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Cariaco Basin

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The Cariaco Basin lies off the north central coast of Venezuela and forms the Gulf of Cariaco . It is bounded on the east by Margarita Island , Cubagua Island, and the Araya Peninsula ; on the north by Tortuga Island and the Tortuga Banks ; on the west by Cape Codera and the rocks known as Farallón Centinela; and on the south by the coast of Venezuela.

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32-436: The Cariaco Basin is an east-west trending pull-apart basin located on the continental shelf off the eastern coast of Venezuela . It is a deep depression composed of two sub-basins, the eastern basin and the western basin, each of about 1,400 metres (4,600 ft) depth, separated by a saddle of approximately 900 metres (3,000 ft) water-depth. To the south, the basin confines with the wide (~50 km) Unare Platform. It

64-399: A frequent trap for hydrocarbons. Likewise intense deformation and rapid subsidence and deposition in pull-aparts creates numerous structural and stratigraphic traps, enhancing their viability as hydrocarbon reservoirs . The shallow extensional regime of pull-apart basins also facilitates the emplacement of felsic intrusive rocks with high copper mineralisation. It is believed to be

96-561: A layer immediately beneath it. Continental crust is produced and (far less often) destroyed mostly by plate tectonic processes, especially at convergent plate boundaries . Additionally, continental crustal material is transferred to oceanic crust by sedimentation. New material can be added to the continents by the partial melting of oceanic crust at subduction zones, causing the lighter material to rise as magma, forming volcanoes. Also, material can be accreted horizontally when volcanic island arcs , seamounts or similar structures collide with

128-439: A steady-state hypothesis argue that the total volume of continental crust has remained more or less the same after early rapid planetary differentiation of Earth and that presently found age distribution is just the result of the processes leading to the formation of cratons (the parts of the crust clustered in cratons being less likely to be reworked by plate tectonics). However, this is not generally accepted. In contrast to

160-451: A straight course and frequently causes bends or step-overs in fault paths. Bends and step-overs of adjacent faults become favorable locations for extensional and compressional stress or transtension and transpression stress, if the shear motion is oblique. Pull-apart basins form in extensional to transtensional environments along fault bends or between two adjacent left-lateral faults or two right-lateral faults. The step-over or bend in

192-467: A very high biomass of plankton, fish, and marine birds and mammals. The seasonal productivity drives the abundance of sardines and attract dolphins . The waters are also home to several species of whales (such as rorquals and humpback ), and orcas . Pull-apart basin In geology, a basin is a region where subsidence generates accommodation space for the deposition of sediments. A pull-apart basin

224-451: Is a structural basin where two overlapping (en echelon) strike-slip faults or a fault bend create an area of crustal extension undergoing tension , which causes the basin to sink down. Frequently, the basins are rhombic or sigmoidal in shape. Dimensionally, basins are limited to the distance between the faults and the length of overlap. The inhomogeneity and structural complexity of continental crust causes faults to deviate from

256-558: Is connected to the open Caribbean Sea through two shallow (around 140 metres (460 ft)) channels, to the north the (Tortuga Channel) and to the west the (Centinela Channel). Water circulation inside the basin is restricted, which, combined with the high annual primary productivity of the region (~500 gCmyr), causes the basin to be permanently anoxic , below ~250 m. This naturally occurring anoxic basin allows for sediments to be deposited without bioturbation , forming varves of alternating light and dark color, which correspond to

288-575: Is considerably thicker than oceanic crust, which has an average thickness of around 7 to 10 km (4.3 to 6.2 mi). Approximately 41% of Earth's surface area and about 70% of the volume of Earth's crust are continental crust. Because the surface of continental crust mainly lies above sea level, its existence allowed land life to evolve from marine life. Its existence also provides broad expanses of shallow water known as epeiric seas and continental shelves where complex metazoan life could become established during early Paleozoic time, in what

320-409: Is little evidence of continental crust prior to 3.5 Ga . About 20% of the continental crust's current volume was formed by 3.0 Ga. There was relatively rapid development on shield areas consisting of continental crust between 3.0 and 2.5 Ga. During this time interval, about 60% of the continental crust's current volume was formed. The remaining 20% has formed during the last 2.5 Ga. Proponents of

