Venus diapirs: Thermal or compositional?

The Venus surface exhibits quasi-circular structures with a bimodal size distribution; there are at least 17 large (∼1300-2600-km diameter) crustal plateaus and volcanic rises, which form major geomorphic features, and ∼515 smaller (200-km median diameter) coronae. All of these features-plateaus, rises, and coronae-are interpreted to be the surface signature of mantle diapirs. Diapirs are driven by buoyancy, which is a function of diapir size and diaper-host density contrast, which can be a function of thermal or compositional differences. Plateaus and rises apparently represent the surface signature of deep-mantle thermal diapirs that interacted, respectively, with ancient thin lithosphere and contemporary thick lithosphere. Existing coronae models either do not specify the nature of diapir buoyancy or assume a density difference resulting from thermal differences. I contrast the geological implications of end-member thermal and compositional buoyancy to provide insight into the mechanisms of Venusian diapirism operating at different scales. The analysis indicates that median-size coronae likely represent the surface expression of compositionally driven, rather than thermal, diapirs, whereas plateaus and rises form from large thermal diapirs. The bimodality and surface distribution of Venus' large (plateaus/rises) and small (coronae) diapiric structures might therefore reflect different mechanisms of diapir formation. Thermally driven deep-mantle plumes, initiated along a warm lower boundary layer, rise through the mantle to the lithosphere, forming plateaus and rises; these plumes would transfer heat from the core. Broad mantle upwellings, which presumably formed in response to a cold upper boundary layer, might generate compositional diapirs at relatively shallow levels in the upper mantle; compositional diapirs, rising to form corona chains, would transfer heat from the mantle. Corona clusters result from compositional diapirs spawned locally by plumes of deep-mantle origin; corona clusters therefore would also represent heat transferred from the mantle, but their formation would be triggered by thermal anomalies from the core-mantle boundary. Isolated coronae probably formed only when the lithosphere was thin, because they occur in regions where the lithosphere is currently too thick to transmit small, diapiric signatures to the surface.