Tectonic evolution of Patagonia
   HOME

TheInfoList



OR:

Patagonia comprises the southernmost region 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 ...
, portions of which lie on either side of the Argentina-Chile border. It has traditionally been described as the region south of the Rio Colorado, although the physiographic border has more recently been moved southward to the
Huincul fault The Huincul Fault or Huincul Fault Zone ( es, Falla de Huincul, Zona de falla Huincul) is an east-to-west-oriented, continental-scale fault that extends from the Neuquén Basin eastwards into the Argentine Shelf. To the west, it has been propos ...
. The region's geologic border to the north is composed of the
Rio de la Plata craton Rio or Río is the Portuguese, Spanish, Italian, and Maltese word for "river". When spoken on its own, the word often means Rio de Janeiro, a major city in Brazil. Rio or Río may also refer to: Geography Brazil * Rio de Janeiro * Rio do Sul, a ...
and several accreted terranes comprising the
La Pampa province La Pampa () is a sparsely populated province of Argentina, located in the Pampas in the center of the country. Neighboring provinces are from the north clockwise San Luis, Córdoba, Buenos Aires, Río Negro, Neuquén and Mendoza. History In ...
. The underlying
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, ...
rocks of the Patagonian region can be subdivided into two large massifs: the
North Patagonian Massif The North Patagonian Massif or Somún Cura Massif (Spanish language, Spanish: ''Macizo Norpatagónico'', ''Macizo Nordpatagónico'' or ''Macizo de Somún Cura'') is a massif in northern Patagonia located in the Argentine provinces of Río Negro Pro ...
and the
Deseado Massif The Deseado Massif (Spanish: ''Macizo del Deseado'') is a massif in southern Patagonia located in the Argentine province of Santa Cruz. On surface the massif is made up of Middle to Late Jurassic-aged felsic volcanic rocks. Analysis of mantle xeno ...
. These massifs are surrounded by
sedimentary basin Sedimentary basins are region-scale depressions of the Earth's crust where subsidence has occurred and a thick sequence of sediments have accumulated to form a large three-dimensional body of sedimentary rock. They form when long-term subsidence ...
s formed in the
Mesozoic The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Cretaceo ...
that underwent subsequent deformation during the
Andean orogeny The Andean orogeny ( es, Orogenia andina) is an ongoing process of orogeny that began in the Early Jurassic and is responsible for the rise of the Andes mountains. The orogeny is driven by a reactivation of a long-lived subduction system along ...
. Patagonia is known for its vast earthquakes and the damage they cause. The rocks comprising Patagonia occurred along the southwestern margin of the ancient supercontinent of
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 ...
. During a period of continental rifting in the
Cambrian The Cambrian Period ( ; sometimes symbolized C with bar, Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million ...
period, a portion of Patagonia was separated from Gondwana, and the resulting passive margin that formed was a site of extensive sedimentation throughout the early-middle
Paleozoic The Paleozoic (or Palaeozoic) Era is the earliest of three geologic eras of the Phanerozoic Eon. The name ''Paleozoic'' ( ;) was coined by the British geologist Adam Sedgwick in 1838 by combining the Greek words ''palaiós'' (, "old") and ' ...
era. During the
Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, whe ...
period, a transition to convergence resulted in the eventual collision of the Patagonian landmass in the late Paleozoic, with contact first occurring in the mid-
Carboniferous The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carbonifero ...
. Several theories exist for the origin of the Patagonian landmass, though there are two that have greater consensus. The first of these theories cites an
allochthon upright=1.6, Schematic overview of a thrust system. The hanging wall block is (when it has reasonable proportions) called a nappe. If an erosional hole is created in the nappe that is called a window (geology)">window. A klippe is a solitary out ...
ous origin of the Patagonian landmass from Gondwana during the Paleozoic, while the other argues that Northern Patagonia is an
autochthonous Autochthon, autochthons or autochthonous may refer to: Fiction * Autochthon (Atlantis), a character in Plato's myth of Atlantis * Autochthons, characters in the novel ''The Divine Invasion'' by Philip K. Dick * Autochthon, a Primordial in the ' ...
component and that only the southern portion is allochthonous. The collision of Patagonia was succeeded by the rifting and eventual breakup of Gondwana during the early
Mesozoic The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Cretaceo ...
, a process which invoked large-scale rotation of the Patagonian landmass. Further extension through the
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 ...
and
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 ...
periods formed the Rocas Verdes back-arc basin, while a transition to a compressional tectonic regime in the Cenozoic concurrent with the Andean orogeny resulted in formation of the foreland
Magallanes basin The Magallanes Basin or Austral Basin is a major sedimentary basin in southern Patagonia. The basin covers a surface of about and has a NNW-SSE oriented shape. The basin is bounded to the west by the Andes mountains and is separated from the Malv ...
.Fildani, A., Romans, B. W., Fosdick, J. C., Crane, W. H., & Hubbard, S. M. (2008). Orogenesis of the Patagonian Andes as reflected by basin evolution in southernmost South America. ''Arizona Geological Society Digest'', ''22'', 259–268.


