A. C. Cook

TOPOGRAPHY OF LOBATE SCARPS ON MERCURY : NEW CONSTRAINTS ON THE PLANET'S CONTRACTION

Lobate scarps are landforms on Mercury that appear to have formed by thrust faulting and are thought to reflect global contraction due to cooling of the planet’s interior. Topographic data for 10 mercurian lobate scarps, derived from Mariner 10 images using photoclinometry and digital stereoanalysis, indicate a range in maximum relief of 0.1 to 1.5 km and a range in horizontal shortening of 0.3 to 3.2 km, assuming fault-plane dips ( θ) of 25°. Previous estimates of cumulative compressional strain recorded in the lobate scarps suggest a decrease in Mercury’s radius of 1 to 2 km. Our estimate of compressional strain based on these new topographic measurements is ~0.056% (θ = 25°). This suggests that Mercury’s radius decreased by <1 km.

Volcanic rifting at Martian grabens

[1] A large fraction of surface extension on Mars occurred at segmented grabens having width/length ratios akin to oceanic rifts on Earth. Association with volcanic landforms such as pit craters clearly suggests interconnection between tectonic and magmatic processes. A Martian rift evolution model is proposed on the basis of new geomorphological and structural interpretations of imagery, high-resolution digital elevation models (DEMs), scaled experimental modeling and three-dimensional boundary element modeling of magmatic and tectonic processes, and a comparison with terrestrial rifts. The DEMs were obtained from Mars Observer Laser Altimeter, Viking Orbiter stereo images, or a combination of both. Comparison of terrestrial rifts included Afar, Iceland, and the East Pacific Rise. The ambient extensional stress field induced by regional body forces is combined at depth with decompression melting and mantle plume thermal anomaly, resulting in emplacement of elongated magma reservoirs along the grabens. Injection of dikes above the reservoirs and flood basalt eruption result in a reservoir underpressurization of up to hundreds of MPa and induces surface collapse. Each collapse event is associated with an eruption of volcanic volumes akin to those of individual flow eruptions in large terrestrial igneous provinces. The geometry and mechanisms of graben formation and surface collapse are described and used to infer reservoir depth and width. We conclude from this study that giant dike swarms akin to typical giant terrestrial dike swarms are unlikely to underlie the volcanic Martian grabens on the basis one graben-one dike. Rather, every volcanic graben segment appears to own a local dike swarm perhaps analogous to the dike swarms in the Icelandic fissure zones. INDEX TERMS: 5475 Planetology: Solid Surface Planets: Tectonics (8149); 5480 Planetology: Solid Surface Planets: Volcanism (8450); 6225 Planetology: Solar System Objects: Mars; 8121 Tectonophysics: Dynamics, convection currents and mantle plumes; 8010 Structural Geology: Fractures and faults; KEYWORDS: rifting, graben, pit crater, magma chamber, dyke

Lunar polar topography derived from Clementine stereoimages

Clementine stereoimagery has been used to produce digital elevation models of the Moon, at a scale of ∼1 km/pixel. These models cover regions poleward of 60° in both hemispheres and reveal topography beyond that covered by the Clementine laser altimeter or Earth-based radar. By combining these polar terrain models with the current Clementine laser altimeter data we have produced a global topographic map of the lunar surface. Several topographic features in the new polar topographic data set are described, including three previously unrecognized pre-Nectarian impact basins. Several known basins have also been mapped, including the southern extent of the South Pole-Aitken basin, and other previously suspected basins have been verified.

Large-scale lobate scarps in the southern hemisphere of Mercury

Abstract Utilizing Mariner 10 images of Mercury, we derived a digital elevation model to examine the topography of the large-scale lobate scarps Adventure Rupes, Resolution Rupes, and Discovery Rupes. The thrust faults that formed these landforms occur along a rough arc that extends for over 1000 km . The new topography shows that vertical uplift occurred on the same side of the three structures suggesting that the fault-planes all dip to the concave side of the arc. These data also show that Adventure and Resolution Rupes are topographically continuous, suggesting the two features were formed by a single thrust fault on Mercury. If this is the case, the Adventure–Resolution Rupes thrust fault is comparable in scale to the Discovery Rupes thrust fault. It is generally believed that Mercurian lobate scarps were formed by compressional stresses induced in the crust as the planets interior cooled and shrank. Global contraction models predict that stresses at the planetary surface are horizontally isotropic (horizontal principal stresses being equal) resulting in randomly distributed thrust faults with no perferred orientations. The location, orientation, and geometry of the Discovery and Adventure–Resolution Rupes thrust faults, may not be randomly distributed. Analysis of the inferred stresses that formed these faults suggests that they were influenced by regional stresses or by mechanical discontinuities in the crust possibly caused by buried impact basins. The new topographic data reveal a broad, roughly circular topographic low interpreted to be an ancient impact basin centered near Schubert crater (43°S, 54°W), not far from an inferred stress center (48°S, 58°W). Thus the Discovery and Adventure–Resolution Rupes thrust faults may have been influenced by mechanical discontinuities in the Mercurian crust introduced by ancient buried impact basins.

Mariner 10 stereo image coverage of Mercury

A computer search was performed on all useable Mariner 10 images of Mercury for which refined camera positions and orientations are available. Over 2000 valid stereo pairs have been found. The results of this search allowed us to construct a map illustrating the available stereo coverage and the corresponding stereo height accuracy across the surface of Mercury. Examples are given to illustrate the quality of the available stereo pairs and the resulting digital elevation models.