Constraints on asthenospheric flow from the depths of oceanic spreading centers: The East Pacific Rise and the Australian‐Antarctic Discordance

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The fastest spreading center, the East Pacific Rise (EPR), is consistently deeper than most other spreading centers. Its average depth along the ∼5000 km length is ∼3100 m, while the mean depth along the adjacent, slower spreading Pacific‐Antarctic Rise (PAR) is ∼2700 m. The deepest spreading center is the ∼4000 m deep, ∼1000 km long Australian Antarctic Discordance (AAD). Analytic and numerical models show that dynamic thinning of asthenosphere can explain the magnitude and wavelength of the depth anomalies along the EPR and AAD. Previous models did not show such significant depth anomalies along spreading centers because the equations used to describe flow of thin viscous asthenosphere were linearized. At the EPR, fast plate divergence thins the asthenosphere by both sequestering it into diverging lithosphere and dragging it with the plates in contrast to the slower spreading but faster migrating PAR. The AAD asthenosphere is starved because of the restriction of asthenospheric flow due to nearby thick continental lithospheric roots combined with a moderately fast spreading rate. A narrow range of asthenospheric properties explains observed depth anomalies for both the AAD and EPR. If the asthenosphere is ∼100°C hotter, and so ∼10 kg/m3 less dense than the underlying asthenosphere, then the model requires an average asthenospheric thickness of ∼250 km and a viscosity of ∼1019 Pa s. Thinner asthenosphere works in the model if it has a lower density and lower viscosity. Although the source of the asthenosphere is not critical to our models, we assume that it is supplied by upwelling of depleted mantle plumes in contrast to enriched plumes that feed oceanic island basalts.