Geologic Analysis of the San Andreas Valley and its Fault System
The San Andreas valley and fault system crosses almost the whole length of California. The fault acts as a divide between major blocks of the crust. Much of the northern and central part of California is covered by the eastward trending boundary. The boundary is composed of the Salinian block, which is composed of granitic and metamorphic rocks bordering on the west and the Franciscan grouping and overlying layers of the Great Valley chain on the east. The fault also has a southern course that slightly curves towards the east and diagonally cuts the Transverse Ranges, which also proceeds to the Gulf of California. In the southern part of California, the basement rocks that are divided by the San Andreas Fault line are mostly of the Precambrian age and are quite young rocks. The rocks are plutonic and metamorphic in nature.
The Blocks found on the crust on each side of the fault do not display a lithologic contrast that is as distinct as the one displayed in the central and northern part of California. The eastern wall of the San Andreas Fault is formed by Franciscan rocks and the rocks form the wall up to the Ranges found in the coast of California. However, the Franciscan rocks are partially concealed in some parts of the fault. The Franciscan rock is a rock made from a mixture of several rocks like, shale, traces of greywacke, traces of mafic rocks, a rare type of limestone and chert. The Franciscan rocks of the volcanic and sedimentary nature are believed to have been formed in a marine environment. This is due to the large deposits found of foraminifers contained in the limestone and the presences of radiolarians contained in the chert.
The age of the rocks are believed to be from the Cretaceous and Late Jurassic periods. However, some of the volcanic rocks and the chert are believed to be as old as the Early Jurassic period. The northern ranges of the coast of California are characterized with Franciscan rocks that are as young as the Late Tertiary period. Determination of the age and origin of the Franciscan rocks of the northern Coast Ranges has proven to be a problem since the limestone deposits found in the Laytonville area are believed to be from the Mid-Cretaceous period. Its origin is on the other hand believed to be from the south from a distance of about several thousand kilometers (Bailey, William and David 177).
The Franciscan rocks from the northwest of the San Francisco area are believed to have moved constantly to the north about 2000 kilometers from its point of origin (Gromme 114). The features described above and several others highly suggest that the most of the Franciscan rocks did not exist there originally but rather have been moved from great distances to settle where they are today. The ophiolite rock found in the Coast Range and the sequence of the Great Valley are believed to be the two overlaying structures of the Franciscan rocks. The separating factor on the other hand is the Coast Range thrust.
The Coast Range ophiolite is a rock that relates greatly to the Great Valley sequence since it represents the oceanic crust onto which most of the sedimentary rock belonging to the Great Valley sequence was deposited. The complete ophiolite sequence is composed of a series of steps. Te first step involves the serpentinized harzburgite tectonite being located at the base. This is then followed by the overlaying by cumulate ultramafic and gabbroic rocks and then these rocks pass upward and go into related plutonic and noncumulate gabbroic rocks. The movement upwards does not stop since the rocks move further upwards into diabase dikes and then finally further upwards into pillow lavas.
The Coast Range ophiolite is however in very thin deposits in the area where the fault creates contact between the Great Valley and the Franciscan Rocks. The rock only exists in full lithologic sequence in very few places with a total thickness ranging from 3 to 5 kilometers. The rock is estimated to have originated from the Middle or Late Jurassic Periods. The rocks are also believed to have been formed in an equatorial setting and then transported through great distances towards the north finally settling in North America and then they were overlain by the Great Valley sequence.
The Great Valley sequence is consisted of sandstone, conglomerate, and marine mudstone, all inter-bedded. The sequence is believed to have been formed in the Late Jurassic to Cretaceous periods. The Great Valley sequence crops out as several sections of layers that are monotonously bedded and are thick and less deformed than the Franciscan ones and they have a greater continuity laterally. It is mostly fully developed especially along the western side of the Valley. In areas such as these, the thickness of the sequence is about more than ten kilometers. Its layers lie in layers as they were deposited except on the areas where fault lines disrupt them.
The Fault also has three segments, the northern, central and southern segments. The southern segment is also known as the Mojave segment. It is located near the Bombay Beach and it borders the Salton Sea at the eastern side and located north of the East Pacific Rise. From here, it runs westward until it meets the Mountains of San Bernardino, then along the mountains on their southern side until it passes through the Cajon Pass. The segment then proceeds in a northwest direction along the San Gabriel Mountains northern base. The mountains mentioned above were all formed because of the activity of the fault and are referred to as the Transverse Range.
As the southern segment passes through Palmdale, the road is cut at Antelope Valley creating a freeway that runs through the area. The fault proceeds then to Frazier Park and then takes a turn to the north, an area commonly known as the big bend. Geologists believe that this area is where the fault locks itself up due to the bend and prevents the plates from moving past each other. It is located in Southern California. The area is also known to have a recurrence of earthquake movement that happens every 140 to 160 years. Past Frazier Park, the fault then moves to the northwest to Carrizo plain where it becomes easily visible. The Central Part of the San Andreas Fault runs from Parkfield to Hollister. This section unlike the Southern one does not experience any seismic activity and is said to possess the aseismic creep. This means that though the fault is slipping slowly, it does not cause any earthquakes in the area.
