Geology and history of the Malea Planum region: A new view of Mars’ oldest large volcanic province

Abstract Characterized by large paterae and late Noachian wrinkle-ridged plains, the ~1.2 million km2 Malea Planum region (MPR) has been grouped into a circum-Hellas volcanic province and likely represents the oldest of the large volcanic areas on Mars. Being key to Mars’ early volcanic, tectonic, and climate evolution, we conducted a comprehensive photogeological investigation of the MPR using multiple datasets including THEMIS-IR as a basemap. We identified 26 geomorphologic units and derived apparent model ages based on crater size-frequency distribution measurements for six of them. Along with stratigraphic, morphologic, hyperspectral, and gravimetric analyses, as well as findings by previous works in the surrounding regions, our chronostratigraphy resulted in a complete landscape formation model of the mapping area. At 3.9–3.8 Ga, Malea and Pityusa Paterae form, probably as volcanic collapse calderas geographically controlled by Hellas-concentric faults. Pityusa Patera hosts folded deposits, possibly pyroclastics emplaced and shortened during patera formation as a piston-type caldera. Around 3.8–3.7 Ga, i.e., during the same time the ridged plains of the Hellas basin are formed, up to ~3.9 million km3 of volcanic and clastic/ballistic deposits partially sourced by Pityusa/Malea activity and/or by now-obscured vents are emplaced and superpose Pityusa and Malea Paterae, thus covering any potential features associated with them. Assuming the wrinkle-ridged plains to entirely consist of basaltic deposits with ~2 wt% H2O, outgassing might have produced ~0.8 m Global Equivalence Layer of water and/or 3.9 hPa of H2, which could have temporarily increased ambient temperatures, potentially enabling fluvial and lacustrine processes across the Malea-Hellas regions. After plains emplacement, doming above a shallow magma chamber and its subsequent partial evacuation forms Amphitrites Patera as a caldera on a ~1.5 km high, broad rise collocated with a positive ~2.6 x 10-3 m/s2 free-air, but no significant Bouguer gravity anomaly. Smooth crater fills throughout the area that often show high thermal inertias as well as enrichments of plagioclase and clay minerals might represent partially leached pyroclastic deposits resulting from this patera formation. Between 3.7 and 3.6 Ga, the northern slope of Amphitrites Patera is heavily dissected by low-viscosity flow processes that drain towards the Hellas basin floor and leave behind the Axius Valles amongst others, forming one of the densest martian valley networks (~0.08 km-1). 1,777 km long Mad Vallis and other smaller channels traversing the entire MPR and connecting the South Pole area with the Hellas basin are also formed around this time. Based on the geologic context and feasibility studies, we favor glacial meltwater/mud or low-viscosity lavas sourced from Amphitrites’ summit over a catastrophic sapping event as cause for the Axius Valles. Following this, the Barnard impact event deposits ejecta on the surrounding flow features southeast of Amphitrites Patera; sinuous valleys and ridges are formed inside Barnard crater, likely by meltwater from ice sheets that might also have occupied Amphitrites Patera. Around 3.5 Ga, ~80-140 m (i.e., up to ~140,000 km3) of layered, friable materials are emplaced across large parts of the MPR as far north as 60°S. These materials are an extension of the circum-south polar Dorsa Argentea Formation (DAF), possible lag deposits from wet-based glaciation. Entrained within these deposits are dark, fine-grained materials, likely pyroclastics potentially sourced from volcanic activity at Peneus Patera, which might have formed around the same time, with bounding faults penetrating the wrinkle-ridged plains but without completely resurfacing its interior floor. Combining structural analyses of radial wrinkle ridges within Peneus Patera with a piston-type caldera model similar to Pityusa Patera ( Bernhardt and Williams, 2021 ) would imply the collapse of a magma chamber at 19.5 to 26 km depth, i.e., potentially in the mid-crust. Up to ~210,000 km3 of friable airfall deposits, possibly sourced by ongoing/recurring Peneus activity, then cover the entire MPR but are eroded except where they are armored by superposing impact ejecta, thus forming numerous pedestal craters. In the Amazonian, these pedestals are themselves covered by up to few 10s of 1000s of km3 of atmosphere-derived volatiles and fines, which are then also sculpted into a second, distinctly younger pedestal crater population. Ongoing erosion of pyroclastic materials entrained in DAF deposits across the MPR and elsewhere continue to provide mafic fines that potentially supply the formation of transversal and barchanoid dune fields in local depressions, e.g., within Pityusa Patera and on the floors of larger impact craters throughout the MPR and Noachis Terra to the northwest, which is contrary to previous theories of more local supplies. In conclusion, our investigation of the MPR, which included a comprehensive map and chronostratigraphic as well as morphometric analyses, shows that the area experienced a complex volcanic, tectonic, eolian as well as most likely (glacio-)fluvial history and acted as corridor between the south polar area and the Hellas basin. In total, ~294,000 km3 of material were eroded from the MPR in multiple episodes, i.e., not just in one catastrophic event. This might have contributed close to a third of the originally one million km3 of hummocky materials on the Hellas basin floor ( Bernhardt et al., 2016a ). Similar to Pityusa Patera, Peneus Patera formed as relatively deep-seated caldera. Activity related to Amphitrites and Peneus Paterae likely contributed to ridged plains formation and the associated volatile release as well as mobilization had significant environmental effects.