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60-712: The Terceira Rift is a geological rift located amidst the Azores islands in the Atlantic Ocean . It runs between the Azores triple junction to the west and the Azores–Gibraltar Transform Fault to the southeast. It separates the Eurasian plate to the north from the African plate to the south. The Terceira Rift is named for Terceira Island through which it passes. It crosses Terceira Island as

120-449: A lacustrine environment or in a restricted marine environment, although not all rifts contain such sequences. Reservoir rocks may be developed in pre-rift, syn-rift and post-rift sequences. Effective regional seals may be present within the post-rift sequence if mudstones or evaporites are deposited. Just over half of estimated oil reserves are found associated with rifts containing marine syn-rift and post-rift sequences, just under

180-468: A common feature at oceanic spreading centers. A feature of the elevated ridges is their relatively high heat flow values, of about 1–10 μcal/cm s, or roughly 0.04–0.4 W/m . Most crust in the ocean basins is less than 200 million years old, which is much younger than the 4.54 billion year age of Earth . This fact reflects the process of lithosphere recycling into the Earth's mantle during subduction . As

240-462: A high angle. These segment boundary zones accommodate the differences in fault displacement between the segments and are therefore known as accommodation zones. Accommodation zones take various forms, from a simple relay ramp at the overlap between two major faults of the same polarity, to zones of high structural complexity, particularly where the segments have opposite polarity. Accommodation zones may be located where older crustal structures intersect

300-403: A kind of orogeneses in extensional settings, which is referred as to rifting orogeny. Once rifting ceases, the mantle beneath the rift cools and this is accompanied by a broad area of post-rift subsidence. The amount of subsidence is directly related to the amount of thinning during the rifting phase calculated as the beta factor (initial crustal thickness divided by final crustal thickness), but

360-469: A location on a mid-ocean ridge above a base-level) is correlated with its age (age of the lithosphere where depth is measured). The depth-age relation can be modeled by the cooling of a lithosphere plate or mantle half-space. A good approximation is that the depth of the seafloor at a location on a spreading mid-ocean ridge is proportional to the square root of the age of the seafloor. The overall shape of ridges results from Pratt isostasy : close to

420-438: A mid-oceanic ridge and a set of conjugate margins separated by an oceanic basin. Rifting may be active, and controlled by mantle convection . It may also be passive, and driven by far-field tectonic forces that stretch the lithosphere. Margin architecture develops due to spatial and temporal relationships between extensional deformation phases. Margin segmentation eventually leads to the formation of rift domains with variations of

480-573: A prominent ESE-WNW fissure zone. The Terceira Rift is 550 km long, and represents the world's slowest spreading center, with plate divergence of 2–4 mm/year. It developed from a transform fault and now operates as a hyper-slow spreading center , as recognized by the relative movement between the African and Eurasian plates. There is a strong resemblance between the Terceira Rift and other ultra- or super- slow spreading ridges, such as

540-407: A quarter in rifts with a non-marine syn-rift and post-rift, and an eighth in non-marine syn-rift with a marine post-rift. Mid-ocean ridge A mid-ocean ridge ( MOR ) is a seafloor mountain system formed by plate tectonics . It typically has a depth of about 2,600 meters (8,500 ft) and rises about 2,000 meters (6,600 ft) above the deepest portion of an ocean basin . This feature

600-485: A ridge axis cools below Curie points of appropriate iron-titanium oxides, magnetic field directions parallel to the Earth's magnetic field are recorded in those oxides. The orientations of the field preserved in the oceanic crust comprise a record of directions of the Earth's magnetic field with time. Because the field has reversed directions at known intervals throughout its history, the pattern of geomagnetic reversals in

660-553: A ship of the Lamont–Doherty Earth Observatory of Columbia University , traversed the Atlantic Ocean, recording echo sounder data on the depth of the ocean floor. A team led by Marie Tharp and Bruce Heezen concluded that there was an enormous mountain chain with a rift valley at its crest, running up the middle of the Atlantic Ocean. Scientists named it the 'Mid-Atlantic Ridge'. Other research showed that

