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34-505: Divergence between the North American and South American Plates began to create oceanic crust off Colombia's Pacific coast by the end of the Jurassic (150 Ma). This divergence, which continued until at least 66 Ma, first resulted in a " proto-Caribbean spreading ridge " between these plates flanked by a perpendicular transform zone on its Pacific side. By 135–130 Ma, the subduction of

68-839: A long span of time. Much of North America west of the Rocky Mountains is composed of such terranes. The southern boundary with the Cocos plate to the west and the Caribbean plate to the east is a transform fault , represented by the Swan Islands Transform Fault under the Caribbean Sea and the Motagua Fault through Guatemala . The parallel Septentrional and Enriquillo–Plantain Garden faults running through Hispaniola and bounding

102-406: A mantle convective current is propelling the plate. Argon-argon dating Argon–argon (or Ar/ Ar ) dating is a radiometric dating method invented to supersede potassium–argon (K/Ar) dating in accuracy. The older method required splitting samples into two for separate potassium and argon measurements, while the newer method requires only one rock fragment or mineral grain and uses

136-602: A mineral or fragments of rock are hand-selected for analysis. These are then irradiated to produce Ar from K via the (n-p) reaction K(n,p) Ar. The sample is then degassed in a high-vacuum mass spectrometer via a laser or resistance furnace. Heating causes the crystal structure of the mineral (or minerals) to degrade, and, as the sample melts, trapped gases are released. The gas may include atmospheric gases, such as carbon dioxide, water, nitrogen, and radiogenic gases like argon and helium, generated from regular radioactive decay over geologic time. The abundance of Ar* increases with

170-572: A rock retains all of its Ar after cooling past the closing temperature and that this was properly sampled during analysis. This technique allows the errors involved in K-Ar dating to be checked. Argon–argon dating has the advantage of not requiring determinations of potassium. Modern methods of analysis allow individual regions of crystals to be investigated. This method is important as it allows crystals forming and cooling during different events to be identified. One problem with argon-argon dating has been

204-466: A sample of known age for a standard. Because this (primary) standard ultimately cannot be determined by Ar/ Ar, it must be first determined by another dating method. The method most commonly used to date the primary standard is the conventional K/Ar technique . An alternative method of calibrating the used standard is astronomical tuning (also known as orbital tuning ), which arrives at a slightly different age. The primary use for Ar/ Ar geochronology

238-546: A second pulse occurred 81-69 Ma. Around 86 Ma the arrival of a large plume initiated the Galápagos hotspot which resulted in volcanism over large parts of the Caribbean Plate and north-west South America. Renewed volcanism about 75 Ma has been attributed to either the Galápagos hotspot, thinning of the lithosphere coupled with associated melting and upwelling of plume-head material, or both. Seismic and geochemical analyses, on

272-460: A single measurement of argon isotopes . Ar/ Ar dating relies on neutron irradiation from a nuclear reactor to convert a stable form of potassium ( K) into the radioactive Ar. As long as a standard of known age is co-irradiated with unknown samples, it is possible to use a single measurement of argon isotopes to calculate the K/ Ar* ratio, and thus to calculate the age of the unknown sample. Ar* refers to

306-528: A thickened zone of oceanic crust between the North American and South American plates. In some places the oceanic crust is 2–3 times as thick as normal oceanic crust (15–20 km (9.3–12.4 mi) vs 7 km (4.3 mi). Its composition is similar to that of the Ontong Java Plateau . Geochemical and geochronological evidences clearly indicate that the Galápagos hotspot initiated the formation of

340-401: Is dating metamorphic and igneous minerals. Ar/ Ar is unlikely to provide the age of intrusions of granite as the age typically reflects the time when a mineral cooled through its closure temperature . However, in a metamorphic rock that has not exceeded its closure temperature the age likely dates the crystallization of the mineral. Dating of movement on fault systems is also possible with

374-506: Is the Ar*/ Ar ratio. The J factor relates to the fluence of the neutron bombardment during the irradiation process; a denser flow of neutron particles will convert more atoms of K to Ar than a less dense one. The Ar/ Ar method only measures relative dates. In order for an age to be calculated by the Ar/ Ar technique, the J parameter must be determined by irradiating the unknown sample along with

