INTEGRATION OF DATA FROM DIFFERENT SPATIAL SCALES: USING THE MICROSTRUCTURAL RECORD OF RHEOLOGY IN A FIELD CONTEXT

The structure and tectonics community has long relied on data from deformation experiments, primarily those focused on volumetrically important phases in the lithosphere, to constrain the ductile rheologies of rocks. Recent efforts to extend this database through microstructural analysis of naturally deformed rocks have focused largely on structurally and mineralogically simple systems deformed under relatively low grade metamorphic conditions. In other words, they have utilized rock types and pressure-temperature-strain rate conditions that can be reasonably extrapolated to those of experimental studies – a critical first step. Investigation of polymineralic rocks deformed naturally at pressures, temperatures, and strain rates that can be ‘simulated' by experimental conditions, as well as at temperatures exceeding our ability to trade temperature for strain rate, is more problematic. Quantifying the rheology of polymineralic systems requires characterization of spatial variations, including the scale, abundance, and distribution of high strain zones and how they relate to material heterogeneity.

An example is given from the Santa Rosa mylonite zone of California, in which we explicitly consider the rheologic significance of distinct structural and mineralogical domains (e.g., folia, compositional bands, and high strain zones) that localize deformation and accumulate strain on a variety of spatial scales. To do this, we focus on the microstructural record of both boundaries between, and deformation mechanisms within, domains identified at both the map and outcrop scales. In this analysis, we propose that it is not necessarily the rheology of the most common phase present, nor the rheology of the weakest phase, that controls strength at the regional scale. Rather, the rheology of the system is dominated by the strain rate of the weakest, regionally continuous domain. In our case study, this domain is a mappable and continuous zone of granodiorite ultramylonite within which microstructures record evidence of superplastic flow.