Anderson, Melissa O., Hannington, Mark D., Haase, Karsten, Schwarz-Schampera, Ulrich, Augustin, Nico , McConachy, Timothy F. and Allen, Katie (2016) Tectonic focusing of voluminous basaltic eruptions in magma-deficient backarc rifts. Earth and Planetary Science Letters, 440 . pp. 43-55. DOI 10.1016/j.epsl.2016.02.002.

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Abstract

Highlights

• Shield-type volcanoes in young backarc rifts may be linked to arc segmentation.
• Arc-transverse faults act as magma conduits for voluminous basaltic eruptions.
• The Nifonea volcano hosts the first large lava lake described in a submarine backarc.
• Effusive eruptions produced short-live hydrothermal venting and an “event plume”.

The Coriolis Troughs of the New Hebrides subduction zone are among the youngest backarc rifts in the world. They reach depths of >3 km, despite their small size (<100 km in length and only 25-45 km wide) and their proximity to the arc front (∼50 km). The narrow, deep graben morphology is characteristic of magma-deficient arc rifts in the early stages of backarc extension, where the rate of extension and subsidence exceeds the magmatic input. Unexpectedly, the youngest graben, the Vate Trough, contains a centrally-located 1000-m tall and 14-km wide shield volcano with a large, 5 × 8 km breached summit caldera. The Nifonea axial volcano has a volume of ∼126 km3, reflecting unusually high extrusion rates, given its young age (<3 Ma), and the summit caldera hosts the remnants of a large lava lake, the first described from a submarine backarc setting. Extensive diffuse hydrothermal venting and several clusters of black smoker chimneys, with the highest recorded fluid temperatures (368 °C) in the SW Pacific, occur on the youngest lava flows. Comparison with similar axial volcanoes on the mid-ocean ridges suggests that the 46 ×106 m3 of sheet flows in the caldera could have been erupted in <30 hours. The focusing of voluminous basaltic eruptions into an otherwise magma-deficient backarc has been linked to strong left-lateral transtension caused by clockwise rotation and segmentation of the southern portion of the arc after collision with d'Entrecasteaux ridge. This study shows that the upper plate stresses can result in dramatic variability in magma supply and hydrothermal activity at the earliest stages of arc rifting and could explain the wide range of melt compositions, volcanic styles and mineral deposit types found in nascent backarc rifts.

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