Misplaced Pages

#899100

59-539: Roughly rhomboid, extending between 50°S 70°W  /  50°S 70°W  / -50; -70 and 63°S 20°W  /  63°S 20°W  / -63; -20 , the plate is 800 km (500 mi) wide and 3,000 km (1,900 mi) long. It is moving WSW at 2.2 cm (0.87 in)/year and the South Sandwich plate is moving east at 5.5 cm (2.2 in)/year in an absolute reference frame. Its boundaries are defined by

118-440: A critically low viscosity enabling them to flow. Subsequently these rocks can decoupled from their base and begin to flow to higher crustal levels along lithostatic pressure gradients that can be caused by melt-induced buoyancy or differences in topography and lateral density contrasts. both of which are affected by erosion. Characteristics of this mode of exhumation include simultaneous normal shearing and reverse shearing along

177-505: A guide for helmsmen. Prior to the modern three-figure method of describing directions (using the 360° of a circle), the 32-point compass was used for directions on most ships, especially among European crews. The smallest unit of measure recognized was 'one point', 1/32 of a circle, or 11 + 1 ⁄ 4 °. In the mariner's exercise of "boxing the compass", all thirty-two points of the compass are named in clockwise order. This exercise became more significant as navigation improved and

236-504: A process that removes parts of the Earth's upper crust by physical processes that occur naturally (e.g. glaciers , wind, water, landslides ). Through this form of exhumation, something previously buried in sediments , for example a landform , is uncovered and can be exposed. Exhumation by tectonic processes refers to any geological mechanism that brings rocks from deeper crustal levels to shallower crustal levels. While erosion or denudation

295-654: A rate of roughly 7.4–9.5 mm/yr that occupies the southern half of the Antarctic-Scotia plate boundary. The relative motion between the Scotia plate and the Antarctic plate on the western boundary is 7.5–8.7 mm/yr. Though the South Scotia Ridge is overall a transform fault, small sections of the ridge are spreading to make up for the somewhat jagged shape of the boundary. At the eastern tip of

354-680: A second exhumation . During the Oligocene (34–23 Ma) South Georgia was reburied again as seafloor spreading took place in the West Scotia Sea. 10 Ma, finally, the South Georgia microcontinent was uplifted as a result of the collision with the Northeast Georgia Rise . The southern edge of the plate is bordered by the Antarctic plate, forming the South Scotia Ridge, a left-lateral transform boundary sliding at

413-419: A series of parameters such as: Detailed and integrated geologic modelling , structural geology , geochronology and metamorphic geological disciplines are key to understanding these processes. Exhumation through denudation could be considered as the process of exposing rock packages solely through the removal of their overlying unconsolidated sediments or solid rock layers. Denudation is here considered as

472-410: Is " Quarto di Tramontana verso Greco "; and northeast-by-north is " Quarto di Greco verso Tramontana ". The table below shows how the 32 compass points are named. Each point has an angular range of 11 + 1 ⁄ 4 degrees where the azimuth midpoint is the horizontal angular direction (clockwise from north) of the given compass bearing; minimum is the lower (counterclockwise) angular limit of

531-414: Is "Greco-Levante"; SSE is "Ostro-Scirocco", etc. The quarter winds are expressed with an Italian phrase, " Quarto di X verso Y" ( pronounced [ˈkwarto di X ˈvɛrso Y ] one quarter from X towards Y), or "X al Y" (X to Y) or "X per Y" (X by Y). There are no irregularities to trip over; the closest principal wind always comes first, the more distant one second, for example: north-by-east

590-414: Is a left-lateral, or sinistral, transform boundary with a transform rate of roughly 7.1 mm/yr. The Magallanes–Fagnano Fault is passing through Tierra del Fuego . The northern ridge stretches from Isla de los Estados off Tierra del Fuego in the west to the microcontinent South Georgia in the east, with a series of shallow banks in between: Burdwood , Davis, Barker, and Shag Rocks . North of

649-459: Is coeval to burial of the structural levels. Tectonics of this kind result in fold and thrust belts or if they are built up over long periods, can form thick-stacked long-hot-orogens, such as the Himalayas . Channel flow typically occurs in long-hot orogens when the orogen is sufficiently thick to promote partial melting in the middle-lower part of the orogen to a point where the rocks reach

SECTION 10

#1732771823900

708-502: Is fundamental in eventually exposing these deeper rocks at the Earth's surface, the geological phenomenon that drive the rocks to shallower crust are still considered exhumation processes. Geological exhumation occurs on a range of scales, from smaller-scale thrusts typically occurring within the shallow crust (less than ca. 10 km deep) which results in exhumation in the order of centimeters to meters scales, to larger-scale features originating at deeper crustal levels along which, exhumation

