Lobate scarps and the Martian crustal dichotomy

Landforms reflecting crustal shortening are found in the ancient highlands of the eastern hemisphere of Mars. These structures, referred to as lobate scarps, are interpreted to be thrust faults. Lobate scarps occur near and are oriented roughly parallel to the Martian crustal dichotomy, a major geologic and topographic boundary that divides the heavily cratered highlands from the relatively smooth, featureless northern lowlands. The long- and short-wavelength topography of lobate scarps in the northern Terra Cimmeria-Amenthes region have been analyzed using photoclinometry, Earth-based radar altimetry, and Mars Orbiter Laser Altimeter data. The measured relief of lobate scarps in this region ranges from ∼110 to 1230 m, and they occur on gentle regional slopes that dip both toward and away from the dichotomy. Estimates of the horizontal shortening across the lobate scarps studied range from roughly 0.24 to 2.6 km (n = 9), assuming fault plane dips of 25°. The displacement-length (D-L) relationships of thrust faults associated with the lobate scarps are consistent with those observed for terrestrial fault populations. The compressional strain in the heavily cratered highlands near the dichotomy, determined using the D-L data for the lobate scarps, is estimated to be ∼0.17%. Topographic data indicate that the dichotomy in the northern Terra Cimmeria-Amenthes region has a distinct topographic signature. The spatial and temporal relationship of the lobate scarps to the boundary suggests that they are related to its formation, supporting models for a tectonic origin of the crustal dichotomy.