Rocks From Somewhere Else

North America once ended far to the east, where the Sierra Nevada is today. All the basement rocks of the San Francisco Bay region have been added to North America, brought by tectonic motion.

Although it is hard to imagine, geologists have determined that the western coastline of North America was once where the Sierra Nevada foothills are today. At that time (the Middle Jurassic period, 171-161 million years ago), the coastline was dominated by a chain of volcanic mountains like those of the Andes Mountains in South America today. The position of the San Francisco Bay region probably was occupied by volcanic islands like those of the Aleutian Islands in Alaska today. However, the huge tectonic plates that make up the Earth's crust constantly move around, forming, colliding, and recycling into the Earth's interior. By the end of the Jurassic period, the volcanic islands had been moved east by this relentless motion. In turn, Jurassic-age and younger rocks have been transported here from elsewhere.

These transported rocks, which are the oldest (basement) rocks of the region, form three groups, each having a distinct history. The first group is the Great Valley complex, probably transported to the San Francisco Bay region and added to North America in Late Jurassic time (about 150 million years ago) along a subduction zone. A subduction zone is the boundary between two converging tectonic plates, where rocks of one plate slide under another plate and down into the Earth's interior. Frequently, some of the rocks on the downgoing plate are scraped off and added to the overlying plate, and rocks of the Great Valley complex were added to North America in this way. The second group is the Franciscan Complex, parts of which originated as far south as the equator. Between Middle Jurassic time (176-161 million years ago) and Miocene time (23-5.3 million years ago), Franciscan Complex rocks were pushed under and scraped off onto rocks of the Great Valley complex along a second subduction zone. In Oligocene time (about 34 million years ago), plate motion changed, and this second subduction zone began to be replaced by the San Andreas Fault system. The third group of basement rocks in the region is the Salinian complex, made up of slivers of continental rocks that have been moved here from southern California by the motion of the San Andreas Fault system. The resulting mosaic of basement rocks, and the younger rocks and sediments deposited over them, is shown below.

illustration - description below

This diagrammatic physical map shows what the western edge of North America looked like 176-161 million years ago (Middle Jurassic time). The present-day positions of San Francisco and Los Angeles, as well as the California state line and the Mexican border (highlighted in yellow), are shown for reference. At that time, the coastline of North America was about 100 miles east of its present position, and two subduction zones were offshore. The Aleutian-type volcanic islands that were in the San Francisco Bay region at that time moved eastward to the eastern subduction zone, where they were scraped off and added to North America. At the same time, Jurassic-age rocks that are now in the San Francisco Bay region came together along the western subduction zone, and these rocks also moved east and eventually were added to North America.

illustration - description below
Map of the San Francisco Bay region, showing basement rocks and overlying Tertiary rocks and Quaternary surficial sediments.