Precambrian-Early Paleozoic setting

Patagonia contain two ancient regions: the
North Patagonian Massif The North Patagonian Massif or Somún Cura Massif (Spanish language, Spanish: ''Macizo Norpatagónico'', ''Macizo Nordpatagónico'' or ''Macizo de Somún Cura'') is a massif in northern Patagonia located in the Argentine provinces of Río Negro Pro ...
and
Deseado Massif The Deseado Massif (Spanish: ''Macizo del Deseado'') is a massif in southern Patagonia located in the Argentine province of Santa Cruz. On surface the massif is made up of Middle to Late Jurassic-aged felsic volcanic rocks. Analysis of mantle xeno ...
. The
lithospheric mantle The lithospheric mantle is the uppermost solid part of Earth's mantle. The lithospheric mantle is subdivided into the subcontinental lithospheric mantle associated with the continental lithosphere and oceanic lithospheric mantle, associated with t ...
beneath Deseado Massif formed 1000–2100 million years ago in the
Paleo __NOTOC__ ''Paleo'' may refer to: Prehistoric Era, Age, or Period * Paleolithic, a prehistoric Era, Age, or Period of human history People * David Strackany, aka "Paleo", an American folk singer-songwriter Art, entertainment, and media * ''P ...
and Mesoproterozoic, evidencing that its lithosphere has a much older history than the ages of crustal rocks exposed at present would suggest (~600 million years). Deseado Massif has formed a single crustal block with the
Falklands Islands The Falkland Islands (; es, Islas Malvinas, link=no ) is an archipelago in the Atlantic Ocean, South Atlantic Ocean on the Patagonian Shelf. The principal islands are about east of South America's southern Patagonian coast and about fro ...
since these times. Like today the Deseado Massif and the Falklands Islands lied next to each other in the Neoproterozoic supercontinent of
Rodinia Rodinia (from the Russian родина, ''rodina'', meaning "motherland, birthplace") was a Mesoproterozoic and Neoproterozoic supercontinent that assembled 1.26–0.90 billion years ago and broke up 750–633 million years ago. were probably ...
. The lithosphere of the North Patagonian Massif formed about the same. Prior to the collision of Patagonia, the nucleus of modern-day South America was contained within a portion of the southwest margin of
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 ...
. This margin consisted of the ancient
Rio de la Plata craton Rio or Río is the Portuguese, Spanish, Italian, and Maltese word for "river". When spoken on its own, the word often means Rio de Janeiro, a major city in Brazil. Rio or Río may also refer to: Geography Brazil * Rio de Janeiro * Rio do Sul, a ...
and a number of accreted
terrane In geology, a terrane (; in full, a tectonostratigraphic terrane) is a crust fragment formed on a tectonic plate (or broken off from it) and accreted or " sutured" to crust lying on another plate. The crustal block or fragment preserves its own ...
s, whose boundaries have been discovered using
paleomagnetic Paleomagnetism (or palaeomagnetismsee ), is the study of magnetic fields recorded in rocks, sediment, or archeological materials. Geophysicists who specialize in paleomagnetism are called ''paleomagnetists.'' Certain magnetic minerals in rock ...
studies. The Rio de la Plata Craton is believed to have been a component of southwest Gondwana since the end of the
Proterozoic The Proterozoic () is a geological eon spanning the time interval from 2500 to 538.8million years ago. It is the most recent part of the Precambrian "supereon". It is also the longest eon of the Earth's geologic time scale, and it is subdivided ...
, likely forming a single body with other Gondwanan crustal blocks. In the late Neoproterozoic-early Cambrian, the
Pampia Pampia was an ancient microcontinent or terrane that collided with Río de la Plata Craton and Río Apas Craton during the Pampean orogeny of late Proterozoic and early Cambrian. It was one of the first terranes to be amalgamated to the old crat ...
terrane collided with the western margin of the Rio de la Plata craton, resulting in the
Pampean orogeny The Pampean orogeny ( es, orogenia pampeana) was an orogeny active in the Cambrian in the western margin of the ancient landmass of Gondwana. The orogen's remains can now be observed in central Argentina, in particular at the Sierras de Córdoba an ...
. Evidence indicates that this Pampia terrane is of parautochthonous Gondwanan origin, separated from Gondwana in an earlier event to later be re-accreted to its margin.


