The highland terrain in southeast Thetis Regio consists of a series of thrust faults and conjugate graben that would usually be classified as tesserae. Unusually, this small area of tesserae is younger than the adjacent plains, a conclusion strengthened by modification of Jumaisat crater, a parabolic dark-halo crater >75 Myr old. Two structural belts are distinguished, the eastern ridged terrain and the central plateau, separated by conjugate graben. Unlike most terrestrial mountain belts, the thrusts in southeast Thetis are outward dipping, implying obduction of the eastern plains onto Thetis. This most likely results from the buoyancy of the Venusian lithosphere, which inhibits subduction. A simple two-stage orogenic model is inferred. Plains crust, driven by extension in Diana Chasma, initially overrides higher-standing tesserae crust in southeast Thetis. Through interaction with a plume, or perhaps through convective instability of the lower lithosphere, a plateau forms that resists further compression and that exhibits extensive volcanism. A new area of thrusting in the ridged terrain develops, while the plateau starts to collapse along graben, the larger of which are filled with lavas. The core of Thetis Regio appears to be surrounded by other tectonic units like southeast Thetis, indicating a piecemeal construction similar to terrestrial continents.
- Thetis Regio