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

Abstract

Flat-rimmed pits lacking emanating flow-like features on pit margins are typically clustered as linear or curvilinear 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 mechanisms. 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.