The application of geophysical modelling techniques to understand the subsurface morphology, eruptive history and magma volumes of maar volcanoes within the Newer Volcanics Province, South-Eastern Australia

<p>Maars are the second most common terrestrial volcano on Earth, and although small, can exhibit great complexity in their eruptive histories. Much of the structure of a maar volcano lies beneath the surface in the form of a diatreme, a pipe-like structure which underlies the crater and is infilled with a mixture of fragmented juvenile and country rock material. The structure of the maar-diatreme (depth and geometry) reflects processes occurring during the eruption, such as phreatomagmatic explosions occurring at deep, shallow or varied levels (reflected in depth of the diatreme), migration of vents (coalesced diatremes) and transitions between eruption styles (presence of dykes and magma ponds). To fully appreciate these volcanic systems, it is necessary to have some understanding of the structure of the maar-diatreme; however, especially in young volcanic fields, they are not always exposed. </p> <p>The Newer Volcanics Province (4.6 Ma-4.5 ka) is an intraplate, basaltic volcanic province comprised of over 400 monogenetic volcanoes, of which approximately 10% are maar volcanoes with no exposures of their diatremes. High-resolution ground gravity and magnetic data is acquired across the volcanic craters to image the depth and geometry of the maar-diatremes. Four case studies representing a range of sizes and eruptive styles were selected, and include the Red Rock and Mt Leura Volcanic Complexes, Ecklin Maar and the Anakies.  </p> <p>The geophysical models of these volcanic centres were produced from interpretations of the gridded gravity and magnetic data, and from using forward and inverse modelling techniques (in 2D and 3D). The models were constrained by integrating data about the maars eruptive styles with measurements of rock density and magnetic susceptibility. However, because potential field models are non-unique, the aim of our modelling technique was to produce multiple models that are consistent with the available geologic and geophysical information. Sensitivity analyses were conducted to assess uncertainty in these models, and to delineate a range of end-member models based on the upper and lower bounds of the petrophysical constraints. </p> <p>Geophysical modelling results suggest these maar volcanoes have broad, shallow diatremes, which form when phreatomagmatic explosions occur at shallow levels of the subsurface. Often, multiple vents are identified within these diatremes, and are possibly related to the weakly lithified host rock collapsing into and blocking the vent, causing it to migrate laterally. Some of these vents are aligned and form multiple, or coalesced craters, indicating vent migration is occurring along the length of a dyke. Other vents appear to be randomly distributed within the maar-diatreme, suggesting that dykes are propagating through the loose debris of the diatreme, causing vertical and lateral variations in the point of fragmentation. </p> <p>Several geophysical trends were identified that correlate to the different eruptive styles of the case studies (i.e., dominantly phreatomagmatic, fluctuating between magmatic and phreatomagmatic, transitional between phreatomagmatic and magmatic). Maars with fluctuating eruptive styles (e.g., Red Rock Volcanic Complex) are characterised by short-wavelength positive gravity and magnetic anomalies superimposed on longer-wavelength gravity and magnetic lows. The irregularly distributed short-wavelength anomalies were reproduced during modelling as dykes and magma ponds within the maar-diatremes. The presence of these intra-diatreme dykes, and observations of fluctuating eruptive styles, suggest the developing diatremes were not completely saturated with water during the eruption, which allowed to magma to fragment by either magmatic or phreatomagmatic styles when conditions were appropriate. </p> <p>Maar volcanoes exhibiting predominantly phreatomagmatic activity (e.g.,  Ecklin and Anakie maar) are characterised by gravity and magnetic lows across the crater, but may contain broader-wavelength, low amplitude positive gravity and magnetic anomalies in the centre of the crater. Modelling indicates that these anomalies are associated with regions containing higher volumes of juvenile material within the diatreme, which is interpreted to represent the entrainment of debris jets into the diatreme fill during the eruption. Volcanic centres experiencing a transition in eruptive style from phreatomagmatic to magmatic (e.g., Mt Leura Volcanic Complex), are characterized by long-wavelength gravity and magnetic highs indicating a large volume of ponded lava infilled the maar crater during the eruption. </p> <p>The total tephra and magma volumes associated with the eruption of these volcanoes can be calculated from the final geophysical models. Based on the average componentry and vesicularity of deposits in the maars ejecta-rims, the dense rock equivalent magma volume of the Ecklin maar, the Red Rock and Mount Leura volcanic complexes is 0.04 x 10⁹ m³, 0.17 x 10⁹ m³ and 0.29 x 10⁹ m³ respectively. The Red Rock and Mount Leura volcanic complexes have magma volumes that are an order of magnitude higher than Ecklin maar, and exhibit far more complex eruptive histories with multiple vents and transitions between explosive phreatomagmatic, magmatic explosive and effusive styles. Based on the total tephra volume, the Volcanic Explosivity Index (VEI) was estimated for each eruption. A VEI magnitude of 2 is assigned to the Ecklin maar, and 3 is assigned to the Mount Leura and Red Rock volcanic complexes. </p>