This is most likely due to the viscous relaxation of the Tethyan icy crust over geologic time.

Inside the crater the rim is composed of arcuate scarps and extends for about 100 km until the floor is reached.

[4] The crater must have originally been deep, with a high mountainous rim and towering central peak.

Over time the crater floor has relaxed to the spherical shape of the Tethyan surface, and the crater's rim and central peak have collapsed (similar relaxation is apparent on Jupiter's moons Callisto and Ganymede).

This indicates that at the time of the Odysseus impact, Tethys must have been sufficiently warm and malleable to allow the topography to collapse; its interior may have even been liquid.