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46-677: 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. The basin extends for more than 500 kilometres (310 miles) from Smith Island (South Shetland Islands) to a portion of the Hero Fracture Zone . The basin can be subdivided into three basins: Western, Central, and Eastern. The Western basin

92-580: A compressional stress on the back-arc region behind the Andes. On the other extreme, the slab going down into the mantle at the Marianas subduction zone is so steep it is nearly vertical. This is the perfect example of an oceanic back-arc basin experiencing extensional forces. The Oriente in Ecuador (the eastern part of the country covered by rainforest) is also a good example of an extensional back-arc basin, this time in

138-423: A continental setting. The continental crust in this area east of the Andes has been stretched out and covered by layers of sediments. Till Till or glacial till is unsorted glacial sediment . Till is derived from the erosion and entrainment of material by the moving ice of a glacier . It is deposited some distance down-ice to form terminal , lateral , medial and ground moraines . Till

184-515: A cover of till. Interpreting the glacial history of landforms can be difficult due to the tendency of overprinting landforms on top of each other. As a glacier melts, large amounts of till are eroded and become a source of sediments for reworked glacial drift deposits. These include glaciofluvial deposits , such as outwash in sandurs , and as glaciolacustrine and glaciomarine deposits, such as varves (annual layers) in any proglacial lakes which may form. Erosion of till may take place even in

230-466: A higher water content behave more fluidly, and thus are more susceptible to flow. There are three main types of flows, which are listed below. In cases where till has been indurated or lithified by subsequent burial into solid rock, it is known as the sedimentary rock tillite . Matching beds of ancient tillites on opposite sides of the south Atlantic Ocean provided early evidence for continental drift . The same tillites also provide some support to

276-415: A spreading center where new oceanic crust is formed. The composition of this new crust is similar to mid-ocean ridge basalt (MORB), although it contains higher amounts of water. The back-arc deformation may be either extensional or compressional. The overriding plate will shorten when its motion is directed towards the trench, resulting in a compression of the back-arc region. This type of deformation

322-402: A subduction zone. The stresses responsible for the deformation in this region of a subduction zone result from a combination of processes. The absolute motion of the upper plate as it moves towards or away from the trench strongly contributes to deformation in the back-arc region. Since the downgoing slab is partly anchored in the viscous layers of the mantle, and therefore its lateral movement

368-462: 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. 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

414-518: 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). The three basins are separated by

460-400: Is a sedimentary rock formed by lithification of till. Glacial till is mostly derived from subglacial erosion and from the entrainment by the moving ice of previously available unconsolidated sediments. Bedrock can be eroded through the action of glacial plucking and abrasion , and the resulting clasts of various sizes will be incorporated to the glacier's bed. Glacial abrasion is

506-740: 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. 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

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552-539: Is associated with a shallow dipping subducted slab. Inversely, an overriding plate moving away from the trench will result in extension, and a back-arc basin will form. This extensional deformation is associated with a steeply dipping slab. The extreme cases of these two types of back-arc deformation can be found in Chile and at the Marianas arc . The shallow dipping slab subducting beneath Chile at an angle of about 10–15° causes

598-441: Is characteristically unsorted and unstratified , and is not usually consolidated . Most till consists predominantly of clay, silt , and sand , but with pebbles, cobbles, and boulders scattered through the till. The abundance of clay demonstrates lack of reworking by turbulent flow, which otherwise would winnow the clay. Typically, the distribution of particle sizes shows two peaks (it is bimodal ) with pebbles predominating in

644-455: Is classified into primary deposits, laid down directly by glaciers, and secondary deposits, reworked by fluvial transport and other processes. Till is a form of glacial drift , which is rock material transported by a glacier and deposited directly from the ice or from running water emerging from the ice. It is distinguished from other forms of drift in that it is deposited directly by glaciers without being reworked by meltwater. Till

690-667: 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 . 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 . Other geoscientists suggest that it

736-460: Is occurring and is related to seamount volcanism and normal faulting within the basin. In August 2020, the largest seismic swarm recorded in the history of the region began to occur. Between 36,000 and 85,000 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

