Formation of Martian Paterae: Insights from Terrestrial Ignimbrite

Introduction: The formation of Martian paterae is poorly understood. ÒTypicalÓ paterae are considered to be constructed primarily of pyroclastic deposits (e.g. Tyrrhena Patera [1], Hadriaca Patera [2], and Apollinaris Patera [3-4]), whereas others are suggested to be constructed primarily of numerous lava flows (e.g. Biblis and Ulysses Paterae [5] and Uranius Patera [6]). Alba Patera is interpreted to have significant components of both pyroclastic and lava flows [78], whereas volcanism may not have been significant in the formation of Orcus Patera [9]. One commonality to all paterae is the feature for which they are named, a shallow summit crater [e.g. 10]. The lack of observed resurgent doming at paterae [e.g. 11] along with the model of basaltdominated volcanism on Mars [e.g. 12] has lead to comparisons between paterae and calderas formed on terrestrial basaltic shield volcanoes [e.g. 11,13]. However, it remains unclear if this is the most valid terrestrial analog. Calderas in basaltic shields are built upon a large edifice dominated by lavas, whereas this is clearly not the case for pyroclastic-dominated paterae. We suggest that a better paterae analog may be ignimbrite shields, which have received comparatively little attention. These are large-volume pyroclastic shields that show neither significant collapse nor resurgent doming [14-16]. Herein, we consider morphologic similarities between Martian paterae and terrestrial ignimbrite shields in order to further explore the processes responsible for the formation of the paterae.