Structural and metamorphic evolution of the Sierras Australes (Buenos Aires Province/Argentina)

Abstract

The Precambrian Basement Complex rocks as well as the Early to Late Palaeozoic cover sediments of the Sierras Australes were affected by one dominant folding and shearing event verging northeastward during Middle to Late Permian times. Strain estimates point to a minimal flattening and lateral shortening of the sedimentary cover sequence of c.20% and c.24% respectively. Continuing rotational deformation within first and second shear and thrust zones is documented by second and third folding and shearing. To the SW the fold belt changes into a fold and thrust belt where imbrication involves the basement. The deformational events were accompanied and outlasted by anchizonal to greenschist facies metamorphism proven by illite crystallinity and quartz deformation and recrystallization data. A temperature increase from ENE to WSW and also from N to S allowed a more ductile deformation of the rock sequence due to different deformation mechanisms operating. The folding and thrusting events were followed by strike-slip shearing on subvertical shear planes and shear zones under an overall sinistral transpressive regime. A model for the tectonic evolution of the Sierras Australes is proposed and some implications for its setting in the Gondwana reconstruction are given.