Flexural Modeling With Dense Seamount Cores: Effect on the Estimation of Elastic Thickness

The conventional approach to flexural modeling beneath seamounts is to approximate the loads using a homogeneous density structure. However, the possibility of misrepresentation of flexural parameters still exists because the modeling ignores dense materials inside seamount cores as recognized by several detailed seismic surveys. In this study, we investigate the effects of inhomogeneous seamount density structures on the estimation of elastic thicknesses. As a starting point, we find the analytic solutions for disc and parabolic loads having dense cores and examine the model predictions for various tectonic settings. The inclusion of a dense core (2900 kg m{}-3) enables us to choose a more realistic value (2500 kg m{}-3) for the outer parts of the load rather than using the higher crustal density. For the given synthetic loads, the amplitudes of the deflection predicted by the uniform density (or simple flexure) model tend to be much larger than those predicted by the dense core model, especially directly beneath the loads. The main reason is that the loading force is decreased due to the difference in the density models. Furthermore, the contribution to the loading force from the materials infilling the deflected crust is reduced when the chosen infill density is less than the crustal density. The maximum departure of the deflection modeled by the dense core model from that by the uniform density model can be achieved when the densities of the core, outer, and infill loads reflect the heterogeneity. The overestimation of the crustal deflection from the uniform density model may approach 35% for geologically reasonable settings (e.g., a large seamount on an old, mechanically strong plate) even if both synthetic load models have the same total mass. However, we generally observe free-air gravity anomalies over the seamounts, which are sensitive to both the density structure of the loads and the flexural deformation underneath; the flexed crust is thus only observed indirectly. The best flexural parameter can be obtained by minimizing the misfits between observed and modeled gravity anomalies. Therefore, the uniform density model inherently has to reduce the elastic thickness, yielding larger deflections and thus a larger negative gravity anomaly, in order to compensate for the extra positive contributions from the load. Here, we explore ways to approximate the dense core model in the Fourier domain, evaluate its effects on gravity modeling, and search for unbiased measures of the thermo-mechanical properties of the oceanic lithosphere.