SPATIAL VARIATION IN ANISOTROPIC SHEAR VELOCITY OF OLD OCEANIC LITHOSPHERE-ASTHENOSPHERE IN THE SOUTHWEST PACIFIC

Regional anisotropic structure of the oceanic lithosphere-asthenosphere system provides important constraints on the formation, evolution, and dynamics of oceanic plates. Data from the Old ORCA (OBS Research into Convecting Asthenosphere) array offer insight into this system in the southwest Pacific. From ambient noise, we observe high signal-to-noise fundamental-mode Love waves in the 5-7s period range, fundamental-mode Rayleigh waves in the 15-25s period band, and first-overtone Rayleigh waves in the 5-10s band. Teleseismic fundamental-mode Rayleigh waves are observed from 20-150s period after tilt and compliance correction. The short-period ambient-noise and long-period teleseismic surface wave observations are inverted for radial and azimuthal anisotropy to depths of 35 km and 300 km, respectively. Inverted velocities indicate a ~1-1.6% difference from the northwest to the southeast across the array, a pattern that aligns with a variation in the gravity anomaly of ~10 mGal. 2𝜃 Azimuthal anisotropy in V_sv is relatively weak and varies spatially across the array in the upper lithosphere to ~70 km depth with 1% amplitude in the southeast and 2% in the northwest. The fast direction is generally oriented North-South which is ~30° clockwise from the fossil-spreading direction (FSD; 330°) suggested by seafloor abyssal-hills. At intermediate depths (70-170 km) azimuthal anisotropy strength increases to 2.2-2.5% with fast direction aligned with absolute plate motion (APM; 300°). Unexpectedly, the fast direction rotates back to FSD azimuth at ~250 km depth. Radial anisotropy consistent with V_sh > V_sv of 1-3% in the shallow mantle and 4-5% in the lower crust. Apparent anisotropic fabric is weaker than observed at intermediate (~70 Ma) and young (~43 Ma) oceanic lithosphere while the velocity variations are greater than observed at these other Pacific arrays. Transition to APM anisotropy, decrease in azimuthal anisotropy strength, and negative velocity gradient suggest a lithosphere-asthenosphere boundary (LAB) at ~70 km depth. We contextualize these results within the framework of crustal thickness variations, effective interaction and transfer of forces at the LAB, and variability with temperature and flow within the asthenosphere and at the ridge during lithosphere formation.