HR: 16:45h
AN: P34A-04    [Abstracts]
TI: Amazonian Mid-Latitude Regional Glaciation on Mars: Lineated Valley Fill, Lobate Debris Aprons and Plateau Deposits at the Dichotomy Boundary and Implications for Climate Change
AU: * Head, J W
EM: James_Head@brown.edu
AF: Brown University, Dept. of Geological Sciences, Box 1846, Providence, RI 02912 United States
AU: Marchant, D R
EM: marchant@bu.edu
AF: Boston University, Dept. of Earth Sciences, 685 Commonwealth Ave., Boston, MA 02215 United States
AB: The dichotomy boundary on Mars represents a distinct geologic, topographic, morphologic and crustal thickness boundary that is characterized by a wide range of modificational processes. The Deuteronilus-Protonilus region represents the area where the boundary reaches its highest latitude. The fretted terrain, located in the vicinity of the dichotomy boundary at these mid-northern latitudes on Mars, displays two enigmatic terrain types: lobate debris aprons (LDA) and lineated valley fill (LVF). The prevailing hypotheses for their origin has been mass wasting from mesa margins and valley walls, with movement periodically assisted by groundwater seepage or atmospheric vapor diffusion into the debris aprons, causing ice-assisted creep. Creep from opposite valley walls and convergence in valley centers has been called on to explain the parallel, along-valley lineations, and little evidence has been found for down-valley movement. New higher-resolution THEMIS and MOC data, however, show compelling evidence for a more integrated picture of LVF formation, suggesting a significant role for regional glaciation. We find evidence for: 1) localized alcoves, sources of hundreds of narrow, lobate concentric-ridged debris flows; 2) bulbous-headed tributary valley systems, which contain converging LVF that feeds into larger valley systems; 3) rounded-sharp-paired intersections of the corners of tributary entrances into main valleys, with sharp corners pointing down-flow; 4) narrow arete-like linear plateau ridge remnants, commonly parallel to LVF; 5) horseshoe-shaped ridges up-valley of topographic obstacles, with deformed and folded upslope LVF; 6) convergence and merging of LVF in the down-valley directions; 7) deformation, distortion and folding of LVF in the vicinity of convergence; 8) distinctive lobe-shaped termini where LVF emerges into the northern lowlands. We interpret these LVF features to have formed as parts of integrated valley glacial systems extending hundreds of km, with accumulation zones on plateaus and in alcoves, converging debris-covered valley glaciers eroding and altering pre-existing valley systems, and ablation zones at distal margins; observed today are sublimation residues preserving morainal remnants of down-valley glacial flow. On the basis of our mapping of the nature and extent of these deposits along the dichotomy boundary, we interpret them to be due to topographically induced adiabatic upwelling of water-rich polar air, cooling and precipitation at the dichotomy boundary to foster snow and ice accumulation, and consequent glacial flow. We are currently considering two end-member options for conditions of origin: 1) emplacement at high obliquity when polar ice deposits are mobilized and transported equatorward, and/or 2) during periods of temporary, outflow channel induced flooding of the adjacent northern lowlands, freezing and sublimation of these deposits, and subsequent atmospheric transport to the dichotomy boundary.
DE: 5416 Glaciation
DE: 5470 Surface materials and properties
DE: 6225 Mars
SC: Planetary Sciences [P]
MN: Fall Meeting 2005