CONSTRAINING DEFORMATION PROCESSES IN THE BRITTLE-DUCTILE TRANSITION REGION BELOW A SUBDUCTION SEISMOGENIC ZONE

The deformation processes in the brittle-ductile transition region are poorly constrained and critical to understanding both the magnitude of earthquakes within the subduction seismogenic zone, and slow slip and tremor downdip of this region. Because this portion of the plate interface in actively deforming zones is impossible to sample, studies of exhumed subduction zones are a way to constrain this transition. Here we present field observations and microstructural data in the form of photomicrographs, thin-section scale x-ray maps and electron backscatter diffraction (EBSD) to constrain the rheology of a paleosubduction interface in the Central Alps during deformation and fluid-rock interaction. We studied three samples across a 10 meter transect within the lowest 10 meters of the overring Austroalpine plate at the contact with the Penninic subduction plate shear zone. Our granodiorite samples are exhumed from paleodepths of 30-35 km. Cataclasites, microfractures, and crack-seal quartz ± calcite veins are evidence of brittle deformation while a ductile foliation fabric indicates viscous deformation at conditions of 400-430ºC & 0.80-0.95 GPa. EBSD data shows progressive increase in viscous deformation and mineralogic changes closer to the contact. As the samples progress towards the shear zone the crystallographic preferred orientation and grain misorientation of quartz and albite decreases suggesting the controlling deformation process changes from dislocation creep to diffusion creep due to pinning of the albite and quartz by phengite. This coincides with progressive fluid-rock modification observed in photomicrographs and chemical x-ray maps. We observe a decrease in albite replaced by an increase in quartz and phengite across the three samples. The breakdown of albite, and its replacement with more water rich minerals like phengite, the quartz +/- calcite veins, and the progressive shift from dislocation to fluid-aided diffusive mechanisms demonstrates increased fluid-rich interactions nearer to the plate interface contact. Increased fluid-rock interaction impacts the viscous deformation mechanisms and strength of these rocks. Diffusion creep occurs in low stress environments, suggesting that fluid- rock interactions lead to a low stress environment below the locked seismogenic zone.