Linear Lobate Debris Aprons, Piedmont-like Lobes, and Crater Fill in the Acheron Fossae Graben Region, Mars: Evidence for Debris-covered Glacier Formation and Flow

Introduction: Lineated valley fill (LVF) and lobate debris aprons (LDA) are widespread in fretted terrain in the northern part of Arabia Terra [1-3], but occur elsewhere in this latitude band in the northern hemisphere, such as at Acheron Fossae, a series of arcuate parallel graben on a rise north of Olympus Mons (217-237° E; 34-40° N) where the graben floors are characterized by viscous flow-like features [4] resembling LDAs and LVF. Questions associated with the analysis of LDAs and LVF are: 1) the range of environments in which LDAs form; 2) the relation of LDA to underlying topography; 3) the nature and direction of flow in LDAs; 4) relationships between the LDA and the LVF; and 5) the mode of origin of the LDA and LVF (e.g., groundwater-fed, ice-assisted rock creep, ice-rich landslides, rock glaciers, debris-covered glaciers) [2-9]. Recent analyses of LVF in the Deuteronilus Men-sae region [10-11] have shown evidence for local sources of LVF in alcoves in valley walls, down-valley flow, merging of flowlines into broad trunk valleys, extensive along-valley flow, and termination in lobate deposits, all features that are similar to valley glacial landsystems on Earth. In this study we analyzed the floors and walls of the gra-ben composing Acheron Fossae [4], examining the viscous flow-like features there and assessing their morphology, topography , relation to underling topography, slope and orientation. We also compared the graben floor structures to lobate deposits found on the pole-facing slopes of impact craters superposed on the Acheron Fossae region (Fig. 1). We previously [12] subdivided LDAs into linear (occurring along valley walls and crater interiors) and circumferential (aprons generally surrounding isolated massifs), a distinction we found useful here. Description of Viscous Flow Features in Acheron Fossae: The Acheron region lies on the NW flank of the Thar-sis rise and forms a broad north-facing topographic arch at least 700 km long, and rising 2-3 km above the surrounding plains; its eastern edge forms a broad dome-like high about 150 km in diameter. Acheron Fossae dissect this basic background topography, forming at least two sets of graben [4] in which viscous flow features of various types and stages of development occur. The Acheron terrain is mapped as Noa-chian in age, and the graben are embayed by plains materials of Hesperian and Amazonian age [13]. Thus, the graben themselves , although showing characteristics suggesting at least two generations of formation [4], have apparently not …