Ocean island densities and models of lithospheric flexure

Estimates of the effective elastic thickness (T-e) of the oceanic lithosphe re based on gravity and bathymetric data from island loads are commonly sig nificantly lower than those based on the wavelength of plate bending at sub duction zones. The anomalously low values for ocean islands have been attri buted to the finite yield strength of the lithosphere, to erosion of the me chanical boundary layer by mantle plumes, to pre-existing thermal stresses and to overprinting of old volcanic loads by younger ones, A fifth possible contribution to the discrepancy is an incorrect assumption about the densi ty of volcanic loads. We suggest that load densities have been systematical ly overestimated in studies of lithospheric flexure, potentially resulting in systematic underestimation of effective elastic thickness and overestima tion of the effects of hotspot volcanism. We illustrate the effect of under estimating load density with synthetic examples and an example from the Mar quesas Islands. This effect, combined with the other effects listed above, in many cases may obviate the need to invoke hotspot reheating to explain l ow apparent elastic thickness.