TheAndean orogeny (Spanish:Orogenia andina) is an ongoing process oforogeny that began in theEarly Jurassic and is responsible for the rise of theAndes mountains. The orogeny is driven by a reactivation of a long-livedsubduction system along the western margin ofSouth America. On a continental scale theCretaceous (90Ma) andOligocene (30 Ma) wereperiods of re-arrangements in the orogeny. The details of the orogeny vary depending on the segment and the geological period considered.
Subduction orogeny has been occurring in what is now western South America since the break-up of thesupercontinentRodinia in theNeoproterozoic.[1] ThePaleozoicPampean,Famatinian andGondwanan orogenies are the immediate precursors to the later Andean orogeny.[2] The first phases of Andean orogeny in theJurassic andEarly Cretaceous were characterized byextensional tectonics,rifting, the development ofback-arc basins and the emplacement of largebatholiths.[1][3] This development is presumed to have been linked to the subduction of coldoceaniclithosphere.[3] During the mid toLate Cretaceous (ca. 90 million years ago) the Andean orogeny changed significantly in character.[1][3] Warmer and younger oceanic lithosphere is believed to have started to be subducted beneath South America around this time. Such kind of subduction is held responsible not only for the intense contractionaldeformation that different lithologies were subject to, but also theuplift anderosion known to have occurred from the Late Cretaceous onward.[3]Plate tectonic reorganization since the mid-Cretaceous might also have been linked to theopening of theSouth Atlantic Ocean.[1] Another change related to mid-Cretaceous plate tectonic changes was the change of subduction direction of the oceanic lithosphere that went from having south-east motion to having a north-east motion at about 90 million years ago.[4] While subduction direction changed it remained oblique (and not perpendicular) to the coast of South America, and the direction change affected severalsubduction zone-parallel faults includingAtacama,Domeyko andLiquiñe-Ofqui.[3][4]
Low angle subduction orflat-slab subduction has been common during the Andean orogeny leading to crustal shortening and deformation and the suppression ofarc volcanism. Flat-slab subduction has occurred at different times in various part of the Andes, with northern Colombia (6–10° N), Ecuador (0–2° S), northern Peru (3–13° S) and north-central Chile (24–30° S) experiencing these conditions at present.[1]
The tectonic growth of the Andes and the regional climate have evolved simultaneously and have influenced each other.[5] The topographic barrier formed by the Andes stopped the income of humid air into the present Atacama desert. This aridity, in turn, changed the normal superficial redistribution of mass via erosion and river transport, modifying the later tectonic deformation.[5] The lack of sediment in the trench to lubricate the subducting plate allows for intense deformation and erosion of the lower crust. The removal of the lower part of the upper plate causes localized extensional tectonics due to gravitational collapse of the upper crust, evident by normal faulting in the forearc.[6]
In the Oligocene theFarallon Plate broke up, forming the modernCocos andNazca plates ushering a series of changes in the Andean orogeny. The new Nazca Plate was then directed into an orthogonal subduction with South America causing ever-since uplift in the Andes, but causing most impact in theMiocene. While the various segments of the Andes have their own uplift histories, as a whole the Andes have risen significantly in last 30 million years (Oligocene–present).[7]
Tectonic blocks ofcontinental crust that had separated from northwestern South America in the Jurassic re-joined the continent in the Late Cretaceous by colliding obliquely with it.[7] This episode ofaccretion occurred in a complex sequence. The accretion of the island arcs against northwestern South America in the Early Cretaceous led to the development of amagmatic arc caused by subduction. TheRomeral Fault in Colombia forms thesuture between the accreted terranes and the rest of South America. Around the Cretaceous–Paleogene boundary (ca. 65 million years ago) theoceanic plateau of theCaribbean large igneous province collided with South America. The subduction of thelithosphere as the oceanic plateau approached South America led to the formation of a magmatic arc now preserved in theCordillera Real of Ecuador and theCordillera Central of Colombia. In the Miocene anisland arc andterrane (Chocó terrane) collided against northwestern South America. This terrane forms parts of what is nowChocó Department and WesternPanama.[1]
