Amphitrites-peneus Paterae / Malea Planum Geology

Introduction: The Amphitrites – Peneus Paterae complex lies on the southern rim of Hellas in Malea Planum [1]. It is a low relief volcanic construct with two large calderas. A volcanic origin was proposed on the basis of Mariner 9 and Viking data [2, 3, 4]. MOLA altimetry and MOC images provide new insight into the volcanic, aeolian and polar processes that have occurred in the region. Fig. 1 illustrates the shaded MOLA-based topography. Background: The Amphitrites-Peneus volcanic complex has two large calderas surrounded by ridged plains and cratered highland beyond. Wrinkle ridges, having several hundred meters of relief, are oriented in a strongly radial pattern around Amphitrites and less strongly around Peneus and Barnard crater. [2] mapped the area as " furrowed shield material " and interpreted it to be old volcanic shields formed by low-viscosity lavas. He identified four calderas: Amphitrites Patera, Barnard, an unnamed one east of Amphitrites and a fourth north of Amphitrites. [3, 4] mapped the surface as " shield material " and defined two calderas: Peneus Patera and an unnamed feature immediately to the north. He also identified a volcanic dome, " Australis Tholus ". [1] map only Amphitrites and Peneus Paterae as calderas. MOLA Interpretation: Based on the MOLA altimetry (Fig. 1) Amphitrites and Peneus Paterae are well-defined calderas located on local topographic highs. Barnard (61.654°S, 298.586°W) is a 120 km complex impact crater, not a volcano. " Australis Tholus " (57.503°S, 322.196°W) is an 11.7 km impact crater. The proposed caldera north of Peneus [2] (56.183°S, 307.511°W) is a circular ridge having a few hundred meters of relief 122 km in diameter. Its eastern side is a piece of older terrain; the western side is an arcuate wrinkle ridge. It is not a caldera and may simply be a buried crater. The other unnamed caldera adjacent to Amphitrites is not observed. Peneus Patera (58.124°S, 307.455°W) is 125 x 136 km, has a relatively flat floor and is surrounded by arcuate faults that step down into the caldera; fault blocks have 100-200 m of offset. The surrounding plains ramp up toward the caldera edge over distances of 50 km. Elevations are highest on the southwest rim (1.1 km) and lowest on the northern rim (500 m). The floor lies at 325 m, 150-800 m below the rim.