Digital Elevation Models Aid the Analysis of Flows at Hrad Vallis, Mars

Abstract:

We have identified several landforms in the Hrad Vallis region of Mars (33.0^o – 35.5^oN, 216^o – 218^oW), which suggest that this area was covered by an ice sheet concurrent with volcanic eruptions. Using digital elevation models derived from High Resolution Imaging Science (HiRISE) and Context Camera (CTX) data, a reexamination of the area reveals a complex history including flow inflation and topographic control by transient topographic barriers. Among Amazonian-age outflow channels, Hrad Vallis is exceptional as it exhibits good evidence of magma/water interactions. It is inferred to have formed in association with a shallow igneous sill that melted part of the martian cryosphere and/or released water from an extensive aquifer to produce enormous lahar-like mud flows. Exposed ~30 m high dikes, 20 m high eroded mounds, and flow paths that are inconsistent with present-day topographic gradients, lead us to speculate that this area was covered by at least ~40 m of material (eolian deposits or ice) at the time of volcanic dike intrusion and flow emplacement. This material was subsequently removed leaving no clear morphologic signs (e.g., wind streaks, if eolian material; moraines, if ice). We favor the ice model because if this area was once ice-covered, it offers a plausible mode of formation (as pingoes) for some enigmatic 30 m high domes in the vicinity. At least one 120 km long flow from Hrad Vallis was emplaced as a pahoehoe-like flow that was confined by topographic obstacles and subsequently inflated to thickness of ~45 m. Although the direct relationship between this flow and Hrad Vallis remains to be determined, the inflated flow suggests a longer period of eruption/emplacement at a slower effusion rate than was previously believed.