Constraining recurrent shear zone activity and the transition from ductile to brittle deformation: White mica geochronology applied to a major Neoproterozoic shear zone in South Brazil

BASEI, MIGUEL A.S.; HEIDELBACH, FLORIAN; HUECK, MATHIAS; PHILIPP, RUY PAULO; OYHANTCABAL, PEDRO; WEMMER, KLAUS; ORIOLO, SEBASTIÁN; SIEGESMUND, SIEGFRIED

Encuentro; Understanding earthquakes using the geological record, Royal Society; 2020

The application of geochronological methods in shear zones is complicated by the recurrent recrystallization of minerals, which has the potential of interfering with the principle of closed isotopic systems of the dated materials. This effect is particularly relevant in structures formed during the transition from ductile to brittle deformation, because of increased fluid circulation and strain localization. K-Ar geochronology of white mica has a good potential of recording processes under these conditions, due to overlapping temperature ranges for mineral crystallization and isotopic closure of different grain-size fractions. This study presents new K-Ar ages of coarse-grained muscovite and multiple clay-sized illite fractions extracted from mylonites and brittle fault gouges of the Dorsal do Canguçu Shear Zone in southernmost Brazil. They are accompanied by detailed macro- to micro-structural and textural analyses, allowing geologically meaningful interpretations of the obtained ages. The new data provide a wide spread of apparent ages of over 100 Myr that are best explained in the context of recurrent deformation under variable temperature conditions and during progressive strain localization. The transition from ductile to brittle activity is constrained by the dating of early fault gouges that are coeval to shortly younger than the youngest fractions extracted from hosting mylonites.