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720-408: A subduction zone drags the rest of the plate along behind it. The slab pull mechanism is considered to be contributing more than the ridge push. A process previously proposed to contribute to plate motion and the formation of new oceanic crust at mid-ocean ridges is the "mantle conveyor" due to deep convection (see image). However, some studies have shown that the upper mantle ( asthenosphere )

780-455: A transform fault, until approximately 3 Ma, when the Terceira Rift axis was initiated. The Azores Plateau began to form around 10 Ma, and is characterized by isotopic and elemental variations indicative of large heterogeneities in the mantle beneath it. There is also evidence of only one magmatic source, and no interaction with other volcanic systems. Additionally, the presence of thick crust and complex volcano-tectonic fabric implies

840-513: Is 2000–4000 m, which is much larger than expected for ultraslow spreading ridges. This irregularity is thought to be due to the association of the rift with the Azores hotspot, in combination with slow spreading rates. Slow spreading results in a strong and thick axial elastic plate, and slower volcanic extrusions from the rift zone, resulting in high topography when merged with plume -related volcanism. The large volume of volcanism associated with

900-452: Is 65,000 km (40,400 mi) long (several times longer than the Andes , the longest continental mountain range), and the total length of the oceanic ridge system is 80,000 km (49,700 mi) long. At the spreading center on a mid-ocean ridge, the depth of the seafloor is approximately 2,600 meters (8,500 ft). On the ridge flanks, the depth of the seafloor (or the height of

960-420: Is a global scale ion-exchange system. Hydrothermal vents at spreading centers introduce various amounts of iron , sulfur , manganese , silicon , and other elements into the ocean, some of which are recycled into the ocean crust. Helium-3 , an isotope that accompanies volcanism from the mantle, is emitted by hydrothermal vents and can be detected in plumes within the ocean. Fast spreading rates will expand

1020-402: Is also affected by the degree to which the rift basin is filled at each stage, due to the greater density of sediments in contrast to water. The simple 'McKenzie model' of rifting, which considers the rifting stage to be instantaneous, provides a good first order estimate of the amount of crustal thinning from observations of the amount of post-rift subsidence. This has generally been replaced by

1080-443: Is in a constant state of 'renewal' at the mid-ocean ridges by the processes of seafloor spreading and plate tectonics. New magma steadily emerges onto the ocean floor and intrudes into the existing ocean crust at and near rifts along the ridge axes. The rocks making up the crust below the seafloor are youngest along the axis of the ridge and age with increasing distance from that axis. New magma of basalt composition emerges at and near

1140-478: Is the result of changes in the volume of the ocean basins which are, in turn, affected by rates of seafloor spreading along the mid-ocean ridges. The 100 to 170 meters higher sea level of the Cretaceous Period (144–65 Ma) is partly attributed to plate tectonics because thermal expansion and the absence of ice sheets only account for some of the extra sea level. Seafloor spreading on mid-ocean ridges

1200-767: Is thinned, the Earth's surface subsides and the Moho becomes correspondingly raised. At the same time, the mantle lithosphere becomes thinned, causing a rise of the top of the asthenosphere. This brings high heat flow from the upwelling asthenosphere into the thinning lithosphere, heating the orogenic lithosphere for dehydration melting, typically causing extreme metamorphism at high thermal gradients of greater than 30 °C. The metamorphic products are high to ultrahigh temperature granulites and their associated migmatite and granites in collisional orogens, with possible emplacement of metamorphic core complexes in continental rift zones but oceanic core complexes in spreading ridges. This leads to

