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Shear strain localization refers to the phenomenon of accumulation of material deformation in narrow slip zones. Many materials exhibit strain localization under different spatial and temporal scales, particularly rocks, metals, soils, and concrete. In the Earth's crust, irreversible deformation can occur in brittle as well as in ductile regimes. Modeling of shear zones is essential in the geodynamic framework. Numerical modeling of strain localization remains challenging due to the non‐linearity and multi‐scale nature of the problem. We develop a numerical approach based on graphical processing units (GPU) to resolve the strain localization in two and three dimensions of a (visco)‐hypoelastic‐perfectly plastic medium. Our approach allows modeling both the compressible and incompressible visco‐elasto‐plastic flows. In contrast to symmetric shear bands frequently observed in the literature, we demonstrate that using sufficiently small strain or strain rate increments, a non‐symmetric strain localization pattern is resolved in two‐ and three‐dimensions, highlighting the importance of high spatial and temporal resolution. We show that elasto‐plastic and visco‐plastic models yield similar strain localization patterns for material properties relevant to applications in geodynamics. We achieve fast computations using three‐dimensional high‐resolution models involving more than 1.3 billion degrees of freedom. We propose a new physics‐based approach explaining spontaneous stress drops in a deforming medium. Plain Language Summary: Strain localization is the accumulation of strain in narrow regions of rocks and other materials like metals, soils, and concrete, occurring at different scales. The strength of most geomaterials, particularly rocks, is strongly pressure‐dependent, with strength increasing with increasing pressure. We developed efficient numerical algorithms using High‐Performance Computing (HPC) and graphical processing units (GPUs) to model strain localization in 2D and 3D for applications in geodynamics and earthquake physics. Unlike previous models, our method reveals non‐symmetrical patterns by using very small strain increments, highlighting the need for high‐detail modeling. We found that elasto‐plastic and visco‐plastic models show similar strain patterns for relevant materials. Our method also achieves fast, detailed computations with over 1.3 billion variables and offers a new explanation for sudden stress drops in deforming materials. Key Points: We resolve material instability during deformation resulting in a non‐symmetric pattern of strain localizationWe demonstrate the similarity in patterns of strain localization between frictional and time‐dependent plasticity modelsWe achieve fast numerical simulations in high‐resolution model setups in three dimensions involving more than 500 million degrees of freedom [ABSTRACT FROM AUTHOR]

Constraining strain localization and the growth of shear fabrics within brittle fault zones at sub‐seismic slip rates is important for understanding fault strength and frictional stability. We conducted direct shear experiments on simulated sandstone‐derived fault gouges at an effective normal stress of 40 MPa, a pore pressure of 15 MPa, and a temperature of 100°C. Using a passive strain marker and X‐ray Computed Tomography, we analyzed the spatial distribution of deformation in gouges deformed in the strain‐hardening, subsequent strain‐softening, and then steady‐state regimes at displacement rates of 1, 30, and 1,000 µm/s. We developed a machine‐learning‐based automatic boundary detection method to recognize the shear fabrics and quantify displacement partitioning between each fabric element. Our results show fabrics oriented along R1 and Y (including boundary) shears are the two major fabric elements. At rates of 1 and 30 µm/s, the relative amount of displacement on R1 shears is displacement dependent, increasing to ∼20% of the total displacement up to the strain‐softening stage, then decreasing to ∼10%–18% at the steady state. This trend is absent at the high rate where ∼18% of the displacement occurs on R1 shears throughout all investigated stages. At all rates, the relative amount of displacement on Y shears increases linearly with displacement to a total of larger than 50% at the steady state. Our study provides constraints on the development of the active slip zone, which is an important factor controlling heating and weakening associated with small‐magnitude earthquakes with limited displacement (mm‐dm), such as induced seismicity. Plain Language Summary: In the past few decades, several studies have focused on the mechanical behavior of simulated fault zones to understand earthquake nucleation. However, minor attention has been paid to the microstructural characterization of fault‐zone gouges due to the difficulties in quantifying deformation. An understanding of brittle fault‐zone fabrics and their development provides crucial constraints on the mechanical strength and stability of faults. We conducted laboratory experiments on simulated sandstone‐derived fault gouges with a passive strain marker under the conditions relevant to earthquake nucleation to explore the development and evolution of shear zone fabrics and their relations to fault strength. We combined X‐ray Computed Tomography (XCT) and a custom‐designed machine‐learning‐based automatic boundary detection method to analyze the spatial distribution of gouge deformation and to quantify displacement partitioning between deformation features. The results show that our samples have similar evolution of the shear fabrics, partitioning of displacement, and mechanical response with increasing shear strain at all tested nucleation velocities. The evolution of the mechanical behavior from strain‐hardening, to softening, to steady‐state stages is related to the transformation of R1 to shear‐parallel shear bands. Up to 50% of the total displacement can be accommodated within shear‐parallel shear bands, facilitating strain weakening of the materials. Key Points: We performed 3‐D analyses on the evolution of shear zone fabrics within sandstone‐derived fault gouges utilizing the X‐ray CT techniqueOur samples show similar evolution of shear fabrics, slip partitioning, and mechanical response with shear strain at all tested velocitiesUp to 50% of the total imposed displacement can be accommodated within shear‐parallel shear bands during earthquake nucleation [ABSTRACT FROM AUTHOR]

