One Hundred Hikes in Yosemite

Evolution of the Yosemite Landscape — The Nevadan Orogeny

For a third time the direction of motion of the oceanic plate became increasingly more nearly perpendicular to the North American plate, and another major compressional period began. This was the Nevadan orogeny, named after the Sierra Nevada, where its record is best preserved. Like earlier orogenies, this one metamorphosed the area's previously existing rocks, including those in today's pendants. The orogeny was quite protracted. Compres sion and uplift may have begun by 176 million years ago, peaked from 163 to 152 million years ago, and then continued in their waning stages until about 140 million years ago, if not longer.

During this orogeny, magma mostly intruded an assemblage of older rocks lying west of the Park, accreting them to western North America. These lands have been called the Foothills belt and the Foothills terrane, but research in the 1980s and '90s, particularly by David Jones of the US Geological Survey, suggests that this belt is composed of at least five terranes. Within the Park, plutonism was minimal. Only one Nevadan-orogeny pluton is known, a small one straddling the Park's northern boundary immediately west of Bigelow Peak. It is composed mostly of diorite and gabbro, both dark-gray rocks. During the orogeny's maximum, the Sierra Nevada was a minor length of a major range, a cordillera, which extended along the western edge of both North America and South America. If a new view of Sierran uplift is correct-one that involves detachment faulting-then the range's lands should have been higher than they are today. To discuss detachment faulting, we first have to elaborate on continental crust and how it reacts to stress.


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