New insights into the evolution of the Mt. Melbourne volcanic field (Northern Victoria Land, Antarctica) from high–resolution aeromagnetic data

The Mt. Melbourne field is interpreted as a quiescent volcanic complex, located in Northern Victoria Land, Antarctica, at the boundary between the Transantarctic Mountains (TAM) and the West Antarctic Rift System (WARS). It is one of the handful Antarctic volcanoes with the potential for large–scale explosive eruptions [1], with resulting key effects on the local environment and potentially on climate. The geological and geophysical structure of this volcanic field remains poorly known, despite its key relevance to better comprehend the Cenozoic tectonic and geodynamic processes responsible for the opening of the WARS and the uplift of the TAM rift flank. Here we present results derived from a novel high–resolution aeromagnetic dataset, collected in the austral summer 2002/2003 during the XVIII Italian Expedition, with the aim of investigating the geophysical structure of the main volcanic centres of the field. Aeromagnetic data were processed and Digital Enhancement and Depth to Magnetic Source analysis performed to reveal the distribution of the main fault systems affecting the Mt. Melbourne volcanic field, particularly beneath the ice–covered areas. The results highlight NNE–SSW, NW–SE and E–W trending structural systems, in agreement with the available tectonic information for the study area [2, 3]. Furthermore, similar NNW–SSE trending pervasive negative anomalies are detected beneath both the Mt. Melbourne edifice and Cape Washington, superimposed by positive ones forming radial patterns. With the aid of laboratory magnetic susceptibility data from rock samples collected in the field [4], we carried out forward and inverse modeling across the volcanic centres in order to image their subglacial internal structure. Based on our results, considering the ambiguity and narrowness of the available geochronological data [1, 5, 6], we propose two (non–mutually exclusive) interpretative models to explain the evolution steps of the Mt. Melbourne volcanic complex. In the former, a major volcanic phase responsible for building of the inner part of the main volcanic centres likely occurred prior to the last magnetic polarity reversal (i.e. before 0.78 Ma, Matuyama Chron), explaining the negative anomalies detected as due to remnant magnetisation. During the Pleistocene–Holocene period, a following second volcanic phase put in place at shallower levels, primarily with present–day magnetization. In the alternative model, magma pulses originated at the lithospheric step between the thick East Antarctic craton and the thinner Ross Sea crust [7] caused i) widespread volcanism at the surface of the volcanic complex, particularly with the building up of the Mt. Melbourne edifice, and ii) a regional upward of the Curie isotherm at depth, causing partial de–magnetisation of the underlying volcanic rocks. References: [1] Giordano et al. (2012). Bull. Volcanol., 74, 1985-2005. [2] Storti et al. (2006). J. Struct. Geol., 28, 50-63. [3] Vignaroli et al. (2015). Tectonophysics, 656, 74-90. [4] Pasquale et al. (2009). Ann. Geophys., 52(2), 197-207. [5] Armstrong (1978). New Zeal. J. Geol. Geophys., 21(6), 685-698. [6] Armienti et al. (1991). Mem. Soc. Geol. Ital., 46, 427-452. [7] Park et al. (2015). Earth Planet. Sci. Lett., 432, 293-299.