Natural and Experimental Constraints on a Flow Law for Dislocation‐Dominated Creep in Wet Quartz

Abstract

Constitutive relationships (i.e., flow laws) relate strain rate () to differential stress (σ), temperature (T), pressure (P), water fugacity ( H O 2 f ), and material properties for a given deformation mechanism. As one of the most abundant minerals in the crust, quartz is thought to control crustal rheology and is commonly used to model and predict the viscous behavior of the middle and lower crust (e.g., Brace & Kohlstedt, 1980;Hirth et al., 2001). At these depths, quartz deforms by dislocation creep, a thermally activated process that is driven by the nucleation, buildup, and motion of dislocations, resulting in viscous flow. Dislocation creep

Quartz rheology constrained from constant-load experiments: Consequences for the strength of the continental crust

“…A careful inspection reveals that the lower n-value studies can be connected with highconfining pressure ones, but exclusively using solid-salt-assemblies (Table 1). More complication can arise from the fact that any theoretical and statistical re-analysis (Fukuda and Shimizu, 2017;Tokle et al, 2019;Lusk et al, 2021) of earlier experimental studies results in a substantially different flow law parameters from what is originally reported (see supplementary text S2, S3). Therefore, a re-evaluation of the wet quartzite GSI flow law for coarse-grained polycrystalline aggregates, considering the above-mentioned contradictions, is needed.…”

“…However, in this way they are essentially treating all the earlier studies, utilizing different techniques, apparatus, and materials for experiments in the same way. On the other hand, Lusk et al (2021) implied that low confining pressures (<0.56 GPa) resulted in higher n-values and higher pressures (0.7-1.6 GPa) in lower ones. As noted by the authors, the low confining pressure conditions are restricted to Paterson-type gas medium apparatus (Table 1), in contrast to Griggs-type apparatus for high confining pressure experiments.…”

“…Recently, Tokle et al (2019) and Lusk et al (2021) compiled and reviewed earlier experimental studies in order to obtain usable flow parameters. They proposed two distinct correlations with the experimental conditions at which those studies were performed.…”

“…The n-value vary from 4 to <2, while the Q value vary from 120 to 242 kJ/mol, in addition to large differences in the A-value. Due to the powerlaw nature of equation ( 2), minor changes in flow law parameters can lead to significant differences in the predicted crustal strength when extrapolated to geological conditions (Hirth et al, 2001;Lusk et al, 2021;Tokle et al, 2019). In several cases the frequently used experimentally-derived flow laws can overestimate the strength of a particular region.…”

Porosity induced by dislocation dynamics in quartz-rich shear bands of granitic rocks

“…When rocks deform by viscous flow, their deformation mechanisms mostly include grain-size-insensitive (GSI) dislocation creep versus grain-size-sensitive (GSS) diffusion creep—or equivalents—that compete with each other to accommodate ductile strain. While diffusion creep operates by migration of vacancies through the crystal lattice in combination with important grain boundary sliding (GBS), dislocation creep involves lattice-controlled crystal plasticity with limited GBS 7 . Because micro-pores are commonly documented in fine-grained, polymineralic shear zones, typically where diffusion creep is expected to be dominant 2 – 4 , the way that pores open at grain boundaries is today visualized as a consequence of GBS according to the model of creep cavitation 2 , 3 , 8 – 13 .…”