Early Paleozoic

The Early Paleozoic tectonic regime in southwestern Gondwana involved a period of rifting during the Cambrian which affected the southern margin of the supercontinent, while at the same time the western margin experienced a compressional setting that saw the accretion of several exotic terranes. It has been hypothesized that following the Cambrian rifting event the Patagonian landmass collided with Antarctica, though evidence for this event is not conclusive.


Cambrian rifting

Early Cambrian rifting of the southwestern Gondwana margin is evidenced by the presence of
granite Granite () is a coarse-grained (phaneritic) intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies undergro ...
s bearing an extensional
geochemical Geochemistry is the science that uses the tools and principles of chemistry to explain the mechanisms behind major geological systems such as the Earth's crust and its oceans. The realm of geochemistry extends beyond the Earth, encompassing the e ...
signature in the
Sierra de la Ventana Sierra de La Ventana is a village in Tornquist Partido in the southwest of the Province of Buenos Aires, Argentina. With a population of 1,819 inhabitants (), it is one of the most attractive tourist centres in the Province and has numerous recreat ...
fold belt north of the Patagonian limits. The occurrence of this rifting event is also documented in the Ellsworth Mountains of Antarctica, the
Cape Fold Belt The Cape Fold Belt is a fold and thrust belt of late Paleozoic age, which affected the sequence of sedimentary rock layers of the Cape Supergroup in the southwestern corner of South Africa. It was originally continuous with the Ventana Mount ...
of South Africa, and the Falkland/Malvinas microplate (present day
Falkland Islands The Falkland Islands (; es, Islas Malvinas, link=no ) is an archipelago in the South Atlantic Ocean on the Patagonian Shelf. The principal islands are about east of South America's southern Patagonian coast and about from Cape Dubouzet ...
), and resulted in the formation of a proto-Pacific passive margin. This rifting stage formed the final outline of southern Gondwana and is thought to have been the beginning of the supercontinent stage in Gondwana. Evidence found in rocks in the
Tierra del Fuego Tierra del Fuego (, ; Spanish for "Land of the Fire", rarely also Fireland in English) is an archipelago off the southernmost tip of the South American mainland, across the Strait of Magellan. The archipelago consists of the main island, Isla G ...
region indicates that this Cambrian rifting event might have resulted in the separation of the southern tip of South America from Gondwana. This rifting event and the detachment of a portion of Patagonia are agreed upon by the two prominent theories regarding Patagonia's origin; however, they disagree on the extent of the displaced terrane. The theory supporting an allochthonous Patagonia cites the entirety of the region, including the North Patagonian Massif, as being separated from southwestern Gondwana. Comparison of the
paleomagnetic Paleomagnetism (or palaeomagnetismsee ), is the study of magnetic fields recorded in rocks, sediment, or archeological materials. Geophysicists who specialize in paleomagnetism are called ''paleomagnetists.'' Certain magnetic minerals in rock ...
poles of Patagonia and Gondwana from the
Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, whe ...
to
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 ...
periods allows for the separation of the two landmasses by up to 1000 kilometres; however, though such a separation is permitted by the evidence, it is not required in order to explain differences in the pole positions. The autochthonous theory meanwhile states that the North Patagonian Massif was not separated during this event, and suggests that rifting resulted only in the separation of a terrane represented by the Deseado Massif. The large, continuous passive margin produced during this rifting event led to the formation of several associated basins.
Sediment Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand an ...
s derived from Gondwana infilled these basins throughout the early Paleozoic until the Devonian period, resulting in the accumulation of thick sedimentary units which later underwent extensive deformation due to the transition to a compressional tectonic regime.


