Lars N. Hansen

Anisotropic viscosity is likely prevalent within the upper mantle, but is usually disregarded in geodynamics models. On a crystal scale, olivine’s intrinsic properties are such that dislocation creep occurs over an order of magnitude more easily along olivine’s [100] symmetry axis than along its [001] axis. However, deforming olivine aggregates generate crystallographic preferred orientations (CPO) with their own macroscopically effective anisotropic viscosities that have proven difficult to estimate from the microscopic anisotropies of individual olivine crystals. Here we present a simple method to derive anisotropic viscosity parameters directly from the CPO mean orientation tensors. To calibrate the method, we created a large database of textures likely to occur in geodynamic simulations. We tested our method within numerical simulations of simple shear with both constant and varying shear directions. Finally, we integrated our method into the geodynamic code ASPECT, where it can be used to explore the geodynamic importance of anisotropic viscosity.

The hydraulic and rheological properties of shear zones that develop in dome‐building eruptions govern the potential for explosive and seismic behavior at those volcanoes. Previous isostatic hot‐pressing experiments demonstrated that crystalline fault gouge undergoes solid‐state sintering at high temperature and pressure, and sintering causes lithification and permeability loss within volcanic shear zones over years. We present results of torsion experiments in which we document, for the first time, gouge rheological behavior at the temperature‐normal stress‐strain rate conditions expected in volcanic shear zones, and determine the effect of shear on solid‐state sintering rate. Gouge sheared at 500°C, 50 or 100 MPa normal stress and 10 ⁻⁴ to 10 ⁻³ s ⁻¹ does not sinter, exhibits strain‐independent behavior, and deforms by distributed granular flow. In contrast, gouge sheared at 900°C, 50 or 100 MPa normal stress and 10 ⁻⁵ to 10 ⁻³ s ⁻¹ sinters, strain‐weakens, and exhibits microstructural evidence of strain localization and significant lithification. Samples sheared at 900°C for <1 hr have final porosities and permeabilities comparable to gouge samples hot‐pressed for 60 hr, indicating sintering is enhanced by a shear stress acting on grain boundaries in addition to a normal stress. These results are used to develop a model for time‐dependent densification by solid‐state sintering that is applicable when gouge is static or shearing. Finally, we propose that sintering‐driven lithification can cause deforming shear zones to transition from aseismic stable sliding to seismogenic sliding, potentially resulting in the drumbeat seismicity frequently observed at dome‐building volcanoes.

Studies of postseismic creep often infer a strong reduction in upper-mantle viscosity after large earthquakes, yet the microphysical mechanism responsible for this behavior remains subject of debate. Here, we present new gas-medium apparatus experiments at 1100–1200°C on pre-dried, annealed Åheim dunite (grain size of ~400 µm) that mimic the postseismic creep after rapid ‘coseismic’ loading. We demonstrate that the transient rheological behavior of these rocks matches remarkably well with predictions of the backstress model, which involves the evolution of dislocation density and long-range interactions among dislocations. Microstructural analyses link the evolution of viscosity up to 1.3 orders of magnitude in our experiments to an increase in dislocation density. Extrapolation to upper mantle conditions illustrates that the backstress model can explain the 1–2 orders of magnitude reduction in upper-mantle viscosity that is inferred from studies of postseismic creep, whereas alternative models involving the plastic anisotropy of olivine cannot.

Dislocation‐based dissipation mechanisms potentially control the viscoelastic response of Earth's upper mantle across a variety of geodynamic contexts, including glacial isostatic adjustment, postseismic creep, and seismic‐wave attenuation. However, there is no consensus on which dislocation‐based, microphysical process controls the viscoelastic behavior of the upper mantle. Although both intergranular (plastic anisotropy) and intragranular (backstress) mechanisms have been proposed, there is currently insufficient laboratory data to discriminate between those mechanisms. Here, we present the results of forced‐oscillation experiments in a deformation‐DIA apparatus at confining pressures of 3–7 GPa and temperatures of 298–1370 K. Our experiments tested the viscoelastic response of polycrystalline olivine—the main constituent of the upper mantle—at stress amplitudes from 70 to 2,800 MPa. Mechanical data are complemented by microstructural analyses of grain size, crystallographic preferred orientation, and dislocation density. We observe amplitude‐ and frequency‐dependent attenuation and modulus relaxation and find that numerical solutions of the backstress model match our results well. Therefore, we argue that interactions among dislocations, rather than intergranular processes (e.g., plastic anisotropy or grain boundary sliding), control the viscoelastic behavior of polycrystalline olivine in our experiments. In addition, we present a linearized version of the constitutive equations of the backstress model and extrapolate it to conditions typical of seismic‐wave propagation in the upper mantle. Our extrapolation demonstrates that the backstress model can explain the magnitude of seismic‐wave attenuation in the upper mantle, although some modification is required to explain the weak frequency dependence of attenuation observed in nature and in previous experimental work.