1260-440: Is too plastic (flexible) to generate enough friction to pull the tectonic plate along. Moreover, mantle upwelling that causes magma to form beneath the ocean ridges appears to involve only its upper 400 km (250 mi), as deduced from seismic tomography and observations of the seismic discontinuity in the upper mantle at about 400 km (250 mi). On the other hand, some of the world's largest tectonic plates such as

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1320-530: Is where seafloor spreading takes place along a divergent plate boundary . The rate of seafloor spreading determines the morphology of the crest of the mid-ocean ridge and its width in an ocean basin. The production of new seafloor and oceanic lithosphere results from mantle upwelling in response to plate separation. The melt rises as magma at the linear weakness between the separating plates, and emerges as lava , creating new oceanic crust and lithosphere upon cooling. The first discovered mid-ocean ridge

1380-638: The Gakkel Ridge in the Arctic Ocean and the Southwest Indian Ridge ). The spreading center or axis commonly connects to a transform fault oriented at right angles to the axis. The flanks of mid-ocean ridges are in many places marked by the inactive scars of transform faults called fracture zones . At faster spreading rates the axes often display overlapping spreading centers that lack connecting transform faults. The depth of

1440-528: The Gakkel Rift and the Southwest Indian Ridge . In particular, high obliquity values of 40°–65°, and a magmatic segmentation wavelength of 100 km, are similar to other very slow rifts. Rift valleys present along the rift are 1000–2200 m deep, and 30–60 km wide, similar to the Mid-Atlantic Ridge median valley. However, the amplitude of the Terceira Rift along-strike topography

1500-423: The Moho topography, including proximal domain with fault-rotated crustal blocks, necking zone with thinning of crustal basement , distal domain with deep sag basins, ocean-continent transition and oceanic domain. Deformation and magmatism interact during rift evolution. Magma-rich and magma-poor rifted margins may be formed. Magma-rich margins include major volcanic features. Globally, volcanic margins represent

1560-585: The North American plate and South American plate are in motion, yet only are being subducted in restricted locations such as the Lesser Antilles Arc and Scotia Arc , pointing to action by the ridge push body force on these plates. Computer modeling of the plates and mantle motions suggest that plate motion and mantle convection are not connected, and the main plate driving force is slab pull. Increased rates of seafloor spreading (i.e.

1620-659: The 'flexural cantilever model', which takes into account the geometry of the rift faults and the flexural isostasy of the upper part of the crust. Some rifts show a complex and prolonged history of rifting, with several distinct phases. The North Sea rift shows evidence of several separate rift phases from the Permian through to the Earliest Cretaceous , a period of over 100 million years. Rifting may lead to continental breakup and formation of oceanic basins. Successful rifting leads to seafloor spreading along

1680-460: The Baikal Rift have segment lengths in excess of 80 km, while in areas of warmer thin lithosphere, segment lengths may be less than 30 km. Along the axis of the rift the position, and in some cases the polarity (the dip direction), of the main rift bounding fault changes from segment to segment. Segment boundaries often have a more complex structure and generally cross the rift axis at

1740-427: The asthenosphere at ocean trenches . Two processes, ridge-push and slab pull , are thought to be responsible for spreading at mid-ocean ridges. Ridge push refers to the gravitational sliding of the ocean plate that is raised above the hotter asthenosphere, thus creating a body force causing sliding of the plate downslope. In slab pull the weight of a tectonic plate being subducted (pulled) below an overlying plate at

1800-478: The axis because of decompression melting in the underlying Earth's mantle . The isentropic upwelling solid mantle material exceeds the solidus temperature and melts. The crystallized magma forms a new crust of basalt known as MORB for mid-ocean ridge basalt, and gabbro below it in the lower oceanic crust . Mid-ocean ridge basalt is a tholeiitic basalt and is low in incompatible elements . Hydrothermal vents fueled by magmatic and volcanic heat are