Collision of Exotic Terranes

While rifting was occurring in the southern portion of Gondwana, the margin further west (along the proto-Andean belt) was experiencing a compressional regime that introduced several allochthonous terranes to the western margin of South America. The first of these, the Pampean orogeny (mentioned above), resulted in the accretion of the Pampia terrane. This was followed by the formation of the Famatina-Eastern Puna
magmatic arc Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natural sa ...
during the
Famatinian orogeny The Famatinian orogeny ( es, Orogenia de Famatina) is an orogeny that predates the rise of the Andes and that took place in what is now western South America during the Paleozoic, leading to the formation of the Famatinian orogen also known as ...
in the Ordovician period, which culminated in the accretion of the
Cuyania The Precordillera Terrane or Cuyania was an ancient microcontinent or terrane whose history affected many of the older rocks of Cuyo in Argentina. It was separated by oceanic crust from the Chilenia terrane which accreted into it at ~420-390 Ma w ...
(Precordillera) terrane. Paleomagnetic evidence suggests that this Cuyania terrane is of Laurentian origin, and it has been hypothesized that the terrane may have been a plateau attached to
Laurentia Laurentia or the North American Craton is a large continental craton that forms the ancient geological core of North America. Many times in its past, Laurentia has been a separate continent, as it is now in the form of North America, although ...
which was involved in the Cambrian rifting event and later accreted during collision between Laurentia and Gondwana.
Geochronological Geochronology is the science of determining the age of rocks, fossils, and sediments using signatures inherent in the rocks themselves. Absolute geochronology can be accomplished through radioactive isotopes, whereas relative geochronology is pr ...
evidence shows that the Famatinian magmatic belt extends south from the Andean margin into the North Patagonian massif, and paleomagnetic studies of these rocks indicate that separation between these bodies has not occurred at least since the Devonian, both lending support to the theory of an autochthonous component of Patagonia.
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 ...
continued along this margin, and in the late Devonian resulted in the collision and accretion of the
Chilenia Chilenia was an ancient microcontinent or terrane whose history affected many of the older rocks of central Chile and western Argentina. It was once separated by oceanic crust from the Cuyania terrane to which it accreted at ~420-390 Ma when Cuyan ...
terrane to the western border of the Precordillera. While collision of Chilenia occurred north of the Huincul lineament south of it the
Chaitenia Chaitenia is a distinct fragment of Earth's crust in southern Chile. Rocks of Chaitenia represents an ancient island arc that existed next to Patagonia which became eventually accreted to it. The accretion of Chaitenia to Patagonia occurred in t ...
terrane accreted to Patagonia in the Devonian. The metamorphosed rocks of Chaitenia crop out mainly in southern Chile and represent remnants of an ancient island arc that existed next to Patagonia. After accretion,
accretionary complex An accretionary wedge or accretionary prism forms from sediments accreted onto the non-subducting tectonic plate at a convergent plate boundary. Most of the material in the accretionary wedge consists of marine sediments scraped off from the d ...
es developed west of Chaitenia, meaning Chaitenia constituted a backstop.