TheCaribbean Plate collided with South America in the Early Cenozoic but shifted then its movement eastward.[7][8]Dextral fault movement between the South American and Caribbean plate started 17–15 million years ago. This movement was canalized along a series ofstrike-slip faults, but these faults alone do not account for all deformation.[9] The northern part of theDolores-Guayaquil Megashear forms part of the dextral fault systems while in the south the megashear runs along the suture between the accreted tectonic blocks and the rest of South America.[10]
Long before the Andean orogeny the northern half of Peru was subject of theaccretion ofterranesin theNeoproterozoic andPaleozoic.[11] Andean orogenic deformation in northern Peru can be traced to theAlbian (Early Cretaceous).[12] This first phase of deformation —the Mochica Phase[A]— is evidenced in thefolding ofCasma Group sediments near the coast.[11]
Sedimentary basins in western Peru changed from marine to continental conditions in theLate Cretaceous as a consequence of a generalized vertical uplift. The uplift in northern Peru is thought to be associated with the contemporary accretion of the Piñónterrane in Ecuador. This stage of orogeny is called the Peruvian Phase.[11] Besides coastal Peru the Peruvian Phase affected or caused crustal shortening along theCordillera Oriental and thetectonic inversion of Santiago Basin in theSub-Andean zone. The bulk of the Sub-Andean zone was however unaffected by the Peruvian Phase.[13]
After a period without much tectonic activity in the Early Eocene the Incaic Phase of orogeny occurred in the Mid and Late Eocene.[12][13] No other tectonic event in the western Peruvian Andes compare with the Incaic Phase in magnitude.[12][13] Horizontal shortening during the Incaic Phase resulted in the formation of theMarañón fold and thrust belt.[12] Anunconformity cutting across the Marañón fold and thrust belt show the Incaic Phase ended no later than 33 million years ago in the earliest Oligocene.[11]
In the period after the Eocene the Northern Peruvian Andes were subject to the Quechua Phase of orogeny. The Quechua Phase is divided into the sub-phases Quechua 1, Quechua 2 and Quechua 3.[B] The Quechua 1 Phase lasted from 17 to 15 million years ago and included a reactivation of Inca Phasestructures in theCordillera Occidental.[C] 9–8 million years ago, in the Quechua 2 Phase, the older parts of the Andes in northern Peru werethrusted to the northeast.[11] Most of theSub-Andean zone of northern Peru deformed 7–5 million years ago (Late Miocene) during the Quechua 3 Phase.[11][13] The Sub-Andean stacked in athrust belt.[11]
The Miocene rise of the Andes in Peru and Ecuador led to increasedorographic precipitation along its eastern parts and to the birth of the modernAmazon River. Onehypothesis links these two changes by assuming that increased precipitation led to increasederosion and this erosion led to filling theAndean foreland basins beyond their capacity and that it would have been the basin over-sedimentation rather than the rise of the Andes that madedrainage basins flow to the east.[13] Previously the interior of northern South America drained to the Pacific.
Early Andean subduction in the Jurassic formed a volcanic arc in northern Chile known asLa Negra Arc.[D] The remnants of this arc are now exposed in theChilean Coast Range. Severalplutons wereemplaced in the Chilean Coast Range in the Jurassic and Early Cretaceous including theVicuña Mackenna Batholith.[15] Further east at similar latitudes, in Argentina and Bolivia, theSalta rift system developed during the Late Jurassic and the Early Cretaceous.[16]Salar de Atacama Basin, which is thought to be thewestern arm of the rift system,[17] accumulated during theLate Cretaceous andEarly Paleogene a >6,000 m thick pile of sediments now known as thePurilactis Group.[18]
Pisco Basin, around latitude 14° S, was subject to amarine transgression in theOligocene andEarly Miocene epochs (25–16Ma[19]).[20] In contrastMoquegua Basin to the southeast and the coast to south of Pisco Basin saw no transgression during this time but a steadily rise of the land.[20]
From theLate Miocene onward the region that would become theAltiplano rose from low elevations to more than 3,000m.a.s.l. It is estimated that the region rose 2000 to 3000 meters in the last ten million years.[21] Together with this uplift several valleys incised in the western flank of the Altiplano. In the Miocene theAtacama Fault moved, uplifting the Chilean Coast Range and creating sedimentary basins east of it.[22] At the same time the Andes around the Altiplano region broadened to exceed any other Andean segment in width.[7] Possibly about 1000 km oflithosphere has been lost due to lithospheric shortening.[23] During subduction the western end of theforearc region[E]flexured downward forming a giantmonocline.[24][25] Somewhat to the south,tectonic inversion belonging during the "Incaic Phase" (Eocene?) have tilted the strata ofPurilactis Group and in some localities alsothrust younger strata on top of it.[26]
