Morphological and structural studies of the Society and Austral hotspot regions in the South Pacific

Abstract

The southeastern extension of the Society (Teahitia hotspot) and Austral (Macdonald hotspot) volcanic chains were extensively studied (1986–1989) using multi-channel echo-sounding (Seabeam), bottom imaging systems and submersible dives. Sea floor within these hotspot regions consists of recently constructed submarine highs built on crust that shows morphological and structural discontinuities with the surrounding, ancient abyssal hill regions (> 4200 m in depth). Recent volcanic and hydrothermal activity is present on several volcanoes within the Society hotspot and on the Macdonald volcano. Previous studies carried out in the Society hotspot region showed several geological provinces: 1. (1) Eight major highs (> 1000 m in height) which extend from above sea level down to a depth of 3750 m. These features have been formed by lateral eruptive events along rift zones which are aligned along four main structural directions—N150°–180°, N80°, N30°–40° and N110°—which correspond to the orientations of ancient ridge axes, transform faults and Riedel-type shear zones, respectively. 2. (2) The sea floor, which slopes away from the base of each major volcano (3750–4200 m depth), has an average slope of 0.5°, and is characterized by smooth topography and volcanic cones less than 500 m high. 3. (3) The surrounding abyssal hill region, which has an average slope of 0.1° near the Society hotspot, consists of faulted terrane with a N170° orientation, and ancient elongated structural highs. From the structural lineations observed in the Society hotspot volcanoes, a reconstruction of volcanic features has been made. It is proposed that the formation of the seamounts occurs in an organized fashion along pre-existing preferential structural orientations that are associated with zones of crustal weakness.