The Sierra Ballena Shear Zone in the southernmost Dom Feliciano Belt (Uruguay): evolution, kinematics, and deformation conditions

Abstract

The Sierra Ballena Shear Zone (SBSZ) is part of a high-strain transcurrent system that divides the Neoproterozoic Dom Feliciano Belt of South America into two different domains. The basement on both sides of the SBSZ shows a deformation stage preceding that of the transcurrent deformation recognized as a high temperature mylonitic foliation associated with migmatization. Grain boundary migration and fluid-assisted grain boundary diffusion enhanced by partial melting were the main deformation mechanisms associated with this foliation. Age estimate of this episode is [658 Ma. The second stage corresponds to the start of transpressional deformation and the nucleation and development of the SBSZ. During this stage, pure shear dominates the deformation, and is characterized by the development of conjugate dextral and sinistral shear zones and the emplacement of syntectonic granites. This event dates to 658-600 Ma based on the age of these intrusions. The third stage was a second transpressional event at about 586 to \560 Ma that was associated with the emplacement of porphyry dikes and granites that show evidence of flattening. Deformation in the SBSZ took place, during the late stages, under regional low-grade conditions, as indicated by the metamorphic paragenesis in the supracrustals of the country rocks. Granitic mylonites show plastic deformation of quartz and brittle behavior of feldspar. A transition from magmatic to solid-state microstructures is also frequently observed in syntectonic granites. Mylonitic porphyries and quartz mylonites resulted from the deformation of alkaline porphyries and quartz veins emplaced in the shear zone. Quartz veins reflect the release of silica associated with the breakdown of feldspar to white mica during the evolution of the granitic mylonites to phyllonites, which resulted in shear zone weakening. Quartz microstructures characteristic of the transition between regime 2 and regime 3, grain boundary migration and incipient recrystallization in feldspar indicate deformation under lower amphibolite to upper greenschist conditions (550-400°C). On the other hand, the mylonitic porphyries display evidence of feldspar recrystallization suggesting magmatic or high-T solid-state deformation during cooling of the dikes.

The Rio de la Plata craton and the adjoining Pan-African/brasiliano terranes: Their origins and incorporation into south-west Gondwana

“…12). The model combines our results with those of recent contributions (e.g., Goscombe and Gray, 200B;Gaucher et aI., 2009;Frimmel et aI., 2010;Oyhantc;:abal et al, 2010b;Saalmann et al, 2010).…”

Major shear zones of southern Brazil and Uruguay: escape tectonics in the eastern border of Rio de La plata and Paranapanema cratons during the Western Gondwana amalgamation

“…Correlations between the major southern Brazilian and Uruguayan shear zones have already been discussed by a number of authors (Basei 1990;Passarelli et al 1993;Fernandes and Koester 1999;Basei et al 2000;Bitencourt and Nardi 2000;Bossi and Gaucher 2004;Oyhantçabal et al 2009a;Passarelli et al 2010) based on the strong suggestion of geometrical and geophysical continuities (Mantovani et al 1989). Together with the Cordilheira (RS) and Sierra Ballena (UY) Shear Zones, the MGSZ is considered part of a lithospheric-scale system of discontinuities that separates geochemical and isotopically distinct terranes .…”

“…The Aiguá Batholith (A) has been correlated with the Pelotas and Florianópolis Batholiths further to the northeast. The oldest value observed in the Aiguá granitoids is a Pb-Pb age on titanite of 614 ± 3 Ma (Oyhantçabal et al 2007(Oyhantçabal et al , 2009a. Late-tectonic granitic bodies within the Aigua Batholith yielded slightly younger U-Pb zircon ages between 590 and 570 Ma (Preciozzi et al 1999(Preciozzi et al , 2001).…”

“…590 Ma (Oyhantçabal et al 2009a, b): (1) an early, associated with the nucleation of conjugate shear zones and (2) a late event, associated with sinistral reactivation of the NS-trending shear zones. The age of the second transpressional episode is constrained by the 40 Ar/ 39 Ar cooling age of muscovite from quartz mylonite (586 ± 2 Ma; Oyhantçabal et al 2009a) similar to U-Pb ages ranging from 570 to 580 Ma obtained for the alkaline felsic magmatism from Uruguay to southern Brazil Leite et al 1998;Hartmann et al 2002;Chemale et al 2003).…”

“…This major lithospheric-scale suture, denominated by Basei et al (2005) as the Sierra Ballena-Major Gercino Lineament (SBMGL), forms a *1,400 km-long shear system and is marked by strong linear negative gravity anomalies (Mantovani et al 1989;Hallinan et al 1993). The SBMGL is a crustal discontinuity that encompasses several anastomosed shear zones, striking NNE and NE with dominant transcurrent kinematics, which controlled the intrusion of calc-alkaline granites, and in which syn-tectonic calc-alkaline, peraluminous and alkaline granites occurred (Picada 1971;Nardi 2000, 2004;Oyhantçabal et al 2007Oyhantçabal et al , 2009a.…”