Constraints on corona formation from an analysis of topographic rims and fracture annuli

Abstract

Venusian coronae are circular to oblong features that are defined by their circumferential fracture annuli. Most coronae also have a circumferential rim. Corona topographies are thought to evolve over time depending on their specific formation mechanisms, which broadly include initiation due to a rising plume or due to plume-induced subduction or delamination. Concentric fractures at coronae are proposed to form when the topographic rim has some threshold curvature, and so fracture patterns should record high rim curvature. Models generally suggest that rims migrate outward, which should leave some fractures interior to the topographic rims. In this study we analyze topography and fracture annuli patterns in 92 coronae, including 58 Type 1 and Type 2 coronae. We find that most coronae have fractures overlapping topographic rims, supporting little rim migration inward or outward. Corona diameters, when defined by topographic rims, are somewhat greater than when defined by fractures. On average, topographic rims take up more than half the coronae diameter. Since Type 1 corona have more complete fracture annuli than Type 2 coronae, it is surprising that there is little difference in the dimensions of rims for the two types. We also find that topographic rim dimensions are not remarkably different in profiles with or without fractures, leaving us to conclude that fracture formation may be strongly influenced by another parameter, such as strain rate or stress field, or that the topographic rims have relaxed since fracture formation.