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408-429: The Ar/ Ar method. Different minerals have different closure temperatures; biotite is ~300°C, muscovite is about 400°C and hornblende has a closure temperature of ~550°C. Thus, a granite containing all three minerals will record three different "ages" of emplacement as it cools down through these closure temperatures. Thus, although a crystallization age is not recorded, the information is still useful in constructing

442-639: The Curaçao Lava Formation and 94–83 ma for the Dumisseau Formation in Haiti, dating both locations back to the original LIP formation 94 Ma. CLIP volcanism originates from the plume -like source distinct from a MORB (mid-ocean ridge basalt) mantle . The long duration of CLIP volcanism can be explained by the interaction between a plume and the Greater Antilles subduction zone. The margins of

476-517: The Eurasian plate and Nubian plate . and westward to the Chersky Range in eastern Siberia. The plate includes both continental and oceanic crust . The interior of the main continental landmass includes an extensive granitic core called a craton . Along most of the edges of this craton are fragments of crustal material called terranes , which are accreted to the craton by tectonic actions over

510-713: The Farallon Plate had begun along this transform zone , effectively modifying it into a subduction zone and beginning the creation of the Great Caribbean Arch. This arch was formed around 120-115 Ma but must have been intersected by the Caribbean spreading ridge until 66 Ma. Hence, the Farallon Plate fed the spreading zone and later became the Caribbean Plate . CLIP formed as a large igneous province and now forms

544-760: The Fifteen-Twenty fracture zone around 16°N. On the northerly boundary is a continuation of the Mid-Atlantic Ridge called the Gakkel Ridge . The rest of the boundary in the far northwestern part of the plate extends into Siberia . This boundary continues from the end of the Gakkel Ridge as the Laptev Sea Rift , on to a transitional deformation zone in the Chersky Range , then the Ulakhan Fault between it and

578-628: The Gonâve microplate , and the parallel Puerto Rico Trench running north of Puerto Rico and the Virgin Islands and bounding the Puerto Rico–Virgin Islands microplate , are also a part of the boundary. The rest of the southerly margin which extends east to the Mid-Atlantic Ridge and marks the boundary between the North American plate and the South American plate is vague but located near

612-747: The Miocene period and are still geologically active, creating earthquakes and volcanoes. The Yellowstone hotspot is most notable for the Yellowstone Caldera and the many calderas that lie in the Snake River Plain , while the Anahim hotspot is most notable for the Anahim Volcanic Belt in the Nazko Cone area. For the most part, the North American plate moves in roughly a southwest direction away from

646-927: The Okhotsk microplate , and finally the Aleutian Trench to the end of the Queen Charlotte Fault system (see also: Aleutian Arc ). The westerly boundary is the Queen Charlotte Fault running offshore along the coast of Alaska and the Cascadia subduction zone to the north, the San Andreas Fault through California , the East Pacific Rise in the Gulf of California , and the Middle America Trench to

680-455: The radiogenic Ar, i.e. the Ar produced from radioactive decay of K. Ar* does not include atmospheric argon adsorbed to the surface or inherited through diffusion and its calculated value is derived from measuring the Ar (which is assumed to be of atmospheric origin) and assuming that Ar is found in a constant ratio to Ar in atmospheric gases. The sample is generally crushed and single crystals of

714-677: The Antilles. One of the least deformed parts is Gorgona Island off Colombia's Pacific coast. The CLIP was created during three phases of eruptions dating between the Aptian and the Maastrichtian : a first phase 124–112 Ma; the main magma production phase 94–83 Ma; and an 80–72 Ma phase. The youngest igneous rocks , in the Dominican Republic and Costa Rica, are from 63 Ma. That the CLIP originated in

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748-651: The Bahamas , extreme northeastern Asia , and parts of Iceland and the Azores . With an area of 76 million km (29 million sq mi), it is the Earth's second largest tectonic plate, behind the Pacific plate (which borders the plate to the west). It extends eastward to the seismically active Mid-Atlantic Ridge at the Azores triple junction plate boundary where it meets

782-600: The CLIP 95-90 Ma in the eastern Pacific. From there it move north-east with the Farallon Plate between the two American plates until it collided with a volcanic arc , the Greater Antilles 60 million years later. Fragments of this voyage is preserved in accreted seamounts along the Central American coast and the Cocos and Carnegie Ridges . Isotopic profiles of Galápagos rocks can be matched with those from CLIP rocks. 92–63 Ma Ar/Ar ages have been reported for

