Tectonic rotations during the Chile Ridge collision and obduction of the Taitao ophiolite (southern Chile)

Abstract

The < 6 Ma young Taitao ophiolite, exposed at the westernmost promontory of the Taitao Peninsula, is located approximately 40 km southeast of the Chile triple junction and consists of a complete sequence of oceanic lithosphere. Systematic sampling for paleomagnetic study was performed to understand the complex obduction processes of the ophiolite onto the forearc of the South American Plate. Two representative demagnetization paths of remanent magnetization vectors were observed. One is characterized by stable univectorial demagnetization paths and was observed in volcaniclastic rocks and dyke complexes. Orientations of their remanent magnetization vectors indicate various degrees of counterclockwise rotations. The other is characterized by multivectorial demagnetization paths and was observed in the plutonic units (gabbros and ultramafic rocks). From these, two distinct stable remanent magnetization vectors were isolated; one has high coercivity and the other has low coercivity along the demagnetization paths with little influence of viscous magnetizations. This suggests that the complex deformation history involved at least two rotational events. The clockwise rotation, inferred from high coercivity remanent magnetization vectors, was attributed to a ridge collision event and the counterclockwise rotation, inferred from the low coercivity remanent magnetization vectors, was attributed to an accommodation phase into the South American forearc during obduction and final emplacement of the ophiolite. Folds developed during this period. Paleomagnetic restorations of the internal structures of the plutonic units and dyke complexes suggest that they probably originated in a mid-oceanic ridge environment near a transform fault. The counterclockwise rotation of the plutonic and dyke complex units during the obduction generated tectonic gaps between these and the basement. The volcaniclastic rocks must have been deposited at nearly their present location, filling the tectonic gaps, as less effect of tectonic rotation was identified on these rocks.

Ophiolite Emplacement and the Effects of the Subduction of the Active Chile Ridge System: Heterogeneous Paleostress Regimes Recorded in the Taitao Ophiolite (Southern Chile)

“…Previous works on the Península de Taitao (e.g., Forsythe and Nelson, 1985;Nelson et al, 1993;Guivel et al, 1996;Lagabrielle et al, 2000;Veloso et al, 2005) showed that the Taitao Ophiolite is in fault contact with surrounding Pre-Jurassic basement and young (<4 Ma) Cabo Raper pluton in the south Guivel et al, 1996;Nelson et al, 1993;Hervé et al, 2003;Veloso et al, 2005). Internal lithological boundaries of the ophiolite (Fig.…”

“…This suggests a NE-SW extension in the northern part of the ophiolite. Veloso et al (2005) suggested that such extension was a direct result of a counterclockwise rotation of the ophiolite sometime after 6 Ma due to its emplacement and accommodation into the forearc. No kinematic indicators -such as those described by Petit (1987) and Doblas (1998)-were observed on these faults, and therefore they were excluded from the further analysis.…”

“…The Chile Margin Unit is composed of thick deposits of highly vesicular pillow lavas and pillow breccias (Bourgois et al, 1993;Nelson et al, 1993;Lagabrielle et al, 2000) interbedded with conglomerates and sandstones. According to Guivel et al (1996Guivel et al ( , 1999 this unit was emplaced close to its actual location, and perhaps as a result of the counterclockwise rotation of the ophiolite package (Veloso et al, 2005).…”

“…1b). All these rock units are northward tilted (Nelson et al, 1993;Lagabrielle et al, 2000;Veloso et al, 2005) and surrounded by young plutonic intrusions (Mpodozis et al, 1985;Lagabrielle et al, 1994;Guivel et al, 1996Guivel et al, , 1999Hervé et al, 2003).…”

“…Several authors have pointed to a close spatial and temporal relation between the processes of 6 Ma ophiolite emplacement (Anma et al, 2006) and the evolution of the active Chile Ridge system that separate the Nazca (to the north) and Antarctic (to the south) oceanic plates (e.g., Cande and Leslie, 1986;Forsythe and Nelson, 1985;Nelson et al, 1993;Keading et al, 1990;Bourgois et al, 1996;Lagabrielle et al, 1994Lagabrielle et al, , 2000Veloso et al, 2005). According to global plate reconstructions and numerical models (e.g., NUVEL1-A) the Nazca plate is presently subducting at a rate of about 8.5 cm/yr in a N80°E direction whereas the Antarctic plate subducts at a rate of ca.…”