Thermal Structure of Eastern Australia's Upper Mantle and its Relationship to Cenozoic Volcanic Activity and Dynamic Topography

Ball, P. W. and Czarnota, K. and White, N. J. and Klöcking, Marthe S. and Davies, D.R. (2021) Thermal Structure of Eastern Australia's Upper Mantle and its Relationship to Cenozoic Volcanic Activity and Dynamic Topography. Geochemistry, Geophysics, Geosystems. ISSN 1525-2027 DOI (In Press)

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Plain Language Summary Earth's convecting mantle lies between the tectonic plates and the core. This mantle layer circulates, driven by the rising and sinking of hot and cold material, respectively. These internal motions can push the tectonic plate above up or down, generating surface topography. Here, we propose that the Eastern Highlands of Australia formed in response to hot mantle material displacing cold mantle at the base of the Australian plate. We arrive at this conclusion using observations gleaned from Australia's volcanic past. Vast amounts of volcanic activity has occurred across Eastern Australia over the last sixty million years. We use the chemical composition of volcanic rocks to estimate mantle temperatures present at the time of eruption. We find that volcanic activity was initiated by wholesale emplacement of hot mantle beneath Eastern Australia, and that this event was coincident with uplift of the Eastern Highlands. However, the largest volcanoes in Eastern Australia developed above localised conduits of especially hot mantle material, known as plumes. Plumes rise from great depths and remain stationary. Therefore, since Australia has moved northwards during Cenozoic times, these larger volcanoes have formed in chains beginning near Townsville 40 million years ago, and ending near Melbourne five million years ago. Abstract Spatio-temporal changes of upper mantle structure play a significant role in generating and maintaining surface topography. Although geophysical models of upper-mantle structure have become increasingly refined, there is a paucity of geologic constraints with respect to its present-day state and temporal evolution. Cenozoic intraplate volcanic rocks that crop out across eastern Australia provide a significant opportunity to quantify mantle conditions at the time of emplacement and to independently validate geophysical estimates. This volcanic activity is divided into two categories: age-progressive provinces that are generated by the sub-plate passage of mantle plumes and age-independent provinces that could be generated by convective upwelling at lithospheric steps. In this study, we acquired and analyzed 78 samples from both types of provinces across Queensland. These samples were incorporated into a comprehensive database of Australian Cenozoic volcanism assembled from legacy analyses. We use geochemical modeling techniques to estimate mantle temperature and lithospheric thickness beneath each province. Our results suggest that melting occurred at depths ≤ 80 km across eastern Australia. Prior to, or coincident with, onset of volcanism, lithospheric thinning as well as dynamic support from shallow convective processes could have triggered uplift of the Eastern Highlands. Mantle temperatures are inferred to be ∼ 50–100 °C hotter beneath age-progressive provinces that demarcate passage of the Cosgrove mantle plume than beneath age-independent provinces. Even though this plume initiated as one of the hottest recorded during Cenozoic times, it appears to have thermally waned with time. These results are consistent with xenolith thermobarometric and geophysical studies.

Item Type: Article
Uncontrolled Keywords: 2021AREP; IA77
Subjects: 02 - Geodynamics, Geophysics and Tectonics
Divisions: 02 - Geodynamics, Geophysics and Tectonics
08 - Green Open Access
Journal or Publication Title: Geochemistry, Geophysics, Geosystems
Identification Number:
Depositing User: Sarah Humbert
Date Deposited: 29 Jul 2021 14:21
Last Modified: 28 Oct 2021 00:01

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