Late-stage water eruptions from Ascraeus Mons volcano, Mars: Implications for its structure and history

Ascraeus Mons was one of the first of the Martian volcanoes to be imaged by the High Resolution Stereo Camera (HRSC) experiment onboard the ESA Mars Express spacecraft. These images show much of the volcano at a higher resolution than previously, and details of its lava flows, sinuous rilles, flank vents and tectonic features indicate an unexpected origin for some of these features. We establish the time-stratigraphic sequence for these features, and use a numerical model on HRSC stereo DTMs of the sinuous rilles, and conclude that they were formed by water erosion. Terrestrial analogues for such features are found at Réunion Island and other volcanoes. We then examine the overall structure of the volcano, which is dissimilar to that of large terrestrial volcanoes in important respects, and perform laboratory analogue experiments of its deformation, concluding that the tectonic features were formed by sinking of the volcano into a substratum that was much weaker than the volcanic edifice. An ice-rich substratum melted by a combination of pressure melting and magmatic heating seems the most likely mechanism. Analogous water-escape structures in a similar volcanic situation have been identified at Mt Haddington in the Antarctic. The possible role of a hydrological cycle and a hydrothermal system within the volcano are discussed. Based on field evidence, we propose that much of the broad aprons of lobate flows issuing from the NE and SSW foot of Ascraeus Mons are composed of mudflows rather than lava flows. These different approaches are linked into a coherent history of this volcano. The similarity of Ascraeus Mons to Pavonis Mons and Arsia Mons (though Ascraeus is younger) suggests that some of our conclusions may apply to these volcanoes too.