Patagonia-Antarctica collision

Following the early Cambrian rifting event, Antarctica underwent a deformational event resulting in uplift of the Transantarctic Mountains in the mid-Cambrian, an event that has been ascribed to the Ross orogeny. It has recently been proposed that during middle to late Cambrian times Patagonia was accreted to East Antarctica, an event that led to the initiation of the Ross orogeny. Following this collision, a transition to extension in the late Ordovician-
Silurian The Silurian ( ) is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the shortest period of the Paleozo ...
resulted in the separation of Patagonia from Antarctica and the formation of a passive margin. Deposition along this passive margin is represented by the Devonian-age lower section of the Beacon Supergroup. Proposed evidence for this connection exists in both the North Patagonian and Deseado Massifs, where remnants of the fossil species of archeocyathids have been discovered, a species preserved in the Shackleton Limestone of the Transantarctic Mountains. Additionally, segments of the Sierra Grande Formation in both massifs show possible correlation with the Beacon Supergroup bearing a common Devonian age. Correlation between granites found in northeastern Patagonia and others involved in the Ross orogeny has also been suggested, but such a connection is lacking geochronological evidence.


Late Paleozoic


Carboniferous-Permian collision

Lasting throughout the early to middle Paleozoic, rifting in the region was interrupted in the mid-Devonian when the tectonic scheme switched from an
extensional In any of several fields of study that treat the use of signs — for example, in linguistics, logic, mathematics, semantics, semiotics, and philosophy of language — an extensional context (or transparent context) is a syntactic environment in w ...
to compressional one, a process that resulted in the collision of the Patagonian terrane with the southwestern Gondwanan margin. Subduction-related igneous rocks from beneath the North Patagonian Massif have been dated at 320–330 million years old, indicating that the subduction process initiated in the early Carboniferous. This was relatively short lived (lasting about 20 million years), and initial contact of the two landmasses occurred in the mid-Carboniferous, with broader collision during the
early Permian 01 or '01 may refer to: * The year 2001, or any year ending with 01 * The month of January * 1 (number) Music * '01 (Richard Müller album), 01'' (Richard Müller album), 2001 * 01 (Son of Dave album), ''01'' (Son of Dave album), 2000 * 01 (Urban ...
. This collision resulted in the formation of two distinct magmatic and metamorphic belts in the North Patagonian Massif, one in the north and one in the west. Isotopic dating of
zircon Zircon () is a mineral belonging to the group of nesosilicates and is a source of the metal zirconium. Its chemical name is zirconium(IV) silicate, and its corresponding chemical formula is Zr SiO4. An empirical formula showing some of the r ...
from the magmatic belts provides evidence that the activity forming the western magmatic arc ceased during the late Carboniferous and may have involved collision of the Antarctic Peninsula with the southwest margin. Deformation and metamorphism resulting from this terrane collision may have begun in the late Carboniferous period and continued into the Permian period. Such deformation is postulated to have had a role in the initiation of the
Gondwanide Orogeny The Gondwanide orogeny was an orogeny active in the Permian that affected parts of Gondwana that are by current geography now located in southern South America, South Africa, Antarctica, Australia and New Guinea. The zone of deformation in Arge ...
and formation of the Gondwanide Fold Belt, which includes the
Sierra de la Ventana Sierra de La Ventana is a village in Tornquist Partido in the southwest of the Province of Buenos Aires, Argentina. With a population of 1,819 inhabitants (), it is one of the most attractive tourist centres in the Province and has numerous recreat ...
mountains north of Patagonia and the
Cape Fold Belt The Cape Fold Belt is a fold and thrust belt of late Paleozoic age, which affected the sequence of sedimentary rock layers of the Cape Supergroup in the southwestern corner of South Africa. It was originally continuous with the Ventana Mount ...
of South Africa. The collisions in this portion of the southwest margin of Gondwana during this time were likely the precursor to the
Terra Australis orogen The Terra Australis Orogen (TAO) was the oceanic southern margin of Gondwana which stretched from South America to Eastern Australia and encompassed South Africa, West Antarctica, New Zealand and Victoria Land in East Antarctica. Origins Terra ...
that later affected this region. Also in the Late Paleozoic the two main landmasses of Patagonia; the North Patagonian Massif and Deseado Massif inferred to have collided following a period of subduction of the plate of Deseado Massif beneath the plate containing the North Patagonian Massif. This subduction is postulated to have eroded the
lithospheric mantle The lithospheric mantle is the uppermost solid part of Earth's mantle. The lithospheric mantle is subdivided into the subcontinental lithospheric mantle associated with the continental lithosphere and oceanic lithospheric mantle, associated with t ...
beneath the North Patagonian Massif. As the prominent tectonic models for Patagonian accretion to Gondwana differ in their interpretation of the extent of the terrane that was detached during the Cambrian rifting, by definition they disagree on the extent of the terrane that was accreted in the Permian.


