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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.<ref name=Garcia>{{cite journal|last1=García|first1=Marga|last2=Ercilla|first2=Gemma|last3=Alonso|first3=Belen|title=Morphology and sedimentary systems in the Central Bransfield Basin, Antarctic Peninsula: sedimentary dynamics fromshelf to basin|journal=Basin Research|date=2009|volume=21|pages=295–314}}</ref>The basin extends for more than 500 kilometers from [[Smith Island]] to a portion of the [[Hero Fracture Zone]].<ref name=Gonzalez-Casado>{{cite journal|last1=González-Casado|first1=José|last2=Jorge|first2=Giner-Robles|last3=Jerónimo|first3=López-Martínez|title=Bransfield Basin, Antarctic Peninsula: Not a normal backarc basin|journal=Geology|date=November 2000|volume=28|issue=11|pages=1043–1046|bibcode=2000Geo....28.1043G|doi=10.1130/0091-7613(2000)28<1043:BBAPNA>2.0.CO;2}}</ref> The basin can be subdivided into three basins: Western, Central, and Eastern.<ref name=Schreider>{{cite journal|last1=Schreider|first1=Al.|last2=Schreider|first2=A.|last3=Evsenko|first3=E.|title=The Stages of the Development of the Basin of the Bransfield Strait|journal=Oceanology|date=2014|volume=54|issue=3|pages=365–373|bibcode=2014Ocgy...54..365S|doi=10.1134/S0001437014020234}}</ref> The Western basin is 130 kilometers long by 70 kilometers wide with a depth of 1.3 kilometers, the Central basin is 230 kilometers long by 60 kilometers wide with a depth of 1.9 kilometers, and the Eastern basin is 150 kilometers long by 40 kilometers wide with a depth of over 2.7 kilometers.<ref name=Schreider />The three basins are separated by the [[Deception Island]] and [[Bridgeman Island]].<ref name=Garcia /> The [[Mohorovičić discontinuity|moho]] depth in the region has been [[Seismology|seismically]] interpreted to be roughly 34 kilometers deep.<ref name=Baranov>{{cite journal|last1=Baranov|first1=A.|title=Moho Depth in Antarctica from Seismic Data|journal=Physics of the Solid Earth|date=2011|volume=47|issue=12|pages=1–13|bibcode=2011IzPSE..47.1058B|doi=10.1134/S1069351311120019}}</ref>
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.<ref name=Garcia>{{cite journal|last1=García|first1=Marga|last2=Ercilla|first2=Gemma|last3=Alonso|first3=Belen|title=Morphology and sedimentary systems in the Central Bransfield Basin, Antarctic Peninsula: sedimentary dynamics fromshelf to basin|journal=Basin Research|date=2009|volume=21|pages=295–314}}</ref>The basin extends for more than 500 kilometers from [[Smith Island]] to a portion of the [[Hero Fracture Zone]].<ref name=Gonzalez-Casado>{{cite journal|last1=González-Casado|first1=José|last2=Jorge|first2=Giner-Robles|last3=Jerónimo|first3=López-Martínez|title=Bransfield Basin, Antarctic Peninsula: Not a normal backarc basin|journal=Geology|date=November 2000|volume=28|issue=11|pages=1043–1046|bibcode=2000Geo....28.1043G|doi=10.1130/0091-7613(2000)28<1043:BBAPNA>2.0.CO;2}}</ref> The basin can be subdivided into three basins: Western, Central, and Eastern.<ref name=Schreider>{{cite journal|last1=Schreider|first1=Al.|last2=Schreider|first2=A.|last3=Evsenko|first3=E.|title=The Stages of the Development of the Basin of the Bransfield Strait|journal=Oceanology|date=2014|volume=54|issue=3|pages=365–373|bibcode=2014Ocgy...54..365S|doi=10.1134/S0001437014020234}}</ref> The Western basin is 130 kilometers long by 70 kilometers wide with a depth of 1.3 kilometers, the Central basin is 230 kilometers long by 60 kilometers wide with a depth of 1.9 kilometers, and the Eastern basin is 150 kilometers long by 40 kilometers wide with a depth of over 2.7 kilometers.<ref name=Schreider />The three basins are separated by the [[Deception Island]] and [[Bridgeman Island]].<ref name=Garcia /> The [[Mohorovičić discontinuity|moho]] depth in the region has been [[Seismology|seismically]] interpreted to be roughly 34 kilometers deep.<ref name=Baranov>{{cite journal|last1=Baranov|first1=A.|title=Moho Depth in Antarctica from Seismic Data|journal=Physics of the Solid Earth|date=2011|volume=47|issue=12|pages=1–13|bibcode=2011IzPSE..47.1058B|doi=10.1134/S1069351311120019}}</ref>


