A COMPARISON OF SHORTENING LANDFORMS ON MARS AND MERCURY

The surfaces of both Mercury and Mars abound with shortening landforms, which are widely accepted to have formed from a combination of thrust faulting and anticlinal folding. These landforms have traditionally been categorized by visual inspection into lobate scarps (LS) and wrinkle ridges (WR). LS are described to have asymmetric positive relief showing fore- and backlimbs with steep and gentle slopes, respectively. WR are reported to consist of broad ridges superposed by narrow ridges, i.e. the wrinkle. To compare between the categories and how they manifest on their host planets, we study morphometric measurements of 100 randomly selected shortening landforms each from Mercury and Mars. We followed standard practice to categorize them into LS (n=121) and WR (n=79). We sampled landforms with lengths, reliefs, and breadths ranging over two orders of magnitude, showing moderate correlations between these parameters on both planets for all structures. Our sampled LS are broader and have more relief than WR on both planets. Landforms on Mercury are broader and have more relief than those sampled on Mars, but relief-to-breadth ratios are remarkably similar for each category. This highlights that the shapes of LS and WR compare well between the two planets. We extracted the lengths and slopes of fore-, and backlimbs of all structures. Forelimb lengths are generally shorter than backlimb lengths for all landforms, but LS have much longer backlimbs than WR on both planets. LS forelimb slopes are most commonly measured at 2° to 6° with WR having even more shallowly sloping forelimbs. Backlimbs slopes are most common at ~1° across all landforms. Both length and slope parameters confirm that LS are more asymmetric than WR. These results also show that steep slopes are not a typical characteristic of any shortening landform sampled in this study but that LS forelimbs are less gently sloping than LS backlimbs. We use two multivariate statistical approaches to identify if our measurements can help us better quantify any differences between LS and WR and Mercury and Mars. Our findings confirm our previous planet-specific results that no combination of parameters can unequivocally indicate the difference between LS and WR and that those categories represent endmembers on a spectrum of shortening landforms that do not adequately capture the complexity of landform shapes observed on Mercury and Mars.