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56-620: The Farallon Trench was a subduction related tectonic formation located off the coast of the western California continental margin during the late to mid Cenozoic era, around 50 miles southeast of modern-day Monterey Bay . The time duration of subduction began from around 165 Ma when the Farallon Plate replaced the Mezcalera promontory, until the San Andreas Fault straightening around 35 Ma. As data accumulated over time,

112-581: A slab window . Other models have been proposed for the Farallon's influence on the Laramide orogeny, including the dewatering of the slab which led to intense uplift and magmatism . Notes Bibliography Seismic tomography Seismic tomography or seismotomography is a technique for imaging the subsurface of the Earth using seismic waves . The properties of seismic waves are modified by

168-403: A characteristic influenced by the presence of oceanic plateaus (or oceanic flood basalts). In addition to influencing slab buoyancy, some oceanic plateaus may have also become accreted to North America. It has been suggested that this deformation may go so far as to include a tear in the slab, where a piece of the subducted Farallon plate has broken off, creating multiple slab remnants. This

224-501: A common view developed that one large oceanic plate, the Farallon Plate, acted as a conveyor belt, conveying accreted terranes onto the North American west coast. As the continent overran the subducting Farallon Plate, the denser plate became subducted into the mantle below the continent. When the plates converged, the dense oceanic plate sank into the mantle to form a slab below the lighter continent. Rapid subduction under

280-410: A large effect on the image created. For example, commonly used tomographic methods work by iteratively improving an initial input model, and thus can produce unrealistic results if the initial model is unreasonable. P-wave data are used in most local models and global models in areas with sufficient earthquake and seismograph density. S- and surface wave data are used in global models when this coverage

336-467: A model limits the resolution it can achieve. Longer wavelengths are able to penetrate deeper into the Earth, but can only be used to resolve large features. Finer resolution can be achieved with surface waves, with the trade off that they cannot be used in models deeper than the crust and upper mantle. The disparity between wavelength and feature scale causes anomalies to appear of reduced magnitude and size in images. P- and S-wave models respond differently to

392-480: A radial path through the Earth, and assumes this profile is valid for every path from the core to the surface. This 1984 study was also the first to apply the term "tomography" to seismology, as the term had originated in the medical field with X-ray tomography . Seismic tomography has continued to improve in the past several decades since its initial conception. The development of adjoint inversions, which are able to combine several different types of seismic data into

448-442: A result of thermal or chemical differences, which are attributed to processes such as mantle plumes, subducting slabs, and mineral phase changes. Larger scale features that can be imaged with tomography include the high velocities beneath continental shields and low velocities under ocean spreading centers . The mantle plume hypothesis proposes that areas of volcanism not readily explained by plate tectonics, called hotspots , are

504-418: A result of thermal upwelling within the mantle. Some researchers have proposed an upper mantle source above the 660km discontinuity for these plumes, while others propose a much deeper source, possibly at the core-mantle boundary . While the source of mantle plumes has been highly debated since they were first proposed in the 1970s, most modern studies argue in favor of mantle plumes originating at or near

560-432: A seismically active region with extensive permanent network coverage. These allow for the imaging of the crust and upper mantle . Regional to global scale tomographic models are generally based on long wavelengths. Various models have better agreement with each other than local models due to the large feature size they image, such as subducted slabs and superplumes . The trade off from whole mantle to whole Earth coverage

616-404: A single inversion, help negate some of the trade-offs associated with any individual data type. Historically, seismic waves have been modeled as 1D rays, a method referred to as "ray theory" that is relatively simple to model and can usually fit travel-time data well. However, recorded seismic waveforms contain much more information than just travel-time and are affected by a much wider path than

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672-435: Is assumed by ray theory. Methods like the finite-frequency method attempt to account for this within the framework of ray theory. More recently, the development of "full waveform" or "waveform" tomography has abandoned ray theory entirely. This method models seismic wave propagation in its full complexity and can yield more accurate images of the subsurface. Originally these inversions were developed in exploration seismology in