The region east of the Altiplano is characterized by deformation and tectonics along a complexfold and thrust belt.[24] Over-all the region surrounding the Altiplano andPuna plateaux has been horizontally shortened since theEocene.[27] In southern Bolivia lithospheric shortening has made theAndean foreland basin to move eastward relative to the continent at an average rate of ca. 12–20 mm per year during most of the Cenozoic.[23][F] Along theArgentine Northwest the Andean uplift has caused Andean foreland basins to separate into several minor isolated intermontane sedimentary basins.[28] Towards the east the piling up of crust in Bolivia and the Argentine Norwest caused a north-southforebulge known asAsunción arch to develop in Paraguay.[29]
The uplift of the Altiplano is thought to be indebted to a combination ofhorizontal shortening of the crust and to increased temperatures in the mantle (thermal thinning).[1][24] The bend in the Andes and the west coast of South America known as theBolivian Orocline was enhanced by Cenozoichorizontal shortening but existed already independently of it.[24]
Meso-scale tectonic processes aside, the particular characteristics of the Bolivian Orocline–Altiplano region have been attributed to a variety of deeper causes. These causes include a local steepening of the subduction angle of Nazca Plate, increased crustal shortening and plate convergence between the Nazca and South American plates, an acceleration in the westward drift of the South American Plate, and a rise in theshear stress between the Nazca and South American plates. This increase in shear stress could in turn be related to the scarcity of sediments in theAtacama trench which is caused by the arid conditions alongAtacama Desert.[7] Capitanioet al. attributes the rise of Altiplano and the bending of the Bolivian Orocline to the varying ages of the subducted Nazca Plate with the older parts of the plate subducting at the centre of the orocline.[30] As Andrés Tassara puts it therigidity of the Bolivian Oroclinecrust is derivative of thethermal conditions. The crust of the western region (forearc) of the orocline has been cold and rigid, resisting and damming up the westward flow of warmer and weakerductile crustal material from beneath the Altiplano.[25]
The Cenozoic orogeny at the Bolivian orocline has produced a significantanatexis of crustal rocks includingmetasediments andgneisses resulting in the formation ofperaluminousmagmas. These characteristics imply that the Cenozoic tectonics and magmatism in parts of Bolivian Andes is similar to that seen incollisionalorogens. The peralumineous magmatism inCordillera Oriental is the cause of the world-classmineralizations of theBolivian tin belt.[31]
The rise of the Altiplano is thought by scientist Adrian Hartley to have enhanced an already prevailingaridity or semi-aridity inAtacama Desert by casting arain shadow over the region.[32]
At the latitudes between 17 and 39° S the Late Cretaceous and Cenozoic development of the Andean orogeny is characterized by an eastward migration of themagmatic belt and the development ofseveral foreland basins.[3] The eastward migration of the arc is thought to be caused bysubduction erosion.[33]
At the latitudes of 32–36° S —that isCentral Chile and most ofMendoza Province— the Andean orogeny proper began in the Late Cretaceous whenbackarc basins wereinverted. Immediately east of the early Andes foreland basins developed and theirflexural subsidence caused the ingression of waters from the Atlantic all the way to the front of the orogen in theMaastrichtian.[34] The Andes at the latitudes of 32–36° S experienced a sequence of uplift in the Cenozoic that started in the west and spread to the east. Beginning about 20 million years ago in theMiocene thePrincipal Cordillera (east of Santiago) began an uplift that lasted until about 8 million years ago.[34] From the Eocene to the early Miocene, sediments[G] accumulated in theAbanico Extensional Basin, a north-south elongated basin in Chile that spanned from 29° to 38° S. Tectonic inversion from 21 to 16 million years ago made the basin to collapse and the sediments to be incorporated to the Andean cordillera.[35] Lavas and volcanic material that are now part of Farellones Formation accumulated while the basin was being inverted and uplifted.[36] The Miocenecontinental divide was about 20 km to the west of the modern water divide that makes up theArgentina–Chile border.[36] Subsequentriver incision shifted the divide to the east leaving old flattish surfaces hanging.[36] Compression and uplift in this part of the Andes has continued into the present.[36] The Principal Cordillera had risen to heights that allowed for the development of valley glaciers about 1 million years ago.[36]
