A Middle Paleozoic shear zone in the Sierra de Valle Fértil, Argentina: records of a continent-arc collision in the Famatinian margin of Gondwana

CRISTOFOLINI, E.A., OTAMENDI, J.E., WALKER JR., B.A., TIBALDI, A.M., ARMAS, P.,BERGANTZ, G.W., MARTINO, R.D.,

JOURNAL OF SOUTH AMERICAN EARTH SCIENCES

PERGAMON-ELSEVIER SCIENCE LTD

Lugar: Amsterdam; Año: 2014 vol. 56 p. 170 - 185

0895-9811

Geological,petrological and structural observations were obtained along a 30-km-longtraverse across a segment of the Valle Fértil shear zone, central-western Argentina. On aregional scale, the shear zone appears as numerous discontinues belts over 25km in width and is approximately 140 km in length, extended on the westernsection of the Sierras Valle Fértil - La Huerta mountain range. The steeply dipping shearzone with a vertical mylonitic lineation is composed of amphibolite faciesribbon mylonites and amphibolite to greenschist facies ultramylonites derivedfrom Early Ordovician plutonic and metasedimentary parent rocks. Locally,syn-kinematic retrogression of mylonites formed greenschist facies phyllonites.During the later stages of deformation, unstrained parent rocks, mylonites,ultramylonites and phyllonites were affected by pervasive cataclasis under lowgreenschist facies conditions associated with localized faulting. One new40Ar/39Ar age on biotite and published 40Ar/39Ar ages on amphibole in the shearzone yield an average cooling rate of 6.2 ºC/Ma for a time period that crossesthe Silurian - Devonian boundary. Since in metasedimentary rocks the youngestzircon rims dated at 465 Ma marks the beginning of cooling, nearly continuousuplift of rocks within the shear zone occurred over a minimum time span of 55Ma.  During the period of activedeformation, dip-slip movement can explain uplift of several kilometers of theEarly Ordovician arc crust. The Valle Fértil shear zone, which was formed nearabove the inferred suture zone between the Famatinian arc and Cuyaniamicrocontinent, is a major structural boundary nucleated within the EarlyOrdovician crust. The simplest geodynamic model to explain the evolution of theValle Fértil shear zone involves the collision of the compositeCuyania/Precodillera microcontinent against the Famatinian arc.