This discussion is interesting.
I just want to add a few points to try to make more sense that Cantha is now also in the "north hemisphere" (if there is any)
1. I suggested that Luxon's area used to be a sea, just like a bigger dead sea in the middle sea.
2. The Kurzick area used to be a forest, just like the jungle climate, with river and very complex biomass. I guess that's part of Maguuma.
Why do Maguuma and Cantha apart?
Plate tectonics theory has the answer.
Two plates, Kytra plate and Cantha plate, moves apart. It produces lines of weakness in the Ocean (what's the name of the ocean, btw...). After repeated vocanic eruptions, melting, cooling, and eruption. There is the ring of Fire in the middle between Tyria and Cantha.
The second wild guess is that Kytra plate and collide towards Ascalon plate. The mountain movement starts and there is the snow mountain area, so does the area east to the crystal desert.
You may also want to ask why the ruin of Ascalon looks like a desert with almost no biomass. The reason is simple: it's destroyed.
The puzzle almost solved.
I hope you may find my guess interesting.