Say there is an Americas-like continent. (The western hemisphere (North America + South America), not the United States of America)
A tyrant civilization that pretty much conquered the continent was located at what it might be Texas latitude, then a cataclysmic event ocurred that wiped out their entire civilization. The used-to-be-oppressed, people from the lower and upper half of the continent managed to survive (south America/upper Canada), as they were far from the Cataclysmic core. Their survival remains a mystery to one another. for the conditions of the post- cataclysmic land are too unbearable. Yet they venture there because of new valuable resources that appeared and later becomes some sort of economy (the lower half attribute divine punishment to the tyrants, and the new resources that arose from their ashes (?) As divine retribution) Because this resource is so difficult to get (land conditions) and already became the focus of economy (like oil) for both halves, the fact that they only meet after centuries of development it's the cause of the conflict, so, how to justify that they only found out of each other's after, say, about 900 years?
What could be this event? How would both halves of the world be climatically impacted?
Since the new found resource grants superhealing, feel free to explore high temperatures/ radiation like things.