There is a geologically brief period during mountain-building when you can get oceans on both sides of a mountain range. Both the Andes and the Himalaya orogeny did it.
Here's how it works. We all know that plates subduct under other plates. Most of us got that in school. The oceans on both sides trick requires the continental collision to happen in the ocean, but with craton (above ocean bits) nearby. The beginning of the collision has one plate subducting under the other. The melted oceanic crust bubbles up (for geological time versions of 'bubbled') and starts forming a volcanic arc well behind the oceanic trench that marks the surface version of the collision.
When the craton hits the trench, it doesn't subduct nearly as well and the continents begin to ruck up like a rug (again, on geological timescales.) Subduction is still happening, but slower perhaps. Energy is going into folding and cracking continental crust. Like pushing your fingers together causes them to steeple, parts of your hand also depress. Sometimes that continental depression is below sea-level and you get a shallow sea. On both sides.
That shallow sea is ephemeral, since the rucking-up eventually raises everything above mean sea level. But it means your fantasy world with a mountain rage in the middle of an ocean isn't as unbelievable as it could seem.
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Here's how it works. We all know that plates subduct under other plates. Most of us got that in school. The oceans on both sides trick requires the continental collision to happen in the ocean, but with craton (above ocean bits) nearby. The beginning of the collision has one plate subducting under the other. The melted oceanic crust bubbles up (for geological time versions of 'bubbled') and starts forming a volcanic arc well behind the oceanic trench that marks the surface version of the collision.
When the craton hits the trench, it doesn't subduct nearly as well and the continents begin to ruck up like a rug (again, on geological timescales.) Subduction is still happening, but slower perhaps. Energy is going into folding and cracking continental crust. Like pushing your fingers together causes them to steeple, parts of your hand also depress. Sometimes that continental depression is below sea-level and you get a shallow sea. On both sides.
That shallow sea is ephemeral, since the rucking-up eventually raises everything above mean sea level. But it means your fantasy world with a mountain rage in the middle of an ocean isn't as unbelievable as it could seem.