AMS method applied to the mapping of the Gastre Fault System, Central Patagonia: an insight in deformation conditions and superimposed magnetic fabrics

ZAFFARANA C. B,; SOMOZA, R.

Foz do Iguazu

Conferencia; 2010 Meeting of the Americas; 2010

American Geophysical Union

The Central Patagonian Batholith (CPB) is a complex of Late Paleozoic - Early Mesozoic I-type, calcalkaline granitoids that crop out in an intracontinental position at Patagonia. One hypothesis suggested that their emplacement was due to a different pre-Jurassic configuration of southern South America, with an embayment in the plate boundary defined by a southern Patagonia block displaced eastward with respect to the main continent, this geometry allowing the hypothesis of a Late Paleozoic - Early Mesozoic subduction zone where the CPB is currently located. To restore this southern Patagonia block to its present day position, the hypothesis invokes Late Triassic to Middle Jurassic dextral motion along the Gastre Fault System (GFS), a large-scale, transcontinental strike-slip zone that would have severely deformed de CPB. To further explore about this interesting model, we performed a detailed study of the petrography, microstructures and magnetic fabric of the CPB in the area were the GFS was defined. In particular, AMS information arises from 100 evenly distributed sites representing the two main magmatic units that form the CPB. All the sampled sites have dominant magmatic fabric with a slight imprint of high-temperature deformation ascribed to the late-magmatic stage. Low temperaturedeformation was locally observed. There is correspondence between petrofabric and magnetic fabric, with preliminary magnetic mineralogy data (hysteresis loops and IRM-backfield curves) supporting AMS interpretations. Most magnetic foliations show a NW-SE subvertical pattern which is shared by all lithologies and defines a 9 km wide, NW-SE structural corridor that extends for 45 km in the studied area, but rapidly vanishes to the north (no magmatic rocks crop out south of this corridor). Magnetic fabrics discordant with the NW-SE trend are common outside the lineament. In turn, magnetic lineations in the above mentioned NW-SE corridor show all the range of plunges from vertical to horizontal. Outside the lineament intermediate inclinations predominate. Low-temperature, superimposed ductile and fragile deformation was observed in 16 sites, affecting older and younger granitoids of the CPB. As observed in magmatic fabrics, magneticfoliation is always parallel to low-temperature tectonic foliation and belongs to the NW-SE structural corridor above mentioned. Magnetic lineations from low temperature deformed rocks have variable orientations. Outside the main lineament, magnetic foliations and lineations can be different, even showing random orientations. In addition, we sampled mafic dikes and felsic veins where we saw examples of both normal and inverse magnetic fabrics. Besides, we took samples from the pair enclave - hosting granitoid in many sites with the purpose of exploring the utility of the AMS method for rheological investigation. Overall, our results may be reconciled with theemplacement of the CPB being controlled by a major NW-SE crustal feature, the rocks showing similar kinematics of deformation during and after the emplacement in magmatic, ductile low-temperature and fragile conditions. No evidence of major post-batholith deformation was found.