BETWEEN A ROCK AND A HARD PLACE: NOACHIAN HIGHLANDS, HESPERIAN LOWLANDS AND TYRRHENA PATERA

The circum-Hellas region of Mars has been volcanically active throughout much of Mars' history, although interest in the low-relief volcanoes in this region is often overshadowed by that in larger volcanoes elsewhere. Tyrrhena Patera (24°S, 252°W) is a volcano with low relief (<1800 m), shallowly sloping flanks (<2°) dissected by radial channels centered on a complicated caldera complex. The friable nature of the shield materials, and their great areal extent (>500 km from the summit) suggest that they are composed of pyroclastic flow deposits. Impact crater size-frequency distributions suggest that the shield materials are Late Noachian/Early Hesperian in age, although apparent mantling of the shield materials distally may obscure these results. Tyrrhena Patera shield materials appear to overlie, or embay, adjacent Noachian highlands to the west, but are embayed by Hesperia Planum materials to the east. In contrast, a large (~1000 km x 250 km) lava flow field, connected to the Tyrrhena Patera summit via a volcano-tectonic rille, shows an impact crater retention age of Late Hesperian/Early Amazonian. Thus, volcanic activity at Tyrrhena Patera began in the Noachian Epoch and may have extended into the Amazonian Epoch. Other highland paterae in the circum-Hellas region do not show extensive recent effusive activity: Hadriaca Patera displays what appear to be late-stage caldera-filling lavas; extant data covering Amphitrites and Peneus Paterae are not yet sufficient to identify lava flows on these volcanoes. The radial channels found on the circum-Hellas paterae do not incise adjacent lava flows and are therefore most likely from the Late Noachian/Early Hesperian Epochs (corresponding with the time of explosive eruptive activity at these volcanoes). Clearly, the volatile and magmatic histories of Mars are closely linked in this region.