The theory of rate and state friction unifies field, laboratory, and theoretical analysis of the evolution of slip on natural faults. While the observational study of earthquakes and aseismic fault slip is hampered by its strong multi‐scale character in space and time, numerical simulations are well‐positioned to link the laboratory study of grain‐scale processes to the scale at which rock masses move. However, challenges remain in accurately representing the complex and permanently evolving sub‐surface fault networks that exist in nature. Additionally, the common representation of faults as interfaces may miss important physical aspects governing volumetric fault system behavior. In response, we propose a transient viscous rheology that produces shear bands that closely mimic the rate‐ and state‐dependent sliding behavior of equivalent fault interfaces. Critically, we show that the expected tendency of the continuum rheology for runaway localization and mesh dependence can be halted by including an artificial diffusion‐type regularization of anelastic strain rate in the softening law. We demonstrate analytically and numerically using a simplified fault transect that important aspects of the frictional behavior are not significantly affected by the introduced regularization. Any discrepancies with respect to the interfacial description of fault behavior are critically evaluated using one dimensional numerical velocity stepping and spring‐slider experiments. Since no new physical parameters are introduced, our model may be straightforwardly used to extend the existing modeling techniques. The model predicts the emergence of complex patterns of shear localization and delocalization that may inform the interpretation of complex damage distributions observed around faults in nature. Plain Language Summary: How, where, and when earthquakes nucleate is one of the great questions in science and society that, despite steady progress, has hardly been answered to any practical degree. Based on field observations, laboratory experiments, and theoretical work it is believed that a cocktail of escalating mechanical, chemical, and thermal grain‐scale processes cause the sudden and rapid onset of earthquakes. The net effect of these processes are characterized by an immediate strengthening and a gradual weakening response to deformation and are unified in simplified form in the theory of "rate and state friction." This theory is commonly used in computer simulations of earthquake sequences. We point out that rate and state friction, unlike some physical theories of earthquake rupture, does not incorporate a diffusion process such as for example heat conduction. We show the introduction of an artificial diffusion process can prevent the mathematical reduction of a fault zone to a two‐dimensional interface while retaining the properties of the original friction law. This in turn enables simulation techniques that rely on an interface‐free description of the earth and promise to provide new insights into the spontaneous organization of seismic and aseismic phenomena in developing fault zones. Key Points: We reformulate the empirical rate and state friction law as a bulk viscous flow law in terms of anelastic shear strain rateWe show how mesh independence is achieved by including a gradient‐like nonlocal anelastic shear strain rate equivalentWe show analytically and numerically that the proposed continuum model closely reproduces existing results of rate and state friction [ABSTRACT FROM AUTHOR]

