The origin of conical mounds at the mouth of Chasma Boreale

One-hundred and six isolated conical mound landforms were identified proximal to the north polar cap of Mars at the mouth of Chasma Boreale. Five unique morphologies were identified including moderately sloped (4°-6°) cratered mound forms including two mounds of Abalos Colles, flat-topped moderately sloped (4°-6°) polygonal mound forms, steeper (6-13°) conical mound forms, subdued mound forms with low-angle slopes (0.5-3°), and small (<2 km diameter) clustered conical forms with low- to moderate-angle (1-5°) flank slopes. From first-order relationships between flank slope and the volcano productivity index (volume/diameter ratio), 16 mounds were found to be morphologically similar to terrestrial low-angle basaltic shield volcanoes. Thirteen mounds exhibit summit craters that range in depth (d) to diameter (D) ratio from 1:9 to 1:320. The circularity and d/D ratios of mound summit craters are consistent with highly modified northern plains impact craters. Image analysis of shield-like mounds reveals morphologic characteristics inconsistent with pristine volcanic landforms for most features. Horizontal flank layering (10-100 m thick), polygonal map-view morphology, topographic relationships, and spatial association with retreating polar scarps suggest that mound forms at the mouth of Chasma Boreale represent remnants of a once thicker, more continuous sequence of near-polar layered material. Evidence for northward scarp retreat is visible along Rupes Tenuis, a sinuous scarp located west of Chasma Boreale. Along this scarp, a single ∼100-m-thick resistant cap unit (part of the Early Amazonian age Rupes Tenuis unit) is being undermined by aeolian erosion, solar ablation, and mass wasting. Remnants of the cap unit rest along the scarp margin and in places form small conical mound clusters at the base of the scarp. Fracture controlled retreat of the scarp is visible along the eastern portion of Rupes Tenuis and along the Escorial crater remnant mesa where small tongue-like promontories have been sculpted into the cap unit. Isolation of these irregularities by scarp retreat has formed several volcanic-like scarp and mesa proximal mounds. Mound forms located closest to the retreating Rupes Tenuis scarp above 78°N were found to be the steepest and tallest mound landforms in the region. Mounds located distal to Rupes Tenuis were found to be broad and subdued. This observation is consistent with long-term retreat of a once more extensive Rupes Tenuis scarp and long-term modification of isolated conical remnants.