Major crustal rotations in the Andean margin: Paleomagnetic results from the Coastal Cordillera of northern Chile

Abstract

Paleomagnetic analyses of Mesozoic lavas and dike swarms from the northern Chilean Coastal Cordillera, between 25.4°S and 26.4°S, reveal a clockwise rotation of about 42°. Magnetizations from lava flows of andesitic‐basaltic composition of the Middle Jurassic La Negra Formation pass both fold and reversal tests and are interpreted as prefolding remanences. Five dike swarms of Middle Jurassic to Early Cretaceous age yield similar directions to that obtained from the La Negra Formation. Four of the five swarms have mixed polarity, suggesting that they too carry a primary or very early remanence. The structural setting of the dikes suggests that they have not suffered any substantial tilting about nonvertical axes since acquisition of the remanence. The clockwise rotation of the area is believed to have been the consequence of transpressional deformation of mid–Late Cretaceous age, post‐100 Ma, associated with abandonment of the Jurassic–Early Cretaceous magmatic arc in this region and its eastward migration to form a new mid–Late Cretaceous magmatic arc in the former back arc region. This younger arc is located east of the Coastal Cordillera and lies in the Central Valley region. The clockwise sense of rotation is consistent with other paleomagnetic data from northern Chile and southern Bolivia, south of the Arica Deflection in the Andean margin, although it is the largest yet reported. To the north of the Arica Deflection, paleomagnetic studies report counterclockwise rotations, and several large‐scale models have been proposed to explain the overall pattern of rotations. Models include oroclinal bending of an originally straight margin, differential shortening across the margin at a preexisting bend which is subsequently tightened by the passive rotation of the limbs of the bend, and distributed shear throughout the margin as a consequence of oblique convergence at a preexisting bend. In contrast to these models, several workers have argued that rotation is better explained in terms of localized in situ rotations. We review these models in light of our results and present a domino‐type model with blocks bounded by left‐lateral faults and rotating clockwise in response to mid‐Late Cretaceous transpression within a crustal scale shear zone. This is consistent with the observed strike‐slip fault systems identified in the Coastal Cordillera.

The Lautaro Basin: A record of inversion tectonics in northern Chile

“…In the other hand, the Paleogene deformation is more widely distributed across the Andes of northern Chile (Arriagada et al, 2000(Arriagada et al, , 2003(Arriagada et al, , 2006bTaylor et al, 2007). Eocene-Oligocene deformation was accompanied by tectonic shortening, mountain uplift and significant clockwise block rota tions (e.g., Hartley et al, 1992;Riley et al, 1993;Forsythe and Chisholm, 1994;Randall et al, 1996;Taylor et al, 1998;Arriagada et al, 2000;Somoza and Tomlinson, 2002;Arriagada et al, 2003Arriagada et al, , 2006bArriagada et al, , 2008. Fission track thermochronology indicates that tectonic uplift and erosion were mainly active during the EoceneEarly Oligocene 'Incaic Orogeny', when at least 4-5 km of rocks were eroded during exhumation of tectonic blocks of the Cordillera de Domeyko between ca.…”

The Southern Central Andes vertical axis tectonic rotations: relations with the deformation pattern

“…Some authors (Beck et al, 1994;Butler et al, 1995) developed models which combine oroclinal bending and local block rotations, then each locality would record a local amount of rotation plus a regional (orocline related) rotation. Domino-style models that relate the rotations detected in northern Chile to observable faults in the forearc have also been published (e.g., Forsythe and Chisholm, 1994;Randall et al, 1996;Taylor et al, 1998;Abels and Bischoff, 1999). However, Arriagada et al (2003) found clockwise tectonic rotations of up to 65° within the forearc domain (Antofagasta region), which would have occurred mainly during the Incaic orogenic event of Eocene-Early Oligocene age.…”

Implications for crustal rotation and tectonic evolution in the central Andes fore arc: New paleomagnetic results from the Copiapó region of northern Chile, 26°–28°S

“…[7] The rocks of the Coastal Cordillera record some of the largest paleomagnetic rotations in the Andes [Forsythe and Chisholm, 1994;Forsythe et al, 1987;Randall et al, 1996;Taylor et al, 1998]. The study area lies at the southern limit of the Central Valley between 26.5 and 27.5°S and east of the city of Copiapó (Figures 2 and 3).…”

“…R is the calculated rotation compared to the appropriate reference 5 Ma synthetic poles from Besse and Courtillot [2002] and its associated error [Demarest, 1983]. Sources of data are 1, Arriagada et al [2006]; 2, Riley et al [1993]; 3, Randall et al [2001]; 4, Randall et al [1996]; 5, Forsythe et al [1987];6, Taylor et al [1996]. sinistral faults and lineaments which could have accommodated clockwise rotation during margin normal contraction [Abels and Bischoff, 1999;Forsythe and Chisholm, 1994;Grocott and Taylor, 2002;Randall et al, 1996;Taylor et al, 1998].…”

“…Sources of data are 1, Arriagada et al [2006]; 2, Riley et al [1993]; 3, Randall et al [2001]; 4, Randall et al [1996]; 5, Forsythe et al [1987];6, Taylor et al [1996]. sinistral faults and lineaments which could have accommodated clockwise rotation during margin normal contraction [Abels and Bischoff, 1999;Forsythe and Chisholm, 1994;Grocott and Taylor, 2002;Randall et al, 1996;Taylor et al, 1998]. Second, although only limited paleomagnetic data are available between the Copiapó and Antofagasta areas [Arriagada et al, 2006;Forsythe and Chisholm, 1994], at least 25°of clockwise rotation is indicated, suggesting that the entire area from 23°to 28°has suffered substantial, and reasonably, uniform crustal rotation.…”