PLAINS VOLCANISM ON MARS REVISITED: THE MORPHOLOGY AND TOPOGRAPHY OF LOW SHIELDS AND ASSOCIATED LANDFORMS

The morphometry of volcanoes provides critical input to the investigation of their tectonic setting and the rheology of their eruption products. It is also an important prerequisite for studies of comparative planetology, e.g., the comparison between terrestrial and planetary surface features. Numerous small and low shield volcanoes on Mars and associated vents and lava flows have previously (e.g., Plescia, 1981) been compared to terrestrial plains volcanism, as defined by Greeley (1982) to be intermediate between flood basalts and the Hawaixian shields. However, the Martian low shields got relatively little attention so far, and despite some work on their topography (e.g., Sakimoto et al., 2003), not many thorough analyses have been carried out so far. This study investigates the topography and morphology of the low shields and associated landforms using MOLA, THEMIS, and MOC data. They display many similarities with terrestrial basaltic volcanic fields, and so far we did not identify any features that are unknown from Earth. They include relatively small and low shields, lava flows, which are often associated with lava channels and –tunnels, and volcanic rift zones. Our results reconfirm the Viking-based conclusion that plains volcanism in the eastern Snake River Plains is perhaps the best terrestrial morphological analogue for these Martian surface features. Icelandic shields, distinct structures in Hawaixi, and other basaltic landforms also show some similarities to Martian plains volcanism. Landforms previously not known from the low shields are sinuous rilles, interpreted as evidence for high eruption rates, spatter cones, and inflation features. Extremely shallow flank slopes of less than 0.5° suggest the eruption of lavas with very low viscosity, which might be the result of high eruption temperatures, high effusion rates, a low Si- and a high Mg-content along with a possible high Fe-content, or a combination of these factors. The distribution of low shields in Tharsis does not show any obvious association with large-scale tectonic features. They might represent recent volcanism, which is not related to mantle plumes, but to a zone of partial melting in an anomalously warm mantle underneath a thickened crust (see the new model recent Martian volcanism of Schumacher and Breuer, 2006, 2007).