Andean orogeny

The Andean orogeny (Spanish: Orogenia andina) 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.

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. 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.

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).

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.

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.