Vertical‐axis rotations determined from paleomagnetism of Mesozoic and Cenozoic strata of the Bolivian Andes

Abstract

Thermal demagnetization and principal component analysis allowed determination of characteristic remanent magnetization (ChRM) directions from 256 sites at 22 localities in Mesozoic and Cenozoic sedimentary strata of the Bolivian Altiplano and Eastern Cordillera. An inclination‐only fold test of site‐mean ChRM directions from Cenozoic units (principally the Santa Lucía Formation) indicates optimum unfolding at 97.1% unfolding, consistent with a primary origin for the ChRM. For Mesozoic strata, optimum unfolding occurred at 89.2%, perhaps indicating secondary remagnetization at some locations. For Cenozoic units, comparison of locality‐mean directions with expected paleomagnetic directions indicates vertical‐axis rotations from 33° counterclockwise to 24° clockwise. Euler pole analysis of along‐strike variation in crustal shortening within the Subandean and Interandean zones indicates 18° clockwise rotation south of the axis of curvature of the Bolivian Andes and 6° counterclockwise rotation northwest of the axis during the past 10 m.y. Along‐strike variation of shortening within the Eastern Cordillera indicates 8° clockwise rotation south of the axis and 8° counterclockwise rotation northwest of the axis from 35 to 10 Ma. These vertical‐axis rotations produced by along‐strike variations in crustal shortening during development of the Bolivian fold‐thrust belt agree well with observed rotations determined from paleomagnetism of Cenozoic rocks in the Eastern Cordillera and in the Subandean and Interandean zones. However, local rotations are required to account for complex rotations in the Cochabamba Basin and within the Altiplano. The curvature of the Bolivian Andes has been progressively enhanced during Cenozoic fold‐thrust belt deformation.

Reinterpretation of the Ordovician rotations in NW Argentina and Northern Chile: a consequence of the Precordillera collision?

“…4b). Different tectonic models have been proposed to account for the systematic pattern of rotations observed along the Central Andes (Somoza et al 1996;Beck 1998;Kley 1999;Arriagada et al 2000;Roperch et al 2000;McQuarrie 2002;Prezzi and Alonso 2002;Richards et al 2004).…”

“…The rotations recorded in the Early Ordovician units of the Eastern Puna could be Paleozoic as well as (Beck 1998;Lamb 2001;Richards et al 2004;Taylor et al 2005;Ré 2008;Maffione et al 2009;IAGA Paleomagnetic Database 2005). c-f DEM of the Ordovician sampling localities and younger localities with the sense and value of rotation that everyone presents.…”

“…Figure 4b shows the distribution of sampling localities of paleomagnetic studies in rocks younger than Ordovician in the Central Andes region under consideration. The database is quite extense (Beck 1998;Lamb 2001;Richards et al 2004;Taylor et al 2005;Ré 2008;Maffione et al 2009;IAGA Paleomagnetic Database 2005). Rotated Mesozoic or Cenozoic units outcropping close to localities of rotated Ordovician rocks suggest that the rotations are Andean, while the lack of rotation of younger rocks would indicate an older age.…”

Paleogeographic and tectonic implications of the first paleomagnetic results from the Middle–Late Cambrian Mesón Group: NW Argentina

“…9) . Richards and Butler (2004) performed a paleomagnetic analysis of Mesozoic and Cenozoic sedimentary strata of Bolivia and found clockwise rotations of approximately 25°for areas south of the axis of the Bolivian Orocline at a similar longitude and some 200 km north of our area.…”

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

“…Local models have also been proposed, but they are focused in the forearc (e.g., Forsythe and Chisholm, 1994;Taylor et al, 1998;Abels and Bischoff, 1999;Arriagada et al, 2003;Taylor et al, 2005). Recently, the quantity, quality, and geographic distribution of paleomagnetic data have expanded greatly (Richards et al, 2004;Prezzi et al, 2004;Arriagada et al, 2006;Roperch et al, 2006;Taylor et al, 2007;Barke and Lamb, 2007;Arriagada et al, 2008b;Maffione et al, 2009). The increased paleomagnetic data set has been accompanied by an increase in the current knowledge of the deformation periods in the Andes (e.g., Oncken et al, 2006;Ege et al, 2007;Hongn et al, 2007;Barnes and Ehlers, 2009), allowing a more detailed analysis of the temporal and spatial distribution of the detected rotations.…”