To study the viscosity of bridgmanite and ferropericlase aggregate, uniaxial compression deformation experiments on pre‐synthesized post‐spinel phase and bridgmanite two‐layered samples were conducted under top lower mantle pressure and 1773 K utilizing DT‐Cup apparatus. Up to the strain of 0.25 ± 0.05, the observed comparable strain of the bridgmanite and post‐spinel samples suggests the bridgmanite dominates the bulk viscosity of the post‐spinel without strain localization in periclase. The microstructures of the deformed post‐spinel samples show evidence of a similar strain of periclase with the bulk strain without strain partitioning. Texture analyses of bridgmanite indicate a dominant slip plane (100), with a steady state fabric strength achieved within the strain of 0.12 ± 0.01. The current experiment has provided no evidence about an onset of strain localization of ∼30 vol.% periclase at 0.25 strain. Our observations provide direct experimental verification of bridgmanite controlled rheology under low strain magnitude, which should be considered in geodynamical models which include mantle compositional and rheological evolution in the lower mantle. Plain Language Summary: The Earth's lower mantle occupies ∼65% volume of Earth's mantle and plays an important role in mantle dynamics. The major constituent mineral of the lower mantle, bridgmanite, might dominate the rheology of the lower mantle. On the other hand, despite its small proportion, there is a chance that ferropericlase controls the rheology of the lower mantle due to its significant softness compared with bridgmanite. In this study, we conducted uniaxial deformation experiments on pre‐synthesized post‐spinel (∼70% bridgmanite +∼30% periclase) and bridgmanite two‐layered samples under top‐most lower mantle conditions up to the strain of 0.25 ± 0.05. The bridgmanite and post‐spinel samples showed similar strains, suggesting that the bridgmanite controls the bulk strength of the post‐spinel. This result is significant for understanding the viscosity structure of Earth's lower mantle. Key Points: Deformation experiments were conducted on pre‐synthesized post‐spinel and bridgmanite two‐layered samples using DT‐Cup apparatusNo strain localization was observed in deformed post‐spinel up to the strain of 0.25 ± 0.05Bridgmanite dominates the bulk rheology of post‐spinel under the current experimental conditions [ABSTRACT FROM AUTHOR]

We investigate the finite deformation of a weak inclusion embedded in a strong matrix. The goal is to understand the possible causes for shear localization that would explain the presence of geochemical heterogeneity in the Earth's lower mantle. The weak phase in the lower mantle is ferropericlase (Fp), and the strong phase is bridgmanite (Br). We herein consider deformation by diffusion creep for which some supporting evidence is available. When an inclusion deforms by diffusion creep, deformation itself modifies its geometry and stress distribution. The modified stress distribution in turn leads to strain‐dependent rheological properties due to the changes in diffusion path length and the stress gradient that drives diffusion flux. We investigate the evolution of deformation of a weak inclusion (i.e., Fp‐like weak grain embedded in Br‐like strong matrix) using the Eshelby theory (for stress and strain rate fields) combined with a theory of diffusional mass transport caused by the gradient in the normal stress within the inclusion. We found that finite strain leads to a significant strain weakening under simple shear deformation but not under axial deformation. Since diffusion creep is thought to be a dominant mechanism of the lower mantle rheology, the results of the present study provide a basis for investigating the nature of shear localization and its important implication for the preservation of geochemical heterogeneity and the distribution of seismic anisotropy in the lower mantle. Plain Language Summary: Localized deformation is a possible explanation for the preservation of geochemical heterogeneity in the lower mantle. The lower mantle is made of two major phases: bridgmanite (Br [∼70%]) and ferropericlase (Fp [∼20%]) (and other minor phases). Lab data show that Fp is much weaker than Br. We investigate the deformation of a weak Fp‐like inclusion embedded in a strong Br‐like matrix as a function of strain. We consider finite strain deformation by diffusion creep. Deformation of a weaker phase evolves with strain caused by the evolution of internal stress (stress acting on the inclusion) and the shape of an inclusion. A substantial weakening of inclusion is observed particularly for shear deformation. These results provide an important implication for the preservation of geochemical heterogeneity and the distribution of seismic anisotropy in the lower mantle of the Earth. Key Points: Diffusion creep rate is highly sensitive to the shape of the Fp grain (embedded in a Br matrix) and boundary normal stress stateIn the setting of a two‐phase mixture, the diffusion creep rate of Fp dramatically increases under remote simple shear deformationThe strain‐dependent diffusion creep of Fp may lead to shear localization that forms the weak boundary layer within the lower mantle [ABSTRACT FROM AUTHOR]

