LARROVERE mariano Alexis
congresos y reuniones científicas
Strain (de)-localization in the ~10 km wide Cuesta de Randolfo mylonite zone, Famatinian arc
CAWOOD, TARRYN; PATERSON, SCOTT R.; RATSCHBACHER, BARBARA; LUSK, ALEXANDER; LARROVERE, MARIANO A.; ALASINO, PABLO H.; RICK, CHRISTOPHER; MEMETI, VALBONE
Conferencia; GSA Annual Meeting; 2017
The ~470 Ma Famatinian Arc in Argentina hosts numerous wide shear zones in its central, S-type pluton dominated part,including the ~10 km Cuesta de Randolfo Mylonite Zone (RMZ). An understanding of the mechanisms of strain localizationactive (or not) in the RMZ may help explain the occurrence of so many wide shear zones in this arc, and contribute to ourunderstanding of strain localization processes in general.We combine field mapping with microscopy to examine the distribution of strain between compositionally- and texturallydifferent map units within the RMZ. We compare how the distance over which protomylonite gives way to ultramylonite inoutcrop, and the proportion and grain size of recrystallized quartz in thin section, change along transects through units ofdifferent composition and/or age.The RMZ is a composite shear zone, comprising two distinct structural domains. Across both domains, much of the shearstrain occurs in bodies of relatively younger alkali-feldspar, two-mica and tourmaline-rich granite. This unit displaysdistributed strain, accommodated by extensive Subgrain Rotation (SGR) recrystallization of quartz. In contrast, bodies ofolder, more biotite-rich but tourmaline-absent granodiorite appear significantly less deformed, with weak to no solid-statefoliation, except where cut by narrow (cm- to m-scale) mylonite to ultramylonite zones.The younger two-mica granites were emplaced during regional shortening, as evidenced by steep magmatic fabrics thatparallel regional solid-state foliations, and indicate a continuous magmatic to solid-state deformation transition. They weretherefore less competent than the older, colder granodiorite, and could accommodate greater strain. The granites thusdeveloped only a moderately intense foliation, distributed over a wide zone of weak rock. Furthermore, we propose thatthe solidus temperature of the tourmaline-bearing granites was lowered by the presence of abundant boron in the melt.Strain localization mechanisms such as strain softening by water addition or reaction, or a transition to grain size sensitivecreep, either did not occur or did not have a significant impact at these near-solidus conditions. The granodiorites,however, were significantly below their solidus, and thus incapable of distributing strain over wide zones.