The topography and morphology of low shields and associated landforms of plains volcanism in the Tharsis region of Mars
The morphology and topography of volcanic landforms provide critical information to the investigation of their tectonic setting and the physical characteristics (e.g., rheology) of their eruption products. Their investigation is also an important prerequisite for studies of comparative planetology, (e.g., the comparison between surface features of the Earth and other planetary bodies). Numerous small and low shield volcanoes on Mars and associated vents and lava flows have previously been compared to terrestrial plains-style volcanism, which is defined as being an intermediate style between flood basalts and the Hawai'ian shields. This study investigates the topography and morphology of Martian landforms associated with plains volcanism using MOLA, MOC, THEMIS, HRSC, and HiRISE data. Low shields and other landforms of plains volcanism on Mars display similarities with terrestrial basaltic volcanic fields, and we do not observe any features that do not have morphologic analogues on Earth. The most typical landforms of Martian plains-style volcanism are low shields, defined as volcanic shields with diameters of typically less than 50 km, heights of a few hundred meters only, and extremely shallow flank slopes of less than 0.5°. Other surface features related to plains-style volcanism on Mars are craters, fissure vents, cinder and spatter cones, lava flows (that are commonly associated with lava channels and tubes), lava inflation features, and volcanic rift zones. Our results reconfirm the Viking Orbiter-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 Hawai'i, and other basaltic landforms also show similarities to Martian plains volcanism. Sinuous rilles, previously not described in association with plains volcanism on Mars, are interpreted as evidence for high eruption rates. The extremely shallow flank slopes of the low shields suggest the eruption of shield-building lavas with 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 spatial distribution of low shield clusters in Tharsis does not show any obvious association with large-scale tectonic features. Plains volcanism might represent a relatively recent type of volcanism on Mars, which is not related to mantle plumes but to a zone of partial melting in an anomalously warm mantle underneath a thickened crust (Schumacher and Breuer, Geophys. Res. Lett. 34, L14202, doi: 10.1029/2007GL030083, 2007).