Flow laws and fabric transitions in wet quartzite

Abstract

We investigated quartzite flow laws based on data from 21 different experimental studies on samples deformed with added water at temperatures from 700 to 1200°C. Two clear trends emerge: one with a power-law stress exponent of n=4±0.3 and the other, at higher stresses and lower temperatures, with a stress exponent of n=2.7±0.3. The change in stress exponent indicates a transition in the rate-limiting process of dislocation creep. The deformation conditions where the change in n occurs also correlate with conditions where a distinctive change in the c-axis fabric is observed for samples deformed in general shear. At low stresses, quartz fabrics are defined by a Y-max (where c-axes align parallel to the Y direction of the strain ellipse), suggesting easy slip on the prism <a> system. At higher stresses quartz fabrics indicate easy slip on the basal <a> system. The correlation between the stress exponents and c-axis fabrics suggests that the flow law transition results from a concomitant switch in the dominant slip system and the slip system that rate-limits deformation. Alternatively, recent studies on both experimental and natural samples provide evidence for dislocation-accommodated grain boundary sliding (disGBS) in quartz aggregates; microstructures and mechanical data acquired on samples deformed at conditions near the transition between the n=4 and n=2.7 flows laws are indicative of this process, providing evidence for a rheological model similar to that proposed for ice and olivine. Extrapolation of these newly constrained flow laws delineates estimated deformation conditions where the Y-max and basal <a> fabrics are preserved in the crust. These results provide further support for the application of quartzite flow laws to investigate the rheology of the continental crust and motivate more analyses on the grain size dependence of creep in quartz aggregates.