Microscopic Defect Dynamics During a Brittle-To-Ductile Transition

The strength of the lithosphere is limited by pressure-dependent, "brittle" deformation of the upper crust and temperature- and strain-rate-dependent "ductile" deformation in the lower crust and upper mantle. From a simplified mechanistic point of view, intersection of these two strength envelopes can be used to estimate the depth of a region known as the brittle-to-ductile transition (BDT) where rocks deform by semi-brittle flow. This BDT is key for estimating the integrated strength of tectonic plates, constraining the earthquake cycle, and utilizing deep geothermal resources. The BDT region is associated with changes in 1) dominant deformation mechanisms, 2) strain localization, and 3) fault stability; however, the transitions in all three attributes are influenced by many parameters, including lithology, grain size, porosity, the presence and migration of pore fluids, the loading conditions, metamorphic reactions, and the geometric complexities of geologic formations and faults. Thus, the transitions in deformation mechanisms, failure mode, and stability do not typically occur simultaneously, resulting in complex behavior that is poorly understood. In this talk, we show that the crossing of a BDT in marble during deformation experiments in the laboratory is accompanied by systematic increase in the frequency of acoustic emissions suggesting a profound change in the mean size and propagation velocity of the active defects. We further identify dominant classes of emitted waveforms using unsupervised learning methods and show that their relative activity systematically changes as the rocks cross the brittle–ductile transition. As pressure increases, long-period signals are suppressed, and short-period signals become dominant. At higher pressures, signals frequently come in avalanche-like patterns. We propose that these classes of waveforms correlate with individual dominant defect types, that is cracks, twins and dislocation avalanches. Complex mixed-mode events indicate that interactions between the defects are common over the whole explored pressure range (0.1 - 250 MPa), in agreement with postmortem microstructural observations. To corroborate these interpretations, we performed deformation experiments on large calcite single crystals recording acoustic emissions and imaging the propagating sources with a high-speed camera. We discuss our results in the framework of a new micro-mechanical model and link the onset of the BDT to stress states at which tensile cracking around spherical cavities ceases to be possible. Our measurements provide unique, real-time data of microscale dynamics over a broad range of pressures and aid in the development of micromechanical models for semi-brittle deformation.