A Young Age of Formation of Rheasilvia Basin on Vesta from Floor Deformation Patterns and Crater Counts

Delivery of significant volumes of howardite-eucrite-diogenite (HED) meteorites from Vesta to Earth is linked to the largest impact events on that protoplanet, especially the giant Rheasilvia basin. Dawn mapping of Rheasilvia reveals a well-preserved impact structure with extensive deformation patterns and moderate superposed cratering. Spiral and radial deformation features across the floor of Rheasilvia are also common to complex craters on Ceres and icy satellites, as well as larger bodies like the Moon, Mars, and Mercury, when floor units are not buried by impact melt and debris. These deformation features predate the solidification of such melt/debris mixtures and are important structural elements in complex craters related to failure in converging floor material during late stages of crater formation. The similarity and commonality of such features and lack of preserved ancient units within Rheasilvia indicate that these patterns formed during the latter stages of the impact event and that the original basin floor and subsequent cratering record are preserved. Updated counts and interpretation of this observed superposed cratering indicate an similar to 0.8-0.9 Ga age of basin formation, supporting a young impact and ejection age. This young age is consistent with recent Ar-Ar ages of feldspar grains in HED (Lindsay et al., 2015), and it may also be consistent with ancient bulk Ar-Ar ages (>similar to 3.2 Ga) for some HEDs because resetting such ages requires prolonged heating from the residual impact heating of rocks left on Vesta during earlier impacts. The complex morphology and recent formation of Rheasilvia basin are also consistent with dynamical estimates for delivery of these meteorites to the Earth-Moon system.