Subglacial catastrophic-flood origin of linear and curvilinear flat-rimmed pit chains on Mars: Evidence from geomorphological mapping and detailed landsystem analysis

Flat-rimmed pits lacking emanating flow-like features on pit margins are typically clustered as linear or curvi-linear chains on Mars. Their formation has been attributed to faulting, volcanism, or roof collapse of subsurface voids. Here we test these end-member models by conducting landform analysis and systematic geomorphological mapping in the central Tharsis region. Our work shows that the mapped flat-rimmed pit chains were formed on a lava-plain landscape that is mantled by a surficial meter-sized boulder layer. The flat-rimmed pit chains are closely associated with a landform assemblage consisting of localized pit-linking/terminating dendritic networks of gullies, U-shaped troughs, isolated sinuous channels, sinuous round-topped boulder-bearing ridges, and rampart craters. The absence of pit-bounding and pit-cutting faults rules out the faulting origin, while the lack of rubble-filled pit floors and erupting flow-like landforms does not favor the volcanic and roof-collapse mecha-nisms. When viewed together, the mapped flat-rimmed pit chains and their closely associated landform assemblage can be explained by a glacial-landsystem model: (1) pits were created by vortex-induced erosion during repeated, violent, catastrophic mega-flooding below a regionally extensive ice sheet possibly covering the entire Tharsis region (i.e., the Tharsis ice cap), (2) the subglacial floods exploited bedrock and/or ice sheet fracture sets resulting in linear and parallel chains, (3) localized pit-linking and pit-terminating U-shaped troughs and round-topped boulder ridges represent subglacial channel bodies including glacial eskers, (4) surficial boulders represent glacial deposits, (5) local trough-bounded pit chains resemble coulees and their hosted rock basins on Earth, and (6) rampart craters with viscous-flow ejecta deposits represent surface or near surface ice generated by the inferred glaciation. We suggest that the excavated pit material was transported by subglacial megafloods along the giant (>6000 km long) circum-Tharsis outflow channels to the outwash plains in the northern-hemisphere lowlands.