Gravity anomalies and crustal section across the Tonga Trench

In 1956, gravity observations with the Vening Meinesz pendulum apparatus were made aboard H.M.S. Telemachus in the southwest Pacific Ocean. For each of several gravity profiles, large free-air anomalies were associated with the Tonga and Kermadec trenches. The seismic structure determined by Raitt, Fisher, and Mason [1955] for the Tonga trench was projected to one of the gravity profiles. The gravity anomaly profile computed on the basis of the seismic structure is in fair agreement with the observed profile, but the two profiles differ by about 100 mgal in the vicinity of the trench. Interposing a seismically ‘masked’ layer under the trench and increasing the crustal thickness there to 23 km improves the fit. A crustal thickness of 36 km is derived from gravity and seismic data for the Tonga-Kermadec ridge. The disagreement between this value and one obtained by Officer [1955] from surface wave dispersion data is discussed.