UC Santa Barbara

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 VSV $\left({V}_{SV}\right)$ and its azimuthal variation (G) $(G)$ to 300‐km depth, and radial anisotropy ξ=VSH/VSV2 $\xi ={\left({V}_{SH}/{V}_{SV}\right)}^{2}$ azimuthal VSH $\left({V}_{SH}\right)$ parameter E $E$ to ∼50 km depth. Both sites show a lithospheric lid over a low‐velocity zone, with lid thickness correlating with plate age, and positive ξ $\xi $ through the crust and upper lithosphere. Young ORCA has moderate lithospheric azimuthal anisotropy (|G| $\vert G\vert $ ∼ 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° ${}^{\circ}$ 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 (|G| $\vert G\vert $ ∼ 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. We used seafloor seismometers to “listen” to naturally sourced seismic waves traveling through the Pacific mantle beneath two areas of different ages: ∼43 million years (Young ORCA) and ∼90 million years (Old ORCA). Subtle changes in wave speed with direction reveal how mantle minerals are lined up by past and present mantle flow. Within the near‐surface Pacific plate (“lithosphere”), the younger site records a strong fabric aligned with the direction plates spread apart when that seafloor formed. The older site shows weaker, rotated fabric, pointing to more complicated spreading or later modification. Deeper beneath the plate (“asthenosphere”), both places show strong alignment that mostly follows today's motion of the Pacific plate over the mantle, consistent with the plate (“lithosphere”) shearing the layer (“asthenosphere”) beneath it. We also identify the transition between these layers at ∼50 km depth under the younger site and ∼70–75 km under the older site. When we compare our local results with global maps, the global maps tend to underestimate the strength and variability of this fabric. Direct ocean‐bottom observations are therefore crucial for revealing small‐scale mantle processes that larger‐scale studies smooth out. New regional models of seismic anisotropy reveal lateral and depth‐dependent variations in Pacific mantle anisotropy that are not well‐captured in global models Strong asthenospheric anisotropy reflects plate‐driven shear in a sub‐lithospheric channel and pressure‐ and/or buoyancy‐driven deformation at greater depth Lithospheric anisotropy is very heterogeneous in strength and direction with respect to spreading, indicating competing sources of ridge‐related deformation New regional models of seismic anisotropy reveal lateral and depth‐dependent variations in Pacific mantle anisotropy that are not well‐captured in global models Strong asthenospheric anisotropy reflects plate‐driven shear in a sub‐lithospheric channel and pressure‐ and/or buoyancy‐driven deformation at greater depth Lithospheric anisotropy is very heterogeneous in strength and direction with respect to spreading, indicating competing sources of ridge‐related deformation