Size and depth of ancient magma reservoirs under atolls and islands of French Polynesia using gravity data

Abstract

New insights into the structural and tectonic evolution of islands and atolls in French Polynesia are derived from the analysis of gravity data. Free‐air anomaly maps were constructed using gravity data from land surveys of 25 islands combined with free‐air anomaly data derived from satellite altimetry and shipborne gravimeters. Residual isostatic anomalies were calculated using a three‐dimensional (3‐D), four‐layer crustal model taking into account the bathymetry of the seafloor, the topography of the islands, and the deflection of the lithosphere under the load of the volcanoes. Twenty of the 25 islands yield a positive residual anomaly, ranging between 9 and 60 mGal. Negative residual anomalies over four islands probably correspond either to limestone deposits or to fragmented material from aerial volcanism. The occurrence of a positive anomaly provides some evidence that there is a solidified magma chamber at depth beneath each of several islands in French Polynesia. A linear relation between the amplitude of the positive gravity anomaly and island volume is also observed. Geological features which generate these positive anomalies are described by simple geometric models (3‐D ellipsoidal dense bodies) which lead to a rough estimate of the size and depth of the magma chambers. We then propose a simple linear relation between the volume of such magma chambers and the volume of islands that can be used for other extinct intraplate basaltic volcanoes. The diameter of the inferred magma chamber and its projection on the surface of the islands imply that calderas correspond mostly to collapses of the flanks of the edifices rather than to caldera vertical subsidence.