A belt of Cenozoic mafic volcanoes in eastern Australia spans >2000 km and encompasses >90 million years. The volcanic record of this belt links Cenozoic plate motion, tectonic reshuffling in the SW Pacific, and east Australian mantle geodynamics. These links are examined in this study based on new paleomagnetic data from >560 samples from six key time periods, new 40Ar/39Ar geochronology from 65 samples, and major element, trace element, and radiogenic isotope results from 62 samples of Cenozoic east Australian mafic rocks. North-to-south age progression of some east Australian volcanic provinces is consistent with hotspot magmatism during the northward movement of Australia away from Antarctica. Other provinces are non-age-progressive, however, leading to a traditional two-fold division between east Australian “central volcanoes” (age-progressive) and “lava-fields” (non-age-progressive). However, animated reconstructions indicate that volcanism is protracted and concurrent in both central volcanoes and lava-fields. Furthermore, new geochemical results indicate that the compositions of the groups are virtually identical: major element, trace element, and radiogenic isotope data suggest generation of both groups below the garnet stability field, and modelling shows that all provinces were likely generated from various degrees of melting of a source similar to the Australian garnet-amphibole lherzolite. The combination of age-progressive and non-age-progressive mafic magmatism (which overlapped spatially, temporally, and in terms of geochemical composition) is consistent with a model wherein both multiple plumes and edge-drive convection along the stepped lithosphere of eastern Australia, accompanied by shear-driven upwelling, contributed to the formation of the Cenozoic volcanic belt. Paleomagnetic and high-resolution 40Ar/39Ar data from the east Australian volcanoes were used to create a new Australian Cenozoic apparent polar wander path (APWP). The new APWP includes a longitudinal excursion at ~25 Ma that corresponds with tectonic collisions in the SW Pacific. Spreading rates between Australia and Antarctica indicate a substantial decrease in plate speed during the late Oligocene, which was larger in the east (10 km/My2) than the west (2 km/My2). The period of reduced plate velocity between 30 and 24 Ma, the discrepancy in half-spreading rates, an eastward offset in the Tasmantid seamounts, the high volume, eruption rates, and intense crustal assimilation at the Tweed province; as well as the divergence of the APWP, correspond to approximately 10o of plate rotation relative to the synthetic reference pole of the global moving hotspot reference frame.
Geosphere, December 2019, v. 15, no. 6, 2057-2061, doi:10.1130/GES02182.1, Animation 2. Animated reconstruction showing the last 60 m.y. of motion of the Australian plate using the global APWP of Besse and Courtillot (2002) applied to the base reconstruction file of Seton et al. (2012), with all plates moving with respect to Australia. Approximate position of New Guinea composite terrains after Davies et al. (1997).
The volume of Cenozoic volcanism in eastern Australia has been estimated previously from the outcrop extent of basalts, but drill holes have subsequently revealed subsurface volcanic flows with a cumulative thickness of up to 60 m. The magnitude of Cenozoic magmatism is therefore greater than previously thought, necessitating a re-evaluation of basalt distribution and estimates of eruptive volumes. Drill-hole data were collected from across the Bowen Basin in eastern Australia to compile a regional basalt isopach map. This large dataset comprising ∼150 000 drill-hole logs was used to derive minimum extrusive volume estimates of the Peak Range, Springsure and Buckland volcanoes of approximately 2600 km3, 1460 km3 and 860 km3, respectively. The southward decrease in the volumes of these volcanoes, as well as a southward diminishing pattern of eruption rate, most likely arose from waning plume magmatism. We estimate that from ca 30 Ma (the average age of the Peak Range volcano in Queensland) to ca 10 Ma (the approximate age of the Canobolas volcano in New South Wales), the eruption rate in the east Australian central volcanoes decreased by roughly 15 km3/Ma. Additionally, our dataset suggests that basalt flows in portions of eastern Australia filled pre-existing channels with maximum depths ranging from 60 to 220 m, implying an early- to mid-Cenozoic topography of moderate relief.
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