Identifying the mechanisms that control the formation of compaction bands is of high interest in reservoir mechanics since these structures may drastically affect the performance of geosystems operations. Considering the difficulty to identify compaction bands in carbonate samples tested in the laboratory, the Digital Volume Correlation technique is applied here and proves to be a relevant method. X‐Ray Computed Tomography (XRCT) images of Saint‐Maximin limestone centimetric samples are recorded before and after several triaxial loading stages and deformation maps are built. A new postprocessing method based on the analysis of the kinematics throughout the observed localization bands is proposed to identify their type. Compaction bands are identified at relatively high‐confining pressures, while shear bands are observed at lower confinements. The brittle‐ductile transitional regime reveals the formation of compactive shear bands, while a diffuse compaction is observed under hydrostatic loading. The effect of porosity heterogeneities on strain localization is explored by computing 3D porosity maps from calibrated XRCT images. Compaction bands are found to dominantly lay inside high‐porosity zones, while shear bands can cross both high‐ and low‐porosity zones. The band orientation is found to be controlled primarily by the confining pressure. Moreover, the porosity heterogeneity strongly affects the volumetric behavior inside the deformation bands, with a dilatant behavior identified in low‐porosity zones in contrast to a compactive behavior observed in high‐porosity zones. Finally, Scanning Electron Microscopy observations reveal that calcite grain crushing is dominant in high‐porosity zones, while intergranular cement cracking occurs in denser zones. Key Points: Computed tomography and digital volume correlation are efficient tools to identify the formation and type of deformation bands in limestone samplesThe porosity heterogeneity controls the onset and propagation of compaction bandsCalcite grain crushing is dominant in high‐porosity zones whereas intergranular cement cracking occurs in low‐porosity zones [ABSTRACT FROM AUTHOR]

Strain localization in the lithosphere and the formation, evolution, and maintenance of resulting plate boundaries play a crucial role in plate tectonics and thermo‐chemical mantle convection. Previously activated lithospheric deformation zones often appear to maintain a "memory" of weakening, leading to tectonic inheritance within plate reorganizations including the Wilson cycle. Different mechanisms have been proposed to explain such strain localization, but it remains unclear which operates on what spatio‐temporal scales, and how to best incorporate them in large‐scale mantle convection models. Here, we analyze two candidates, (1), grain‐size sensitive rheology and, (2), damage‐style parameterizations of yield, stress which are sometimes used to approximate the former. Grain‐size reduction due to dynamic recrystallization can drive localization in the ductile domain, and grain growth provides a time‐dependent rheological hardening component potentially enabling the preservation of rheological heterogeneities. We compare the dynamic weakening and hardening effects as well as the timescales of strength evolution for a composite rheology including grain‐size dynamics with a pseudo‐plastic rheology including damage‐ (or "strain"‐) dependent weakening. We explore the implications of different proposed grain‐size evolution laws, and test to which extent strain‐dependent rheologies can mimic the weakening and hardening effects of the more complex micro‐physical behavior. Such an analysis helps to better understand the parallels and differences between various strain‐localization modeling approaches used in different tectonics and geodynamics communities. More importantly, our results contribute to efforts to identify the key ingredients of strain‐localization and damage hysteresis within plate tectonics and how to represent those in planetary‐scale modeling. Key Points: Comparative analysis of strain localization and damage memory for grain‐size dependent and strain/damage parameterized rheologiesIdentification of key ingredients of strain localization and damage hysteresis and how to represent those in planetary‐scale modelingPlastic strain softening enables hysteresis with a memory duration similar to grain growth at lithospheric temperature conditions [ABSTRACT FROM AUTHOR]

