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How do fault-block mountains form?
Answer 1:

When fault blocks move, there is usually one block that goes up and another that goes down, and the one that goes up becomes a mountain or mountain range, while the one that goes down becomes a valley. Erosion then sculpts the shape of the fault blocks into the peaks and ridges you more commonly think of when you think of mountains.

The fault blocks move in the first place due to movements in the Earth's crust driven by plate tectonics. For example, the North America plate is moving west north-west from the Mid-Atlantic Ridge, and the Pacific Plate is moving north north-west from the East Pacific Rise. A few million years ago, the part of the East Pacific Rise of California got run over by the North America plate, which means that the North America Plate now meets the Pacific Plate directly, instead of having another plate (namely the Farallon Plate) in-between. Because the Pacific Plate is moving northwest slightly faster than the North America plate, the junction between the two is sliding northward and stretching out western North America. This sliding is what causes the San Andreas Fault, and the stretching is what causes the basin-and-range faulting that covers most of Nevada. Due to a historical accident, the western edge of the North America plate is not straight, however, but zigs just north of the Santa Barbara Channel. Thus, while the Pacific Plate is moving north, it slams right into the North America Plate, creating fault blocks that push up the Santa Ynez Mountains and the rest of the Transverse Ranges. The Sierra Nevada and Wasatch mountains, meanwhile, are moving up because they are being tilted as the valleys in Nevada fall downward into the hole created by the stretching.


Answer 2:

Fault block mountains typically form in extensional tectonic regimes which means the land is being pulled apart during rifting or thinning. The land "breaks" apart in blocks that create horsts and grabens making highs and lows in the topography. They can form outside of extensional regimes for example when there is a slight bend in a strike slip fault.

That's it!


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