Creep Events and Shear Localization in a Polyphase Material: Insight into the Brittle-Ductile Transition

Abstract:

The co-occurrence of brittle and ductile deformation underlies many large-scale tectonic processes, geodetically and seismologically monitored strain transients, and even several applied geoscience problems in hydrocarbon production and CO₂ sequestration. Evidence for such mixed-deformation is preserved in the rock record, such as in mid-crustal shear zones where, depending on the temperature and pressure conditions, some mineral phases undergo brittle and others ductile deformation during the same overall bulk deformation.

Here we combine physical experiments with an analytical approach to investigate the impact of the semi-brittle material on the deformation localization and on stick-slip and creep events. Two sets of experiments are performed: 1) shear of a mixture of elastic-frictional grains within a viscously deforming media, and 2) propagation of wetted fractures in a visco-elasto-plastic interlinked polymer gel during shear. We measure the force and displacement while imaging the shear cell. In both cases the rock analogues show a semi-brittle behavior where the deformation localization is enhanced and slip events get damped leading to creep. Several parameters were then extracted from the experiments and directly used as input parameters in an analytical solution for semi-brittle flow. In the analytical model localization and slip is modeled as a damped oscillator mimicking the behavior of discrete fractures originating in a strong material that get viscously damped in a surrounding continuous weaker material. That the measured and calculated decay times of slip events lead to comparable results is supportive of the models physical premise. This study suggest that in a mixed brittle-ductile system, localization can be efficient, frictional responses can be dampened but still present, and basic rheological descriptions may apply, all of which has implications for understanding strain transients and coseismic release.