767-401: Is in the order of hundreds of meters to kilometers. The geological mechanisms that drive deep crustal exhumation can occur in a variety of tectonic settings but are ultimately driven by the convergence of tectonic plates through subduction . Depending on the type of convergent boundary , exhumation occurs by thrusting in the accretionary wedge , by obduction and/or as a process during

826-594: The Antarctic Peninsula , the beginning of South Scotia Ridge, a small and heavily dissected bank degenerates into several outcrops dominated by Paleozoic and Cretaceous rocks. A small basin, Powel Basin, separates this cluster from the South Orkney microcontinent composed of Triassic and younger rocks. The eastern continuation of the ridge, the Scotia Arc east of the South Sandwich plate , are

885-719: The Eocene . Before that, this basin went through a series of geological transformations during the Cretaceous , through which South Georgia was first buried, then made a topographic feature again by the Late Cretaceous. At about 45 Ma, South Georgia, still part of the South American plate , got buried again and something, possibly rotation of the Fuegian Andes, completed the break-up and allowed South Georgia

944-694: The Mid-Atlantic Ridge grew from 1200 km to 7000 km. This led to compressional deformations along the western margin of the South American plate and the obduction of the Rocas Verdes basement onto this margin. Structures associated with this obduction are found from Tierra del Fuego to South Georgia. The acceleration of the westward motion of South America and the inversion of the Rocas Verde Basin finally lead to

1003-649: The Middle Ages (with no obvious connection to the twelve classical compass winds of the ancient Greeks and Romans). The traditional mariner's wind names were expressed in Italian , or more precisely, the Italianate Mediterranean lingua franca common among sailors in the 13th and 14th centuries, which was principally composed of Genoese ( Ligurian ), mixed with Venetian , Sicilian , Provençal , Catalan , Greek , and Arabic terms from around

1062-563: The South Sandwich island arc and trench . This volcanic active island arc has submerged ancestors in Jane and Discovery banks in the southern ridge. The western edge of the plate is bounded by the Antarctic plate, forming the Shackleton Fracture Zone and the southern Chile Trench . The Southern Chile Trench is a southern extension of the subduction of the Antarctic and Nazca plates below South America. Heading south along

1121-849: The South Sandwich Islands on the Sandwich Plate and the Lesser Antilles on the Caribbean plate, and both plates also had a major impact on global climate when they closed the two major gateways between the Pacific and Atlantic Oceans during the Mesozoic and Cenozoic . The northern edge of the Scotia plate is bounded by the South American plate, forming the North Scotia Ridge. The North Scotia ridge

1180-482: The clockwise direction and "CCW" counterclockwise . The final three columns show three common naming conventions: No "by" avoids the use of "by" with fractional points. Colour coding shows whether each of the three naming systems matches the "CW" or "CCW" column. The traditional compass rose of eight winds (and its 16-wind and 32-wind derivatives) was invented by seafarers in the Mediterranean Sea during

1239-490: The orogenic cycle (i.e. mountain building and collapse cycle). During the subduction of an oceanic plate underneath the continental crust, some fragments of the oceanic crust can be trapped above the continental crust through obduction . The resulting rocks obducted on the continental crust are called ophiolites .   While the exact mechanism behind the formation of ophiolites is still up for debate, those rocks still show an example of rocks being exhumed and exposed at

SECTION 20

#1732771823900

1298-511: The 32-wind compass rose comes from the eight principal winds, eight half-winds, and sixteen quarter-winds combined, with each compass point at an 11 + 1 ⁄ 4 ° angle from the next. By the middle of the 18th century, the 32-point system had been further extended by using half- and quarter-points to give a total of 128 directions. These fractional points are named by appending, for example, ⁠ 1 / 4 ⁠ east, ⁠ 1 / 2 ⁠ east, or ⁠ 3 / 4 ⁠ east to

1357-843: The Burdwood Bank on the western North Scotia Ridge south of the Falkland Islands. The Rocas Verdes basin was filled with turbidites derived from the volcanic arc on its Pacific margin and partly from its continental margin. The Weddell Sea continued to expand which led to the extension between the Patagonian Andes and the Antarctic Peninsula. In the Mid-Cretaceous (100 Ma) the spreading rate in South Atlantic increased significantly and

1416-789: The Central Scotia Sea at that time but has since been translated eastward by the back-arc spreading centre of the East Scotia Ridge. One of the most prominent features in the Scotia plate itself is the median valley known as the West Scotia Ridge. It was produced by two plates that are no longer independently active: the Magallanes and Central Scotia plates so is an extinct spreading center. The ridge consists of seven segments separated by right-lateral transform faults of various lengths. The western region of

