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32-678: The Piemont-Liguria basin or the Piemont-Liguria Ocean (sometimes only one of the two names is used, for example: Piemonte Ocean) was a former piece of oceanic crust that is seen as part of the Tethys Ocean . Together with some other oceanic basins that existed between the continents Europe and Africa , the Piemont-Liguria Ocean is called the Western or Alpine Tethys Ocean. The Piemont-Liguria Ocean
64-423: A strict order but occur all simultaneously over a depth range of 4–18 km suggesting that these processes can occur already in the upper mantle. The mantle melts are most commonly modified by fractional crystallisation due to cooling and by assimilation of crustal rocks. The most important parameter controlling the processes operating in the lower oceanic crust is the magma supply, this is further controlled by
96-561: Is 6KM long. The lower oceanic crust connects the Earth's mantle with the MORB , where around 60% of the total magma production of the Earth happens. The three main processes happening in this region of the oceanic crust are partial melting of the Earth's mantle, melt accumulation at various depths and the chemical modification of this melts during ascent,. This three processes do not happen in
128-410: Is continuously being created at mid-ocean ridges. As continental plates diverge at these ridges, magma rises into the upper mantle and crust. As the continental plates move away from the ridge, the newly formed rocks cool and start to erode with sediment gradually building up on top of them. The youngest oceanic rocks are at the oceanic ridges, and they get progressively older away from the ridges. As
160-449: Is in the west Pacific and north-west Atlantic — both are about up to 180-200 million years old. However, parts of the eastern Mediterranean Sea could be remnants of the much older Tethys Ocean , at about 270 and up to 340 million years old. The oceanic crust displays a pattern of magnetic lines, parallel to the ocean ridges, frozen in the basalt . A symmetrical pattern of positive and negative magnetic lines emanates from
192-477: Is the lower part of the oceanic crust and represents the major part of it (volumetrically biggest part). It is generally located 4–8 km below the ocean floor and the major lithologies are mafic ( ultramafic and gabbroic rocks) which derive from melts rising from the Earth's mantle . This part of the oceanic crust is an important zone for processes such as melt accumulation and melt modification ( fractional crystallisation and crustal assimilation). And
224-619: Is the result of the cooling of magma derived from mantle material below the plate. The magma is injected into the spreading center, which consists mainly of a partly solidified crystal mush derived from earlier injections, forming magma lenses that are the source of the sheeted dikes that feed the overlying pillow lavas. As the lavas cool they are, in most instances, modified chemically by seawater. These eruptions occur mostly at mid-ocean ridges, but also at scattered hotspots, and also in rare but powerful occurrences known as flood basalt eruptions. But most magma crystallises at depth, within
256-401: Is the so-called "gabbro glacier", where crystals settle in a shallow melt-dominated lens beneath the ridge axis. The weight of the accumulating crystals settling to the bottom of the magma lens induces a ductile flow and deformation within the gabbros, just like the ice in a glacier responds to accumulated snow. Nevertheless, the model fails to explain the layered variations in mineral types,
288-556: The Azores and Iceland . Prior to the Neoproterozoic Era 1000 Ma ago the world's oceanic crust was more mafic than present-days'. The more mafic nature of the crust meant that higher amounts of water molecules ( OH ) could be stored the altered parts of the crust. At subduction zones this mafic crust was prone to metamorphose into greenschist instead of blueschist at ordinary blueschist facies . Oceanic crust
320-411: The blueschist or eclogite facies . Oceanic crust Oceanic crust is the uppermost layer of the oceanic portion of the tectonic plates . It is composed of the upper oceanic crust, with pillow lavas and a dike complex, and the lower oceanic crust , composed of troctolite , gabbro and ultramafic cumulates . The crust overlies the rigid uppermost layer of the mantle . The crust and
352-404: The lower oceanic crust . There, newly intruded magma can mix and react with pre-existing crystal mush and rocks. Although a complete section of oceanic crust has not yet been drilled, geologists have several pieces of evidence that help them understand the ocean floor. Estimations of composition are based on analyses of ophiolites (sections of oceanic crust that are thrust onto and preserved on