Allochthonous theory

The allochthonous theory hypothesizes that the entirety of Patagonia, including both basement massifs were separated from Gondwana during rifting in the Cambrian. One version of this model includes a hypothetical independent collision of the two massifs following rifting in the Cambrian to form the Patagonia terrane prior to its eventual collision with Gondwana. The two magmatic belts found in the North Patagonian Massif are inferred to represent collision of this portion of Patagonia against the margin of Gondwana following the closure of an ocean basin during convergence and subduction. By this model, the western belt in the North Patagonian Massif would have been formed due to subduction of oceanic crust under its southern margin, with termination of subduction resulting from collision of the Antarctic Peninsula during the mid-late Carboniferous. Meanwhile, the northern belt would have been created during subduction of oceanic crust underneath its northern margin. Geophysical studies in the region discovered a large subsurface structure along the northern Patagonian border, which cuts off the suture boundaries between the Rio de la Plata craton and its westward accreted terranes. This feature has been hypothesized as representing the suture zone of Patagonia with Gondwana.


Autochthonous theory

According to the autochthonous theory, the North Patagonian Massif is an autochthonous portion of former Gondwana that has been in its current position since at least the Ordovician. The Deseado Massif, however, is believed to be an allochthonous (or parautochthonous) component, detached from Gondwana during the Cambrian rifting episode and accreted again in the Permian. There is evidence that crustal magnetic signatures on either side of the Huincul fault zone are the same, indicating the North Patagonian Massif and southwestern Gondwana may have been one continuous landmass through the Paleozoic. The late Paleozoic collision then is believed to have occurred between the Deseado Massif and the southwestern edge of the North Patagonian Massif, with oceanic crust being subducted below the southern margin of the North Patagonian Massif to form the observed magmatic belts in that region. Late Permian break off of the subducting slab resulted in upwelling of the mantle and extensive melting of the crust followed by a transition to
post-orogenic collapse In geology, orogenic collapse is the thinning and lateral spread of thickened crust. It is a broad term referring to processes which distribute material from regions of high gravitational potential energy to regions of low gravitational potential ...
, an episode which is preserved in the
Choiyoi Group Choiyoi Group ( es, Grupo Choiyoi) is a Permian and Triassic-aged group of volcano-sedimentary formations in Argentina and Chile. The group bears evidence of bimodal-style volcanism related to an ancient subduction zone that existed along the w ...
volcanic province. There is a possibility that the collision of the Deseado Massif terrane with the Gondwana margin may have produced a tear in the subducting slab, leading to its eventual break-off.


