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

Abstract

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.

A Model of Failure and Localization of Basalt at Temperature and Pressure Conditions Spanning the Brittle‐Ductile Transition

“…Our predictions of the depth of the BDT agree well with observations of volcanic events (Ágústsdóttir et al., 2019; Sigmundsson et al., 2015) and natural (Blanck et al., 2019) and drilling‐induced seismicity (Friðleifsson et al., 2018) in Iceland, where basalt is extensively present as the main lithology. The model results are in good agreement with experimental evidence (Adelinet et al., 2013; Döhmann et al., 2019; Meyer et al., 2019; Nardini et al., 2018; Violay et al., 2012, 2017), in situ observations (Ágústsdóttir et al., 2019; Friðleifsson et al., 2014, 2017), and theoretical arguments (Carcione et al., 2018; Horii & Nemat‐Nasser, 1986; Parisio et al., 2019). The approach taken can be extended to study other lithologies such as carbonatic basements in which the depth of the BDT would be shallower (Parisio et al., 2019), or above‐average geothermal gradient around local temperature anomalies, such as shallow‐depth magma pockets (Elders et al., 2014).…”

“…Since inclusions, weak spots, and heterogeneities are likely to be ubiquitous in the Earth's crust, we believe that this assumption well represents the actual conditions. Laboratory experiments have also confirmed that a single inclusion could trigger localized shear strain under pressure and temperature conditions that would instead lead to homogeneous strain patterns in homogeneous samples (Nardini et al, 2018) and large deformation creep models have shown the necessity of including strain-softening in order to agree with the observations (Döhmann et al, 2019). Nevertheless, localization is possible in nonassociated plasticity and is triggered by the appearance of singularities in the acoustic tensor (Grassl & Jirásek, 2006;Rudnicki & Rice, 1975), themselves associated with acceleration waves (Lemaitre et al, 2009).…”

Controls on Asymmetric Rift Dynamics: Numerical Modeling of Strain Localization and Fault Evolution in the Kenya Rift

“…Note that the employed random number generator srand can yield variable patterns for different library configurations; hence, individual model details are currently not reproducible on other computers, only the overall ensemble behavior. To facilitate localization, we use softening laws for (i) brittle layers (e.g., Brune, 2014; Jourdon et al., 2021; Persaud et al., 2017; Petersen & Schiffer, 2016; Salazar‐Mora et al., 2018) and (ii) ductile parts of the crust (e.g., Döhmann et al., 2019; Gerbi et al., 2010; Pérez‐Gussinyé et al., 2020) (Table 1). Brittle softening is implemented as a linear decrease of the friction coefficient by up to 90% between accumulated strain values of 0–0.5.…”

Finite Thickness of Shear Bands in Frictional Viscoplasticity and Implications for Lithosphere Dynamics

“…However, the physical processes within shear bands, which control their width, are often considered to be beyond the scope of current geodynamic models. Hence, for sake of simplicity, strain localization is often induced by a priori-defined strain-softening functions (Buck & Lavier, 2001;Buiter et al, 2006;Döhmann et al, 2019;Huismans & Beaumont, 2002;Lavier et al, 1999), which are meant to take into account the role of complex thermo-hydro-chemico-mechanical interactions within faults in a phenomenological sense.…”