The Spatial and Azimuthal Distribution of Lobate Scarps and High-relief Ridges on Mercury

Introduction: An understanding of the spatial and azimuthal distribution of lobate scarps on Mercury is important in constraining thermal history models and models for the origin of tectonic stresses [see 1]. Existing models involve either global (radial) contraction due to secular cooling of the interior, tidal despinning, or a combination of the two [1, 2, 3, 4, 5]. A comprehensive database of tectonic features on the hemisphere of Mercury imaged by Mariner 10 is being compiled to rigorously test these models. Map of Tectonic Features: We have digitized the locations of over 90 lobate scarps and high-relief ridges (Fig. 1). Lobate scarps and high-relief ridges are interpreted to be the surface expression of thrust faults and high-angle reverse faults [1, 2, 3, 6, 7, 8]. The structures were digitized from Mariner 10 image mosaics with improved geometric rectification [9]. Topographic data derived from Mariner 10 stereo coverage was also used where available [6, 7, 8]. The tectonic features were divided into segments with roughly uniform orientations generally greater than 5 km in length. Thus far, the locations of over 450 segments have been digitized and coded according to the structure-type and the geologic unit in which it occurs. The majority of the lobate scarps and high relief ridges identified occur in intercrater plains, with only a few of these structures found in smooth plains units. Wrinkle ridges, which occur in Calorian smooth plains units, were not included in this study. Analysis: The spatial distribution of the tectonic features was analyzed by plotting their cumulative length in 20° latitude bins, normalized by the area of the bins (Fig. 2). This clearly demonstrates that the distribution of lobate scarps and high-relief ridges is not uniform. The greatest cumulative length of these tectonic features occurs in the southern hemisphere. Over ~52% of the total cumulative length is between 30°S to 90°S (Fig. 2). The most dramatic result is the relatively large number of tectonic features near the region of the south pole. The greatest area normalized cumulative length of tectonic features is found between 70°S to 90°S (Fig. 2). In the northern hemisphere, there is a gradual decrease in the area normalized cumulative length from the equator toward the pole. However, the largest area normalized cumulative length of tectonic features in the northern hemisphere is in the north polar region (70°N to 90°N). The azimuthal distribution is also not uniform. A plot of …