Heterogeneous Mantle Flow and Deviations from Half-Space Cooling Observed Beneath the Central Pacific

Age dependence of oceanic upper mantle structure provides insights into the thermal evolution of the lithosphere-asthenosphere system and mantle convection processes. The half-space cooling (HSC) model is commonly invoked to describe thermal evolution of oceanic lithosphere at ages <75 Ma. Here, we utilize new high-resolution profiles of shear velocity (VSV) and anisotropy at the Young ORCA experiment (~43 Ma) along with previous observations at the NoMelt experiment (~70 Ma), to evaluate performance of the HSC model. We measure Rayleigh- (5150 s) and Love-wave (57 s) phase velocities at Young ORCA using ambient noise and teleseismic earthquakes and invert for VSV and azimuthal anisotropy to 300 km depth. In the upper lithospheric mantle, we observe relatively weak azimuthal anisotropy (33.5%) with fast axis parallel to fossil spreading, indicating horizontally aligned olivine associated with corner flow at the mid-ocean ridge. Below ~35 km depth, azimuthal anisotropy rotates to and surpasses absolute plate motion (APM) by ~30 below ~150 km depth. Although APM is similar at the Young ORCA and NoMelt sites separated by only ~2100 km, both exhibit asthenospheric anisotropy that is rotated from APM as well as rotated from one another by ~35. We attribute this to heterogeneity in mantle flow at a length scale 2100 km that is controlled by pressure- or density-driven flow in the asthenosphere, rather than shear at the base of the plate. VS converted from VSV at Young ORCA reaches a minimum of ~4.2 km/s at 110 km depth, while the minimum at NoMelt is ~4.3 km/s at 120 km. Using the tool Perple_X and including anelastic effects, we estimate VS for a realistic mantle composition and HSC temperature profile at each location. We find that HSC can explain mantle VS at NoMelt for a mantle potential temperature of 1380C, but predictions overestimate velocities at Young ORCA from 70200 km depth. Instead, slow velocities at Young ORCA are consistent with an elevated mantle potential temperature of ~1480C and/or 0.61% melt from 70200 km. Proximity of Young ORCA to the Marquesas hotspot ~750 km away may contribute to increased mantle temperatures or presence of melt. Our observations of heterogeneous mantle flow as well as deviations from the HSC model are consistent with the presence of small-scale convection beneath the central Pacific.