Combining natural microstructures with composite flow laws: an improved approach for the extrapolation of lab data to nature

So far, rheological predictions that are based on experimentally derived flow laws appear inconsistent with microstructural observations in natural calcite mylonites because grain size sensitive (GSS) diffusion creep is predicted while microstructural criteria suggest grain size insensitive (GSI) dislocation creep as the dominant deformation mechanism. A way out of this discrepancy between experiment and nature can be found by combining full grain size distributions with composite GSS+GSI flow laws. We applied this approach to natural carbonate mylonites from the Helvetic Alps, Switzerland. For steady state microstructures, our calculations indicate an increasing GSS component with increasing temperature at geologically constrained strain rates of 10−10–10−11 s−1. The modeling results are consistent with microstructural observations of the natural mylonites: in these rocks, dynamically steady state microfabrics show a systematic change in grain size, grain size distribution and grain aspect ratio with temperature while crystallographic preferred orientations remain of similar strength. When single value mean grain sizes are used rather than grain size distributions, rheological modeling fails to give reasonable results. Further, paleostresses estimated from conventional recrystallized grain size piezometers were found to be unrealistically high, but were reduced to 80–100 MPa when results of the composite modeling were used. Hence, combining composite flow laws with microstructural data from natural mylonites forms a promising approach for a better extrapolation of experimental data to nature and therefore provides new insight into changes in rheology during large-scale geodynamic processes.