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The Troodos Ophiolite on the island of Cyprus represents a Late Cretaceous spreading axis (mid-ocean ridge) that has since been uplifted due to its positioning on the overriding Anatolian Plate at the Cyprus arc and ongoing subduction to the south of the Eratosthenes Seamount. [1]
The lowest units of the ophiolite are the Lower Pillow Lavas, controversially separated from the Upper Pillow Lavas. Filling spaces in between the pillows in the pillow lava units are dispersed metal oxide sediments that can also be seen as veins filling cooling fractures within the lavas. The metal oxides are ferruginous with ferromanganese oxides, clays, carbonates, volcanic glass and pelagic sediments.
Above the pillow lava units lies a layer of ferromaganiferous mudstones and clastic volcanics (the epiclastics). The epiclastites are massive altered lava fragments in a mud matrix, usually ferromanganiferous. Overlying this is the massive-finely laminated ferromanganese muds. Between the epiclastics and muds lie background accumulations of pelagic sediment.
To the south there is the Mathiati-Margi massive sulfide ore body and stockwork mineralisation. The sulfide ore occurs at the same stratigraphic level as the Lower and Upper pillow lava contact, and is overlain by unmineralised lavas. [2]
Dunite bodies (olivine) are common in the mantle series of the Troodos, and contain chromite concentrations.
The sheeted dykes show a general tholeiitic trend, of basalts, andesites and dacites. There is no obvious boundary for the compositional differences, but the lower lavas are generally more enriched and evolved (silicic) while the upper lavas are less evolved and depleted.
The geochemical evidence implies that the Troodos ophiolite has come from mantle that has already been depleted, with extraction of mid ocean ridge basalt, but then subsequently enriched in certain trace elements as well as water. Along with the alkaline character of the plagiogranites it can be assumed that the spreading ridge of the Troodos was situated above a subduction zone, but the mantle from which lavas were extruded was that of mantle that had recently lost a melt fraction.
The Troodos is a unique ophiolite in terms of observing hydrothermal alteration, because it has not been metamorphosed to a high extent or deformed extensively. Therefore, it is easy to see the successions and relationships of the hydrothermal processes to the structure of the ridge. This is difficult to observe in modern ridges due to accessibility problems, and so the Troodos gives a unique view into these processes. The fact that the same kinds of alteration can be seen in modern axes implies the same processes happened at the Troodos, even though it was formed in a supra-subduction zone.
Alteration of the lavas is related to both axial hydrothermal systems and crustal aging in a submarine environment. [3] Fluid can be shown to have penetrated at least to the base of the plutonic sequence where high temperature and secondary phases in the plutonics and cumulates imply alteration close to the ridge axis.
The presence of alteration in all of the extrusive levels but the very highest imply a succession of numerous hydrothermal convection cells active during eruption. [4]
As the crustal sequence gradually moved off of the spreading axis, there was cessation of the main metalliferous deposition and progressive restriction of water/rock action and eventually water interaction was restricted to within rock units as the crust was sealed off. This caused precipitation of late stage zeolites and carbonates.
The massive sulfide deposits can also be shown to have formed at the same temperature as modern day black smokers, which provides evidence that these could be formed from the smokers.[ citation needed ]
In terms of the physical mechanism of spreading the Troodos spreading axis is broadly comparable to that of a modern intermediate spreading ridge. [5] The eruption rates along the ridge are high so there is little time for sediment accumulation during active periods. In terms of lava geochemistry and stratigraphy, however, Troodos is more likely to have formed in a subduction initiation setting [6]
Research on the Troodos flourished after the late 1960s revolution on the fact that ophiolites represented fragments of ocean crust, where then petrologic and secondly structural studies were done on various ophiolites around the world. Interpretations of the Troodos have advanced understandings of the construction of ocean lithosphere, the nature of the seismic layering of the oceanic crust and the magmatic, structural and hydrothermal processes at the ridges. Also, importantly it has helped with understandings of the mechanisms associated with plate collision.
In the early 1970s it began to be widely accepted that the ophiolite represented sea-floor spreading, and subsequently that the Troodos showed geochemical signatures like that of arc volcanics.[ citation needed ] This last fact was first pushed by Akiho Miyashiro in 1973 who challenged the common conception of Troodos Ophiolite and proposed an island arc origin for it. [7] This was done arguing that numerous lavas and dykes in the ophiolite had calc-alkaline chemistries. [7] In the early 1980s the term supra-subduction zone was coined to infer the formation of lavas above a subducting lithospheric slab, with no specification of where in relation to the subducting slab they form. From subsequent studies of other ophiolites it has been found that these generally have a similar geochemical signature, and so it is inferred that most are supra-subduction zone related.[ citation needed ]
In the Troodos ophiolite it was observed from the variation in magma types, which can be seen to go from evolved to less evolved mafic rocks in localised cross cutting field relationships implying the presence of more than one magma chamber that cuts other exhausted ones. This has now been shown to be supported from other ophiolite bodies such as Oman.
