Emplacement and tectonic deformation of smooth plains in the Caloris basin, Mercury

Thomas R. Watters, Scott L. Murchie, Mark Robinson, Sean C. Solomon, Brett W. Denevi, Sarah L. André, James W. Head

Research output: Contribution to journalArticlepeer-review

49 Scopus citations


MESSENGER's first flyby of Mercury provided the first complete view of the Caloris basin and its interior smooth plains. Multispectral imaging shows that the interior plains are spectrally distinct from surrounding terrain, with a reflectance about 15-20% higher and a steeper spectral slope than the global average. The Caloris basin rim massifs and hummocky material, in contrast, have lower reflectance and a shallower spectral slope than the global average and extend around the entire basin. Isolated, relatively red patches along the margins of Caloris each surround irregularly shaped, scalloped-edged, rimless depressions that are interpreted to be volcanic vents, some with associated pyroclastic deposits. A nearly continuous annulus of smooth plains exterior to the basin displays spectral characteristics that contrast sharply with those of the basin-interior smooth plains. The exterior smooth plains are lower in reflectance and spectrally similar to the Caloris rim massifs and the hummocky materials. The rims and floors of larger impact craters in the interior plains expose low-reflectance material. There is widespread evidence of contractional and extensional tectonic deformation of the Caloris interior smooth plains. A set of linear radial graben, Pantheon Fossae, radiates outward from a zone near the basin center, and some of these graben extend to and intersect a set of generally basin-concentric graben distributed along the outer margin of the Caloris basin floor. The average areal extensional strain expressed by the mapped system of graben is ~ 0.08%. Wrinkle ridges also deform the interior smooth plains and predate both basin-radial and basin-concentric graben. The average areal contractional strain expressed by the mapped wrinkle ridges is ~ 0.07%. The density of wrinkle ridges on the western interior smooth plains is less than on the eastern interior plains, in contrast to the more even distribution of graben. A similar difference is seen between the density of wrinkle ridges on the western and eastern exterior smooth plains, with far fewer wrinkle ridges per unit area on the western annular plains. Superposition relations, spectral contrast, and the occurrence of volcanic vents and possible pyroclastic deposits all support a volcanic origin for the Caloris interior plains. Crater counts indicate that the Caloris exterior plains and hummocky material are younger than the Caloris rim and interior plains and are therefore also likely volcanic in origin. The contrast in spectral properties between the interior and exterior plains indicates a difference in the composition of the two expanses of volcanic material. Models for extensional tectonic deformation of the Caloris basin interior inferred from the distribution of graben viewed by Mariner 10 must be reevaluated to account for the discovery of Pantheon Fossae.

Original languageEnglish (US)
Pages (from-to)309-319
Number of pages11
JournalEarth and Planetary Science Letters
Issue number3-4
StatePublished - Aug 15 2009


  • Caloris basin
  • Mercury
  • geologic history
  • tectonics

ASJC Scopus subject areas

  • Geophysics
  • Geochemistry and Petrology
  • Earth and Planetary Sciences (miscellaneous)
  • Space and Planetary Science


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