The mesas are not being held up by their own structure - they would collapse/ sink into the mantle really soon if that were the case. They're being held up by a constant welling up of energy along the magnetic poles of the planet. Although the planet has a floating crust, the mesas are thick cratons that have been fixed to the magnetic poles for billions of years (since the formation of the planet). They may rotate a bit over time as the land moves around them, but they probably contain some material that attracts to the very strong telluric (bull**** word but it'll do) field at those points. The magnetic poles do not wander nearly as much as those on Earth, although the polarity switches a few times per million years - with temporary catastrophic results.

I want the coastline of the Patagonia-like region bordering the northern mesa to have been charted by Thurian explorers, but I don't want them to have realized that the adjoining mountain range is actually five times as high as a regular one. Do you think it would be possible that the continual glaciation has carved away the slopes of the mesa enough that the curvature of the world prevents a traveller at sea level from truly seeing the unbroken limb of the central plateau on the horizon, instead seeing a jagged, glaciated mountain range?