A STRUCTURAL MAP OF THE RHEASILVIA BASIN ON VESTA

The Rheasilvia Basin is a 450-km-diameter impact crater at the south pole of Vesta. This diameter is ~85% of the mean diameter of the asteroid, thus occupying most of the southern hemisphere. Poles of vertical planes defined along the Divalia Fossae, large-scale troughs encircling two-third of the equator, cluster near the center of the Rheasilvia impact basin at the south pole, which was interpreted as evidence for an impact-induced origin of these troughs. We produced a structural map of the basin that shows various types of structures, including scarps, ridges, and lineaments. The mapping was performed on the 60 m/pixel Dawn Framing Camera Global Mosaic and four hillshade images computed with different solar azimuths derived from the ~93 m/pixel global digital terrain model at a fixed scale of 1:200000 using a polar stereographic projection. The mapped structures were further examined with slope and aspect maps to ensure consistency of classification. Length-weighted rose diagrams were plotted to visualize the orientations of each structure type across the basin floor in 36 bins. We also included the Divalia Fossae in our map to investigate any cross-cutting relationships with the basin. We identified 30 basin-bounding scarps with a total length of 1,311 km, 563 scarps within the basin with a total of 8,586 km, 494 ridges with a total of 5,814 km, and 4,393 undefined lineaments with a total length of 29,340 km. The scarps show a pervasive spiral pattern within the basin. The rose diagrams quantitatively capture this pattern with the scarps furthest from the basin center showing distinctively concentric orientations, but their orientation increasingly and systematically deviating from concentric orientations the closer they are located toward the basin center. The majority of ridges are concentric to the basin, whereas no distinctive pattern is observed for the lineaments. The southernmost trough of Divalia Fossae clearly crosscuts the Rheasilvia basin structures. Thus, the trough must have formed after the basin emplacement, which makes their previously assumed simultaneous formation questionable. This structural map presents important patterns and cross-cutting relationships, and future analyses of them will contribute towards understanding the post-emplacement basin-, and global tectonics on Vesta.