Geoid anomalies across Pacific fracture zones

Abstract

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, Ah, 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, Ah, divided by the age offset, At, is plotted as a function of plate age. Except for Mendocino and Clipperton FZs along which the variation of AhlAt with age is consistent with the prediction of the thermal plate model with a plate thickness of 90-100km, the dependence on age of AhlAt along the other FZs is different from that predicted by conductive cooling models. Large fluctuations of the AhlAt-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 Ahl At-age behaviour.

Mechanical erosion and reheating of the lithosphere: A numerical model for hotspot swells

“…The geoid anomaly is given by [e.g., Parsons and Daly, 1983] with a power law viscosity proportional to the third power of the stress result in similar convective patterns than experiments with a NewtonJan viscosity where temperature and pressure dependence is reduced by a factor of 2 to 4 [Christensen, 1984]. As far as geoid and topography data are concerned, several observations support the existence of a sublithospheric low-viscosity zone (LVZ) where the viscosity drops by a factor 10 to 100 [Cathies, 1975;Craig and McKenzie, 1986;Robinson et al, 1988;Marty et al, 1988]. In the present study we have not considered all these parameters separately.…”

“…LVZ) where the viscosity drops by a factor 10 to 100[Cathies, 1975;Craig and McKenzie, 1986;Robinson et al, 1988;Marty et al, 1988]. In the present study we have not considered all these parameters separately.…”

Stress in the Lithosphere and the Strength of Active Faults

“…Uplift of the transform side of the rift might also result, at least in part, from twisting moments induced by an increase in shear stress with depth along the transform [ Chen , 1989]. (Other references discussing stresses, displacements, geoid anomalies or bathymetry along transform faults and fracture zones include Marty et al ., 1988; Robinson et al ., 1988; Argus et al ., 1989; Wiens and Petroy , 1989; Wesssel , 1990; and Mueller and Phillips , 1991. )…”

Geoid anomalies and the structure of continental and oceanic lithospheres

“…The observed flattening of the topography as a function of age seems to be the consequence of heat brought at the base of the lithosphere by small-scale convection [Parsons and Sclater, 1977], rather than a "dynamical" effect of deeper mass anomalies in the mantle [Colin and Fleitout, 1990]. Geoid jumps across transform faults [Sandwell and Schubert, 1982;Gibert et al, 1987;Marty et al, 1988;Freedman and Parsons, 1990] also indicate that some convective heat transfer at the base of the lithosphere occurs, but dispersion is large. To try to resolve some of these issues, we carried out a study of geoid as a function of age in the intermediate-wavelength range corresponding to harmonics 5-30 to 13-30.…”