The Andean orogeny () is an ongoing process of orogeny that began in the Early Jurassic and is responsible for the rise of the Andes mountains. The orogeny is driven by a reactivation of a long-lived subduction system along the western margin of South America. On a continental scale the Cretaceous (90 Ma) and Oligocene (30 Ma) were periods of re-arrangements in the orogeny. The details of the orogeny vary depending on the segment and the geological period considered.
Overview Subduction orogeny has been occurring in what is now western South America since the break-up of the supercontinent Rodinia in the Neoproterozoic. The Paleozoic Pampean, Famatinian and Gondwanan orogenies are the immediate precursors to the later Andean orogeny.Charrier et al. 2006, pp. 113–114. The first phases of Andean orogeny in the Jurassic and Early Cretaceous were characterized by extensional tectonics, rifting, the development of back-arc basins and the emplacement of large batholiths. This development is presumed to have been linked to the subduction of cold oceanic lithosphere. During the mid to Late Cretaceous (ca. 90 million years ago) the Andean orogeny changed significantly in character. 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 contractional deformation that different lithologies were subject to, but also the uplift and erosion known to have occurred from the Late Cretaceous onward. Plate tectonic reorganization since the mid-Cretaceous might also have been linked to the opening of the South Atlantic Ocean. 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. While subduction direction changed it remained oblique (and not perpendicular) to the coast of South America, and the direction change affected several subduction zone-parallel faults including Atacama, Domeyko and Liquiñe-Ofqui.Charrier et al. 2006, pp. 45–46.
Low angle subduction or flat-slab subduction has been common during the Andean orogeny leading to crustal shortening and deformation and the suppression of arc 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.
The tectonic growth of the Andes and the regional climate have evolved simultaneously and have influenced each other. 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. 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.
In the Oligocene the Farallon Plate broke up, forming the modern Cocos and Nazca 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 the Miocene. 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).
Orogeny by segment
Colombia, Ecuador and Venezuela (12° N–3° S)
Tectonic blocks of continental crust that had separated from northwestern South America in the Jurassic re-joined the continent in the Late Cretaceous by colliding obliquely with it. This episode of accretion occurred in a complex sequence. The accretion of the island arcs against northwestern South America in the Early Cretaceous led to the development of a magmatic arc caused by subduction. The Romeral Fault in Colombia forms the suture between the accreted terranes and the rest of South America. Around the Cretaceous–Paleogene boundary (ca. 65 million years ago) the oceanic plateau of the Caribbean large igneous province collided with South America. The subduction of the lithosphere as the oceanic plateau approached South America led to the formation of a magmatic arc now preserved in the Cordillera Real of Ecuador and the Cordillera Central of Colombia. In the Miocene an island arc and terrane (Chocó terrane) collided against northwestern South America. This terrane forms parts of what is now Chocó Department and Western Panama.
The Caribbean Plate collided with South America in the Early Cenozoic but shifted then its movement eastward. Dextral fault movement between the South American and Caribbean plate started 17–15 million years ago. This movement was canalized along a series of strike-slip faults, but these faults alone do not account for all deformation. The northern part of the Dolores-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.
Northern Peru (3–13° S)
Long before the Andean orogeny the northern half of Peru was subject of the accretion of terranes in the Neoproterozoic and Paleozoic. Andean orogenic deformation in northern Peru can be traced to the Albian (Early Cretaceous). This first phase of deformation —the Mochica Phase— is evidenced in the folding of Casma Group sediments near the coast.
Sedimentary basins in western Peru changed from marine to continental conditions in the Late 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 Piñón terrane in Ecuador. This stage of orogeny is called the Peruvian Phase. Besides coastal Peru the Peruvian Phase affected or caused crustal shortening along the Cordillera Oriental and the tectonic inversion of Santiago Basin in the Sub-Andean zone. The bulk of the Sub-Andean zone was however unaffected by the Peruvian Phase.
