Regional context

• Interested in the last 100 million years of tectonic evolution when Australia separated from Antarctica.
• The region now known as “Zealandia” is most likely a fragment of continental crust that was rafted off mainland Australia.

← Muller et al. (2016), EPSL

Mortimer et al. (2017), GSA today

Volcanic activity

• 3 major hotspot chains:
  • Tasmantid seamount chain
  • Lord Howe seamount chain
  • Cosgrove track
• These chains are made up of central volcanoes with leucitite-bearing eruptions.
• However, smaller lava field volcanoes that exhibit no time dependence.
• Lava field volcanoes outnumber eruptions from central volcanoes.

What drives non-age progressive eruptions
in Eastern Australia and Zealandia?

Edge-driven convection

Convective eddy that forms at the trailing edge of sharp discontinuities in lithospheric thickness

Davies et al. 2014, Farrington et al 2010, Demidjuk et al. 2007

Asthenospheric shear

Shear-driven upwellings in the mantle that induce partial melting at the base of the lithosphere

Conrad et al. 2011

Previous geodynamic studies

• The thickness of the lithosphere across the eastern margin of Australia and Zealandia does not vary considerably.
• Eruption products from lava field volcanoes do not align to the direction of plate motion.
• Unclear whether these convective cells can be sustained through time.

• High rates of shear are not continuous across Zealandia
• Unclear whether asthenospheric shear alone can explain older spikes in volcanism.

Geochemical evidence

Park et al. 2019, Nature Geoscience

The Slab Cemetery

• Chemical heterogeneities in the mantle are introduced by recycling oceanic lithosphere through subduction.
• Flat subduction occurs in the Western Pacific due to the thermal buoyancy of oceanic lithosphere.
• Subducted oceanic lithosphere congregate at the transition zone (410-660km depth)
• A slab burial ground underlies all of Eastern Australia and Zealandia.
• We propose that spikes in intraplate volcanism are triggered by pulses in slab flux due to the release of subducted volatiles in the transition zone inherited from past subduction.

Our proposed model

King et al. 2015, Geology

Plate kinematics & “slab flux”

The most pronounced pulse of volcanism are correlated with bursts in slab flux.

21 Ma: Rapid eastward migration of the Tonga-Kermadec trench. Ontong Java Plateau choked the Melanesian Trench and forced subduction to migrate inbound to the Trobriand Trough.

4 Ma: Reversal in subduction polarity along the northern margin of the Australian plate may have detached the formerly subducting Pacific plate slab along the Melanesian Trench.

Slab flux: the volumetric rate of material being consumed at subduction zones

i

Seismic
Evidence

Slow S-wave velocity in the transition zone (averaged 410-660km) suggests thermo-chemical anomalies - possibly associated with melt.

P-wave anomaly images indicate flat subduction in the north of the Tonga-Kermadec Trench, and transitions to steeper subduction angles in the south.

S40RTS

SL2013SV

TX2019

Volcanoes sample recycled volatiles in the transition zone

We compared the variation in isotope ratios across lava field volcanoes and age-progressive central volcanoes and leucitite suites (hotspot chains).

• Non age-progressive volcanoes in E Australia and Zealandia sample fertile source reservoirs in the transition zone.
  • EM1/2 ~ consistent with the subduction of sediments
  • HIMU ~ recycled oceanic crust that is metasomatically enriched
• Above any categorical sorting by volcano type, the strongest variation in isotopic signature is controlled by latitude in this region.
• Zealandia-Antarctic anomalous mantle does continue north.

Geochemical variation controlled by slab consumption rates

• Mantle reservoir transitions from EM2 in the north to HIMU in the south.
  • EM1/2 ~ consistent with the subduction of sediments
  • HIMU ~ ancient recycled oceanic crust that is metasomatically enriched
• Relatively unimpeded subduction in the northern segment of the trench supplies a constant flux of volatile-enriched oceanic lithosphere into the transition zone.
• Volcanoes at higher latitudes reflect recycled oceanic lithosphere that has lost Rb due to a longer residence time in the transition zone.
  • Subduction has been congested by the Hikurangi Plateau

25 Ma

0 Ma

Conclusions

Intraplate volcanism is triggered by the release of subducted volatiles

in the transition zone during pulses in slab flux.

Multiple plumes are not required to explain the majority of volcanism in eastern Australia - Zealandia.

Supported by:

• Thick piles of oceanic lithosphere have accumulated at the transition zone over 100 Myrs.
• Increases in eruption frequency at 21 Ma and 4 Ma correlate with slab flux.
• Isotope geochemisty show volcanoes sample a mantle reservoir consistent with recycled volatiles in slabs.