LINEATED VALLEY FILL AT THE DICHOTOMY BOUNDARY ON MARS: EVIDENCE FOR REGIONAL MID-LATITUDE GLACIATION

The fretted terrain, located at mid-northern latitudes on Mars along the dichotomy boundary, 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 movement down the valley axis. 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, as 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 intersections of the corners of tributary entrances into main valleys, with corners pointing down-flow; 4) narrow arete-like plateau 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.