Subduction of the Bougainville seamount (Vanuatu): mechanical and geodynamic implications

Abstract

New bathymetric data gathered during a Seabeam survey (SEAPSO cruise, leg 1) enable us to re-examine the flexural response of the oceanic lithosphere subducting under the New Hebrides (Vanuatu) island arc. The Bougainville seamount and Sabine bank are interpreted as immerged fossil atolls, the recent subsidence of which is related to the subduction of the oceanic lithosphere. The position and altitude of the different fossil atolls which belong to the d'Entrecasteaux or Loyalty ridges are in good agreement with predictions of elastic flexure of the lithosphere. We deduce an average effective elastic thickness of the lithosphere of about 22 km for the area under study. This value is slightly smaller than the one corresponding to the lithospheric age as given by the magnetic anomalies, but is in good agreement with the age after a correction for thermal rejuvenation. This assumption of a thermal rejuvenation of the North Loyalty basin is also supported by previously reported high heat-flow values and attenuation of S n seismic waves. However, the location and depth of the trench in front of the North Loyalty basin do not agree with the model which fits the other data. This discrepancy is interpreted as the result of variations of the value of P b (vertical force per unit length of trench applied at the edge of the plate) along the subduction zone. Such variations may be related to the length of the subducted slab, which is shorter in front of the d'Entrecasteaux and Loyalty ridges than in front of the North Loyalty basin, according to the hypocentral distribution of earthquakes and a tomographic image of the slab.