FRACTURE-SIZE SCALING PATTERNS IN THE CAMBRIAN MESóN GROUP, ARGENTINA

The Cambrian Mesón Group, a thoroughly cemented quartzarenite, features a variety of opening- and shear-mode fractures, consistently arranged in parallel or conjugate sets (respectively). Fracture sets studied include thoroughly cemented fractures that cut across multiple bedding planes as well as poorly cemented fractures that are typically confined to individual beds. The aperture-size scaling properties of these fracture sets were studied in the field and using a scanning electron microscope equipped with a cathodoluminescence detector (SEM-CL). Samples collected from all field datasets revealed microscopic fractures (“microfractures”) whose orientation, kinematic mode, and crosscutting relationships suggest coeval development with large fractures. All field (outcrop-scale) datasets show an exponential relationship between fracture size and cumulative frequency. Microfracture size and cumulative frequency are related by a power law when episodic fracture opening is evident using SEM-CL; this relationship is present in both opening- and shear-mode fractures. Exponential size scaling is present in microfracture sets that lack an episodic opening signature. These results suggest that power-law scaling is the result of accumulated fracture strain involving positive feedback, whereby later fracturing episodes develop preferentially along pre-existing fractures. Power-law size distributions, where present, only persist at the microfracture scale, suggesting that this positive feedback mechanism is weakened among the larger fractures. The mechanical effects of sedimentary bedding poorly explain this weakening, as exponential distributions persist at the outcrop scale among bedding-bounded and non-bedding-bounded fractures alike.