Quartz Flow Law Revisited: The Significance of Pressure Dependence of the Activation Enthalpy

Abstract

An accurate flow law for dislocation creep of quartz aggregates is critical for the understanding of continental rheology, numerical modeling of lithospheric processes, and the interpretation of microstructures of natural quartz-bearing mylonites. Despite many decades of research, considerable discrepancies still exist among quartz flow laws determined from different experiments. We demonstrate that the key to reconcile these discrepancies is to consider the pressure dependence of the activation enthalpy. From existing high-quality creep experiments on quartz aggregates, we critically identified test runs having microstructures and stress-strain curves indicating steady state regimes 2 and 3 dislocation creep and used the data to obtain a set of flow law parameters most consistent among existing experiments. Because estimates of strain rate and stress from natural mylonites are bound with large uncertainties, they cannot be used to construct a flow law better than one determined from well-controlled creep experiments. A large number of geological studies and modern GPS observations suggest that crustal scale ductile shear zones likely flow at a strain rate range between 10 −13 and 10 −11 s −1 . In this strain rate range, our flow law predicts flow stresses broadly consistent with stress estimates from many natural mylonites based on well-calibrated piezometers for dynamically recrystallized quartz grains.

Assessment of Quartz Grain Growth and the Application of the Wattmeter to Predict Quartz Recrystallized Grain Sizes

“…All rights reserved.experiments conducted with varying confining pressure at constant temperature (see Figure16inChernak et al, 2009). Pressure-stepping experiments conducted at a constant strain rate, temperature, and fluid content also show agreement between the mechanical results ofKronenberg and Tullis (1984),Chernak et al (2009) and Holyoke and Kronenberg (2013).An additional complicating factor in the analysis byLu and Jiang (2019) is that the role of water fugacity has been analyzed using data (the three data points fromKronenberg and Tullis, 1984) that follow the two different flow laws determined by Tokle et al (2019) (see Figure 7a in Tokle et al, 2019). After accounting for this issue, Tokle et al (2019) determined r values for both flow laws in the range between 1.0 -1.5 (neglecting the possible role of the activation volume).…”

“…Recent analyses have highlighted that extrapolation of quartzite flow laws can be impacted by neglecting the activation volume term in the activation enthalpy (Lu and Jiang, 2019). The activation volume for quartz grain growth has not previously been calculated.…”

“…Recent analyses have highlighted that extrapolation of quartzite flow laws can be impacted by neglecting the activation volume term in the activation enthalpy (Lu & Jiang, 2019). Using a compiled data set of select TOKLE AND HIRTH 10.1029/2020JB021475 14 of 21 2019) grain growth law.…”

The Deep Structure and Rheology of a Plate Boundary-Scale Shear Zone: Constraints from an Exhumed Caledonian Shear Zone, NW Scotland

“…Variability in values of H, n, and r in equation 2, which may stem from differences in starting materials, experimental conditions (including deformation apparatus), poorly constrained fluid content, among other uncertainties, can result in order-of-magnitude differences in predicted strain rate. More recent work also introduces a pressure sensitivity for activation enthalpy (H), adding further complexity [133]. We contend that experimental-based constitutive laws describing rock rheology should be thoroughly tested, and if necessary altered, to better fit well-constrained fieldbased data, as suggested by Hirth et al [113].…”

“…(c) Significant variability exists between published calibrations of flow laws for dislocation creep in quartz [113,114,[131][132][133][134][135]. Variability in values of H, n, and r in equation 2, which may stem from differences in starting materials, experimental conditions (including deformation apparatus), poorly constrained fluid content, among other uncertainties, can result in order-of-magnitude differences in predicted strain rate.…”