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

Abstract

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.

Radar Albedo Feature

“…2 and 3). Locally, volcanic deposits flood structural lows (Bindschadler et al 1992a;Bindschadler 1995;Hansen andWillis 1996, 1998;Ivanov and Head 1996;Gilmore et al 1998;Hansen et al 2000;Romeo et al 2005;Hansen 2006). Although there is debate about whether folds dominantly predate or postdate ribbons, most workers agree that (1) fold and ribbon formation broadly overlapped in time (or at least the formation of short-and intermediate-wavelength folds and ribbons structures), (2) graben formed late relative to folds and ribbons, and (3) volcanic activity accompanied ribbon tessera fabric formation -that is, the formation of ribbons, folds, and graben.…”

“…Ribbon character and spacing, functions of brittle layer thickness (Hansen and Willis 1998), require a high geothermal gradient (minimum heat flow similar to average terrestrial mid-ocean ridges) during ribbon formation (Gilmore et al 1998;Ruiz 2007). Regular ribbon spacing across 10 6 km 2 (Ghent and Tibuleac 2000) implies a thermal control for the base of the upper strong layer corresponding to a regional brittle-ductile transition depth (Hansen and Willis 1998;Hansen 2006;Ruiz 2007). Calculated heat-flow values for ribbon formation and the estimated depth to the crustal solidus require either extremely thin crust or a crustal magma reservoir across areas of ribbon tessera formation; thus, ribbon tessera records regional-scale environmental conditions different than those of contemporary Venus (Ruiz 2007).…”

“…Opposite trough walls either join via V-shaped terminations forming extension fracture ribbons or join via ramped structures, evidence of true graben structures (shear-fracture ribbons) (Hansen and Willis 1998). Ribbon fabric is comprised of periodic alternating long, narrow, troughs and alternating flat-topped ridges (Hansen andWillis 1996, 1998;Pritchard et al 1997;Ghent and Tibuleac 2002). Ribbon troughs, which are long (many 10's of km) relative to their width (generally 1-4 km), are named for their long aspect ratios and periodic spacing, appearing like ribbons across a landscape.…”

“…Ribbon tesserae are probably among the oldest local terrain on the planet (Hansen andWillis 1996, 1998); however, not all ribbon tesserae formed at the same time, though they likely formed during an ancient era on Venus (Hansen and López 2010). In some cases, ribbons may be the oldest structures of tesserae on crustal plateaus (Hansen and Willis 1998;Ghent and Tibuleac 2002); in other cases, they may have formed after the formation of short-wavelength folds (wavelengths of <1 km; Hansen 2006) and/or either before, synchronous with, or after, intermediate wavelength folds (wavelengths of $2-5 km; Hansen 2006).…”

Geologic map of the Hecate Chasma quadrangle (V-28), Venus

“…Three outcrops of materials with two or more dominant fracture and (or) ridge orientations were mapped as tessera material (unit t) Willis, 1996, 1998;Hansen, 2006), following the convention for Venus (Tanaka, 1994), while units with a single or very dominant direction of fractures were mapped as unit pfH or unit peH. Tessera materials tend to be relatively radar-bright or rough at the Magellan radar wavelength (12.6 cm), and stand higher than the surrounding units (Barsukov and others, 1986).…”