Constraints on the Mantle Rheology From Observations of Flexural Rigidity of Lithosphere at Seamounts

The deformation of oceanic lithosphere in response to surface loads such as seamounts can provide important constraints on the mantle rheology. Such deformation is often characterized in terms of flexural rigidity or elastic thickness T_e. For lithosphere with given elastic parameters and a load with a given age, T_e near the load is controlled by lithospheric viscosity structure which determines the stress relaxation and thus stress distribution in the lithosphere where the load is supported. Recognizing that oceanic lithospheric viscosity depends on mantle rheology and lithospheric thermal structure which is well constrained for a given seafloor age, Watts and Zhong [2000] used a 2-D Cartesian multi-layer visco-elastic loading model to investigate the controls of rheological parameters (i.e., activation energy E and asthenospheric viscosity η ₀) on T_e. They fit the observed T_e with a single pair of values for E and η ₀. In a continuation of their work, a similar model, but using a more realistic cylindrical geometry has been developed. Model results using a range of E and η ₀ for a Newtonian rheology were compared to T_e from seamount data. We found that small η ₀ needs to pair with large E to fit the T_e for seamounts younger than 10 Ma. For example, the data can be modeled equally well with pairs (90 kJ/mol, 1e20 Pa s), (120 kJ/mol, 1e19 Pa s), or (240 kJ/mol, 1e16 Pa s). For E=360 kJ/mol, η ₀ is required to be significantly less than 1e16 Pa s. Consideration of the yield strength envelope leads to moderate increase of η ₀ estimates for a given E. We will also discuss the effects of non-Newtonian (i.e., power-law) rheology on our results.