The Unique Formation of the Nili Patera Caldera on Mars

Abstract:

Caldera formation is common on the large volcanoes of Mars. Of these, the Nili Patera caldera in Syrtis Major appears unique. Set at one end of a 200 km long depression, the caldera manifests both extrusive [Christensen et al., 2005] and potentially intrusive [Wray et al., 2013] evolved silica-rich units, hydrothermal systems [Skok et al., 2010], and other post-caldera-formation volcanic structures such as the Nili Tholus cone. Nili Patera distinguishes itself by the depth of collapse. While other Martian caldera collapse less than thickness of the volcanics, Nili Patera’s floor lies ~1800 m below the plains elevations: ~1300 m below the estimated 500 m mean thickness of Syrtis Major [Hiesinger and Head, 2004]. Other calderas lack morphologic evidence of late-stage volcanic constructs, while dust cover precludes spectroscopic evidence of evolved compositions in many calderas. If Nili Patera is unique, the question then is why? We examine the qualities that distinguish Nili Patera not just from calderas on other volcanoes but also from the neighboring Meroe Patera, due south within the same elongate depression. The Noachian-aged material below Syrtis Major have been observed to be altered and phyllosilicate-rich [Ehlmann et al., 2009, Marzo et al., 2010]. Access to this water-rich substrate could have driven the surface hydrothermal system. The eruption of evolved lavas would not be caused by the substrate composition, but could instead result from the collapse. Basaltic volcanoes on Earth often have evolved plutons created by repeated partial melting. The massive collapse of Nili Patera could have provided a mechanism for this unit to erupt, while remaining buried in other volcanoes. We examine multiple hypotheses for why this deep collapse is only observed in Nili Patera and how the observed lithologies here may relate to the evolution of other Martian volcanoes. The regional-scale compositional signature, that the Syrtis Major province shares with other major volcanic constructs [Taylor et al., 2010], provides a broader context to such evolutionary pathways.