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 the ridge to be subducted. [1] 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 the established tectonic and magmatic regimes.[1] In general, the data used to identify possible slab windows comes from seismic tomography and heat flow studies.[2]

Effects

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 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.[1] Mantle flowing upward through the slab window in order to compensate for 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.[3]

Geometry

The geometry of a slab window depends primarily on 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.[1]

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 the second is when the ridge is parallel to the trench, causing a rectangular window to form.[4]

Examples

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 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.[5][6][7] The northward younging of Pemberton Belt volcanism in southwestern British Columbia, Canada was probably related to a northward moving slab window edge under North America 29 to 6.8 million years ago.[6]

In addition to 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.[8]

See also

  • Slab (geology) – The portion of a tectonic plate that is being subducted
  • Slab gap hypothesis – Explanation for several instances of crustal extension that occur inland near former subduction zones

References

  1. ^ a b c d Thorkelson, Derek J., 1996, Subduction of diverging plates and the principles of slab window formation, Tectonophysics, v. 255, p. 47-63
  2. ^ van Wijk, J.W., Govers, R., Furlong, K.P., 2001, Three-dimensional thermal modeling of the California upper mantle: a slab window vs. stalled slab, Earth and Planetary Letters, v. 186, p. 175-186
  3. ^ Groome, Wesley G., Thorkelson, Derek J., 2009, The three-dimensional thermo-mechanical signature of ridge subduction and slab window migration: Tectonophysics v. 464, p. 70-83
  4. ^ Guillaume, Benjamin et al. 2010, mantle flow and dynamic topography associated with slab window openings: Insights from laboratory models: Tectonophysics v. 496, p. 83-98
  5. ^ Thorkelson, Derek J., Taylor, Richard P., 1989, Cordilleran slab windows: Geology, v. 17, p. 833-836
  6. ^ a b Madson, J.K.; Thorkelson, D.J.; Friedman, R.M.; Marshall, D.D. (2006). "Cenozoic to Recent plate configurations in the Pacific Basin: Ridge subduction and slab window magmatism in western North America". Geosphere. 2 (1): 11–34. Bibcode:2006Geosp...2...11M. doi:10.1130/GES00020.1.
  7. ^ Zandt, G., Humphreys, E., 2008, Toroidal mantle flow through the western U.S. slab window: Geology v. 36, p. 295-298, doi:10.1130/G24611A.1
  8. ^ McCrory, P.A., Wilson, D.S., Stanley, R.G., 2009, Continuing evolution of the Pacific-Juan de Fuca-North America slab window system - A trench-ridge-transform example from the Pacific Rim: Tectonophysics v. 464, p. 30-42

External links

Bahama Banks

The Bahama Banks are the submerged carbonate platforms that make up much of the Bahama Archipelago. The term is usually applied in referring to either the Great Bahama Bank around Andros Island, or the Little Bahama Bank of Grand Bahama Island and Great Abaco, which are the largest of the platforms, and the Cay Sal Bank north of Cuba. The islands of these banks are politically part of the Bahamas. Other banks are the three banks of the Turks and Caicos Islands, namely the Caicos Bank of the Caicos Islands, the bank of the Turks Islands, and wholly submerged Mouchoir Bank. Further southeast are the equally wholly submerged Silver Bank and Navidad Bank north of the Dominican Republic.

Challis Arc

The Challis Arc was an Eocene volcanic field that stretched from southwestern British Columbia through Washington to Idaho, United States. The volcanic field extended between 42 and 49 degrees north latitude and was about 1500 kilometers in length. It exhibited volcanic activity for about 10 million years. Remnants of the Challis Arc are found as granitic plutons in the North Cascades, the Okanagan Highlands and in southcentral Idaho.

It was first theorized in 1979 that the volcanic field formed as a result of subduction of the eastern block of the Kula Plate between 57 and 37 million years ago. More recent publications argue that the Challis Arc was formed by more complex tectonic interactions. One proposed model theorizes that the Farallon plate underwent subduction and imbrication beneath the North American plate to form the Challis Arc. Another model suggests that intracontinental rifting and igneous activity between the Pacific and North American plates formed the Challis arc. By definition, a volcanic arc is formed via subduction, so the Challis Arc's naming as a volcanic arc is a matter of debate among geologists. The current limited availability of historical geochemical data prevents any of the proposed theories from being confirmed or falsified, so there is still no consensus on the Challis Arc's formation.

