Lithospheric cooling trends and deviations in oceanic PP-P and SS-S differential traveltimes

[1] The thermal and compositional structure of oceanic lithosphere, which exerts an important control on plate behavior, is still debated. Our set of 60,000 PP-P and SS-S traveltime differences with oceanic PP and SS bounce points provides a good constraint on both compressionaland shear-wave velocity. By calculating traveltimes for thermal models that are converted to seismic structures with a thermodynamic approach, we test whether lithospheric cooling can explain PP-P and SS-S traveltime variations with plate age. The PP-P and SS-S traveltimes have substantial scatter but, on average, decrease by 0.2 and 0.7 s/Myr, respectively, when the PP and SS waves reflect off progressively older oceanic crust. Both a half-space and a plate cooling model with a mid-ocean ridge basalt-source mantle potential temperature (1315 50 C) explain the average values of the PP-P and SS-S anomalies and their decrease with plate age. Residual PP-P and SS-S anomalies relative to a cooling model reveal large-scale patterns. Along a few paths (e.g., Tonga–Fiji to western North America), seismic heterogeneity in the deep mantle is responsible for a significant fraction of the PP-P and SS-S traveltime variation. Most anomalies probably correspond to broad temperature variations in the upper mantle, such as a very slow central–northern Pacific (which may require a 100 C excess temperature) and highand low-velocity anomalies along the ridges that correlate with deep and shallow bathymetry, respectively.