Mesozoic extension


Rifting of Gondwana

Following the period of late Paleozoic convergence that led to the collision of Patagonia with Gondwana, a transition to extensional tectonics in the Triassic period induced rifting within Patagonia. This extension began northwest of Patagonia along the pre-existing suture zone between the Chilenia and Cuyania terranes and led to the formation of the
Cuyo basin Cuyo Basin ( es, Cuenca Cuyana) is a sedimentary basin in Mendoza Province, western Argentina. The Cuyo Basin has a NNW-SSE elongated shape and is limited to the west by the Sierra Pintada System and to the east by the Pampean pericraton. To the ...
, among others. More widespread rifting began in the Jurassic, by which time the breakup of Gondwana was underway. This was accompanied by the initiation and evolution of the Southern Atlantic rift system which led to the opening of the South Atlantic Ocean. The extension led to formation of fault-bounded basins, including the Cañadon Asfalto Basin and Rocas Verdes Basin. Though initially a continental rift basin, the Rocas Verdes Basin transitioned to a
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 ...
with the introduction of proto-oceanic crust, for which evidence is preserved in the Sarmiento and Tortuga ophiolite sequences.


Rotations within Patagonia

During the early breakup of Gondwana and associated continental rifting, the Patagonian landmass underwent large-scale rotations. Paleomagnetic data from late Jurassic-early Cretaceous rocks in the southern portion of the North Patagonian Massif indicate that clockwise rotation of up to 30 degrees occurred in that area during the Early Cretaceous, affecting a crustal block at least tens of kilometers in size. Additional findings from within the Deseado Massif revealed that similar rotations occurred in that area as well, either during the same Early Cretaceous episode or in an earlier deformational event during the Late Jurassic. Similar processes are reported to have affected the Falkland Islands, and may have been operating at the same time as those in Patagonia. The mechanism behind these rotations is unclear, and evidence of associated deformational structures is scarce. It has been proposed that the Gastre Fault System is one such structure that accommodated much of the deformation involved with crustal rotations; however, this hypothesis lacks supporting evidence.


Cretaceous-Cenozoic


Compression and foreland basin formation

During the Cretaceous, accelerating spreading rates of mid-ocean ridges in the Pacific and Atlantic Oceans as well as increased subduction below the western margin caused a shift from extensional tectonics towards compression, concurrent with the initiation of the Andean orogeny. This transition resulted in
inversion Inversion or inversions may refer to: Arts * , a French gay magazine (1924/1925) * ''Inversion'' (artwork), a 2005 temporary sculpture in Houston, Texas * Inversion (music), a term with various meanings in music theory and musical set theory * ...
of the Rocas Verdes Basin and ultimately led to its closure in the Late Cretaceous. Associated with closure of the basin was the development of the Andean fold and thrust belt. Deep-water sediment deposition during the Late Cretaceous contractional phase formed the thick Cerro Toro Formation, and subsequent formations record the progressive movement of deposition from deep-water to shallow-water and ultimately
deltaic A river delta is a landform shaped like a triangle, created by deposition of sediment that is carried by a river and enters slower-moving or stagnant water. This occurs where a river enters an ocean, sea, estuary, lake, reservoir, or (more rare ...
environments.


