Almost the entire basin is a graben region, bearing basin and range characteristics, formed by uplifting and subsidence along several northsouth faults.
Surface features include the large southern polar cap, older cratered planes cross-cut by graben and scarps, as well as youthful features caused by endogenic resurfacing.
In this new model, the northern portion is an ENE-facing half-graben, extending further east than had been thought, while the southern portion is an east-facing half graben.