Before the Miocene uplift of the Principal Cordillera was over, theFrontal Cordillera to the east started a period of uplift that lasted from 12 to 5 million years ago. Further east thePrecordillera was uplifted in the last 10 million years and theSierras Pampeanas has experienced a similar uplift in the last 5 million years. The more eastern part of the Andes at these latitudes had their geometry controlled by ancient faults dating to theSan Rafael orogeny of thePaleozoic.[34] TheSierras de Córdoba (part of the Sierras Pampeanas) where the effects of the ancientPampean orogeny can be observed, owes it modern uplift and relief to the Andean orogeny in the lateCenozoic.[37][38] Similarly theSan Rafael Block east of the Andes and south of Sierras Pampeanas was raised in the Miocene during the Andean orogeny.[39] In broad terms the most active phase of orogeny in area of southern Mendoza Province and northern Neuquén Province (34–38° S) happened in the Late Miocene whilearc volcanism occurred east of the Andes.[39]
At more southern latitudes (36–39° S) various Jurassic and Cretaceousmarine transgressions from the Pacific are recorded in the sediments ofNeuquén Basin.[H] In the Late Cretaceous conditions changed. Amarine regression occurred and thefold and thrust belts of Malargüe (36°00 S), Chos Malal (37° S) and Agrio (38° S) started to develop in the Andes and did so in untilEocene times. This meant an advance of the orogenic deformation since the Late Cretaceous that caused the western part ofNeuquén Basin to stack in the Malargüe and Agrio fold and thrust belts.[40][39] In theOligocene the western part of the fold and thrust belt was subject to a short period ofextensional tectonics whose structures were inverted in theMiocene.[40][I] After a period of quiescence the Agrio fold and thrust belt resumed limited activity in the Eocene and then again in the Late Miocene.[39]
In the south of Mendoza Province the Guañacos fold and thrust belt (36.5° S) appeared and grew in thePliocene andPleistocene consuming the western fringes of the Neuquén Basin.[40][39]
![[icon]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aWiki_letter_w_cropped.svg) | This sectionneeds expansion. You can help byadding to it.(August 2016) |
| This sectionneeds expansion. You can help byadding to it.(December 2015) |
The early development of the Andean orogeny in southernmost South America affected also theAntarctic Peninsula.[43] In southernPatagonia at the onset of the Andean orogeny in theJurassic,extensional tectonics created theRocas Verdes Basin, aback-arc basin whose southeastern extension survives as theWeddell Sea in Antarctica.[43][44] In theLate Cretaceous the tectonic regime of Rocas Verdes Basin changed leading to its transformation into a compressionalforeland basin –theMagallanes Basin– in theCenozoic. This change was associated with an eastward move of the basindepocenter and theobduction ofophiolites.[43][44] The closure of Rocas Verdes Basin in the Cretaceous is linked to thehigh-grade metamorphism ofCordillera Darwin Metamorphic Complex in southernTierra del Fuego.[45]
As the Andean orogeny went on, South America drifted away from Antarctica during the Cenozoic leading first to the formation of anisthmus and then to the opening of theDrake Passage 45 million years ago. The separation from Antarctica changed the tectonics of the Fuegian Andes into atranspressive regime withtransform faults.[43][J]
About 15 million years ago in theMiocene theChile Ridge began to subduct beneath the southern tip of Patagonia (55° S). The point of subduction, thetriple junction has gradually moved to the north and lies at present at 47° S. The subduction of the ridge has created a northward moving "window" orgap in the asthenosphere beneath South America.[46]
{{cite book}}:|journal= ignored (help){{cite book}}:|journal= ignored (help)Major South American geological features | |||||
|---|---|---|---|---|---|
| Tectonic plates |  | ||||
| Cratons andshields | |||||
| Structures undergoingsubduction | |||||
| Faults andshear zones | |||||
| Rifts andgrabens | |||||
| Sedimentary basins | |||||
| Orogenies | |||||
| Metallogenetic provinces | |||||
| Volcanism |
| ||||
