Lineated Valley Fill and Lobate Debris Aprons in the Deuteronilus Mensae Region, Mars: Implications for Regional Glaciation

Introduction: Mamers Valles is a fretted valley along the dichotomy boundary in the Deuteronilus Men-sae region of Mars (Fig. 1). Analyses of Mamers Valles from Viking images [1-2] characterized it as a fretted channel filled with textured terrain. The channel is interpreted to have formed in the Early Hesperian or earlier [3-4], however, the terrain filling the valley is recent, as young as Late Amazonian [5-7]. Depending on the morphology of this terrain, it came to be known as either lineated valley fill (LVF), or lobate debris aprons (LDA). LDA and LVF surround massifs and fill valleys, troughs, and large craters throughout the northern and southern mid-latitudes. More recent studies have proposed a variety of different models for the formation of LVF and LDA [1-5, 8-9], including 1) debris flow lubricated by interstitial ice, and 2) debris-covered glacier. Here we map and compare both LVF and LDA to assess their mode of origin. Observations: We find that lobate debris aprons and lineated valley fill represent two endmember morphologies of the same material. We have observed the type morphologies of LDA and LVF as well as deposits that are apparently transitions between the two (Fig. 1, A-F). A typical LDA is lobe-shaped in map view, with linea-tions generally parallel to the outer lobe-shaped margin and flow direction normal to the massif or valley wall from which it emanates (e.g., Fig. 1, A and B). Typical LVF displays lineations parallel to valley walls and flow direction also parallel to valley walls; flow direction is interpreted by the orientation of lobes from alcoves in valley walls becoming entrained in the LVF (e.g., Fig. 1, E). Figure 1 C and D show deposits that appear to be intermediate stages between LDA and LVF, having characteristics of both. C displays linear LDA [10], lobes emanating from the walls of Mamers Valles, with internal lineations parallel to the overall lobe shape. In C, in contrast to B, when the lobes meet in the center of the valley, they clearly deform and begin to flow down the local topographic gradient, parallel to the valley walls. D, in Ismenius Lacus, ~600 km east of Mamers Valles, shows further that LDA lobes from the valley wall meet, deform, and flow down the local topographic gradient until the lobes are indistinguishable from each other, internal lineations and flow direction become parallel to the valley walls, as in LVF. LVF can …