Bransfield Basin | |
---|---|
Type | Back-arc basin |
Location | |
Region | Northwest of the Antarctic Peninsula |
Type section | |
Country | Antarctica |
The Bransfield Basin is a back-arc rift basin located off the northern tip of the Antarctic Peninsula. The basin lies within a Northeast and Southwest trending strait that separates the peninsula from the nearby South Shetland Islands to the Northwest. [1] The basin extends for more than 500 kilometres (310 miles) from Smith Island (South Shetland Islands) to a portion of the Hero Fracture Zone. [2] The basin can be subdivided into three basins: Western, Central, and Eastern. [3] The Western basin is 130 kilometres (81 miles) long by 70 kilometres (43 miles) wide with a depth of 1.3 kilometres (1,400 yards), the Central basin is 230 kilometres (140 miles) long by 60 kilometres (37 miles) wide with a depth of 1.9 kilometres (2,100 yards), and the Eastern basin is 150 kilometres (93 miles) long by 40 kilometres (25 miles) wide with a depth of over 2.7 kilometres (3,000 yards). [3] The three basins are separated by the Deception Island and Bridgeman Island. [1] The moho depth in the region has been seismically interpreted to be roughly 34 kilometres (21 miles) deep. [4]
The Bransfield Basin is considered to be a back-arc basin that is located behind the South Shetland Islands. The Islands are believed to have formed from a period of subduction that occurred between the Phoenix Plate and the Antarctic Plate starting roughly 200 million years ago during the Mesozoic. [5] [6] [7] It is believed that the Phoenix plate stopped subducting under the Antarctic plate at least 4 million years ago during the Pliocene. [3] [5] [6] [7] Once the subduction ceased, it is believed that the extension that created the basin was initiated. Aeromagnetic surveys have provided evidence that the extension occurred 1.8 million years ago during the Pleistocene at a rate of 0.25 to 0.75 centimetres (1⁄10 to 3⁄10 inch) per year. [7]
It is widely accepted that the Bransfield Basin formed from extension caused by slab rollback. [3] [5] [6] [7] New geophysical and structural data contradicts previously believed theories about slab rollback being the main mechanism for the opening of the basin. [1] [2] A newer theory for the opening of the basin is attributed to sinistral strike-slip motion between the Scotia plate and Antarctic plates. [2] [8] It is theorized that the trench between the Phoenix and Antarctic plates is locked in place and there is not any motion within the trench. The new data suggests trench retreat is not attributed as a mechanism for extension because there is a lack of seismic activity in the South Shetland Trench area, and that slab rollback is not a mechanism for extension either because if it were then Northwest-Southeast extension should be observed in the entire South Shetland region but instead compression can be observed. It is proposed that the motion between the Scotia plate and Antarctic plate are pushing the Phoenix plate to the Northwest creating compression. [2] [8]
There are 10 identified volcanoes along the 300 km long ridge from Bridgeman Island to Deception Island. Deception (30 km diameter base), Penguin (8 km diameter base), and Bridgeman (25 km diameter base) islands are the tops of Pleistocene-Recent stratovolcanoes, while 7 additional submarine volcanoes exist as seamounts, with the Orca Seamount being the largest (20 km diameter base). [9]
The main factor that controls deposition inside the Bransfield Basin is glacial cyclicity. Additional contributing factors include physiography, tectonics, and oceanography. [1] [10] Three stratigraphic units have been identified on the margins. The oldest unit is an over- consolidated diamicton from subglacial processes. The middle unit is a pebbly-sandy stratified mud from the proximal-ice or sub-ice shelf. The youngest unit consists of diatomaceous mud originating from open marine conditions. Sedimentary systems occur on the margins that are related to glacial and glacial marine, mass wasting, seabed fluid-escape, and contour current processes. [10]
Glacial processes have deposited a subglacial deformation till. The sediment that makes up this unit is derived from pressure melting of the glacier and from the substrate the glacier passed over. The subglacial deformation till unit is composed of a matrix-supported diamicton. [10]