Based on the experimental study on Saint‐Maximin limestone presented in a companion paper (Abdallah et al., 2020), a gradient‐dependent plasticity model is developed to account for the key role of porosity heterogeneity in the formation and propagation of compaction bands. The constitutive law, developed in the frame of a micromorphic continuum theory, contains two hardening variables: the porosity and its second gradient. A calibration method for the additional micromechanical parameters is proposed. Data from the companion paper are used in a four‐step calibration procedure. First, the standard Cauchy component is calibrated by means of the macroscopic stress‐strain curves. An Asymmetric Cam‐Clay (ACC) model is adopted for the yield surface. The dominant wavelength of the porosity heterogeneity of the material is evaluated by applying the fast Fourier transformation (FFT) on several porosity profiles obtained on samples before loading. This wavelength is interpreted as the material length that appears in the model. In addition to the deformation maps computed from digital volume correlation (DVC), a method that evaluates the second gradient of porosity is developed to calibrate the hardening laws. A linear stability analysis (LSA) is performed to calibrate the higher‐order elastic modulus. The constitutive model is implemented in a finite element code, and triaxial loading experiments are simulated. The numerical results are consistent with the experimental data in terms of the onset of compaction bands, their thickness, and plastic strain. The size effect of the sample is explored, and a regular band spacing, comparable to the band thickness, is obtained. Key Points: The key role of porosity heterogeneity in compaction banding of a weak limestone is modeled using gradient‐dependent plasticityThe constitutive model is calibrated from triaxial experiments data combined with imaging techniques to compute porosity and strain mapsFinite element simulations can correctly reproduce the dominant features of the compaction bands as observed in the experiments [ABSTRACT FROM AUTHOR]

We performed deformation and grain growth experiments on natural olivine aggregates with olivine water contents (COH = 600 ± 300 H/106 Si) similar to upper mantle olivine, at 1000–1200°C and 1,400 ± 100 MPa confining pressure. Our experiments differ from published grain growth studies in that most were (1) conducted on natural olivine cores rather than hot‐pressed aggregates and (2) dynamically recrystallized prior to or during grain growth. We combine our results with similar experiments performed at 1200–1300°C and fit the data to a grain growth relationship, yielding a growth exponent (p) of 3.2, activation energy (EG) 620 ± 145 kJ mol−1 (570 ± 145 kJ mol−1 when accounting for the role of temperature on water content), activation volume (VG) ~5 × 10−6 m3 mol−1, and rate constant (k0) 1.8 × 103 mp s−1. Our EG is within uncertainty of that predicted for dislocation creep of wet olivine (E* = 480 ± 40 kJ mol−1). Grain size in strain rate‐stepping samples adjusted to the olivine piezometer within 1.3–7.9% strain. The active grain boundary migration processes during deformation and dynamic recrystallization affect the kinetics of postdeformation grain growth, as grain boundary migration driven by strain energy density (ρGBM) may delay the onset of grain growth driven by interfacial energy (γGBM). We compared our postdeformation grain growth rates with data from previously published hydrostatic annealing experiments on synthetic olivine. At geologic timescales, the growth rates are much slower than predicted by the existing wet olivine grain growth law. Key Points: Postdeformation grain growth of moderately wet olivine is slower than predicted by the previously published wet olivine grain growth lawGrain boundary migration driven by gradients in strain energy density may delay the onset of grain growth driven by interfacial energyGrain size reduction by dynamic recrystallization can play an important role in the longevity of strain localization in Earth's lithosphere [ABSTRACT FROM AUTHOR]

The presence of pressurized fluids influences the mechanical behavior of faults. To test the roles of normal stress and fluid pressure on shear strength and localization behavior of calcite gouges, we conducted a series of rotary‐shear experiments with pore fluid pressures up to 10.5 MPa and difference between normal stress and fluid pressure up to 11.2 MPa. Calcite gouges were sheared for displacements of 0.3 m to several meters at slip rates of 1 mm/s and 1 m/s. Drainage conditions in experiments were constrained from estimates of the hydraulic diffusivity. Gouges were found to be drained at 1 mm/s, but possibly partially undrained during sliding at 1 m/s. Shear strength obeys an effective‐stress law with an effective‐stress coefficient close to unity with a friction coefficient of ~0.7 that decreases to 0.19 due to dynamic weakening. The degree of comminution and slip localization constrained from experimental microstructures depends on the effective normal stress. Slip localization in calcite gouges does not occur at low effective normal stress. The presence of pore fluids lowers the shear strength of gouges sheared at 1 mm/s and causes an accelerated weakening at 1 m/s compared to dry gouges, possibly due to enhanced subcritical crack growth and intergranular lubrication. Thermal pressurization occurs only after dynamic weakening when friction is generally low and relatively independent of normal stress and therefore unaffected by thermal pressurization. The experimental results are consistent with the view that the presence of pressurized fluid in carbonate‐bearing faults can facilitate earthquake nucleation. Key Points: Normal stress and fluid pressure equally affect shear strength of calcite gouges at relatively low effective normal stresses (≤11 MPa)The degree of slip localization in calcite gouges sheared at seismic slip rates increases with effective normal stressThermal pressurization has small effect on shear stress as it occurs after change from pressure‐ to temperature‐controlled slip behavior [ABSTRACT FROM AUTHOR]

