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50-551: Due to the relative plate motions, the triple junction has been migrating northwards for the past 25–30 million years, and assuming rigid plates, the geometry requires that a void, called slab window , develop southeast of the MTJ. At this point, removal of the subducting Gorda lithosphere from beneath North America causes asthenospheric upwelling. This instigates different tectonic processes, which include surficial uplift, crustal deformation, intense seismic activity, high heat flow, and even

100-405: A decollement . Extensional decollements can grow to great dimensions and form detachment faults , which are low-angle normal faults with regional tectonic significance. Due to the curvature of the fault plane, the horizontal extensional displacement on a listric fault implies a geometric "gap" between the hanging and footwalls of the fault forms when the slip motion occurs. To accommodate into

150-829: A plate boundary. This class is related to an offset in a spreading center , such as a mid-ocean ridge , or, less common, within continental lithosphere , such as the Dead Sea Transform in the Middle East or the Alpine Fault in New Zealand. Transform faults are also referred to as "conservative" plate boundaries since the lithosphere is neither created nor destroyed. Dip-slip faults can be either normal (" extensional ") or reverse . The terminology of "normal" and "reverse" comes from coal mining in England, where normal faults are

200-574: A fault hosting valuable porphyry copper deposits is northern Chile's Domeyko Fault with deposits at Chuquicamata , Collahuasi , El Abra , El Salvador , La Escondida and Potrerillos . Further south in Chile Los Bronces and El Teniente porphyry copper deposit lie each at the intersection of two fault systems. Faults may not always act as conduits to surface. It has been proposed that deep-seated "misoriented" faults may instead be zones where magmas forming porphyry copper stagnate achieving

250-500: A fault is locked, and when it reaches a level that exceeds the strength threshold, the fault ruptures and the accumulated strain energy is released in part as seismic waves , forming an earthquake . Strain occurs accumulatively or instantaneously, depending on the liquid state of the rock; the ductile lower crust and mantle accumulate deformation gradually via shearing , whereas the brittle upper crust reacts by fracture – instantaneous stress release – resulting in motion along

300-410: A fault often forms a discontinuity that may have a large influence on the mechanical behavior (strength, deformation, etc.) of soil and rock masses in, for example, tunnel , foundation , or slope construction. The level of a fault's activity can be critical for (1) locating buildings, tanks, and pipelines and (2) assessing the seismic shaking and tsunami hazard to infrastructure and people in

350-408: A fault's age by studying soil features seen in shallow excavations and geomorphology seen in aerial photographs. Subsurface clues include shears and their relationships to carbonate nodules , eroded clay, and iron oxide mineralization, in the case of older soil, and lack of such signs in the case of younger soil. Radiocarbon dating of organic material buried next to or over a fault shear

400-477: A result of rock-mass movements. Large faults within Earth 's crust result from the action of plate tectonic forces, with the largest forming the boundaries between the plates, such as the megathrust faults of subduction zones or transform faults . Energy release associated with rapid movement on active faults is the cause of most earthquakes . Faults may also displace slowly, by aseismic creep . A fault plane

450-543: Is often critical in distinguishing active from inactive faults. From such relationships, paleoseismologists can estimate the sizes of past earthquakes over the past several hundred years, and develop rough projections of future fault activity. Many ore deposits lie on or are associated with faults. This is because the fractured rock associated with fault zones allow for magma ascent or the circulation of mineral-bearing fluids. Intersections of near-vertical faults are often locations of significant ore deposits. An example of

500-444: Is the plane that represents the fracture surface of a fault. A fault trace or fault line is a place where the fault can be seen or mapped on the surface. A fault trace is also the line commonly plotted on geologic maps to represent a fault. A fault zone is a cluster of parallel faults. However, the term is also used for the zone of crushed rock along a single fault. Prolonged motion along closely spaced faults can blur

550-456: The Chesapeake Bay impact crater . Ring faults are the result of a series of overlapping normal faults, forming a circular outline. Fractures created by ring faults may be filled by ring dikes . Synthetic and antithetic are terms used to describe minor faults associated with a major fault. Synthetic faults dip in the same direction as the major fault while the antithetic faults dip in

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600-487: The Franciscan Complex . This unit is made of sandstones, shales, cherts, metagraywackes, melanges, as well as mafic volcanics, and is mostly metamorphosed to blueschist and eclogite facies. The spatial distribution of heat flow in the vicinity of the MTJ is similar to what would be expected in a subduction environment. That is, heat flow is low above the subducting Gorda slab, between 40 and 50 mW/m. South of

