THE TECTONICS OF IMPACT BASINS ON MERCURY

Tectonic landforms on Mercury can be generally described as either globally distributed or basin-localized. Basin-localized tectonic landforms can be either contractional or extensional. The only clear evidence of extensional deformation on Mercury found to date is within three impact basins, Caloris (diameter D ~ 1550 km), Rembrandt (D ~ 715 km), and Raditladi (D ~ 250 km). The interior plains of Caloris and Rembrandt have diverse arrays of contractional wrinkle ridges and extensional troughs or graben. The interior plains of Raditladi contain basin-concentric graben confined within the peak ring, but they show no evidence of contractional features. In the Caloris basin, wrinkle ridges form basin-concentric and basin-radial patterns that are similar to those found in lunar mascon basins. Extension in Caloris is expressed as a complex pattern of basin-radial and basin-concentric graben. The radial graben of Pantheon Fossae radiate outward from a zone near the basin center, and some extend to basin-concentric graben distributed along the outer basin margin. In the Rembrandt basin, wrinkle ridges and graben also form a basin-radial and basin-concentric pattern. Basin-concentric wrinkle ridges in Rembrandt form an almost complete ring in the interior. Unlike the radial graben of Pantheon Fossae, the Rembrandt basin-radial graben are confined to a zone that extends inward from the interior ridge ring. Some radial and concentric graben in the basin form a polygonal pattern near the outer margin of the interior ridge ring, similar to the polygonal pattern formed by graben near the margin of the Caloris basin. The basin-radial graben and wrinkle ridges of Rembrandt, however, form a unique wheel-and-spoke pattern of tectonic landforms. The overall tectonic patterns in Caloris and Rembrandt are in sharp contrast to those in lunar maria where concentric and radial wrinkle ridges generally occur in the basin interior and concentric graben occur along the margins. Models for basin deformational history must be reevaluated to account for the patterns of tectonic structures seen in Mercury’s basins.