Experimental results demonstrate that amorphous materials can be generated by frictional sliding at a wide range of conditions and that once formed the frictional strength of the fault is reduced. Nevertheless, amorphous materials have not been described in many natural faults, and their importance in natural systems is not well understood. We have identified amorphous materials within a 3‐mm‐thick slip zone (i.e., narrow band) in the Chihshang Fault, Taiwan, as well as within a submicrometer thick band in a distributed network of scaly clays. The scaly clays resemble those developed in other faults that demonstrably accommodated slow creep, and they also impart very low permeability implying that fluid‐related alteration (which could result in amorphization) will be inefficient. For this reason, and because of its throughgoing nature and position within a deforming zone, we infer a shear‐related origin for the amorphous materials. Therefore, amorphous materials can be observed in near‐surface environments because they have been formed during recent deformation under limited fluid‐circulation circumstances. The narrow slip zone containing amorphous materials is throughgoing and is interpreted to have acted as a frictional interface that accommodated brittle deformation at the macroscopic scale, whereas the surrounding mélange accommodated distributed "ductile" shear. The formation of amorphous materials therefore typifies the "mixed‐mode" style of deformation in this creeping fault zone. Key Points: Amorphous materials (AMs), derived from shear‐induced amorphization, were observed in the creeping Chihshang Fault, TaiwanAMs within a localized narrow band and along scaly clay fabrics can be treated brittle and ductile deformation in bulk fault rocksThe presence of AMs was resulted from lasting on‐fault amorphization driven by tectonic loading and stress relaxation [ABSTRACT FROM AUTHOR]

Recent laboratory evidence shows that compaction creep in porous rocks may develop through stages of acceleration, especially if the material is susceptible to strain localization. This paper provides a mechanical interpretation of compaction creep based on viscoplasticity and nonlinear dynamics. For this purpose, a constitutive operator describing the evolution of compaction creep is defined to evaluate the spontaneous accumulation of pore collapse within an active compaction band. This strategy enables the determination of eigenvalues associated with the stability of the response which is able to differentiate decelerating from accelerating strain. This mathematical formalism was linked to a constitutive law able to simulate compaction localization. Material point simulations were then used to identify the region of the stress space where unstable compaction creep is expected, showing that accelerating strains correspond to pulses of inelastic strain rate. Such pulses were also found in full‐field numerical analyses of delayed compaction, revealing that they correspond to stages of inception and propagation of new bands across the volume of the simulated sample. These results illustrate the intimate relation between the spatial patterns of compaction and their temporal dynamics, showing that while homogeneous compaction develops with decaying rates of accumulation, localized compaction occurs through stages of accelerating deformation caused by the loss of strength taking place during the formation of a band. In addition, they provide a predictive modeling framework to simulate and explain the spatio‐temporal dynamics of compaction in porous sedimentary formations. Key Points: Delayed compaction banding is interpreted as a viscoplastic material instabilityStability indices predicting impending unstable creep in compacting rock have been proposedIt has been shown that unstable compaction creep involves transient pulses of inelasticity both at local and global levels [ABSTRACT FROM AUTHOR]