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#1732773382357384-498: The magma supply and therefore the heat flow is low and can't maintain a persistent liquid magma chamber . Sinton and Detrick (1992) modelled a schematic cross section of an axial magma chamber beneath a slow-spreading ridge such as the Mid-Atlantic Ridge . Due to the reduced heat and magma supply, a steady-state eruptible magma lens is relinquished in favor of a sill-like mush zone and a smaller transition zone beneath
416-468: The ocean floor . The IODP expedition 345 was one of the first drilling project, which sampled a significant thickness of layered igneous rocks. A shallow melt can erupt through cool crust and produce sheeted dikes and volcanics , but the small chamber seems difficult to resolve with traditional ideas of fractional crystallization and crystal settling to form the thick sequence of layered gabbros and foliated gabbros and ultramafics. One proposed model
448-448: The common detection of small lens/mush zone at fast-spreading ridges emphasize the small magma chamber model. Modally and compositionally layered gabbroic rock is often found (or abundant) in the lower crustal sections of ophiolite . The layered lower crust is thus one of the key features of all models of fast-spreading lower crust. Nevertheless, distinct modal layering as observed in major ophiolites has rarely been observed or sampled on
480-504: The continents), comparisons of the seismic structure of the oceanic crust with laboratory determinations of seismic velocities in known rock types, and samples recovered from the ocean floor by submersibles , dredging (especially from ridge crests and fracture zones ) and drilling. Oceanic crust is significantly simpler than continental crust and generally can be divided in three layers. According to mineral physics experiments, at lower mantle pressures, oceanic crust becomes denser than
512-531: The correlated layering in mineral compositional variations, and the apparently primary near-vertical fabrics in the upper gabbros that appear to represent subvertical melt conduits. Kelemen and co-workers concluded that most of the lower oceanic crust crystallized in place, and proposed "the sheeted sill" model. In the model the sills form when porous flow of rising basaltic liquids (or small melt-filled fractures) are stopped beneath permeability (earth sciences) barriers of earlier crystallized melts and pond to form
544-797: The formation of the Alps and the Apennines in the Tertiary . Fragments of Piemont-Ligurian oceanic crust were preserved as ophiolites in the Penninic nappes of the Alps and the Tuscan nappes of the Apennines. These nappes were subducted, sometimes to great depths in the mantle , before being obducted again. Due to the high pressures at these depths, much of the material had been metamorphosed in
576-401: The formed cumulates are then "dragged" up by mantle flow to form the lower oceanic crust. Today, a model intermediate between these two has become popular. This model is referred to as a "plum pudding", where the lower oceanic crust is constructed from a number of nested plutons that crystallize within the mantle or crust. Schwartz et al. (2005) describes another variant. He postulates that
608-406: The mantle rises it cools and melts, as the pressure decreases and it crosses the solidus . The amount of melt produced depends only on the temperature of the mantle as it rises. Hence most oceanic crust is the same thickness (7±1 km). Very slow spreading ridges (<1 cm·yr half-rate) produce thinner crust (4–5 km thick) as the mantle has a chance to cool on upwelling and so it crosses
640-476: The mid-ocean ridge. New rock is formed by magma at the mid-ocean ridges, and the ocean floor spreads out from this point. When the magma cools to form rock, its magnetic polarity is aligned with the then-current positions of the magnetic poles of the Earth. New magma then forces the older cooled magma away from the ridge. This process results in parallel sections of oceanic crust of alternating magnetic polarity. Lower oceanic crust The lower oceanic crust
672-713: The northwest and the Apulian plate (a sub-plate of the African tectonic plate ) in the southeast. When the Apulian plate started moving to the northwest in the late Cretaceous, Piemont-Ligurian crust began to subduct beneath it. In the Paleocene the Piemont-Ligurian Ocean had completely disappeared under the Apulian plate and continental collision started between Apulia and Europe, which would lead to
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#1732773382357704-508: The oceanic crust can be used to estimate the (thermal) thickness of the lithosphere, where young oceanic crust has not had enough time to cool the mantle beneath it, while older oceanic crust has thicker mantle lithosphere beneath it. The oceanic lithosphere subducts at what are known as convergent boundaries . These boundaries can exist between oceanic lithosphere on one plate and oceanic lithosphere on another, or between oceanic lithosphere on one plate and continental lithosphere on another. In
