Microscopic Defect Dynamics of 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 of the lower crust and upper mantle. From a simplified mechanistic point of view, intersection of these two strength envelopes can be used to estimate a depth of a region known as the brittle–to–ductile transition (BDT) where rocks deform by semi-brittle flow. The transitions in deformation mechanisms, failure mode and stability need not to occur simultaneously, resulting in complex behavior that is poorly understood. The fundamental agents of deformation that control rock strength are microscale defects such as point defects, dislocations, twins, grain boundaries and cracks . Their collective motion under stress leads to the emergence of larger-scale features that ultimately determine whether rocks fracture or flow during tectonic processes. Here, in collaboration with EAPS rock physics laboratories team, I show progressive evolution in dynamics of “defects” encoded as emitted ultrasound emissions in room-temperature deformation of dry Carrara marble. Three dominant classes of waveforms are identified: First, long-period crack-like signals, which are abundant in low confining pressures. The second class of waves- due to twining process- are more localized in frequency domain with evolving frequencies as confining pressure increases. Increasing confining pressure from 0.1 to 150MPa results shift in the dominant frequency of the signals transits from ~500kHz to ~20 MHz. The third class of waveforms- as signature of dislocation avalanches-, often weak and mixed with the twin waveforms, are more common in higher pressures (>70MPa) and extend to frequencies above 40MHz (i.e., ultra-high frequency acoustic emissions with duration as short as 200ns) . Further increasing confining pressure to 200MPa and higher, the latter signals are more dominant and single out as multiple avalanches in single emission ; in 250MPa semi-continuous emissions of type III signals are distinct possibly indicating continuous gliding of collective dislocations. Modeling ultrasound emissions from the sources with different sizes and front velocities, indicates for a stable finite size source for all range of pressures, the velocity of propagation must increase in factor of 40-50 with regard to 10MPa confining pressure. The results provide a novel comprehensive picture of dynamics of nucleation, propagation and interaction of plastic defects of geomaterials in transition from brittle to ductile deformations.

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