One Hundred Hikes in Yosemite

Evolution of the Yosemite Landscape — The Antler and Sonoma Orogenies

In the vicinity of Yosemite National Park, the early geologic record is too incomplete to permit a reconstruction. However, evidence elsewhere makes a reconstruction possible, and it indicates that in the Sierra, volcanism began about 420 million years ago (and has continued on and off until geologically recent time). Before the creation of the Sierra Nevada range proper, there were several periods of arc volcanism, eruptions produced by a curved line of volcanoes, such as Alaska's Aleutian Islands or the Ryukyu Islands between Japan and Taiwan. Just as a back-arc basin-the shallow South China Sea-separates that chain of volcanic islands from the coast of China, a similar basin separated a chain of volcanic islands in the vicinity of the Sierra Nevada from the coast of North America, located in today's Nevada. After initial sputtering, volcanoes began to erupt in earnest around 400 million years ago and did so until the Antler orogeny, which occurred about 365 to 350 million years ago. In the vicinity of today's range, existing sediments were metamorphosed as the chain of volcanic islands was thrust eastward, compressing, heating, and disrupting them.

After the Antler orogeny, which would have raised the compressed lands, they underwent extension, which caused them to sink and to become buried under accumulating marine sediments. After tens of millions of years the sea retreated and the upper marine sediments were eroded away through uplift, leaving about a 50-million-year gap in the geologic record. (The Santa Lucia Range, mentioned earlier, had a similar, if considerably more recent, uplift-extension-subsidence-uplift history.) Eventually, land in our area formed once again with a second round of arc volcanism, which began about 260 million years ago. This lasted at least until the Sonoma orogeny, which occurred from about 250 to 240 million years ago, around the time the Paleozoic era gave way to the Mesozoic era. As before, the orogeny was due to a chain of volcanic islands being thrust eastward.

During each orogeny North America proper grew slightly westward as masses of crustal rocks were compressed and metamorphosed and then became accreted terranes. Probably included among them are ones that later would be reduced to the Snow Lake, Sachse Monument, Saddlebag Lake, and northern Ritter Range pendants. Terranes do not necessarily stay in place; today in California (and elsewhere), some are on the move, riding passively atop the earth's plates (their motions are called plate tectonics). The terranes in today's Sierra Nevada became fixed because magma rose beneath them, solidifying to form a very stable basement composed of one or more plutons. The earliest surviving plutons in the Sierra Nevada are located in its southern part. They are about 240 million years old and formed as the Sonoma orogeny was waning. The earliest ones in the Yosemite area are about 210 million years old, and are found east of the crest, close to their relatively contemporaneous volcanic rocks. You'll see these granitic rocks above the north shore of Lundy Lake, in middle Lee Vining Canyon near its Warren Fork, and at Kidney and Lower Sardine lakes.


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