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.
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.
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