1475-612: The Early Oligocene (90–30 Ma) little changed in the region, except for the subduction of the Phoenix plate along the Shackleton Fracture Zone. The Late Paleocene and Early Eocene (60–50 Ma) saw the formation of South Scotia Sea and South Scotia Ridge — the first sign of separation of the southern Andes and the Antarctic Peninsula ;— which resulted in seafloor spreading in the West Scotia Sea and hence

1534-482: The Earth's surface) are brought towards the Earth's surface (i.e.shallower crustal levels) by crustal thickening (see compared also tectonic uplift ) and/or extensional tectonics and are subsequently exposed by erosion. Often exhumation involves a complex interaction between crustal thickening, extensional tectonics and erosion. Notably, there are overlapping characteristics between the different modes of burial and exhumation and distinction and between them relies on

1593-573: The East Scotia Ridge, the North Scotia Ridge, the South Scotia Ridge, and the Shackleton Fracture Zone . The Scotia plate is made of oceanic crust and continental fragments now distributed around the Scotia Sea . Before the formation of the plate began 40  million years ago (40Ma), these fragments formed a continuous landmass from Patagonia to the Antarctic Peninsula along an active subduction margin. At present,

1652-551: The Mediterranean basin. This Italianate patois was used to designate the names of the principal winds on the compass rose found in mariners' compasses and portolan charts of the 14th and 15th centuries. The traditional names of the eight principal winds are: Local spelling variations are far more numerous than listed, e.g. Tramutana, Gregale, Grecho, Sirocco, Xaloc, Lebeg, Libezo, Leveche, Mezzodi, Migjorn, Magistro, Mestre, etc. Traditional compass roses will typically have

1711-519: The Scotia plate can be dated to 26–5.5 Ma suggesting spreading was active before spreading completely translated to the East Scotia Ridge at about 6 Ma. It was assumed that spreading cessation occurred where the W7 segment joins the North Scotia Ridge. However basalt lava sampled from the eastern side of the W7 segment were found to be between 93 and 137 million years old and the geochemistry of arc-like basalts suggests no sea floor spreading actually occurred in

1770-647: The Scotia plate is a spreading ridge bounded by the South Sandwich microplate , forming the East Scotia Ridge. The East Scotia Ridge is a back-arc spreading ridge that formed due to subduction of the South American plate below the South Sandwich plate along the South Sandwich Island arc. Exact spreading rates are still being disputed in the literature, but it has been agreed that rates range between 60 and 90 mm/yr. The banks of northern Central Scotia Sea are superposed on oceanic basement and

1829-511: The South Shetland Islands of Antarctica was a small opening with limited circulation. A change in relative motion between the South American plate and the Antarctic plate would have severe effects, causing seafloor spreading and the formation of the Scotia plate. Marine geophysical data indicates that motion between the South American plate and the Antarctic plate shifted from N–S to WNW–ESE accompanied by an eightfold increase in

Scotia plate - Misplaced Pages Continue

1888-499: The United States Navy to box from north and south toward east and west, with the exception that divisions adjacent to a cardinal or inter-cardinal point are always referred to that point." The Royal Navy used the additional "rule that quarter points were never read from a point beginning and ending with the same letter." Compass roses very rarely named the fractional points and only showed small, unlabelled markers as

1947-579: The W7 segment itself. This has resulted in the suggestion that the W7 segment is a downfaulted block of the North Scotia Ridge of the Fuegian Andes continental margin arc, or is potentially related to the putative Cretaceous Central Scotia Sea. The timing of the formation of the Scotia plate and opening of the Drake Passage have long been the subject of much debate due to the important implications for changes in ocean currents and shifts in paleoclimate . The thermal isolation of Antarctica , engendering

2006-727: The compass point; and maximum is the upper (clockwise) angular limit of the compass point. Navigation texts dating from the Yuan , Ming , and Qing dynasties in China use a 24-pointed compass with named directions. These are based on the twelve Earthly Branches , which also form the basis of the Chinese zodiac. When a single direction is specified, it may be prefaced by the character 單 (meaning single) or 丹 . Headings mid-way in-between are compounds as in English. For instance, 癸子 refers to

2065-491: The compass with additional azimuths. Within European tradition, a fully defined compass has 32 "points" (and any finer subdivisions are described in fractions of points). Compass points or compass directions are valuable in that they allow a user to refer to a specific azimuth in a colloquial fashion, without having to compute or remember degrees. The names of the compass point directions follow these rules: In summary,

