Patterns of strain localization in heterogeneous, polycrystalline rocks – a numerical perspective

Gardner, Robyn; Piazolo, Sandra; Evans, Lynn; Daczko, Nathan R.

doi:10.1016/j.epsl.2017.01.039

Cited by 38 publications

(35 citation statements)

References 48 publications

(87 reference statements)

Supporting

Mentioning

Contrasting

Order By: Relevance

“…In an attempt to model rock weakening and strain localization, different types of softening parameterizations have been previously used in order to describe the weakening behavior of natural materials. For example, Gardner et al () studied strain localization using different load bearing framework geometries. They found that interconnected weak layers are hard to form without a dynamic weakening process, which was also observed in an experimental study by Holyoke and Tullis ().…”

Section: Discussionmentioning

confidence: 99%

“…This implies the necessity of using softening laws to properly model strain localization and thus shear zone formation. Gardner et al () used a different implementation to simulate weakening. They introduced stress dependent softening combined with time dependent hardening focusing on the transition from nonlinear to linear flow.…”

Section: Discussionmentioning

confidence: 99%

“…Other modeling studies focused on the effect of rheological weakening and hardening mechanisms. Weakening mechanisms have been formulated as a function of strain (Cyprych et al, ; Mazzotti & Gueydan, ), stress (Gardner et al, ), and deformation work or grain size in combination with grain size‐dependent flow laws (e.g., Bercovici & Ricard, ; Cross et al, ; Herwegh et al, ; Jessell et al, ). However, all these formulations have been shown to strongly influence the localization behavior in numerical models.…”

Section: Introductionmentioning

confidence: 99%

See 2 more Smart Citations

Strain Localization and Weakening Processes in Viscously Deforming Rocks: Numerical Modeling Based on Laboratory Torsion Experiments

et al. 2019

View full text Add to dashboard Cite

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.

show abstract

Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Strain Localization and Weakening Processes in Viscously Deforming Rocks: Numerical Modeling Based on Laboratory Torsion Experiments

et al. 2019

View full text Add to dashboard Cite

show abstract

“…1020 Ma. Given the uncertainty of the dating, as well as other observations about the temporal and spatial pattern of strain localization (e.g., Jessell et al, 2005;Gardner et al, 2017), the shear zones on the eastern and western margins of the granite need not have formed synchronously.…”

Section: Geologic Context For Strain Localizationmentioning

confidence: 99%

Timing and anatomy of granitic strain gradients in the Grenville Front tectonic zone, Ontario, Canada

et al. 2017

View full text Add to dashboard Cite

Observations of physical and chemical changes across strain gradients can provide information about the processes that lead to localization and therefore provide better tools for prediction of spatial and temporal strain patterns. In contrast to the many chemical and microstructural studies of natural shear zones in metapelites and mafic lithologies, few have described kilometer-scale granitic strain gradients despite the fact that granitoid bodies make up much of the orogenic crust. This study reports microstructural and compositional data across two amphibolite-facies strain gradients from middle crust of the Grenville orogen. The kilometer-scale gradients, defined by stretched enclaves and foliation intensity, form parts of the Boundary and Bad River shear zones, located along the east and west borders of the Bad River granite in the southwestern Grenville Front tectonic zone in Ontario, Canada. The granite is bounded on both sides by older orthogneiss (ca. 1720 Ma igneous crystallization age). Zircon U-Pb ages indicate that the granite crystallized from a magma at ca. 1465 Ma, with metamorphic growth at ca. 1020 Ma. Zircons in the orthogneiss record metamorphic growth during these two younger episodes. These age determinations are consistent with other regional dates and indicate that the strain gradients in the Bad River granite formed during the Grenville orogeny. Whole-rock analyses reveal minor heterogeneity in major-element distribution in the granite that can be attributed to igneous processes, and homogeneity in the trace elements, indicating that strain did not affect the bulk-rock composition. In contrast, microstructural and chemical analyses across the two strain gradients indicate some correlation with strain: slight changes in mineral compositions, development of crystallographic preferred orientations in quartz, an increase in recrystallized fraction, a reduction in recrystallized grain size, and the development of a mixed-phase matrix. However, most of these variations are subtle and occur at different positions across the gradients. The spatial distribution of the microscale changes suggests a change in deformation mechanisms toward the margin, accompanying increased localization. The rheological weakening at the Bad River granite margins was the product of microstructural, rather than mineralogical, change, in contrast to nearby shear zones in more mafic units.

