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

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

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-long traverse across a segment of the Valle Fertil shear zone, central-western Argentina. On a regional scale, the shear zone appears as numerous discontinues belts over 25 km in width and is approximately 140 km in length, extended on the western section of the Sierras Valle Fertil e La Huerta mountain range. The steeply dipping shear zone with a vertical mylonitic lineation is composed of amphibolite facies ribbon mylonites and amphibolite to greenschist facies ultramylonites derived from 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 low greenschist facies conditions associated with localized faulting. One new 40Ar/39Ar age on biotite and published 40Ar/39Ar ages on amphibole in the shear zone yield an average cooling rate of 6.2 C/Ma for a time period that crosses the Siluriane Devonian boundary. Since in metasedimentary rocks the youngest zircon´s rims dated at 465 Ma marks the beginning of cooling, nearly continuous uplift of rocks within the shear zone occurred over a minimum time span of 55 Ma. During the period of active deformation, dip-slip movement can explain uplift of several kilometers of the Early Ordovician arc crust. The Valle Fertil shear zone, which was formed near above the inferred suture zone between the Famatinian arc and Cuyania microcontinent, is a major structural boundary nucleated within the Early Ordovician crust. The simplest geodynamic model to explain the evolution of the Valle Fertil shear zone involves the collision of the composite Cuyania/Precodillera microcontinent against the Famatinian arc.