The curved Magallanes fold and thrust belt: Tectonic insights from a paleomagnetic and anisotropy of magnetic susceptibility study

Abstract

The Magallanes fold and thrust belt (FTB) presents a large-scale curvature from N-S oriented structures north of 52°S to nearly E-W in Tierra del Fuego Island. We present a paleomagnetic and anisotropy of magnetic susceptibility (AMS) study from 85 sites sampled in Cretaceous to Miocene marine sediments. Magnetic susceptibility is lower than 0.0005 SI for 76 sites and mainly controlled by paramagnetic minerals. AMS results indicate that the sedimentary fabric is preserved in the undeformed areas of Tierra del Fuego and the more external thrust sheets units, where an incipient lineation due to layer parallel shortening is recorded. Prolate AMS ellipsoids, indicating a significant tectonic imprint in the AMS fabric, are observed in the internal units of the belt. AMS results show a good correlation between the orientation of the magnetic lineation and the fold axes. However, in Península Brunswick, the AMS lineations are at~20°counterclockwise to the strike of the fold axes. Pretectonic stable characteristic remanent magnetizations (ChRM) were determined in seven sites. A counterclockwise rotation (21.2°± 9.2°) is documented by ChRM data from four sites near the hinge of the belt in Península Brunswick and near Canal Whiteside while there is no evidence of rotation near the nearly E-W oriented Vicuña thrust within Tierra del Fuego. The curved shape of the Cenozoic Magallanes FTB is not related to vertical axis rotation, and thus, the Magallanes FTB can be considered as a primary arc.

Constraints on deformation of the Southern Andes since the Cretaceous from anisotropy of magnetic susceptibility

“…Similar AMS parameters and lithological features were observed in Cretaceous-Oligocene sedimentary units from the southern and western Magallanes fold-thrust belt (Diraison, 1998;Maffione et al, 2010;Poblete et al, 2014). This implies that the magnetic fabric of the foreland basin formations from the Southern Andes very likely preserve a record of the syn-sedimentary tectonics.…”

“…The primary nature of the curvature of the Magallanes fold-and-thrust belt, supported by paleomagnetic data (Maffione et al, 2010;Poblete et al, 2014), requires more complex processes (different from oroclinal bending) to explain the radial strain field documented by this study. Paleomagnetic constraints indicate that the presentday shape of the Southern Andes was likely already acquired in the Late Cretaceousearly Cenozoic before the formation of the Magallanes fold-and-thrust belt.…”

“…Considering the existing AMS database from the Southern Andes (Diraison, 1998;Maffione et al, 2010;Poblete et al, 2014), we infer that the most internal structural domains of the fold-and-thrust belt, exposing…”

“…Latest models proposed a two-stage evolution of the curvature of the Southern Andes consisting of a Late Cretaceous phase of oroclinal bending during closure of the Rocas Verdes basin, followed by formation of the primary curvature of the Magallanes fold-thrust belt during the Cenozoic (Poblete et al, 2014).…”

“…This initial hypothesis was subsequently supported by sparse A C C E P T E D M A N U S C R I P T paleomagnetic data from the magmatic units, mainly within the internal sectors of the Fuegian Andes, which documented the occurrence of ~90° of counterclockwise rotation (see review by Rapalini, 2007). Tectonic models proposing oroclinal bending in the Cenozoic, related to the opening of the Drake Passage between South America and Antarctica, initially received the largest consensus (Dalziel et al, 1973;Cunningham, 1993;Kraemer, 2003), but were recently challenged by paleomagnetic data from the sedimentary units of the Magallanes fold-and-thrust belt (Maffione et al, 2010;Poblete et al, 2014). Although alternative hypotheses have also emerged, suggesting a primary origin of the curvature of the Southern Andes (Ramos and Aleman, 2000), oroclinal bending models related to the closure of the Rocas Verdes marginal basin in the Late Cretaceous, proposed originally by Burns et al (1980), represent today a viable mechanism congruent with the known rotation pattern of the Southern Andes (Maffione et al, 2010;Poblete et al, 2014).…”