Evidence of Low Northern Midlatitude (~33n) Valley Glacier Deposits along the Dichotomy Boundary: Nilosyrtis

Introduction: Lineated valley fill (LVF) and lobate debris aprons (LDA) form in association with fretted terrain and fretted channels in the northern part of Ara-bia Terra [1-3]. Geological mapping in the region [4] shows that LVF and LDA represent Amazonian-aged modification of the fretted topography, which itself formed prior to middle Hesperian. Expanding on [1-11], we report observations of LVF in a multiple-tributary valley system at the eastern end of the Arabia Terra-Terra Sabea portion of the dichotomy boundary, near Nilosyrtis Mensae (68.5°E, 33.5°N (Fig. 1). Questions about LDA/LVF include: 1) the direction of flow in LDA/LVF; 2) the extent and continuity of flow in LVF [5]; 3) relationships between LDA and LVF; and 4) the origins of the LDA/LVF [2-11]. Recent analyses of LVF in northern Arabia Terra [10-11] have shown evidence for local sources of LVF in alcoves in valley walls, down-valley flow, merging of flowlines into trunk valleys, along-valley flow, and termination in lo-bate deposits; all features similar to valley glacial systems on Earth. The unique physical setting and relatively low latitude of the region under study (Fig. 1) provides information on lobate LVF, the confluence of multiple sources of LVF, potential LVF stratigraphy, stages of evolution of LVF, and LDA/LVF origin. Our observations point towards an improved understanding of the origins of LVF deposits in the context of terrestrial debris-covered glacier analogs [e.g., 12,13] and of their surface textures [9,14]. Description of Nilosyrtis Mensae LDA/LVF: The valley system in Nilosyrtis Mensae (Fig. 1) is composed of four radially linked valleys with a common exit valley , which is in turn linked to the Nilosyrtis Mensae valley system. The valleys are ~5-10 km long, ~5 km wide, and ~1.5 km deep. In the southernmost valley terminal alcove (Fig. 1), an LDA with pitted distal margin is observed extending down-slope at the head of the valley (Fig. 1); this appears superposed on LVF in the flatter part of this valley floor. We interpret these observations to mean that the most recent deposit is the LDA, but at earlier times, the LDA expanded and deformed into LVF as is seen elsewhere along the dichotomy boundary [e.g., 15-17]. Closely spaced lineations interpreted as convergence of flowing material are observed at the juncture of the radially linked valleys (Figs. 1-2). Here flow extending out of valley 2 is compressed by flow emerging from valley 1; this then merges with material from …