New Constraints on Fault Nucleation and Propagation Using Dynamic Microtomography Experiments

<p>Experimental rock deformation research plays an important role in understanding the mechanical behavior, deformation microstructures, and physical properties of rocks and minerals. In practice, most experiments are designed to isolate a given process, limiting access to the interplay between various processes that takes place in nature. This is in part because changes in microstructure are commonly documented after an experiment has ended. The loss of information during deformation makes quantifying feedback of different mechanisms extremely challenging. However, natural processes often involve concurrent inelastic deformation mechanisms and simultaneous metamorphic or diagenetic reactions. Quantitative accessment of these processes demands better constraints of the feedback between rock deformation and the evolving rock properties and microstructures.</p><p>Recent dynamic microtomography experiments have shown great potential in characterizing the evolution of microstructure and strain distribution during fault growth at in-situ pressure and temperature conditions. Using an X-ray transparent deformation apparatus that operates at crustal stress conditions, we have imaged the process of fault nucleation and propagation in natural rocks undergoing brittle faulting.  Applying the digital volume correlation technique to time-resolved 3-dimensional microtomographic datasets, we documented the evolution of strain distribution within a deforming rock. These results elucidate how fractures open, slide, coalesce, and propagate in rock samples responding to increasing shear stress.  </p><p>Using dynamic microtomography, it is now possible to address the effect of chemo-mechanical coupling on the emergent properties of rocks by conducting deformation experiments in which several mechanisms operate simultaneously. We studied the effect of chemo-mechanical coupling on fracturing induced by hydration reaction in serpentinite. Quantitative characterization of evolving mechanical behavior and microstructure enables us to understanding the feedback between thermal load, chemical reaction rate, and mechanical failure. Dynamic microtomography provides a promising approach to link evolving mechanical behavior with evolving microstructures. New experimental constraints on microstructural and internal stress-strain evolution can lead to more robust extrapolations of laboratory results to large scale geologic processes.</p>

Temporal evolution of a shear-type rock fracture process zone (FPZ) along continuous, sequential, and spontaneously well-separated laboratory instabilities—from intact rock to thick gouged fault

Summary The development of shear-type fault analogues from intact rock at the laboratory scale provides a unique opportunity for investigating tectonic-scale phenomena through the lens of geophysics. The transition from rock fracture creation to laboratory fault slip must exist. We observe three spontaneously temporally well-separated mechanical instabilities attributed to the continuous evolution of a shear-type rock fracture between two artificial flaws. Their separation is validated with rapid mechanical stress drops and stabilizations, periodical AE behaviors (AE event number and AE moment release rate), and b-value drops. One instability occurs near the stress peak and corresponds to fracture incipience where fault development is mostly identified via optical observations; the other two instabilities are in the post-stress-peak domain and correspond to the fault nucleation and slip stages, respectively, with distinguishable AE releases from the fault region. The macroscale fracture has been created at the moment of rapid-stress drop for the second instability; off-fault damage, increasing gouge powder generation, and slip acceleration can be identified within the fault slip stage. AE behavior throughout fault nucleation shows a reversal of the Omori-Utsu (O-U) law. AEs attributed to the fault slip display regular O-U law decay and the distinction between the AE behavior for fault nucleation and fault slip is pronounced. These observations and analyses can provide further understanding on the analogue relationship between a laboratory loading-induced fault and a natural fault.

Investigation of the Surface Morphology and Stacking Fault Nucleation on the (000-1)C Facet of Heavily Nitrogen-Doped 4H-SiC Boules

The stacking fault formation during physical vapor transport growth of heavily nitrogen-doped (mid-1019 cm−3) 4H-SiC crystals was investigated. Low-voltage scanning electron microscopy (LVSEM) observations detected the stacking fault formation on the (000-1) facet of heavily nitrogen-doped 4H-SiC crystals. Stacking faults showed characteristic morphologies, and atomic force microscopy (AFM) studies revealed that these morphologies of stacking faults stemmed from the interaction between surface steps and stacking faults. Based on these results, the stacking fault formation mechanism in heavily nitrogen-doped 4H-SiC crystals is discussed.