Strain rates recorded in the perimeter-area relationship of recrystallized quartz aggregates from the Sandhill Corner shear zone, Maine, USA

Abstract

Current available methods for determining strain rates during viscous deformation are limited, generally relying on the preservation of appropriate microstructures. A lesser-known method to determine strain rates of monomineralic quartz aggregates relates the perimeter-area fractal dimension (D-value) of grain boundaries to deformation conditions. We explore this method by applying it to the Sandhill Corner shear zone (SCSZ), a seismogenic shear zone exhumed from the base of the frictional-to-viscous transition. Strain-rate patterns from D-values are similar to those from flow laws combined with grain size piezometry and predict comparable magnitudes to rates from flow laws using the theoretical piezometer of Shimizu (2012). We compare quartz D-values with proxies for grain shape and grain boundary roughness and find a clear correlation between D-value and grain size, suggesting it may record flow stress. We interpret the disparity in spatial trends between D-value and recrystallized grain size within the inner shear zone as reflecting the influence of transient deformation events associated with the seismic cycle. While the D-value is commonly referred to as a measure of the degree of serration or roughness of grain boundaries, this study suggests a more complex interrelationship of the D-value, grain size, aspect ratio, and roundness. Future field work and experimental calibrations are encouraged to test our findings and refine strain-rate estimates derived from D-values.