728-418: Is not sufficient, such as in ocean basins and away from subduction zones. First-arrival times are the most widely used, but models utilizing reflected and refracted phases are used in more complex models, such as those imaging the core. Differential traveltimes between wave phases or types are also used. Local tomographic models are often based on a temporary seismic array targeting specific areas, unless in

784-487: Is supported by tomography studies and provides some more explanation of the formation of Laramide structures that are further inland from the edge. A 2013 study proposed two additional now-subducted plates that would account for some of the unexplained complexities of the accreted terranes, suggesting that the Farallon should be partitioned into Northern Farallon, Angayucham , Mezcalera and Southern Farallon segments based on recent tomographic models. Under this model,

840-473: Is the coarse resolution (hundreds of kilometers) and difficulty imaging small features (e.g. narrow plumes). Although often used to image different parts of the subsurface, P- and S-wave derived models broadly agree where there is image overlap. These models use data from both permanent seismic stations and supplementary temporary arrays. Seismic tomography can resolve anisotropy, anelasticity, density, and bulk sound velocity. Variations in these parameters may be

896-451: The convergent plate boundary than is typical of a subduction-generated orogeny . Significant deformation of the slab also occurred due to this flat subduction phenomenon, which has been imaged by seismic tomography. There is a concentration of velocity anomalies in the tomography that is thicker than the slab itself should be, indicating that folding and deformation occurred beneath the surface during subduction. In other words, more of

952-409: The lower-mantle . Further study will be needed to understand this inconsistency in data and will, with all luck, provide a solid and concrete understanding of the western continental margin of North America and its complexities upon completion. Farallon Plate Over time, the central part of the Farallon plate was subducted under the southwestern part of the North American plate. The remains of

1008-431: The underlying magmatic system . These images have most commonly been used to estimate the depth and volume of magma stored in the crust, but have also been used to constrain properties such as the geometry, temperature, or chemistry of the magma. It is important to note that both lab experiments and tomographic imaging studies have shown that recovering these properties from seismic velocity alone can be difficult due to

1064-400: The 1980s and 1990s and were too computationally complex for global and regional scale studies, but development of numerical modeling methods to simulate seismic waves has allowed waveform tomography to become more common. Seismic tomography uses seismic records to create 2D and 3D models of the subsurface through an inverse problem that minimizes the difference between the created model and

1120-671: The Farallon Plate as far inland as Utah and Arizona. The earliest record of subhorizontal subduction of the Farallon slab is the extinguishing of magmatism in the Sierra Nevada batholith of California roughly 85 Ma. As the Farallon Plate subducted below the California continental margin an accretionary wedge was formed in the trench, which yielded unique rock types as a result of regional metamorphism . The formation of Franciscan Melange and blueschist units along paleo-coastlines resulted from this subduction and are direct evidence of

1176-703: The Farallon Plate by the Pacific Plate, created the Juan de Fuca Plate to the north and the Cocos Plate to the south. The final stages of the evolution of California's continental margin was the growth of the San Andreas transform fault system, which formed as the Pacific Plate came into contact with the continental margin and the MTJ was formed. As subduction of the Pacific Plate continued along this margin, and

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1232-549: The Farallon Plate's past existence. Other forms of evidence include the Farallon Islands, Catalina Islands, and uplift of the Diablo Mountain Range as a result of the clogged subduction zone mentioned above. These observations can be explained by a model for the weakening and ultimate falling apart of the uppermost part of the subducted oceanic plate in the 20–30 m.y. after the end of rapid subduction. As

1288-818: The Farallon plate are the Explorer , Gorda , and Juan de Fuca plates, which are subducting under the northern part of the North American plate ; the Cocos plate subducting under Central America ; and the Nazca plate subducting under the South American plate . The Farallon plate is also responsible for transporting old island arcs and various fragments of continental crust , which have rifted off of other distant plates. These fragments from elsewhere are called terranes (sometimes, "exotic" terranes). During