650-709: The Gorda Deformation Zone (GDZ) and resulting in abundant seismicity. Motion along the Mendocino Transform Fault (MTF) is right-lateral on E–W oriented, vertically dipping planes. Within the portion of North American crust overlying the Gorda slab, motion on faults is reverse, and in April 1992, a M 7.1 earthquake ruptured the southern portion of the Cascadia subduction zone. Similar to the general seismicity patterns in

700-534: The North American plate . These effects include distinct fore-arc volcanism and extension in the plate which may be a contributing factor to the formation of the Basin and Range Province . The northward younging of Pemberton Belt volcanism in southwestern British Columbia , Canada, may have been related to a northward moving slab window edge under North America 29 to 6.8 million years ago. In addition to

750-545: The MTJ, heat flow through the California coast is higher, around 90 mW/m. The distance south of the MTJ over which heat flow increases gives an indication of the timing of development of the heat flow anomaly. The observed surface heat flow doubles over a distance of ~200 km, corresponding to a timeframe of migration of 4–5 Ma. It is also consistent with a source for the anomaly, thought to be asthenospheric mantle material, emplaced at shallow depths of 15–25 km, i.e. in

800-410: The adjacent lithosphere, eventually welding to it and moving along with it, analogous to the motion of a conveyor belt. Lower crust-upper mantle viscous coupling plays a dominant role in converting accretionary margin materials into continent-like crust. Researchers were able to demonstrate that in this 'conveyor belt' mechanism, the crust is first thickened north of the triple junction, and after passage,

850-408: The angle the ridge intersects the subduction zone and the dip angle of the down-going plate. Other influential factors include the rates of divergence and subduction as well as heterogeneities found within specific systems. There are two end-member scenarios in terms of the geometry of a slab window: the first is when the subducted ridge is perpendicular to the trench, producing a V-shaped window, and

900-478: The crust is thinned south of the triple junction. In this way, as the MTJ migrates, there is production of a basal conveyor belt beneath North America that transports material from the south to the north. This is consistent with an observed pattern of anomalous crustal structure determined by local-earthquake crustal tomography in the region. The region is dominated by Mesozoic-to-Cretaceous aged rocks which make up an uplifted subduction zone accretionary wedge called

950-414: The crust. A thrust fault has the same sense of motion as a reverse fault, but with the dip of the fault plane at less than 45°. Thrust faults typically form ramps, flats and fault-bend (hanging wall and footwall) folds. A section of a hanging wall or foot wall where a thrust fault formed along a relatively weak bedding plane is known as a flat and a section where the thrust fault cut upward through

1000-432: The decreased lithospheric volume can also produce decompression melting. Slab window melts are distinguished from calc-alkaline subduction-related magmas by their different chemical compositions. The increase in temperature caused by the presence of a slab window can also produce anomalous high temperature metamorphism in the region between the trench and the volcanic arc. The geometry of a slab window depends primarily on

1050-433: The direction of extension or shortening changes during the deformation but the earlier formed faults remain active. The hade angle is defined as the complement of the dip angle; it is the angle between the fault plane and a vertical plane that strikes parallel to the fault. Ring faults , also known as caldera faults , are faults that occur within collapsed volcanic calderas and the sites of bolide strikes, such as

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1100-431: The distinction, as the rock between the faults is converted to fault-bound lenses of rock and then progressively crushed. Due to friction and the rigidity of the constituent rocks, the two sides of a fault cannot always glide or flow past each other easily, and so occasionally all movement stops. The regions of higher friction along a fault plane, where it becomes locked, are called asperities . Stress builds up when

1150-405: The established tectonic and magmatic regimes. In general, the data used to identify possible slab windows comes from seismic tomography and heat flow studies. As a slab window develops, the mantle in that region becomes increasingly hot and dry. The decrease in hydration causes arc volcanism to diminish or stop entirely, as magma production in subduction zones generally results from hydration of

1200-425: The extrusion of volcanic rocks. This activity is centered on the current triple junction position, but evidence for its migration is found in the geology all along the California coast, starting as far south as Los Angeles. The passage of the MTJ causes mantle material to flow into the region vacated by the Gorda plate. Once this hot mantle material is south of the triple junction, it cools, stiffens, and accretes to

