What question did this study set out to answer?

To investigate the mantle melting processes and lithospheric structure under Cenozoic volcanoes in eastern Australia using advanced imaging techniques.

March 21, 2026Open Access

Mantle melting and lithospheric structure beneath eastern Australia’s Cenozoic volcanoes from 3D magnetotellurics

RMRelly MargionoMeteorological, Climatological, And Geophysical Agency GHGraham HeinsonThe University of Adelaide

Key Points

To investigate the mantle melting processes and lithospheric structure under Cenozoic volcanoes in eastern Australia using advanced imaging techniques.
Conducted a 3D inversion of magnetotelluric data across approximately 800 locations in eastern Australia.
Analyzed resistivity models to characterize the lithosphere and asthenosphere properties beneath volcanoes.
Evaluated compositional characteristics of upper and lower crustal materials based on resistivity readings.
Identified anomalously low resistivity (around 100 Ohm.m) in the upper mantle beneath volcanoes at depths below 125 km.
Found hydrated mineral compositions in the lower crust with resistivity measurements of around 50 Ohm.m.
Suggested decompression melting processes raise the geotherm and influence volatile migration across extensive areas.

Abstract

One of the most remarkable features of the eastern Australian continent are Cenozoic age (65 Ma to present) volcanoes over 3,000 km span north-south, with no significant age progression, even though the continent has moved at a rate of up to 75 km/Ma in an NNE direction over much of this time. Three theorems have been advanced to explain age-independent volcanism: (1) decompression melting from the transition zone due to volatile content from subducted slab stagnation; (2) edge-driven convection at the margins of steps in lithospheric thickness that drive upwelling from volatile-rich mantle reservoirs; and (3) melting of low-viscosity pockets of sub-lithospheric mantle due to asthenospheric shear. In this paper we have undertaken a 3D inversion of magnetotelluric (MT) sites across the Tasmanide accretionary orogens in eastern Australia (~ 800 locations) to define the broad-scale resistivity of the lithosphere and asthenosphere. The upper mantle directly beneath the age-independent Cenozoic volcanoes has anomalously low-resistivity (~ 100 Ohm.m) below 125 km depth, compatible with dry lherzolite-harzburgite-wehrlite compositions at temperatures of ~ 1,400 °C. The resistivity model suggests that inland from the volcanos, there is a step-like increase in lithospheric thickness. In the lower crust, beneath volcanic centres a resistivity of ~ 50 Ohm.m is consistent with a hydrated clinopyroxene-orthopyroxene-plagioclase composition at ~ 900 °C. No additional conduction mechanisms (such as graphite, sulphides or partial melt) are required. Results suggest that decompression melting from the transition zone raises the geotherm to adiabatic below 125 km and efficiently removes volatiles to the crust over a wide area (hundreds of kilometres) of subducted slab. Edge-driven convention may also occur at the step in the lithosphere thickness, particularly for the New Volcanic Zone and the leucite Cosgrove Track volcanoes. However, surface volcanism occurs in a much narrower zone where the melt solidus is intersected at lower crustal depths.

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