An analysis of isostasy in the world's oceans 1. Hawaiian‐Emperor Seamount Chain

Cross‐spectral techniques have been used to analyze the relationship between gravity and bathymetry on 14 profiles of the Hawaiian‐Emperor seamount chain. The resulting filter or transfer function has been used to evaluate the state of isostasy along the chain. The transfer function can be best explained by a simple model in which the oceanic lithosphere is treated as a thin elastic plate overlying a weak fluid. The best‐fitting estimate of the elastic thickness of the plate is in the range 20–30 km. Analysis of individual profiles shows significant differences in the elastic thickness along the seamount chain. Relatively low estimates of the elastic thickness were obtained for the Emperor Seamounts north of 40°N, and relatively high estimates for the Emperor Seamounts south of 40°N and the Hawaiian Ridge. These differences cannot be explained by a simple model in which there is a viscous reaction to the seamount loads through time. The best explanation is a simple model in which the elastic thickness depends on age and hence temperature gradient of the lithosphere. The low values can be explained if the Emperor Seamounts north of 40°N loaded a relatively young hot plate, and the high values can be explained if the Emperor Seamounts south of 40°N and the Hawaiian Ridge loaded a relatively old cold plate. These estimates of the elastic thickness along with determinations from other loads on the Pacific lithosphere suggest that the elastic thickness corresponds closely to the 450 ± 150°C isotherm, based on simple cooling models. Thus the large deformations and associated flexural stresses (>1 kbar) at seamount loads do not appear to change appreciably through time. This conclusion is in agreement with subsidence data along the seamount chain and with some gravity observations in the continents.