Memory of brittle-ductile yielding within near surface fault zone sediments

The interactions between fluid and solids in fault zones are governed by slip, slip rate, and constituent properties. These interactions are recorded by particle shape and size distributions, fracture patterns, and the geochemical composition of material within the deformation zone. The evolution of near-surface sediment microstructures and yielding behaviors under tectonic loading and at variable fluid saturation remains an open question. We collect undisturbed 10 x 40 mm cores from unconsolidated silt-sized sediments (fines) surrounding, and along, a fault strand that slipped while saturated, and likely experienced aseismic slip under variable saturation over the past 300 years. We use X-ray microtomography to analyze voids within the fines and found that they are ellipsoidal, have volume distributions that are best fit by a truncated power-law, orient sub-parallel to the fault strike, and sometimes merge into tabular or irregularly shaped fractures. The volume range for power-law scaling in the distributions separates a smaller population of voids with markedly different distributions in sphericity, tortuosity, aspect ratio, and minor/major axis lengths from a larger population of voids. The power-law truncation is likely due to the finite core size. We interpret the voids as initially small gas bubbles that nucleated where cavities existed within the fines and then grew via diffusion of immiscible gases when saturated, or via brittle/ductile yielding of the fines under variable saturation. Several fractures cross-cut or branch off some voids, indicating multiple deformation events and suggesting that the void boundaries are weak spots within the fines that accommodate tectonic strain. Similar growth mechanisms have been observed in magmatic systems, where ductile yielding of the melt occurs from the merging of bubbles that primarily orient at acute angles from the maximum extension direction. These findings suggest that, in addition to sands, pore structures in finer-grained sediments preserve a record of near-surface aseismic slip and may provide a relative estimate of near-surface strain. The findings further imply that a process akin to ductile yielding deformed the fines and, in turn, the pore voids.