We estimate the elastic thickness Te for the Pacific plate at the time of volcanism for approximately 60 guyots of Cretaceous age in the Japanese, Wake, and Mid-Pacific Mountain seamount groups of the northwest Pacific. The values of Te are constrained by comparing synthetic gravity and deflection of the vertical computed from digital bathymetry assuming regional compensation models to potential field data derived from Seasat radar altimetry. The amplitude of the gridded Seasat gravity field over the seamounts is consistent with Te values between 5 and 15 km, but these values represent only lower bounds since the 100-km spacing of satellite tracks may undersample the gravity for seamounts lying between adjacent passes. Direct modeling of the deflection of the vertical along individual satellite tracks avoids this distribution bias but can lead to errors if the bathymetric feature is mislocated with respect to the satellite coordinate system. Nevertheless, for no seamount does the root-mean-square (RMS) difference between observed and predicted deflection of the vertical show a distinct minimum for elastic plate thickness greater than 15 km.

However, for some features the RMS minimum is unconstrained, allowing Te greater than 15 km. Given the possible bias in the modeling of satellite data, the low values for elastic plate thickness were confirmed for the Japanese and Wake group using shipborne gravity data and multibeam bathymetry collected during the Roundabout leg 10 expedition, supplemented with published Navy sonar array sounding system (SASS) bathymetry. The analysis of shipborne data constrains most elastic plate thickness values to between 10 and 15 km for crustal densities between 2600 and 2800 kg/m3. The low values for elastic plate thickness for these Cretaceous guyots that formed in the area of the ''Darwin Rise'' suggest either that they formed on lithosphere less than 40 m.y. old or that older lightosphere was reheated near the time of volcanism. While the first possiblity is permitted by age data from the Mid-Pacific Mountains, radiometric dates and magnetic lineations from the Japanese and Wakes are more consistent with formation of these seamount groups on older, thermally anomalous lithosphere, such as that which is thought to exist presently beneath the volcanoes of French Polynesia. ¿American Geophysical Union 1990