Tectonic Features in the Equatorial Lowlands of Mercury Viewed at High Incidence Angles

The spatial distribution of tectonic features on Mercury, although not fully understood, is related to the stress regime and the mechanical properties of the lithosphere during the time that the features formed and remained active. Lobate scarps and high-relief ridges, compressional features that generally have ~1 km of relief and are hundreds of kilometers long, were identified on Mercury from images acquired during the Mariner 10 and MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) flybys. Images taken from orbit during the primary MESSENGER mission, with full coverage of the surface, confirmed that these scarps and ridges appear to be concentrated in three broad, north-south bands. Images at high incidence angles, collected since April 2012 during the MESSENGER extended mission, provide a more complete picture of the spatial extent and orientations of these features, and of their relationship to neighboring landforms. Digital elevation models, from laser altimetry and stereo imaging, additionally allow for comparisons between tectonic landforms and elevation and for measurements of slope and relief across individual features. Scarps and ridges are found at a wide range of elevations on Mercury. The greatest concentration of such features in an equatorial lowland setting is in an area (40°N-40°S, 220°-270°E) that is within one of the three north-south bands of tectonic features. Within this area, the 48 previously mapped features generally do not display preferred orientations or a consistent relationship to topography. Of these scarps, 47 were identified in flyby images and one in orbital images. Three follow the rim of Beethoven basin (10°-30°S, 225-245°E, ~600 km diameter), likely having formed along earlier zones of weakness in the crust created during formation of the basin. From recent images taken at high incidence angles, which currently have ~75% coverage in this equatorial lowland area, we are able to identify only seven additional tectonic features, all within Beethoven basin. Six of these newly identified features are also subparallel to the basin rim. However, no other scarps in our study area are so clearly connected to a particular topographic or geologic feature. The 22 lobate scarps and high-relief ridges in the northeastern quadrant of our study area have similar base elevations (average of -0.78 km, standard deviation of 0.17 km) and relief. Maximum measured relief (along one scarp) averages 0.59 km (standard deviation of 0.13 km), with a median of 0.56 km. Additionally, the scarps often terminate at a neighboring scarp, in six cases such that the two scarps are tangent to each other, and in four cases such that they intersect at an angle of ≥45°. These similarities and relationships suggest that the 22 features may be tectonically linked and may have therefore formed as an assemblage within a relatively short interval of time. This assemblage of faults is located in an area of apparently limited lateral variation in crustal thickness, as indicated by crustal models consistent with long-wavelength topography and gravity. If the limited range in crustal thickness in this area was paralleled by a limited range in mechanical lithosphere thickness, this may have facilitated the formation of an assemblage of linked tectonic features.