Subaqueous Mega-slides on the Floor of Hellas

Introduction: The floor of the Hellas basin (Hellas Planitia) features unique landforms which have defied unambiguous interpretation [1-5]. We suggest that extensive subaqueous mass movements have been involved in the emplacement and deformation of the basin floor deposits, followed by intensive eolian erosion. Topography, Landforms, and Previous Interpretations: Hellas Planitia is characterized by a rough central plateau (Alpheus Colles [AC] and Hellas Chaos [HC]) surrounded on the south, west, and north sides by lower terrain adjacent to the interior walls of Hellas Basin, including the lowest elevations on Mars (Fig. 1). The Hellas Basin probably hosted deep lakes during the Noachian and Hesperian [2, 6] Honeycomb and reticulate deposits. In the north and western portions of this marginal depression occur eroded and highly deformed " honeycomb " sediments (unit h of [2], AHh of [5] and Arr of [4]). Recent CTX images indicate this unit is more widespread than previously mapped, including exposures within depressions in the AC. The unit has been interpreted to be the result of soft-sediment deformation possibly due to the weight of blocks of ice from remnants of an ice-covered lake [2]. Alternatively, deformation has been interpreted to have resulted from penetrative cellular convection in soft sediment (salt-doming) or crustal doming [7]. The deformation has a strong structural grain oriented parallel to the Hellas rim and to the northwestern margin of the higher AC plateau (Fig. 2), and individual " cells " have dimensions of 5-10 km. Because of the low crater density, freshness of exposures, and interpretations of superposition relationships in MOC images, this unit has been interpreted to be the youngest basin-floor deposit in Hellas [2, 4, 5].. A similar, but topographically higher deformed " reticulate " unit (r of [2] and Hr of [5]) flanks the bottom of the western Hellas interior rim and likewise has a structural grain paralleling the rim. Alpheus Colles and Hellas Chaos plateau. The materials forming the interior Hellas plateau have been interpreted as a stack of Hesperian sediments of possible eolian, volcanic, fluvial, or lacustrine origin [1, 2, 4, 5]. A variety of subunits have been mapped based largely on morphology, but we emphasize the general patterns. The largest features are a series of irregular or elongate ridges up to 1 km high spaced about 100-200 km apart separating broad depressions either smooth-floored or filled with clusters of knobs