1344-606: The Farallon slab appears as a velocity anomaly on the tomography model. Multiple studies show that the subduction of the Farallon plate was characterized by a period of " flat-slab subduction ," which is the subduction of a plate at a relatively shallow angle to the overriding crust (in this case, North America). This phenomenon is one that accounts for the far-inland orogenisis of the Rocky Mountains and other ranges in North America which are much farther from

1400-418: The North American continent overrode a series of subduction trenches, and several microcontinents (similar to those in the modern-day Indonesian Archipelago ) were added to it. These microcontinents must have had adjacent oceanic plates that are not represented in previous models of Farallon subduction, so this interpretation brings forth a different perspective on the history of collision. Based on this model,

1456-475: The accuracy of a model. As early as 1972, researchers successfully used some of the underlying principles of modern seismic tomography to search for fast and slow areas in the subsurface. The first widely cited publication that largely resembles modern seismic tomography was published in 1976 and used local earthquakes to determine the 3D velocity structure beneath Southern California. The following year, P-wave delay times were used to create 2D velocity maps of

1512-504: The complexity of seismic wave propagation through focused zones of hot, potentially melted rocks. While comparatively primitive to tomography on Earth, seismic tomography has been proposed on other bodies in the solar system and successfully used on the Moon . Data collected from four seismometers placed by the Apollo missions have been used many times to create 1-D velocity profiles for

1568-540: The complexity of this coast line. As of 2013, it is generally accepted that the western quarter of North America consists of accreted terrane accumulated over roughly the past 200 m.y as the remnant Farallon Plate (the Juan De Fuca and Cocos plates) continues to convey oceanic terrane onto the continental margin . This model, however, was unable to explain many terrane complexities, and is inconsistent with seismic tomographic images of subducting slabs which penetrate

1624-587: The contact zone grew, the San Andreas proportionally grew as well. Evidence of the existence of the Farallon Trench and past subduction of the Farallon Plate is evident in specific geologic units observed along paleo-coastlines of the west coast of the United States and California continental region. Late Cretaceous–Paleogene magma can be seen overlying subhorizontally subducted sediments from

1680-437: The core-mantle boundary. This is in large part due to tomographic images that reveal both the plumes themselves as well as large low-velocity zones in the deep mantle that likely contribute to the formation of mantle plumes. These large low-shear velocity provinces as well as smaller ultra low velocity zones have been consistently observed across many tomographic models of the deep Earth Subducting plates are colder than

1736-463: The geologic setting, seismometer coverage, distance from nearby earthquakes, and required resolution. The model created by tomographic imaging is almost always a seismic velocity model , and features within this model may be interpreted as structural, thermal, or compositional variations. Geoscientists apply seismic tomography to a wide variety of settings in which the subsurface structure is of interest, ranging in scale from whole-Earth structure to

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1792-518: The interiors of other planetary bodies when only a single seismometer is available. For example, data gathered by the SEIS (Seismic Experiment for Interior Structure) instrument on InSight on Mars has been able to detect the Martian core. Global seismic networks have expanded steadily since the 1960s, but are still concentrated on continents and in seismically active regions. Oceans, particularly in

1848-616: The late-20th century, tomography is only capable of viewing changes in velocity structure over decades. For example, tectonic plates only move at millimeters per year, so the total amount of change in geologic structure due to plate tectonics since the development of seismic tomography is several orders of magnitude lower than the finest resolution possible with modern seismic networks. However, seismic tomography has still been used to view near-surface velocity structure changes at time scales of years to months. Tomographic solutions are non-unique. Although statistical methods can be used to analyze

1904-451: The location of the earthquake hypocenter . CT scans use linear x-rays and a known source. In the early 20th century, seismologists first used travel time variations in seismic waves from earthquakes to make discoveries such as the existence of the Moho and the depth to the outer core. While these findings shared some underlying principles with seismic tomography, modern tomography itself

