The origin of the Andes’ curved shape discovered

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Through the analysis of new paleomagnetic data from Ecuador, the rotations of the Western Cordillera – which are reflected in the curved shape of the Andean chain – were highlighted and the nature of the Inter-Andean Valley was put in evidence

A team of researchers from the Istituto Nazionale di Geofisica e Vulcanologia (Ingv), the ESPOL University of Guayaquil in Ecuador and the University of Toronto in Canada, have shown that the Western Cordillera of Ecuador has undergone a clockwise rotation of about 20° in the last 10 million years. Comparing this result with previous investigations along the Andes, they found out that the typical convex shape of the Andes in Ecuador was generated by paleomagnetic rotations in opposite directions, clockwise and counterclockwise, of the two edges of the orogenic bend. The orogenesis process has therefore determined the bending of the mountain range which previously had a more rectilinear shape.

The study Significance of northern Andes terrane extrusion and genesis of the Interandean Valley: Paleomagnetic evidence from the “Ecuadorian Orocline”  has been published in the AGU journal ‘Tectonics’.

The Andes represent one of the most spectacular mountain ranges in the globe. They run for an impressive 8,000 km length along the western edge of the South American plate, from the Caribbean to Tierra del Fuego. Their origin is due to the subduction (that is, to the sinking of a lithospheric plate below the one immediately adjacent) that has lasted for at least 200 million years of the oceanic sectors of various plates (Caribbean, Nazca, Antarctic plate) below the South American plate.

New paleomagnetic data were obtained by Ingv researchers (Gaia Siravo and Fabio Speranza) in collaboration with researchers from the ESPOL University of Guayaquil (Ecuador) and the University of Toronto (Canada).

During a three-week campaign in October 2019, which was conducted for the Ingv by Gaia Siravo and Fabio Speranza, over 300 oriented samples were taken in the Western and Royal Cordillera of Ecuador, constituting the first ever paleomagnetic dataset obtained in this area. The samples, coming from volcanic rocks between 10 and 40 million years old, were brought to Rome and analyzed in the Ingv’s Paleomagnetism Laboratory. These data highlighted the rotations that accompanied the Andean chain’s orogenic events.

The researchers also dubbed the Ecuadorian Andean arc the “Ecuadorian orocline” and discovered further implications for its formation.

“From our studies it emerged that its geometry is similar to that of the Amazonian Craton, a very ancient and rigid portion of the South American plate, dating back to over 1 billion years ago”, says Gaia Siravo, Ingv researcher and first author of the study, which continues “It is probable that the original shape of this rigid block limited the propagation of the orogenic deformation, imposing the same geometry on the adjacent mountain range, and that therefore the Andes followed the curvature of the western edge of the Amazonian Craton”.

“The new paleomagnetic data have made it possible to understand also the formation of the Inter-Andean Valley. It is a depression placed at mean elevation of 2,500 m which, along the eastern edge of the Western Cordillera, longitudinally interrupts the Andean chain, formed by peaks and plateaus around 3,000-4,000 meters”, explains Fabio Speranza, Director of the Ingv’s Rome 2 Section.

“The new data show that the formation age of the Valley dates back to about 10 million years and is substantially coeval with the age of the clockwise rotation observed in the Western Cordillera. It is therefore likely that the Valley is the reflection of the emplacement of the rotational thrust of the Western Cordillera, which would have created a long and narrow tectonic depression in front of the moving tectonic unit”, concludes Fabio Speranza.