==Tectonic Development==
==Tectonic Development==
[[File:Bransfield Basin cartoon.tif|thumbnail|left|Schematic cartoon of the Bransfield Basin tectonic setting.]]
[[File:Bransfield Basin cartoon.tif|thumbnail|left|Schematic cartoon of the Bransfield Basin tectonic setting.]]
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 [[subduction]] event that occurred between the [[Phoenix Plate]] and the [[Antarctic plate]] roughly 200 million years ago during the [[Mesozoic]].<ref name=Lawver>{{cite book|last1=Lawver|first1=Lawrence|last2=Keller|first2=Randall|last3=Fisk|first3=Martin|last4=Strelin|first4=Jorge|title=Backarc Basins: Tectonics and Magmatism|date=1995|publisher=Plenum Press|location=New York|pages=316–342}}</ref>
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 [[subduction]] event that occurred between the [[Phoenix plate]] and the [[Antarctic plate]] roughly 200 million years ago during the [[Mesozoic]].<ref name=Lawver>{{cite book|last1=Lawver|first1=Lawrence|last2=Keller|first2=Randall|last3=Fisk|first3=Martin|last4=Strelin|first4=Jorge|title=Backarc Basins: Tectonics and Magmatism|date=1995|publisher=Plenum Press|location=New York|pages=316–342}}</ref>
<ref name=Galindo-Zaldivar>{{cite book|last1=Galindo-Zaldivar|first1=Jesus|last2=Gamboa|first2=Luiz|last3=Maldonado|first3=Andres|last4=Nakao|first4=Seizo|last5=Bochu|first5=Yao|title=Antarctica: Contributions to global earth sciences|date=2006|publisher=Spring-verlag|location=New York|pages=243–248}}</ref><ref name="Eulalia Gracia">{{cite journal|last1=Gracia|first1=Eulalia|last2=Canals|first2=Miquel|last3=Farran|first3=Marcel|last4=Prieto|first4=Maria|last5=Sorribas|first5=Jordi|last6=Team|first6=Gebra|title=Morphostructure and Evolutionn of the Central and Eastern Bransfield Basins (NW Antarctic Peninsula|journal=Marine Geophysical Researches|date=1995|volume=18|pages=429–448}}</ref> It is believed that the Pheonix plate stopped subducting under the Antarctic plate at least 4 million years ago during the [[Pliocene]].<ref name=Schreider /><ref name=Lawver /><ref name=Galindo-Zaldivar /><ref name="Eulalia Gracia" />. Once the subduction event ceased, it is believed that the extension that created the basin was initiated. [[Aeromagnetic_survey|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 cenimeters per year. <ref name="Eulalia Gracia" />
<ref name=Galindo-Zaldivar>{{cite book|last1=Galindo-Zaldivar|first1=Jesus|last2=Gamboa|first2=Luiz|last3=Maldonado|first3=Andres|last4=Nakao|first4=Seizo|last5=Bochu|first5=Yao|title=Antarctica: Contributions to global earth sciences|date=2006|publisher=Spring-verlag|location=New York|pages=243–248}}</ref><ref name="Eulalia Gracia">{{cite journal|last1=Gracia|first1=Eulalia|last2=Canals|first2=Miquel|last3=Farran|first3=Marcel|last4=Prieto|first4=Maria|last5=Sorribas|first5=Jordi|last6=Team|first6=Gebra|title=Morphostructure and Evolutionn of the Central and Eastern Bransfield Basins (NW Antarctic Peninsula|journal=Marine Geophysical Researches|date=1995|volume=18|pages=429–448}}</ref> It is believed that the Pheonix plate stopped subducting under the Antarctic plate at least 4 million years ago during the [[Pliocene]].<ref name=Schreider /><ref name=Lawver /><ref name=Galindo-Zaldivar /><ref name="Eulalia Gracia" />. Once the subduction event ceased, it is believed that the extension that created the basin was initiated. [[Aeromagnetic_survey|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 centimeters per year. <ref name="Eulalia Gracia" />