Basin uplift and deformation

Continued compression through 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 ...
period and the associated horizontal shortening resulted in uplift and associated deformation of the Andean fold and thrust belt and provided exposure of formations within the
Magallanes Basin The Magallanes Basin or Austral Basin is a major sedimentary basin in southern Patagonia. The basin covers a surface of about and has a NNW-SSE oriented shape. The basin is bounded to the west by the Andes mountains and is separated from the Malv ...
. Though the sequence of deformational events leading to the present day formation is unclear, observational evidence from the region indicates that there were three notable periods of deformation, occurring in the Late Cretaceous period, the
Oligocene epoch 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 th ...
, and some time following the Miocene epoch. Reconstructions of the events that resulted in deformation of the basin are complicated by variability in the style and extent of deformation along the Patagonian Andes, ranging from intense folding and steep thrust faulting with accompanying metamorphism to broad fold sequences lacking metamorphism. However, evidence from sedimentary relationships and the absolute dating of igneous rocks cutting across sedimentary layers allow for the inference of the relative ages given above. Additional data from the dating of metamorphosed layers provide bracket ages for the timing of exhumation of the Magallanes Basin, and suggest that this basin, as well as the Andean fold-thrust belt, were exhumed from below the surface between 10 and 4 million years ago. The Antarctic Plate started to subduct beneath South American 14 million years ago in the Miocene epoch. At first it subducted only in the southernmost tip of Patagonia, meaning that the
Chile Triple Junction The Chile Triple Junction (or Chile Margin Triple Junction) is a geologic triple junction located on the seafloor of the Pacific Ocean off Taitao and Tres Montes Peninsula on the southern coast of Chile. Here three tectonic plates meet: the South ...
lay near the
Strait of Magellan The Strait of Magellan (), also called the Straits of Magellan, is a navigable sea route in southern Chile separating mainland South America to the north and Tierra del Fuego to the south. The strait is considered the most important natural pass ...
. As the southern part of the
Nazca Plate The Nazca Plate or Nasca Plate, named after the Nazca region of southern Peru, is an oceanic tectonic plate in the eastern Pacific Ocean basin off the west coast of South America. The ongoing subduction, along the Peru–Chile Trench, of the Na ...
and the
Chile Rise The Chile Ridge, also known as the Chile Rise, is a submarine oceanic ridge formed by the divergent plate boundary between the Nazca Plate and the Antarctic Plate. It extends from the triple junction of the Nazca, Pacific, and Antarctic plates t ...
became consumed by subduction the more northerly regions of the Antarctic Plate began to subduct beneath Patagonia so that the Chile Triple Junction lies at present offshore Taitao Peninsula at 46°15' S. As the Andes rose in the
Middle Miocene The Middle Miocene is a sub-epoch of the Miocene Epoch made up of two stages: the Langhian and Serravallian stages. The Middle Miocene is preceded by the Early Miocene. The sub-epoch lasted from 15.97 ± 0.05 Ma to 11.608 ± 0.005 Ma (million y ...
(14–12 million years ago) a
rain shadow A rain shadow is an area of significantly reduced rainfall behind a mountainous region, on the side facing away from prevailing winds, known as its leeward side. Evaporated moisture from water bodies (such as oceans and large lakes) is carrie ...
developed to the east giving origin to the
Patagonian Desert The Patagonian Desert, also known as the Patagonian Steppe, is the largest desert in Argentina and is the 8th largest desert in the world by area, occupying 673,000 square kilometers (260,000 mi2). It is located primarily in Argentina and ...
.


Quaternary tectonics

The eastern coast of Patagonia has experienced some considerable uplift during the
Pleistocene The Pleistocene ( , often referred to as the ''Ice age'') is the geological Epoch (geology), epoch that lasted from about 2,580,000 to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was fina ...
as evidenced by marine terraces and beaches found above sea level. Uplift rates eastern coast of Patagonia exceeds those of the Atlantic coast of South America (except
Recife That it may shine on all ( Matthew 5:15) , image_map = Brazil Pernambuco Recife location map.svg , mapsize = 250px , map_caption = Location in the state of Pernambuco , pushpin_map = Brazil#South A ...
). Uplift in Patagonia contrast starkly with the Río de la Plata which has been a region of
subsidence Subsidence is a general term for downward vertical movement of the Earth's surface, which can be caused by both natural processes and human activities. Subsidence involves little or no horizontal movement, which distinguishes it from slope move ...
. The causes of the uplift trend have been attributed to a decreased downward drag induced by flow in Earth's mantle beneath Patagonia. This change is in turn derived from the geologically recent
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 ...
of the Antarctic Plate beneath South America, which as a new subducting plate with a short subducting slab, would have less capacity to induce flow in Earth's mantle.


Notes


References

{{Major South American geological formations, state=collapsed . Plate tectonics Geology of South America