1250-409: The fault (called a piercing point ). In practice, it is usually only possible to find the slip direction of faults, and an approximation of the heave and throw vector. The two sides of a non-vertical fault are known as the hanging wall and footwall . The hanging wall occurs above the fault plane and the footwall occurs below it. This terminology comes from mining: when working a tabular ore body,

1300-532: The fault is the vertical component of the separation and the heave of the fault is the horizontal component, as in "Throw up and heave out". The vector of slip can be qualitatively assessed by studying any drag folding of strata, which may be visible on either side of the fault. Drag folding is a zone of folding close to a fault that likely arises from frictional resistance to movement on the fault. The direction and magnitude of heave and throw can be measured only by finding common intersection points on either side of

1350-461: The fault movement. Faults are mainly classified in terms of the angle that the fault plane makes with the Earth's surface, known as the dip , and the direction of slip along the fault plane. Based on the direction of slip, faults can be categorized as: In a strike-slip fault (also known as a wrench fault , tear fault or transcurrent fault ), the fault surface (plane) is usually near vertical, and

1400-406: The fault. A fault in ductile rocks can also release instantaneously when the strain rate is too great. Slip is defined as the relative movement of geological features present on either side of a fault plane. A fault's sense of slip is defined as the relative motion of the rock on each side of the fault concerning the other side. In measuring the horizontal or vertical separation, the throw of

1450-428: The footwall moves laterally either left or right with very little vertical motion. Strike-slip faults with left-lateral motion are also known as sinistral faults and those with right-lateral motion as dextral faults. Each is defined by the direction of movement of the ground as would be seen by an observer on the opposite side of the fault. A special class of strike-slip fault is the transform fault when it forms

1500-531: The footwall. The dip of most normal faults is at least 60 degrees but some normal faults dip at less than 45 degrees. A downthrown block between two normal faults dipping towards each other is a graben . A block stranded between two grabens, and therefore two normal faults dipping away from each other, is a horst . A sequence of grabens and horsts on the surface of the Earth produces a characteristic basin and range topography . Normal faults can evolve into listric faults, with their plane dip being steeper near

1550-665: The fossil slab windows of the Cenozoic seen in North America, there are other regions along the Pacific Rim (e.g. in California, Mexico, Costa Rica, Patagonia and the Antarctic Peninsula) that exhibit active ridge subduction producing slab windows. Fault (geology)#Strike-slip faults In geology , a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as

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1600-429: The geometric gap, and depending on its rheology , the hanging wall might fold and slide downwards into the gap and produce rollover folding , or break into further faults and blocks which fil in the gap. If faults form, imbrication fans or domino faulting may form. A reverse fault is the opposite of a normal fault—the hanging wall moves up relative to the footwall. Reverse faults indicate compressive shortening of

1650-491: The implied mechanism of deformation. A fault that passes through different levels of the lithosphere will have many different types of fault rock developed along its surface. Continued dip-slip displacement tends to juxtapose fault rocks characteristic of different crustal levels, with varying degrees of overprinting. This effect is particularly clear in the case of detachment faults and major thrust faults . The main types of fault rock include: In geotechnical engineering ,

1700-456: The intrusion of plutons within the overlying crust in the region. An example of volcanic bodies that formed by magma upwelling and solidification are the Nine Sisters , located between Morro Bay and San Luis Obispo in California. The source of the material which flows into the slab window is a matter of debate, specifically whether it is derived directly from the underlying mantle, or from

1750-464: The largest faults on Earth and give rise to the largest earthquakes. A fault which has a component of dip-slip and a component of strike-slip is termed an oblique-slip fault . Nearly all faults have some component of both dip-slip and strike-slip; hence, defining a fault as oblique requires both dip and strike components to be measurable and significant. Some oblique faults occur within transtensional and transpressional regimes, and others occur where

1800-409: The mantle wedge due to de-watering of the subducting slab. Slab-window magmatism may then replace this melting, and can be produced by multiple processes, including increased temperatures, mantle circulation producing interaction of supra- and sub-slab mantle, partial melting of subducted slab edges and extension in the upper plate. Mantle flowing upward through the slab window in order to compensate for

1850-404: The mantle wedge to the east. The chemistry of erupted basalts associated with the MTJ are typical of mantle wedge–derived melts, characterized by enrichment in the large-ion lithophile elements and depletion in the high-field-strength elements. In general, mantle wedge-derived melts are relatively more hydrous, have lower viscosity and temperatures than melts derived from sub-slab mantle. Most of

