Interpreting subsurface volcanic structures using geologically constrained 3‐D gravity inversions: Examples of maar‐diatremes, Newer Volcanics Province, southeastern Australia

Abstract

We present results and a method to geophysically image the subsurface structures of maar volcanoes to better understand eruption mechanisms and risks associated with maar-forming eruptions. High-resolution ground gravity and magnetic data were acquired across several maar volcanoes within the Newer Volcanics Province of southeastern Australia, including the Ecklin maar, Red Rock Volcanic Complex, and Mount Leura Volcanic Complex. The depth and geometry of subsurface volcanic structures were determined by interpretation of gridded geophysical data and constrained 2.5-D forward and 3-D inverse modeling techniques. Bouguer gravity lows identified across the volcanic craters reflect lower density lake sediments and pyroclastic debris infilling the underlying maar-diatremes. These anomalies were reproduced during modeling by shallow coalesced diatremes. Short-wavelength positive gravity and magnetic anomalies identified within the center of the craters suggest complex internal structures. Modeling identified feeder vents, consisting of higher proportions of volcanic debris, intrusive dikes, and ponded magma. Because potential field models are nonunique, sensitivity analyses were undertaken to understand where uncertainty lies in the interpretations, and how the models may vary between the bounds of the constraints. Rather than producing a single "ideal" model, multiple models consistent with available geologic information are created using different inversion techniques. The modeling technique we present focuses on maar volcanoes, but there are wider implications for imaging the subsurface of other volcanic systems such as kimberlite pipes, scoria cones, tuff rings, and calderas.

How Polygenetic are Monogenetic Volcanoes: Case Studies of Some Complex Maar‐Diatreme Volcanoes

“…For instance, Valentine and White [29] propose an alternative model that allows multiple levels of country rock disruption and fragmentation, based on effective mixing by debris jets, an important subsurface transport phenomenon in phreatomagmatic vent complexes that is defined as an upward-moving stream of volcaniclastic debris, magmatic gases, and water vapor ± liquid water droplets, occurring on multiple vertical levels within a growing subsurface diatreme (e.g., [144]). This conceptual model is in accordance with the observed irregular distribution of accidental lithics in ejecta rings (e.g., [145]), field examples on diatreme geometry (e.g., [79]), but also on experimental cratering studies (e.g., [109,124,146]) and geophysical modeling (e.g., [80,81,147]). Chako Tchamabé et al [11] also suggested that the variation of juvenile populations within the stratigraphic sequence of maars might reflect a potential mode of explosions during maar-diatreme formation (Figure 7).…”

The Al-Du’aythah volcanic cones, Al-Madinah City: implications for volcanic hazards in northern Harrat Rahat, Kingdom of Saudi Arabia

“…Understanding the subsurface architecture of a volcano or a volcanic field is important in order to fully comprehend eruption histories and processes (Blaikie et al 2014). Usually, subsurface architecture is inferred based on known structural geology (i.e.…”

“…Geophysical methods may also provide information that can be related to the subsurface geology (e.g. Mrlina et al 2009;Blaikie et al 2014). For example, magnetotelluric (MT) surveys produce images of electrical resistivity contrasts according to analysis of electromagnetic impedances of the subsurface structure.…”

The erupted volumes of tephra from maar volcanoes and estimates of their VEI magnitude: Examples from the late Cenozoic Newer Volcanics Province, south-eastern Australia

“…These volcanoes have previously been the subject of geophysical investigations into the geometry of the underlying maar-diatreme and results from these studies are presented from a methodological point of view in Blaikie et al (2014b) and from a volcanological point of view in Blaikie et al (2012Blaikie et al ( , 2014a. Some statistical data reporting the bulk volumes of the maar diatremes was presented by (Blaikie et al 2014a, b), but calculating the volumes of the ejecta rings and total magma volume was beyond the scope of those papers and is addressed here.…”