Over
the past seven years, a combination of New Zealand-,
Australian- and German-led research voyages using the research vessels
Tangaroa, Southern Surveyor, and Sonne respectively, have
revolutionised our understanding of the Kermadec-Tonga arc-backarc systems. This
convergent margin is the most seismically-active subduction zone system on Earth
with convergence rates steadily increasing northwards from the Hikurangi Trench
to ~250 mm/year at northern Tonga. In combination with the
intra-oceanic setting, factors potentially producing magmatic, tectonic, and
hydrothermal variability in this system include an increase in potentially
subductable sediment northwards along-strike, intersection of the Louisville
Ridge (hot-spot chain) and Osbourn Trough (paleo-spreading centre) with the arc
at ~25S, and the possibility of Samoan (hot-spot) plume ingression in the
northern Lau Basin. The bulk of the magmatism is submarine; whereas only a few
emergent (some transient) volcanic islands exist, about 80 submarine volcanic
centres of significant size (>1000 m above surrounding sea floor) are present
between White Island in the south to north of Tafahi in
Tonga. About 35 per cent of these
centres are hydrothermally active. Important petrogenetic features of the
volcanic rocks along this arc system are the abundance of rhyolite (some
quartz-amphibole-bearing), and the outcrop of primitive
olivine-clinopyroxene-phyric basalts in satellite cones of the major edifices.
Many of the edifices consist of complex caldera systems. Eruptions from and
degradation of these edifices were probably tsunamigenic. Many of the
hydrothermal systems developed within the edifices are likely destroyed during
the sector collapses which are the characteristic edifice degradation mode.
There are three backarc spreading systems along this margin: the Havre Trough,
Valu Fa-Eastern Lau Spreading Centre, and the nascent Fonualei Rifts. The latter
are particularly notable in having captured the entire suprasubduction zone
magmatic flux for a distance of about 150 km, shutting down the adjacent
volcanic front arc volcanoes.