No thinning of the lithosphere beneath northern part of the Cook-Austral volcanic chains

Abstract

The Cook-Austral volcanic chains are situated in the South Pacific. They are composed of several volcanic chains and isolated volcanoes corresponding to distinct events from 55 Ma to present time. The tectonic history of the volcanoes emplacement is quite complex. It involves the activity of at least three hot spots, and sometimes several volcanic stages coexist on the same volcano. The purpose of this study is to bring new information on the loading history of the northern part of the Austral chain by studying the variation of the elastic thickness of the lithosphere. We have used the high-quality data acquired during the ZEPOLYF2 cruise which conducted in July 1999, a multibeam survey of this area, and we got several new ages on seamounts. In this region where several volcanic stages overlap, the simple elastic model applied on the present-day bathymetry is not appropriate. We then reconstructed the loading history of each volcano or group of volcanoes using information provided by the radiometric ages and chemical signature of the volcanic stages or the volcanoes morphology. We show that the lithosphere is characterized by normal elastic thickness if one correctly isolates the different volcanic stages occurring on this area. The abnormally low values of the effective elastic thickness obtained previously over this area may be explained either by not taking into account the overlapping volcanism or by poor quality of the geophysical data used for the determination. Copyright 2008 by the American Geophysical Union.