NATURE AND TECTONIC SETTING OF THE SOUTHERN KERMADEC SUBMARINE ARC VOLCANOS - AN OVERVIEW

The 1000-km long Kermadec subduction system (northern New Zealand), and adjacent Havre Trough back-arc basin, associated with the Pacific-Australian plate boundary, is an archetypal example of an active ''island arc'' convergent margin. Although the position and tectonic setting of the frontal arc is critical to understanding tectonism and magmatism within an arc-back-arc system, this has not been, unequivocally established for the Kermadec subduction system. Swath surveys from the 35-degrees-37-degrees-S segment of the Kermadec system reveal that the frontal arc comprises seven large arc stratovolcanoes (Rumble II, III, IV, Silent II, Rumble V, Tangaroa, and Clark). The frontal arc continues south with the Whakatane volcano sited at the northern extremity of the Taupo Volcanic Zone. Of these eight volcanoes, the latter four are newly identified, and all have constructional volumes varying between 50 and 260 km3. All of these arc edifices are sited 15-25 km west of the Kermadec frontal ridge. Bathymetric data similarly indicate that other Kermadec arc volcanoes between 32-degrees and 35-degrees-S also lie west of the ridge axis, within the ''back-arc'' region, while north of 32-degrees-S the arc volcanoes lie along the frontal ridge. This apparent variation in the tectonic position of the frontal arc coincides with other changes in the morphology of the subduction system, north and south of 32-degrees-S, and as suggested by Pelletier and Dupont (1990), may reflect differences in the length of the underlying subducted slab. The 35-degrees-37-degrees-S frontal arc segment is sited within 35-45 km of the present-day back-arc rift axis, and may control the initial loci of rift graben evolution. Sidescan imagery and seafloor photography of the southern Kermadec arc volcanoes reveal a complex volcanic stratigraphy with intertwined pillow lavas, proximal pillow breccias, highly vesicular scoriaceous lapilli, and pervasively mass-wasted volcaniclastic and epiclastic detritus. All volcanic deposits interdigitate with pelagic sediments. A single K-Ar dated andesite from Whakatane volcano suggests that the history of the present frontal arc may extend back to at least 0.77 Ma while Rumble III and V volcanoes are the major, although not exclusive, sites of geologically recent edifice construction. The historical acoustic record of southern Kermadec submarine eruptions since 1958 is now regarded as equivocal, with the July 1986 eruption of Rumble III the only unambiguous volcanic activity recorded in modem times.