After a period without much tectonic activity in the Early Eocene the Incaic Phase of orogeny occurred in the Mid and Late Eocene. No other tectonic event in the western Peruvian Andes compare with the Incaic Phase in magnitude. Horizontal shortening during the Incaic Phase resulted in the formation of the Marañón fold and thrust belt. An unconformity cutting across the Marañón fold and thrust belt show the Incaic Phase ended no later than 33 million years ago in the earliest Oligocene.
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. The Quechua 1 Phase lasted from 17 to 15 million years ago and included a reactivation of Inca Phase structures in the Cordillera Occidental. 9–8 million years ago, in the Quechua 2 Phase, the older parts of the Andes in northern Peru were thrusted to the northeast. Most of the Sub-Andean zone of northern Peru deformed 7–5 million years ago (Late Miocene) during the Quechua 3 Phase. The Sub-Andean stacked in a thrust belt.
The Miocene rise of the Andes in Peru and Ecuador led to increased orographic precipitation along its eastern parts and to the birth of the modern Amazon River. One hypothesis links these two changes by assuming that increased precipitation led to increased erosion and this erosion led to filling the Andean foreland basins beyond their capacity and that it would have been the basin over-sedimentation rather than the rise of the Andes that made drainage basins flow to the east. Previously the interior of northern South America drained to the Pacific.
Bolivian Orocline (13–26° S) Early Andean subduction in the Jurassic formed a volcanic arc in northern Chile known as La Negra Arc. The remnants of this arc are now exposed in the Chilean Coast Range. Several plutons were emplaced in the Chilean Coast Range in the Jurassic and Early Cretaceous including the Vicuña Mackenna Batholith.Charrier et al. 2006, pp. 47–48. Further east at similar latitudes, in Argentina and Bolivia, the Salta rift system developed during the Late Jurassic and the Early Cretaceous. Salar de Atacama Basin, which is thought to be the western arm of the rift system, accumulated during the Late Cretaceous and Early Paleogene a >6,000 m thick pile of sediments now known as the Purilactis Group.
Pisco Basin, around latitude 14° S, was subject to a marine transgression in the Oligocene and Early Miocene epochs (25–16 Ma). In contrast Moquegua 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.
From the Late Miocene onward the region that would become the Altiplano rose from low elevations to more than 3,000 m.a.s.l. It is estimated that the region rose 2000 to 3000 meters in the last ten million years.Charrier et al. 2006, pp. 100–101. Together with this uplift several valleys incised in the western flank of the Altiplano. In the Miocene the Atacama Fault moved, uplifting the Chilean Coast Range and creating sedimentary basins east of it.Charrier et al. 2006, p. 97. At the same time the Andes around the Altiplano region broadened to exceed any other Andean segment in width. Possibly about 1000 km of lithosphere has been lost due to lithospheric shortening. During subduction the western end of the forearc region flexured downward forming a giant monocline. Somewhat to the south, tectonic inversion belonging during the "Incaic Phase" (Eocene?) have tilted the strata of Purilactis Group and in some localities also thrust younger strata on top of it.
The region east of the Altiplano is characterized by deformation and tectonics along a complex fold and thrust belt. Over-all the region surrounding the Altiplano and Puna plateaux has been horizontally shortened since the Eocene. In southern Bolivia lithospheric shortening has made the Andean 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. Along the Argentine Northwest the Andean uplift has caused Andean foreland basins to separate into several minor isolated intermontane sedimentary basins. Towards the east the piling up of crust in Bolivia and the Argentine Norwest caused a north-south forebulge known as Asunción arch to develop in Paraguay.
The uplift of the Altiplano is thought to be indebted to a combination of horizontal shortening of the crust and to increased temperatures in the mantle (thermal thinning). The bend in the Andes and the west coast of South America known as the Bolivian Orocline was enhanced by Cenozoic horizontal shortening but existed already independently of it.