The

Antarctic Peninsula 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 Antarctic ...

, roughly south of

South America South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere at the northern tip of the continent. It can also be described as the southe ...

, is the northernmost portion of the continent of

Antarctica Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean, it contains the geographic South Pole. Antarctica is the fifth-largest contine ...

. Like the associated

Andes The Andes, Andes Mountains or Andean Mountains (; ) are the longest continental mountain range in the world, forming a continuous highland along the western edge of South America. The range is long, wide (widest between 18°S – 20°S ...

, the Antarctic Peninsula is an excellent example of ocean-continent collision resulting in

subduction Subduction is a geological process in which the oceanic lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, the ...

. The peninsula has experienced continuous subduction for over 200 million years, but changes in continental configurations during the amalgamation and breakup of continents have changed the orientation of the peninsula itself, as well as the underlying

volcanic rocks Volcanic rock (often shortened to volcanics in scientific contexts) is a Rock (geology), rock formed from lava erupted from a volcano. In other words, it differs from other igneous rock by being of Volcano, volcanic origin. Like all rock types, ...

associated with the subduction zone.

Tectonic evolution and geology of the Antarctic Peninsula

The geology of the Antarctic Peninsula occurred in three stages: #Pre-subduction stage of marginal basin deposition, later separated by the Gondwanian orogeny during the

Permian The Permian ( ) is a geologic period and stratigraphic system which spans 47 million years from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.9 Mya. It is the last period of the Paleoz ...

-Late

Triassic The Triassic ( ) is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.36 Mya. The Triassic is the first and shortest period ...

#The middle subduction phase, characterized by the formation of the Antarctic Peninsula (inner) and

South Shetland Islands The South Shetland Islands are a group of Antarctic islands with a total area of . They lie about north of the Antarctic Peninsula, and between southwest of the nearest point of the South Orkney Islands. By the Antarctic Treaty of 195 ...

(outer) magmatic arcs, during the middle

Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The J ...

Miocene The Miocene ( ) is the first geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and means "less recen ...

. #The late subduction phase, when the opening of the Bransfield Rift and

back-arc basin A back-arc basin is a type of geologic basin, found at some convergent plate boundaries. Presently all back-arc basins are submarine features associated with island arcs and subduction zones, with many found in the western Pacific Ocean. Most of ...

s occur. This is followed by contemporaneous terrestrial and submarine volcanic activity, from the

Oligocene The Oligocene ( ) is a geologic epoch of the Paleogene Period and extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that define the epoch are well identified but the ...

-present day.

Pre-subduction history

Gondwana Gondwana () was a large landmass, often referred to as a supercontinent, that formed during the late Neoproterozoic (about 550 million years ago) and began to break up during the Jurassic period (about 180 million years ago). The final stages ...

broke apart, the

started to take on its modern shape. Roughly 220 million years ago, the continents of Antarctica, South America, and Africa rifted apart. This rifting created low relief basins which allowed for the transport of sediments and subsequent deposition of sedimentary rocks. These rocks, the oldest on the peninsula, belong to the Trinity Peninsula Group (TPG), which are mostly composed of siliciclastic turbidite deposits, ~1200–3000 m thick, deposited in a marginal marine basin. Unfortunately their age is poorly constrained, but they are most likely from the upper

and

. The clastic component of these sediments was derived from the weathering, erosion, and subsequent transportation of metamorphic, igneous and sedimentary material from

, then to the northeast.

Gondwanian orogeny

During this time the Trinity Peninsula Group sediments were folded and slightly metamorphosed, particularly at the peninsula's northernmost point. Retroarc

thrusting Thrust is a reaction force described quantitatively by Newton's third law. When a system expels or accelerates mass in one direction, the accelerated mass will cause a force of equal magnitude but opposite direction to be applied to that sys ...

also occurred at this time. Both events were most likely caused by the incipient subduction of the south-east

Pacific Plate The Pacific Plate is an oceanic tectonic plate that lies beneath the Pacific Ocean. At , it is the largest tectonic plate. The plate first came into existence 190 million years ago, at the triple junction between the Farallon, Phoenix, and Iza ...

under the

supercontinent. As a result, marginal basin clastics from the Pacific plate's oceanic basement were obducted onto the continental margin of

, composed of older crystalline

basement A basement or cellar is one or more floors of a building that are completely or partly below the ground floor. It generally is used as a utility space for a building, where such items as the furnace, water heater, breaker panel or fuse box, ...

Middle subduction phase

Inner magmatic arc

The inner magmatic arc, despite being older than the outer magmatic arc, has higher topographic relief. It forms the mainland of the Antarctic Peninsula. The creation of the inner magmatic arc is characterized by terrestrial clastic deposition and the early stages of acidic volcanism and plutonism. The Mesozoic clastic sequence (Number 2-Figure 2) consists of the Mount Flora Formation (MFF), which is a 270 m thick package of plant-bearing coarse sedimentary

breccia Breccia () is a rock composed of large angular broken fragments of minerals or rocks cemented together by a fine-grained matrix. The word has its origins in the Italian language, in which it means "rubble". A breccia may have a variety of di ...

s and conglomerates, with a limited amount of interbedded sandstones and shales. The clastic beds overlay the TPG sediments, and are separated by angular

unconformities An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval o ...

