California: Beauty of Tectonic Proportions

Plate Tectonics

California’s landscape is shaped in large part by plate tectonics, and in particular, the movement of the North American, Pacific, and Farallon plates over the past 150 million years. The existence of our coastal mountains, large central valley, and Sierra Nevada Mountains (to focus on the state’s main features) are due primarily to the movements of these three plates. You have probably heard of the North American Plate (almost all of North America), and the Pacific Plate (most of the Pacific Ocean). But you may not have heard of the Farallon Plate, which was a different large plate in between the Pacific and the North American Plates from about 150 to 25 million years ago.

The Farallon Plate was subducted under (went underneath) the North American Plate. As it was subducted, the accumulated sediments on the top of the Farallon Plate were scraped onto the North American Plate, creating much of the present day coastal mountain ranges.

The Sierra is Born

As the subducting Farallon Plate plunged deep beneath the North American Plate, it melted and created an inland volcanic chain. By about 60-70 million years ago the deep centers of these volcanoes had cooled and were beginning to be uplifted; they formed the present day granites of the Sierra Nevada.

A long with the volcanic chain you get a basin in front of them and this basin filled with sediment and became California’s Central Valley.

Creation of the San Andreas Fault

By about 25-30 million years ago the Farallon Plate and the spreading center that created it were entirely subducted. This left the Pacific Plate in contact with the North American Plate. Subduction stopped and the San Andreas Fault was born. The movement between this new contact was mostly side-to-side with the Pacific Plate heading northwest and the North American Plate heading southeast. Technically it is called a right-lateral strike-slip fault.

The San Andreas Fault is part of a complex system of several faults. Its motion is not just side-to-side (at an average 2 inches per year) but also involves steady uplift of the North American Plate relative to the Pacific Plate.

In Santa Cruz, my home turf, we are on what is called the Salinian Block, a chunk of granite brought northward, and also upward, by the San Gregorio and San Andreas faults. Much of this transport happened during the Miocene while the present day Salinian Block was at the bottom of the ocean accumulating sediments. Some of these sediments became the Santa Cruz Mudstone and the Purisima Formation.