Age constraints on the tectonic activity of the Guacha Corral Shear Zone, Sierra de Comechingones, Córdoba.

MARTINO, R.D., ; STEENKEN, A; SIEGESMUND. S.; LOPEZ DE LUCHI, M.G.; WEMMER, K,

Mendoza

Simposio; Gondwana 12 Geological and biological heritage of Gondwana; 2005

Academia Nacional de Ciencias

The Guacha Corral Shear Belt (GCSB), a major N-NNW/S-SSE trending shear zone that is developed of 32° SL, constitutes one of the more complex and largest (20 km wide, 120 km long) shear belts of the Sierras de Córdoba. It extends from the Achala Batholith up to the southern tip of the Sierra de Comechingones where it disappears under the modern sedimentary cover. The eastern limit is the Guacha Corral lineament. The western limit is developed in its northern part along the entire width of the Sierra de Comechingones whereas the southern part is made up by a series of minor shear belts that are interfingered with undeformed rocks. (Fagiano et al. 1999, Martino 2003). Mylonites of GCSB are developed on diatexites, metatexites, gneisses and amphibolites which show an intense retrogression that turned them into phyllonites close to modern faults.  High and low temperature deformations were observed (Martino 2003, Withmeyer and Simpson 2003). The anastomosing mylonitic foliation shows highly variable strike and dip with average values of N 0-350º/26-65º E. A N 70-74°/25-50° mica stretching lineation is recognized. Theta and sigma clasts are present, the latter indicating a reverse shear sense. The GCSB is a major tectonic boundary within the Sierras de Córdoba because it overthrusts and reworks rocks of different metamorphic grade and deformation style. It juxtaposed the rocks of the high to medium grade Cambrian Comechingones and Monte Guazú Metamorphic complexes (hanging wall) on top of the medium grade Cambrian to Early Ordovician Conlara Metamorphic Complex (Steenken et al. 2005 submitted). The Faja Altautina is part of this footwall block (Martino 2003). This situation is analogous to what is observed along the Pachango shear belt that overthrusts the La Aguadita  Sil + Ms stromatites over Bt + Grt + Ms metamorphic rocks indicating a metamorphic inversion and the limit between units with different metamorphism and deformation (Martino 2003). A correlation between the GCSB and the Pachango shear belt is proposed. Both belts were active since the Ocloyic phase of the Famatinian Orogeny. All along the GCSB, but specially in the northern tract, sheet like Qtz + Kfs + Ms pegmatoids of variable thickness are emplaced. These pegmatoids constitute two groups: one concordant with the mylonitc foliation and showing ductile deformation along their borders and another discordant and undeformed. Four K-Ar cooling ages were obtained from muscovite booklets. Deformed pegmatoids yielded cooling ages between 418.5-429.0 Ma whereas the undeformed pegmatoids indicated ages between 420.7-431.0 Ma. A minimum middle to late Silurian age is assigned to the cessation of the tectonic activity of the GCSB which is coetaneous with the 440 Ma (K/Ar Ms age) regional basement cooling and orogenic relaxation of the footwall in the Sierra de San Luis. (Steenken et al. 2004). Later, the GCSB would have acted as weaker zone that partially controlled the emplacement of the Devonian batholiths during the Achalian cycle.