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

Abstract

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 compressional‐ and 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/Myr½, 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 high‐ and low‐velocity anomalies along the ridges that correlate with deep and shallow bathymetry, respectively.