It is widely accepted that the Bransfield basin formed from extension caused by [[Oceanic trench|slab rollback]].<ref name=Schreider /><ref name=Lawver /><ref name=Galindo-Zaldivar /><ref name="Eulalia Gracia" /> New geophysical and structural data contradicts previously believed theories about slab rollback being the main mechanism for the opening of the basin.<ref name=Garcia /><ref name=Gonzalez-Casado /> A newer theory for the opening of the basin is attributed to [[Sinistral and dextral|sinistral strike-slip motion]] between the [[Scotia Plate]] and Antarctic plates.<ref name=Gonzalez-Casado /><ref name=Fretzdorff>{{cite journal|last1=Fretzdorff|first1=Susanne|last2=Worthington|first2=Time|last3=Haase|first3=Karsten|last4=Hekinian|first4=Roger|last5=Franz|first5=Leander|last6=Keller|first6=Randall|last7=Stoffers|first7=Peter|title=Magmatism in the Bransfield Basin:Rifting of the South Shetland Arc?|journal=Journal of Geophysical Research|date=2004|volume=109|pages=1–19|bibcode=2004JGRB..10912208F|doi=10.1029/2004JB003046}}</ref> It is theorized that the trench between the the Phoenix and Antarctic plates is locked in place and there is not any motion within the trench. The new data suggests [[Oceanic trench|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 too 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.<ref name=Gonzalez-Casado /><ref name=Fretzdorff />
It is widely accepted that the Bransfield Basin formed from extension caused by [[Oceanic trench|slab rollback]].<ref name=Schreider /><ref name=Lawver /><ref name=Galindo-Zaldivar /><ref name="Eulalia Gracia" /> New geophysical and structural data contradicts previously believed theories about slab rollback being the main mechanism for the opening of the basin.<ref name=Garcia /><ref name=Gonzalez-Casado /> A newer theory for the opening of the basin is attributed to [[Sinistral and dextral|sinistral strike-slip motion]] between the [[Scotia plate]] and Antarctic plates.<ref name=Gonzalez-Casado /><ref name=Fretzdorff>{{cite journal|last1=Fretzdorff|first1=Susanne|last2=Worthington|first2=Time|last3=Haase|first3=Karsten|last4=Hekinian|first4=Roger|last5=Franz|first5=Leander|last6=Keller|first6=Randall|last7=Stoffers|first7=Peter|title=Magmatism in the Bransfield Basin:Rifting of the South Shetland Arc?|journal=Journal of Geophysical Research|date=2004|volume=109|pages=1–19|bibcode=2004JGRB..10912208F|doi=10.1029/2004JB003046}}</ref> It is theorized that the trench between the the Phoenix and Antarctic plates is locked in place and there is not any motion within the trench. The new data suggests [[Oceanic trench|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.<ref name=Gonzalez-Casado /><ref name=Fretzdorff />


==Geology==
==Geology==
[[File:Bransfield Basin cross section(revised).png|thumbnail|right|Cross section of the Bransfield Basin during alternating phases of glaciation]]
[[File:Bransfield Basin cross section(revised).png|thumbnail|right|Cross section of the Bransfield Basin during alternating phases of glaciation]]
The main factor that controls deposition inside the Bransfield basin is [[Glacial period|Glacial cyclicity]]. Additional contributing factors include [[Physical geography|Physiography]], [[tectonics]], and [[oceanography]].<ref name=Garcia /><ref name="Marga Garcia">{{cite journal|last1=Garcia|first1=Marga|last2=Ercilla|first2=Gemma|last3=Alonso|first3=Belen|last4=Casas|first4=David|last5=Dowdeswell|first5=Julian|title=Sediment lithofacies, processes and sedimentary models in the central Bransfield Basin, Antarctic Peninsula, since the Last Glacial Maximum|journal=Marine Geology|date=2011|pages=1–16}}</ref> Three [[Stratigraphy|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 diatmaceous mud originating from open marine conditions. Sedimentary systems occur on the margins that are related to glacial and glacialmarine, mass wasting, seabed fluid-escape, and countour current processes.<ref name="Marga Garcia" />
The main factor that controls deposition inside the Bransfield basin is [[Glacial period|glacial cyclicity]]. Additional contributing factors include [[Physical geography|physiography]], [[tectonics]], and [[oceanography]].<ref name=Garcia /><ref name="Marga Garcia">{{cite journal|last1=Garcia|first1=Marga|last2=Ercilla|first2=Gemma|last3=Alonso|first3=Belen|last4=Casas|first4=David|last5=Dowdeswell|first5=Julian|title=Sediment lithofacies, processes and sedimentary models in the central Bransfield Basin, Antarctic Peninsula, since the Last Glacial Maximum|journal=Marine Geology|date=2011|pages=1–16}}</ref> Three [[Stratigraphy|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 diatmaceous mud originating from open marine conditions. Sedimentary systems occur on the margins that are related to glacial and glacialmarine, mass wasting, seabed fluid-escape, and countour current processes.<ref name="Marga Garcia" />