736-573: The recycling of this part of the oceanic crust, together with the upper mantle has been suggested as a significant source component for tholeiitic magmas in Hawaiian volcanoes. Although the lower oceanic crust builds the link between the mantle and the MORB , and can't be neglected for the understanding of MORB evolution, the complex processes operating in this zone remain unclear and there is an ongoing debate in Earth Sciences about this. It
768-446: The ridge axis is underlain by a crystal mush containing a small percentage of melt , capped by a thin melt lens containing a generally high, but variable melt fraction. The completely liquid body is a thin and narrow sill -like lens (10 to 150 m [33 to 492 ft] thick and < 2 km [1.2 mi] wide). The lens is maintained by reinjection of primitive magma. The lack of any detectable large magma chamber and
800-470: The rigid upper mantle layer together constitute oceanic lithosphere . Oceanic crust is primarily composed of mafic rocks, or sima , which is rich in iron and magnesium. It is thinner than continental crust , or sial , generally less than 10 kilometers thick; however, it is denser, having a mean density of about 3.0 grams per cubic centimeter as opposed to continental crust which has a density of about 2.7 grams per cubic centimeter. The crust uppermost
832-485: The second situation, the oceanic lithosphere always subducts because the continental lithosphere is less dense. The subduction process consumes older oceanic lithosphere, so oceanic crust is seldom more than 200 million years old. The process of super-continent formation and destruction via repeated cycles of creation and destruction of oceanic crust is known as the Wilson Cycle . The oldest large-scale oceanic crust
864-601: The sills. Cooling rates are generally sufficiently slow that crystals and their interstitial liquids are in chemical equilibrium , as long as the liquid is immobile. However, buoyancy and/or compaction (geology) may induce liquid migration through the mush, resulting a significant compositional and microstructural modification. Slow- and intermediate-spreading ridges form typically valleys about 30 to 50 km (19 to 31 mi) wide and 1 to 5 km (0.62 to 3.11 mi) deep, with step-like inward-facing scarps, similar to rift valleys on land. Compared to fast spreading-ridges,
896-538: The solidus and melts at lesser depth, thereby producing less melt and thinner crust. An example of this is the Gakkel Ridge under the Arctic Ocean . Thicker than average crust is found above plumes as the mantle is hotter and hence it crosses the solidus and melts at a greater depth, creating more melt and a thicker crust. An example of this is Iceland which has crust of thickness ~20 km. The age of
928-491: The spreading rate, and therefore, spreading rate is a critical variable in models for the formation of the lower oceanic crust. The rate at which plate divergence occurs at mid-ocean ridges is not the same for all ridge segments. Ridges with a spreading rate less than 3 cm/a are considered slow-spreading ridges, while those with a rate greater than 5 cm/a are considered fast-spreading ridges Intensive search spanning over three decades of seismic imaging have shown that
960-525: The surrounding mantle. The most voluminous volcanic rocks of the ocean floor are the mid-oceanic ridge basalts, which are derived from low- potassium tholeiitic magmas . These rocks have low concentrations of large ion lithophile elements (LILE), light rare earth elements (LREE), volatile elements and other highly incompatible elements . There can be found basalts enriched with incompatible elements, but they are rare and associated with mid-ocean ridge hot spots such as surroundings of Galapagos Islands ,
992-405: The well-developed rift valley. Convection and mixing in the magma chamber is far less likely than at fast ridges. Thermal constrains led to the development of different models to reconstruct the accretion history. The "infinite leek" model suggests small magma batches, forming small "nested" intrusions. Another model proposed that crystallization could occur at depth, where temperatures are higher,
Piemont-Liguria Ocean - Misplaced Pages Continue
1024-568: Was formed in the Jurassic period, when the paleocontinents Laurasia (to the north, with Europe) and Gondwana (to the south, with Africa) started to move away from each other. The oceanic crust that formed in between the two continents became the Piemont-Liguria Ocean. In the Cretaceous period the Piemont-Liguria Ocean lay between Europe (and a smaller plate called the Iberian plate ) in
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