The Northern segment continues on from Hollister and passes through the Santa Cruz Mountains. These mountains were the epicenter for the Loma Prieta earthquake that happened in 1989. The fault then continues on to the San Francisco Peninsula. This is where, in 1895, it was discovered by Professor Lawson. It the moves to Mussel Rock at a place called Pacifica. Pacifica is believed to have been the epicenter for the earthquake that hit San Francisco in 1906. The fault then passes through the Bolinas Lagoon that is in the north of the Stinson Beach. From here, it proceeds onwards to the Tomales bay to the Bodega bay then to Fort Ross. From there it forms a valley that helps the Gualala River to flow and then heads to Point Arena then to Cape Mendocino.
The San Andreas Fault forms the sliding fault boundary between two plates, the North American Plate and the Pacific Plate. Geographically it divides the state of California into two parts from Cape Mendocino to the border of Mexico. The Pacific Plate houses Los Angeles, San Diego and Big Sur. The North American Plate houses Sierra Nevada, Sacramento and San Francisco. Though San Francisco was hit by a huge earthquake in 1906, it does not lie in the line of the fault. The fault however covers areas like Point Reyes Station, Wrightwood, San Bernardino, Palmdale, Frazier Park, Gorman, Daly City, Desert Hot Springs, Bodega Bay and San Bernardino. Moreover, they are at great risk due to the activity of the fault line.
The Fault is a transform fault. This means that it a fault line formed by two plates diagonally rubbing against one another. For instance if one had two slices of bread places on a table and then place them alongside one another on one straight edge. Then if they were rubbed against one another, the resultant effect would be the crumbling of the crumbs on either side of the pieces onto the anchovy side. This similarly happens to the fault lines and the landforms that form them. The geology of the areas adjacent to the fault line then greatly changes due to this effect. Geologists say that the plates are still in motion and move a couple of inches every year and compare this rate of movement to the rate of growth of the human fingernails.
The motion however is said not to be steady but rather an average motion. For several years now, the plates have been locked together and have had no movement at all. The plates shall then be pushing against one another and after a few years of built-up strain, the plates shall slip a few inches across one another after the rocks in between them have been broken up and crushed. While the rocks break and move, the waves that are sent out in all directions are the ones that are felt as earthquakes. There are several places where the fault can be seen clearly on the ground as the Carrizo Plain located in San Luis Obispo County among others. The fault is seen on the ground like a series of pressure ridges and scarps (Gromme 115).
However, in other areas the faults cannot be seen clearly due to the lack of activity from the fault line. Moreover, it is covered with deposits of alluvium and bushes have grown all over. Most of the roads that fall along the fault line pass through the mountains formed by the crumbled rock that has been formed by the moving plates. The part of the San Andreas Fault that acts as the most intriguing and beautiful thing is the presence of a vast number of different rocks that cover every side of the fault. Since the fault is believed to be about 28 million years old from the Late Jurassic period, it contains rocks that originated from very great distances and very different locations. For instance, a rock, the Salinian block that is composed of granite, is believed to have originated from either of two places, northern Mexico or South California. In Monterey County, there exists another hallmark created from the fault. The Pinnacles National Monument is a half of a volcanic complex but it is such a great wonder and its other half, the Neenach Volcanics, exists about 200 miles to the southeast in Los Angeles County.
There have been several myths, theories and legends formulated about the San Andreas Fault. It has been said that the fault shall one day slide so far that the state of California shall slide over and fall into the sea. Another theory is that there shall arise a kind of weather that shall include earthquakes hitting the area around the fault anytime of day. These myths and theories are wrong and geologically, the fault cannot crack open and there cannot be earthquakes at random times of the day. The fault is also a good site for tourists to visit or for family outings that involve hiking or climbing. This is because it offers beautiful scenery and it has very good access roads that snake through it (Bailey Blake and David C79).
Due to the state’s warm temperate climate and the accessibility of the fault, the visiting of the site becomes more enjoyable and it can be considered a holiday destination. There are also many species of birds and other wildlife together with secure locations that can promote camping. There are also many species of plants different types of rocks and immense natural beauty. This makes the area a hot spot for tourism and the national parks that surround the fault are very many. People from all over the world travel to California to witness the Great San Andreas Fault for themselves.
Works Cited
Bailey, Edgar, Blake, M.C., Jr., and David Jones. “On-land Mesozoic oceanic crust in California Coast Ranges.” Geological Survey research, 1970: U.S. Geological Survey Professional Paper 700-C (1970): C70-C81. Print.
Bailey, Edgar, William Irwin, and David Jones. Franciscan and Related Rocks and Their Significance in the Geology of Western California. San Francisco: California Division of Mines and Geology, 1964. Print.
Gromme, Charles. “Paleomagnetism of Franciscan basalt, Marin County, California.” Blake, M. C. Jr., ed., Franciscan geology of northern California: Los Angeles, Society of Economic Paleontologists and Mineralogists, Pacific Section, v. 43, p. 113-119. 1984. Print.
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