782-413: Is produced by glacial grinding, and the longer the till remains at the ice-bedrock interface, the more thoroughly it is crushed. However, the crushing process appears to stop with fine silt. Clay in till is likely eroded from bedrock rather than being created by glacial processes. The sediments carried by a glacier will eventually be deposited some distance down-ice from its source. This takes place in

828-419: Is significantly slower than the surface plate, then any motion of the overriding plate will cause extensional or compressional stress in the back-arc region depending on the direction of motion. In addition, mantle convection in the upper mantle wedge caused by the downward movement of the subducted slab causes stress in the upper plate and the high heat flow that characterizes back-arcs. The pulling effect of

874-514: Is widely accepted that the Bransfield Basin formed from extension caused by slab rollback . New geophysical and structural data contradicts previously believed theories about slab rollback being the main mechanism for the opening of the basin. A newer theory for the opening of the basin is attributed to sinistral strike-slip motion between the Scotia plate and Antarctic plates. It is theorized that

920-460: The ablation zone , which is the part of the glacier where the rate of ablation (removal of ice by evaporation, melting, or other processes) exceeds the rate of accumulation of new ice from snowfall. As ice is removed, debris are left behind as till. The deposition of glacial till is not uniform, and a single till plain can contain a wide variety of different types of tills due to the various erosional mechanisms and location of till with respect to

966-689: The Deception Island and Bridgeman Island . The moho depth in the region has been seismically interpreted to be roughly 34 kilometres (21 miles) deep. 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

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1012-625: The Mesozoic . It is believed that the Phoenix plate stopped subducting under the Antarctic plate at least 4 million years ago during the Pliocene . 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. It

1058-548: The Precambrian Snowball Earth glaciation event hypothesis. Tills sometimes contain placer deposits of valuable minerals such as gold. Diamonds have been found in glacial till in the north-central United States and in Canada. Till prospecting is a method of prospecting in which tills are sampled over a wide area to determine if they contain valuable minerals, such as gold, uranium, silver, nickel, or diamonds, and

1104-466: The basin. Undersea volcanoes experience what is called bimodal volcanism . 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 . The source of this phenomenon is interpreted as a result from indicate formation from partial melting or fractional crystallization . This type of volcanism

1150-582: The careful statistic work by geologist Chauncey D. Holmes in 1941 that elongated clasts in tills tend to align with the direction of ice flow. Clasts in till may also show slight imbrication , with the clasts dipping upstream. Though till is generally unstratified, till high in clay may show lamination due to compaction under the weight of overlying ice. Till may also contain lenses of sand or gravel , indicating minor and local reworking by water transitional to non-till glacial drift. The term till comes from an old Scottish name for coarse, rocky soil. It

1196-467: The coarser peak. The larger clasts (rock fragments) in till typically show a diverse composition, often including rock types from outcrops hundreds of kilometers away. Some clasts may be rounded, and these are thought to be stream pebbles entrained by the glacier. Many of the clasts are faceted, striated, or polished, all signs of glacial abrasion . The sand and silt grains are typically angular to subangular rather than rounded. It has been known since

1242-413: The difficulties in accurately classifying different tills, which are often based on inferences of the physical setting of the till rather than detailed analysis of the till fabric or particle size. Subglacial lodgement tills are deposits beneath the glacier that are forced, or "lodged" into the bed below. As glaciers advance or retreat, the clasts that are deposited by the ice may have a lower velocity than

1288-490: 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. 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

1334-411: The glacier. Since the rate of deposition is controlled by the rate of basal melting, it is worth considering the factors that contribute to melting. These can be the geothermal heat flux, frictional heat generated by sliding, ice thickness, and ice-surface temperature gradients. Subglacial deformation tills refer to the homogenization of glacial sediments that occur when the stresses and shear forces from

1380-401: The glacier. These consist of clasts and debris that become exposed due to melting via solar radiation. These debris are either just debris that have a high relative position on the glacier, or clasts that have been transported up from the base of the glacier. Debris accumulation has a feedback-loop relationship with melting. Initially, the darker colored debris absorb more heat and thus accelerate

