Discussion and Conclusions

Morphology and Distribution

In many of the rose diagrams the dominance of hexagonal craters is manifested by peak intervals of about 60° (see, e.g., Fig. 8b and 9b). Most polygonal craters in greater Hellas region have a tendency towards hexagonal outline, whereas clearly square-shaped craters are more or less a rarity, even among small polygonal craters. Large square-shaped craters are absent. "Perfect" pentagons are no anomaly, although they are not nearly as common sight as "perfect" hexagons. Typical are also craters where two rim segments have "overgrown" turning a "would-be-hexagon" into a pentagonal crater. Several craters also exhibit features that justify the use of the name "octagonal" crater.

The distribution of polygonal craters (Fig. 1) shows that in greater Hellas region they are generally more abundant in the northern part of the study area. This results in part from the resolution of the images, which gets worse the further the image is from the equator. The principal reason for the distribution is simply the fact that in the southern part of the study area the total amount of craters is much smaller than in the north, mainly due to volcanic and fluvial deposits covering large areas around Hellas. We believe, however, that the target properties, especially the existence of prominent fracture directions, also play an important role in the distribution of polygonal craters. A good example of this is the concentration of polygonal craters in Hellespontus Montes, a known locality of intense tectonic modification (e.g., Wichman and Schultz 1989). Figure 13 displays the main tectonic features from the study area along with the information from the rose diagrams.

The fact that not all impact craters display polygonal features requires some attention. We are well aware that the distribution pattern of polygonal craters displayed in Fig. 1 has errors due to the subjectivity of recognizing polygonal craters. The true amount of polygonal craters in the study area is larger, since we only accepted craters, which were indicated as such by both researchers dealing with the particular block. Many polygonal craters were left out simply because of this subjectivity. Therefore, Fig. 1 displays only the minimum estimate of polygonal craters in greater Hellas region. In any case, a huge number of craters are more or less circular, and obviously not affected by, e.g., fracturing caused by Hellas and Isidis impact basins. Several factors can explain this. In some places, thick layers of younger sediments or volcanic deposits may have covered the ancient fractured crust so that impacts have not penetrated deep enough to be influenced by old fracturing. Some areas, like Malea Planum, might be composed of mainly pyroclastic material that has been too porous for a dominant fracture pattern to develop. In other localities, impacts and endogenic processes could have created so dense and complex fracture pattern that no dominating directions are present anymore (see Fulmer and Roberts 1963). Given the highly versatile nature of the geology in greater Hellas region, such local variations are not surprising.

Rim Strikes

The dominant east-west striking rim directions in Hellespontus Montes (Fig. 7b) and the area immediately to west from it (Fig. 7a) are easily explained as an indication of radial fracturing caused by the Hellas basin. The approximately north-south component, which is stronger closer to Hellas, most likely originates from concentric fracturing around Hellas. This Hellas concentric fracturing in Hellespontus Montes is a well-known phenomenon and can clearly be seen as roughly north-south trending grabens and scarps (e.g., Wichman and Schultz 1989; Leonard and Tanaka 2001). It is interesting to see that Hellas radial fracturing seems to extend further than concentric, or that at least radial fracturing is easier than concentric to see using polygonal craters.

The concentration of polygonal craters close to the Isidis basin and the fracture directions indicated by the crater rims can be understood as a result of intense radial fracturing related to Isidis basin. This can be seen best southwest from Isidis, where north-easterly rim directions strongly predominate (Fig. 8a and 8b). Southeast from Isidis the picture is not so clear (Fig. 9a and 9b, see also Fig. 11a and 11b), but the radial fracturing seems to exist also there. Weaker concentric fracturing also appears to be present on both sides of the Isidis basin.

Fig. 13. A sketch map of major geologic and tectonic features (basins, paterae and the volcanic plains, ridges, grabens, lineaments) of the study area with the rose diagrams presenting strikes of polygonal crater rims. Correlation between, e.g., the structural pattern of Hellespontus Montes or Hesperia Planum and the rim orientations is striking. Note the rim strikes radial and concentric to Isidis and Hellas basins, interpreted to indicate basin-centered radial and concentric fracturing, respectively. Shading denotes areas where rim strike measurements were made.

