POLYGONAL TERRAIN, UTOPIA
PLANITIA, MARSTopographic Analysis
The inferred drape folding associated with martian polygonal terrain results from differential compaction of the cover materials over an older surface with significant topography [McGill and Hills, 1992] or an undulating surface created by Rayleigh circulation [Lane and Christensen, 2000]. The surface relief produced by this differential compaction is a function of the relief of the buried basement floor, the average percent compaction and the total thickness of the cover deposit. For a given thickness of cover the surface relief will equal the basement relief times the average fractional compaction of the cover. Also, the percent compaction at any depth within the cover must be a function of the total overburden pressure. This means that percent compaction should increase with the depth of the cover deposit, and thus that the average fractional compaction should be proportional to cover thickness.
Throughout the polygonal terrains are found circular grabens that are inferred to overlie the rims of buried impact craters [Carr, 1981; McGill, 1986]. How these circular grabens form must be intrinsically linked to the formation of the polygonal terrains they are found in.
Circular grabens are the ideal locations to test for differential compaction. Basement relief beneath circular grabens can be modeled using empirical equations [Pike and Davis, 1984] which describe martian craters; surface relief can be determined with data from the Mars Orbiter Laser Altimeter (MOLA). Differential compaction models predict that the circular grabens will bound topographic depressions, because total cover thickness will be greater over the centers of completely buried craters than over their rims. Since large craters are deeper than small craters, the models also predict that surface relief will be proportional to ring fracture diameters.
Quasi-circular depressions (QCDs) are observed around Mars in the high-precision gridded data set of MOLA topography [Frey et al., 1999, 2000, 2001, 2002], many having no corresponding structural feature. It is proposed that these QCDs are surface representations of buried impact craters [Frey et al., 1999, 2000, 2001, 2002]. If so, the same reasoning explained above will apply, and in regions of comparable cover thickness, QCD surface relief should be proportional to diameter. Showing that this relationship holds true for QCDs throughout the northern lowlands supports the suppositition that QCDs, of which circular graben depressions are a subset, do represent buried impact craters. Since counting impact craters is a common way of determining the age of a unit, this verification could help determine the age of the buried basement floor of the northern lowlands, by substituting QCD counts for crater counts [Frey et al, 2002]. Furthermore, showing that the surface relief vs. diameter relationship holds throughout the northern plains, including the regions of giant polygons, supports differential compaction of the material covering the lowlands. Since cooling volcanics do not produce the horizontal strains [Cas and Wright, ] necessary for producing grabens the size of those in the polygonal regions, establishing the surface relief vs. diameter relationship also supports a sedimentary origin for the cover material, which would be consistent with the past existence of an “ocean” in the lowlands.
Modelling
The circular grabens in Acidalia Planitia are singular, but a majority of the circular grabens in southwest Utopia Planitia are comprised of two, nested ring fractures, and the published drape-fold models do not provide an explanation for this. Drape folding would increase the probability of fracturing over a drape anticline formed over a buried crater rim, but this should only produce one fracture. The spacing between the concentric rings does not correlate with diameter of the circular graben but does correlate with its proximity to the center of the Utopia basin [Buczkowski et al., 2003]. Many researchers [eg. Tanaka et al., 2001] have deduced that cover thickness should increase towards the center of the basin, thus suggesting a correlation between ring spacing and thickness of cover material.
The work proposed here is to determine if analytical and/or numerical models of compaction, drape folding and horizontal extension could 1) explain the doubling of circular grabens and 2) explain the observed dependency of graben spacing on cover thickness. It is postulated that two separate mechanisms act to produce the two grabens; drape folding associated with differential compaction yields a graben just inside of the crater rim and extension of the shrinking material covering the buried crater yields a second graben outside of the crater rim [Buczkowski et al., 2003].
Image Analysis
The Mars Orbiter Camera (MOC) produces images of exceedingly high resolution, as does the Thermal Emission Imaging System (THEMIS). There are structural features, such as relay structures, terminal and lateral ramps and antithetic faults, that should be present in the polygonal fractures if they are true grabens. Ramps and relay structures can indicate the nature of graben propagation. Determination of the dip of the fracture walls can also differentiate whether the fractures are grabens with inward dipping walls, or extensional cracks with nearly vertical walls.
Image analysis can also provide insight into the nature of the polygonal materials. If the material are sedimentary they should resemble the fine-grained, homogeneous, bedded material seen when sediments are deposited in standing water. On the other hand, the discovery of volcanic features would exclude a sedimentary origin of the giant polygons, unless they were obviously younger than the bounding fractures.