Counterclockwise rotation of late Eocene–Oligocene fore‐arc deposits in southern Peru and its significance for oroclinal bending in the central Andes

Abstract

The results of a paleomagnetic study along the fore arc of southern Peru (15–18°S) and northern Chile (18–19°S) are reported from middle to late Miocene ignimbrites (7 sites), late Oligocene to early Miocene ignimbrites (72 sites), Paleogene sediments (20 sites), and Mesozoic and Paleocene volcanics and intrusions (31 sites). Comparison of locality‐mean directions with expected paleomagnetic directions indicates vertical axis rotations ranging from 5.2 ± 11.3° clockwise to 55.6 ± 7.0° counterclockwise. Spatially, the magnitude of counterclockwise rotations increases northward from ∼0° within the Chilean fore arc south of 18°30′S to >45° north of 16°30′S. In southern Peru, paleomagnetic rotations recorded in Paleogene red beds decrease from late Eocene to late Oligocene, whereas Miocene ignimbrites display no evidence of rotation. These new results confirm that the rotations recorded in the fore arc of southern Peru were acquired at least before ∼15 Ma, and probably before 25 Ma, and thus prior to the late Neogene shortening of the sub‐Andes. The onset of major Andean shortening in the Eastern Cordillera during the latest Eocene–earliest Oligocene is interpreted to have triggered the bending of the Peruvian fore arc. The region of the Peruvian fore arc with the largest rotations appears to be the fore‐arc counterpart of the Abancay deflection, a remarkable NE‐SW offset in the axis of the Eastern Cordillera induced by a major regional preorogenic structure. We underline that the Abancay deflection should be seen as the northwestern boundary, and therefore as a key element, of the Bolivian Orocline.

Slow Long‐Term Exhumation of the West Central Andean Plate Boundary, Chile

“…Subduction of the Nazca (and previously Farallon) Plate under the South American Plate has resulted in a mountain belt characterized by the largest trench‐to‐peak relief on the planet, frequent seismic activity ( M w  > 8.5), and a long‐standing history of volcanism (e.g., Armijo et al, 2015). Notably, three distinctly oriented sections of the range (Figure 1) may be the result of differential shortening recording periods of Andean deformation and uplift (Allmendinger et al, 1997; Isacks, 1988; Roperch et al, 2006), though this uniquely bent geometry may itself lead to focused deformation in the upper plate, particularly at plate corners (e.g., Bendick & Ehlers, 2014; Jordan et al, 1983). Other tectonic controls have been invoked to explain variation in Andean morphology (i.e., angle of convergence, convergence velocity, age of oceanic crust, and trench orientation), though no clear consensus exists on their relative importance (e.g., Barnes & Ehlers, 2009; Maloney et al, 2013; Oncken et al, 2006).…”

Constricción neógena en la Cordillera de la Costa, norte de Chile: neotectónica y datación de superficies con 21Ne cosmogónico

“…La discontinuidad cortical más relevante en la zona en estudio es sin duda el Sistema de Fallas de Atacama (SFA), que se dispone orientado paralelo a la fosa, y cuya geometría cretácica induce una importante herencia en la deformación actual. El registro paleomagnético de esta porción del oroclino no evidencia rotaciones mayores a 5º después del Mioceno (Arriagada et al, 2000;Coutand et al, 1999;Roperch et al, , 2006. La información geodésica del campo de deformación actual, indica que la mecánica de deformación del oroclino aun está activa (Allmendinger et al, 2005b), sin embargo no ha sido lo sufi cientemente efectiva después del Mioceno para producir deformación permanente que registre una rotación de bloques mayor a 5º.…”

Seismic-stratigraphic architecture of the Oligocene-Pliocene Camaná Formation, southern Peruvian forearc (Province of Arequipa)

“…This arrangement consists of (aparently) sinistral ~NW-SE wrench faulting that is interpreted to have facilitated uplift of the Coastal Cordillera (probably showing also sinistral behavior, i.e., IIIFS, as interpreted in Western Cordillera (Sempere and Jacay, 2006;Alván et al, 2015). A transtensive tectonic arrangement for southern Perú could be consistent with tectonic rotations as proposed Roperch et al (2006). Schildgen et al (2009), Decou et al (2013), Alván et al (2015) and Alván (2015) demonstrated that Western Cordillera in the study area experimented uplift with consequente denudation and deposition of sediments (Fig.…”

“…Fault scarps are mostly ~NW-SE oriented, and are visible along the sea floor up to the offshore of northern Chile (von Huene et al, 1996). If we prolongate alignments of the submarine canyons back to onshore, we observe that the thickest stackings of the Camaná Formation coincides with the river mouths of the large valleys at La Chira (16°30' S), Camaná (16°38' S), La Virgen (16°43' S), and Punta de Bombón (17°15' S) (Pecho and Morales, 1969;Sempere et al, 2004;Roperch et al, 2006).…”

“…According to Roperch et al (2006) and Sempere and Jacay (2006), transtensional tectonics occurred as "pulses" along the forearc during Cenozoic and it was progressive, being favorable for creation of single ~NE-SW elongated depocentres (or sub-basins, e.g., Caravelí sub-basin, Marocco et al, 1985;Huamán, 1985) such as stackings of sedimentary rocks defined as Camaná, La Virgen, and Punta de Bombón offshore depocentres (orange circles in Fig. 7A).…”