Glacial marine processes have deposited two different units within the region. One of the units comprises proglacial debris flows have deposited a matrix-supported diamicton with interbeds of laminated mud on the lower portion of the continental slope. The other depositional process is a mixture of rain out from the ice from either melting or instantaneous dumping from the surface of an overturned portion of ice, and from marine rain out. The terrigenous and biogenic material compounds together to form sandy muds with sparse clasts. [10]
Open marine processes have deposited three units within the region. One of the units is a fining-upwards turbidity current deposit can be observed within the lower slope of the basin. Layers of volcanic ash around 1 to 4 centimetres (1⁄3 to 1+2⁄3 inches) thick are within the deposit. Another unit is a contorted/disturbed mud that makes up a slide unit. This unit is distinct because its angular contacts and disturbed structures that form from sediment reworking and plastic deformation from sliding. The third unit is a stratified mud with clast layers at the lower slope's foot. This unit is deposited from contour currents, and differences in clast size is attributed to shifting current conditions. [10]
The subduction event between the Phoenix plate and the Antarctic plate have built a volcanic arc consisting of low potassium to medium potassium content along the Antarctic Peninsula and South Shetland Islands. Volcanism occurred in multiple events during 130–110, 90–70, 60–40, and 30–20 million years ago. The paucity can be interpreted as subducting younger crust or subsidence the post 20 million years arc after the basin formed. [8] Volcanism is widespread within the Quaternary which created a series of submarine volcanoes. The submarine volcanoes produce glassy lavas ranging in compositions similar to what would be expected in arcs higher in large-ion lithophile elements to enriched mid-ocean ridge basalts. [8]
The Bransfield Basin is abnormal when it comes to the style of volcanism that can be observed within the basin. Undersea volcanoes experience what is called bimodal volcanism. [11] Igneous rocks within the basin are andesite and basalt. The closer to the center of the undersea volcanoes the composition of the rocks shifts towards more felsic rock types such as rhyolite, rhyodacite, and dacite. [11] The source of this phenomenon is interpreted as a result from indicate formation from partial melting or fractional crystallization. This type of volcanism is commonly observed in Phanerozoic volcanic massive sulfide systems, and is not commonly observed in modern back-arc basins. Examples of where bimodal volcanism can be observed are the Okinawa Trough and the Sumizu Rift. [11]
The occurrence of incipient seafloor spreading in the basin is under controversy. Some researchers suggest that it does not occur within the basin because of the crustal thickness, magnetic anomaly patterns, and intracrustal diapirism. [2] Other geoscientists suggest that it is occurring and is related to seamount volcanism and normal faulting within the basin. [1] [2]
In August 2020, the largest seismic swarm recorded in the history of the region began to occur. Between 36,000 [12] and 85,000 [13] earthquakes were detected in just a few months, with earthquakes up to magnitude 6.0. The swarm was located off King George Island, just a few kilometers from the Orca Seamount, which was thought to be extinct. Some studies indicate that the earthquakes were produced by magmatic intrusion, although there is no precise evidence that the volcano has erupted due to low instrumentation in the area. [13]
The geography of Antarctica is dominated by its south polar location and, thus, by ice. The Antarctic continent, located in the Earth's southern hemisphere, is centered asymmetrically around the South Pole and largely south of the Antarctic Circle. It is washed by the Southern Ocean or, depending on definition, the southern Pacific, Atlantic, and Indian Oceans. It has an area of more than 14.2 million km2. Antarctica is the largest ice desert in the world.
Deception Island is in the South Shetland Islands close to the Antarctic Peninsula with a large and usually "safe" natural harbor, which is occasionally affected by the underlying active volcano. This island is the caldera of an active volcano, which seriously damaged local scientific stations in 1967 and 1969. The island previously held a whaling station. It is now a tourist destination with over 15,000 visitors per year. Two research stations are operated by Argentina and Spain during the summer season. While various countries have asserted sovereignty, it is still administered under the Antarctic Treaty System.
The Ring of Fire is a tectonic belt of volcanoes and earthquakes.
