Assuming that map is a complete planet, in cylindrical Mercator projection? It seems to be, with the characteristic lateral smudging of textures at the extreme top and bottom.
That smudged-looking mountain range at the top is actually an elevated north pole, with a roughly circular aspect.
It just requires a planet that took a hefty thump early in its life. Like Mars, where the whole North pole region is 6km lower than the South pole region.
Your map is the same, just inverted. North is high, and the lower south is occupied by an ocean.
This is your world on https://www.maptoglobe.com/ByWPZtkcw
Quite a nice looking globe, it is!
