Flexure of the ocean lithosphere from island uplift, bathymetry and geoid height observations: the Society Islands

Summary In flexure models of the response of the ocean lithosphere to sea‐mount loading three key unknowns must generally be considered; the density distribution of the load, the degree of fill‐in of the deflection‐created moat and the flexural rigidity. The models are usually constrained by one of three types of observations; gravity or geoid height anomalies, the moat and arch geometry and the amount of uplift of islands on this arch. The determination of the model parameters from geoid anomalies is critically dependent on the interpretation of the broad and regional geoid rise that is seen over many of the younger seamount chains and likewise the flexure parameters deduced from the moat and arch geometry are a function of the interpretation of the associated bathymetric swell. The uplift data require that the tectonic, erosional and sea‐level history of the region be known, although this requirement can be relaxed if, rather than the uplift relative to sea‐level, the change in uplift with distance from the load is used as the observed quantity. All these types of observations have been used in this paper to constrain the flexure model for the Society Islands and while any one data type on its own does not lead to reliable flexure parameters, a consistent solution is found when the three are taken together. The results point to a relatively low load density of 2.5 g cm‐3 (apart from a high density core) a moat that is partly filled in with sediment or volcanic debris, and a flexural rigidity of about 3 times 1029 dyne cm.