Coquihalla Mountain

Coquihalla Mountain is an extinct stratovolcano in Similkameen Country, southwestern British Columbia, Canada, located 10 km (6.2 mi) south of Falls Lake and 22 km (14 mi) west of Tulameen between the Coquihalla and Tulameen rivers. With a topographic prominence of 816 m (2,677 ft), it towers above adjacent mountain ridges. It is the highest mountain in the Bedded Range of the northern Canadian Cascades with an elevation of 2,157 m (7,077 ft) and lies near the physiographic boundaries with the Coast Mountains on the west and the Interior Plateau on the east.

Farallon Plate

The Farallon Plate was an ancient oceanic plate that began subducting under the west coast of the North American Plate—then located in modern Utah—as Pangaea broke apart during the Jurassic period. It is named for the Farallon Islands, which are located just west of San Francisco, California.

Over time, the central part of the Farallon Plate was completely subducted under the southwestern part of the North American Plate. The remains of the Farallon Plate are the Juan de Fuca, Explorer and Gorda Plates, 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 crustal material rifted off from other distant plates and accreting them to the North American Plate.

These fragments from elsewhere are called terranes (sometimes, "exotic" terranes). Much of western North America is composed of these accreted terranes.

Franklin Glacier Complex

The Franklin Glacier Complex is a deeply eroded volcano in the Waddington Range of southwestern British Columbia, Canada. Located about 65 km (40 mi) northeast of Kingcome, this sketchily known complex resides at Franklin Glacier near Mount Waddington. It is over 2,000 m (6,600 ft) in elevation and because of its considerable overall altitude, a large proportion of the complex is covered by glacial ice.

Magmatic activity of the Franklin Glacier Complex spanded roughly four million years from the Late Miocene to the Early Pleistocene, with the most recently identified volcanic eruption having taken place around 2.2 million years ago. The existence of thermal springs near the complex implies that magmatic heat is still present. It has therefore been of interest to geothermal exploration.

Fueguino

Fueguino is a volcanic field in Chile. The southernmost volcano in the Andes, it lies on Tierra del Fuego's Cook Island and also extends over nearby Londonderry Island. The field is formed by lava domes, pyroclastic cones, and a crater lake.

Volcanic activity at Fueguino is part of the Austral Volcanic Zone, which is formed by the subduction of the Antarctic Plate beneath the South America Plate. The subducting plate has not reached a depth sufficient for proper volcanic arc volcanism, however.

The field bears no trace of glacial erosion on its volcanoes, and reports exist of volcanic activity in 1712, 1820 and 1926.

Mendocino Triple Junction

The Mendocino Triple Junction (MTJ) is the point where the Gorda plate, the North American plate, and the Pacific plate meet, in the Pacific Ocean near Cape Mendocino in northern California. This triple junction is the location of a change in the broad plate motions which dominate the west coast of North America, linking convergence of the northern Cascadia subduction zone and translation of the southern San Andreas Fault system. The Gorda plate is subducting, towards N50ºE, under the North American plate at 2.5 – 3 cm/yr, and is simultaneously converging obliquely against the Pacific plate at a rate of 5 cm/yr in the direction N115ºE. The accommodation of this plate configuration results in a transform boundary along the Mendocino Fracture Zone, and a divergent boundary at the Gorda Ridge.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 the extrusion of volcanic rocks. This activity is centred 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.

Mount Barr

Mount Barr is a mountain in the Skagit Range of the Cascade Mountains of southern British Columbia, Canada, located on the northeast side of Wahleach Lake and just southwest of Hope. It is a ridge highpoint with an elevation of 1,907 m (6,257 ft).

Mount Barr is one of several magmatic features just north of the Chilliwack batholith. It is part of

a large circular igneous intrusion that was placed along the Fraser Fault 16 to 21 million years ago. The intrusion is part of the Pemberton Volcanic Belt, an eroded volcanic belt that formed as a result of subduction of the Farallon Plate starting 29 million years ago.

Northern Cordilleran Volcanic Province

The Northern Cordilleran Volcanic Province (NCVP), formerly known as the Stikine Volcanic Belt, is a geologic province defined by the occurrence of Miocene to Holocene volcanoes in the Pacific Northwest of North America. This belt of volcanoes extends roughly north-northwest from northwestern British Columbia and the Alaska Panhandle through Yukon to the Southeast Fairbanks Census Area of far eastern Alaska, in a corridor hundreds of kilometres wide. It is the most recently defined volcanic province in the Western Cordillera. It has formed due to extensional cracking of the North American continent—similar to other on-land extensional volcanic zones, including the Basin and Range Province and the East African Rift. Although taking its name from the Western Cordillera, this term is a geologic grouping rather than a geographic one. The southmost part of the NCVP has more, and larger, volcanoes than does the rest of the NCVP; further north it is less clearly delineated, describing a large arch that sways westward through central Yukon.

