Structural and metamorphic evolution of a Paleozoic middle- to lower-crustal section in the western Sierras Pampeanas of northwest Argentina

The Sierra de Pie de Palo and Loma de las Chacras of the western Sierras Pampeanas, northwest Argentina, preserve a middle- to lower-crustal sequence of greenschist through granulite facies rocks. Top-to-the-west deformation associated with east-dipping subduction beneath the Famatina arc began by 515 Ma and requires that the region is not basement to the accreted Precordillera terrane but was autochthonous to the Gondwana margin by 515 Ma. Ordovician shear zones associated with collision of the Precordillera terrane propagated towards the foreland from east to west, as recorded by U-Pb zircon ages from granitic and intermediate melts, Lu-Hf garnet ages of prograde metamorphism, and 40Ar/39 cooling ages. Muscovite and hornblende 40Ar/39Ar cooling ages suggest syn-convergent extension began at or prior to 439 Ma and led to regional cooling to 415 Ma.Regional granulite facies metamorphism associated with magmatic accretion resulted in isothermal conditions throughout the middle and lower crust of the arc. Melting was synchronous with the peak episode of shortening related to collision of the Precordillera terrane. Decompression to middle crustal levels along a counterclockwise pressure-temperature path did not occur until after 407 Ma as recorded by Lu-Hf garnet ages U-Pb titanite ages and was likely associated with a change in plate boundary conditions.Shear zones formed prior to 439 Ma occurred at high temperatures in the lower crust where extrapolation of empirically derived ow laws predict many lithologies to be uniformly weak. The Duraznos shear zone preserves variations in temperature, grain size, and fluid conditions across the shear zone. The observations are used to calculate a four order of magnitude variation in strain rate and effective viscosity across the shear zone. Temperature, grain size, and fluid conditions vary over relatively small distances within the middle and lower crust of deforming orogens and therefore differential stress, strain rate, and effective viscosity should be expected to vary.