How to Renew Old Mountain Ranges

Question

Conventionally, there are two ways to build mountains--direct collision (as is the case with the Himalayas) and one side sinking beneath the other (which explains why the Pacific Ring of Fire is volcanic.)

But in an alternate Earth, we have found an Appalachian Mountain chain in which the highest peak is 20,308 feet above sea level and nearly all are shaped like the Tetons. The mystery is that we have found no evidence that this came from subduction or continent-continent collision, but something did push the mountain range back up after millions of years of inactivity and erosion. What else could bring old mountain ranges like the Appalchians back up?

Answer 1

The mountains become lighter, and float higher.

In our world, the Appalachians formed from plate tectonics in the remote Ordovician. But much later, in the Cretaceous, they rose again. Plate tectonics were not responsible for this late rise. Instead, the mountains rose because they became lighter.

Appalachians Get a Face-Lift From Earth's Mantle

The Cullasaja River basin, part of the southern Appalachians in western North Carolina, holds scores of mountains and valleys, but parts of its terrain are more rugged than others... Usually...rugged peaks point to younger terrain with recent or active tectonic mountain-building processes. But geologists know the Appalachians have been tectonically quiet for more than 200 million years. What could have rejuvenated the southern Appalachians?

Plate tectonics and the usual mountain-building suspects probably weren't responsible, since the Appalachians were tectonically quiet by that time. But by turning to previous surveys of the region, Gallen found another possible culprit.
Regional uplift driven by the Earth's mantle — the hot, flowing layer below the outer crust — could be the culprit, according to a team of researchers led by geologist Sean Gallen of North Carolina State University.

One way to explain those features is that the dense "root" of the mountain range delaminated, or peeled off, from the rest of the crust around 8 million years ago. "Sometimes the mountain root becomes more dense than the mantle below, and it's not gravitationally stable," Gallen said. "It will basically drip off the base of the crust, and the remaining crust, which is lighter, will bob up on top of the mantle."

The heavy bottom of the mantle falls away into the earth under gravity - "delaminates" and separates from the lighter upper parts of the mountains. These lighter parts then bob higher.

In the alternate world, the processes in motion in the Cretaceous continue to this day (and to some degree they are in our world). Perhaps more of the base delaminated and what was left above was even lighter. Perhaps the mantle below was composed of denser materials and the mountains floated higher.

In either case, this is a real world process that may be responsible for the real uplift of this mountain range in our world. Augmenting a real thing for a fictional world is a fine world building strategy.

Answer 2

Shrinking due to cooling.

As the deeper layers of a planet cool down, the planet shrinks. Of course this will result in stresses on the surface, which has to adapt to a lower volume.

This will produce wrinkles on that very surface, and those wrinkles are the renewed mountains you talk about.