2124-403: The direction halfway between point 子 and point 癸 , or 7 + 1 ⁄ 2 °. This technique is referred to as a double-needle ( 雙針 ) compass. Exhumation (geology) In geology , exhumation is the process by which a parcel of buried rock approaches Earth's surface. It differs from the related ideas of rock uplift and surface uplift in that it is explicitly measured relative to

2183-738: The eastern trench migration, but both plates have moved very slowly since. 57°S 46°W  /  57°S 46°W  / -57; -46 Points of the compass#16-wind compass rose The points of the compass are a set of horizontal, radially arrayed compass directions (or azimuths ) used in navigation and cartography . A compass rose is primarily composed of four cardinal directions — north , east , south , and west —each separated by 90 degrees , and secondarily divided by four ordinal (intercardinal) directions—northeast, southeast, southwest, and northwest—each located halfway between two cardinal directions. Some disciplines such as meteorology and navigation further divide

2242-745: The formation of the Antarctic ice sheet , has largely been attributed to the opening of the Drake Passage. The Scotia plate originated about 80  million years ago (Ma), during the late Mesozoic at the Panthalassic margins of the Gondwana supercontinent between two Precambrian cratons, the Kalahari and East Antarctic Cratons , now located in Africa and Antarctica. Its development

2301-524: The half- and quarter-point system increased the number of directions to include in the 'boxing'. Points remained the standard unit until switching to the three-figure degree method. These points were also used for relative measurement, so that an obstacle might be noted as 'two points off the starboard bow', meaning two points clockwise of straight ahead, 22 + 1 ⁄ 2 ° This relative measurement may still be used in shorthand on modern ships, especially for handoffs between outgoing and incoming helmsmen, as

2360-493: The initial opening of a deep Drake Passage. The ongoing lengthening of the North Scotia Ridge beyond the Burdwood Bank caused South Georgia to move further east. The banks of the North Scotia Ridge contain volcanic rocks similar to those found in Tierra del Fuego, including on Isla de los Estados on the easternmost tip. During the early Eocene (50 Ma), the Drake Passage between the southern tip of South America at Cape Horn and

2419-399: The initials T, G, L, S, O, L, P, and M on the main points. Portolan charts also colour-coded the compass winds: black for the eight principal winds, green for the eight half-winds, and red for the sixteen quarter-winds. Each half-wind name is simply a combination of the two principal winds that it bisects, with the shortest name usually placed first, for example: NNE is "Greco-Tramontana"; ENE

Scotia plate - Misplaced Pages Continue

2478-584: The initiation of the Scotia Arc. This inversion had a strike-slip component which can be seen in the Cooper Bay dislocation on South Georgia. This geological regime lead to the uplift and elongation of the Andes and the embryonic North Scotia Ridge, which resulted in the initial eastward relocation of the South Georgia microcontinent and formation of the Central Scotia Sea. Between the Late Cretaceous and

2537-505: The islands indicating the possible shift of the transform fault to an area south of the island. It has also been suggested that the plate bearing the islands may have broken off from the Scotia plate, forming a new independent South Georgia microplate, yet there is little evidence to make this conclusion. The South Georgia microcontinent was originally connected to the Roca Verdes back-arc basin (southernmost Tierra del Fuego ) until

2596-408: The loss of granularity is less significant than the brevity and simplicity of the summary. The table below shows how each of the 128 directions are named. The first two columns give the number of points and degrees clockwise from north. The third gives the equivalent bearing to the nearest degree from north or south towards east or west. The "CW" column gives the fractional-point bearings increasing in

2655-404: The name of one of the 32 points. Each of the 96 fractional points can be named in two ways, depending on which of the two adjoining whole points is used, for example, N ⁠ 3 / 4 ⁠ E is equivalent to NbE ⁠ 1 / 4 ⁠ N. Either form is easily understood, but alternative conventions as to correct usage developed in different countries and organisations. "It is the custom in

2714-459: The orogen that was built up during collision. During collapse, high-grade rocks from the core of the orogen are exhumed through upward flow towards now thinned crustal areas forming domal shaped metamorphic core complexes . Alternatively, or in conjunction with the extension of the center of the orogen, propagation of the rock-mass towards the margin may lead to exhumation along a series of brittle or ductile thrusts and normal faults and ultimately

2773-582: The plate is almost completely submerged, with only the small exceptions of South Georgia on its northeastern edge and the southern tip of South America. Together with the South Sandwich Plate , the Scotia plate joins the southernmost Andes to the Antarctic Peninsula, just like the Caribbean plate joins the northernmost Andes to North America, and these two plates are comparable in several ways. Both have volcanic arcs at their eastern ends,

