisotropy ( ∼ 3%) with fast axes subparallel to absolute plate motion in a narrow channel between ∼70 and 200 km depth, suggesting plate‐induced shear within a weak shallow asthenosphere. At greater depths, anisotropy amplitudes decrease and azimuths rotate (N–S beneath Young; NW–SE beneath Old), consistent with superposed pressure or buoyancy‐driven flow in the deeper asthenosphere. Global models of azimuthal anisotropy under‐predict strength, depth variability, and rotation of fabric, underscoring the utility of dense OBS arrays for resolving smaller‐scale deformation processes within the oceanic lithosphere‐asthenosphere system.