Geologic constraints on crustal plateau surface histories, Venus: The lava pond and bolide impact hypotheses

Venusian crustal plateau formation is hotly debated. Crustal plateaus are large (∼1500-2500 km) quasi-circular ∼0.5-to 4-km-high plateaus that host distinctive tectonic fabrics, called ribbon tessera. Debate centers on plateau support and fabric formation. Detailed geologic mapping of an ∼360,000 km² region, eastern Ovda Regio, provides critical clues for plateau evolution. Ribbon-tessera fabrics record broadly synchronous layer contraction, extension, and flooding, as an initially strong, thin layer (<100 m) developed across Ovda. As the layer underwent ductile shortening and brittle extension, subsurface lava flooded structural lows. With progressive deformation the layer thickened, leading to formation of progressively longer-wavelength structures. Early-formed shorter-wavelength structures and lava-filled valleys were carried piggyback on longer-wavelength folds. This history indicates an extremely large strength contrast that is inconsistent with a conductive geotherm, reflecting instead the presence of a near-surface molten layer. I propose the lava-pond hypothesis: Ribbon-tessera fabrics represent progressively solidified "scum" of a huge lava pond, leading to crustal plateau formation. I further propose that individual lava ponds formed as a result of large bolide (∼20-30 km diameter) impact with ancient thin lithosphere, causing massive partial melting in the upper mantle. Melt rose buoyantly to form huge (∼1500-2000 km diameter) lava ponds, completely resurfacing the local area. In the upper mantle depleted residuum would be compositionally more buoyant and stronger than adjacent undepleted mantle. Isostatic adjustment resulted in pond uplift and plateau formation. Subsequent secular cooling could "freeze" a plateau in place, or mantle convection could strip away the residuum root, leading to plateau collapse.