Outgassing of volcanic systems is a ubiquitous phenomenon. Yet the mechanisms facilitating gas escape from vesiculating magmas and lavas remain poorly understood. Pervasive outgassing is thought to depend on the efficient and abundant formation of permeable pathways. Here we present results from experiments designed to identify the conditions and mechanisms needed to form such permeable pathways. We use a foamed silicate melt (FOAMGLAS®) in our experiments as a proxy for natural vesicular melts. FOAMGLAS® cores are compressed under a range of temperatures and strain rates, and results are evaluated against the state of melt relaxation (parameterized using the Deborah number). We find that foam microstructure and rheological and outgassing behaviors evolve with strain and as a function of melt relaxation state. Relaxed melt foams harden during deformation but remain impermeable. As foams become less relaxed at higher strain rate and/or melt viscosity, they show complex responses to deformation (strain weakening and hardening) yet remain impermeable. Strain localization and formation of high‐permeability bands occur only in highly strained, unrelaxed foams. However, these bands are thin and oriented perpendicular to the principal stress, resulting in limited outgassing. Permeable pathways do not readily form in foams; rather, high‐porosity melts are "persistently impermeable." Our results imply that vesicular silicic lavas may not outgas as efficiently as previously thought. Instead, sustained impermeability allows the lava to maintain low effective viscosities, and to flow extended distances. In addition, pore pressures may rise in deforming impermeable lavas, perhaps priming the lava for later explosive behavior. Key Points: High‐temperature deformation experiments on cores of FOAMGLAS® explore melt foam rheological and outgassing behaviorsDeforming melt foams show strain weakening, hardening, and localization, but undergo little or no outgassing (i.e., are impermeable)Crystal‐free bubble‐rich silicic lavas outgas inefficiently, allowing for sustained low effective viscosities and efficient transport [ABSTRACT FROM AUTHOR]

Grain size reduction via dynamic recrystallization (DRX) is intimately related to the softening of crystalline materials under an applied stress, and plays a role in numerous geodynamic processes that involve strain‐weakening feedbacks. Despite its importance, few studies have provided empirical constraints on the rates of DRX and grain size reduction. Here we examine DRX rates using the Johnson‐Mehl‐Avrami‐Kolmogorov theory for phase transformation kinetics. Our analysis uses a compilation of published and newly‐derived DRX data from experimental studies on a wide range of geologic and engineering materials. We find that DRX rates are strongly dependent on the homologous temperature at which deformation occurs, with DRX occurring over a broad range of shear strains (0.01 < γ < 200), and more rapidly with increasing homologous temperature. Moreover, the experimental data can be described by a single fit to a modified form of the Avrami equation that incorporates homologous temperature dependence, implying that, to first order, DRX rates are largely material independent. We also examine the influence of stress, deformation work rate, crystal orientation, and initial grain size on DRX rates and compare DRX data from experimental and natural conditions. Overall, we show that microstructures can evolve over long transient intervals, and provide a framework for determining DRX kinetics from empirical data, which can ultimately be incorporated into geodynamic models of grain size evolution in the Earth. Key Points: Dynamic recrystallization (DRX) rates are examined for a compilation of experimental data using the Avrami theory for transformation kineticsDRX rates are strongly dependent on homologous temperature, with DRX occurring over a broad range of shear strains (0.01 < γ < 200)Experimental DRX data can be fit using a single, material‐independent relationship that incorporates homologous temperature [ABSTRACT FROM AUTHOR]

Localization processes in the viscous lower crust generate ductile shear zones over a broad range of scales affecting long‐term lithosphere deformation and the mechanical response of faults during the seismic cycle. Here we use centimeter‐scale numerical models in order to gain detailed insight into the processes involved in strain localization and rheological weakening in viscously deforming rocks. Our 2‐D Cartesian models are benchmarked to high‐temperature and high‐pressure torsion experiments on Carrara marble samples containing a single weak Solnhofen limestone inclusion. The models successfully reproduce bulk stress‐strain transients and final strain distributions observed in the experiments by applying a simple, first‐order softening law that mimics rheological weakening. We find that local stress concentrations forming at the inclusion tips initiate strain localization inside the host matrix. At the tip of the propagating shear zone, weakening occurs within a process zone, which expands with time from the inclusion tips toward the matrix. Rheological weakening is a precondition for shear zone localization, and the width of this shear zone is found to be controlled by the degree of softening. Introducing a second softening step at elevated strain, a high strain layer develops inside the localized shear zone, analogous to the formation of ultramylonite bands in mylonites. These results elucidate the transient evolution of stress and strain rate during inception and maturation of ductile shear zones. Key Points: First‐order numerical softening laws successfully reproduce strain localization and stress transients observed in laboratory torsion testsOur models provide a virtual way of analyzing viscous process zone evolution as it nucleates near an inclusion and propagates into a matrixThe degree of weakening controls the process zone geometry, the finite width of shear bands, and the potential formation of ultramylonites [ABSTRACT FROM AUTHOR]

