In my sci-fi universe, I have an earth-sized planet orbiting a K-type star. This planet (which has life) has about 0.6 G of surface gravity, an atmospheric pressure of 0.8 bars, and no continents. Water covers 60% of the surface.
Now, here’s the tricky part: I would like this planet to be girdled by a giant mountain range or plateau, about 10 km above the average altitude or sea level. This plateau should be continuous, separating the two hemispheres and preventing their indigenous life forms from mixing to any great extent.
What natural/cosmic phenomenon could explain this geographical oddity?