Structural relationships in and around the Rheasilvia basin on Vesta

Abstract

The Rheasilvia basin is an impact structure occupying most of Vesta's southern hemisphere. Divalia Fossae, a set of circum-equatorial troughs, were previously proposed to be concentric around the basin, which is widely regarded as evidence that the Rheasilvia impact directly caused the formation of the troughs. Here, we produce a structural map of Rheasilvia that allows for geologic interpretations and quantitative analyses of structure orientations and densities. We mapped basin-bounding scarps, scarps within the basin, ridges, and undefined lineaments. Scarps abound near the basin rim, with ridges being densely located on the basin floor near the central mound, forming a spiral pattern. This pattern is well-preserved on the basin floor except in the area superposing the older Veneneia basin, indicating that pre-existing Veneneia structures substantially influenced those of Rheasilvia. This implies that the lithosphere must have remained highly shattered after the Veneneia impact until Rheasilvia was emplaced. The Divalia Fossae cross-cut the Rheasilvia basin, and reanalysis of the geometric relationship between both landforms reveals that the troughs are not concentric around the basin center. These results are inconsistent with the previous hypothesis of trough formation and require a reconsideration of Vesta's tectonic history.