===Glacial processes===
===Glacial processes===
Line 53: Line 53:


===Glacialmarine processes===
===Glacialmarine processes===
Glacialmarine processes have deposited two different units within the region. One of the processes is a proglacial debris flows have deposited a matrix-supported diamicton with interbeds of [[Lamination (geology)|laminated]] [[mud]] on the lower portion of the [[Continental margin|continental slope]]. The other 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 and biogenic material compounds together to form sandy muds with sparse clasts.<ref name="Marga Garcia" />
Glacialmarine processes have deposited two different units within the region. One of the processes is a proglacial debris flows have deposited a matrix-supported diamicton with interbeds of [[Lamination (geology)|laminated]] [[mud]] on the lower portion of the [[Continental margin|continental slope]]. The other 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 and [[biogenic]] material compounds together to form sandy muds with sparse clasts.<ref name="Marga Garcia" />


===Open Marine processes===
===Open Marine processes===
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 volanic ash around 1 to 4 cenitmeters 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 [[Contourite|contour currents]], and differences in clast size is attributed to shifting current conditions.<ref name="Marga Garcia" />
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 volanic ash around 1 to 4 cenitmeters 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 [[Deformation (engineering)|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 [[Contourite|contour currents]], and differences in clast size is attributed to shifting current conditions.<ref name="Marga Garcia" />


==Magmatism==
==Magmatism==
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.<ref name=Fretzdorff /> 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 [[Incompatible element|large-ion lithophile elements]] to [[Mid-ocean ridge|enriched mid-ocean ridge basalts]].<ref name=Fretzdorff />
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.<ref name=Fretzdorff /> 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 [[Incompatible element|large-ion lithophile elements]] to [[Mid-ocean ridge|enriched mid-ocean ridge basalts]].<ref name=Fretzdorff />


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 [[Multimodal distribution|bimodal]] volcanism.<ref name="Sven Peterson">{{cite journal|last1=Peterson|first1=Sven|last2=Herzig|first2=Peter|last3=Schampera|first3=Ulrich|last4=Hannington|first4=Mark|last5=Jonasson|first5=Ian|title=Hydrothermal precipitates associated with bimodal volcanism in the Central Bransfield Strait, Antarctica|journal=Mineralium Deposita|date=2004|volume=39|pages=358-379}}</ref> [[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]].<ref name="Sven Peterson" />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 [http://irvents-new3.whoi.edu/ventfield/sumisu-rift Sumizu Rift]<ref name="Sven Peterson" />.
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 [[Multimodal distribution|bimodal]] volcanism.<ref name="Sven Peterson">{{cite journal|last1=Peterson|first1=Sven|last2=Herzig|first2=Peter|last3=Schampera|first3=Ulrich|last4=Hannington|first4=Mark|last5=Jonasson|first5=Ian|title=Hydrothermal precipitates associated with bimodal volcanism in the Central Bransfield Strait, Antarctica|journal=Mineralium Deposita|date=2004|volume=39|pages=358-379}}</ref> [[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]].<ref name="Sven Peterson" /> 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 [http://irvents-new3.whoi.edu/ventfield/sumisu-rift Sumizu Rift]<ref name="Sven Peterson" />.


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.<ref name=Gonzalez-Casado /> Other geoscientists suggest that it is occurring and is related to seamount volcanism and normal faulting within the basin.<ref name=Garcia /><ref name=Gonzalez-Casado />
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|magnetic anomaly]] patterns, and intracrustal [[Diapir|diapirism]].<ref name=Gonzalez-Casado /> Other geoscientists suggest that it is occurring and is related to seamount volcanism and [[normal faulting]] within the basin.<ref name=Garcia /><ref name=Gonzalez-Casado />


==See Also==
==See Also==

Revision as of 17:38, 6 March 2015

Bransfield Basin
Bathymetry map of the Bransfield Basin created using Geomap App software with a bathymetry map overlay with a distance and elevation scale.
TypeBack-arc basin
Location
RegionNorthwest of the Antarctic Peninsula
Type section
CountryAntarctica


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 kilometers from Smith Island 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 kilometers long by 70 kilometers wide with a depth of 1.3 kilometers, the Central basin is 230 kilometers long by 60 kilometers wide with a depth of 1.9 kilometers, and the Eastern basin is 150 kilometers long by 40 kilometers wide with a depth of over 2.7 kilometers.[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 kilometers deep.[4]

Tectonic Development

Schematic cartoon of the Bransfield Basin tectonic setting.

