Tectonic evolution of Patagonia
[2] 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.
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.
[3] 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.
[10] 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.
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.
[11] 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.
[11] 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.
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.
[2] Geochronological evidence shows that the Famatinian magmatic belt extends south from the Andean margin into the North Patagonian massif,[2] and paleomagnetic studies of these rocks indicate that separation between these bodies has not occurred at least since the Devonian,[13] both lending support to the theory of an autochthonous component of Patagonia.
[11] It has recently been proposed that during middle to late Cambrian times Patagonia was accreted to East Antarctica,[5] an event that led to the initiation of the Ross orogeny.
[5] Additionally, segments of the Sierra Grande Formation in both massifs show possible correlation with the Beacon Supergroup bearing a common Devonian age.
[12] 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.
[17] 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.
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.
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.
[2] 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, an episode which is preserved in the Choiyoi Group volcanic province.
[6] 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.
[7] 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.
[8] Continued compression through the Tertiary 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.
As the southern part of the Nazca Plate and the Chile Rise 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.[19][20] As the Andes rose in the Middle Miocene (14–12 million years ago) a rain shadow developed to the east giving origin to the Patagonian Desert.
