Positions of ancient continents, 550 million years ago

The Pampean orogeny ( es, orogenia pampeana) was an

orogeny Orogeny is a mountain building process. An orogeny is an event that takes place at a convergent plate margin when plate motion compresses the margin. An ''orogenic belt'' or ''orogen'' develops as the compressed plate crumples and is uplifted t ...

active 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 ...

in the western margin of the ancient landmass 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 ...

. The orogen's remains can now be observed in central

Argentina Argentina (), officially the Argentine Republic ( es, link=no, República Argentina), is a country in the southern half of South America. Argentina covers an area of , making it the second-largest country in South America after Brazil, th ...

, in particular at the

Sierras de Córdoba The Sierras de Córdoba is a mountain range in central Argentina, located between the Pampas to the east and south and the Chaco to the north and east. Most of the range is located in Córdoba Province, except for the southwestern margin whic ...

and other parts of the eastern

Sierras Pampeanas The Sierras Pampeanas (also called Central Sierras or Pampas Sierras) (English: Pampas Mountains) is a geographical region of Argentina. The Sierras Pampeanas are a chain of mountains that rise sharply from the surrounding pampa region of ...

. It is uncertain if the orogeny involved at some point a

continental collision In geology, continental collision is a phenomenon of plate tectonics that occurs at convergent boundaries. Continental collision is a variation on the fundamental process of subduction, whereby the subduction zone is destroyed, mountains prod ...

. The Pampean orogen can be considered both part of the larger Terra Australis orogen and of the

Brasiliano orogeny Brasiliano orogeny or Brasiliano cycle ( pt, Orogênese Brasiliana and ''Ciclo Brasiliano'') refers to a series of orogenies of Neoproterozoic age exposed chiefly in Brazil but also in other parts of South America. The Brasiliano orogeny is a reg ...

. The Pampean orogeny was succeeded by 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 ...

further west.

Magmatic belts

The Pampean orogen contains a magmatic belt including

granodiorite Granodiorite () is a coarse-grained (phaneritic) intrusive igneous rock similar to granite, but containing more plagioclase feldspar than orthoclase feldspar. The term banatite is sometimes used informally for various rocks ranging from gra ...

monzogranite Monzogranites are biotite granite rocks that are considered to be the final fractionation product of magma. Monzogranites are characteristically felsic (SiO2 > 73%, and FeO + MgO + TiO2 < 2.4), weakly

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

s, all of them of

calc-alkaline The calc-alkaline magma series is one of two main subdivisions of the subalkaline magma series, the other subalkaline magma series being the tholeiitic series. A magma series is a series of compositions that describes the evolution of a mafic mag ...

chemistry. The igneous rocks of this belt formed at various times in over the period from 555 to 525

million years ago The abbreviation Myr, "million years", is a unit of a quantity of (i.e. ) years, or 31.556926 teraseconds. Usage Myr (million years) is in common use in fields such as Earth science and cosmology. Myr is also used with Mya (million years ago). ...

. From 525 million years ago onward another magmatic belt of peralumineous and

mafic A mafic mineral or rock is a silicate mineral or igneous rock rich in magnesium and iron. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks include ...

rocks developed further amidst

gneiss Gneiss ( ) is a common and widely distributed type of metamorphic rock. It is formed by high-temperature and high-pressure metamorphic processes acting on formations composed of igneous or sedimentary rocks. Gneiss forms at higher temperatures an ...

schist Schist ( ) is a medium-grained metamorphic rock showing pronounced schistosity. This means that the rock is composed of mineral grains easily seen with a low-power hand lens, oriented in such a way that the rock is easily split into thin flakes o ...

amphibolite Amphibolite () is a metamorphic rock that contains amphibole, especially hornblende and actinolite, as well as plagioclase feldspar, but with little or no quartz. It is typically dark-colored and dense, with a weakly foliated or schistose (flaky ...

s and

carbonate rock Carbonate rocks are a class of sedimentary rocks composed primarily of carbonate minerals. The two major types are limestone, which is composed of calcite or aragonite (different crystal forms of CaCO3), and dolomite rock (also known as dolosto ...

s. The igneous rocks of this belt formed in the period from 525 to 515 million years ago.

Tectonic interpretation

The Pampean orogeny can be considered part of the larger Terra Australis orogeny or of the

. The Pampean orogeny developed at a similar time as the Paraguai Belt of the Brasiliano Orogeny, but in difference to the Paraguay Belt that ended up in the interior of

the Pampean Orogen remained at a continental margin. The orogen eventually ceased activity and was succeeded by the

further west. The eastern magmatic belt of the Pampean orogeny is interpreted as the remains of a

volcanic arc A volcanic arc (also known as a magmatic arc) is a belt of volcanoes formed above a subducting oceanic tectonic plate, with the belt arranged in an arc shape as seen from above. Volcanic arcs typically parallel an oceanic trench, with the arc lo ...

associated with an east-dipping subduction zone while the western one is thought to represent a younger volcanic arc that developed on what was once the

accretionary prism 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 do ...

of the orogen. There have been differing views among geologists on the tectonic and paleogeographic position of the Puncoviscana Basin in relation to the events of the Pampean orogeny. The Pampean orogeny is believed by some geologists to be associated with the accretion of a " Pampia Terrane" to the

Río de la Plata Craton The Rio de la Plata Craton (RPC) is a medium-sized continental block found in Uruguay, eastern Argentina and southern Brazil. During its complex and protracted history it interacted with a series other blocks and is therefore considered importan ...

, resulting from the closure of a sea that existed in-between. This sea would have been the Puncoviscana Basin. Víctor Ramos proposes instead that the Puncoviscana Basin was a

foreland basin A foreland basin is a structural basin that develops adjacent and parallel to a mountain belt. Foreland basins form because the immense mass created by crustal thickening associated with the evolution of a mountain belt causes the lithospher ...

located west of a "Pampia block" that collided with Río de la Plata Craton. Contrasting to this view Aceñolaza and Toselli claim instead that the Puncoviscana Basin originated from an

aulacogen An aulacogen is a failed arm of a triple junction. Aulacogens are a part of plate tectonics where oceanic and continental crust is continuously being created, destroyed, and rearranged on the Earth’s surface. Specifically, aulacogens are a ri ...

splitting Arequipa-Antofalla Craton from

Río de la Plata The Río de la Plata (, "river of silver"), also called the River Plate or La Plata River in English, is the estuary formed by the confluence of the Uruguay River and the Paraná River at Punta Gorda. It empties into the Atlantic Ocean and fo ...

and Guaporé Craton. Following this interpretation the aulacogen would have closed during the Pampean orogeny.

Notes

References

{{Major South American geological formations Orogenies of South America Geology of Argentina Geology of Córdoba Province, Argentina Cambrian orogenies Cambrian South America Sierras Pampeanas