In terms of ophiolite emplacement, there was a problem of how to uplift dense oceanic lithosphere through 5–6 km of water and onto continents. [8] This process, however it happened, was coined obduction. The processes could possibly vary depending on the active or passive type of margin encountered, such as Tethyan or Cordilleran margins. In Tethyan passive margins gravity sliding over accretionary terranes via low angle thrust faults was proposed. On the Cordilleran margin, lithospheric fragments are incorporated into accretionary terranes. In the Troodos, gravity surveys have implied that the ophiolite is underlain by continental crust whose relative buoyancy uplifted the ocean crust, which in some circumstances could eventually lead to sliding onto the accretionary wedge (or now Eratostines seamount subducted for Troodos).
In the supra-subduction zone, spreading is not controlled like in mid-ocean ridge settings, as the extension is mainly facilitated by slab rollback that creates space independently of the availability of magma. Therefore, the fastest spreading rates are caused by the most rapid rollback and thus favours a magmatic spreading as in many cases the mantle may not be able to keep up with the spreading. Therefore, there is unusually thinned crust, large low-angle extensional faults are common and much crustal rotation. [9]
Oceanic trenches are prominent, long, narrow topographic depressions of the ocean floor. They are typically 50 to 100 kilometers wide and 3 to 4 km below the level of the surrounding oceanic floor, but can be thousands of kilometers in length. There are about 50,000 km (31,000 mi) of oceanic trenches worldwide, mostly around the Pacific Ocean, but also in the eastern Indian Ocean and a few other locations. The greatest ocean depth measured is in the Challenger Deep of the Mariana Trench, at a depth of 10,994 m (36,070 ft) below sea level.
Subduction is a geological process in which the oceanic lithosphere and some continental lithosphere is recycled into the Earth's mantle at the convergent boundaries between tectonic plates. Where one tectonic plate converges with a second plate, the heavier plate dives beneath the other and sinks into the mantle. A region where this process occurs is known as a subduction zone, and its surface expression is known as an arc-trench complex. The process of subduction has created most of the Earth's continental crust. Rates of subduction are typically measured in centimeters per year, with rates of convergence as high as 11 cm/year.
Obduction is a geological process whereby denser oceanic crust is scraped off a descending ocean plate at a convergent plate boundary and thrust on top of an adjacent plate. When oceanic and continental plates converge, normally the denser oceanic crust sinks under the continental crust in the process of subduction. Obduction, which is less common, normally occurs in plate collisions at orogenic belts or back-arc basins.
An ophiolite is a section of Earth's oceanic crust and the underlying upper mantle that has been uplifted and exposed, and often emplaced onto continental crustal rocks.
A convergent boundary is an area on Earth where two or more lithospheric plates collide. One plate eventually slides beneath the other, a process known as subduction. The subduction zone can be defined by a plane where many earthquakes occur, called the Wadati–Benioff zone. These collisions happen on scales of millions to tens of millions of years and can lead to volcanism, earthquakes, orogenesis, destruction of lithosphere, and deformation. Convergent boundaries occur between oceanic-oceanic lithosphere, oceanic-continental lithosphere, and continental-continental lithosphere. The geologic features related to convergent boundaries vary depending on crust types.
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 the rigid upper mantle layer together constitute oceanic lithosphere.
Forearc is a plate tectonic term referring to a region in a subduction zone between an oceanic trench and the associated volcanic arc. Forearc regions are present along convergent margins and eponymously form 'in front of' the volcanic arcs that are characteristic of convergent plate margins. A back-arc region is the companion region behind the volcanic arc.
Serpentinization is a hydration and metamorphic transformation of ferromagnesian minerals, such as olivine and pyroxene, in mafic and ultramafic rock to produce serpentinite. Minerals formed by serpentinization include the serpentine group minerals, brucite, talc, Ni-Fe alloys, and magnetite. The mineral alteration is particularly important at the sea floor at tectonic plate boundaries.
The Isua Greenstone Belt is an Archean greenstone belt in southwestern Greenland, aged between 3.7 and 3.8 billion years. The belt contains variably metamorphosed mafic volcanic and sedimentary rocks, and is the largest exposure of Eoarchaean supracrustal rocks on Earth. Due to its age and low metamorphic grade relative to many Eoarchaean rocks, the Isua Greenstone Belt has become a focus for investigations on the emergence of life and the style of tectonics that operated on the early Earth.
A back-arc basin is a type of geologic basin, found at some convergent plate boundaries. Presently all back-arc basins are submarine features associated with island arcs and subduction zones, with many found in the western Pacific Ocean. Most of them result from tensional forces, caused by a process known as oceanic trench rollback, where a subduction zone moves towards the subducting plate. Back-arc basins were initially an unexpected phenomenon in plate tectonics, as convergent boundaries were expected to universally be zones of compression. However, in 1970, Dan Karig published a model of back-arc basins consistent with plate tectonics.