1860-490: The axis changes in a systematic way with shallower depths between offsets such as transform faults and overlapping spreading centers dividing the axis into segments. One hypothesis for different along-axis depths is variations in magma supply to the spreading center. Ultra-slow spreading ridges form both magmatic and amagmatic (currently lack volcanic activity) ridge segments without transform faults. Mid-ocean ridges exhibit active volcanism and seismicity . The oceanic crust

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1920-422: The central axis of most mid-ocean ridges , where new oceanic crust and lithosphere is created along a divergent boundary between two tectonic plates . Failed rifts are the result of continental rifting that failed to continue to the point of break-up. Typically the transition from rifting to spreading develops at a triple junction where three converging rifts meet over a hotspot . Two of these evolve to

1980-399: The current direction of spreading motion, which is approximately N70°. Evidence of pre-rift structures include ancient transform directions of N110°–N125°, reactivating as transtensional fault zones, and N–S directions from former middle-oceanic rift faults, reactivating as left-lateral fault zones. It is important to note other hotspot associated islands do not display these features, and thus,

2040-407: The discovery of the worldwide extent of the mid-ocean ridge in the 1950s, geologists faced a new task: explaining how such an enormous geological structure could have formed. In the 1960s, geologists discovered and began to propose mechanisms for seafloor spreading . The discovery of mid-ocean ridges and the process of seafloor spreading allowed for Wegener's theory to be expanded so that it included

2100-474: The floor of the Atlantic, as it keeps spreading, is continuously tearing open and making space for fresh, relatively fluid and hot sima [rising] from depth". However, Wegener did not pursue this observation in his later works and his theory was dismissed by geologists because there was no mechanism to explain how continents could plow through ocean crust , and the theory became largely forgotten. Following

2160-418: The hotspot is mainly controlled by regional extension between Africa and Eurasia, as indicated by strikes of extension fractures. Furthermore, the majority of inter-plate deformation is focused on the Terceira Rift, although a clear deformation pattern has not yet been recognized. The structures present along the Terceira Rift have directions likely associated with pre-rift geometry, as they do not correspond to

2220-402: The larger bounding faults. Subsequent extension becomes concentrated on these faults. The longer faults and wider fault spacing leads to more continuous areas of fault-related subsidence along the rift axis. Significant uplift of the rift shoulders develops at this stage, strongly influencing drainage and sedimentation in the rift basins. During the climax of lithospheric rifting, as the crust

2280-567: The majority of passive continental margins. Magma-starved rifted margins are affected by large-scale faulting and crustal hyperextension. As a consequence, upper mantle peridotites and gabbros are commonly exposed and serpentinized along extensional detachments at the seafloor. Many rifts are the sites of at least minor magmatic activity , particularly in the early stages of rifting. Alkali basalts and bimodal volcanism are common products of rift-related magmatism. Recent studies indicate that post-collisional granites in collisional orogens are

2340-532: The mid-ocean ridge causing basalt reactions with seawater to happen more rapidly. The magnesium/calcium ratio will be lower because more magnesium ions are being removed from seawater and consumed by the rock, and more calcium ions are being removed from the rock and released into seawater. Hydrothermal activity at the ridge crest is efficient in removing magnesium. A lower Mg/Ca ratio favors the precipitation of low-Mg calcite polymorphs of calcium carbonate ( calcite seas ). Slow spreading at mid-ocean ridges has

2400-617: The mid-ocean ridge from the South Atlantic into the Indian Ocean early in the twentieth century. Although the first-discovered section of the ridge system runs down the middle of the Atlantic Ocean, it was found that most mid-ocean ridges are located away from the center of other ocean basins. Alfred Wegener proposed the theory of continental drift in 1912. He stated: "the Mid-Atlantic Ridge ... zone in which

2460-402: The movement of oceanic crust as well as the continents. Plate tectonics was a suitable explanation for seafloor spreading, and the acceptance of plate tectonics by the majority of geologists resulted in a major paradigm shift in geological thinking. It is estimated that along Earth's mid-ocean ridges every year 2.7 km (1.0 sq mi) of new seafloor is formed by this process. With