1426-403: The ice itself. When the friction between the clast and the bed exceeds the forces of the ice flowing above and around it, the clast will cease to move, and it will become a lodgement till. Subglacial meltout tills are tills that are deposited via the melting of the ice lobe. Clasts are transported to the base of the glacier over time, and as basal melting continues, they are slowly deposited below

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1472-412: 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. Open marine processes have deposited three units within the region. One of

1518-459: 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. Glacial processes have deposited

1564-650: The melting process. After a significant amount of melting has occurred, the thickness of the till insulates the ice sheet and slows the melting process. Supraglacial meltout tills typically end up forming moraines. Supraglacial flow tills refer to tills that are subject to a dense concentration of clasts and debris from meltout. These debris localities are then subsequently affected by ablation . Due to their unstable nature, they are subject to downslope flow, and thus named "flow till." Properties of flow tills vary, and can depend on factors such as water content, surface gradient, and debris characteristics. Generally, flow tills with

1610-470: The moving glacier rework the topography of the bed. These contain preglacial sediments (non glacial or earlier glacial sediments), which have been run over and thus deformed by meltout processes or lodgement. The constant reworking of these deposited tills leads to a highly homogenized till. Supraglacial meltout tills are similar to subglacial meltout tills. Rather than being the product of basal melting, however, supraglacial meltout tills are imposed on top of

1656-495: The post 20 million years arc after the basin formed. 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 . The Bransfield Basin is abnormal when it comes to the style of volcanism that can be observed within

1702-419: The slab as it goes down into the mantle causes a rollback motion of the trench, which also applies stress on the back-arc region of the upper plate. However, this last process has less of an impact on deformation compared to upper plate motion. Back-arcs can form on either oceanic crust or continental crust. In the case of oceanic crust, most back-arc regions are subjected to tensional stresses and thus develop

1748-464: The subglacial environment, such as in tunnel valleys . There are various types of classifying tills: Traditionally (e.g. Dreimanis , 1988 ) a further set of divisions has been made to primary deposits, based upon the method of deposition. Van der Meer et al. 2003 have suggested that these till classifications are outdated and should instead be replaced with only one classification, that of deformation till. The reasons behind this are largely down to

1794-413: The top of the stratigraphic sediment sequence, which has a major influence on land usage. Till is deposited as the terminal moraine , along the lateral and medial moraines and in the ground moraine of a glacier, and moraine is often conflated with till in older writings. Till may also be deposited as drumlins and flutes , though some drumlins consist of a core of stratified sediments with only

1840-415: 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). The main factor that controls deposition inside the Bransfield Basin is glacial cyclicity . Additional contributing factors include physiography , tectonics , and oceanography . Three stratigraphic units have been identified on

1886-454: The transporting glacier. The different types of till can be categorized between subglacial (beneath) and supraglacial (surface) deposits. Subglacial deposits include lodgement, subglacial meltout, and deformation tills. Supraglacial deposits include supraglacial meltout and flow till. Supraglacial deposits and landforms are widespread in areas of glacial downwasting (vertical thinning of glaciers, as opposed to ice-retreat. They typically sit at

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1932-483: 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

1978-474: 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

2024-404: The weathering of bedrock below a flowing glacier by fragmented rock on the basal layer of the glacier. The two mechanisms of glacial abrasion are striation of the bedrock by coarse grains moved by the glacier, thus gouging the rock below, and polishing of the bedrock by smaller grains such as silts. Glacial plucking is the removal of large blocks from the bed of a glacier. Much of the silt in till

2070-401: Was first used to describe primary glacial deposits by Archibald Geikie in 1863. Early researchers tended to prefer the term boulder clay for the same kind of sediments, but this has fallen into disfavor. Where it is unclear whether a poorly sorted, unconsolidated glacial deposit was deposited directly from glaciers, it is described as diamict or (when lithified ) as diamictite . Tillite

2116-407: 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. Back-arc Back-arc deformation is a product of subduction at convergent plate tectonic boundaries. It initiates and evolves behind the volcanic arc on the overriding plate of

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