We studied a structurally oversimplified, extensional fault zone developed in poorly lithified, quartz-rich, high-porosity sandy sediments of the seismically active Crotone Basin (southern Italy). The fault zone consists of a cm-thick, discrete fault core embedded in virtually undeformed wall sediments. By combining grain size, shape, and microstructural analyses with mineralogical analyses and permeability measurements, we investigated the influence of initial sedimentological characteristics of sands on the final faulted granular products and related hydrologic properties. Faulting produces a general grain-size and porosity reduction by changing both the grain-size and shape distributions. We document a combination of intragranular fracturing, spalling, and flaking of grain edges in the fault core, which do not depend on grain mineralogy. The dominance of cataclasis, also confirmed by fractal dimensions >2.6, is generally not expected at a deformation depth <1 km. Initial grain size exerts a fundamental control on the comminution process and on the resulting permeability variations up to four orders of magnitude. Coarse-grained sand shows a much higher comminution intensity, grain-shape variations, and permeability drop than fine-grained sands. This is because coarser aggregates have (1) fewer grain-to-grain contacts for a given area, which results in higher stress concentration at contact points, and (2) a higher probability of preexisting, intragranular microstructural defects that result in a lower grain strength. The peculiar structural architecture, the dominance of cataclasis over nondestructive particulate flow, and the compositional variations of clay minerals in the fault core strongly suggest that the studied fault zone developed by a coseismic rupture.

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