Abstract

On Mars, the three large volcanoes of Tharsis (Arsia, Pavonis, and Ascraeus Montes) are aligned with other smaller edifices, conforming the 'Alignment 3' defined in previous works. These volcanoes show a wide variation in morphologies despite being formed by the same migrating mantle plume under the lithosphere. In this work, the volcanic edifices associated with the Alignment 3 were examined in terms of: 1) their morphologies; and 2) their ages as determined from the Southern Polar Giant Impact (SPGI) model compared to the crater counting. Using Mars Orbiter Laser Altimeter (MOLA) data, we have analysed the geomorphological and morphometric characteristics of the volcanic centres and identified possible composite volcanoes along the alignment. We suggest that the possible existence of composite volcanoes might not be related to felsic compositions as it occurs on Earth, but rather to their peculiar eruptive histories. Furthermore, a comparison with the morphometry of terrestrial composite volcanoes suggests that the Alignment 3 contains a majority of low composite volcanoes, two in the Uranius Group (Uranius and Ceraunius Tholus), the three large volcanoes of Tharsis, and two in the Southern Group (Sirenum Tholus and Sirenum Mons). The beginning of the Alignment 3 is characterized by Labeatis Mons whilst the end by the low sloping and possibly sub-glacial Chronius Mons. Based on the size difference between the large central volcanoes of Tharsis and the smaller volcanoes located at the extreme ends of the alignment, we conclude that the peak of the migrating mantle plume coincides with the large volcanoes of the Tharsis Group. The analysis of the existing crater count ages suggests evident inconsistencies with the SPGI ages along the loxodromic trajectory of the Alignment 3.