352-554: Is now called the Cambrian explosion . All continental crust is ultimately derived from mantle-derived melts (mainly basalt ) through fractional differentiation of basaltic melt and the assimilation (remelting) of pre-existing continental crust. The relative contributions of these two processes in creating continental crust are debated, but fractional differentiation is thought to play the dominant role. These processes occur primarily at magmatic arcs associated with subduction . There

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384-474: Is sometimes called sial because its bulk composition is richer in aluminium silicates (Al-Si) and has a lower density compared to the oceanic crust , called sima which is richer in magnesium silicate (Mg-Si) minerals. Changes in seismic wave velocities have shown that at a certain depth (the Conrad discontinuity ), there is a reasonably sharp contrast between the more felsic upper continental crust and

416-456: The Atlantic Ocean , for example) are termed passive margins . The high temperatures and pressures at depth, often combined with a long history of complex distortion, cause much of the lower continental crust to be metamorphic – the main exception to this being recent igneous intrusions . Igneous rock may also be "underplated" to the underside of the crust, i.e. adding to the crust by forming

448-704: The Dead Sea , the Salton Sea , and the Sea of Marmara . Pull-apart basins are amenable to research because sediments deposited in the basin provide a timeline of activity along the fault. The Salton Trough is an active pull-apart located in a step-over between the dextral San Andreas Fault and the Imperial Fault . Displacement on the fault is approximately 6 cm/yr. The current transtensional state generates normal growth faults and some strike slip motion. The growth faults in

480-484: The Mediterranean Sea at about 340 Ma. Continental crust and the rock layers that lie on and within it are thus the best archive of Earth's history. The height of mountain ranges is usually related to the thickness of crust. This results from the isostasy associated with orogeny (mountain formation). The crust is thickened by the compressive forces related to subduction or continental collision. The buoyancy of

512-620: The Salton Buttes . Pull-apart basins represent an important exploration target for oil and gas, porphyry copper mineralisation, and geothermal fields. The Matzen fault system in the Matzen oil field has been recast as extensional grabens produced by pull-apart basins of the Vienna Basin . The Dead Sea has been studied extensively and thinning of the crust in pull-aparts may generate differential loading and instigate salt diapirs to rise,

544-468: The CARIACO ocean time series program has demonstrated that this anoxic basin is quite dynamic and has helped understand the paleoclimatic record stored in the basin's sediments. The Cariaco Basin is an example of upwelling zone ecosystems . The seasonal upwelling cycle and sea surface temperature changes are linked to the intensity of the trade winds , and cause events of high primary production that support

576-481: The Cariaco Basin has also a unique chemistry. Bacteria inhabit both the oxic and anoxic portions of the water column, with a maximum around the interface where oxygen disappears. This 'interface' oscillates between 200 and 300 meters. As such unique location, the Cariaco Basin has been the site of a variety of studies since the mid-1950s. Since 1995, an international ( Venezuela and United States ) program has expanded

608-411: The continental crust is about, 2.83 g/cm (0.102 lb/cu in), less dense than the ultramafic material that makes up the mantle , which has a density of around 3.3 g/cm (0.12 lb/cu in). Continental crust is also less dense than oceanic crust, whose density is about 2.9 g/cm (0.10 lb/cu in). At 25 to 70 km (16 to 43 mi) in thickness, continental crust

640-432: The crust forces it upwards, the forces of the collisional stress balanced by gravity and erosion. This forms a keel or mountain root beneath the mountain range, which is where the thickest crust is found. The thinnest continental crust is found in rift zones, where the crust is thinned by detachment faulting and eventually severed, replaced by oceanic crust. The edges of continental fragments formed this way (both sides of

672-419: The dominant mode of continental crust formation and destruction. It is a matter of debate whether the amount of continental crust has been increasing, decreasing, or remaining constant over geological time. One model indicates that at prior to 3.7 Ga ago continental crust constituted less than 10% of the present amount. By 3.0 Ga ago the amount was about 25%, and following a period of rapid crustal evolution it

