Modification of the Walls of a Noachian Crater in Northern Arabia Terra (24°e, 39°n) during Northern Mid-latitude Amazonian Glacial Epochs on Mars: Nature and Evolution of Lobate Debris Aprons and Their Relationships to Lineated Valley Fill

Introduction: Fretted terrain and fretted channels are very well developed in the northern part of Arabia Terra [1] and are the site of development of lineated valley fill (LVF) and lobate debris aprons (LDA) [2-3]. The detailed geological history and chronology of this region provide [4] a framework for the interpretation of modification processes operating on and near the dichotomy boundary. This study [4] underlines the conclusion that fretted channels and valleys formed early in Mars history (prior to middle Hesperian) and that the associated LVF and LDAs represent Amazonian-aged modification of the fretted topography. The LDA and LVF are characterized by very low slopes and thus have historically been interpreted to be related to viscous gravity flow of debris lubricated by groundwater or ice, with ice thought to have been supplied by vapor diffusion and condensation in slope talus. Uncertain has been 1) the direction of flow in LDA and LVF, either normal to the valley [2], or possibly parallel (down-valley) [5]; 2) detailed structure and texture of the LDA; 3) the relationships between the LDA and the LVF; 4) the relationship of LDA and LVF to adjacent walls; 5) the origin of the lubricating agent; 6) the mode of origin of the LDA and LVF (e.g., ice-assisted rock creep, ice-rich landslides , rock glaciers, debris-covered glaciers) [2-6]; and 7) the implications for the geological history of Mars. Recently , regional analyses of LVF [7-8] 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 analysis we mapped a ~50,000 km 2 area just north of an area of LVF interpreted to be a valley glacial landsystem (Fig. 1) and addressed the questions listed above. We found it useful to subdivide LDAs into linear (occurring along valley walls and crater interiors) and circumferential (aprons generally surrounding isolated massifs). 1) Direction of flow in LVF: In the mapped region, flow directions are away from the massif walls in circum-ferential LDA (Fig. 2) and away from valley and crater walls in linear LDA (Fig. 3); however, when massif/wall LDA/LVF meets in central valley floors (Fig. 4) or between massifs (Fig. 5) it merges and commonly follows the local topographic gradient to create along-valley flow, forming complex folds and …