Constraining the timing and evolution of a long-lived tectonic boundary: An example from the Early Paleozoic, Argentina

Abstract

The Guacha Corral shear zone (GCSZ) is one of the most prominent crustal-scale shear zones in the Eastern Sierras Pampeanas of Argentina, juxtaposing two different metamorphic terranes: the eastern hanging wall, which was metamorphosed in the 550-510 Ma Pampean Orogen, and the western footwall, metamorphosed in the 500-440 Ma Famatinian Orogen. In this study, the tectono-thermal evolution of the footwall was constrained using U–Pb dating of zircon, monazite and apatite, and results were compared to the broader evolution of the shear zone. The main episode of migmatization was recorded in the footwall at ~475 Ma by zircon and monazite, indicating anatexis during the Famatinian Orogeny at amphibolite facies conditions (~700 °C – Ti-in-zircon thermometer). It was at this time that the major thrusting event was taking place due to the docking of the Precordillera/Cuyania terrane to the proto-Andean margin. Anatexis in the footwall was followed by cooling reaching upper greenschist facies conditions recorded by apatite U/Pb ages at ~400 Ma. The cooling rates during retrogression of footwall migmatites from ~690 °C to ~500 °C were of ~3–4 °C/Ma. The cooling processes after peak metamorphism were temporally related to the waning of the Famatinian Orogeny during the Early Devonian. After crossing apatite closure temperature, rocks entered greenschist facies conditions and continued motion on the shear zone favored the influx of fluids to retrogress the footwall. This late motion is temporally related to the collision of the Chilenia terrane to the proto-Andean margin at ~390 Ma, marking the start of the Chanic/Achalian Orogeny.