Wrinkle ridge plains

Wrinkle ridge plains are found on every planet that has lava plains. They therefore offer a good basis for comparing the planets.

Wrinkle ridges are low, broad ridges that form along a fault under compression. Sharp, sinuous crenulations run along them. On the plains, wrinkle ridges form parallel to one another, at almost constant spacing.

They form through contraction at depth, linked to the cooling of lava, under the weight of a lava layer several kilometres thick, as in lava-filled basins (such as the lunar maria). The compression is usually explained by newly erupted basalt lava rising from depth and weighing down on the layers that were at the surface before. On Venus another cause has been suggested: the upward push of mantle diapirs from below. Wrinkle ridges can also form in sedimentary material by a similar mechanism.

On Venus these plains are also called “regional plains”. They covered 40% of the area surveyed during global geological mapping (Ivanov and Head 2011). Two types can be told apart on Venus. The more widespread, stratigraphically lower unit (Rusalka Formation) is radar-dark, and canali are common on it. An upper (younger) unit (Ituana Formation) has a uniform, intermediate radar brightness, or it is patterned by lobate lava flows.

These plains occur mainly in lower-lying areas. Their thickness is estimated at 400–500 m. On Venus the ridges typically repeat every 10–30 km (Jindal 2019).