A sheeted dyke complex, or sheeted dike complex, is a series of sub-parallel intrusions of igneous rock, forming a layer within the oceanic crust. At mid-ocean ridges, dykes are formed when magma beneath areas of tectonic plate divergence travels through a fracture in the earlier formed oceanic crust, feeding the lavas above and cooling below the seafloor forming upright columns of igneous rock. Magma continues to cool, as the existing seafloor moves away from the area of divergence, and additional magma is intruded and cools. In some tectonic settings slices of the oceanic crust are obducted (emplaced) upon continental crust, forming an ophiolite.
The Izu–Bonin–Mariana (IBM) arc system is a tectonic plate convergent boundary in Micronesia. The IBM arc system extends over 2800 km south from Tokyo, Japan, to beyond Guam, and includes the Izu Islands, the Bonin Islands, and the Mariana Islands; much more of the IBM arc system is submerged below sealevel. The IBM arc system lies along the eastern margin of the Philippine Sea Plate in the Western Pacific Ocean. It is the site of the deepest gash in Earth's solid surface, the Challenger Deep in the Mariana Trench.
The southern margin of the Anatolian Plate is in collision with the African Plate, which has created the uplift of the Cyprus arc and Cyprus itself.
High pressure terranes along the ~1200 km long east-west trending Bangong-Nujiang suture zone (BNS) on the Tibetan Plateau have been extensively mapped and studied. Understanding the geodynamic processes in which these terranes are created is key to understanding the development and subsequent deformation of the BNS and Eurasian deformation as a whole.
A subduction zone is a region of the Earth's crust where one tectonic plate moves under another tectonic plate; oceanic crust gets recycled back into the mantle and continental crust gets produced by the formation of arc magmas. Arc magmas account for more than 20% of terrestrially produced magmas and are produced by the dehydration of minerals within the subducting slab as it descends into the mantle and are accreted onto the base of the overriding continental plate. Subduction zones host a unique variety of rock types formed by the high-pressure, low-temperature conditions a subducting slab encounters during its descent. The metamorphic conditions the slab passes through in this process generates and alters water bearing (hydrous) mineral phases, releasing water into the mantle. This water lowers the melting point of mantle rock, initiating melting. Understanding the timing and conditions in which these dehydration reactions occur, is key to interpreting mantle melting, volcanic arc magmatism, and the formation of continental crust.
Flat slab subduction is characterized by a low subduction angle beyond the seismogenic layer and a resumption of normal subduction far from the trench. A slab refers to the subducting lower plate. A broader definition of flat slab subduction includes any shallowly dipping lower plate, as in western Mexico. Flat slab subduction is associated with the pinching out of the asthenosphere, an inland migration of arc magmatism, and an eventual cessation of arc magmatism. The coupling of the flat slab to the upper plate is thought to change the style of deformation occurring on the upper plate's surface and form basement-cored uplifts like the Rocky Mountains. The flat slab also may hydrate the lower continental lithosphere and be involved in the formation of economically important ore deposits. During the subduction, a flat slab itself may deform or buckle, causing sedimentary hiatus in marine sediments on the slab. The failure of a flat slab is associated with ignimbritic volcanism and the reverse migration of arc volcanism. Multiple working hypotheses about the cause of flat slabs are subduction of thick, buoyant oceanic crust (15–20 km) and trench rollback accompanying a rapidly overriding upper plate and enhanced trench suction. The west coast of South America has two of the largest flat slab subduction zones. Flat slab subduction is occurring at 10% of subduction zones.
An upper mantle body is a geological region where upper mantle rocks (peridotite) outcrop on the surface of the Earth.
Divergent double subduction, also called outward dipping double-sided subduction, is a special type of subduction process in which two parallel subduction zones with different directions are developed on the same oceanic plate. In conventional plate tectonics theory, an oceanic plate subducts under another plate and new oceanic crust is generated somewhere else, commonly along the other side of the same plates However, in divergent double subduction, the oceanic plate subducts on two sides. This results in the closure of ocean and arc–arc collision.
The geology of New Caledonia includes all major rock types, which here range in age from ~290 million years old (Ma) to recent. Their formation is driven by alternate plate collisions and rifting. The mantle-derived Eocene Peridotite Nappe is the most significant and widespread unit. The igneous unit consists of ore-rich ultramafic rocks thrust onto the main island. Mining of valuable metals from this unit has been an economical pillar of New Caledonia for more than a century.
The Chile Ridge, also known as the Chile Rise, is a submarine oceanic ridge formed by the divergent plate boundary between the Nazca Plate and the Antarctic Plate. It extends from the triple junction of the Nazca, Pacific, and Antarctic plates to the Southern coast of Chile. The Chile Ridge is easy to recognize on the map, as the ridge is divided into several segmented fracture zones which are perpendicular to the ridge segments, showing an orthogonal shape toward the spreading direction. The total length of the ridge segments is about 550–600 km.
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