Geoid anomalies across Pacific fracture zones

Abstract

SUMMARY Cooling of the oceanic lithosphere and related subsidence of the ocean floor are usually understood in terms of simple conductive thermal models: half-space and plate-cooling models, which predict the same behaviour for heat flow, sea-floor depth and geoid on very young sea floor but large deviations at greater ages. Since geoid data are much more sensitive to the thermal structure of the lithosphere at depth than other kinds of data, satellite-derived geoid anomalies, in particular geoid steps observed across fracture zones, are currently analysed to discriminate between the two thermal models. In this study we present results for the geoid step, Δh, estimated along altimeter profiles of the Seasat satellite across five fracture zones (FZs) of the NE Pacific: Clipperton, Clarion, Molokai, Murray and Mendocino, and across the Udintsev FZ in the S Pacific. For each FZ, the quantity geoid offset, Δh, divided by the age offset, Δt, is plotted as a function of plate age. Except for Mendocino and Clipperton FZs along which the variation of Δh/Δt with age is consistent with the prediction of the thermal plate model with a plate thickness of 90-100 km, the dependence on age of Δh/Δt along the other FZs is different from that predicted by conductive cooling models. Large fluctuations of the Δh/Δt-age relationship are observed along Murray, Clarion and Udintsev FZs, suggesting that the thermal structure of the oceanic lithosphere under a FZ is more complex than is to be expected from conductive cooling alone. According to recent convection models, small-scale convective instabilities developed underneath the FZ in a shallow low-viscosity layer could explain the observed Δh/Δt-age behaviour.