Search for: All records

Abstract The onset of brittle failure in rocks includes dilatancy and strain localization. To better understand this nucleation process, we analyze the evolution of the local three‐dimensional strain tensor using X‐ray tomograms acquired during triaxial compression experiments on granite and sandstone. The onset of the localization of the compaction, dilation, and shear strain occurs when ∼65% of the rock volume experiences dilation. Tracking the locations of the high strains throughout loading suggests that the deformation that occurs early in loading influences the location of the system‐sized fracture network that produces macroscopic failure. This influence is larger in the sandstone experiments than the granite experiments, likely due to the microstructure of the sandstone. These results have important implications for detecting precursors to catastrophic failure. 

Abstract Stress‐driven melt segregation may have important geochemical and geophysical effects but remains a poorly understood process. Few constraints exist on the permeability and distribution of melt in deformed partially molten rocks. Here, we characterize the 3D melt network and resulting permeability of an experimentally deformed partially molten rock containing several melt‐rich bands based on an X‐ray microtomography data set. Melt fractions range from 0.08 to 0.28 in the ∼20‐μm‐thick melt‐rich bands, and from 0.02 to 0.07 in the intervening ∼30‐μm‐thick regions. We simulated melt flow through subvolumes extracted from the reconstructed rock at five length scales ranging from the grain scale (3 μm) to the minimum length required to fully encompass two melt‐rich bands (64 μm). At grain scale, few subvolumes contain interconnected melt, and permeability is isotropic. As the length scale increases, more subvolumes contain melt that is interconnected parallel to the melt bands, but connectivity diminishes in the direction perpendicular to them. Even if melt is connected in all directions, permeability is lower perpendicular to the bands, in agreement with the elongation of melt pockets. Permeability parallel to the bands is proportional to melt fraction to the power of an exponent that increases from ∼2 to 5 with increasing length scale. The permeability in directions parallel to the bands is comparable to that for an isotropic partially molten rock. However, no flow is possible perpendicular to the bands over distances similar to the band spacing. Melt connectivity limits sample scale melt flow to the plane of the melt‐rich bands. 

Key Points The distance from the trench to the arc is negatively correlated with the slab dip within each subduction zone The thermal structure of the upper plate, which is tied to the depth of the slab coupling/decoupling transition, controls arc location Variations in the location of the spreading center in a given subduction zone can be linked to changes in slab dip during trench retreat 

Key Points High pore fluid pressure stabilizes fault propagation in porous sandstone deformed under drained conditions Slow faulting was associated with pervasive microcracking and diffuse shear bands only in samples deformed sufficiently slow Pervasive subcritical cracking enables slow faulting at high pore fluid pressure under drained conditions at the sample scale