Cooling of the oceanic lithosphere—evidence from geoid anomalies across the Udintsev and Eltanin fracture zones

Abstract The often step-like variation in the marine geoid across fracture zones is believed to reflect the change in age of the ocean floor, and hence in temperature structure of the plate across the fracture zone. The rate of change of the geoid with age has been estimated as a function of age from geoid offsets across fracture zones and used to constrain thermal models of lithospheric cooling. Results from three studies of the geoid offsets across the Mendocino fracture zone are consistent with a thermal plate cooling model having a plate thickness between 100 and 125 km. This is in contrast to results obtained from the combined measurements from fracture zones in the South Pacific, which are not consistent with either the plate model or boundary layer model of cooling. To examine this inconsistency we have measured geoid offsets on Seasat profiles across the Udintsev and Eltanin fracture zones in the South Pacific and tested the robustness of the measurement technique. We used a second-order polynomial that included a step function centered on the fracture zone to estimate the regional geoid and the geoid offset. The polynomials were fit to the geoid profiles using least-squares minimization criteria. Data within a certain distance of the fracture zone were excluded from the fitting procedure in order to minimize the effects of horizontal cooling and lithospheric deformation near the fracture zone. We find that the estimate of the geoid offset is very sensitive to the length of data excluded and that generally, an exclusion width of 100–200 km is needed to avoid anomalies (unrelated to the regional geoid offset) near the fracture zone. Exclusion widths of less than 100 km generally lead to unpredictable errors in the geoid offset estimates. Using exclusion widths of between 150 and 200 km on most profiles, we constructed geoid slope (geoid offset divided by the age offset) versus average age plots for each of the limbs of the two fracture zones. Similar trends were found on both the east and west limbs of the Eltanin and the east limb of the Udintsev. The trends were inconsistent with a single thermal plate thickness and instead showed an initial decrease in slope between 0 and 35 m.y., followed by an abrupt increase followed again by a decrease in slope. In contrast, the west branch of the Udintsev shows an increase in geoid slope values between 20 and 35 m.y., followed by a decrease. The difference in observed trends across the western limb of the Udintsev may be due to the effects of the Louisville Ridge (north of the fracture zone) on the regional geoid. The similarity in trends on the remaining three limbs argues against the effects of isolated thermal or bathymetric anomalies and appears instead, to reflect a general feature of the geoid-slope versus average age relationship across fracture zones. Although the thermal plate cooling model is successful in predicting both seafloor depths and heat flow values out to ages of at least 80 m.y. B.P., it cannot explain the observed geoid slope values for these two fracture zones. It is not clear at this point whether this is due to inadequacies in the cooling model or to peculiarities in fracture zone evolution, possibly caused by small-scale convection driven by the horizontal temperature gradients beneath fracture zones.