APPLYING POTENTIAL FIELD GEOPHYSICAL METHODS TO UNDERSTAND THE SUBSURFACE MORPHOLOGY OF MAAR VOLCANOES, EXAMPLES FROM THE NEWER VOLCANICS PROVINCE OF SOUTH-EASTERN AUSTRALIA

When the original or partly eroded edifice of a volcano is preserved, studies will tend to focus on the characteristics of surrounding deposits, with features of the volcanoes subsurface often inferred from surface observations. Potential field geophysical modelling techniques can be applied to better understand the subsurface morphology of volcanoes and when linked with observations of surface geology can be used to develop a more complete understanding of the volcanic centres eruptive history.

High resolution ground gravity and magnetic data was acquired across several maar volcanoes located within the intraplate basaltic Newer Volcanics Province (NVP) of Western Victoria, south-eastern Australia. The maar volcanoes surveyed are both simple and complex, representing a range of the different sizes and styles of eruptions observed within maar volcanoes of the NVP. Maar volcanoes form as ascending magma comes into contact with ground water resulting in phreatomagmatic explosions that excavate a deep crater which is infilled by pyroclastic debris during and after the eruption. This crater is known as a diatreme and has a high petrophysical contrasts (lower density and higher magnetic susceptibility) with the surrounding host rock, making maar volcanoes ideal for gravity and magnetic modelling.

Gravity and magnetic data was subject to 2D forward and 3D inverse modelling in order to reveal details on the depth, geometry and petrophysical property distributions of the volcanoes diatreme and feeder dykes. Gravity lows with corresponding magnetic highs are observed across the maar craters and were reproduced during modelling with the presence of a diatreme. Smaller wavelength gravity and magnetic anomalies detected in the centre of the more complex volcanic craters can be explained by the presence of intrusive dykes or vents filled with a higher proportion of denser volcanic debris.

Modelling suggests that multiple coalescing diatreme structures exist below the volcanic edifices, some containing intrusive dykes or a denser central vent filled in with volcanic debris. Multiple diatreme structures suggest a complex eruption history involving vent migration, while preserved dykes within the diatreme suggest short-lived fluctuations between phreatomagmatic and magmatic eruption styles.