Strain Localization and Weakening Processes in Viscously Deforming Rocks: Numerical Modeling Based on Laboratory Torsion Experiments. Issue 1 (30th January 2019)

• Record Type: Journal Article
• Authors: Döhmann, M. J. E. A.; Brune, S.; Nardini, L.; Rybacki, E.; Dresen, G.
• Is Part Of: Journal of geophysical research. Volume 124:Issue 1(2019)
• Page Start: 1120
• Page End: 1137
• Publication Date: 2019-01-30
• Subjects: dislocation creep -- torsion -- 2‐D numerical model -- rheological weakening -- two phase aggregates -- strain localization; Geomagnetism -- Periodicals; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 551.1
• DOI: 10.1029/2018JB016917
• Languages: English
• ISSNs: 2169-9313

Abstract

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. Key Points: First‐order numerical softening laws successfully reproduce strain localization and stress transients observed in laboratory torsion tests Our models provide a virtual way of analyzing viscous process zone evolution as it nucleates near an inclusion and propagates into a matrix The degree of weakening controls the process zone geometry, the finite width of shear bands, and the potential formation of ultramylonites