Paleomagnetism of upper Miocene ignimbrites at the Puna: An analysis of vertical‐axis rotations in the Central Andes

Abstract

The origin of the rotations detected paleomagnetically in the Central Andes is controversial. Tectonic models proposed to explain it involve Late Cenozoic oroclinal bending (the Bolivian Orocline), and/or small‐block rotations driven by oblique subduction. In this paper, we report paleomagnetic data from upper Miocene ignimbrites from the Puna, in the presumed southern limb of the orocline. These rocks showed high unblocking temperature and high coercivity magnetization. The resulting pole position (latitude 85.7°S, longitude 80.5°E, A95 = 7.9°, K = 22, N = 17) indicates that the study area has not been significantly affected by vertical‐axis rotation since late Miocene onward. This implies that the whole Puna has not undergone significant regional rigid‐body rotation during the same time span. A farther analysis based on Neogene paleomagnetic data from 31 Andean localities distributed from 11°S to 31°S shows the presence of rotated and unrotated areas. Systematic counterclockwise rotations in Perü and northern Bolivia are observed, whereas clockwise rotations are present in southern Bolivia, northern Chile, and perhaps northwestern Argentina. No systematic rotations are found farther south. The overall time‐spatial distributions of these Neogene paleomagnetic data suggest that if orogenic bending occurred, it must have taken place before the middle Miocene. Thus the oroclinal hypothesis cannot explain the rotations observed in middle Miocene rocks or younger ones, which also show orocline‐like declination anomalies. We suggest that small‐block rotations driven by distributed shear may be a single process that can account for the whole rotations in the Central Andes. It is hypothesized that horizontal shear in the Andean crust could be controlled by ancient structures.

Collisional model for rapid fore‐arc block rotations, arc curvature, and episodic back‐arc rifting in subduction settings

“…Subduction of the Nazca Plate beneath South America occurs at the Chile and Peru Trenches; several aseismic ridges currently underthrust the margin. Semicircular gray arrows depict the opposing rotations (observed from paleomagnetic studies [e.g., Beck, 1987;Roperch and Carlier, 1992;Randall, 1998;Somoza et al, 1996;Lamb, 2001]) north and south of the Bolivian Orocline, and the straight arrows show the direction of decreasing back-arc shortening away from the orocline, consistent with the sense of rotation of the margin [e.g., Isacks, 1988;Kley, 1999;Macedo-Sanchez et al, 1992]. Elevation data from ETOPO5.…”

“…Figure A17) have documented large rotations of the Andean margin that appear to be associated with development of the Bolivian orocline. Anticlockwise rotations are prevalent in the northern limb of the orocline, while clockwise rotation dominates the southern limb [e.g., Beck, 1987;Roperch and Carlier, 1992;Randall, 1998;Somoza et al, 1996;Lamb, 2001]. Observed gradients in shortening within the Andean orogenic belt (i.e., decreased shortening away from the center of the orocline) are consistent with tectonic rotation of the Andean margin as large fore-arc blocks [e.g., Isacks, 1988;Kley, 1999;Macedo-Sanchez et al, 1992], and there is some indication that these rotations may continue to the present day [Lamb, 2000].…”

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

“…According to these authors, Neogene shortening in the sub-Andean zone only slightly enhanced the orogenic curvature of the Central Andes. Alternatively, models suggesting the existence of local block rotations in response to distributed shear have also been proposed (e.g., Beck, 1988;Dewey and Lamb, 1992;Somoza et al, 1996;Beck, 1998). Such distributed shear would result from partitioning of the oblique convergence vector between the Nazca and South American plates into coast parallel and perpendicular components.…”

“…Along the Central Andes a pattern of vertical axis tectonic rotations has been paleomagnetically identified (e.g., Isacks, 1988;Beck, 1988;Dewey and Lamb, 1992;Somoza et al, 1996). This Central Andean Rotation Pattern (CARP, Somoza et al, 1996) is characterized by counterclockwise block rotations north of Arica Deflection (~19°S) (Gephart's symmetry plane; Gephart, 1994) and clockwise rotations to the south.…”

“…This Central Andean Rotation Pattern (CARP, Somoza et al, 1996) is characterized by counterclockwise block rotations north of Arica Deflection (~19°S) (Gephart's symmetry plane; Gephart, 1994) and clockwise rotations to the south. Different hypothesis and models have been proposed to explain the CARP.…”