Tectonic landforms
Ridged plains, Ridge belt, Mountain belt
Many kinds of linear tectonic forms can deform the material of the lava plains. One of them is a bundle of ridges several kilometres wide and tens of kilometres long. Besides these complex but rather hilly zones, Venus also has a few really high fold mountain belts.
The ridges are arranged in bundles that form ridge belts (Frank and Head 1990, Ivanov and Head 2011). Where they spread over a wider area, the area can be mapped as ridged plains.
Each ridge is 5–10 km wide and tens or a hundred kilometres long. Their shape varies from steep ridges to a series of gently sloping hills. The ridges are usually symmetrical, with smooth, rounded surfaces. They formed by compression in basaltic lava plains (see the plate, trending NE–SW).
The ridges formed after the tesserae, but they were folded up before the wrinkle ridges. The stratigraphic relations along their contacts with these two landforms show this. Their relation to the tesserae is more complex. At the margins of tesserae, ridge belts are deformed in a tessera-like way, as densely lineated plains are. This suggests that at one point in the history of Venus, the tesserae and the ridged plains were (de)formed at the same time.
One unusual, unique arrangement of ridge belts lies near 70°N, 200°E. It has been known as “the Ridge Belt Fan” since the Venera 15/16 missions (Barsukov et al. 1986).
Their descriptor term is Dorsa. On Earth we would call them low mountains, or orogenic (mountain-building) areas.
The type examples of mountain belts are the ranges folded around Lakshmi Planum. They are really high: 5–7 km, and Maxwell Montes reaches 11 km. The isostatic thickening of the crust also played a part in their formation, so they have a root zone (Zuber et al. 1994). They consist of closely spaced ridges 5–15 km wide and a few hundred kilometres long.
Lakshmi Planum and the mountains around it are unique on Venus. Their origin may be linked to the Artemis centre on the far side of the planet. The mantle material that rises there may sink here, creating compression at the surface (Hansen 2018).