. Overlying the MFF clastic sequence are the acidic volcanics of the Kenny Glacier Formation (KGF). This volcanic sequence is a 215 m thick group of

rhyolite Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture, but may be porphyritic, containing larger mineral crystals (phenocrysts) in an otherwise fine-grained groundmass. The mineral ...

dacite Dacite () is a volcanic rock formed by rapid solidification of lava that is high in silica and low in alkali metal oxides. It has a fine-grained (aphanitic) to porphyritic texture and is intermediate in composition between andesite and rhyolite. ...

lava Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a moon onto its surface. Lava may be erupted at a volcano or through a fracture in the crust, on land or un ...

s, ignimbrites,

tuff Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock cont ...

s, and agglomerates. The acidic

dikes Dyke (UK) or dike (US) may refer to: General uses * Dyke (slang), a slang word meaning "lesbian" * Dike (geology), a subvertical sheet-like intrusion of magma or sediment * Dike (mythology), ''Dikē'', the Greek goddess of moral justice * Dikes, ...

and sills which intrude the MFF and TPG sediments may be due to the KGF

stratovolcano A stratovolcano, also known as a composite volcano, is a conical volcano built up by many layers (strata) of hardened lava and tephra. Unlike shield volcanoes, stratovolcanoes are characterized by a steep profile with a summit crater and per ...

. The acidic volcanism that created the KGF sequence is associated with plutonic intrusions during the Middle

-Early

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of th ...

in the northern Antarctic Peninsula. These plutonic intrusions could have been caused by the doming and rifting in the continental margin of Gondwana at the onset of oceanic slab subduction.

Outer magmatic arc

The outer magmatic arc, of which the

are a part, is a westward migration of the inner magmatic arc. Similar to the inner magmatic arc, the outer is composed of subduction-related acidic volcanism. A study on Alexander Island that focused on the conditions required for the generation of andesitic lavas postulated that the source for the andesitic lavas could be either the development of a slab-window due to the subduction of a

spreading ridge A mid-ocean ridge (MOR) is a seafloor mountain system formed by plate tectonics. It typically has a depth of about and rises about above the deepest portion of an ocean basin. This feature is where seafloor spreading takes place along a diverge ...

or the breakup of the subducted slab beneath the

fore-arc basin Forearc is a plate tectonic term referring to a region between an oceanic trench, also known as a subduction zone, and the associated volcanic arc. Forearc regions are present along a convergent margins and eponymously form 'in front of' the vol ...

. The South Shetland Islands are bisected by two systems of

strike-slip In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectonic ...

faults. The older system, which is parallel with the island arc, is characterized by right-lateral faults, and was active on King George Island during most of the

Tertiary Tertiary ( ) is a widely used but obsolete term for the geologic period from 66 million to 2.6 million years ago. The period began with the demise of the non-avian dinosaurs in the Cretaceous–Paleogene extinction event, at the start ...

. The younger system of faults, also a series of strike-slip faults, displaced the older system and formed transverse to the island arc. Movement of the fault activity was caused by the counterclockwise rotation of the Antarctic Continent with respect to the subduction zone.

Late subduction phase, opening of the Bransfield Rift

The last and most recent stage in the evolution of the

subduction zone Subduction is a geological process in which the oceanic lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, the ...

is the opening of the Bransfield Rift, creating the Bransfield

from the

to the present day. This basin separates the inner, older magmatic arc (mainland Antarctic Peninsula) from the outer, younger magmatic arc (South Shetland Islands).

Alkaline In chemistry, an alkali (; from ar, القلوي, al-qaly, lit=ashes of the saltwort) is a base (chemistry), basic, ionic compound, ionic salt (chemistry), salt of an alkali metal or an alkaline earth metal. An alkali can also be defined as ...

and tholeiitic volcanic activity is associated with this rifting event. The trenchward migration of the spreading center is attributed to the subduction of the

Phoenix Plate The Phoenix Plate (also known as the Aluk Plate or Drake Plate) was a tectonic plate that existed during the early Paleozoic through late Cenozoic time. It formed a triple junction with the Izanagi and Farallon plates in the Panthalassa Ocean as ...

under the Antarctic Plate. Slab rollback and South Shetland Trench oceanward retreat have led to extensional forces acting on the leading edge of the overriding plate. The Bransfield Strait, the result of this extension, is presumed to be four million years old or less; magnetic anomalies created by the formation of new basaltic crust and aligned with the axis of the Bransfield Rift indicate that the newly formed

oceanic crust Oceanic crust is the uppermost layer of the oceanic portion of the tectonic plates. It is composed of the upper oceanic crust, with pillow lavas and a dike complex, and the lower oceanic crust, composed of troctolite, gabbro and ultramafic cumu ...

in the Bransfield Strait is roughly 1.3 million years old. Unfortunately the deposition of sediments and extensive intrusions into the rift render computer modeling unreliable. Isolated occurrences of terrestrial volcanic activity are present, and are predominantly alkaline to tholeiitic in composition.

References

{{Reflist, 2

External links

Geologic map of the Antarctic PeninsulaGeological map of Eastern Graham Land
Antarctic Peninsula Geology of Antarctica