High strain-rate deformation fabrics characterize a kilometers-thick Paleozoic fault zone in the Eastern Sierras Pampeanas, central Argentina

Abstract

High strain rate fabrics that transgress a crustal depth range of ca. 8–22 km occur within a major Paleozoic fault zone along the western margin of the Sierras de Córdoba, central Argentina. The NNW-striking, east-dipping ‘Tres Arboles’ fault zone extends for at least 250 km and separates two metamorphic terranes that reached peak temperatures in the middle Cambrian and Ordovician, respectively. Exposed fault zone rocks vary from a 16-km-thickness of ultramylonite and mylonite in the southern, deepest exposures to <5 km in the northern, shallower-level exposures. Three transects across the fault zone have been examined in detail. In the deepest section, newly crystallized sillimanite needles define the foliation and wrap garnet and feldspar theta- and delta-type porphyroclasts in a biotite-rich ultramylonite. Geothermometry and preserved microstructures in feldspar and quartz indicate deformation at temperatures >520 °C. Reaction-enhanced grain size reduction and grain boundary sliding were the predominant deformation mechanisms in these high strain rate rocks. Ultramylonites in the intermediate depth section also contain evidence for grain boundary sliding and diffusional mass transfer, although overprinted by late stage chlorite. In the shallowest exposed section, rocks were deformed at or near to the brittle–ductile transition to produce mylonite, cataclasite, shear bands and pseudotachylyte. The overall structure of the Tres Arboles zone is consistent with existing fault zone models and suggests that below the brittle–ductile transition, strain compatibility may be accommodated through very thick zones of high temperature ultramylonite.