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704-576: The dry or rainy season. Its unique geography and undisturbed sediment record provides an excellent history of tropical climate change and is particularly sensitive to shifts in the Intertropical Convergence Zone (ITCZ) and has been the subject of extensive paleoclimatological research, amongst other sedimentological studies, geochemical studies, with alkenones , Mg/Ca , and micropaleontological, with foraminifera , pollen and spores , dinocysts and coccoliths . Because of its anoxia,

736-411: The fault must be the same direction as sense of motion on the fault otherwise the area will be subject to transpression. For example, two overlapping left lateral fault must have a left-step-over to create a pull-apart basin. This is illustrated in the accompanying figures. A regional strike slip fault is referred to as a principle displacement zone (PDZ). Connecting the tips of step over faults to

768-535: The lower continental crust, which is more mafic in character. Most continental crust is dry land above sea level. However, 94% of the Zealandia continental crust region is submerged beneath the Pacific Ocean , with New Zealand constituting 93% of the above-water portion. The continental crust consists of various layers, with a bulk composition that is intermediate (SiO 2 wt% = 60.6). The average density of

800-502: The main structural control on the giant Escondida deposit in Chile . Geothermal fields are located in pull-aparts for the same reason due to the high heat flow associated with rising magmas. Continental crust Continental crust is the layer of igneous , metamorphic , and sedimentary rocks that forms the geological continents and the areas of shallow seabed close to their shores, known as continental shelves . This layer

832-615: The oldest rocks on Earth are within the cratons or cores of the continents, rather than in repeatedly recycled oceanic crust ; the oldest intact crustal fragment is the Acasta Gneiss at 4.01 Ga , whereas the oldest large-scale oceanic crust (located on the Pacific plate offshore of the Kamchatka Peninsula ) is from the Jurassic (≈180 Ma ), although there might be small older remnants in

864-631: The opposite fault are bounding basin sidewall faults. The tectonic subsidence of strike-slip basins is mainly episodic, short lived (typically less than 10 Ma), and end abruptly with commonly very high tectonic subsidence rates (greater than 0.5 km/Ma) compared to all other basin types. Recent sandbox models have shown that the geometry and evolution of pull-apart basins varies greatly in pure-strike slip situations versus transtensional settings. Transtensional settings are believed to generate greater surface subsidence than pure-strike slip alone. Famous localities for continental pull-apart basins are

896-466: The persistence of continental crust, the size, shape, and number of continents are constantly changing through geologic time. Different tracts rift apart, collide and recoalesce as part of a grand supercontinent cycle . There are currently about 7 billion cubic kilometres (1.7 billion cubic miles) of continental crust, but this quantity varies because of the nature of the forces involved. The relative permanence of continental crust contrasts with

928-414: The region strike N15E, have steep dips (~70 deg), and vertical displacements of 1–4 mm/yr. Eight large slip events have occurred on these faults with throw ranging from 0.2 to 1.0 meters. These produce earthquakes greater than magnitude six and are responsible for the majority of extension in the basin and consequently thermal anomalies, subsidence, and localization of rhyolite buttes such as

960-567: The research in the basin. The CARIACO ( Ca rbon R etention i n a C olored O cean; [1] Archived 2013-12-06 at the Wayback Machine ) program consists of a time series station in the eastern deep of the basin which is visited on a monthly basis to collect hydrographic, nutrient and primary productivity measurements. A suite of other measurements, including a sediment trap mooring, microbiological studies and current measurements are also conducted at this site. The work that has resulted from

992-521: The short life of oceanic crust. Because continental crust is less dense than oceanic crust, when active margins of the two meet in subduction zones, the oceanic crust is typically subducted back into the mantle. Continental crust is rarely subducted (this may occur where continental crustal blocks collide and overthicken, causing deep melting under mountain belts such as the Himalayas or the Alps ). For this reason

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1024-494: The side of the continent as a result of plate tectonic movements. Continental crust is also lost through erosion and sediment subduction, tectonic erosion of forearcs, delamination, and deep subduction of continental crust in collision zones. Many theories of crustal growth are controversial, including rates of crustal growth and recycling, whether the lower crust is recycled differently from the upper crust, and over how much of Earth history plate tectonics has operated and so could be

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