Fig. 13. A sketch map of major geologic and tectonic features (basins, paterae and the volcanic plains, ridges, grabens, lineaments) of the study area with the rose diagrams presenting strikes of polygonal crater rims. Correlation between, e.g., the structural pattern of Hellespontus Montes or Hesperia Planum and the rim orientations is striking. Note the rim strikes radial and concentric to Isidis and Hellas basins, interpreted to indicate basin-centered radial and concentric fracturing, respectively. Shading denotes areas where rim strike measurements were made.

According to Wichman and Schultz (1989), a trough and scarp system radial to Isidis can be traced over 1000 km southeast of Isidis. This distant indication of Isidis-centered deformation seems to appear also on our data as a peak of northwestern rim directions in Fig. 11a and 11b, although there are also other stronger peaks. This is interesting because the blocks 22°S/264°W and especially 22°S/248°W are situated mainly on the volcanic plains of Hesperia Planum, not on older highlands. Nevertheless, our results seem to support those gained by Wichman and Schultz (1989). The same northwesterly trend can also be seen in Raitala's study (1988; see also Crown et al. 1992) of the wrinkle ridges in Hesperia Planum. It is important to note in Fig. 11a and 11b that in these blocks a fracture system radial to Isidis has approximately the same direction as a fracture system concentric to Hellas would have, and vice versa. Therefore in this area it is impossible to differentiate between fractures radial to Isidis and concentric to Hellas using polygonal craters alone. It seems likely that the strong 020°-040° peak in the block 22°S/264°W (Fig. 11a) is caused by a fracture system radial to Hellas. This Hellas radial fracturing can also be present in block 22°S/248°W (Fig. 11b) as the 020°-050° peak. The numerous east-west trending rims in the blocks are tentatively interpreted as resulting from fracturing caused by the volcano-tectonism of Hadriaca and Tyrrhena Paterae. This prominent rim direction peak coincides with one major direction of wrinkle ridges in Hesperia Planum (Raitala 1988), emphasizing its significance.

The two blocks in the northeastern corner of our study area, 6°S/232°W (Fig. 10a) and 6°S/214°W (Fig. 10b), sample not only craters in old highland material, but also in the volcanic plains of Elysium Planitia. The rim strike peaks at 030°-040° are roughly radial to Hellas and could be induced by it. However, we feel that radial fracturing caused by the rise of Elysium Mons is a much easier way to explain this dominant fracture direction. The strong east-west trending peak in 6°S/214°W could also have its origin in the concentric fracturing caused by the volcano-tectonic evolution of Elysium Planitia. Another possibility is that the peak reflects the global dichotomy, since the boundary between the southern highlands and the northern lowlands trends also approximately east-west.

The relatively few polygonal craters on Malea Planum display a directional pattern (Fig. 12a-12d), which, on a first look, differs from other studied areas. However, the number of measurements in each four blocks in Malea Planum is very low - only about 10-20% of the number of measurements in other blocks - which may lead to random errors (57°S/328°W n=22 (Fig. 12a), 57°S/312°W n=18 (Fig. 12b), 57°S/296°W n=16 (Fig. 12c), 57°S/280 n=16 (Fig. 12d)). Thus, conclusions drawn from the data of polygonal craters on Malea Planum should be taken as speculative, since the number of measurements does not completely match the statistical criteria. The conclusion that can be drawn is that the distribution of rim directions does not appear to be completely random. One may, with some confidence, hypothesize that the direct influence of the Early Noachian Hellas impact itself is rather weak and that the major cause of the stresses in Malea Planum is related to the rise of the shield of Amphitrites Patera during Late Noachian - Early Hesperian. However, the effect of later Hellas-centered deformation on polygonal craters in Malea Planum cannot be ruled out, because the formation of probably basin-related wrinkle ridges continued into Early Hesperian (see, e.g., Leonard and Tanaka 2001 and references therein).

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