show abstract

“…Previous studies have shown that connectivity and continuity of phases play a significant role in Journal of Geophysical Research: Solid Earth 10.1002/2017JB014000 the constitutive behavior of rocks [Burg and Wilson, 1987;Jordan, 1988;Handy, 1990;Bloomfield and Covey-Crump, 1993;Handy, 1994]. In addition, transitioning from a strong-phase supported framework (i.e., quartz dispersed around stronger feldspar grains that support the load) to a weak-phase supported network (i.e., weak interconnected quartz layers support the load) dramatically reduces a rock's bulk strength [Arzi, 1978;Nicolas, 1992;Gardner et al, 2017]. In the case of the Seven Gables outcrop, it appears that the dike's composition facilitated the development of monomineralic quartz layers, which led to weakening compared to the granodiorite.…”

Section: Strength Contrast Between Granodiorite and Dikementioning

confidence: 99%

Testing constitutive equations for brittle‐ductile deformation associated with faulting in granitic rock

2017

View full text Add to dashboard Cite

Uncertainty in constitutive equations for brittle‐ductile deformation limits our understanding of earthquake nucleation and propagation at the base of the seismogenic lithosphere. To reduce this uncertainty, we investigate exhumed strike‐slip faults and related deformation features in the Lake Edison granodiorite (central Sierra Nevada, CA) that developed at 250–500°C and ~250 MPa. The Seven Gables outcrop contains a 10 cm wide contractional fault step separating 2 m‐scale left‐lateral faults. Within the step, an ~ 4 cm thick leucocratic dike is stretched and rotated, thus constraining the kinematics of deformation, and the dike and surrounding granodiorite are strongly mylonitized. Petrographic and electron backscatter diffraction analyses reveal evidence for brittle and plastic deformation mechanisms, including dislocation creep, diffusion creep, microfracturing, and cataclasis. We present a 2‐D finite element model of the Seven Gables outcrop that tests a series of candidate constitutive equations: Von Mises elastoplasticity, Drucker‐Prager elastoplasticity, power law creep viscoelasticity, two‐layer elastoviscoplasticity, and coupled elastoviscoplasticity. Models based on Von Mises yielding most accurately match the outcrop deformation. Frictional plastic yield criteria (i.e., Drucker‐Prager) are incapable of reproducing the outcrop deformation due to the elevated mean compressive stress and reduced plastic yielding within the model fault step. Furthermore, the power law creep viscoelastic model requires a high strain rate (~10−4 s−1) to resolve slip on faults and fails to localize strain within the step region. Comparing model results and elastic stress fields with field observations suggests that deformation localizes in regions of elevated mean compressive stress and Mises equivalent stress.

show abstract

Patterns of strain localization in heterogeneous, polycrystalline rocks – a numerical perspective

Cited by 38 publications

References 48 publications

Strain Localization and Weakening Processes in Viscously Deforming Rocks: Numerical Modeling Based on Laboratory Torsion Experiments

Strain Localization and Weakening Processes in Viscously Deforming Rocks: Numerical Modeling Based on Laboratory Torsion Experiments

Timing and anatomy of granitic strain gradients in the Grenville Front tectonic zone, Ontario, Canada

Testing constitutive equations for brittle‐ductile deformation associated with faulting in granitic rock

Contact Info

Product

Resources

About