816-510: The CLIP have been uplifted and are exposed above sea level, which makes it unique among oceanic plateaus. It stretches 2,500 km (1,600 mi) east to west and 1,300 km (810 mi) north to south. The CLIP is composed of irregularly thickened (up to 20 km (12 mi)) oceanic crust of the Caribbean Plate and the deformed associated magmatic terranes obducted onto the Pacific coasts of northern South America, Central America, and

850-622: The CLIP remains unclear, especially considering the subduction in the Costa Rica-Panama arc initiated during the Campanian (83–72 Ma). The Galápagos hotspot is probably responsible for the main plume-related magmatic event 90 Ma, whilst the 76 Ma and 55 Ma event are related to lithospheric thinning in the Central Caribbean. Ar/Ar dating have determined that the main magmatism occurred 95 to 83 million years ago (Ma) while

884-677: The Farallon plate. The boundary along the Gulf of California is complex. The gulf is underlain by the Gulf of California Rift Zone , a series of rift basins and transform fault segments from the northern end of the East Pacific Rise in the mouth of the gulf to the San Andreas Fault system in the vicinity of the Salton Trough rift/ Brawley seismic zone . It is generally accepted that a piece of

918-536: The Mid-Atlantic Ridge at a rate of about 2.3 centimeters (~1 inch) per year. At the same time, the Pacific plate is moving to the northwest at a speed of between 7 and 11 centimeters (~3-4 inches) per year. The motion of the plate cannot be driven by subduction as no part of the North American plate is being subducted, except for a small section comprising part of the Puerto Rico Trench ; thus other mechanisms continue to be investigated. One study in 2007 suggests that

952-448: The North American plate was broken off and transported north as the East Pacific Rise propagated northward, creating the Gulf of California. However, it is as yet unclear whether the oceanic crust between the rise and the mainland coast of Mexico is actually a new plate beginning to converge with the North American plate, consistent with the standard model of rift zone spreading centers generally. A few hotspots are thought to exist below

986-522: The North American plate. The most notable hotspots are the Yellowstone (Wyoming), Jemez Lineament (New Mexico), and Anahim (British Columbia) hotspots. These are thought to be caused by a narrow stream of hot mantle convecting up from the Earth's core–mantle boundary called a mantle plume , although some geologists think that upper mantle convection is a more likely cause. The Yellowstone and Anahim hotspots are thought to have first arrived during

1020-504: The Pacific is obvious because fragments of oceanic crust accreted to the margins of the Caribbean, for example on Hispaniola and Puerto Rico , contain fauna of Pacific provenance. The Farallon Plate's eastward movement forced the northern half of the CLIP into the ocean basin that had opened between North and South America starting in the Jurassic. However, the mechanisms causing the NE movement of

1054-458: The age of the sample, though the rate of increase decays exponentially with the half-life of K, which is 1.248 billion years. The age of a sample is given by the age equation: where λ is the radioactive decay constant of K (approximately 5.5 x 10 year , corresponding to a half-life of approximately 1.25 billion years), J is the J-factor (parameter associated with the irradiation process), and R

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1088-432: The other hand, suggest the CLIP consists of several oceanic plateaus and palaeo- hotspot tracks formed 139-83 Ma some of which have been overprinted by later magmatism. If these first volcanic activities were generated by the Galápagos hotspot, it would make it the oldest still active hotspot on Earth. North American Plate The North American plate is a tectonic plate containing most of North America , Cuba ,

1122-494: The south. On its western edge, the Farallon plate has been subducting under the North American plate since the Jurassic period. The Farallon plate has almost completely subducted beneath the western portion of the North American plate, leaving that part of the North American plate in contact with the Pacific plate as the San Andreas Fault. The Juan de Fuca , Explorer , Gorda , Rivera , Cocos and Nazca plates are remnants of

1156-428: The thermal history of the rock. Dating minerals may provide age information on a rock, but assumptions must be made. Minerals usually only record the last time they cooled down below the closure temperature, and this may not represent all of the events which the rock has undergone, and may not match the age of intrusion. Thus, discretion and interpretation of age dating is essential. Ar/ Ar geochronology assumes that

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