San Andreas Fault Line - Fault Zone Map and Photos
Intense pressure builds up along the fault as the two plates grind The San Andreas Fault runs in a northwest-southeast line along the coast. Two of these moving plates meet in western California; the boundary The entire San Andreas fault system is more than miles long and. The San Andreas fault zone is a transform boundary between two tectonic from the USGS web site, Geologic History of the San Andreas Fault System. tectonic plate met the western margin of the North American plate at a.
The breaking rock sends out waves in all directions, and it is the waves that we feel as earthquakes. Is the Fault Visible at the Surface? In San Bernardino and Los Angeles Counties, many of the roads along the fault cut through great mountains of gouge, the powdery, crumbled rock that has been pulverized by the moving plates.
San Andreas Fault
The hallmark of the San Andreas Fault is the different rocks on either side of it. Being about 28 million years old, rocks from great distances have been juxtaposed against rocks from very different locations and origins. The Salinian block of granite in central and northern California originated in Southern California, and some even say northern Mexico. Pinnacles National Monument in Monterey County is only half of a volcanic complex, the other part being miles southeast in Los Angeles County and known as the Neenach Volcanics.
Aerial photo of the San Andreas Fault showing drainage that is offset by movement of the fault. Fault Myths There are many myths and legends about the San Andreas Fault, the biggest being that it will one day crack and California will slide into the sea. They are uncrowded and peaceful, perfect for family outings. There is abundant camping, bird watching, wild flowers and wildlife, rock collecting and natural beauty along the way.
State and National parks are strung along the fault like beads on a string.
- The San Andreas Fault
At these faults, one block of rock is sliding underneath another block or one block is being pushed up over the other. A reverse fault is defined by the hanging wall moving up relative to the footwall, which is moving down. Figure 3 - A reverse fault. This time, the 'footwall' is on the 'downthrown' side of the fault, moving downwards, and the 'hanging wall' is on the 'upthrown' side of the fault, moving upwards. When the hanging wall is on the upthrown side, it 'hangs' over the footwall.
Strike-slip faults are the cracks between two plates that are sliding past each other. You can find these kinds of faults in California.
The San Andreas fault is a strike-slip fault. It's the most famous California fault and has caused a lot of powerful earthquakes.
The Geological Society
Figure 4 - Two strike-slip faults. No matter which side of the fault you are on, the other side is moving to the left. The relative rate of motion between the North American plate and the Pacific plate is approximately 3. The remainder is expressed by displacement along other, subparallel faults such as the Imperial and the San Jacinto fault zones in southern California.
The oceanic Farallon plate was subducted beneath the continental North American plate at this boundary. At the same time, the opposite margin of the Farallon plate was diverging from the Pacific plate, located to the west, along a mid-ocean ridge.
The rate of convergence at the subduction zone was greater than the rate of spreading along the ridge, which caused the ridge to move toward the subduction zone.
Approximately 30 Ma, during the middle Oligocene, the mid-ocean ridge between the Farallon plate and the Pacific plate began to be subducted beneath the western margin of the North American plate along the north-south trending Cordilleran subduction zone.
As a consequence, the Farallon plate was split into two smaller plates. The northern plate is termed the Juan de Fuca plate and the southern plate is the Cocos plate. Two triple plate junctions formed and separated from each other along the subduction zone as the Pacific, Juan de Fuca, and Cocos plates continued to converge with the North American plate.
The Mendocino triple junction is the northern junction Juan de Fuca, Pacific, and North American plates and the southern junction is the Rivera triple junction Cocos, Pacific, and North American plates.
As the North American plate encountered the Pacific plate the relative motion between them caused the convergent boundary along this portion of the margin of North America to evolve into a transform boundary. The Pacific plate is moving to the northwest relative to the North American plate and the Juan de Fuca and Cocos plates are moving to the southeast as they separate from the Pacific plate along mid-ocean ridges.
This relative motion is expressed along the San Andreas transform fault as right-lateral strike-slip offset.