The support mechanism of the young Foundation Seamounts inferred from bathymetry and gravity

Abstract

SUMMARY The youngest volcanoes of the Foundation volcanic chain were built on a plate less than 5 Myr near the Pacific‐Antarctic ridge (PAR) axis. The progressive approach of the spreading ridge to the hotspot during the last 10 Myr resulted in the building of the edifices upon a progressively younger plate. Estimates of the effective elastic thickness, Te, using two independent methods show that the elastic thickness of the plate diminishes towards the spreading ridge, from 5‐3 km to 2‐0 km. The spatial variation of Te correlates with a significant change in both the morphology and the volume of the volcanoes, suggesting an important control of the elastic thickness on the volcanic processes. The reduction in Te also explains conveniently the reduction in the distance between the North and the South Systems, the double volcanic line that characterizes the young part of the Foundation chain, in the direction of young loading ages. The South System formed at the top of a flexural bulge created by the emplacement of the North System, the main volcanic line. The low Te estimate for the South System and the shape of the Bouguer anomalies confirm this model. The effective elastic thicknesses obtained in this study are well matched by isotherms between 200 ◦ C and 300 ◦ C and support the hypothesis of a weak plate. Three mechanisms possibly acted to lower the plate rigidity: Re-heating of a ‘normal’ plate by the hotspot (North System); local fracturing (South System) and the creation of a thin lithosphere at the PAR axis by the coupling of a hotspot and the ridge-related melting zones (near-ridge volcanoes).