Modification of Graben along the Dichotomy Boundary in Eastern Arabia Terra (coloe Fossae; 53-59°e, 37-41°n): Nature and Evolution of Lobate Debris Aprons and Their Relationships to Lineated Valley Fill and Glacial Systems

Introduction: The dichotomy boundary represents a fundamental transition between the northern lowlands and southern uplands. The most well-defined portion of the boundary occurs as a complex set of scarps and reentrants where the boundary extends into northern mid-latitudes (~25-47°N), between 0° and 80°E. A considerable portion of the boundary (northern part of Arabia Terra) is comprised of well-developed fretted terrain and fretted channels [1 and is the site of lineated valley fill (LVF) and lo-bate debris aprons (LDA) [2-3]. A framework for the interpretation of modification processes operating on and near the dichotomy boundary is provided by a detailed geological history and chronology of this region [4]; the fretted channels and valleys formed prior to middle Hesperian and 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. Major outstanding questions include: 1) the detailed structure and texture of the LDA; 2) the origin of structures in the LDA and LVF; 3) the direction of flow in LDA and LVF [2,5]; 4) the relationships between the LDA and the LVF; 5) whether LDAs represent single or multiple events; 6) the origin of the lubricating agent; and 7) the mode of origin of the LDA and LVF (e.g., ice-assisted rock creep, ice-rich landslides, rock glaciers, debris-covered glaciers) [2-8]. Recently, regional analyses of LVF [9-10] 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 studied an unusual region (36-42°N 53.5-58°E) characterized by a series of narrow linear graben (Coloe Fossae), extending for about 200 km NW-SE along and just south of the dichotomy boundary (Fig. 1). We compared the LDAs and LVF found here with areas characterized by mesas and broad sinuous valleys of the fretted terrain [11-13] and to Acheron Fossae [14] an area of LDA/LVF development outside northern Arabia Terra. We found it useful to subdivide LDAs into linear (occurring along valley walls and crater interiors) and circumferential (aprons generally surrounding isolated massifs) [12]. Detailed structure of …