Lineated valley fill and lobate debris apron stratigraphy in Nilosyrtis Mensae, Mars: Evidence for phases of glacial modification of the dichotomy boundary

The Nilosyrtis Mensae region is important among dichotomy boundary fretted terrain outcrops, as it provides evidence of overprinting of ancient landscapes by a suite of glacial features, providing a composite view of the variety of midlatitude glacial modification processes that can occur during recent Martian ice ages. On the basis of a series of criteria developed for the identification of a glacial origin for lineated valley fill and lobate debris aprons, we interpret stratigraphic, topographic, and textural relationships between lineated valley fill and lobate debris apron morphological units as evidence of local and regional glacial overprinting of the landscape during the recent Amazonian. We document ( 1) stratigraphic relationships between lineated valley fill subunits, including the presence of apparently superposed and small-scale lobate features, ( 2) the regional integration and flow of lineated valley fill material, ( 3) lineated valley fill degradation, and ( 4) the nature and stratigraphic position of lobate debris aprons. These observations suggest multiple phases or episodes of midlatitude valley glacier activity. These observations, together with those of surface units elsewhere in northern Arabia Terra interpreted as glacially modified landforms, suggest the possibility of midlatitude glacial deposits extending over broad portions of the Martian dichotomy boundary within the past several hundred million years.