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 subduction event that occurred between the Phoenix plate and the Antarctic plate roughly 200 million years ago during the Mesozoic.[5] [6][7] It is believed that the Pheonix plate stopped subducting under the Antarctic plate at least 4 million years ago during the Pliocene.[3][5][6][7]. Once the subduction event 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 centimeters 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 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]

Geology

Cross section of the Bransfield Basin during alternating phases of glaciation

The main factor that controls deposition inside the Bransfield basin is glacial cyclicity. Additional contributing factors include physiography, tectonics, and oceanography.[1][9] 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 diatmaceous mud originating from open marine conditions. Sedimentary systems occur on the margins that are related to glacial and glacialmarine, mass wasting, seabed fluid-escape, and countour current processes.[9]

Glacial processes

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.[9]

Glacialmarine processes

Glacialmarine processes have deposited two different units within the region. One of the processes is a proglacial debris flows have deposited a matrix-supported diamicton with interbeds of laminated mud on the lower portion of the continental slope. The other 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 and biogenic material compounds together to form sandy muds with sparse clasts.[9]

Open Marine processes

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 volanic ash around 1 to 4 cenitmeters 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.[9]

Magmatism

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.[10] 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.[10] 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[10].

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]

See Also

References

  1. ^ a b c d e García, Marga; Ercilla, Gemma; Alonso, Belen (2009). "Morphology and sedimentary systems in the Central Bransfield Basin, Antarctic Peninsula: sedimentary dynamics fromshelf to basin". Basin Research. 21: 295–314.
  2. ^ a b c d e f González-Casado, José; Jorge, Giner-Robles; Jerónimo, López-Martínez (November 2000). "Bransfield Basin, Antarctic Peninsula: Not a normal backarc basin". Geology. 28 (11): 1043–1046. Bibcode:2000Geo....28.1043G. doi:10.1130/0091-7613(2000)28<1043:BBAPNA>2.0.CO;2.
  3. ^ a b c d Schreider, Al.; Schreider, A.; Evsenko, E. (2014). "The Stages of the Development of the Basin of the Bransfield Strait". Oceanology. 54 (3): 365–373. Bibcode:2014Ocgy...54..365S. doi:10.1134/S0001437014020234.
  4. ^ Baranov, A. (2011). "Moho Depth in Antarctica from Seismic Data". Physics of the Solid Earth. 47 (12): 1–13. Bibcode:2011IzPSE..47.1058B. doi:10.1134/S1069351311120019.
  5. ^ a b c Lawver, Lawrence; Keller, Randall; Fisk, Martin; Strelin, Jorge (1995). Backarc Basins: Tectonics and Magmatism. New York: Plenum Press. pp. 316–342.
  6. ^ a b c Galindo-Zaldivar, Jesus; Gamboa, Luiz; Maldonado, Andres; Nakao, Seizo; Bochu, Yao (2006). Antarctica: Contributions to global earth sciences. New York: Spring-verlag. pp. 243–248.
  7. ^ a b c d Gracia, Eulalia; Canals, Miquel; Farran, Marcel; Prieto, Maria; Sorribas, Jordi; Team, Gebra (1995). "Morphostructure and Evolutionn of the Central and Eastern Bransfield Basins (NW Antarctic Peninsula". Marine Geophysical Researches. 18: 429–448.
  8. ^ a b c d Fretzdorff, Susanne; Worthington, Time; Haase, Karsten; Hekinian, Roger; Franz, Leander; Keller, Randall; Stoffers, Peter (2004). "Magmatism in the Bransfield Basin:Rifting of the South Shetland Arc?". Journal of Geophysical Research. 109: 1–19. Bibcode:2004JGRB..10912208F. doi:10.1029/2004JB003046.
  9. ^ a b c d e Garcia, Marga; Ercilla, Gemma; Alonso, Belen; Casas, David; Dowdeswell, Julian (2011). "Sediment lithofacies, processes and sedimentary models in the central Bransfield Basin, Antarctic Peninsula, since the Last Glacial Maximum". Marine Geology: 1–16.
  10. ^ a b c Peterson, Sven; Herzig, Peter; Schampera, Ulrich; Hannington, Mark; Jonasson, Ian (2004). "Hydrothermal precipitates associated with bimodal volcanism in the Central Bransfield Strait, Antarctica". Mineralium Deposita. 39: 358–379.
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