Chapter 6 Section 4
Building California by Plate Tectonics
• The region that we know as California has been at an active plate boundary for the past 225 million years.
• As a result, plate tectonics has been the most important force shaping California’s geologic history.
• Before about 225 million years ago, North America’s western edge was much farther east than it is now.
• The area where Nevada and the eastern deserts of California are today was the west coast of North
America.
• Most of what is now California was either part of a distant oceanic plate or did not exist.
• When Pangaea began to break up, the North American plate moved west.
• The continent’s western edge became an active convergent plate boundary.
• A long period of subduction began, which was an important period of geologic “building” in California.
• Three major tectonic plates influenced California’s geologic history: the North American plate, the
Farallon plate, and the Pacific plate.
• A convergent boundary existed between the North American and Farallon plates.
• The Farallon plate subducted beneath the North American plate.
• The ancient Farallon plate lay between the North American and Pacific plates.
• About 25 million years ago, the entire Farallon plate was subducted at one part of the boundary.
• The Pacific Plate touched North America for the first time, forming a transform boundary.
• As the Farallon plate continues to subduct, the transform boundary continues to grow longer.
• Today, it is about 2,600 km long.
• The Juan de Fuca plate off northern California is part of what remains of the ancient Farallon plate.
Subduction and Volcanism
• The subduction of the Farallon plate caused rocks to melt and caused chunks of rock to collide with the
North American continent.
• Subduction caused a great deal of magma, or molten rock, to form in the lithosphere.
• This magma solidified to form a huge mass of granite called the Sierra Nevada batholith.
• A batholith is a large mass of igneous rock that forms deep below the surface.
• Batholiths are the “roots” of subduction zone volcanoes.
• A chain of huge volcanoes must have formed above the giant magma chamber.
• These volcanoes probably stood twice as tall as today’s Sierra Nevadas.
• The granite batholith is exposed in the Sierra Nevada mountain range.
• Today, the Juan de Fuca plate is subducting beneath the North American plate.
• This area is known as the Cascadia subduction zone.
• A chain of active volcanoes is present in this zone, in the Cascade Mountains of California, Oregon, and
Washington.
Subduction and Accretion
• During subduction, pieces of the plate that subducts may be scraped off and attached to the overriding
plate.
• This process, called accretion, forms mountain chains.
• These mountain chains are parallel to the plate boundary.
• The rocks in California’s Coast Ranges and Transverse Ranges are though to have been formed by
accretion.
• The Central Valley, Los Angeles Basin, and Ventura Basin separate some of these mountain ranges.
Accreted Terranes
• The chunks of lithosphere that are scraped off of subduction plates and added to the edge of a continent
are called accreted terranes.
• The rocks of a terrane differ from the surrounding rocks by age or composition.
California Gold
• The foothills along the western side of the Sierra Nevadas contain rocks filled with gold.
• These rocks are thought to be accreted terranes.
• This rock formed near submarine volcanic vents.
• After the terranes were accreted, the gold became concentrated in the quartz veins of the Mother Lode.
• Gold is an important part of California’s history.
The San Andreas Fault System
•
California is home to the most famous transform plate boundary in the world, the San Andreas fault
system.
• The San Andreas fault system extends for about 1,000 km.
• The San Andreas fault forms the boundary between the Pacific and North American plates.
• Most of California is on the North American plate.
• A small part of California, west of the San Andreas fault, lies on the Pacific plate.
• The Pacific plate is moving to the northwest relative to the North American plate.
• Not all plate movement takes place on the San Andreas fault itself.
• In the San Francisco area and southern California, motion takes place on other faults of the system.
• These faults lie west and east of the San Andreas fault.
• In these areas, it is best to think of the boundary between the North American and Pacific plates as a zone,
not a line.
The Pacific and North American plates have been moving along the San Andreas fault system for about 25
million years.
• During the last 16 million years, the separation, or offset, along the fault has been 315 km.
• Geologists use rocks to estimate the amount and rate of movement along the fault.
• Geologists determine offset by matching unusual rocks that have been separated by the fault.
• They date these rocks to determine when they formed.
• Then geologists use that date to determine when the areas were not separated.
• In southern California, the San Andreas fault makes a huge bend as it passes east of Los Angeles.
• Because of this bend, the Pacific and North American plates collide as they move past each other.
• As a result, the motion along this boundary is partly convergent.
• Because southern California is being compressed, areas near the bend are being uplifted or dropped down
by active faults.
• The San Bernardino Mountains and the San Gabriel Mountains are tectonically created mountain ranges.
• The Los Angeles basin is a large depression bordered by active faults.
Plate Tectonics and the California Landscape
• Much of California’s landscape has been formed by plate tectonics.
• Compression has recently uplifted California’s rugged mountains.
• The steep, rocky coastlines have been formed by uplift along the plate boundary.
• Major river valleys, mountain ranges, and the coastline are oriented in a northwesterly direction.
• A northwesterly orientation is parallel to the faults of the plate boundary.
• The Transverse Ranges are oriented east-west, due to motion from the San Andreas fault system.