1900-408: The miner stood with the footwall under his feet and with the hanging wall above him. These terms are important for distinguishing different dip-slip fault types: reverse faults and normal faults. In a reverse fault, the hanging wall displaces upward, while in a normal fault the hanging wall displaces downward. Distinguishing between these two fault types is important for determining the stress regime of

1950-435: The most common. With the passage of time, a regional reversal between tensional and compressional stresses (or vice-versa) might occur, and faults may be reactivated with their relative block movement inverted in opposite directions to the original movement (fault inversion). In such a way, a normal fault may therefore become a reverse fault and vice versa. In a normal fault, the hanging wall moves downward, relative to

2000-482: The opposite direction. These faults may be accompanied by rollover anticlines (e.g. the Niger Delta Structural Style). All faults have a measurable thickness, made up of deformed rock characteristic of the level in the crust where the faulting happened, of the rock types affected by the fault and of the presence and nature of any mineralising fluids . Fault rocks are classified by their textures and

2050-464: The orientation of compression in the GDZ northwest of the MTJ. 40°22′N 124°36′W  /  40.367°N 124.600°W  / 40.367; -124.600 Slab window In geology , a slab window is a gap that forms in a subducted oceanic plate when a mid-ocean ridge meets with a subduction zone and plate divergence at the ridge and convergence at the subduction zone continue, causing

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2100-512: The region, the majority of the aftershocks for this event had vertical strike-slip motions and were located within the Gorda plate, or on the MTF at depths between 23 and 35 km. None of the aftershocks were consistent with northeast underthrusting of the Gorda plate beneath North America, as was the case in the main event. This set of earthquake geometries implies a stress field characterized by N–NW, horizontal principal compressive stress consistent with

2150-399: The ridge to be subducted. Formation of a slab window produces an area where the crust of the over-riding plate is lacking a rigid lithospheric mantle component and thus is exposed to hot asthenospheric mantle (for a diagram of this, see the link below). This produces anomalous thermal, chemical and physical effects in the mantle that can dramatically change the over-riding plate by interrupting

2200-412: The right time for—and type of— igneous differentiation . At a given time differentiated magmas would burst violently out of the fault-traps and head to shallower places in the crust where porphyry copper deposits would be formed. As faults are zones of weakness, they facilitate the interaction of water with the surrounding rock and enhance chemical weathering . The enhanced chemical weathering increases

2250-520: The second is when the ridge is parallel to the trench, causing a rectangular window to form. The North American Cordillera is a well-studied plate margin that provides a good example of the effects a slab window can have on an over-riding continental plate. Beginning in the Cenozoic , the fragmentation of the Farallon plate as it subducted caused slab windows to open that then generated anomalous features in

2300-458: The seismicity near the MTJ is offshore, concentrated along the Mendocino Transform Fault. Seismicity is also distributed within the Gorda plate itself, owing to its small size, young age, and relatively thin lithosphere. Oblique convergence of the Gorda plate towards the Pacific plate causes intense north–south compression, and induces anomalously strong internal deformation in the former, giving rise to

2350-405: The slab window. This rise of the heat flow anomaly time implies that there is probably only a thin crustal lid above the slab window. The presence of hot asthenospheric mantle at shallow levels beneath the western margin of North America is likely to generate melt and cause magmatism. Accordingly, a sequence of volcanoes in the wake of the MTJ passing were activated; this magmatism likely leads to

2400-416: The stratigraphic sequence is known as a ramp . Typically, thrust faults move within formations by forming flats and climbing up sections with ramps. This results in the hanging wall flat (or a portion thereof) lying atop the foot wall ramp as shown in the fault-bend fold diagram. Thrust faults form nappes and klippen in the large thrust belts. Subduction zones are a special class of thrusts that form

2450-400: The surface, then shallower with increased depth, with the fault plane curving into the Earth. They can also form where the hanging wall is absent (such as on a cliff), where the footwall may slump in a manner that creates multiple listric faults. The fault panes of listric faults can further flatten and evolve into a horizontal or near-horizontal plane, where slip progresses horizontally along

2500-570: The vicinity. In California, for example, new building construction has been prohibited directly on or near faults that have moved within the Holocene Epoch (the last 11,700 years) of the Earth's geological history. Also, faults that have shown movement during the Holocene plus Pleistocene Epochs (the last 2.6 million years) may receive consideration, especially for critical structures such as power plants, dams, hospitals, and schools. Geologists assess

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