1960-451: The mantle into which they are moving. This creates a fast anomaly that is visible in tomographic images. Tomographic images have been made of most subduction zones around the world and have provided insight into the geometries of the crust and upper mantle in these areas. These images have revealed that subducting plates vary widely in how steeply they move into the mantle. Tomographic images have also seen features such as deeper portions of

2016-483: The material through which they travel. By comparing the differences in seismic waves recorded at different locations, it is possible to create a model of the subsurface structure. Most commonly, these seismic waves are generated by earthquakes or man-made sources such as explosions. Different types of waves, including P- , S- , Rayleigh , and Love waves can be used for tomographic images, though each comes with their own benefits and downsides and are used depending on

2072-432: The measured seismic waveform to be fit during the inversion. Seismic tomography is similar to medical x-ray computed tomography (CT scan) in that a computer processes receiver data to produce a 3D image, although CT scans use attenuation instead of travel-time difference. Seismic tomography has to deal with the analysis of curved ray paths which are reflected and refracted within the Earth, and potential uncertainty in

2128-402: The moon, and less commonly 3-D tomographic models. Tomography relies on having multiple seismometers, but tomography-adjacent methods for constraining Earth structure have been used on other planets. While on Earth these methods are often used in combination with seismic tomography models to better constrain the locations of subsurface features, they can still provide useful information about

2184-468: The observed seismic data. Various methods are used to resolve anomalies in the crust , lithosphere , mantle , and core based on the availability of data and types of seismic waves that pass through the region. Longer wavelengths penetrate deeper into the Earth, but seismic waves are not sensitive to features significantly smaller than their wavelength and therefore provide a lower resolution. Different methods also make different assumptions, which can have

2240-442: The plate falls apart, not only is compressional stress relieved, but significant back-slip along the old subduction zone is also possible, perhaps bringing blueschist rapidly upward from 20- to 30-km depths, where it can be observed along the California coast to this day. To understand the subduction of the Farallon Plate, the creation of the Farallon Trench, and the present location of the subducted plate, detailed seismic tomography

2296-457: The plate moved west, causing the following geologic events to occur: When the final archipelago , the Siletzia archipelago, lodged as a terrane, the associated trench stepped west. When this happened, the trench that had been characterized as an oceanic-oceanic subduction environment approached the North American margin and eventually became the current Cascadia subduction zone . This created

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2352-614: The position at which the flexed subducting slab begins to descend beneath and deform the continental plate margin. By 43 Ma, during the Eocene, worldwide plate motions changed and the Pacific Plate began to move away from North America and subduction of the Farallon Plate slowed dramatically. By around 36 Ma, the easternmost part of the East Pacific Rise, located between the Pioneer and Murray fracture zones at that time, approached

2408-409: The reflection and refraction of these waves. The location and magnitude of variations in the subsurface can be calculated by the inversion process, although solutions to tomographic inversions are non-unique. Most commonly, only the travel time of the seismic waves is considered in the inversion. However, advances in modeling techniques and computing power have allowed different parts, or the entirety, of

2464-420: The slab should be in the lower mantle, but the deformation has caused it to remain shallower, in the upper mantle. Multiple hypotheses have been proposed to explain this shallow subduction angle and resulting deformation. Some studies suggest that the faster movement of the North American plate caused the slab to flatten, resulting in slab rollback . Another cause of flat slab subduction may be slab buoyancy ,

2520-581: The southern hemisphere, are under-covered. Temporary seismic networks have helped improve tomographic models in regions of particular interest, but typically only collect data for months to a few years. The uneven distribution of earthquakes biases tomographic models towards seismically active regions. Methods that do not rely on earthquakes such as active source surveys or ambient noise tomography have helped image areas with little to no seismicity, though these both have their own limitations as compared to earthquake-based tomography. The type of seismic wave used in

