How could a mountain range naturally form in a roughly clean, complete shape?

In this universe, the planet Paveiha has a mountain range that completely surrounds a region in a semi square/rectangle(ish) shape. Inside the borders of this vast mountain range is a verdant land with an isolated ecosystem and a civilization that is completely detached and protected from the outside world.

I emphasise that the region is completely surrounded by this mountain range on all sides. The borders are partly irregular but the shape is still complete and roughly even on three of its four sides. There's a picture below showing the size (in comparison to the continental United States) and shape of the mountain range.

First of all, would it be possible for a mountain range like this to form at all? As far as I know, there's nothing like this on Earth so I wonder if it's even possible.

If so, what are some natural ways a mountain range could form in a roughly clean, complete shape? By clean I mean pretty even on most sides, as it is on 3 of its 4. Instead of like some monstrous blob with arms all over the place.

(If it's actually impossible, what would need to be changed to make something like this happen?)

Below is a picture showing the size of the region in comparison to the continental United States. I cut the region out from a map and put it in this to scale map of Earth. Generally, the mountain range encompasses its borders, (blue lines). The two mountains on the inside are irrelevant to the question.

posted over 7 years ago

Noble‭

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