El Oroclino del Maipo: Un rasgo estructural de primer orden en la evolución geodinámica Mioceno a Reciente en los Andes de Chile central.

Abstract

ABSTRACT. In this paper we show the results of a paleomagnetic study carried out along the western Andes of central Chile from 30° to 36°S. Whereas paleomagnetic analysis from Jurassic to Late Cretaceous rocks in the Pampean flat slab segment shows small or non significant clockwise vertical-axis rotations, results obtained in Late Jurassic to Neogeone rocks to the south, on the normal subduction segment, show sistematically clockwise rotations up to 40º. Paleomagnetic rotations are coeval with thrusting along the High Andes in Late Miocene times. We propose a coupled Bolivian Orocline-Juan Fernández Ridge (JFR) model where a far-field component of clockwise rotation related to the formation of the Bolivian Orocline was first acquired by the margin between 31º and 33ºS. The same area was later rotated by a slight counterclockwise component related to the subduction of the JFR over the last 10 Ma. South of 33ºS the clockwise rotations related to the Bolivian Orocline component were amplified by the subduction of the JFR. Keywords: Central Andes, Maipo Orocline, Bolivian Orocline, Juan Fernández Ridge, Paleomagnetism. RESUMEN. El Oroclino del Maipo: Un rasgo estructural de primer orden en la evolución geodinámica Mioceno a Reciente en los Andes de Chile central. En este trabajo se muestran los resultados de un estudio paleomagnético realizado a lo largo de los Andes chilenos entre los 30º y 36ºS. Mientras que los análisis paleomagnéticos en rocas del Jurásico al Cretácico Tardío del segmento Pampeano de subducción plana muestran rotaciones de eje vertical en sentido horario pequeñas o poco significantes, los resultados obtenidos en las rocas del Jurásico Tardío al Neógeno, al sur, en el segmento de subducción normal, sistemáticamente muestran rotaciones horarias de hasta 40º. Estas rotaciones paleomagnéticas son coetáneas con cabalgamientos a lo largo de los Andes Principales durante el Mioceno Tardío. Nosotros proponemos un modelo acoplado entre el Oroclino Boliviano y el Ridge de Juan Fernández (RJF) donde una componente de campo lejano de rotación horaria relacionada con la formación del Oroclino Boliviano fue en una primera etapa adquirida por el margen entre los 31º y 33ºS. La misma región fue posteriormente rotada por una leve componente antihoraria asociada con la subducción del RJF cerca de los 10 Ma. Al sur de los 33ºS, las rotaciones horarias relacionadas con la componente del Oroclino Boliviano fueron amplificadas por la subducción del RJF.

Latitudinal and Longitudinal Patterns of Exhumation in the Andes of North‐Central Chile

“…Some authors have interpreted that the main periods of exhumation and shortening in north-central Chile are a consequence of the arrival to the studied region of the southward propagating subduction of the Juan Fernandez ridge (Arriagada et al, 2013;Cembrano et al, 2003). The ridge reached the northern limit of studied region at 28°S around 12 Ma ago and the southern limit at 32°S around 10 Ma ago (Yáñez et al, 2001).…”

“…However, the measured AFT ages approximately 43-30 Ma (Figure 8b) cannot be ruled out as they indicate that rocks from the Main Cordillera next to the topographic front south of 31.5°S were cooled through PAZ coevally or shortly after their emplacement given by U-Pb ages at approximately 46 Ma (Figure 3). Although no late Eocene reverse faults have been recognized south of 30°S, paleomagnetic reconstructions show that block rotations between 31.5 and 33°S mainly occurred in the early to mid-Miocene but after an initial Eocene-Oligocene phase (Arriagada et al, 2013). Thus, AFT ages approximately 43-30 Ma close to the topographic front south of 31.5°S are probably related to this earlier phase.…”

Neogene landscape evolution in the Andes of north-central Chile between 28 and 32°S: interplay between tectonic and erosional processes

“…The Incaic Orogenic Phase corresponds to an important episode of shortening, uplift and exhumation widely recognized throughout the Domeyko Cordillera in northern Chile during the Eocene and Oligocene. Palaeomagnetic data indicate that Eocene -Oligocene clockwise palaeomagnetic rotations become mostly zero south of 318S (Arriagada et al 2009(Arriagada et al , 2013. Therefore, it has been interpreted that the study area includes the southern limit of Incaic deformation (Arriagada et al 2009(Arriagada et al , 2013.…”

“…Palaeomagnetic data indicate that Eocene -Oligocene clockwise palaeomagnetic rotations become mostly zero south of 318S (Arriagada et al 2009(Arriagada et al , 2013. Therefore, it has been interpreted that the study area includes the southern limit of Incaic deformation (Arriagada et al 2009(Arriagada et al , 2013. According to structural and geochronological data, Eocene compression in the Huasco Valley was associated with inversion of previous Lower Cretaceous extensional basins by a series of low-angle faults located between the San Félix and La Totora Faults in the western and central Frontal Cordillera (Figs 3 & 5, Salazar 2012).…”

“…During the Eocene and Oligocene the Incaic Range was the main palaeogeographic feature to the north of 318S ( Fig. 1; Maksaev & Zentilli 1999;Charrier et al 2007;Arriagada et al 2009Arriagada et al , 2013Bissig & Riquelme 2010), while an extensional volcanosedimentary basin, known as the Abanico Basin (Charrier et al 2002), developed south of 328S.…”