Dislocation-based dissipation mechanisms potentially control the viscoelastic response of Earth’s upper mantle across a variety of geodynamic contexts, including glacial isostatic adjustment, postseismic creep, and seismic-wave attenuation. However, there is no consensus on which dislocation-based, microphysical process controls the viscoelastic behavior of the upper mantle. Although both intergranular (plastic anisotropy) and intragranular (backstress) mechanisms have been proposed, there is currently insufficient laboratory data to discriminate between those mechanisms. Here, we present the results of forced-oscillation experiments in a deformation-DIA apparatus at confining pressures of 3–7 GPa and temperatures of 298–1370 K. Our experiments tested the viscoelastic response of polycrystalline olivine—the main constituent of the upper mantle—at stress amplitudes from 70 to 2800 MPa. Mechanical data are complemented by microstructural analyses of grain size, crystallographic preferred orientation, and dislocation density. We observe amplitude- and frequency-dependent attenuation and modulus relaxation and find that numerical solutions of the backstress model match our results well. Therefore, we argue that interactions among dislocations, rather than intergranular processes (e.g., plastic anisotropy or grain boundary sliding), control the viscoelastic behavior of polycrystalline olivine in our experiments. In addition, we present a linearized version of the constitutive equations of the backstress model and extrapolate it to conditions typical of seismic-wave propagation in the upper mantle. Our extrapolation demonstrates that the backstress model can explain the magnitude of seismic-wave attenuation in the upper mantle, although some modification is required to explain the weak frequency dependence of attenuation observed in nature and in previous experimental work.

Subduction zone dynamics depend on the rheological behavior of shear zones defining the plate interface, which can accommodate a spectrum of slip from earthquakes to creep. In cold subduction zones, high-pressure, low-temperature metamorphism of the oceanic crust forms blueschist-facies rocks within the interface. Geologic observations demonstrate that deformation at the plate interface is often localized into blueschists with fabrics dominated by glaucophane, a sodic amphibole. Microstructural observations of exhumed blueschists suggest that glaucophane typically deforms by dislocation-accommodated mechanisms. Recent experiments have made progress in developing new flow laws for diffusion creep and dislocation creep. However, the rheological behavior of blueschists remains under-constrained due to challenges arising from phase stability that limits experimental temperatures. Therefore, we conducted experiments at very high pressures to suppress brittle deformation and favor dislocation-accommodated mechanisms to investigate low-temperature plasticity in glaucophane. We conducted cyclical loading experiments in the deformation-DIA on blueschist cores, both parallel and perpendicular to foliation, and cold-pressed glaucophane powders. Experimental conditions covered confining pressures of 6–8 GPa and temperatures of 20–800 °C at strain rates of ~10–4 s–1. We observe temperature-dependent yield and flow stresses and nearly temperature-independent backstresses. These mechanical results as well as the microstructures are characteristic of deformation dominated by dislocation glide. These experiments provide the foundation for a constitutive law describing low-temperature plasticity in glaucophane, which will serve as an input for geodynamic models that aim to understand the physics of strain localization, quantify steady-state interface strength, and explain the mechanics of slip transients.

To examine the microstructural evolution that occurs during transient creep, we deformed samples of polycrystalline olivine to different strains that spanned the initial transient deformation. Two sets of samples with different initial grain sizes of 5 μm and 20 μm were deformed in torsion at T = 1,523 K, P = 300 MPa, and a constant shear strain rate of 1.5 × 10⁻⁴ s⁻¹, during which both sets of samples experienced strain hardening. We characterized the microstructures at the end of each experiment using high‐angular resolution electron backscatter diffraction (HR‐EBSD) and dislocation decoration. In the coarse‐grained samples, dislocation density increased from 1.5 × 10¹¹ m⁻² to 3.6 × 10¹² m⁻² with strain. Although the same final dislocation density was reached in the fine‐grained samples, it did not vary significantly at small strains, potentially due to concurrent grain growth during deformation. In both sets of samples, HR‐EBSD analysis revealed that intragranular stress heterogeneity increased in magnitude with strain and that elevated stresses are associated with regions of high geometrically necessary dislocation density. Further analysis of the stresses and their probability distributions indicate that the stresses are imparted by dislocations and cause long‐range elastic interactions among them. These characteristics indicate that dislocation interactions were the primary cause of strain hardening during transient creep in our samples. A comparison of the results to the predictions of three recent models reveals that the models do not correctly predict the evolution in stress and dislocation density with strain in our experiments due to a lack of previous such data in their calibrations.

The effect of small concentrations of intracrystalline water on the strength of olivine is significant at asthenospheric temperatures but is poorly constrained at lower temperatures applicable to the shallow lithosphere. We examined the effect of water on the yield stress of olivine during low‐temperature plasticity using room‐temperature Berkovich nanoindentation. The presence of water in olivine (1,600 ppm H/Si) does not affect hardness or yield stress relative to dry olivine (≤40 ppm H/Si) outside of uncertainty but may slightly reduce Young’s modulus. Differences between water‐bearing and dry crystals in similar orientations were minor compared to differences between dry crystals in different orientations. These observations suggest water content does not affect the strength of olivine at low homologous temperatures. Thus, intracrystalline water does not play a role in olivine deformation at these temperatures, implying that water does not lead to weakening in the coldest portions of the mantle.