THE CENTRAL ANDEAN ROTATION PATTERN IN NORTHERN CHILE: IMPLICATIONS FOR UPLIFT OF THE ANDEAN PLATEAU AND BOLIVIAN OROCLINE FORMATION

Recent paleomagnetic studies we have carried out in the modern forearc of northern Chile show that a large part of the rotations leading to the formation of the Central Andean Rotation Pattern (CARP) are closely related to the earliest stages of development of the Bolivian Orocline during the Eocene-Oligocene. A further detailed paleomagnetic study has allowed us to precise the general CARP geometry in northern and central Chile which there includes three distinct “rotation domains”: From Arica to Antofagasta (18º-22ºS) no evidences of tectonic rotations have been found along a 400 km long section of the Coastal Cordillera. Further south, a ~800 km long domain that shows systematically clockwise rotations up to 60º embracing the whole forearc region occurs between Antofagasta and Ovalle (22º-31ºS). Finally, south of Ovalle to, at least, the latitude of Santiago (31º-33ºS), there is another domain showing no significant, paleomagnetically determined, tectonic rotations.

The NE trending “Calama transfer zone” abruptly marks the boundary between the northern and central domains while, in contrast, the transition between the central and southern domains occur as by a progressive southwards decrease in the magnitude of rotations, which finally disappear south of 31ºS. We hypothesize that the Arica-Antofagasta forearc region was displaced eastwards in response to horizontal shortening occurring in the Bolivian Eastern Cordillera (EC) during the early stages of the formation of the Bolivian Orocline in the Eocene-Oligocene. EC shortening may have been equilibrated by of lateral mass transfer and crustal flow to the NE along the Antofagasta-Ovalle clockwise-rotated domain as well as by SE-directed mass transfer in the anticlockwise-rotated southern Peru domain. As attenuation of the tectonic rotations to the south coincides with a diminution in the width of the Andean Cordillera we propose that overall plan view geometry of the Central Andes (not including the Sierras Pampeanas basement uplifts) could have been established during the Paleogene