2832-525: The ridge is the 3 km (1.9 mi) deep Falkland Trough . Experts in plate tectonics have been unable to determine whether the South Georgian Islands are part of the Scotia plate or have been recently accreted to the South American plate. Surface expressions of the plate boundary are found north of the islands suggesting a long-term presence of the transform fault there. Yet seismic studies have identified strain and thrusting south of

2891-575: The ridge, the subduction rate decreases until its remaining oblique motion evolves into the Shackleton Fracture Zone transform boundary. The south-western edge of the plate is bounded by the Shetland microplate separating the Shackleton Fracture Zone and the South Scotia Ridge. North of South Shetland Islands and along the southern half of the Shackleton Fracture Zone is the remnant of the Phoenix plate (also known as Drake or Aluk Plate). Around 47 Ma

2950-402: The roof and the base of the channel respectively, high-temperature retrograde metamorphic assemblages, cooing ages should be younger to the front of the channel and P-T-t paths suggesting prolonged burial and synchronous exhumation throughout the channel. Post-convergent gravitational collapse (extension) occurs once the convergence forces can no longer support the gravitational force of

3009-469: The separation rate. This shift in spreading initiated crustal thinning and by 30–34 Ma, the West Scotia Ridge formed. A complex pattern of spreading prior to 26 Ma is probably present in the oldest parts of this spreading regime; i.e. west of Terror Rise (north of Elephant Island ) and on the shelf slope of Tierra del Fuego. Until 17 Ma the Central Scotia plate moved quickly eastwards, fuelled by

SECTION 50

#1732771823900

3068-552: The southern margin of the Falkland Plateau and into the Rocas Verdes back-arc basin which extends from South Georgia and along the Patagonian Andes. Fragments of the inverted oceanic basement of this basin are preserved as ophiolitic complexes in this area, including the Larsen Harbour complex on South Georgia. This makes it possible to restore the original position of the South Georgia microcontinent south of

3127-462: The spreading centre of the West Scotia Sea. Analyses of samples of volcaniclastic rocks from these sites indicate they are constructs of a continental arc and in some cases oceanic arc similar to those being formed in the currently active South Sandwich Arc. The oldest volcanic arc activity in the central and eastern regions of the Scotia Sea are 28.5 Ma. The South Sandwich forarc originated in

3186-506: The subduction of the Phoenix plate started as the propagation of the Pacific-Antarctic Ridge continued. The last collision between Phoenix ridge segments and the subduction zone was 6.5 Ma and at 3.3 Ma movements had stopped and the remnants of the Phoenix plate was incorporated into the Antarctic plate. The southern part of the Shackleton Fracture Zone is the former eastern edge of the Phoenix plate. The eastern edge of

3245-459: The subduction to the collisional phases of the orogenic cycle, a tectonic wedge forms on the prowedge (side of the subducting plate) and commonly the retrowedge (continental side) of the orogen. During the continued convergence, the wedge maintains its shape by maintaining its critical angle of taper by the interaction of thickening through basal accretion or foreland propagation (frontal accretion) and thinning through normal faulting and erosion at

3304-478: The surface by the tectonic process of obduction and then exposed. Exhumation of deep crustal rocks during an orogenic cycle occurs mainly during continental collision or during post-collision extension and is thus, is broadly grouped into the three mechanisms which are used to describe the burial and exhumation of the cycle namely, syn-convergent orogenic wedges, channel flow (also known as ductile extrusion) and post-convergence gravitational collapse. During

3363-405: The surface of the Earth, rather than with reference to some absolute reference frame, such as the Earth's geoid . Exhumation of buried rocks should be considered as two different categories namely, exhumation by denudation / erosion or exhumation by tectonic processes followed by erosion . In the latter case, rocks (or rock packages) from deeper crustal levels (meter to kilometer depths below

3422-441: The upper part of the wedge. Erosion of the wedge significantly impacts the dynamics within the wedge, which promotes exhumation by thrusting mid-crustal rocks to the hanging wall of the wedge. Characteristics of this mode of exhumation include, evidence for strong non coaxial reverse-shearing, pro-grade metamorphism, cooling ages are progressively younger towards deeper structural levels and that exhumation at higher structural levels

3481-722: Was also influenced by the Río de la Plata Craton in South America. The initial cause for its formation was the break-up of Gondwana in what became the south-west Indian Ocean. The earliest marine fossils found on the Maurice Ewing Bank , on the eastern end of the Falkland Plateau , are associated with the Indian and Tethys Oceans and probably slightly more than 150 Ma. The Weddell Sea opened and spread along

#899100