Thomas R. Watters

Radar and photoclinometric studies of wrinkle ridges on Mars

Earth-based radar altimetry and image derived photoclinometric profiles were analyzed to examine both the long- and short-wavelength topography associated with wrinkle ridges on Mars. Photoclinometrically derived elevation data across wrinkle ridges were evaluated to determine the sensitivity of profiles to two empirical photoclinometric parameters, the horizontal digital number (HDN) and the scattered light value (SLV). The photoclinometric profiles are extremely sensitive to small variations in HDN. The sense of slope of a profile can be completely reversed over a range in HDN of as little as ±1. Comparably small variations in the SLV have relatively minor effects on the photoclinometrically derived elevations. The existence of elevation offsets from one side of the ridge to the other, reported in previous photoclinometric studies of martian wrinkles, were not confirmed through photoclinometry. In addition, no evidence of elevation offsets were found in Earth-based radar altimetry profiles across wrinkle ridges. In order to more accurately model wrinkle ridge topography, we controlled photoclinometrically derived elevations with long-wavelength topography obtained from the radar altimetry. The results of this study do not support kinematic models for the origin of planetary wrinkle ridges that involve deeply rooted thrust faults which separate crustal blocks at different elevations. A kinematic model involving buckling of shallow crustal layers into concentric folds that close, leading to the development of thrust faults, is consistent with wrinkle ridge morphology and terrestrial analogs. Recent geophysical studies of terrestrial analogs and the influence of shallow subsurface structures, particularly buried craters, on the localization of many wrinkle ridges on Mars suggest that thrust faults associated with the ridges are confined to the ridged plains material and do not extend into the lithosphere.

Planetary Tectonics: The tectonics of Mercury

Summary Mercury has a remarkable number of landforms that express widespread deformation of the planet’s crustal materials. Deformation on Mercury can be broadly described as either distributed or basin-localized. The distributed deformation on Mercury is dominantly compressional. Crustal shortening is reflected by three landforms, lobate scarps, high-relief ridges, and wrinkle ridges. Lobate scarps are the expression of surface-breaking thrust faults and are widely distributed on Mercury. High-relief ridges are closely related to lobate scarps and appear to be formed by high-angle reverse faults. Wrinkle ridges are landforms that reflect folding and thrust faulting and are found largely in smooth plains material within and exterior to the Caloris basin. The Caloris basin has an array of basin-localized tectonic features. Basin-concentric wrinkle ridges in the interior smooth plains material are very similar to those found in lunar mascon basins. The Caloris basin also has the only clear evidence of broad-scale, extensional deformation. Extension of the interior plains materials is expressed as a complex pattern of basin-radial and basin-concentric graben. The graben crosscut the wrinkle ridges in Caloris, suggesting that they are among the youngest tectonic features on Mercury. The tectonic features have been used to constrain the mechanical and thermal structure of Mercury’s crust and lithosphere and to test models for the origin of tectonic stresses. Modeling of lobate scarp thrust faults suggests that the likely depth to the brittle–ductile transition (BDT) is 30 to 40 km. Plausible thermal and mechanical structures for Mercury at the time of faulting suggest an elastic thickness Te of 25 to 30 km and a heat flux of roughly 30 mWm −2 . The thickness of the crust is also constrained to be <140 km. A combination of despinning and thermal contraction