At least four large volcanoes are grouped with the Northern Cordilleran Volcanic Province, including Hoodoo Mountain in the Boundary Ranges, the Mount Edziza volcanic complex on the Tahltan Highland, and Level Mountain and Heart Peaks on the Nahlin Plateau. These four volcanoes have volumes of more than 15 km3 (3.6 cu mi), the largest and oldest which is Level Mountain with an area of 1,800 km2 (690 sq mi) and a volume of more than 860 km3 (210 cu mi). Apart from the large volcanoes, several smaller volcanoes exist throughout the Northern Cordilleran Volcanic Province, including cinder cones which are widespread throughout the volcanic zone. Most of these small cones have been sites of only one volcanic eruption; this is in contrast to the larger volcanoes throughout the volcanic zone, which have had more than one volcanic eruption throughout their history.

The Northern Cordilleran Volcanic Province is part of an area of intensive earthquake and volcanic activity around the Pacific Ocean called the Pacific Ring of Fire. However, the Northern Cordilleran Volcanic Province is commonly interpreted to be part of a gap in the Pacific Ring of Fire between the Cascade Volcanic Arc further south and the Aleutian Arc further north. But the Northern Cordilleran Volcanic Province is recognized to include over 100 independent volcanoes that have been active in the past 1.8 million years. At least three of them have erupted in the past 360 years, making it the most active volcanic area in Canada. Nevertheless, the dispersed population within the volcanic zone has witnessed few eruptions due to remoteness and the infrequent volcanic activity.

Pali-Aike volcanic field

Pali-Aike volcanic field is a volcanic field in Argentina which straddles the border with Chile. It is part of a province of back-arc volcanoes in Patagonia, which formed from processes involving the collision of the Chile Rise with the Peru–Chile Trench. It lies farther east than the Austral Volcanic Zone, the volcanic arc which forms the Andean Volcanic Belt at this latitude.

Pali-Aike formed over a Jurassic basin starting from the late Miocene as a consequence of regional tectonic events and local extension. It consists of an older plateau basalt formation and younger volcanic centres in the form of pyroclastic cones, scoria cones, maars and associated lava flows. These vents often form local alignments along lineaments or faults. The volcanic field is noteworthy for the presence of large amounts of xenoliths in its rocks and because the maar Laguna Potrok Aike is located here. The field was active starting from 3.78 million years ago. The latest eruptions occurred during the Holocene, as indicated by the burial of archeological artifacts; Laguna Azul maar formed about 3,400 years before present.

Río Murta (volcano)

Río Murta is a volcano in Chile.

The volcano consists of a complex of lava flows along the valleys at the Río Murta. These flows display columnar joints, lava tubes and pillow lavas, and have volumes of less than 1 cubic kilometre (0.24 cu mi). These landforms along with the presence of palagonite indicate that the eruptions happened beneath glaciers.Volcanic activity in the region is in part influenced by the Chile Triple Junction, the point where the Chile Rise is subducted into the Peru-Chile Trench. This point forms a gap in the Andean Volcanic Belt, with Southern Volcanic Zone volcanism north of the gap generated by the fast subduction of the older and colder Nazca Plate beneath the South America Plate and Austral Volcanic Zone volcanism south of the gap formed by the slow subduction of the younger and warmer Antarctic Plate. In between these two subduction processes, a slab window opened up and allowed the rise of alkali basalt magmas.Río Murta rocks are basalts with a low content of potassium. They contain phenocrysts of clinopyroxene, olivine and plagioclase. The chemical composition is unlike that of other regional basaltic volcanoes, and reflects the influence of oceanic asthenosphere.The basement in the region is formed by various Paleozoic to Mesozoic sediments and volcanic rocks. The plutons of the Northern Patagonian Batholith were intruded into this basement and may have an origin in the subduction of the Nazca Plate-Farallon Plate.The age of these flows is controversial. Potassium-argon dating has yielded ages of 900,000 - 850,000 years before present, some flows are too young to date and the relatively well conserved appearance suggest a Holocene age. 40 kilometres (25 mi) northwest of Río Murta lies Cerro Hudson, an active arc volcano.

Seal Nunataks

The Seal Nunataks are a group of 16 islands called nunataks emerging from the Larsen Ice Shelf east of Graham Land, Antarctic Peninsula. The Seal Nunataks have been described as separate volcanic vents of ages ranging from Miocene to Pleistocene. There are unconfirmed reports of Holocene volcanic activity.