Digital volume correlation analysis of time‐resolved X‐ray microtomography scans acquired during in situ triaxial compression of Green River shale cores provided time series of 3‐D incremental strain fields that elucidated evolving deformation processes by quantifying microscopic strain localization. With these data, we investigated the impact of mechanical anisotropy on microscopic strain localization culminating in macroscopic shear failure. We conducted triaxial compression experiments with the maximum compressive stress, σ1, aligned perpendicular and parallel to lamination planes in order to investigate end‐member stress states that arise within sedimentary basins. When the preexisting laminations were perpendicular to σ1, a lamination‐parallel region with high axial compaction developed within the macroscopically linear deformation phase of the experiment and then thickened with increasing applied differential stress. Scanning electron microscopy images indicate that this axial compaction occurred within a lower density lamination and that more axial compaction occurred within the center of the core than near its sides. Boundary element method simulations suggest that this compacting volume promoted shear fracture development within the upper portion of the shale. When the laminations were parallel to σ1, lamination‐parallel dilation bands formed, thickened, and intensified in dilation. Population densities of the distributions of incremental shear strain, radial dilation, and axial contraction calculated by digital volume correlation analysis enabled quantification of the evolving overall impact of, and interplay between, these various deformation modes. Key Points: Time‐resolved in situ X‐ray tomography imaging captured the path to macroscopic shear failure within shaleLamination‐parallel zones of high incremental axial contraction and radial dilation developed prior to macroscopic failureDigital volume correlation and numerical modeling results suggest that axial compaction promoted shear strain localization [ABSTRACT FROM AUTHOR]

In Southern California, the Pacific-North America relative plate motion is accommodated by the complex southern San Andreas Fault system that includes many young faults (<2 Ma). The initiation of these young faults and their impact on strain partitioning and fault slip rates are important for understanding the evolution of this plate boundary zone and assessing earthquake hazard in Southern California. Using a three-dimensional viscoelastoplastic finite element model, we have investigated how this plate boundary fault system has evolved to accommodate the relative plate motion in Southern California. Our results show that when the plate boundary faults are not optimally configured to accommodate the relative plate motion, strain is localized in places where new faults would initiate to improve the mechanical efficiency of the fault system. In particular, the Eastern California Shear Zone, the San Jacinto Fault, the Elsinore Fault, and the offshore dextral faults all developed in places of highly localized strain. These younger faults compensate for the reduced fault slip on the San Andreas Fault proper because of the Big Bend, a major restraining bend. The evolution of the fault system changes the apportionment of fault slip rates over time, which may explain some of the slip rate discrepancy between geological and geodetic measurements in Southern California. For the present fault configuration, our model predicts localized strain in western Transverse Ranges and along the dextral faults across the Mojave Desert, where numerous damaging earthquakes occurred in recent years. [ABSTRACT FROM AUTHOR]

We use xenoliths from young cinder cones in the eastern Mojave region of Southern California to investigate deformation fabrics and their implications for strain localization, lithospheric viscosity, and controls on mineral lattice-preferred orientations (LPOs) and seismic anisotropy at Moho depths. Lower crustal gabbros and upper mantle peridotites were collected from two areas separated by ∼80 km-the Cima and Deadman Lake Volcanic Fields. At both localities, mantle peridotites exhibit solid-state deformation fabrics that show strong LPOs and other evidence for dislocation creep as the dominant deformation mechanism. The preservation of microstructures ranging from annealed, granular, to mylonitic indicates that there is substantial strain localization in the Mojave mantle near the Moho. Paleopiezometry conducted on subgrains in olivine indicates that stress magnitudes during deformation were 12-26 MPa. Calculations using olivine flow laws yield strain rates on the order of ∼10−12/s and an associated viscosity of ∼1019 Pa s, consistent with estimates of mantle lid viscosity from postseismic relaxation studies. Two types of olivine LPOs were observed in the peridotites: A-type and E-type fabrics, both of which predict seismic fast axes that align parallel to the lineation within the foliation plane. The occurrences of the two fabric types appear to correlate with strain magnitude but not with water contents measured using Fourier transform infrared spectroscopy. Lower crustal fabrics are dominated by magmatic foliations and LPOs in plagioclase, which produce seismic fast axes oriented perpendicular to the foliation plane. This suggests that even where mantle and lower crustal fabrics are kinematically linked, their seismic fast axes may appear anticorrelated across the Moho. [ABSTRACT FROM AUTHOR]