Basement Complex of the Rockies

Teton Mountains

 

The Tetons are an exception for the basement complex. The basement did not become exposed from the Laramide orogeny. In fact the Teton range was formed only a few million years ago in contrast to the other ranges in the same area some 75-40 million years ago. A major fault instead caused the Tetons to become present. The Teton fault extends 40 miles along the base of the Teton Range. About ten million years ago, this region began to stretch and the Earth's crust cracked forming faults. Each time the crust broke; an earthquake up to magnitude 7.5 shook the land tilting the mountains skyward and dropping the valley floor. These random bursts of energy created the abrupt front of the Teton Range as it towers above Jackson Hole. At 13,770 feet, the summit of the Grand Teton rises 6,000 feet above the valley floor. Amazingly, this fault may be up to 30,000 feet or over 5 miles! The floor of Jackson Hole may have dropped over 24,000 feet, roughly three times as much as the mountains rose.

 

The older sediments that are present below the Flathead Sandstone are composed mainly of gneiss that had been heavily folded during faulting then eroded by the glaciation some 18,000 years ago.