^aCentre National d'Etudes Spatiales, Groupe de Recherche de Géodésie Spatiale, 18, Avenue Edouard Belin, 31055 Toulouse Cedex France

Cooling of the lithosphere causes a short-wavelength step in the geoid across fracture zones that can also be used to constrain thermal models. We have analyzed geoid data at fracture zones from the SEASAT altimeter measurements in the entire Pacific Ocean and redetermined parameters of the cooling models. We find that the data reveal two distinct regimes of cooling; one for seafloor ages in the range 0–30 Ma, the other beyond 30 Ma; this does not appear to be correlated with particular fracture zones but rather it is representative of the whole area studied, i.e., the entire south Pacific and northeast Pacific Ocean. These two trends may be interpreted in terms of two different (asymptotic) thermal thicknesses of the plate model. The smaller thermal thickness ( 65 km) found for ages <30 Ma—compared to 90 km in the age range 30–50 Ma—calls for some kind of thermal perturbation in the vicinity of the ridge crest.

From the results obtained in this study, we conclude that the half-space cooling model is unable to explain the data, that beyond 30 Ma, a simple plate model gives a satisfactory fit to the data but in the younger plate portion (ages < 30 Ma) the cooling history of the oceanic lithosphere is much more complex than predicted by the usual cooling models. Furthermore, the depth-age relationship obtained from the geoid-derived thermal parameters departs significantly beyond 30 Ma from the widely used Parsons and Sclater's depth-age curve, predicting a lesser subsidence.