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2520-698: The ocean crust can be used as an indicator of age; given the crustal age and distance from the ridge axis, spreading rates can be calculated. Spreading rates range from approximately 10–200 mm/yr. Slow-spreading ridges such as the Mid-Atlantic Ridge have spread much less far (showing a steeper profile) than faster ridges such as the East Pacific Rise (gentle profile) for the same amount of time and cooling and consequent bathymetric deepening. Slow-spreading ridges (less than 40 mm/yr) generally have large rift valleys , sometimes as wide as 10–20 km (6.2–12.4 mi), and very rugged terrain at

2580-474: The ocean floor appears similar to the seam of a baseball . The mid-ocean ridge system thus is the longest mountain range on Earth, reaching about 65,000 km (40,000 mi). The mid-ocean ridges of the world are connected and form the Ocean Ridge, a single global mid-oceanic ridge system that is part of every ocean , making it the longest mountain range in the world. The continuous mountain range

2640-403: The oceanic crust and lithosphere moves away from the ridge axis, the peridotite in the underlying mantle lithosphere cools and becomes more rigid. The crust and the relatively rigid peridotite below it make up the oceanic lithosphere , which sits above the less rigid and viscous asthenosphere . The oceanic lithosphere is formed at an oceanic ridge, while the lithosphere is subducted back into

2700-411: The onset of rifting, the upper part of the lithosphere starts to extend on a series of initially unconnected normal faults , leading to the development of isolated basins. In subaerial rifts, for example, drainage at the onset of rifting is generally internal, with no element of through drainage. As the rift evolves, some of the individual fault segments grow, eventually becoming linked together to form

2760-455: The opposite effect and will result in a higher Mg/Ca ratio favoring the precipitation of aragonite and high-Mg calcite polymorphs of calcium carbonate ( aragonite seas ). Experiments show that most modern high-Mg calcite organisms would have been low-Mg calcite in past calcite seas, meaning that the Mg/Ca ratio in an organism's skeleton varies with the Mg/Ca ratio of the seawater in which it

2820-478: The plateau developed through the migration of the Terceira Rift towards the NE, causing a constant position over a fixed hotspot. As previously mentioned, the Azores hotspot is thought to be the primary source for the excess magmatism along the Terceira Rift, resulting in the anomalously high relief of along-strike topography, and is considered to be sampling a relatively undegassed, primitive reservoir. Lines of evidence for

2880-493: The plume-shaped structure in this area include mantle seismic anomalies, bathymetry and gravity anomalies, and plume noble gas signatures associated with the hotspot. However, there is also a possibility the increased magmatic activity on the Terceira Rift is a partial result of the involvement with the rift–rift–rift triple junction between the Eurasian, African, and North American plates. Rift Major rifts occur along

2940-456: The point of seafloor spreading, while the third ultimately fails, becoming an aulacogen . Most rifts consist of a series of separate segments that together form the linear zone characteristic of rifts. The individual rift segments have a dominantly half-graben geometry, controlled by a single basin-bounding fault. Segment lengths vary between rifts, depending on the elastic thickness of the lithosphere. Areas of thick colder lithosphere, such as

3000-404: The product of rifting magmatism at converged plate margins. The sedimentary rocks associated with continental rifts host important deposits of both minerals and hydrocarbons . SedEx mineral deposits are found mainly in continental rift settings. They form within post-rift sequences when hydrothermal fluids associated with magmatic activity are expelled at the seabed. Continental rifts are

3060-574: The rate of expansion of the mid-ocean ridge) have caused the global ( eustatic ) sea level to rise over very long timescales (millions of years). Increased seafloor spreading means that the mid-ocean ridge will then expand and form a broader ridge with decreased average depth, taking up more space in the ocean basin. This displaces the overlying ocean and causes sea levels to rise. Sealevel change can be attributed to other factors ( thermal expansion , ice melting, and mantle convection creating dynamic topography ). Over very long timescales, however, it

