Patagonia comprises the southernmost region of South America, 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 region's geologic border to the north is composed of the Rio de la Plata craton and several accreted terranes comprising the La Pampa province. The underlying basement rocks of the Patagonian region can be subdivided into two large massifs: the North Patagonian Massif and the Deseado Massif. These massifs are surrounded by sedimentary basins formed in the Mesozoic that underwent subsequent deformation during the Andean orogeny. 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. During a period of continental rifting in the Cambrian 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 era. During the Devonian 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. 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 allochthonous origin of the Patagonian landmass from Gondwana during the Paleozoic, while the other argues that Northern Patagonia is an autochthonous 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, a process which invoked large-scale rotation of the Patagonian landmass. Further extension through the Jurassic and Cretaceous 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.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 and Deseado Massif. The lithospheric mantle beneath Deseado Massif formed 1000–2100 million years ago in the Paleo 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 since these times. Like today the Deseado Massif and the Falklands Islands lied next to each other in the Neoproterozoic supercontinent of Rodinia. 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. This margin consisted of the ancient Rio de la Plata craton and a number of accreted terranes, whose boundaries have been discovered using paleomagnetic studies. The Rio de la Plata Craton is believed to have been a component of southwest Gondwana since the end of the Proterozoic, likely forming a single body with other Gondwanan crustal blocks. In the late Neoproterozoic-early Cambrian, the Pampia terrane collided with the western margin of the Rio de la Plata craton, resulting in the Pampean orogeny. 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 granites bearing an extensional geochemical signature in the Sierra de la Ventana 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 of South Africa, and the Falkland/Malvinas microplate (present day Falkland Islands), 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 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 poles of Patagonia and Gondwana from the Devonian to Permian 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. Sediments 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 during the Famatinian orogeny in the Ordovician period, which culminated in the accretion of the Cuyania (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 which was involved in the Cambrian rifting event and later accreted during collision between Laurentia and Gondwana. Geochronological 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 continued along this margin, and in the late Devonian resulted in the collision and accretion of the Chilenia terrane to the western border of the Precordillera.
While collision of Chilenia occurred north of the Huincul lineament south of it the Chaitenia 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 complexes 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 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 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. 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 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 and formation of the Gondwanide Fold Belt, which includes the Sierra de la Ventana mountains north of Patagonia and the Cape Fold Belt 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 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 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.