Paleomagnetism and tectonics of the southern Atacama Desert (25–28°S), northern Chile

Abstract

International audienceWe report paleomagnetic results for 131 sites from the modern forearc of northern Chile (25°S and 28°S). Remanent magnetization in volcanic and intrusive rocks is mostly primary, while a secondary magnetization is observed in sedimentary rocks. Comparison of locality-mean directions with expected paleomagnetic directions indicates vertical axis rotations from -7.3° +/- 21.6° counterclockwise to 52.7° +/- 17° clockwise. Jurassic to Early Cretaceous rocks from the Coastal Cordillera and Cretaceous to Paleocene rocks from the Central Depression show similar magnitude (>30°) clockwise rotations, while more variable rotations occur in Mesozoic to Eocene rocks of the Precordillera. Clockwise rotations in Mesozoic and Paleogene rocks occur in the Chilean Frontal Cordillera south of 27°30'S. Paleomagnetic results in three large Miocene ignimbrite sheets overlying rotated and nonrotated older rocks in the Precordillera and Pre-Andean Depression which show no relative rotation between sites indicate that most rotations within the study area occurred prior to 18 Ma (early Miocene) and likely during and after the ``Incaic'' tectonic event, which affected large tracts of the central Andes. The postulated onset of rotations in the north Chilean forearc was contemporaneous with the beginning of horizontal shortening and uplift of the Eastern Cordillera in Bolivia and northwestern Argentina. Rotation of the Chilean forearc, enhancement of the curvature of the central Andes, and the formation of the Bolivian Orocline seem to be, for the most part, closely linked to the evolution of the Eocene-Oligocene tectonics of the Eastern Cordillera

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

“…The average rotation of all localities in this area is 30.8 ± 17.9° [ Arriagada et al, 2006] which is similar to that observed in the Copiapó region. Like the Copiapó region the timing of rotation is believed to be synpost-Paleocene to Eocene, with the younger bound being constrained by the nonrotation of Oligocene-Miocene sediments and ignimbrites [Arriagada et al, 2003;Somoza et al, 1999;Somoza and Tomlinson, 2002].…”

“…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. Finally we note that some of the largest rotations in the Antofagasta area (localities a, b, and c of Arriagada et al [2000]) occur not in the regions of NE-SW deflected geological strike but in the apparently least deflected N-S striking area on the southernmost fringe of the postulated dextral shear zone [Arriagada et al, 2003].…”

“…[36] Around the DFS the pattern of rotations is very heterogeneous with much localized variation in the recorded rotations ranging from clockwise 76 ± 18°to counterclockwise À7 ± 22° [Arriagada et al, 2006;Fernández et al, 2000;Randall et al, 2001;Riley et al, 1993] (Table 3 and Figures 1 and 2). East of the DFS, data are limited and ambiguous as to whether the rotations decrease rapidly or whether the results from this part of the area are also highly variable.…”

“…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). Our study seeks to determine the relationship between the large rotations recorded in the Coastal Cordillera and the more variable rotations recorded in the Precordillera from around the Domeyko Fault System [Arriagada et al, 2006;Randall et al, 2001;Riley et al, 1993].…”

“…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].…”