Global volcanism and tectonism on Mercury: comparison with the Moon

Both morphologic and tectonic studies indicate that Mercury and the Moon have quite different internal histories, despite their apparently similar morphologies. The evaluation of the volcanic surfaces indicates a decreasing volcanism on Mercury at the largest impacting time, despite short and local reactivations. On the Moon, the basaltic volcanism was increasing at the same time and continued for 1 billion years. That indicates a strongly different thermal evolution for these two planetary bodies.A widespread graben pattern is present on the Moon, with a statistical dominance of radial or tangential orientation with respect to the Imbrium basin, thus suggesting a relation between this major basin and the expansion of the Moon.Azimuthal studies show that the compressive structures, observed on the stereographic covered surface of Mercury are not randomly oriented, but seem radial towards the Caloris basin, thus indicating a possible influence of this largest basin on Mercurian contraction.The qualitative and quantitative formulations of these tectonic perturbations induced by large basins will be developed in a companion paper [1].     

Far-field tectonics associated with a large impact basin: applications to Caloris on Mercury and Imbrium on the Moon

Lithospheric readjustments after the formation of a large impact crater have been computed. The models predict tectonic perturbations over a major portion of the surface of the planet. The weight of the ejecta and the mechanical perturbations in the crater area give rise to membrane stresses. If the planetary lithosphere is in compression, the direction of maximum compression after the impact is perpendicular to a meridian (in a referential where the basin would be at the North pole). On Mercury the preferential orientation of compressive scarps radial to Caloris is explained in this manner. For a planet in extension grabens perpendicular and parallel to a meridian can appear successively as the mechanical state of the crater area is modified. On the Moon this result is consistent with the large-scale tectonics associated with Imbrium.