A multi-scale analysis of factors controlling the dynamics of basaltic volcanic fields: Newer Volcanics Province, Australia

Abstract

Basaltic Volcanic Fields (BVFs) occur on all continents, in all tectonic environments and host a diverse range of small scale basaltic volcanoes, which include the most numerous types of volcanic edifices on Earth. They are associated with small, often monogenetic, eruptions with long periods of quiescence. BVF are poorly studied compared with more obviously threatening, and high eruptive flux volcanic systems (volcanic arcs, mid-ocean ridges, continental rifts, and mantle plumes). The range of models used to explain what drives volcanism at the dozens of BVFs studied globally, stems from the variety of tectonic settings in which they occur. The complexity and inconsistency between the various models accounts for the need for further research on how these fields develop and the hazards they pose to society. BVFs that have been studied in detail are predominantly geometrically smaller examples, with characteristics that suggest relationships with local tectonic processes. The Newer Volcanics Province (NVP) is an expansive Pliocene to Recent intraplate basaltic plains province, located in south-eastern Australia. The NVP has several aspects that make it an interesting and unique BVF to study. It is not readily relatable to any tectonic processes expressed at the surface, it occurs in a compressional lithospheric stress field setting, and it is host to some of the world’s largest maar volcanoes. The NVP therefore provides an ideal case study for larger end-member examples of both volcanoes and BVFs. The NVP as a whole, and many of its volcanoes, have been the subject of geochemical, deep geophysical imaging, physical volcanology, and limited age dating studies over the past half century. Despite this, there are significant gaps in understanding what controls on volcanism in a compressive stress regime, and the formation of very large maar volcanoes. Potential field modelling and spatial analysis methods are proven effective methods in studying basaltic volcanoes and volcanic fields. Hence, they are used in this thesis to investigate the aforementioned unique aspects of the NVP, with the implications being relevant to cases of basaltic volcanoes and BVFs worldwide. Lake Purrumbete Maar (LPM) is a ~50 ka yrs old, large maar volcano with a crater that is up to 2,800 m in diameter. Despite its age, Lake Purrumbete’s near circular crater is well preserved, having undergone only minor erosion and is. It is one of a number of maar volcanoes of the NVP that rank amongst the largest examples in the world. Forward and inverse potential field modelling is used to constrain the subsurface structures related to the maar to assist in determining the factors that control the formation of such a large maar. Results show that LPM is the result of at least four coalesced vents that have produced a large shallow bowl shaped diatreme system, and not a deep conical feature. This is consistent with features typical of maars hosted in unconsolidated sediments, which is suggested for LPM by the occurrence of irregular marl lithic clasts with peperitic textures in the tephra ring deposits. Geometry inversions of the magnetic data indicate that the vents extend to a greater depth than inferred by accidental lithics present in the volcanic deposits (1472 boreholes. Results show the NVP is large (23,100±530 km2) voluminous (680–900 km3 DRE) and high-flux (0.15–0.2 km3/ka) example relative to comparable low-flux IBVFs (0.0001 – 0.1 km3/ka). All the BVFs used for comparison have eruptive fluxes an order of magnitude or more less than examples of plume related volcanoes (Kilauea) and BVFs (Eastern Snake River Plains). Most lower flux BVFs also show no systematic age migration pattern in volcanism suggestive of a fixed mantle plume, and those with detailed geochronology and volume data often show a correlation between their eruptive flux and the rate of local tectonic processes. It is suggested that the NVP and most low- and high-flux BVFs are the result of upwelling occurring in the asthenosphere, related to tectonic processes; without requiring additional thermal input from a deep mantle source, as is inferred for several cases. Considering a control on volcanism by tectonic processes, the range of eruptive flux of tectonically controlled BVFs is related to variations in the rate of the effecting tectonic process, mantle composition, and the size of the mantle source zone where melt generation and accumulation is taking place.