Other
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.p41a1355p&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #P41A-1355
Other
5415 Erosion And Weathering, 5416 Glaciation, 5419 Hydrology And Fluvial Processes
Scientific paper
Many Martian outflow channels and valley networks terminate at "contacts" between the southern highlands and northern plains that were identified based on Viking Orbiter images over 20 years ago and interpreted by Parker and others as regressive shorelines of an ancient Martian ocean. Accumulating high-resolution image data and gridded MOLA topography is enabling a reassessment of the elevations and morphologies of proposed shoreline contacts. Many of the better-preserved, lowest-elevation of these contacts exhibit lobate flow-front morphology suggestive of low-viscosity lava or debris flows advancing up the highland margin from the northern plains. As progressively more degraded, higher-elevation contacts are examined, the lobate flow-front morphology is less apparent, and can be found in association with terrace-like morphologies that resemble terrestrial strandlines. In the most-degraded contacts at higher elevations, flow morphology is largely or completely unrecognizable, giving way to topographic terraces on the highland margin that are evident only in the MOLA topography, and that resemble uplifted and abandoned terrestrial coastlines where most of the coastal morphology has been destroyed. The best-defined of the proposed shoreline contacts, the "Deuteronilus Level" or Contact 2 in Parker et al. 1989, exhibits lobate flow fronts at the margins that suggest a flow direction from the plains interior and up the flanks of the highlands to about an elevation of -3900m locally. This contact is the most extensively traced of the proposed shoreline features. For the most part, it does appear to define a formerly level surface, though it ranges from about -3900m elevation in west Deuteronilus Mensae, to -4200m in west Cydonia Mensae. Changes in elevation along the contact are typically abrupt, and associated with recognizable structural features such a wrinkle ridges, so vertical offsets of once-level features is a plausible explanation for these variations in elevation. In Parker et al 1993, high albedo lobate flow fronts were described that appear to have advanced up-channel in the northern Chryse region, and a similar margin was identified in western Utopia. These were interpreted as backfill deposits associated with ponding within the northern plains associated with the circum-Chryse floods. However, post-Viking high-resolution images from MGS, Odyssey, and MRO have revealed this material to be similar in texture to that in west Deuteronilus Mensae, and MOLA topography reveals the margin in this area to have as much as 50m of relief associated with the flows. Plainward of this margin, the topography drops off, suggesting lowering of the surface of the plains interior by up to several hundred meters (even kilometers, in the basin center) via removal of material. It is suggested, then, that these lobate deposits are debris-rich flow fronts at the margin of a late transgressive phase of a debris and ice-covered ocean that has since receded.
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