The Antarctic Peninsula, known as O'Higgins Land in Chile and Tierra de San Martín in Argentina, and originally as Graham Land in the United Kingdom and the Palmer Peninsula in the United States, is the northernmost part of mainland Antarctica.
Bridgeman Island is one of the South Shetland Islands in Antarctica. It is an almost circular, volcanic island marked by steep sides, measuring 900 by 600 metres with a maximum elevation of 240 m (787 ft) high, lying 45 kilometres (28 mi) east of King George Island.
Penguin Island is one of the smaller of the South Shetland Islands of Antarctica.
Bransfield Strait or Fleet Sea is a body of water about 100 kilometres (60 mi) wide extending for 300 miles (500 km) in a general northeast – southwest direction between the South Shetland Islands and the Antarctic Peninsula.
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.
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The following outline is provided as an overview of and introduction to Oceanography.
The Mariana Plate is a micro tectonic plate located west of the Mariana Trench which forms the basement of the Mariana Islands which form part of the Izu–Bonin–Mariana Arc. It is separated from the Philippine Sea Plate to the west by a divergent boundary with numerous transform fault offsets. The boundary between the Mariana and the Pacific Plate to the east is a subduction zone with the Pacific Plate subducting beneath the Mariana. This eastern subduction is divided into the Mariana Trench, which forms the southeastern boundary, and the Izu–Ogasawara Trench the northeastern boundary. The subduction plate motion is responsible for the shape of the Mariana plate and back arc.
The Shetland Plate, or South Shetland Plate, is a tectonic microplate located off the tip of the Antarctic Peninsula that contains the South Shetland Islands. The plate is bordered on three sides by the Antarctic Plate, while the fourth side is bordered by the Scotia Plate. The northwestern border is defined by the South Shetland Trench, separating the Shetland Plate to the south from the Antarctic Plate to the north. This trench is the remnant of a subduction zone where the defunct Phoenix Plate, now part of the Antarctic Plate, subducted under the Antarctic Peninsula and the Shetland Islands. The southeastern border is a rift zone, with the Antarctic Plate creating the Bransfield Basin. The southwestern and northeastern boundaries are each part of larger fracture zones. The southwestern border is the Hero Fracture Zone and separates the Antarctic Plate to the southwest from the Shetland Plate to the northeast. The northeastern boundary is the Shackleton Fracture Zone and separates the Shetland Plate to the southwest from the Scotia Plate.
Orca Seamount is a seamount near King George Island in Antarctica, in the Bransfield Strait. While it is inactive, last volcanic activity at Orca Seamount is judged to have occurred in the recent past as there are temperature anomalies in the seawater around the seamount. Thermophilic and hyperthermophilic microorganisms have been found at the seamount.
Melville Peak is a prominent peak surmounting Cape Melville, the eastern cape of King George Island, in the South Shetland Islands off Antarctica. It represents an eroded stratovolcano of unknown age and contains a volcanic crater at its summit. A volcanic ash layer similar in composition to Melville Peak has been identified 30 km (19 mi) away from the volcano and may indicate Melville Peak has been volcanically active in the last few thousand years.
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Monowai Seamount is a volcanic seamount to the north of New Zealand. It is formed by a large caldera and a volcanic cone just south-southeast from the caldera. The volcanic cone rises to depths of up to 100 metres (330 ft) but its depth varies with ongoing volcanic activity, including sector collapses and the growth of lava domes. The seamount and its volcanism were discovered after 1877, but only in 1980 was it named "Monowai" after a research ship of the same name.
Kemp Caldera and Kemp Seamount form a submarine volcano south of the South Sandwich Islands, in a region where several seamounts are located. The seamount rises to a depth of 80 metres (260 ft) below sea level; the caldera has a diameter of 8.3 by 6.5 kilometres and reaches a depth of 1,600 metres (5,200 ft). The caldera contains several Hydrothermal vents, including white smokers and diffuse venting areas, which are host to chemolithotrophic ecological communities. The seamount and caldera, which were discovered by seafloor mapping in 2009, are part of the South Georgia and the South Sandwich Islands Marine Protected Area.