Siletzia

Siletzia is the massive formation of early to middle Eocene epoch marine basalts and interbedded sediments in the forearc of the Cascadia subduction zone; this forms the basement rock under western Oregon and Washington and the southern tip of Vancouver Island. It is now fragmented into the Siletz and Crescent terranes.Siletzia corresponds geographically to the Coast Range Volcanic Province (or Coast Range basalts), but is distinguished from slightly younger basalts that erupted after Siletzia accreted to the continent and differ in chemical composition. The Siletzia basalts are tholeiitic, a characteristic of mantle-derived magma erupted from a spreading ridge between plates of oceanic crust. The younger basalts are alkalic or calc-alkaline, characteristic of magmas derived from a subduction zone. This change of composition reflects a change from marine to continental volcanism that becomes evident around 48 to 42 Ma (millions of years ago), and is attributed to the accretion of Siletzia against the North American continent.Various theories have been proposed to account for the volume and diversity of Siletzian magmatism, as well as the approximately 75° of rotation, but the evidence is insufficient to determine Siletzia's origin; the question remains open.The accretion of Siletzia against the North American continent approximately 50 million years ago (contemporaneous with the initiation of the bend in the Hawaiian-Emperor seamount chain) was a major tectonic event associated with a reorganization of the earth's tectonic plates. This is believed to have a caused a shift in the subduction zone, termination of the Laramide orogeny that was uplifting the Rocky Mountains, and major changes in tectonic and volcanic activity across much of western North America.

Slab (geology)

In geology, a slab is the portion of a tectonic plate that is being subducted.Slabs constitute an important part of the global plate tectonic system. They drive plate tectonics – both by pulling along the lithosphere to which they are attached in a processes known as slab pull and by inciting currents in the mantle (slab suction). They cause volcanism due to flux melting of the mantle wedge, and they affect the flow and thermal evolution of the Earth's mantle. Their motion can cause dynamic uplift and subsidence of the Earth's surface, forming shallow seaways and potentially rearranging drainage patterns.Geologists have imaged slabs down to the seismic discontinuities between the upper and lower mantle and to the core–mantle boundary. About 100 slabs have been described at depth, and where and when they subducted. Slab subduction is the mechanism by which lithospheric material is mixed back into the Earth's mantle.

Slesse Mountain

Slesse Mountain, usually referred to as Mount Slesse, is a mountain just north of the US-Canada border, in the Cascade Mountains of British Columbia, near the town of Chilliwack. It is notable for its large, steep local relief. For example, its west face drops over 1,950 m (6,398 ft) to Slesse Creek in less than 3 km (2 mi). It is also famous for its huge Northeast Buttress; see the climbing notes below. The name means "fang" in the Halkomelem language. Notable nearby mountains include Mount Rexford and Canadian Border Peak in British Columbia, and American Border Peak, Mount Shuksan, and Mount Baker, all in the US state of Washington.

Undersea mountain range

Undersea mountain ranges are mountain ranges that are mostly or entirely underwater, and specifically under the surface of an ocean. If originated from current tectonic forces, they are often referred to as a mid-ocean ridge. In contrast, if formed by past above-water volcanism, they are known as a seamount chain. The largest and best known undersea mountain range is a mid-ocean ridge, the Mid-Atlantic Ridge. It has been observed that, "similar to those on land, the undersea mountain ranges are the loci of frequent volcanic and earthquake activity".

Wave base

The wave base, in physical oceanography, is the maximum depth at which a water wave's passage causes significant water motion. For water depths deeper than the wave base, bottom sediments and the seafloor are no longer stirred by the wave motion above.

Wells Gray-Clearwater volcanic field

The Wells Gray-Clearwater volcanic field, also called the Clearwater Cone Group, is a potentially active monogenetic volcanic field in east-central British Columbia, Canada, located approximately 130 km (81 mi) north of Kamloops. It is situated in the Cariboo Mountains of the Columbia Mountains and on the Quesnel and Shuswap Highlands. As a monogenetic volcanic field, it is a place with numerous small basaltic volcanoes and extensive lava flows.Most of the Wells Gray-Clearwater volcanic field is encompassed within a large wilderness park called Wells Gray Provincial Park. This 5,405 km2 (2,087 sq mi) park was established in 1939 to protect Helmcken Falls and the unique features of the Clearwater River drainage basin, including this volcanic field. Five roads enter the park and provide views of some of the field's volcanic features. Short hikes lead to several other volcanic features, but some areas are accessible only by aircraft.

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