2576-399: The southwestern North America continent began 40 to 60 million years ago (Ma), during the mid Paleocene to mid Eocene epochs. This convergent subduction margin created a distinctive geomorphologic feature called an oceanic trench , which occurs at a convergent plate boundaries as a heavy metal rich, lithospheric plate moves below a light silica rich continental plate . The trench marks

2632-793: The subducting plate tearing off from the upper portion. Tomography can be used to image faults to better understand their seismic hazard . This can be through imaging the fault itself by seeing differences in seismic velocity across the fault boundary or by determining near-surface velocity structure, which can have a large impact on the magnitude on the amplitude of ground-shaking during an earthquake due to site amplification effects . Near-surface velocity structure from tomographic images can also be useful for other hazards, such as monitoring of landslides for changes in near-surface moisture content which has an effect on both seismic velocity and potential for future landslides. Tomographic images of volcanoes have yielded new insights into properties of

2688-445: The subduction of the Farallon plate, it accreted these island arcs and terranes to the North American plate . Much of western North America is composed of these accreted terranes. As an ancient tectonic plate, the Farallon plate must be studied using methods that allow researchers to see deep beneath the Earth's surface. The understanding of the Farallon plate has evolved as details from seismic tomography provide improved details of

2744-461: The submerged remnants. Since the North American west coast has a convoluted structure, significant work has been required to resolve the complexity. Seismic tomography can be used to image the remainder of the subducted plate because it is still "cold," as in, it has not reached thermal equilibrium with the mantle. This is important for the use of tomography because seismic waves have different velocities in materials of different temperatures, so

2800-562: The trench and the young, hot, buoyant lithosphere appears to have clogged part of the subduction zone, resulting in widespread dramatic uplift on land. The eventual complete subduction of this plate, consequential contact of the Pacific Plate with the California continental margin, and creation of the Mendocino Triple Junction (MTJ), took place around 30 to 20 Ma. The partial complete subduction and division of

2856-523: The types of anomalies. Models based solely on the wave that arrives first naturally prefer faster pathways, causing models based on these data to have lower resolution of slow (often hot) features. This can prove to be a significant issue in areas such as volcanoes where rocks are much hotter than their surroundings and oftentimes partially melted. Shallow models must also consider the significant lateral velocity variations in continental crust. Because seismometers have only been deployed in large numbers since

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2912-474: The upper few meters below the surface. Tomography is solved as an inverse problem . Seismic data are compared to an initial Earth model and the model is modified until the best possible fit between the model predictions and observed data is found. Seismic waves would travel in straight lines if Earth was of uniform composition, but structural , chemical, and thermal variations affect the properties of seismic waves, most importantly their velocity , leading to

2968-490: The validity of a model, unresolvable uncertainty remains. This contributes to difficulty comparing the validity of different model results. Computing power limits the amount of seismic data, number of unknowns, mesh size, and iterations in tomographic models. This is of particular importance in ocean basins, which due to limited network coverage and earthquake density require more complex processing of distant data. Shallow oceanic models also require smaller model mesh size due to

3024-473: The whole Earth at several depth ranges, representing an early 3D model. The first model using iterative techniques, which improve upon an initial model in small steps and are required when there are a large number of unknowns, was done in 1984. The model was made possible by iterating upon the first radially anisotropic Earth model , created in 1981. A radially anisotropic Earth model describes changes in material properties, specifically seismic velocity, along

3080-620: Was not developed until the 1970s with the expansion of global seismic networks. Networks like the World-Wide Standardized Seismograph Network were initially motivated by underground nuclear tests , but quickly showed the benefits of their accessible, standardized datasets for geoscience . These developments occurred concurrently with advancements in modeling techniques and computing power that were required to solve large inverse problems and generate theoretical seismograms, which are required to test

3136-457: Was used to render images of the existing submerged remnants. The plate can now be seen at depths of around 200 km below the central continental United States. Since the North American coast shows an extremely complicated geologic structure, intensive work has been required to understand the complexity of this system. In 2013 a new explanation emerged from recent research, proposing two additional now fully subducted plates, accounting for some of

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