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3120-412: The reactivated structures are most likely a result of the complex tectonic setting associated with the Azores triple junction instability. The above observations may represent earlier stages of development between the rifting of Eurasia and Africa. The first stage began in 25 Ma and lasted until 8 Ma, corresponding to the initial rifting of the oceanic plateau. At 8 Ma, extension began along

3180-584: The ridge axis, there is a hot, low-density mantle supporting the oceanic crust. As the oceanic plate cools, away from the ridge axis, the oceanic mantle lithosphere (the colder, denser part of the mantle that, together with the crust, comprises the oceanic plates) thickens, and the density increases. Thus older seafloor is underlain by denser material and is deeper. Spreading rate is the rate at which an ocean basin widens due to seafloor spreading. Rates can be computed by mapping marine magnetic anomalies that span mid-ocean ridges. As crystallized basalt extruded at

3240-758: The ridge crest that can have relief of up to 1,000 m (3,300 ft). By contrast, fast-spreading ridges (greater than 90 mm/yr) such as the East Pacific Rise lack rift valleys. The spreading rate of the North Atlantic Ocean is ~ 25 mm/yr, while in the Pacific region, it is 80–145 mm/yr. The highest known rate is over 200 mm/yr in the Miocene on the East Pacific Rise. Ridges that spread at rates <20 mm/yr are referred to as ultraslow spreading ridges (e.g.,

3300-440: The ridge crest was seismically active and fresh lavas were found in the rift valley. Also, crustal heat flow was higher here than elsewhere in the Atlantic Ocean basin. At first, the ridge was thought to be a feature specific to the Atlantic Ocean. However, as surveys of the ocean floor continued around the world, it was discovered that every ocean contains parts of the mid-ocean ridge system. The German Meteor expedition traced

3360-875: The rift axis. In the Gulf of Suez rift, the Zaafarana accommodation zone is located where a shear zone in the Arabian-Nubian Shield meets the rift. Rift flanks or shoulders are elevated areas around rifts. Rift shoulders are typically about 70 km wide. Contrary to what was previously thought, elevated passive continental margins (EPCM) such as the Brazilian Highlands , the Scandinavian Mountains and India's Western Ghats , are not rift shoulders. The formation of rift basins and strain localization reflects rift maturity. At

3420-431: The seafloor were analyzed by oceanographers Matthew Fontaine Maury and Charles Wyville Thomson and revealed a prominent rise in the seafloor that ran down the Atlantic basin from north to south. Sonar echo sounders confirmed this in the early twentieth century. It was not until after World War II , when the ocean floor was surveyed in more detail, that the full extent of mid-ocean ridges became known. The Vema ,

3480-521: The sites of significant oil and gas accumulations, such as the Viking Graben and the Gulf of Suez Rift . Thirty percent of giant oil and gas fields are found within such a setting. In 1999 it was estimated that there were 200 billion barrels of recoverable oil reserves hosted in rifts. Source rocks are often developed within the sediments filling the active rift ( syn-rift ), forming either in

3540-486: Was grown. The mineralogy of reef-building and sediment-producing organisms is thus regulated by chemical reactions occurring along the mid-ocean ridge, the rate of which is controlled by the rate of sea-floor spreading. The first indications that a ridge bisects the Atlantic Ocean basin came from the results of the British Challenger expedition in the nineteenth century. Soundings from lines dropped to

3600-457: Was the Mid-Atlantic Ridge , which is a spreading center that bisects the North and South Atlantic basins; hence the origin of the name 'mid-ocean ridge'. Most oceanic spreading centers are not in the middle of their hosting ocean basis but regardless, are traditionally called mid-ocean ridges. Mid-ocean ridges around the globe are linked by plate tectonic boundaries and the trace of the ridges across

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