Venus's shield terrain

Plains, planitiae, or lowlands--expanses of gentle, long-wavelength ( approximately 1000 km) basins--cover approximately 80% of Venus's surface. These regions are widely accepted as covered by volcanic flows, although the mechanism(s) responsible for resurfacing remains elusive; in addition, a volcanic origin for the lowland surface may be open to question. Lowland resurfacing is typically attributed to catastrophic emplacement (10-100 m.y.) of globally extensive, thick (1-3 km) flood-type lava. This model of resurfacing has been postulated on the basis of impact crater distribution, taken together with a lack of obvious volcanic flows or edifices, and a lack of viable alternative models. Ongoing geologic mapping of approximately 15,000,000 km (super 2) (0-25N/90-150E) using 225 m/pixel and 75 m/pixel NASA Magellan SAR (synthetic aperture radar) data indicates that small edifices, called shields (1-15-km-diameter edifices, <<1 km high), play a major role in lowland resurfacing. Individual shields are radar-smooth or -rough, quasi-circular to circular features with or without a central pit. Shield shapes, which have been previously documented, range from shield, dome, or cone, to flat-topped or flat. Shield deposits typically coalesce, forming a thin, regionally extensive but discontinuous, mechanically strong layer, herein called shield paint. Shield paint conforms to delicate local topography, providing evidence of its thin character and indicating generally low viscosity during emplacement. Shield terrain (shields and shield paint) covers more than 10,000,000 km (super 2) within the study area. Detailed mapping of five 2 degrees X2 degrees regions using coregistered normal and inverted right- and left-illumination SAR imagery indicates shield densities of 3500-33,500 shields/10 (super 6) km (super 2) ; thus, the map area hosts more than 35,000-335,000 shields. Shield terrain generally postdates, but is also locally deformed by, fractures and wrinkle ridges, indicating time-transgressive formation relative to local deformation and/or reactivation. The regional scale crust was strong throughout shield-terrain formation. Shield terrain may extend across much of Venus's surface.