The Vallenar Discontinuity and the Maipo Orocline: Regional significance of clockwise vertical-axis rotations in the central Chilean Andes.
Abstract
One of the most prominent tectonic features of the Andes is the Central Andean Rotation Pattern (CARP), which is closely related to the Bolivian Orocline and characterized by paleomagnetically determined clockwise rotations in northern Chile and counterclockwise rotations in southern Peru (Arriagada et al., 2008). Along the Chilean margin, between 29°S and 38°S, three prominent curvatures are observed. The Vallenar Discontinuity near ∼29°S corresponds to the southern limit of the Bolivian Orocline. North of 29°S the major structural elements (Paleozoic basement highs and thrusts) are NNE oriented while from 29°S down to 32°S the structures are mainly NS. The central Chilean margin presents also significant bends near Santiago (∼33°S, Maipo Orocline) and in the Arauco region (∼38°S). Near Santiago, the Maipo bend coincides with the subduction of the Juan Fernandez Ridge (JFR). During the last five years we have undertaken new paleomagnetic and structural studies along the forearc of northern and central Chile in order to understand the origin of the bends in the Chilean margin and the consequence of its indentation by the JFR. Clockwise rotations are, consistently large (30°S- 45°) north of the Vallenar discontinuity, but south of the Vallenar discontinuity, the segment between 29°S to 32°S was not subjected to significant clockwise rotation. South of ∼33°S, significant clockwise deflections up to 39° of the declinations are again observed. Rotations occur both in Mesozoic rocks of the Coastal Cordillera and Tertiary rocks of the Main Cordillera. Whereas most of the CARP rotations, involving bending of the Bolivian Orocline and clockwise rotations north of the Vallenar Discontinuity, occurred essentially during the Paleogene, the paleomagnetic rotations obtained in Tertiary formations of central Chile constrain the maximum possible age for the occurrence of rotations of the Maipo Orocline to the Miocene. Neogene shortening in the foreland belt induced only slight orogenic curvature near the Vallenar Discontinuity. In the southern Central Andes, the along-strike variation in the magnitude of Neogene shortening may have been large enough to produce the Maipo Orocline. From 22 to 10Ma, the location of the subduction of the JFR moved rapidly southward from Arica (18°S) to Santiago (33°S) (Yanez et al., 2002). During the past 10 My, the JFR has been subducting continuously along the same part of the continental margin inducing flat slab and tectonic shortening. We will discuss the origins of the bends observed along the forearc of northern and central Chile and examine the role of the subduction of the JFR in bending the margin and inducing the observed clockwise rotations south of Santiago. Arriagada C., P. Roperch, C. Mpodozis, P. R. Cobbold (2008), Paleogene building of the Bolivian Orocline: Tectonic restoration of the central Andes in 2-D map view, Tectonics, 27, TC6014, doi:10.1029/2008TC002269. Yanez, G., J. Cembrano, M. Pardo, C. Ranero, D. Selles, (2002) The Challenger-Juan Fernandez-Maipo major tectonic transition of the Nazca-Andean subduction system at 33-34°S: geodynamic evidence and implications, Journal of South American Earth Sciences, 15, 23-38,doi: 10.1016/S0895-9811(02)00004-4.
- Publication:
-
AGU Spring Meeting Abstracts
- Pub Date:
- May 2009
- Bibcode:
- 2009AGUSMGP72A..04A
- Keywords:
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- 1525 Paleomagnetism applied to tectonics: regional;
- global;
- 8108 Continental tectonics: compressional