I want to write a story that has a [Pangaean](https://en.wikipedia.org/wiki/Pangaea) like world, but human civilization already existed during that era. Basically, the earth just has one giant piece of land while the rest is the sea.

People don't venture to the seas, because there are no lands to find, only fish. And every single humans living there are culturally the same, due to being ruled by one ruler in the center of the unified land.

But, something happened, something tore apart the land, tearing the unified land apart into countless islands. It was so sudden, the people couldn't believe that their world could be torn apart literally.

Now my question :

What is a believable reason for the land to be split apart, but still have at least 50% of the human population survive? If that is not naturally possible, what kind of magic that will produce that result, but still makes it seem natural? I want my story to be low-fantasy.

Note :
The era of civilization when the "split" happens is the medieval era, the days of swords and shields.