GRAVITY-ANOMALIES AND FLEXURE OF THE LITHOSPHERE AT ASCENSION ISLAND

Ascension Island, in the northern South Atlantic, forms the summit of a volcanic edifice 60 km in diameter which places a substantial load o n the underlying young oceanic lithosphere. An analysis of a combined data set of recent and historical surface-gravity and bathymetry measu rements on and around the island suggests that the lithosphere respond s to this load by flexure equivalent to that of an elastic plate only approximate to 3 km thick, and that the mean density of the volcanic e difice is approximate to 2500 kg m(-3). A steep gravity gradient acros s the island cannot be explained by a simple flexural model and must b e attributed to lateral density variations within the volcano itself. The effective elastic thickness is considerably less than the expected approximate to 12 km mechanical thickness of the approximate to 6 Ma lithosphere loaded by the volcano, and less even than zero-age elastic thicknesses commonly observed at slow-spreading ridges with axial rif t valleys. The unusually small elastic thickness may be attributed to the combined effects of the high curvature beneath the island, which p roduces bending stresses that are limited by the yield stress envelope , localized heating of the lithosphere during emplacement of the islan d, and crustal thickening. When these factors are taken into account, the observed flexure is consistent with rheological models based on ex perimental rock mechanics.