Magnetic fabric and microstructures across the Andes of Tierra del Fuego.

FEDERICO ESTEBAN; AUGUSTO RAPALINI; ALEJANDRO TASSONE; MARÍA ELENA CERREDO; JUAN F. VILAS; HORACIO LIPPAI

19-20 de noviembre. Santiago. Chile

Simposio; International Geological Congress on the Southern Hemisfere (Geosur 2).; 2007

GEOSUR

Magnetic fabric and microstructural studies in orogenic belts provide significant information regarding the type and degree of tectonic deformation affecting the main lithologic units exposed along them. Despite its resolution, versatility and speed, the magnetic fabric studies are still very rare in South America and especially so in orogenic belts. As part of a long-term and interdisciplinary project to study the tectonic evolution of the Fuegian Andes, a magnetic fabric transect across the Andean chain in the Argentine side of the Tierra del Fuego island was carried out. This was done by measuring the anisotropy of magnetic susceptibility (AMS) and was accompanied by petrographic observations of oriented thin sections from representative samples. Two-hundred and twelve oriented samples (mainly drilled cores) were collected at 25 sites distributed across the Fuegian Andes from Paso Garibaldi in the north to the Lapataia bay in the south. Different lithologies were sampled, although a majority of sites was located on basic sills (2 to more than 20 meters thick) intruded in the Late Jurassic Lemaire or the Early Cretaceous Yahgan formations. In most of these rocks the original magmatic textures are poorly preserved, with a significant static ocean-floor metamorphism, overprinted by a low-grade metamorphism associated with the Andean deformation that has produced a penetrative foliation defined by isoriented chlorite-prehnite assemblages. Preliminary AMS results show moderate bulk magnetic susceptibility between 0.5 and 1.5 x 10-3 SI for most sites, suggesting that paramagnetic contributions are significant. Anisotropy degrees range widely from a few to over forty per cent, and the ellipsoid shape tends to fall in the oblate field (T>0). Pole to magnetic foliation (K3) tends to agree with the pole to well developed cleavage or tectonic foliation in the shales surrounding the basaltic sills. However, some minor systematic departure is apparent. Magnetic lineation (K1) trends systematically N-NNE to S-SW, with shallow inclination towards the south, in the northern part of the study area (from Valle Carvajal to Paso Garibaldi). This is consistent with deformation governed by the NE-SW thrust tectonics that produced significant shortening and tectonic transport of the thrust and fold belt towards the NNE. It is interpreted that K1 (defined by aligned neocrystallized minerals) is parallel to tectonic transport direction. In areas to the south (Valle Carvajal and Beagle Channel areas), K1 is generally subhorizontal and tends to point to the E-W to ESE-WNW direction; while the magnetic foliation plane is subvertical with an E to ESE orientation. This magnetic fabric pattern suggests deformation associated with the significant E-ESE transcurrent zones of Valle Carvajal and Beagle Channel, confirming that important strike-slip deformation took place along these lineaments during the development of the Andean orogeny in the Fuegian Andes. The Late Cretaceous (ca. 77 Ma) porphyritic dacite exposed on the Beagle Channel near Ushuaia shows AMS characteristics consistent with lack of significant internal deformation, also supported by the microstructural analysis, which suggests end of main tectonic deformation in the area before its intrusion.